Introduction to Programming Using Java, Seventh Edition

Version 7.0, August 2014

(Version 7.0.3, with minor corrections, May 2018)

Author:  David J. Eck  (eck@hws.edu)

This is an experimental ebook version of a free Java Programming textbook. Please excuse any irregularities in formatting. In particular, your reader might have problems with overly wide code segments. Note that PDF and web versions are available on-line at http://math.hws.edu/javanotes/. Source code for examples is not included in this ebook but can be accessed on-line at the same link. You can email comments to the author.

©1996--2018, David J. Eck.

This work is licensed under a Creative Commons Attribution-Noncommercial-ShareAlike 3.0 License. (This license allows you to redistribute this book in unmodified form for non-commercial purposes. It allows you to make and distribute modified versions for non-commercial purposes, as long as you include an attribution to the original author, clearly describe the modifications that you have made, and distribute the modified work under the same license as the original. Permission might be given by the author for other uses. See the license for full details.) The most recent version of this book is always available, at no charge, for downloading and for on-line use at the Web address: http://math.hws.edu/javanotes/

Introduction to Programming Using Java
Version 7.0, August 2014

Version 7.0.3, with minor corrections, December 2018

Preface

Introduction to Programming Using Java is a free introductory computer programming textbook that uses Java as the language of instruction. It is suitable for use in an introductory programming course and for people who are trying to learn programming on their own. There are no prerequisites beyond a general familiarity with the ideas of computers and programs. There is enough material for a full year of college-level programming. Chapters 1 through 7 can be used as a textbook in a one-semester college-level course or in a year-long high school course. The remaining chapters can be covered in a second course.

The Seventh Edition of the book covers "Java 7." The most recent version of Java is 8, but this book has only a few very short mentions of the new features in Java 8.

The home web site for this book is http://math.hws.edu/javanotes/. The page at that address contains links for downloading a copy of the web site and for downloading PDF versions of the book. The web site -- and the web site download -- includes source code for the sample programs that are discussed in the text, answers to end-of-chapter quizzes and a discussion and solution for each end-of-chapter exercises. Readers are encouraged to download the source code for the examples and to read and run the programs as they read the book. Readers are also strongly encouraged to read the exercise solutions if they want to get the most out of this book.

In style, this is a textbook rather than a tutorial. That is, it concentrates on explaining concepts rather than giving step-by-step how-to-do-it guides. I have tried to use a conversational writing style that might be closer to classroom lecture than to a typical textbook. This is certainly not a Java reference book, and it is not a comprehensive survey of all the features of Java. It is not written as a quick introduction to Java for people who already know another programming language. Instead, it is directed mainly towards people who are learning programming for the first time, and it is as much about general programming concepts as it is about Java in particular. I believe that Introduction to Programming using Java is fully competitive with the conventionally published, printed programming textbooks that are available on the market. (Well, all right, I'll confess that I think it's better.)

There are several approaches to teaching Java. One approach uses graphical user interface programming from the very beginning. Some people believe that object oriented programming should also be emphasized from the very beginning. This is not the approach that I take. The approach that I favor starts with the more basic building blocks of programming and builds from there. After an introductory chapter, I cover procedural programming in Chapters 2, 3, and 4. Object-oriented programming is introduced in Chapter 5. Chapter 6 covers the closely related topic of event-oriented programming and graphical user interfaces. Arrays are introduced in Chapter 3 with a full treatment in Chapter 7. Chapter 8 is a short chapter that marks a turning point in the book, moving beyond the fundamental ideas of programming to cover more advanced topics. Chapter 8 is about writing robust, correct, and efficient programs. Chapters 9 and 10 cover recursion and data structures, including the Java Collection Framework. Chapter 11 is about files and networking. Chapter 12 covers threads and parallel processing. Finally, Chapter 13 returns to the topic of graphical user interface programming to cover some of Java's more advanced capabilities.

The Seventh Edition of "Introduction to Programming using Java" is not a huge update from the sixth edition. In fact, my main motivation for the new version was to remove any use of applets or coverage of applets from the book. Applets are Java programs that run on a web page. When Java first came out, they were exciting, and it seemed like they would become a major way of creating active content for the Web. Up until the sixth edition, the web pages for this book included applets for running many of the sample programs. However, because of security issues and the emergence of other technologies, applets are no longer widely used. Furthermore, the most recent versions of Java made it fairly difficult and unpleasant to use the applets in the book. In place of applets, I have tried to make it as easy as possible for readers to download the sample programs and run them on their own computers.

Another significant change in the seventh edition is that arrays are now introduced in Chapter 3 in a basic form that is used throughout the next three chapters. Previously, arrays were not introduced until Chapter 7, after objects and GUI programming had already been covered. Much of the more advanced coverage of arrays is still in Chapter 7.

Aside from that, there are many small improvements throughout, mostly related to features that were new in Java 7. Version 7.0.3 is the final release of the seventh edition.

The latest complete edition of Introduction to Programming using Java is available on line at http://math.hws.edu/javanotes/. The first version of the book was written in 1996, and there have been several editions since then. All editions are archived (at least until my retirement) at the following Web addresses:

- First edition: http://math.hws.edu/eck/cs124/javanotes1/ (Covers Java 1.0.)

- Second edition: http://math.hws.edu/eck/cs124/javanotes2/ (Covers Java 1.1.)

- Third edition: http://math.hws.edu/eck/cs124/javanotes3/ (Covers Java 1.1.)

- Fourth edition: http://math.hws.edu/eck/cs124/javanotes4/ (Covers Java 1.4.)

- Fifth edition: http://math.hws.edu/eck/cs124/javanotes5/ (Covers Java 5.0.)

- Sixth edition: http://math.hws.edu/eck/cs124/javanotes6/ (Covers Java 5.0, with a bit of 6.0.)

- Seventh edition: http://math.hws.edu/eck/cs124/javanotes7/ (Covers Java 7.)

Introduction to Programming using Java is free, but it is not in the public domain. Version 7 is published under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike 3.0 License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-sa/3.0/. For example, you can:

- Post an unmodified copy of the on-line version on your own Web site (including the parts that list the author and state the license under which it is distributed!).

- Give away unmodified copies of this book or sell them at cost of production, as long as they meet the requirements of the license.

- Make modified copies of the complete book or parts of it and post them on the web or otherwise distribute them non-commercially, provided that attribution to the author is given, the modifications are clearly noted, and the modified copies are distributed under the same license as the original. This includes translations to other languages.

For uses of the book in ways not covered by the license, permission of the author is required.

While it is not actually required by the license, I do appreciate hearing from people who are using or distributing my work.

A technical note on production: The on-line and PDF versions of this book are created from a single source, which is written largely in XML. To produce the PDF version, the XML is processed into a form that can be used by the TeX typesetting program. In addition to XML files, the source includes DTDs, XSLT transformations, Java source code files, image files, a TeX macro file, and a couple of scripts that are used in processing. The scripts work on Linux and on Mac OS.

I have made the complete source files available for download at the following address:

http://math.hws.edu/eck/cs124/downloads/javanotes7-full-source.zip

These files were not originally meant for publication, and therefore are not very cleanly written. Furthermore, it requires a fair amount of expertise to use them. However, I have had several requests for the sources and have made them available on an "as-is" basis. For more information about the sources and how they are used see the README file from the source download.

Professor David J. Eck
Department of Mathematics and Computer Science
Hobart and William Smith Colleges
300 Pulteney Street
Geneva, New York 14456, USA
Email: eck@hws.edu
WWW: http://math.hws.edu/eck/

David Eck

Chapter 1

Overview: The Mental Landscape

When you begin a journey, it's a good idea to have a mental map of the terrain you'll be passing through. The same is true for an intellectual journey, such as learning to write computer programs. In this case, you'll need to know the basics of what computers are and how they work. You'll want to have some idea of what a computer program is and how one is created. Since you will be writing programs in the Java programming language, you'll want to know something about that language in particular and about the modern computing environment for which Java is designed.

As you read this chapter, don't worry if you can't understand everything in detail. (In fact, it would be impossible for you to learn all the details from the brief expositions in this chapter.) Concentrate on learning enough about the big ideas to orient yourself, in preparation for the rest of the book. Most of what is covered in this chapter will be covered in much greater detail later in the book.

Contents of Chapter 1:

- Section 1: The Fetch and Execute Cycle: Machine Language

- Section 2: Asynchronous Events: Polling Loops and Interrupts

- Section 3: The Java Virtual Machine

- Section 4: Fundamental Building Blocks of Programs

- Section 5: Objects and Object-oriented Programming

- Section 6: The Modern User Interface

- Section 7: The Internet and Beyond

- Quiz on This Chapter

Section 1.1

The Fetch and Execute Cycle: Machine Language

A computer is a complex system consisting of many different components. But at the heart -- or the brain, if you want -- of the computer is a single component that does the actual computing. This is the Central Processing Unit, or CPU. In a modern desktop computer, the CPU is a single "chip" on the order of one square inch in size. The job of the CPU is to execute programs.

A program is simply a list of unambiguous instructions meant to be followed mechanically by a computer. A computer is built to carry out instructions that are written in a very simple type of language called machine language. Each type of computer has its own machine language, and the computer can directly execute a program only if the program is expressed in that language. (It can execute programs written in other languages if they are first translated into machine language.)

When the CPU executes a program, that program is stored in the computer's main memory (also called the RAM or random access memory). In addition to the program, memory can also hold data that is being used or processed by the program. Main memory consists of a sequence of locations. These locations are numbered, and the sequence number of a location is called its address. An address provides a way of picking out one particular piece of information from among the millions stored in memory. When the CPU needs to access the program instruction or data in a particular location, it sends the address of that information as a signal to the memory; the memory responds by sending back the data contained in the specified location. The CPU can also store information in memory by specifying the information to be stored and the address of the location where it is to be stored.

On the level of machine language, the operation of the CPU is fairly straightforward (although it is very complicated in detail). The CPU executes a program that is stored as a sequence of machine language instructions in main memory. It does this by repeatedly reading, or fetching, an instruction from memory and then carrying out, or executing, that instruction. This process -- fetch an instruction, execute it, fetch another instruction, execute it, and so on forever -- is called the fetch-and-execute cycle. With one exception, which will be covered in the next section, this is all that the CPU ever does.

The details of the fetch-and-execute cycle are not terribly important, but there are a few basic things you should know. The CPU contains a few internal registers, which are small memory units capable of holding a single number or machine language instruction. The CPU uses one of these registers -- the program counter, or PC -- to keep track of where it is in the program it is executing. The PC simply stores the memory address of the next instruction that the CPU should execute. At the beginning of each fetch-and-execute cycle, the CPU checks the PC to see which instruction it should fetch. During the course of the fetch-and-execute cycle, the number in the PC is updated to indicate the instruction that is to be executed in the next cycle. (Usually, but not always, this is just the instruction that sequentially follows the current instruction in the program.)

A computer executes machine language programs mechanically -- that is without understanding them or thinking about them -- simply because of the way it is physically put together. This is not an easy concept. A computer is a machine built of millions of tiny switches called transistors, which have the property that they can be wired together in such a way that an output from one switch can turn another switch on or off. As a computer computes, these switches turn each other on or off in a pattern determined both by the way they are wired together and by the program that the computer is executing.

Machine language instructions are expressed as binary numbers. A binary number is made up of just two possible digits, zero and one. Each zero or one is called a bit. So, a machine language instruction is just a sequence of zeros and ones. Each particular sequence encodes some particular instruction. The data that the computer manipulates is also encoded as binary numbers. In modern computers, each memory location holds a byte, which is a sequence of eight bits. (A machine language instruction or a piece of data generally consists of several bytes, stored in consecutive memory locations.)

A computer can work directly with binary numbers because switches can readily represent such numbers: Turn the switch on to represent a one; turn it off to represent a zero. Machine language instructions are stored in memory as patterns of switches turned on or off. When a machine language instruction is loaded into the CPU, all that happens is that certain switches are turned on or off in the pattern that encodes that instruction. The CPU is built to respond to this pattern by executing the instruction it encodes; it does this simply because of the way all the other switches in the CPU are wired together.

So, you should understand this much about how computers work: Main memory holds machine language programs and data. These are encoded as binary numbers. The CPU fetches machine language instructions from memory one after another and executes them. It does this mechanically, without thinking about or understanding what it does -- and therefore the program it executes must be perfect, complete in all details, and unambiguous because the CPU can do nothing but execute it exactly as written. Here is a schematic view of this first-stage understanding of the computer:

Section 1.2

Asynchronous Events: Polling Loops and Interrupts

The CPU spends almost all of its time fetching instructions from memory and executing them. However, the CPU and main memory are only two out of many components in a real computer system. A complete system contains other devices such as:

- A hard disk or solid state drive for storing programs and data files. (Note that main memory holds only a comparatively small amount of information, and holds it only as long as the power is turned on. A hard disk or solid state drive is used for permanent storage of larger amounts of information, but programs have to be loaded from there into main memory before they can actually be executed. A hard disk stores data on a spinning magnetic disk, while a solid state drive is a purely electronic device with no moving parts.)

- A keyboard and mouse for user input.

- A monitor and printer which can be used to display the computer's output.

- An audio output device that allows the computer to play sounds.

- A network interface that allows the computer to communicate with other computers that are connected to it on a network, either wirelessly or by wire.

- A scanner that converts images into coded binary numbers that can be stored and manipulated on the computer.

The list of devices is entirely open ended, and computer systems are built so that they can easily be expanded by adding new devices. Somehow the CPU has to communicate with and control all these devices. The CPU can only do this by executing machine language instructions (which is all it can do, period). The way this works is that for each device in a system, there is a device driver, which consists of software that the CPU executes when it has to deal with the device. Installing a new device on a system generally has two steps: plugging the device physically into the computer, and installing the device driver software. Without the device driver, the actual physical device would be useless, since the CPU would not be able to communicate with it.

A computer system consisting of many devices is typically organized by connecting those devices to one or more busses. A bus is a set of wires that carry various sorts of information between the devices connected to those wires. The wires carry data, addresses, and control signals. An address directs the data to a particular device and perhaps to a particular register or location within that device. Control signals can be used, for example, by one device to alert another that data is available for it on the data bus. A fairly simple computer system might be organized like this:

Now, devices such as keyboard, mouse, and network interface can produce input that needs to be processed by the CPU. How does the CPU know that the data is there? One simple idea, which turns out to be not very satisfactory, is for the CPU to keep checking for incoming data over and over. Whenever it finds data, it processes it. This method is called polling, since the CPU polls the input devices continually to see whether they have any input data to report. Unfortunately, although polling is very simple, it is also very inefficient. The CPU can waste an awful lot of time just waiting for input.

To avoid this inefficiency, interrupts are generally used instead of polling. An interrupt is a signal sent by another device to the CPU. The CPU responds to an interrupt signal by putting aside whatever it is doing in order to respond to the interrupt. Once it has handled the interrupt, it returns to what it was doing before the interrupt occurred. For example, when you press a key on your computer keyboard, a keyboard interrupt is sent to the CPU. The CPU responds to this signal by interrupting what it is doing, reading the key that you pressed, processing it, and then returning to the task it was performing before you pressed the key.

Again, you should understand that this is a purely mechanical process: A device signals an interrupt simply by turning on a wire. The CPU is built so that when that wire is turned on, the CPU saves enough information about what it is currently doing so that it can return to the same state later. This information consists of the contents of important internal registers such as the program counter. Then the CPU jumps to some predetermined memory location and begins executing the instructions stored there. Those instructions make up an interrupt handler that does the processing necessary to respond to the interrupt. (This interrupt handler is part of the device driver software for the device that signaled the interrupt.) At the end of the interrupt handler is an instruction that tells the CPU to jump back to what it was doing; it does that by restoring its previously saved state.

Interrupts allow the CPU to deal with asynchronous events. In the regular fetch-and-execute cycle, things happen in a predetermined order; everything that happens is "synchronized" with everything else. Interrupts make it possible for the CPU to deal efficiently with events that happen "asynchronously," that is, at unpredictable times.

As another example of how interrupts are used, consider what happens when the CPU needs to access data that is stored on a hard disk. The CPU can access data directly only if it is in main memory. Data on the disk has to be copied into memory before it can be accessed. Unfortunately, on the scale of speed at which the CPU operates, the disk drive is extremely slow. When the CPU needs data from the disk, it sends a signal to the disk drive telling it to locate the data and get it ready. (This signal is sent synchronously, under the control of a regular program.) Then, instead of just waiting the long and unpredictable amount of time that the disk drive will take to do this, the CPU goes on with some other task. When the disk drive has the data ready, it sends an interrupt signal to the CPU. The interrupt handler can then read the requested data.

Now, you might have noticed that all this only makes sense if the CPU actually has several tasks to perform. If it has nothing better to do, it might as well spend its time polling for input or waiting for disk drive operations to complete. All modern computers use multitasking to perform several tasks at once. Some computers can be used by several people at once. Since the CPU is so fast, it can quickly switch its attention from one user to another, devoting a fraction of a second to each user in turn. This application of multitasking is called timesharing. But a modern personal computer with just a single user also uses multitasking. For example, the user might be typing a paper while a clock is continuously displaying the time and a file is being downloaded over the network.

Each of the individual tasks that the CPU is working on is called a thread. (Or a process; there are technical differences between threads and processes, but they are not important here, since it is threads that are used in Java.) Many CPUs can literally execute more than one thread simultaneously -- such CPUs contain multiple "cores," each of which can run a thread -- but there is always a limit on the number of threads that can be executed at the same time. Since there are often more threads than can be executed simultaneously, the computer has to be able switch its attention from one thread to another, just as a timesharing computer switches its attention from one user to another. In general, a thread that is being executed will continue to run until one of several things happens:

- The thread might voluntarily yield control, to give other threads a chance to run.

- The thread might have to wait for some asynchronous event to occur. For example, the thread might request some data from the disk drive, or it might wait for the user to press a key. While it is waiting, the thread is said to be blocked, and other threads, if any, have a chance to run. When the event occurs, an interrupt will "wake up" the thread so that it can continue running.

- The thread might use up its allotted slice of time and be suspended to allow other threads to run. Not all computers can "forcibly" suspend a thread in this way; those that can are said to use preemptive multitasking. To do preemptive multitasking, a computer needs a special timer device that generates an interrupt at regular intervals, such as 100 times per second. When a timer interrupt occurs, the CPU has a chance to switch from one thread to another, whether the thread that is currently running likes it or not. All modern desktop and laptop computers, and even typical smartphones and tablets, use preemptive multitasking.

Ordinary users, and indeed ordinary programmers, have no need to deal with interrupts and interrupt handlers. They can concentrate on the different tasks or threads that they want the computer to perform; the details of how the computer manages to get all those tasks done are not important to them. In fact, most users, and many programmers, can ignore threads and multitasking altogether. However, threads have become increasingly important as computers have become more powerful and as they have begun to make more use of multitasking and multiprocessing. In fact, the ability to work with threads is fast becoming an essential job skill for programmers. Fortunately, Java has good support for threads, which are built into the Java programming language as a fundamental programming concept. Programming with threads will be covered in Chapter 12.

Just as important in Java and in modern programming in general is the basic concept of asynchronous events. While programmers don't actually deal with interrupts directly, they do often find themselves writing event handlers, which, like interrupt handlers, are called asynchronously when specific events occur. Such "event-driven programming" has a very different feel from the more traditional straight-through, synchronous programming. We will begin with the more traditional type of programming, which is still used for programming individual tasks, but we will return to threads and events later in the text, starting in Chapter 6

By the way, the software that does all the interrupt handling, handles communication with the user and with hardware devices, and controls which thread is allowed to run is called the operating system. The operating system is the basic, essential software without which a computer would not be able to function. Other programs, such as word processors and Web browsers, are dependent upon the operating system. Common operating systems include Linux, various versions of Windows, and Mac OS.

Section 1.3

The Java Virtual Machine

Machine language consists of very simple instructions that can be executed directly by the CPU of a computer. Almost all programs, though, are written in high-level programming languages such as Java, Fortran, or C++. A program written in a high-level language cannot be run directly on any computer. First, it has to be translated into machine language. This translation can be done by a program called a compiler. A compiler takes a high-level-language program and translates it into an executable machine-language program. Once the translation is done, the machine-language program can be run any number of times, but of course it can only be run on one type of computer (since each type of computer has its own individual machine language). If the program is to run on another type of computer it has to be re-translated, using a different compiler, into the appropriate machine language.

There is an alternative to compiling a high-level language program. Instead of using a compiler, which translates the program all at once, you can use an interpreter, which translates it instruction-by-instruction, as necessary. An interpreter is a program that acts much like a CPU, with a kind of fetch-and-execute cycle. In order to execute a program, the interpreter runs in a loop in which it repeatedly reads one instruction from the program, decides what is necessary to carry out that instruction, and then performs the appropriate machine-language commands to do so.

One use of interpreters is to execute high-level language programs. For example, the programming language Lisp is usually executed by an interpreter rather than a compiler. However, interpreters have another purpose: they can let you use a machine-language program meant for one type of computer on a completely different type of computer. For example, one of the original home computers was the Commodore 64 or "C64". While you might not find an actual C64, you can find programs that run on other computers -- or even in a web browser -- that "emulate" one. Such an emulator can run C64 programs by acting as an interpreter for the C64 machine language.

The designers of Java chose to use a combination of compilation and interpreting. Programs written in Java are compiled into machine language, but it is a machine language for a computer that doesn't really exist. This so-called "virtual" computer is known as the Java Virtual Machine, or JVM. The machine language for the Java Virtual Machine is called Java bytecode. There is no reason why Java bytecode couldn't be used as the machine language of a real computer, rather than a virtual computer. But in fact the use of a virtual machine makes possible one of the main selling points of Java: the fact that it can actually be used on any computer. All that the computer needs is an interpreter for Java bytecode. Such an interpreter simulates the JVM in the same way that a C64 emulator simulates a Commodore 64 computer. (The term JVM is also used for the Java bytecode interpreter program that does the simulation, so we say that a computer needs a JVM in order to run Java programs. Technically, it would be more correct to say that the interpreter implements the JVM than to say that it is a JVM.)

Of course, a different Java bytecode interpreter is needed for each type of computer, but once a computer has a Java bytecode interpreter, it can run any Java bytecode program, and the same program can be run on any computer that has such an interpreter. This is one of the essential features of Java: the same compiled program can be run on many different types of computers.

Why, you might wonder, use the intermediate Java bytecode at all? Why not just distribute the original Java program and let each person compile it into the machine language of whatever computer they want to run it on? There are several reasons. First of all, a compiler has to understand Java, a complex high-level language. The compiler is itself a complex program. A Java bytecode interpreter, on the other hand, is a relatively small, simple program. This makes it easy to write a bytecode interpreter for a new type of computer; once that is done, that computer can run any compiled Java program. It would be much harder to write a Java compiler for the same computer.

Furthermore, some Java programs are meant to be downloaded over a network. This leads to obvious security concerns: you don't want to download and run a program that will damage your computer or your files. The bytecode interpreter acts as a buffer between you and the program you download. You are really running the interpreter, which runs the downloaded program indirectly. The interpreter can protect you from potentially dangerous actions on the part of that program.

When Java was still a new language, it was criticized for being slow: Since Java bytecode was executed by an interpreter, it seemed that Java bytecode programs could never run as quickly as programs compiled into native machine language (that is, the actual machine language of the computer on which the program is running). However, this problem has been largely overcome by the use of just-in-time compilers for executing Java bytecode. A just-in-time compiler translates Java bytecode into native machine language. It does this while it is executing the program. Just as for a normal interpreter, the input to a just-in-time compiler is a Java bytecode program, and its task is to execute that program. But as it is executing the program, it also translates parts of it into machine language. The translated parts of the program can then be executed much more quickly than they could be interpreted. Since a given part of a program is often executed many times as the program runs, a just-in-time compiler can significantly speed up the overall execution time.

I should note that there is no necessary connection between Java and Java bytecode. A program written in Java could certainly be compiled into the machine language of a real computer. And programs written in other languages can be compiled into Java bytecode. However, the combination of Java and Java bytecode is platform-independent, secure, and network-compatible while allowing you to program in a modern high-level object-oriented language.

(In the past few years, it has become fairly common to create new programming languages, or versions of old languages, that compile into Java bytecode. The compiled bytecode programs can then be executed by a standard JVM. New languages that have been developed specifically for programming the JVM include Groovy, Clojure, and Processing. Jython and JRuby are versions of older languages, Python and Ruby, that target the JVM. These languages make it possible to enjoy many of the advantages of the JVM while avoiding some of the technicalities of the Java language. In fact, the use of other languages with the JVM has become important enough that several new features have been added to the JVM specifically to add better support for some of those languages. And this improvement to the JVM has in turn made possible some of the new features in Java 7 and Java 8.)

I should also note that the really hard part of platform-independence is providing a "Graphical User Interface" -- with windows, buttons, etc. -- that will work on all the platforms that support Java. You'll see more about this problem in Section 1.6.

Section 1.4

Fundamental Building Blocks of Programs

There are two basic aspects of programming: data and instructions. To work with data, you need to understand variables and types; to work with instructions, you need to understand control structures and subroutines. You'll spend a large part of the course becoming familiar with these concepts.

A variable is just a memory location (or several consecutive locations treated as a unit) that has been given a name so that it can be easily referred to and used in a program. The programmer only has to worry about the name; it is the compiler's responsibility to keep track of the memory location. As a programmer, you need to keep in mind that the name refers to a kind of "box" in memory that can hold data, even though you don't have to know where in memory that box is located.

In Java and in many other programming languages, a variable has a type that indicates what sort of data it can hold. One type of variable might hold integers -- whole numbers such as 3, -7, and 0 -- while another holds floating point numbers -- numbers with decimal points such as 3.14, -2.7, or 17.0. (Yes, the computer does make a distinction between the integer 17 and the floating-point number 17.0; they actually look quite different inside the computer.) There could also be types for individual characters ('A', ';', etc.), strings ("Hello", "A string can include many characters", etc.), and less common types such as dates, colors, sounds, or any other kind of data that a program might need to store.

Programming languages always have commands for getting data into and out of variables and for doing computations with data. For example, the following "assignment statement," which might appear in a Java program, tells the computer to take the number stored in the variable named "principal", multiply that number by 0.07, and then store the result in the variable named "interest":

interest = principal * 0.07;

There are also "input commands" for getting data from the user or from files on the computer's disks, and there are "output commands" for sending data in the other direction.

These basic commands -- for moving data from place to place and for performing computations -- are the building blocks for all programs. These building blocks are combined into complex programs using control structures and subroutines.

A program is a sequence of instructions. In the ordinary "flow of control," the computer executes the instructions in the sequence in which they occur in the program, one after the other. However, this is obviously very limited: the computer would soon run out of instructions to execute. Control structures are special instructions that can change the flow of control. There are two basic types of control structure: loops, which allow a sequence of instructions to be repeated over and over, and branches, which allow the computer to decide between two or more different courses of action by testing conditions that occur as the program is running.

For example, it might be that if the value of the variable "principal" is greater than 10000, then the "interest" should be computed by multiplying the principal by 0.05; if not, then the interest should be computed by multiplying the principal by 0.04. A program needs some way of expressing this type of decision. In Java, it could be expressed using the following "if statement":

if (principal > 10000)
interest = principal * 0.05;
else
interest = principal * 0.04;

(Don't worry about the details for now. Just remember that the computer can test a condition and decide what to do next on the basis of that test.)

Loops are used when the same task has to be performed more than once. For example, if you want to print out a mailing label for each name on a mailing list, you might say, "Get the first name and address and print the label; get the second name and address and print the label; get the third name and address and print the label..." But this quickly becomes ridiculous -- and might not work at all if you don't know in advance how many names there are. What you would like to say is something like "While there are more names to process, get the next name and address, and print the label." A loop can be used in a program to express such repetition.

Large programs are so complex that it would be almost impossible to write them if there were not some way to break them up into manageable "chunks." Subroutines provide one way to do this. A subroutine consists of the instructions for performing some task, grouped together as a unit and given a name. That name can then be used as a substitute for the whole set of instructions. For example, suppose that one of the tasks that your program needs to perform is to draw a house on the screen. You can take the necessary instructions, make them into a subroutine, and give that subroutine some appropriate name -- say, "drawHouse()". Then anyplace in your program where you need to draw a house, you can do so with the single command:

drawHouse();

This will have the same effect as repeating all the house-drawing instructions in each place.

The advantage here is not just that you save typing. Organizing your program into subroutines also helps you organize your thinking and your program design effort. While writing the house-drawing subroutine, you can concentrate on the problem of drawing a house without worrying for the moment about the rest of the program. And once the subroutine is written, you can forget about the details of drawing houses -- that problem is solved, since you have a subroutine to do it for you. A subroutine becomes just like a built-in part of the language which you can use without thinking about the details of what goes on "inside" the subroutine.

Variables, types, loops, branches, and subroutines are the basis of what might be called "traditional programming." However, as programs become larger, additional structure is needed to help deal with their complexity. One of the most effective tools that has been found is object-oriented programming, which is discussed in the next section.

Section 1.5

Objects and Object-oriented Programming

Programs must be designed. No one can just sit down at the computer and compose a program of any complexity. The discipline called software engineering is concerned with the construction of correct, working, well-written programs. The software engineer tries to use accepted and proven methods for analyzing the problem to be solved and for designing a program to solve that problem.

During the 1970s and into the 80s, the primary software engineering methodology was structured programming. The structured programming approach to program design was based on the following advice: To solve a large problem, break the problem into several pieces and work on each piece separately; to solve each piece, treat it as a new problem which can itself be broken down into smaller problems; eventually, you will work your way down to problems that can be solved directly, without further decomposition. This approach is called top-down programming.

There is nothing wrong with top-down programming. It is a valuable and often-used approach to problem-solving. However, it is incomplete. For one thing, it deals almost entirely with producing the instructions necessary to solve a problem. But as time went on, people realized that the design of the data structures for a program was at least as important as the design of subroutines and control structures. Top-down programming doesn't give adequate consideration to the data that the program manipulates.

Another problem with strict top-down programming is that it makes it difficult to reuse work done for other projects. By starting with a particular problem and subdividing it into convenient pieces, top-down programming tends to produce a design that is unique to that problem. It is unlikely that you will be able to take a large chunk of programming from another program and fit it into your project, at least not without extensive modification. Producing high-quality programs is difficult and expensive, so programmers and the people who employ them are always eager to reuse past work.

So, in practice, top-down design is often combined with bottom-up design. In bottom-up design, the approach is to start "at the bottom," with problems that you already know how to solve (and for which you might already have a reusable software component at hand). From there, you can work upwards towards a solution to the overall problem.

The reusable components should be as "modular" as possible. A module is a component of a larger system that interacts with the rest of the system in a simple, well-defined, straightforward manner. The idea is that a module can be "plugged into" a system. The details of what goes on inside the module are not important to the system as a whole, as long as the module fulfills its assigned role correctly. This is called information hiding, and it is one of the most important principles of software engineering.

One common format for software modules is to contain some data, along with some subroutines for manipulating that data. For example, a mailing-list module might contain a list of names and addresses along with a subroutine for adding a new name, a subroutine for printing mailing labels, and so forth. In such modules, the data itself is often hidden inside the module; a program that uses the module can then manipulate the data only indirectly, by calling the subroutines provided by the module. This protects the data, since it can only be manipulated in known, well-defined ways. And it makes it easier for programs to use the module, since they don't have to worry about the details of how the data is represented. Information about the representation of the data is hidden.

Modules that could support this kind of information-hiding became common in programming languages in the early 1980s. Since then, a more advanced form of the same idea has more or less taken over software engineering. This latest approach is called object-oriented programming, often abbreviated as OOP.

The central concept of object-oriented programming is the object, which is a kind of module containing data and subroutines. The point-of-view in OOP is that an object is a kind of self-sufficient entity that has an internal state (the data it contains) and that can respond to messages (calls to its subroutines). A mailing list object, for example, has a state consisting of a list of names and addresses. If you send it a message telling it to add a name, it will respond by modifying its state to reflect the change. If you send it a message telling it to print itself, it will respond by printing out its list of names and addresses.

The OOP approach to software engineering is to start by identifying the objects involved in a problem and the messages that those objects should respond to. The program that results is a collection of objects, each with its own data and its own set of responsibilities. The objects interact by sending messages to each other. There is not much "top-down" in the large-scale design of such a program, and people used to more traditional programs can have a hard time getting used to OOP. However, people who use OOP would claim that object-oriented programs tend to be better models of the way the world itself works, and that they are therefore easier to write, easier to understand, and more likely to be correct.

You should think of objects as "knowing" how to respond to certain messages. Different objects might respond to the same message in different ways. For example, a "print" message would produce very different results, depending on the object it is sent to. This property of objects -- that different objects can respond to the same message in different ways -- is called polymorphism.

It is common for objects to bear a kind of "family resemblance" to one another. Objects that contain the same type of data and that respond to the same messages in the same way belong to the same class. (In actual programming, the class is primary; that is, a class is created and then one or more objects are created using that class as a template.) But objects can be similar without being in exactly the same class.

For example, consider a drawing program that lets the user draw lines, rectangles, ovals, polygons, and curves on the screen. In the program, each visible object on the screen could be represented by a software object in the program. There would be five classes of objects in the program, one for each type of visible object that can be drawn. All the lines would belong to one class, all the rectangles to another class, and so on. These classes are obviously related; all of them represent "drawable objects." They would, for example, all presumably be able to respond to a "draw yourself" message. Another level of grouping, based on the data needed to represent each type of object, is less obvious, but would be very useful in a program: We can group polygons and curves together as "multipoint objects," while lines, rectangles, and ovals are "two-point objects." (A line is determined by its two endpoints, a rectangle by two of its corners, and an oval by two corners of the rectangle that contains it. The rectangles that I am talking about here have sides that are vertical and horizontal, so that they can be specified by just two points; this is the common meaning of "rectangle" in drawing programs.) We could diagram these relationships as follows:

DrawableObject, MultipointObject, and TwoPointObject would be classes in the program. MultipointObject and TwoPointObject would be subclasses of DrawableObject. The class Line would be a subclass of TwoPointObject and (indirectly) of DrawableObject. A subclass of a class is said to inherit the properties of that class. The subclass can add to its inheritance and it can even "override" part of that inheritance (by defining a different response to some method). Nevertheless, lines, rectangles, and so on are drawable objects, and the class DrawableObject expresses this relationship.

Inheritance is a powerful means for organizing a program. It is also related to the problem of reusing software components. A class is the ultimate reusable component. Not only can it be reused directly if it fits exactly into a program you are trying to write, but if it just almost fits, you can still reuse it by defining a subclass and making only the small changes necessary to adapt it exactly to your needs.

So, OOP is meant to be both a superior program-development tool and a partial solution to the software reuse problem. Objects, classes, and object-oriented programming will be important themes throughout the rest of this text. You will start using objects that are built into the Java language in the next chapter, and in Chapter 5 you will begin creating your own classes and objects.

Section 1.6

The Modern User Interface

When computers were first introduced, ordinary people -- including most programmers -- couldn't get near them. They were locked up in rooms with white-coated attendants who would take your programs and data, feed them to the computer, and return the computer's response some time later. When timesharing -- where the computer switches its attention rapidly from one person to another -- was invented in the 1960s, it became possible for several people to interact directly with the computer at the same time. On a timesharing system, users sit at "terminals" where they type commands to the computer, and the computer types back its response. Early personal computers also used typed commands and responses, except that there was only one person involved at a time. This type of interaction between a user and a computer is called a command-line interface.

Today, of course, most people interact with computers in a completely different way. They use a Graphical User Interface, or GUI. The computer draws interface components on the screen. The components include things like windows, scroll bars, menus, buttons, and icons. Usually, a mouse is used to manipulate such components or, on "touchscreens," your fingers. Assuming that you have not just been teleported in from the 1970s, you are no doubt already familiar with the basics of graphical user interfaces!

A lot of GUI interface components have become fairly standard. That is, they have similar appearance and behavior on many different computer platforms including Mac OS, Windows, and Linux. Java programs, which are supposed to run on many different platforms without modification to the program, can use all the standard GUI components. They might vary a little in appearance from platform to platform, but their functionality should be identical on any computer on which the program runs.

Shown below is an image of a very simple Java program that demonstrates a few standard GUI interface components. When the program is run, a window similar to the picture shown here will open on the computer screen. There are four components in the window with which the user can interact: a button, a checkbox, a text field, and a pop-up menu. These components are labeled. There are a few other components in the window. The labels themselves are components (even though you can't interact with them). The right half of the window is a text area component, which can display multiple lines of text. A scrollbar component appears alongside the text area when the number of lines of text becomes larger than will fit in the text area. And in fact, in Java terminology, the whole window is itself considered to be a "component."

(If you would like to run this program, the source code, GUIDemo.java, as well as a compiled program, , are available on line. For more information on using this and other examples from this textbook, see Section 2.6.)

Now, Java actually has two complete sets of GUI components. One of these, the AWT or Abstract Windowing Toolkit, was available in the original version of Java. The other, which is known as Swing, was introduced in Java version 1.2, and is used in preference to the AWT in most modern Java programs. The program that is shown above uses components that are part of Swing.

When a user interacts with GUI components, "events" are generated. For example, clicking a push button generates an event, and pressing return while typing in a text field generates an event. Each time an event is generated, a message is sent to the program telling it that the event has occurred, and the program responds according to its program. In fact, a typical GUI program consists largely of "event handlers" that tell the program how to respond to various types of events. In this example, the program has been programmed to respond to each event by displaying a message in the text area. In a more realistic example, the event handlers would have more to do.

The use of the term "message" here is deliberate. Messages, as you saw in the previous section, are sent to objects. In fact, Java GUI components are implemented as objects. Java includes many predefined classes that represent various types of GUI components. Some of these classes are subclasses of others. Here is a diagram showing just a few of Swing's GUI classes and their relationships:

Don't worry about the details for now, but try to get some feel about how object-oriented programming and inheritance are used here. Note that all the GUI classes are subclasses, directly or indirectly, of a class called JComponent, which represents general properties that are shared by all Swing components. In the diagram, two of the direct subclasses of JComponent themselves have subclasses. The classes JTextArea and JTextField, which have certain behaviors in common, are grouped together as subclasses of JTextComponent. Similarly JButton and JToggleButton are subclasses of JAbstractButton, which represents properties common to both buttons and checkboxes. (JComboBox, by the way, is the Swing class that represents pop-up menus.)

Just from this brief discussion, perhaps you can see how GUI programming can make effective use of object-oriented design. In fact, GUIs, with their "visible objects," are probably a major factor contributing to the popularity of OOP.

Programming with GUI components and events is one of the most interesting aspects of Java. However, we will spend several chapters on the basics before returning to this topic in Chapter 6.

Section 1.7

The Internet and Beyond

Computers can be connected together on networks. A computer on a network can communicate with other computers on the same network by exchanging data and files or by sending and receiving messages. Computers on a network can even work together on a large computation.

Today, millions of computers throughout the world are connected to a single huge network called the Internet. New computers are being connected to the Internet every day, both by wireless communication and by physical connection using technologies such as DSL, cable modems, and Ethernet.

There are elaborate protocols for communication over the Internet. A protocol is simply a detailed specification of how communication is to proceed. For two computers to communicate at all, they must both be using the same protocols. The most basic protocols on the Internet are the Internet Protocol (IP), which specifies how data is to be physically transmitted from one computer to another, and the Transmission Control Protocol (TCP), which ensures that data sent using IP is received in its entirety and without error. These two protocols, which are referred to collectively as TCP/IP, provide a foundation for communication. Other protocols use TCP/IP to send specific types of information such as web pages, electronic mail, and data files.

All communication over the Internet is in the form of packets. A packet consists of some data being sent from one computer to another, along with addressing information that indicates where on the Internet that data is supposed to go. Think of a packet as an envelope with an address on the outside and a message on the inside. (The message is the data.) The packet also includes a "return address," that is, the address of the sender. A packet can hold only a limited amount of data; longer messages must be divided among several packets, which are then sent individually over the net and reassembled at their destination.

Every computer on the Internet has an IP address, a number that identifies it uniquely among all the computers on the net. (Actually, the claim about uniqueness is not quite true, but the basic idea is valid, and the full truth is complicated.) The IP address is used for addressing packets. A computer can only send data to another computer on the Internet if it knows that computer's IP address. Since people prefer to use names rather than numbers, most computers are also identified by names, called domain names. For example, the main computer of the Mathematics Department at Hobart and William Smith Colleges has the domain name math.hws.edu. (Domain names are just for convenience; your computer still needs to know IP addresses before it can communicate. There are computers on the Internet whose job it is to translate domain names to IP addresses. When you use a domain name, your computer sends a message to a domain name server to find out the corresponding IP address. Then, your computer uses the IP address, rather than the domain name, to communicate with the other computer.)

The Internet provides a number of services to the computers connected to it (and, of course, to the users of those computers). These services use TCP/IP to send various types of data over the net. Among the most popular services are instant messaging, file sharing, electronic mail, and the World-Wide Web. Each service has its own protocols, which are used to control transmission of data over the network. Each service also has some sort of user interface, which allows the user to view, send, and receive data through the service.

For example, the email service uses a protocol known as SMTP (Simple Mail Transfer Protocol) to transfer email messages from one computer to another. Other protocols, such as POP and IMAP, are used to fetch messages from an email account so that the recipient can read them. A person who uses email, however, doesn't need to understand or even know about these protocols. Instead, they are used behind the scenes by computer programs to send and receive email messages. These programs provide the user with an easy-to-use user interface to the underlying network protocols.

The World-Wide Web is perhaps the most exciting of network services. The World-Wide Web allows you to request pages of information that are stored on computers all over the Internet. A Web page can contain links to other pages on the same computer from which it was obtained or to other computers anywhere in the world. A computer that stores such pages of information is called a web server. The user interface to the Web is the type of program known as a web browser. Common web browsers include Internet Explorer, Firefox, Chrome, and Safari. You use a Web browser to request a page of information. The browser sends a request for that page to the computer on which the page is stored, and when a response is received from that computer, the web browser displays it to you in a neatly formatted form. A web browser is just a user interface to the Web. Behind the scenes, the web browser uses a protocol called HTTP (HyperText Transfer Protocol) to send each page request and to receive the response from the web server.

Now just what, you might be thinking, does all this have to do with Java? In fact, Java is intimately associated with the Internet and the World-Wide Web. When Java was first introduced, one of its big attractions was the ability to write applets. An applet is a small program that is transmitted over the Internet and that runs on a web page. Applets make it possible for a web page to perform complex tasks and have complex interactions with the user. Alas, applets have suffered from a variety of security problems, and fixing those problems has made them more difficult to use. Applets have become much less common on the Web, and in any case, there are other options for running programs on Web pages.

But applets are only one aspect of Java's relationship with the Internet. Java can be used to write complex, stand-alone applications that do not depend on a Web browser. Many of these programs are network-related. For example many of the largest and most complex web sites use web server software that is written in Java. Java includes excellent support for network protocols, and its platform independence makes it possible to write network programs that work on many different types of computer. You will learn about Java's network support in Chapter 11.

Its support for networking is not Java's only advantage. But many good programming languages have been invented only to be soon forgotten. Java has had the good luck to ride on the coattails of the Internet's immense and increasing popularity.

As Java has matured, its applications have reached far beyond the Net. The standard version of Java already comes with support for many technologies, such as cryptography and data compression. Free extensions are available to support many other technologies such as advanced sound processing and three-dimensional graphics. Complex, high-performance systems can be developed in Java. For example, Hadoop, a system for large scale data processing, is written in Java. Hadoop is used by Yahoo, Facebook, and other Web sites to process the huge amounts of data generated by their users.

Furthermore, Java is not restricted to use on traditional computers. Java can be used to write programs for many smartphones (though not for the iPhone). It is the primary development language for Android-based devices. (Some mobile devices use a version of Java called Java ME ("Mobile Edition"), but Android uses Google's own version of Java and does not use the same graphical user interface components as standard Java.) Java is also the programming language for the Amazon Kindle eBook reader and for interactive features on Blu-Ray video disks.

At this time, Java certainly ranks as one of the most widely used programming languages. It is a good choice for almost any programming project that is meant to run on more than one type of computing device, and is a reasonable choice even for many programs that will run on only one device. It is probably still the most widely taught language at Colleges and Universities. It is similar enough to other popular languages, such as C, C++, and Python, that knowing it will give you a good start on learning those languages as well. Overall, learning Java is a great starting point on the road to becoming an expert programmer. I hope you enjoy the journey!

Quiz on Chapter 1

Question 1:

One of the components of a computer is its CPU. What is a CPU and what role does it play in a computer?

Question 2:

Explain what is meant by an "asynchronous event." Give some examples.

Question 3:

What is the difference between a "compiler" and an "interpreter"?

Question 4:

Explain the difference between high-level languages and machine language.

Question 5:

If you have the source code for a Java program, and you want to run that program, you will need both a compiler and an interpreter. What does the Java compiler do, and what does the Java interpreter do?

Question 6:

What is a subroutine?

Question 7:

Java is an object-oriented programming language. What is an object?

Question 8:

What is a variable? (There are four different ideas associated with variables in Java. Try to mention all four aspects in your answer. Hint: One of the aspects is the variable's name.)

Question 9:

Java is a "platform-independent language." What does this mean?

Question 10:

What is the "Internet"? Give some examples of how it is used. (What kind of services does it provide?)

See the Answers

Chapter 2

Programming in the Small I: Names and Things

On a basic level (the level of machine language), a computer can perform only very simple operations. A computer performs complex tasks by stringing together large numbers of such operations. Such tasks must be "scripted" in complete and perfect detail by programs. Creating complex programs will never be really easy, but the difficulty can be handled to some extent by giving the program a clear overall structure. The design of the overall structure of a program is what I call "programming in the large."

Programming in the small, which is sometimes called coding, would then refer to filling in the details of that design. The details are the explicit, step-by-step instructions for performing fairly small-scale tasks. When you do coding, you are working "close to the machine," with some of the same concepts that you might use in machine language: memory locations, arithmetic operations, loops and branches. In a high-level language such as Java, you get to work with these concepts on a level several steps above machine language. However, you still have to worry about getting all the details exactly right.

This chapter and the next examine the facilities for programming in the small in the Java programming language. Don't be misled by the term "programming in the small" into thinking that this material is easy or unimportant. This material is an essential foundation for all types of programming. If you don't understand it, you can't write programs, no matter how good you get at designing their large-scale structure.

The last section of this chapter discusses programming environments. That section contains information about how to compile and run Java programs, and you should take a look at it before trying to write and use your own programs or trying to use the sample programs in this book.

Contents of Chapter 2:

- Section 1: The Basic Java Application

- Section 2: Variables and the Primitive Types

- Section 3: Strings, Classes, Objects, and Subroutines

- Section 4: Text Input and Output

- Section 5: Details of Expressions

- Section 6: Programming Environments

- Programming Exercises

- Quiz on This Chapter

Section 2.1

The Basic Java Application

A program is a sequence of instructions that a computer can execute to perform some task. A simple enough idea, but for the computer to make any use of the instructions, they must be written in a form that the computer can use. This means that programs have to be written in programming languages. Programming languages differ from ordinary human languages in being completely unambiguous and very strict about what is and is not allowed in a program. The rules that determine what is allowed are called the syntax of the language. Syntax rules specify the basic vocabulary of the language and how programs can be constructed using things like loops, branches, and subroutines. A syntactically correct program is one that can be successfully compiled or interpreted; programs that have syntax errors will be rejected (hopefully with a useful error message that will help you fix the problem).

So, to be a successful programmer, you have to develop a detailed knowledge of the syntax of the programming language that you are using. However, syntax is only part of the story. It's not enough to write a program that will run -- you want a program that will run and produce the correct result! That is, the meaning of the program has to be right. The meaning of a program is referred to as its semantics. More correctly, the semantics of a programming language is the set of rules that determine the meaning of a program written in that language. A semantically correct program is one that does what you want it to.

Furthermore, a program can be syntactically and semantically correct but still be a pretty bad program. Using the language correctly is not the same as using it well. For example, a good program has "style." It is written in a way that will make it easy for people to read and to understand. It follows conventions that will be familiar to other programmers. And it has an overall design that will make sense to human readers. The computer is completely oblivious to such things, but to a human reader, they are paramount. These aspects of programming are sometimes referred to as pragmatics. (I will often use the more common term style.)

When I introduce a new language feature, I will explain the syntax, the semantics, and some of the pragmatics of that feature. You should memorize the syntax; that's the easy part. Then you should get a feeling for the semantics by following the examples given, making sure that you understand how they work, and, ideally, writing short programs of your own to test your understanding. And you should try to appreciate and absorb the pragmatics -- this means learning how to use the language feature well, with style that will earn you the admiration of other programmers.

Of course, even when you've become familiar with all the individual features of the language, that doesn't make you a programmer. You still have to learn how to construct complex programs to solve particular problems. For that, you'll need both experience and taste. You'll find hints about software development throughout this textbook.

We begin our exploration of Java with the problem that has become traditional for such beginnings: to write a program that displays the message "Hello World!". This might seem like a trivial problem, but getting a computer to do this is really a big first step in learning a new programming language (especially if it's your first programming language). It means that you understand the basic process of:

1. getting the program text into the computer,

2. compiling the program, and

3. running the compiled program.

The first time through, each of these steps will probably take you a few tries to get right. I won't go into the details here of how you do each of these steps; it depends on the particular computer and Java programming environment that you are using. See Section 2.6 for information about creating and running Java programs in specific programming environments. But in general, you will type the program using some sort of text editor and save the program in a file. Then, you will use some command to try to compile the file. You'll either get a message that the program contains syntax errors, or you'll get a compiled version of the program. In the case of Java, the program is compiled into Java bytecode, not into machine language. Finally, you can run the compiled program by giving some appropriate command. For Java, you will actually use an interpreter to execute the Java bytecode. Your programming environment might automate some of the steps for you -- for example, the compilation step is often done automatically -- but you can be sure that the same three steps are being done in the background.

Here is a Java program to display the message "Hello World!". Don't expect to understand what's going on here just yet; some of it you won't really understand until a few chapters from now:

/** A program to display the message
*  "Hello World!" on standard output.
*/
public class HelloWorld {

public static void main(String[] args) {
System.out.println("Hello World!");
}

}   // end of class HelloWorld

The command that actually displays the message is:

System.out.println("Hello World!");

This command is an example of a subroutine call statement. It uses a "built-in subroutine" named System.out.println to do the actual work. Recall that a subroutine consists of the instructions for performing some task, chunked together and given a name. That name can be used to "call" the subroutine whenever that task needs to be performed. A built-in subroutine is one that is already defined as part of the language and therefore automatically available for use in any program.

When you run this program, the message "Hello World!" (without the quotes) will be displayed on standard output. Unfortunately, I can't say exactly what that means! Java is meant to run on many different platforms, and standard output will mean different things on different platforms. However, you can expect the message to show up in some convenient or inconvenient place. (If you use a command-line interface, like that in Oracle's Java Development Kit, you type in a command to tell the computer to run the program. The computer will type the output from the program, Hello World!, on the next line. In an integrated development environment such as Eclipse, the output might appear somewhere in one of the environment's windows.)

You must be curious about all the other stuff in the above program. Part of it consists of comments. Comments in a program are entirely ignored by the computer; they are there for human readers only. This doesn't mean that they are unimportant. Programs are meant to be read by people as well as by computers, and without comments, a program can be very difficult to understand. Java has two types of comments. The first type begins with // and extends to the end of a line. There is a comment of this form on the last line of the above program. The computer ignores the // and everything that follows it on the same line. The second type of comment starts with /* and ends with */, and it can extend over more than one line. The first three lines of the program are an example of this second type of comment. (A comment that actually begins with /**, like this one does, has special meaning; it is a "Javadoc" comment that can be used to produce documentation for the program.)

Everything else in the program is required by the rules of Java syntax. All programming in Java is done inside "classes." The first line in the above program (not counting the comment) says that this is a class named HelloWorld. "HelloWorld," the name of the class, also serves as the name of the program. Not every class is a program. In order to define a program, a class must include a subroutine named main, with a definition that takes the form:

public static void main(String[] args) {
statements
}

When you tell the Java interpreter to run the program, the interpreter calls this main() subroutine, and the statements that it contains are executed. These statements make up the script that tells the computer exactly what to do when the program is executed. The main() routine can call other subroutines that are defined in the same class or even in other classes, but it is the main() routine that determines how and in what order the other subroutines are used.

The word "public" in the first line of main() means that this routine can be called from outside the program. This is essential because the main() routine is called by the Java interpreter, which is something external to the program itself. The remainder of the first line of the routine is harder to explain at the moment; for now, just think of it as part of the required syntax. The definition of the subroutine -- that is, the instructions that say what it does -- consists of the sequence of "statements" enclosed between braces, { and }. Here, I've used statements as a placeholder for the actual statements that make up the program. Throughout this textbook, I will always use a similar format: anything that you see in this style of text (italic in angle brackets) is a placeholder that describes something you need to type when you write an actual program.

As noted above, a subroutine can't exist by itself. It has to be part of a "class". A program is defined by a public class that takes the form:

public class program-name {

optional-variable-declarations-and-subroutines

public static void main(String[] args) {
statements
}

optional-variable-declarations-and-subroutines

}

The name on the first line is the name of the program, as well as the name of the class. (Remember, again, that program-name is a placeholder for the actual name!)

If the name of the class is HelloWorld, then the class must be saved in a file called HelloWorld.java. When this file is compiled, another file named HelloWorld.class will be produced. This class file, HelloWorld.class, contains the translation of the program into Java bytecode, which can be executed by a Java interpreter. HelloWorld.java is called the source code for the program. To execute the program, you only need the compiled class file, not the source code.

The layout of the program on the page, such as the use of blank lines and indentation, is not part of the syntax or semantics of the language. The computer doesn't care about layout -- you could run the entire program together on one line as far as it is concerned. However, layout is important to human readers, and there are certain style guidelines for layout that are followed by most programmers.

Also note that according to the above syntax specification, a program can contain other subroutines besides main(), as well as things called "variable declarations." You'll learn more about these later, but not until Chapter 4.

Section 2.2

Variables and the Primitive Types

Names are fundamental to programming. In programs, names are used to refer to many different sorts of things. In order to use those things, a programmer must understand the rules for giving names to them and the rules for using the names to work with them. That is, the programmer must understand the syntax and the semantics of names.

According to the syntax rules of Java, the most basic names are identifiers. Identifiers can be used to name classes, variables, and subroutines. An identifier is a sequence of one or more characters. It must begin with a letter or underscore and must consist entirely of letters, digits, and underscores. ("Underscore" refers to the character '_'.) For example, here are some legal identifiers:

N   n   rate  x15   quite_a_long_name   HelloWorld

No spaces are allowed in identifiers; HelloWorld is a legal identifier, but "Hello World" is not. Upper case and lower case letters are considered to be different, so that HelloWorld, helloworld, HELLOWORLD, and hElloWorLD are all distinct names. Certain words are reserved for special uses in Java, and cannot be used as identifiers. These reserved words include: class, public, static, if, else, while, and several dozen other words. (Remember that reserved words are not identifiers, since they can't be used as names for things.)

Java is actually pretty liberal about what counts as a letter or a digit. Java uses the Unicode character set, which includes thousands of characters from many different languages and different alphabets, and many of these characters count as letters or digits. However, I will be sticking to what can be typed on a regular English keyboard.

The pragmatics of naming includes style guidelines about how to choose names for things. For example, it is customary for names of classes to begin with upper case letters, while names of variables and of subroutines begin with lower case letters; you can avoid a lot of confusion by following this standard convention in your own programs. Most Java programmers do not use underscores in names, although some do use them at the beginning of the names of certain kinds of variables. When a name is made up of several words, such as HelloWorld or interestRate, it is customary to capitalize each word, except possibly the first; this is sometimes referred to as camel case, since the upper case letters in the middle of a name are supposed to look something like the humps on a camel's back.

Finally, I'll note that in addition to simple identifiers, things in Java can have compound names which consist of several simple names separated by periods. (Compound names are also called qualified names.) You've already seen an example: System.out.println. The idea here is that things in Java can contain other things. A compound name is a kind of path to an item through one or more levels of containment. The name System.out.println indicates that something called "System" contains something called "out" which in turn contains something called "println".

2.2.1  Variables

Programs manipulate data that are stored in memory. In machine language, data can only be referred to by giving the numerical address of the location in memory where the data is stored. In a high-level language such as Java, names are used instead of numbers to refer to data. It is the job of the computer to keep track of where in memory the data is actually stored; the programmer only has to remember the name. A name used in this way -- to refer to data stored in memory -- is called a variable.

Variables are actually rather subtle. Properly speaking, a variable is not a name for the data itself but for a location in memory that can hold data. You should think of a variable as a container or box where you can store data that you will need to use later. The variable refers directly to the box and only indirectly to the data in the box. Since the data in the box can change, a variable can refer to different data values at different times during the execution of the program, but it always refers to the same box. Confusion can arise, especially for beginning programmers, because when a variable is used in a program in certain ways, it refers to the container, but when it is used in other ways, it refers to the data in the container. You'll see examples of both cases below.

(In this way, a variable is something like the title, "The President of the United States." This title can refer to different people at different times, but it always refers to the same office. If I say "the President is playing basketball," I mean that Barack Obama is playing basketball. But if I say "Hillary Clinton wants to be President" I mean that she wants to fill the office, not that she wants to be Barack Obama.)

In Java, the only way to get data into a variable -- that is, into the box that the variable names -- is with an assignment statement. An assignment statement takes the form:

variable = expression;

where expression represents anything that refers to or computes a data value. When the computer comes to an assignment statement in the course of executing a program, it evaluates the expression and puts the resulting data value into the variable. For example, consider the simple assignment statement

rate = 0.07;

The variable in this assignment statement is rate, and the expression is the number 0.07. The computer executes this assignment statement by putting the number 0.07 in the variable rate, replacing whatever was there before. Now, consider the following more complicated assignment statement, which might come later in the same program:

interest = rate * principal;

Here, the value of the expression "rate * principal" is being assigned to the variable interest. In the expression, the * is a "multiplication operator" that tells the computer to multiply rate times principal. The names rate and principal are themselves variables, and it is really the values stored in those variables that are to be multiplied. We see that when a variable is used in an expression, it is the value stored in the variable that matters; in this case, the variable seems to refer to the data in the box, rather than to the box itself. When the computer executes this assignment statement, it takes the value of rate, multiplies it by the value of principal, and stores the answer in the box referred to by interest. When a variable is used on the left-hand side of an assignment statement, it refers to the box that is named by the variable.

(Note, by the way, that an assignment statement is a command that is executed by the computer at a certain time. It is not a statement of fact. For example, suppose a program includes the statement "rate = 0.07;". If the statement "interest = rate * principal;" is executed later in the program, can we say that the principal is multiplied by 0.07? No! The value of rate might have been changed in the meantime by another statement. The meaning of an assignment statement is completely different from the meaning of an equation in mathematics, even though both use the symbol "=".)

2.2.2  Types

A variable in Java is designed to hold only one particular type of data; it can legally hold that type of data and no other. The compiler will consider it to be a syntax error if you try to violate this rule by assigning a variable of the wrong type to a variable. We say that Java is a strongly typed language because it enforces this rule.

There are eight so-called primitive types built into Java. The primitive types are named byte, short, int, long, float, double, char, and boolean. The first four types hold integers (whole numbers such as 17, -38477, and 0). The four integer types are distinguished by the ranges of integers they can hold. The float and double types hold real numbers (such as 3.6 and -145.99). Again, the two real types are distinguished by their range and accuracy. A variable of type char holds a single character from the Unicode character set. And a variable of type boolean holds one of the two logical values true or false.

Any data value stored in the computer's memory must be represented as a binary number, that is as a string of zeros and ones. A single zero or one is called a bit. A string of eight bits is called a byte. Memory is usually measured in terms of bytes. Not surprisingly, the byte data type refers to a single byte of memory. A variable of type byte holds a string of eight bits, which can represent any of the integers between -128 and 127, inclusive. (There are 256 integers in that range; eight bits can represent 256 -- two raised to the power eight -- different values.) As for the other integer types,

- short corresponds to two bytes (16 bits). Variables of type short have values in the range -32768 to 32767.

- int corresponds to four bytes (32 bits). Variables of type int have values in the range -2147483648 to 2147483647.

- long corresponds to eight bytes (64 bits). Variables of type long have values in the range -9223372036854775808 to 9223372036854775807.

You don't have to remember these numbers, but they do give you some idea of the size of integers that you can work with. Usually, for representing integer data you should just stick to the int data type, which is good enough for most purposes.

The float data type is represented in four bytes of memory, using a standard method for encoding real numbers. The maximum value for a float is about 10 raised to the power 38. A float can have about 7 significant digits. (So that 32.3989231134 and 32.3989234399 would both have to be rounded off to about 32.398923 in order to be stored in a variable of type float.) A double takes up 8 bytes, can range up to about 10 to the power 308, and has about 15 significant digits. Ordinarily, you should stick to the double type for real values.

A variable of type char occupies two bytes in memory. The value of a char variable is a single character such as A, *, x, or a space character. The value can also be a special character such a tab or a carriage return or one of the many Unicode characters that come from different languages. Values of type char are closely related to integer values, since a character is actually stored as a 16-bit integer code number. In fact, we will see that chars in Java can actually be used like integers in certain situations.

It is important to remember that a primitive type value is represented using ony a certain, finite number of bits. So, an int can't be an arbitrary integer; it can only be an integer in a certain finite range of values. Similarly, float and double variables can only take on certain values. They are not true real numbers in the mathematical sense. For example, the mathematical constant π can only be approximated by a value of type float or double, since it would require an infinite number of decimal places to represent it exactly. For that matter, simple numbers like 1/3 can only be approximated by floats and doubles.

2.2.3  Literals

A data value is stored in the computer as a sequence of bits. In the computer's memory, it doesn't look anything like a value written on this page. You need a way to include constant values in the programs that you write. In a program, you represent constant values as literals. A literal is something that you can type in a program to represent a value. It is a kind of name for a constant value.

For example, to type a value of type char in a program, you must surround it with a pair of single quote marks, such as 'A', '*', or 'x'. The character and the quote marks make up a literal of type char. Without the quotes, A would be an identifier and * would be a multiplication operator. The quotes are not part of the value and are not stored in the variable; they are just a convention for naming a particular character constant in a program. If you want to store the character A in a variable ch of type char, you could do so with the assignment statement

ch = 'A';

Certain special characters have special literals that use a backslash, \, as an "escape character". In particular, a tab is represented as '\t', a carriage return as '\r', a linefeed as '\n', the single quote character as '\'', and the backslash itself as '\\'. Note that even though you type two characters between the quotes in '\t', the value represented by this literal is a single tab character.

Numeric literals are a little more complicated than you might expect. Of course, there are the obvious literals such as 317 and 17.42. But there are other possibilities for expressing numbers in a Java program. First of all, real numbers can be represented in an exponential form such as 1.3e12 or 12.3737e-108. The "e12" and "e-108" represent powers of 10, so that 1.3e12 means 1.3 times 10^12 and 12.3737e-108 means 12.3737 times 10-108. This format can be used to express very large and very small numbers. Any numeric literal that contains a decimal point or exponential is a literal of type double. To make a literal of type float, you have to append an "F" or "f" to the end of the number. For example, "1.2F" stands for 1.2 considered as a value of type float. (Occasionally, you need to know this because the rules of Java say that you can't assign a value of type double to a variable of type float, so you might be confronted with a ridiculous-seeming error message if you try to do something like "x = 1.2;" if x is a variable of type float. You have to say "x = 1.2F;". This is one reason why I advise sticking to type double for real numbers.)

Even for integer literals, there are some complications. Ordinary integers such as 177777 and -32 are literals of type byte, short, or int, depending on their size. You can make a literal of type long by adding "L" as a suffix. For example: 17L or 728476874368L. As another complication, Java allows binary, octal (base-8), and hexadecimal (base-16) literals. I don't want to cover number bases in detail, but in case you run into them in other people's programs, it's worth knowing a few things: Octal numbers use only the digits 0 through 7. In Java, a numeric literal that begins with a 0 is interpreted as an octal number; for example, the octal literal 045 represents the number 37, not the number 45. Octal numbers are rarely used, but you need to be aware of what happens when you start a number with a zero. Hexadecimal numbers use 16 digits, the usual digits 0 through 9 and the letters A, B, C, D, E, and F. Upper case and lower case letters can be used interchangeably in this context. The letters represent the numbers 10 through 15. In Java, a hexadecimal literal begins with 0x or 0X, as in 0x45 or 0xFF7A. Finally, binary literals start with 0b or 0B and contain only the digits 0 and 1; for example: 0b10110.

As a final complication, numeric literals in Java 7 can include the underscore character ("_"), which can be used to separate groups of digits. For example, the integer constant for seven billion could be written 7_000_000_000, which is a good deal easier to decipher than 7000000000. There is no rule about how many digits have to be in each group. Underscores can be especially useful in long binary numbers; for example, 0b1010_1100_1011.

I will note that hexadecimal numbers can also be used in character literals to represent arbitrary Unicode characters. A Unicode literal consists of \u followed by four hexadecimal digits. For example, the character literal '\u00E9' represents the Unicode character that is an "e" with an acute accent.

For the type boolean, there are precisely two literals: true and false. These literals are typed just as I've written them here, without quotes, but they represent values, not variables. Boolean values occur most often as the values of conditional expressions. For example,

rate > 0.05

is a boolean-valued expression that evaluates to true if the value of the variable rate is greater than 0.05, and to false if the value of rate is not greater than 0.05. As you'll see in Chapter 3, boolean-valued expressions are used extensively in control structures. Of course, boolean values can also be assigned to variables of type boolean. For example, if test is a variable of type boolean, then both of the following assignment statements are legal:

test = true;
test = rate > 0.05;

2.2.4  Strings and String Literals

Java has other types in addition to the primitive types, but all the other types represent objects rather than "primitive" data values. For the most part, we are not concerned with objects for the time being. However, there is one predefined object type that is very important: the type String. (String is a type, but not a primitive type; it is in fact the name of a class, and we will return to that aspect of strings in the next section.)

A value of type String is a sequence of characters. You've already seen a string literal: "Hello World!". The double quotes are part of the literal; they have to be typed in the program. However, they are not part of the actual String value, which consists of just the characters between the quotes. A string can contain any number of characters, even zero. A string with no characters is called the empty string and is represented by the literal "", a pair of double quote marks with nothing between them. Remember the difference between single quotes and double quotes! Single quotes are used for char literals and double quotes for String literals! There is a big difference between the String "A" and the char 'A'.

Within a string literal, special characters can be represented using the backslash notation. Within this context, the double quote is itself a special character. For example, to represent the string value

I said, "Are you listening!"

with a linefeed at the end, you would have to type the string literal:

"I said, \"Are you listening!\"\n"

You can also use \t, \r, \\, and Unicode sequences such as \u00E9 to represent other special characters in string literals.

2.2.5  Variables in Programs

A variable can be used in a program only if it has first been declared. A variable declaration statement is used to declare one or more variables and to give them names. When the computer executes a variable declaration, it sets aside memory for the variable and associates the variable's name with that memory. A simple variable declaration takes the form:

type-name  variable-name-or-names;

The variable-name-or-names can be a single variable name or a list of variable names separated by commas. (We'll see later that variable declaration statements can actually be somewhat more complicated than this.) Good programming style is to declare only one variable in a declaration statement, unless the variables are closely related in some way. For example:

int numberOfStudents;
String name;
double x, y;
boolean isFinished;
char firstInitial, middleInitial, lastInitial;

It is also good style to include a comment with each variable declaration to explain its purpose in the program, or to give other information that might be useful to a human reader. For example:

double principal;    // Amount of money invested.
double interestRate; // Rate as a decimal, not percentage.

In this chapter, we will only use variables declared inside the main() subroutine of a program. Variables declared inside a subroutine are called local variables for that subroutine. They exist only inside the subroutine, while it is running, and are completely inaccessible from outside. Variable declarations can occur anywhere inside the subroutine, as long as each variable is declared before it is used in any way. Some people like to declare all the variables at the beginning of the subroutine. Others like to wait to declare a variable until it is needed. My preference: Declare important variables at the beginning of the subroutine, and use a comment to explain the purpose of each variable. Declare "utility variables" which are not important to the overall logic of the subroutine at the point in the subroutine where they are first used. Here is a simple program using some variables and assignment statements:

/**
* This class implements a simple program that
* will compute the amount of interest that is
* earned on $17,000 invested at an interest
* rate of 0.027 for one year.  The interest and
* the value of the investment after one year are
* printed to standard output.
*/

public class Interest {

public static void main(String[] args) {

/* Declare the variables. */

double principal;     // The value of the investment.
double rate;          // The annual interest rate.
double interest;      // Interest earned in one year.

/* Do the computations. */

principal = 17000;
rate = 0.027;
interest = principal * rate;   // Compute the interest.

principal = principal + interest;
// Compute value of investment after one year, with interest.
// (Note: The new value replaces the old value of principal.)

/* Output the results. */

System.out.print("The interest earned is $");
System.out.println(interest);
System.out.print("The value of the investment after one year is $");
System.out.println(principal);

} // end of main()

} // end of class Interest

This program uses several subroutine call statements to display information to the user of the program. Two different subroutines are used: System.out.print and System.out.println. The difference between these is that System.out.println adds a linefeed after the end of the information that it displays, while System.out.print does not. Thus, the value of interest, which is displayed by the subroutine call "System.out.println(interest);", follows on the same line as the string displayed by the previous System.out.print statement. Note that the value to be displayed by System.out.print or System.out.println is provided in parentheses after the subroutine name. This value is called a parameter to the subroutine. A parameter provides a subroutine with information it needs to perform its task. In a subroutine call statement, any parameters are listed in parentheses after the subroutine name. Not all subroutines have parameters. If there are no parameters in a subroutine call statement, the subroutine name must be followed by an empty pair of parentheses.

All the sample programs for this textbook are available in separate source code files in the on-line version of this text at http://math.hws.edu/javanotes/source. They are also included in the downloadable archives of the web site, in a folder named source. The source code for the Interest program, for example, can be found in the file Interest.java in subfolder named chapter2 inside the source folder.

Section 2.3

Strings, Classes, Objects, and Subroutines

The previous section introduced the eight primitive data types and the type String. There is a fundamental difference between the primitive types and String: Values of type String are objects. While we will not study objects in detail until Chapter 5, it will be useful for you to know a little about them and about a closely related topic: classes. This is not just because strings are useful but because objects and classes are essential to understanding another important programming concept, subroutines.

2.3.1  Built-in Subroutines and Functions

Recall that a subroutine is a set of program instructions that have been chunked together and given a name. A subroutine is designed to perform some task. To get that task performed in a program, you can "call" the subroutine using a subroutine call statement. In Chapter 4, you'll learn how to write your own subroutines, but you can get a lot done in a program just by calling subroutines that have already been written for you. In Java, every subroutine is contained either in a class or in an object. Some classes that are standard parts of the Java language contain predefined subroutines that you can use. A value of type String, which is an object, contains subroutines that can be used to manipulate that string. These subroutines are "built into" the Java language. You can call all these subroutines without understanding how they were written or how they work. Indeed, that's the whole point of subroutines: A subroutine is a "black box" which can be used without knowing what goes on inside.

Let's first consider subroutines that are part of a class. One of the purposes of a class is to group together some variables and subroutines, which are contained in that class. These variables and subroutines are called static members of the class. You've seen one example: In a class that defines a program, the main() routine is a static member of the class. The parts of a class definition that define static members are marked with the reserved word "static", such as the word "static" in public static void main...

When a class contains a static variable or subroutine, the name of the class is part of the full name of the variable or subroutine. For example, the standard class named System contains a subroutine named exit. To use that subroutine in your program, you must refer to it as System.exit. This full name consists of the name of the class that contains the subroutine, followed by a period, followed by the name of the subroutine. This subroutine requires an integer as parameter, so you would actually use it with a subroutine call statement such as

System.exit(0);

Calling System.exit will terminate the program and shut down the Java Virtual Machine. You could use it if you had some reason to terminate the program before the end of the main routine. (The parameter tells the computer why the program was terminated. A parameter value of 0 indicates that the program ended normally. Any other value indicates that the program was terminated because an error was detected, so you could call System.exit(1) to indicate that the program is ending because of an error. The parameter is sent back to the operating system; in practice, the value is usually ignored by the operating system.)

System is just one of many standard classes that come with Java. Another useful class is called Math. This class gives us an example of a class that contains static variables: It includes the variables Math.PI and Math.E whose values are the mathematical constants π and e. Math also contains a large number of mathematical "functions." Every subroutine performs some specific task. For some subroutines, that task is to compute or retrieve some data value. Subroutines of this type are called functions. We say that a function returns a value. Generally, the returned value is meant to be used somehow in the program that calls the function.

You are familiar with the mathematical function that computes the square root of a number. The corresponding function in Java is called Math.sqrt. This function is a static member subroutine of the class named Math. If x is any numerical value, then Math.sqrt(x) computes and returns the square root of that value. Since Math.sqrt(x) represents a value, it doesn't make sense to put it on a line by itself in a subroutine call statement such as

Math.sqrt(x);   // This doesn't make sense!

What, after all, would the computer do with the value computed by the function in this case? You have to tell the computer to do something with the value. You might tell the computer to display it:

System.out.print( Math.sqrt(x) );  // Display the square root of x.

or you might use an assignment statement to tell the computer to store that value in a variable:

lengthOfSide = Math.sqrt(x);

The function call Math.sqrt(x) represents a value of type double, and it can be used anyplace where a numeric literal of type double could be used.

The Math class contains many static member functions. Here is a list of some of the more important of them:

- Math.abs(x), which computes the absolute value of x.

- The usual trigonometric functions, Math.sin(x), Math.cos(x), and Math.tan(x). (For all the trigonometric functions, angles are measured in radians, not degrees.)

- The inverse trigonometric functions arcsin, arccos, and arctan, which are written as: Math.asin(x), Math.acos(x), and Math.atan(x). The return value is expressed in radians, not degrees.

- The exponential function Math.exp(x) for computing the number e raised to the power x, and the natural logarithm function Math.log(x) for computing the logarithm of x in the base e.

- Math.pow(x,y) for computing x raised to the power y.

- Math.floor(x), which rounds x down to the nearest integer value that is less than or equal to x. Even though the return value is mathematically an integer, it is returned as a value of type double, rather than of type int as you might expect. For example, Math.floor(3.76) is 3.0. The function Math.round(x) returns the integer that is closest to x, and Math.ceil(x) rounds x up to an integer. ("Ceil" is short for "ceiling", the opposite of "floor.")

- Math.random(), which returns a randomly chosen double in the range 0.0 <= Math.random() < 1.0. (The computer actually calculates so-called "pseudorandom" numbers, which are not truly random but are effectively random enough for most purposes.) We will find a lot of uses for Math.random in future examples.

For these functions, the type of the parameter -- the x or y inside the parentheses -- can be any value of any numeric type. For most of the functions, the value returned by the function is of type double no matter what the type of the parameter. However, for Math.abs(x), the value returned will be the same type as x; if x is of type int, then so is Math.abs(x). So, for example, while Math.sqrt(9) is the double value 3.0, Math.abs(9) is the int value 9.

Note that Math.random() does not have any parameter. You still need the parentheses, even though there's nothing between them. The parentheses let the computer know that this is a subroutine rather than a variable. Another example of a subroutine that has no parameters is the function System.currentTimeMillis(), from the System class. When this function is executed, it retrieves the current time, expressed as the number of milliseconds that have passed since a standardized base time (the start of the year 1970, if you care). One millisecond is one-thousandth of a second. The return value of System.currentTimeMillis() is of type long (a 64-bit integer). This function can be used to measure the time that it takes the computer to perform a task. Just record the time at which the task is begun and the time at which it is finished and take the difference.

Here is a sample program that performs a few mathematical tasks and reports the time that it takes for the program to run. On some computers, the time reported might be zero, because it is too small to measure in milliseconds. Even if it's not zero, you can be sure that most of the time reported by the computer was spent doing output or working on tasks other than the program, since the calculations performed in this program occupy only a tiny fraction of a millisecond of a computer's time.

/**
* This program performs some mathematical computations and displays the
* results.  It also displays the value of the constant Math.PI.  It then
* reports the number of seconds that the computer spent on this task.
*/

public class TimedComputation {

public static void main(String[] args) {

long startTime; // Starting time of program, in milliseconds.
long endTime;   // Time when computations are done, in milliseconds.
double time;    // Time difference, in seconds.

startTime = System.currentTimeMillis();

double width, height, hypotenuse;  // sides of a triangle
width = 42.0;
height = 17.0;
hypotenuse = Math.sqrt( width*width + height*height );
System.out.print("A triangle with sides 42 and 17 has hypotenuse ");
System.out.println(hypotenuse);

System.out.println("\nMathematically, sin(x)*sin(x) + "
+ "cos(x)*cos(x) - 1 should be 0.");
System.out.println("Let's check this for x = 1:");
System.out.print("      sin(1)*sin(1) + cos(1)*cos(1) - 1 is ");
System.out.println( Math.sin(1)*Math.sin(1)
+ Math.cos(1)*Math.cos(1) - 1 );
System.out.println("(There can be round-off errors when"
+ " computing with real numbers!)");

System.out.print("\nHere is a random number:  ");
System.out.println( Math.random() );

System.out.print("The value of Math.PI is ");
System.out.println( Math.PI );

endTime = System.currentTimeMillis();
time = (endTime - startTime) / 1000.0;

System.out.print("\nRun time in seconds was:  ");
System.out.println(time);

} // end main()

} // end class TimedComputation

2.3.2  Classes and Objects

Classes can be containers for static variables and subroutines. However classes also have another purpose. They are used to describe objects. In this role, the class is a type, in the same way that int and double are types. That is, the class name can be used to declare variables. Such variables can only hold one type of value. The values in this case are objects. An object is a collection of variables and subroutines. Every object has an associated class that tells what "type" of object it is. The class of an object specifies what subroutines and variables that object contains. All objects defined by the same class are similar in that they contain similar collections of variables and subroutines. For example, an object might represent a point in the plane, and it might contain variables named x and y to represent the coordinates of that point. Every point object would have an x and a y, but different points would have different values for these variables. A class, named Point, for example, could exist to define the common structure of all point objects, and all such objects would then be values of type Point.

As another example, let's look again at System.out.println. System is a class, and out is a static variable within that class. However, the value of System.out is an object, and System.out.println is actually the full name of a subroutine that is contained in the object System.out. You don't need to understand it at this point, but the object referred to by System.out is an object of the class PrintStream. PrintStream is another class that is a standard part of Java. Any object of type PrintStream is a destination to which information can be printed; any object of type PrintStream has a println subroutine that can be used to send information to that destination. The object System.out is just one possible destination, and System.out.println is a subroutine that sends information to that particular destination. Other objects of type PrintStream might send information to other destinations such as files or across a network to other computers. This is object-oriented programming: Many different things which have something in common -- they can all be used as destinations for information -- can all be used in the same way -- through a println subroutine. The PrintStream class expresses the commonalities among all these objects.

The dual role of classes can be confusing, and in practice most classes are designed to perform primarily or exclusively in only one of the two possible roles. Fortunately, you will not need to worry too much about it until we start working with objects in a more serious way, in Chapter 5.

By the way, since class names and variable names are used in similar ways, it might be hard to tell which is which. Remember that all the built-in, predefined names in Java follow the rule that class names begin with an upper case letter while variable names begin with a lower case letter. While this is not a formal syntax rule, I strongly recommend that you follow it in your own programming. Subroutine names should also begin with lower case letters. There is no possibility of confusing a variable with a subroutine, since a subroutine name in a program is always followed by a left parenthesis.

As one final general note, you should be aware that subroutines in Java are often referred to as methods. Generally, the term "method" means a subroutine that is contained in a class or in an object. Since this is true of every subroutine in Java, every subroutine in Java is a method. The same is not true for other programming languages, and for the time being, I will prefer to use the more general term, "subroutine." However, I should note that some people prefer to use the term "method" from the beginning.

2.3.3  Operations on Strings

String is a class, and a value of type String is an object. That object contains data, namely the sequence of characters that make up the string. It also contains subroutines. All of these subroutines are in fact functions. For example, every string object contains a function named length that computes the number of characters in that string. Suppose that advice is a variable that refers to a String. For example, advice might have been declared and assigned a value as follows:

String advice;
advice = "Seize the day!";

Then advice.length() is a function call that returns the number of characters in the string "Seize the day!". In this case, the return value would be 14. In general, for any variable str of type String, the value of str.length() is an int equal to the number of characters in the string. Note that this function has no parameter; the particular string whose length is being computed is the value of str. The length subroutine is defined by the class String, and it can be used with any value of type String. It can even be used with String literals, which are, after all, just constant values of type String. For example, you could have a program count the characters in "Hello World" for you by saying

System.out.print("The number of characters in ");
System.out.print("the string \"Hello World\" is ");
System.out.println( "Hello World".length() );

The String class defines a lot of functions. Here are some that you might find useful. Assume that s1 and s2 are variables of type String:

- s1.equals(s2) is a function that returns a boolean value. It returns true if s1 consists of exactly the same sequence of characters as s2, and returns false otherwise.

- s1.equalsIgnoreCase(s2) is another boolean-valued function that checks whether s1 is the same string as s2, but this function considers upper and lower case letters to be equivalent. Thus, if s1 is "cat", then s1.equals("Cat") is false, while s1.equalsIgnoreCase("Cat") is true.

- s1.length(), as mentioned above, is an integer-valued function that gives the number of characters in s1.

- s1.charAt(N), where N is an integer, returns a value of type char. It returns the Nth character in the string. Positions are numbered starting with 0, so s1.charAt(0) is actually the first character, s1.charAt(1) is the second, and so on. The final position is s1.length() - 1. For example, the value of "cat".charAt(1) is 'a'. An error occurs if the value of the parameter is less than zero or is greater than or equal to s1.length().

- s1.substring(N,M), where N and M are integers, returns a value of type String. The returned value consists of the characters of s1 in positions N, N+1,..., M-1. Note that the character in position M is not included. The returned value is called a substring of s1. The subroutine s1.substring(N) returns the substring of s1 consisting of characters starting at position N up until the end of the string.

- s1.indexOf(s2) returns an integer. If s2 occurs as a substring of s1, then the returned value is the starting position of that substring. Otherwise, the returned value is -1. You can also use s1.indexOf(ch) to search for a char, ch, in s1. To find the first occurrence of x at or after position N, you can use s1.indexOf(x,N). To find the last occurance of x in s1, use s1.lastIndexOf(x).

- s1.compareTo(s2) is an integer-valued function that compares the two strings. If the strings are equal, the value returned is zero. If s1 is less than s2, the value returned is a number less than zero, and if s1 is greater than s2, the value returned is some number greater than zero. (If both of the strings consist entirely of lower case letters, or if they consist entirely of upper case letters, then "less than" and "greater than" refer to alphabetical order. Otherwise, the ordering is more complicated.)

- s1.toUpperCase() is a String-valued function that returns a new string that is equal to s1, except that any lower case letters in s1 have been converted to upper case. For example, "Cat".toUpperCase() is the string "CAT". There is also a function s1.toLowerCase().

- s1.trim() is a String-valued function that returns a new string that is equal to s1 except that any non-printing characters such as spaces and tabs have been trimmed from the beginning and from the end of the string. Thus, if s1 has the value "fred  ", then s1.trim() is the string "fred", with the spaces at the end removed.

For the functions s1.toUpperCase(), s1.toLowerCase(), and s1.trim(), note that the value of s1 is not changed. Instead a new string is created and returned as the value of the function. The returned value could be used, for example, in an assignment statement such as "smallLetters = s1.toLowerCase();". To change the value of s1, you could use an assignment "s1 = s1.toLowerCase();".

Here is another extremely useful fact about strings: You can use the plus operator, +, to concatenate two strings. The concatenation of two strings is a new string consisting of all the characters of the first string followed by all the characters of the second string. For example, "Hello" + "World" evaluates to "HelloWorld". (Gotta watch those spaces, of course -- if you want a space in the concatenated string, it has to be somewhere in the input data, as in "Hello " + "World".)

Let's suppose that name is a variable of type String and that it already refers to the name of the person using the program. Then, the program could greet the user by executing the statement:

System.out.println("Hello, "  +  name  +  ".  Pleased to meet you!");

Even more surprising is that you can actually concatenate values of any type onto a String using the + operator. The value is converted to a string, just as it would be if you printed it to the standard output, and then that string is concatenated with the other string. For example, the expression "Number" + 42 evaluates to the string "Number42". And the statements

System.out.print("After ");
System.out.print(years);
System.out.print(" years, the value is ");
System.out.print(principal);

can be replaced by the single statement:

System.out.print("After " + years +
" years, the value is " + principal);

Obviously, this is very convenient. It would have shortened some of the examples presented earlier in this chapter.

2.3.4  Introduction to Enums

Java comes with eight built-in primitive types and a large set of types that are defined by classes, such as String. But even this large collection of types is not sufficient to cover all the possible situations that a programmer might have to deal with. So, an essential part of Java, just like almost any other programming language, is the ability to create new types. For the most part, this is done by defining new classes; you will learn how to do that in Chapter 5. But we will look here at one particular case: the ability to define enums (short for enumerated types).

Technically, an enum is considered to be a special kind of class, but that is not important for now. In this section, we will look at enums in a simplified form. In practice, most uses of enums will only need the simplified form that is presented here.

An enum is a type that has a fixed list of possible values, which is specified when the enum is created. In some ways, an enum is similar to the boolean data type, which has true and false as its only possible values. However, boolean is a primitive type, while an enum is not.

The definition of an enum type has the (simplified) form:

enum enum-type-name { list-of-enum-values }

This definition cannot be inside a subroutine. You can place it outside the main() routine of the program. The enum-type-name can be any simple identifier. This identifier becomes the name of the enum type, in the same way that "boolean" is the name of the boolean type and "String" is the name of the String type. Each value in the list-of-enum-values must be a simple identifier, and the identifiers in the list are separated by commas. For example, here is the definition of an enum type named Season whose values are the names of the four seasons of the year:

enum Season { SPRING, SUMMER, FALL, WINTER }

By convention, enum values are given names that are made up of upper case letters, but that is a style guideline and not a syntax rule. An enum value is a constant; that is, it represents a fixed value that cannot be changed. The possible values of an enum type are usually referred to as enum constants.

Note that the enum constants of type Season are considered to be "contained in" Season, which means -- following the convention that compound identifiers are used for things that are contained in other things -- the names that you actually use in your program to refer to them are Season.SPRING, Season.SUMMER, Season.FALL, and Season.WINTER.

Once an enum type has been created, it can be used to declare variables in exactly the same ways that other types are used. For example, you can declare a variable named vacation of type Season with the statement:

Season vacation;

After declaring the variable, you can assign a value to it using an assignment statement. The value on the right-hand side of the assignment can be one of the enum constants of type Season. Remember to use the full name of the constant, including "Season"! For example:

vacation = Season.SUMMER;

You can print out an enum value with an output statement such as System.out.print(vacation). The output value will be the name of the enum constant (without the "Season."). In this case, the output would be "SUMMER".

Because an enum is technically a class, the enum values are technically objects. As objects, they can contain subroutines. One of the subroutines in every enum value is named ordinal(). When used with an enum value, it returns the ordinal number of the value in the list of values of the enum. The ordinal number simply tells the position of the value in the list. That is, Season.SPRING.ordinal() is the int value 0, Season.SUMMER.ordinal() is 1, Season.FALL.ordinal() is 2, and Season.WINTER.ordinal() is 3. (You will see over and over again that computer scientists like to start counting at zero!) You can, of course, use the ordinal() method with a variable of type Season, such as vacation.ordinal().

Using enums can make a program more readable, since you can use meaningful names for the values. And it can prevent certain types of errors, since a compiler can check that the values assigned to an enum variable are in fact legal values for that variable. However, we will in fact use them only occasionally in this book. For now, you should just appreciate them as the first example of an important concept: creating new types. Here is a little example that shows enums being used in a complete program:

public class EnumDemo {

// Define two enum types -- remember that the definitions
// go OUTSIDE The main() routine!

enum Day { SUNDAY, MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY }

enum Month { JAN, FEB, MAR, APR, MAY, JUN, JUL, AUG, SEP, OCT, NOV, DEC }

public static void main(String[] args) {

Day tgif;     // Declare a variable of type Day.
Month libra;  // Declare a variable of type Month.

tgif = Day.FRIDAY;    // Assign a value of type Day to tgif.
libra = Month.OCT;    // Assign a value of type Month to libra.

System.out.print("My sign is libra, since I was born in ");
System.out.println(libra);   // Output value will be:  OCT
System.out.print("That's the ");
System.out.print( libra.ordinal() );
System.out.println("-th month of the year.");
System.out.println("   (Counting from 0, of course!)");

System.out.print("Isn't it nice to get to ");
System.out.println(tgif);   // Output value will be:  FRIDAY

System.out.println( tgif + " is the " + tgif.ordinal()
+ "-th day of the week.");
}

}

Section 2.4

Text Input and Output

We have seen that it is very easy to display text to the user with the functions System.out.print and System.out.println. But there is more to say on the topic of outputting text. Furthermore, most programs use data that is input to the program at run time, so you need to know how to do input as well as output. This section explains how to get data from the user, and it covers output in more detail than we have seen so far. It also has a section on using files for input and output.

2.4.1  Basic Output and Formatted Output

The most basic output function is System.out.print(x), where x can be a value or expression of any type. If the parameter, x, is not already a string, it is converted to a value of type String, and the string is then output to the destination called standard output. (Generally, this means that the string is displayed to the user; however, in GUI programs, it outputs to a place where a typical user is unlikely to see it. Furthermore, standard output can be "redirected" to write to a different output destination. Nevertheless, for the type of program that we are working with now, the purpose of System.out is to display text to the user.)

System.out.println(x) outputs the same text as System.out.print, but it follows that text by a line feed, which means that any subsequent output will be on the next line. It is possible to use this function with no parameter, System.out.println(), which outputs nothing but a line feed. Note that System.out.println(x) is equivalent to

System.out.print(x);
System.out.println();

You might have noticed that System.out.print outputs real numbers with as many digits after the decimal point as necessary, so that for example π is output as 3.141592653589793, and numbers that are supposed to represent money might be output as 1050.0 or 43.575. You might prefer to have these numbers output as, for example, 3.14159, 1050.00, and 43.58. Java has a "formatted output" capability that makes it easy to control how real numbers and other values are printed. A lot of formatting options are available. I will cover just a few of the simplest and most commonly used possibilities here.

The function System.out.printf can be used to produce formatted output. (The name "printf," which stands for "print formatted," is copied from the C and C++ programming languages, where this type of output originated.) System.out.printf takes one or more parameters. The first parameter is a String that specifies the format of the output. This parameter is called the format string. The remaining parameters specify the values that are to be output. Here is a statement that will print a number in the proper format for a dollar amount, where amount is a variable of type double:

System.out.printf( "%1.2f", amount );

The output format of a value is specified by a format specifier. In this example, the format specifier is %1.2f. The format string (in the simple cases that I cover here) contains one format specifier for each of the values that is to be output. Some typical format specifiers are %d, %12d, %10s, %1.2f, %15.8e and %1.8g. Every format specifier begins with a percent sign (%) and ends with a letter, possibly with some extra formatting information in between. The letter specifies the type of output that is to be produced. For example, in %d and %12d, the "d" specifies that an integer is to be written. The "12" in %12d specifies the minimum number of spaces that should be used for the output. If the integer that is being output takes up fewer than 12 spaces, extra blank spaces are added in front of the integer to bring the total up to 12. We say that the output is "right-justified in a field of length 12." A very large value is not forced into 12 spaces; if the value has more than 12 digits, all the digits will be printed, with no extra spaces. The specifier %d means the same as %1d -- that is, an integer will be printed using just as many spaces as necessary. (The "d," by the way, stands for "decimal" -- that is, base-10 -- numbers. You can replace the "d" with an "x" to output an integer value in hexadecimal form.)

The letter "s" at the end of a format specifier can be used with any type of value. It means that the value should be output in its default format, just as it would be in unformatted output. A number, such as the "20" in %20s, can be added to specify the (minimum) number of characters. The "s" stands for "string," and it can be used for values of type String. It can also be used for values of other types; in that case the value is converted into a String value in the usual way.

The format specifiers for values of type double are more complicated. An "f", as in %1.2f, is used to output a number in "floating-point" form, that is with digits after a decimal point. In %1.2f, the "2" specifies the number of digits to use after the decimal point. The "1" specifies the (minimum) number of characters to output; a "1" in this position effectively means that just as many characters as are necessary should be used. Similarly, %12.3f would specify a floating-point format with 3 digits after the decimal point, right-justified in a field of length 12.

Very large and very small numbers should be written in exponential format, such as 6.00221415e23, representing "6.00221415 times 10 raised to the power 23." A format specifier such as %15.8e specifies an output in exponential form, with the "8" telling how many digits to use after the decimal point. If you use "g" instead of "e", the output will be in exponential form for very small values and very large values and in floating-point form for other values. In %1.8g, the 8 gives the total number of digits in the answer, including both the digits before the decimal point and the digits after the decimal point.

For numeric output, the format specifier can include a comma (","), which will cause the digits of the number to be separated into groups, to make it easier to read big numbers. In the United States, groups of three digits are separated by commas. For example, if x is one billion, then System.out.printf("%,d",x) will output 1,000,000,000. In other countries, the separator character and the number of digits per group might be different. The comma should come at the beginning of the format specifier, before the field width; for example: %,12.3f. If you want the output to be left-justified instead of right justified, add a minus sign to the beginning of the format specifier: for example, %-20s.

In addition to format specifiers, the format string in a printf statement can include other characters. These extra characters are just copied to the output. This can be a convenient way to insert values into the middle of an output string. For example, if x and y are variables of type int, you could say

System.out.printf("The product of %d and %d is %d", x, y, x*y);

When this statement is executed, the value of x is substituted for the first %d in the string, the value of y for the second %d, and the value of the expression x*y for the third, so the output would be something like "The product of 17 and 42 is 714" (quotation marks not included in output!).

To output a percent sign, use the format specifier %% in the format string. You can use %n to output a line feed. You can also use a backslash, \, as usual in strings to output special characters such as tabs and double quote characters.

2.4.2  A First Text Input Example

For some unfathomable reason, Java has never made it very easy to read data typed in by the user of a program. You've already seen that output can be displayed to the user using the subroutine System.out.print. This subroutine is part of a pre-defined object called System.out. The purpose of this object is precisely to display output to the user. There is a corresponding object called System.in that exists to read data input by the user, but it provides only very primitive input facilities, and it requires some advanced Java programming skills to use it effectively.

Java 5.0 finally made input a little easier with a new Scanner class. However, it requires some knowledge of object-oriented programming to use this class, so it's not ideal for use here at the beginning of this course. Java 6 introduced the Console class for communicating with the user, but Console has its own problems. (It is not always available, and it can only read strings, not numbers.) Furthermore, in my opinion, Scanner and Console still don't get things quite right. Nevertheless, I will introduce Scanner briefly at the end of this section, in case you want to start using it now. However, we start with my own version of text input.

Fortunately, it is possible to extend Java by creating new classes that provide subroutines that are not available in the standard part of the language. As soon as a new class is available, the subroutines that it contains can be used in exactly the same way as built-in routines. Along these lines, I've written a class named TextIO that defines subroutines for reading values typed by the user. The subroutines in this class make it possible to get input from the standard input object, System.in, without knowing about the advanced aspects of Java that are needed to use Scanner or to use System.in directly. TextIO also has a few other capabilities that I will discuss later in this section.

To use the TextIO class, you must make sure that the class is available to your program. What this means depends on the Java programming environment that you are using. In general, you just have to add the source code file, TextIO.java, to the same directory that contains your main program. See Section 2.6 for information about how to use TextIO.

The input routines in the TextIO class are static member functions. (Static member functions were introduced in the previous section.) Let's suppose that you want your program to read an integer typed in by the user. The TextIO class contains a static member function named getlnInt that you can use for this purpose. Since this function is contained in the TextIO class, you have to refer to it in your program as TextIO.getlnInt. The function has no parameters, so a complete call to the function takes the form "TextIO.getlnInt()". This function call represents the int value typed by the user, and you have to do something with the returned value, such as assign it to a variable. For example, if userInput is a variable of type int (created with a declaration statement "int userInput;"), then you could use the assignment statement

userInput = TextIO.getlnInt();

When the computer executes this statement, it will wait for the user to type in an integer value. The user must type a number and press return before the program can continue. The value that the user typed will then be returned by the function, and it will be stored in the variable, userInput. Here is a complete program that uses TextIO.getlnInt to read a number typed by the user and then prints out the square of that number:

/**
* A program that reads an integer that is typed in by the
* user and computes and prints the square of that integer.
*/

public class PrintSquare {

public static void main(String[] args) {

int userInput;  // The number input by the user.
int square;     // The userInput, multiplied by itself.

System.out.print("Please type a number: ");
userInput = TextIO.getlnInt();
square = userInput * userInput;

System.out.println();
System.out.println("The number that you entered was " + userInput);
System.out.println("The square of that number is " + square);
System.out.println();

} // end of main()

} //end of class PrintSquare

When you run this program, it will display the message "Please type a number:" and will pause until you type a response, including a carriage return after the number. Note that it is good style to output a question or some other prompt to the user before reading input. Otherwise, the user will have no way of knowing exactly what the computer is waiting for, or even that it is waiting for the user to do something.

2.4.3  Basic TextIO Input Functions

TextIO includes a variety of functions for inputting values of various types. Here are the functions that you are most likely to use:

j = TextIO.getlnInt();     // Reads a value of type int.
y = TextIO.getlnDouble();  // Reads a value of type double.
a = TextIO.getlnBoolean(); // Reads a value of type boolean.
c = TextIO.getlnChar();    // Reads a value of type char.
w = TextIO.getlnWord();    // Reads one "word" as a value of type String.
s = TextIO.getln();        // Reads an entire input line as a String.

For these statements to be legal, the variables on the left side of each assignment statement must already be declared and must be of the same type as that returned by the function on the right side. Note carefully that these functions do not have parameters. The values that they return come from outside the program, typed in by the user as the program is running. To "capture" that data so that you can use it in your program, you have to assign the return value of the function to a variable. You will then be able to refer to the user's input value by using the name of the variable.

When you call one of these functions, you are guaranteed that it will return a legal value of the correct type. If the user types in an illegal value as input -- for example, if you ask for an int and the user types in a non-numeric character or a number that is outside the legal range of values that can be stored in a variable of type int -- then the computer will ask the user to re-enter the value, and your program never sees the first, illegal value that the user entered. For TextIO.getlnBoolean(), the user is allowed to type in any of the following: true, false, t, f, yes, no, y, n, 1, or 0. Furthermore, they can use either upper or lower case letters. In any case, the user's input is interpreted as a true/false value. It's convenient to use TextIO.getlnBoolean() to read the user's response to a Yes/No question.

You'll notice that there are two input functions that return Strings. The first, getlnWord(), returns a string consisting of non-blank characters only. When it is called, it skips over any spaces and carriage returns typed in by the user. Then it reads non-blank characters until it gets to the next space or carriage return. It returns a String consisting of all the non-blank characters that it has read. The second input function, getln(), simply returns a string consisting of all the characters typed in by the user, including spaces, up to the next carriage return. It gets an entire line of input text. The carriage return itself is not returned as part of the input string, but it is read and discarded by the computer. Note that the String returned by TextIO.getln() might be the empty string, "", which contains no characters at all. You will get this return value if the user simply presses return, without typing anything else first.

TextIO.getln() does not skip blanks or end-of-lines before reading a value. But the input functions getlnInt(), getlnDouble(), getlnBoolean(), and getlnChar() behave like getlnWord() in that they will skip past any blanks and carriage returns in the input before reading a value. When one of these functions skips over an end-of-line, it outputs a '?' to let the user know that more input is expected.

Furthermore, if the user types extra characters on the line after the input value, all the extra characters will be discarded, along with the carriage return at the end of the line. If the program executes another input function, the user will have to type in another line of input, even if they had typed more than one value on the previous line. It might not sound like a good idea to discard any of the user's input, but it turns out to be the safest thing to do in most programs.

Using TextIO for input and output, we can now improve the program from Section 2.2 for computing the value of an investment. We can have the user type in the initial value of the investment and the interest rate. The result is a much more useful program -- for one thing, it makes sense to run it more than once! Note that this program uses formatted output to print out monetary values in their correct format.

/**
* This class implements a simple program that will compute
* the amount of interest that is earned on an investment over
* a period of one year.  The initial amount of the investment
* and the interest rate are input by the user.  The value of
* the investment at the end of the year is output.  The
* rate must be input as a decimal, not a percentage (for
* example, 0.05 rather than 5).
*/

public class Interest2 {

public static void main(String[] args) {

double principal;  // The value of the investment.
double rate;       // The annual interest rate.
double interest;   // The interest earned during the year.

System.out.print("Enter the initial investment: ");
principal = TextIO.getlnDouble();

System.out.print("Enter the annual interest rate (as a decimal): ");
rate = TextIO.getlnDouble();

interest = principal * rate;       // Compute this year's interest.
principal = principal + interest;  // Add it to principal.

System.out.printf("The amount of interest is $%1.2f%n", interest);
System.out.printf("The value after one year is $%1.2f%n", principal);

} // end of main()

} // end of class Interest2

(You might be wondering why there is only one output routine, System.out.println, which can output data values of any type, while there is a separate input routine for each data type. For the output function, the computer can tell what type of value is being output by looking at the parameter. However, the input routines don't have parameters, so the different input routines can only be distinguished by having different names.)

2.4.4  Introduction to File I/O

System.out sends its output to the output destination known as "standard output." But standard output is just one possible output destination. For example, data can be written to a file that is stored on the user's hard drive. The advantage to this, of course, is that the data is saved in the file even after the program ends, and the user can print the file, email it to someone else, edit it with another program, and so on. Similarly, System.in has only one possible source for input data.

TextIO has the ability to write data to files and to read data from files. TextIO includes output functions TextIO.put, TextIO.putln, and TextIO.putf. Ordinarily, these functions work exactly like System.out.print, System.out.println, and System.out.printf and are interchangeable with them. However, they can also be used to output text to files and to other destinations.

When you write output using TextIO.put, TextIO.putln, or TextIO.putf, the output is sent to the current output destination. By default, the current output destination is standard output. However, TextIO has subroutines that can be used to change the current output destination. To write to a file named "result.txt", for example, you would use the statement:

TextIO.writeFile("result.txt");

After this statement is executed, any output from TextIO output statements will be sent to the file named "result.txt" instead of to standard output. The file will be created if it does not already exist. Note that if a file with the same name already exists, its previous contents will be erased without any warning!

When you call TextIO.writeFile, TextIO remembers the file and automatically sends any output from TextIO.put or other output functions to that file. If you want to go back to writing to standard output, you can call

TextIO.writeStandardOutput();

Here is a simple program that asks the user some questions and outputs the user's responses to a file named "profile.txt." As an example, it uses TextIO for output to standard output as well as to the file, but System.out could also have been used for the output to standard output.

public class CreateProfile {

public static void main(String[] args) {

String name;     // The user's name.
String email;    // The user's email address.
double salary;   // the user's yearly salary.
String favColor; // The user's favorite color.

TextIO.putln("Good Afternoon!  This program will create");
TextIO.putln("your profile file, if you will just answer");
TextIO.putln("a few simple questions.");
TextIO.putln();

/* Gather responses from the user. */

TextIO.put("What is your name?           ");
name = TextIO.getln();
TextIO.put("What is your email address?  ");
email = TextIO.getln();
TextIO.put("What is your yearly income?  ");
salary = TextIO.getlnDouble();
TextIO.put("What is your favorite color? ");
favColor = TextIO.getln();

/* Write the user's information to the file named profile.txt. */

TextIO.writeFile("profile.txt");  // subsequent output goes to file
TextIO.putln("Name:            " + name);
TextIO.putln("Email:           " + email);
TextIO.putln("Favorite Color:  " + favColor);
TextIO.putf( "Yearly Income:   %,1.2f%n", salary);

/* Print a final message to standard output. */

TextIO.writeStandardOutput();
TextIO.putln("Thank you.  Your profile has been written to profile.txt.");

}

}

In many cases, you want to let the user select the file that will be used for output. You could ask the user to type in the file name, but that is error-prone, and users are more familiar with selecting a file from a file dialog box. The statement

TextIO.writeUserSelectedFile();

will open a typical graphical-user-interface file selection dialog where the user can specify the output file. This also has the advantage of alerting the user if they are about to replace an existing file. It is possible for the user to cancel the dialog box without selecting a file. TextIO.writeUserSelectedFile is a function that returns a boolean value. The return value is true if the user selected a file, and is false if the user canceled the dialog box. Your program can check the return value if it needs to know whether it is actually going to write to a file or not.

TextIO can also read from files, as an alternative to reading from standard input. You can specify an input source for TextIO's various "get" functions. The default input source is standard input. You can use the statement TextIO.readFile("data.txt") to read from a file named "data.txt" instead, or you can let the user select the input file with a GUI-style dialog box by saying TextIO.readUserSelectedFile(). After you have done this, any input will come from the file instead of being typed by the user. You can go back to reading the user's input with TextIO.readStandardInput().

When your program is reading from standard input, the user gets a chance to correct any errors in the input. This is not possible when the program is reading from a file. If illegal data is found when a program tries to read from a file, an error occurs that will crash the program. (Later, we will see that it is possible to "catch" such errors and recover from them.) Errors can also occur, though more rarely, when writing to files.

A complete understanding of input/output in Java requires a knowledge of object oriented programming. We will return to the topic later, in Chapter 11. The file I/O capabilities in TextIO are rather primitive by comparison. Nevertheless, they are sufficient for many applications, and they will allow you to get some experience with files sooner rather than later.

2.4.5  Other TextIO Features

The TextIO input functions that we have seen so far can only read one value from a line of input. Sometimes, however, you do want to read more than one value from the same line of input. For example, you might want the user to be able to type something like "42 17" to input the two numbers 42 and 17 on the same line. TextIO provides the following alternative input functions to allow you to do this:

j = TextIO.getInt();     // Reads a value of type int.
y = TextIO.getDouble();  // Reads a value of type double.
a = TextIO.getBoolean(); // Reads a value of type boolean.
c = TextIO.getChar();    // Reads a value of type char.
w = TextIO.getWord();    // Reads one "word" as a value of type String.

The names of these functions start with "get" instead of "getln". "Getln" is short for "get line" and should remind you that the functions whose names begin with "getln" will consume an entire line of data. A function without the "ln" will read an input value in the same way, but will then save the rest of the input line in a chunk of internal memory called the input buffer. The next time the computer wants to read an input value, it will look in the input buffer before prompting the user for input. This allows the computer to read several values from one line of the user's input. Strictly speaking, the computer actually reads only from the input buffer. The first time the program tries to read input from the user, the computer will wait while the user types in an entire line of input. TextIO stores that line in the input buffer until the data on the line has been read or discarded (by one of the "getln" functions). The user only gets to type when the buffer is empty.

Note, by the way, that although the TextIO input functions will skip past blank spaces and carriage returns while looking for input, they will not skip past other characters. For example, if you try to read two ints and the user types "42,17", the computer will read the first number correctly, but when it tries to read the second number, it will see the comma. It will regard this as an error and will force the user to retype the number. If you want to input several numbers from one line, you should make sure that the user knows to separate them with spaces, not commas. Alternatively, if you want to require a comma between the numbers, use getChar() to read the comma before reading the second number.

There is another character input function, TextIO.getAnyChar(), which does not skip past blanks or carriage returns. It simply reads and returns the next character typed by the user, even if it's a blank or carriage return. If the user typed a carriage return, then the char returned by getAnyChar() is the special linefeed character '\n'. There is also a function, TextIO.peek(), that lets you look ahead at the next character in the input without actually reading it. After you "peek" at the next character, it will still be there when you read the next item from input. This allows you to look ahead and see what's coming up in the input, so that you can take different actions depending on what's there.

The TextIO class provides a number of other functions. To learn more about them, you can look at the comments in the source code file, TextIO.java.

Clearly, the semantics of input is much more complicated than the semantics of output! Fortunately, for the majority of applications, it's pretty straightforward in practice. You only need to follow the details if you want to do something fancy. In particular, I strongly advise you to use the "getln" versions of the input routines, rather than the "get" versions, unless you really want to read several items from the same line of input, precisely because the semantics of the "getln" versions is much simpler.

2.4.6  Using Scanner for Input

TextIO makes it easy to get input from the user. However, since it is not a standard class, you have to remember to make TextIO.java available to any program that uses it. Another option for input is the Scanner class. One advantage of using Scanner is that it's a standard part of Java and so is always there when you want it.

It's not that hard to use a Scanner for user input, and it has some nice features, but using it requires some syntax that will not be introduced until Chapter 4 and Chapter 5. I'll tell you how to do it here, without explaining why it works. You won't understand all the syntax at this point. (Scanners will be covered in more detail in Subsection 11.1.5.)

First, you should add the following line to your program at the beginning of the source code file, before the "public class...":

import java.util.Scanner;

Then include the following statement at the beginning of your main() routine:

Scanner stdin = new Scanner( System.in );

This creates a variable named stdin of type Scanner. (You can use a different name for the variable if you want; "stdin" stands for "standard input.") You can then use stdin in your program to access a variety of subroutines for reading user input. For example, the function stdin.nextInt() reads one value of type int from the user and returns it. It is almost the same as TextIO.getInt() except for two things: If the value entered by the user is not a legal int, then stdin.nextInt() will crash rather than prompt the user to re-enter the value. And the integer entered by the user must be followed by a blank space or by an end-of-line, whereas TextIO.getInt() will stop reading at any character that is not a digit.

There are corresponding methods for reading other types of data, including stdin.nextDouble(), stdin.nextLong(), and stdin.nextBoolean(). (stdin.nextBoolean() will only accept "true" or "false" as input.) These subroutines can read more than one value from a line, so they are more similar to the "get" versions of TextIO subroutines rather than the "getln" versions. The method stdin.nextLine() is equivalent to TextIO.getln(), and stdin.next(), like TextIO.getWord(), returns a string of non-blank characters.

As a simple example, here is a version of the sample program Interest2.java that uses Scanner instead of TextIO for user input:

import java.util.Scanner;  // Make the Scanner class available.

public class Interest2WithScanner {

public static void main(String[] args) {

Scanner stdin = new Scanner( System.in );  // Create the Scanner.

double principal;  // The value of the investment.
double rate;       // The annual interest rate.
double interest;   // The interest earned during the year.

System.out.print("Enter the initial investment: ");
principal = stdin.nextDouble();

System.out.print("Enter the annual interest rate (as a decimal): ");
rate = stdin.nextDouble();

interest = principal * rate;       // Compute this year's interest.
principal = principal + interest;  // Add it to principal.

System.out.printf("The amount of interest is $%1.2f%n", interest);
System.out.printf("The value after one year is $%1.2f%n", principal);

} // end of main()

} // end of class Interest2With Scanner

Note the inclusion of the two lines given above to import Scanner and create stdin. Also note the substitution of stdin.nextDouble() for TextIO.getlnDouble(). (In fact, stdin.nextDouble() is really equivalent to TextIO.getDouble() rather than to the "getln" version, but this will not affect the behavior of the program as long as the user types just one number on each line of input.)

I will continue to use TextIO for input for the time being, but I will give a few more examples of using Scanner in the on-line solutions to the end-of-chapter exercises. There will be more detailed coverage of Scanner later in the book.

Section 2.5

Details of Expressions

This section takes a closer look at expressions. Recall that an expression is a piece of program code that represents or computes a value. An expression can be a literal, a variable, a function call, or several of these things combined with operators such as + and >. The value of an expression can be assigned to a variable, used as a parameter in a subroutine call, or combined with other values into a more complicated expression. (The value can even, in some cases, be ignored, if that's what you want to do; this is more common than you might think.) Expressions are an essential part of programming. So far, this book has dealt only informally with expressions. This section tells you the more-or-less complete story (leaving out some of the less commonly used operators).

The basic building blocks of expressions are literals (such as 674, 3.14, true, and 'X'), variables, and function calls. Recall that a function is a subroutine that returns a value. You've already seen some examples of functions, such as the input routines from the TextIO class and the mathematical functions from the Math class.

The Math class also contains a couple of mathematical constants that are useful in mathematical expressions: Math.PI represents π (the ratio of the circumference of a circle to its diameter), and Math.E represents e (the base of the natural logarithms). These "constants" are actually member variables in Math of type double. They are only approximations for the mathematical constants, which would require an infinite number of digits to specify exactly. The standard class Integer contains a couple of constants related to the int data type: Integer.MAX_VALUE is the largest possible int, 2147483647, and Integer.MIN_VALUE is the smallest int, -2147483648. Similarly, the class Double contains some constants related to type double. Double.MAX_VALUE is the largest value of type double, and Double.MIN_VALUE is the smallest positive value. It also has constants to represent infinite values, Double.POSITIVE_INFINITY and Double.NEGATIVE_INFINITY, and the special value Double.NaN to represent an undefined value. For example, the value of Math.sqrt(-1) is Double.NaN.

Literals, variables, and function calls are simple expressions. More complex expressions can be built up by using operators to combine simpler expressions. Operators include + for adding two numbers, > for comparing two values, and so on. When several operators appear in an expression, there is a question of precedence, which determines how the operators are grouped for evaluation. For example, in the expression "A + B * C", B*C is computed first and then the result is added to A. We say that multiplication (*) has higher precedence than addition (+). If the default precedence is not what you want, you can use parentheses to explicitly specify the grouping you want. For example, you could use "(A + B) * C" if you want to add A to B first and then multiply the result by C.

The rest of this section gives details of operators in Java. The number of operators in Java is quite large. I will not cover them all here, but most of the important ones are here.

2.5.1  Arithmetic Operators

Arithmetic operators include addition, subtraction, multiplication, and division. They are indicated by +, -, *, and /. These operations can be used on values of any numeric type: byte, short, int, long, float, or double. (They can also be used with values of type char, which are treated as integers in this context; a char is converted into its Unicode code number when it is used with an arithmetic operator.) When the computer actually calculates one of these operations, the two values that it combines must be of the same type. If your program tells the computer to combine two values of different types, the computer will convert one of the values from one type to another. For example, to compute 37.4 + 10, the computer will convert the integer 10 to a real number 10.0 and will then compute 37.4 + 10.0. This is called a type conversion. Ordinarily, you don't have to worry about type conversion in expressions, because the computer does it automatically.

When two numerical values are combined (after doing type conversion on one of them, if necessary), the answer will be of the same type. If you multiply two ints, you get an int; if you multiply two doubles, you get a double. This is what you would expect, but you have to be very careful when you use the division operator /. When you divide two integers, the answer will always be an integer; if the quotient has a fractional part, it is discarded. For example, the value of 7/2 is 3, not 3.5. If N is an integer variable, then N/100 is an integer, and 1/N is equal to zero for any N greater than one! This fact is a common source of programming errors. You can force the computer to compute a real number as the answer by making one of the operands real: For example, when the computer evaluates 1.0/N, it first converts N to a real number in order to match the type of 1.0, so you get a real number as the answer.

Java also has an operator for computing the remainder when one number is divided by another. This operator is indicated by %. If A and B are integers, then A % B represents the remainder when A is divided by B. (However, for negative operands, % is not quite the same as the usual mathematical "modulus" operator, since if one of A or B is negative, then the value of A % B will be negative.) For example, 7 % 2 is 1, while 34577 % 100 is 77, and 50 % 8 is 2. A common use of % is to test whether a given integer is even or odd: N is even if N % 2 is zero, and it is odd if N % 2 is 1. More generally, you can check whether an integer N is evenly divisible by an integer M by checking whether N % M is zero.

The % operator also works with real numbers. In general, A % B is what is left over after you remove as many copies of B as possible from A. For example, 7.52 % 0.5 is 0.02.

Finally, you might need the unary minus operator, which takes the negative of a number. For example, -X has the same value as (-1)*X. For completeness, Java also has a unary plus operator, as in +X, even though it doesn't really do anything.

By the way, recall that the + operator can also be used to concatenate a value of any type onto a String. When you use + to combine a string with a value of some other type, it is another example of type conversion, since any type can be automatically converted into type String.

2.5.2  Increment and Decrement

You'll find that adding 1 to a variable is an extremely common operation in programming. Subtracting 1 from a variable is also pretty common. You might perform the operation of adding 1 to a variable with assignment statements such as:

counter  =  counter + 1;
goalsScored  =  goalsScored + 1;

The effect of the assignment statement x = x + 1 is to take the old value of the variable x, compute the result of adding 1 to that value, and store the answer as the new value of x. The same operation can be accomplished by writing x++ (or, if you prefer, ++x). This actually changes the value of x, so that it has the same effect as writing "x = x + 1". The two statements above could be written

counter++;
goalsScored++;

Similarly, you could write x-- (or --x) to subtract 1 from x. That is, x-- performs the same computation as x = x - 1. Adding 1 to a variable is called incrementing that variable, and subtracting 1 is called decrementing. The operators ++ and -- are called the increment operator and the decrement operator, respectively. These operators can be used on variables belonging to any of the numerical types and also on variables of type char. ('A'++ is 'B'.)

Usually, the operators ++ or -- are used in statements like "x++;" or "x--;". These statements are commands to change the value of x. However, it is also legal to use x++, ++x, x--, or --x as expressions, or as parts of larger expressions. That is, you can write things like:

y = x++;
y = ++x;
TextIO.putln(--x);
z = (++x) * (y--);

The statement "y = x++;" has the effects of adding 1 to the value of x and, in addition, assigning some value to y. The value assigned to y is the value of the expression x++, which is defined to be the old value of x, before the 1 is added. Thus, if the value of x is 6, the statement "y = x++;" will change the value of x to 7, but it will change the value of y to 6 since the value assigned to y is the old value of x. On the other hand, the value of ++x is defined to be the new value of x, after the 1 is added. So if x is 6, then the statement "y = ++x;" changes the values of both x and y to 7. The decrement operator, --, works in a similar way.

Note in particular that the statement x = x++; does not change the value of x! This is because the value that is being assigned to x is the old value of x, the one that it had before the statement was executed. The net result is that x is incremented but then immediately changed back to its previous value! You also need to remember that x++ is not the same as x + 1. The expression x++ changes the value of x; the expression x + 1 does not.

This can be confusing, and I have seen many bugs in student programs resulting from the confusion. My advice is: Don't be confused. Use ++ and -- only as stand-alone statements, not as expressions. I will follow this advice in almost all examples in these notes.

2.5.3  Relational Operators

Java has boolean variables and boolean-valued expressions that can be used to express conditions that can be either true or false. One way to form a boolean-valued expression is to compare two values using a relational operator. Relational operators are used to test whether two values are equal, whether one value is greater than another, and so forth. The relational operators in Java are: ==, !=, <, >, <=, and >=. The meanings of these operators are:

A == B       Is A "equal to" B?
A != B       Is A "not equal to" B?
A < B        Is A "less than" B?
A > B        Is A "greater than" B?
A <= B       Is A "less than or equal to" B?
A >= B       Is A "greater than or equal to" B?

These operators can be used to compare values of any of the numeric types. They can also be used to compare values of type char. For characters, < and > are defined according the numeric Unicode values of the characters. (This might not always be what you want. It is not the same as alphabetical order because all the upper case letters come before all the lower case letters.)

When using boolean expressions, you should remember that as far as the computer is concerned, there is nothing special about boolean values. In the next chapter, you will see how to use them in loop and branch statements. But you can also assign boolean-valued expressions to boolean variables, just as you can assign numeric values to numeric variables. And functions can return boolean values.

By the way, the operators == and != can be used to compare boolean values too. This is occasionally useful. For example, can you figure out what this does:

boolean sameSign;
sameSign = ((x > 0) == (y > 0));

One thing that you cannot do with the relational operators <, >, <=, and >= is to use them to compare values of type String. You can legally use == and != to compare Strings, but because of peculiarities in the way objects behave, they might not give the results you want. (The == operator checks whether two objects are stored in the same memory location, rather than whether they contain the same value. Occasionally, for some objects, you do want to make such a check -- but rarely for strings. I'll get back to this in a later chapter.) Instead, you should use the subroutines equals(), equalsIgnoreCase(), and compareTo(), which were described in Subsection 2.3.3, to compare two Strings.

Another place where == and != don't work as you would expect is with Double.NaN, the constant that represents an undefined value of type double. The values of x == Double.NaN and x != Double.NaN are both defined to be false in all cases, whether or not x is Double.NaN! To test whether a real value x is the undefined value Double.NaN, use the boolean-valued function Double.isNaN(x).

2.5.4  Boolean Operators

In English, complicated conditions can be formed using the words "and", "or", and "not." For example, "If there is a test and you did not study for it...". "And", "or", and "not" are boolean operators, and they exist in Java as well as in English.

In Java, the boolean operator "and" is represented by &&. The && operator is used to combine two boolean values. The result is also a boolean value. The result is true if both of the combined values are true, and the result is false if either of the combined values is false. For example, "(x == 0) && (y == 0)" is true if and only if both x is equal to 0 and y is equal to 0.

The boolean operator "or" is represented by ||. (That's supposed to be two of the vertical line characters, |.) The expression "A || B" is true if either A is true or B is true, or if both are true. "A || B" is false only if both A and B are false.

The operators && and || are said to be short-circuited versions of the boolean operators. This means that the second operand of && or || is not necessarily evaluated. Consider the test

(x != 0) && (y/x > 1)

Suppose that the value of x is in fact zero. In that case, the division y/x is undefined mathematically. However, the computer will never perform the division, since when the computer evaluates (x != 0), it finds that the result is false, and so it knows that ((x != 0) && anything) has to be false. Therefore, it doesn't bother to evaluate the second operand. The evaluation has been short-circuited and the division by zero is avoided. (This may seem like a technicality, and it is. But at times, it will make your programming life a little easier.)

The boolean operator "not" is a unary operator. In Java, it is indicated by ! and is written in front of its single operand. For example, if test is a boolean variable, then

test = ! test;

will reverse the value of test, changing it from true to false, or from false to true.

2.5.5  Conditional Operator

Any good programming language has some nifty little features that aren't really necessary but that let you feel cool when you use them. Java has the conditional operator. It's a ternary operator -- that is, it has three operands -- and it comes in two pieces, ? and :, that have to be used together. It takes the form

boolean-expression
?
expression1
:
expression2

The computer tests the value of boolean-expression . If the value is true, it evaluates expression1 ; otherwise, it evaluates expression2 . For example:

next = (N % 2 == 0) ? (N/2) : (3*N+1);

will assign the value N/2 to next if N is even (that is, if N % 2 == 0 is true), and it will assign the value (3*N+1) to next if N is odd. (The parentheses in this example are not required, but they do make the expression easier to read.)

2.5.6  Assignment Operators and Type Conversion

You are already familiar with the assignment statement, which uses the symbol "=" to assign the value of an expression to a variable. In fact, = is really an operator in the sense that an assignment can itself be used as an expression or as part of a more complex expression. The value of an assignment such as A=B is the same as the value that is assigned to A. So, if you want to assign the value of B to A and test at the same time whether that value is zero, you could say:

if ( (A=B) == 0 )...

Usually, I would say, don't do things like that!

In general, the type of the expression on the right-hand side of an assignment statement must be the same as the type of the variable on the left-hand side. However, in some cases, the computer will automatically convert the value computed by the expression to match the type of the variable. Consider the list of numeric types: byte, short, int, long, float, double. A value of a type that occurs earlier in this list can be converted automatically to a value that occurs later. For example:

int A;
double X;
short B;
A = 17;
X = A;    // OK; A is converted to a double
B = A;    // illegal; no automatic conversion
//       from int to short

The idea is that conversion should only be done automatically when it can be done without changing the semantics of the value. Any int can be converted to a double with the same numeric value. However, there are int values that lie outside the legal range of shorts. There is simply no way to represent the int 100000 as a short, for example, since the largest value of type short is 32767.

In some cases, you might want to force a conversion that wouldn't be done automatically. For this, you can use what is called a type cast. A type cast is indicated by putting a type name, in parentheses, in front of the value you want to convert. For example,

int A;
short B;
A = 17;
B = (short)A;  // OK; A is explicitly type cast
//      to a value of type short

You can do type casts from any numeric type to any other numeric type. However, you should note that you might change the numeric value of a number by type-casting it. For example, (short)100000 is -31072. (The -31072 is obtained by taking the 4-byte int 100000 and throwing away two of those bytes to obtain a short -- you've lost the real information that was in those two bytes.)

When you type-cast a real number to an integer, the fractional part is discarded. For example, (int)7.9453 is 7. As another example of type casts, consider the problem of getting a random integer between 1 and 6. The function Math.random() gives a real number between 0.0 and 0.9999..., and so 6*Math.random() is between 0.0 and 5.999.... The type-cast operator, (int), can be used to convert this to an integer: (int)(6*Math.random()). Thus, (int)(6*Math.random()) is one of the integers 0, 1, 2, 3, 4, and 5. To get a number between 1 and 6, we can add 1: "(int)(6*Math.random()) + 1". (The parentheses around 6*Math.random() are necessary because of precedence rules; without the parentheses, the type cast operator would apply only to the 6.)

The type char is almost an integer type. You can assign char values to int variables, and you can assign numerical constants in the range 0 to 65535 to char variables. You can also use explicit type-casts between char and the numeric types. For example, (char)97 is 'a', (int)'+' is 43, and (char)('A' + 2) is 'C'.

Type conversion between String and other types cannot be done with type-casts. One way to convert a value of any type into a string is to concatenate it with an empty string. For example, "" + 42 is the string "42". But a better way is to use the function String.valueOf(x), a static member function in the String class. String.valueOf(x) returns the value of x, converted into a string. For example, String.valueOf(42) is the string "42", and if ch is a char variable, then String.valueOf(ch) is a string of length one containing the single character that is the value of ch.

It is also possible to convert certain strings into values of other types. For example, the string "10" should be convertible into the int value 10, and the string "17.42e-2" into the double value 0.1742. In Java, these conversions are handled by built-in functions.

The standard class Integer contains a static member function for converting from String to int. In particular, if str is any expression of type String, then Integer.parseInt(str) is a function call that attempts to convert the value of str into a value of type int. For example, the value of Integer.parseInt("10") is the int value 10. If the parameter to Integer.parseInt does not represent a legal int value, then an error occurs.

Similarly, the standard class Double includes a function Double.parseDouble. If str is a String, then the function call Double.parseDouble(str) tries to convert str into a value of type double. An error occurs if str does not represent a legal double value.

Getting back to assignment statements, Java has several variations on the assignment operator, which exist to save typing. For example, "A += B" is defined to be the same as "A = A + B". Every operator in Java that applies to two operands, except for the relational operators, gives rise to a similar assignment operator. For example:

x -= y;     // same as:   x = x - y;
x *= y;     // same as:   x = x * y;
x /= y;     // same as:   x = x / y;
x %= y;     // same as:   x = x % y;
q &&= p;    // same as:   q = q && p;  (for booleans q and p)

The combined assignment operator += even works with strings. Recall that when the + operator is used with a string as one of the operands, it represents concatenation. Since str += x is equivalent to str = str + x, when += is used with a string on the left-hand side, it appends the value on the right-hand side onto the string. For example, if str has the value "tire", then the statement str += 'd'; changes the value of str to "tired".

2.5.7  Precedence Rules

If you use several operators in one expression, and if you don't use parentheses to explicitly indicate the order of evaluation, then you have to worry about the precedence rules that determine the order of evaluation. (Advice: don't confuse yourself or the reader of your program; use parentheses liberally.)

Here is a listing of the operators discussed in this section, listed in order from highest precedence (evaluated first) to lowest precedence (evaluated last):

Unary operators:              ++, --, !, unary -, unary +, type-cast
Multiplication and division:  *,  /,  %
Addition and subtraction:     +,  -
Relational operators:         <,  >,  <=,  >=
Equality and inequality:      ==,  !=
Boolean and:                  &&
Boolean or:                   ||
Conditional operator:         ?:
Assignment operators:         =,  +=,  -=,  *=,  /=,  %=

Operators on the same line have the same precedence. When operators of the same precedence are strung together in the absence of parentheses, unary operators and assignment operators are evaluated right-to-left, while the remaining operators are evaluated left-to-right. For example, A*B/C means (A*B)/C, while A=B=C means A=(B=C). (Can you see how the expression A=B=C might be useful, given that the value of B=C as an expression is the same as the value that is assigned to B?)

Section 2.6

Programming Environments

Although the Java language is highly standardized, the procedures for creating, compiling, and editing Java programs vary widely from one programming environment to another. There are two basic approaches: a command line environment, where the user types commands and the computer responds, and an integrated development environment (IDE), where the user uses the keyboard and mouse to interact with a graphical user interface. While there is just one common command line environment for Java programming, there are several common IDEs, including Eclipse, NetBeans, and BlueJ. I cannot give complete or definitive information on Java programming environments in this section, but I will try to give enough information to let you compile and run the examples from this textbook. (Readers are strongly encouraged to read, compile, and run the examples. Source code can be downloaded from the book's web page, http://math.hws.edu/javanotes.)

One thing to keep in mind is that you do not have to pay any money to do Java programming (aside from buying a computer, of course). Everything that you need can be downloaded for free on the Internet.

2.6.1  Java Development Kit

The basic development system for Java programming is usually referred to as the JDK (Java Development Kit). It is a part of Java SE, the Java "Standard Edition" (as opposed to Java EE for servers or Java ME for mobile devices). Note that Java SE comes in two versions, a Development Kit version (the JDK) and a Runtime Environment version (the JRE). The Runtime can be used to run Java programs, but it does not allow you to compile your own Java programs. The Development Kit includes the Runtime but also lets you compile programs. You need a JDK for use with this textbook.

Java was developed by Sun Microsystems, Inc., which is now a part of the Oracle corporation. Oracle makes the JDK for Windows, Mac OS, and Linux available for free download at its Java Web site. Many Windows computers come with a Java Runtime already installed, but you might need to install the JDK. Some versions of Linux come with the JDK either installed by default or on the installation media. Mac OS does not currently come with Java pre-installed. If you need to download and install the JDK, be sure to get the JDK for Java 7, Java 8, or later. As of summer, 2014, it can be downloaded from

http://www.oracle.com/technetwork/java/javase/downloads/index.html

If a JDK is properly installed on your computer, you can use the command line environment to compile and run Java programs. An IDE will also require a JDK, but it might be included with the IDE download.

2.6.2  Command Line Environment

Many modern computer users find the command line environment to be pretty alien and unintuitive. It is certainly very different from the graphical user interfaces that most people are used to. However, it takes only a little practice to learn the basics of the command line environment and to become productive using it.

To use a command line programming environment, you will have to open a window where you can type in commands. In Windows, you can open such a command window by running the program named cmd. (In Windows 7, click "Start / Program Files / Accessories / Command Prompt." In Windows 8, press the Windows and X keys together to bring up the "Power User Menu," and select "Command Prompt.") In Mac OS, you want to run the Terminal program, which can be found in the Utilities folder inside the Applications folder. In Linux, there are several possibilities, including an old program called xterm; try looking for "Terminal" in your applications menu.

No matter what type of computer you are using, when you open a command window, it will display a prompt of some sort. Type in a command at the prompt and press return. The computer will carry out the command, displaying any output in the command window, and will then redisplay the prompt so that you can type another command. One of the central concepts in the command line environment is the current directory which contains files that can be used by the commands that you type. (The words "directory" and "folder" mean the same thing.) Often, the name of the current directory is part of the command prompt. You can get a list of the files in the current directory by typing in the command dir (on Windows) or ls (on Linux and Mac OS). When the window first opens, the current directory is your home directory, where all your files are stored. You can change the current directory using the cd command with the name of the directory that you want to use. For example, to change into your Desktop directory, type in the command cd Desktop and press return.

You should create a directory (that is, a folder) to hold your Java work. For example, create a directory named javawork in your home directory. You can do this using your computer's GUI; another way to do it is to open a command window, cd to the directory that you want to contain the new dirctory, and enter the command mkdir javawork. When you want to work on programming, open a command window and use the cd command to change into your work directory. Of course, you can have more than one working directory for your Java work; you can organize your files any way you like.

The most basic commands for using Java on the command line are javac and java; javac is used to compile Java source code, and java is used to run Java stand-alone applications. If a JDK is correctly installed on your computer, it should recognize these commands when you type them in on the command line. Try typing the commands java -version and javac -version which should tell you which version of Java is installed. If you get a message such as "Command not found," then Java is not correctly installed. If the "java" command works, but "javac" does not, it means that a Java Runtime is installed rather than a Development Kit. (On Windows, after installing the JDK, you need to modify the Windows PATH environment variable to make this work. See the JDK installation instructions on Oracle's download site for information about how to do this.)

To test the javac command, place a copy of TextIO.java into your working directory. (If you downloaded the Web site of this book, you can find it in the directory named source; you can use your computer's GUI to copy-and-paste this file into your working directory. Alternatively, you can navigate to TextIO.java on the book's Web site and use the "Save As" command in your Web browser to save a copy of the file into your working directory.) Type the command:

javac  TextIO.java

This will compile TextIO.java and will create a bytecode file named TextIO.class in the same directory. Note that if the command succeeds, you will not get any response from the computer; it will just redisplay the command prompt to tell you it's ready for another command.

To test the java command, copy a sample program such as Interest2.java from this book's source directory into your working directory, or download it from the web site. First, compile the program with the command

javac  Interest2.java

Remember that for this to succeed, TextIO must already be in the same directory. Then you can execute the program using the command

java  Interest2

Be careful to use just the name of the program, Interest2, with the java command, not the name of the Java source code file or the name of the compiled class file. When you give this command, the program will run. You will be asked to enter some information, and you will respond by typing your answers into the command window, pressing return at the end of the line. When the program ends, you will see the command prompt, and you can enter another command. (Note that "java TextIO" would not make sense, since TextIO does not have a main() routine, and so it doesn't make sense to try to execute it as a program.)

You can follow a similar procedure to run all of the examples in this book. Some examples require additional classes, such as TextIO, in addition to the main program. Remember to place any required classes in the same folder as the program that uses them. (You can use either the .java or the .class files for the required classes.)

To create your own programs, you will need a text editor. A text editor is a computer program that allows you to create and save documents that contain plain text. It is important that the documents be saved as plain text, that is without any special encoding or formatting information. Word processor documents are not appropriate, unless you can get your word processor to save as plain text. A good text editor can make programming a lot more pleasant. Linux comes with several text editors. On Windows, you can use notepad in a pinch, but you will probably want something better. For Mac OS, you might download the free TextWrangler application. One possibility that will work on any platform is to use jedit, a good programmer's text editor that is itself written in Java and that can be downloaded for free from www.jedit.org.

To work on your programs, you can open a command line window and cd into the working directory where you will store your source code files. Start up your text editor program, such as by double-clicking its icon or selecting it from a Start menu. Type your code into the editor window, or open an existing source code file that you want to modify. Save the file into your working directory. Remember that the name of a Java source code file must end in ".java", and the rest of the file name must match the name of the class that is defined in the file. Once the file is saved in your working directory, go to the command window and use the javac command to compile it, as discussed above. If there are syntax errors in the code, they will be listed in the command window. Each error message contains the line number in the file where the computer found the error. Go back to the editor and try to fix one or more errors, save your changes, and then try the javac command again. (It's usually a good idea to just work on the first few errors; sometimes fixing those will make other errors go away.) Remember that when the javac command finally succeeds, you will get no message at all. Then you can use the java command to run your program, as described above. Once you've compiled the program, you can run it as many times as you like without recompiling it.

That's really all there is to it: Keep both editor and command-line window open. Edit, save, and compile until you have eliminated all the syntax errors. (Always remember to save the file before compiling it -- the compiler only sees the saved file, not the version in the editor window.) When you run the program, you might find that it has semantic errors that cause it to run incorrectly. In that case, you have to go back to the edit/save/compile loop to try to find and fix the problem.

2.6.3  Eclipse

In an Integrated Development Environment, everything you need to create, compile, and run programs is integrated into a single package, with a graphical user interface that will be familiar to most computer users. There are a number of different IDEs for Java program development, ranging from fairly simple wrappers around the JDK to highly complex applications with a multitude of features. For a beginning programmer, there is a danger in using an IDE, since the difficulty of learning to use the IDE, on top of the difficulty of learning to program, can be overwhelming. However, for my own programming, I generally use the Eclipse IDE, and I introduce my students to it after they have had some experience with the command line. I will discuss Eclipse in some detail and two other IDEs, NetBeans and BlueJ, in much less detail. All of these IDEs have features that are very useful even for a beginning programmer, although a beginner will want to ignore many of their advanced features.

You can download an Eclipse IDE from eclipse.org. It is a free program. Eclipse is itself written in Java. It requires a Java Runtime Environment, but not necessarily a JDK, since it includes its own compiler. You should make sure that the JRE or JDK, Version 7 or higher is installed on your computer, as described above, before you install Eclipse. There are several versions of the Eclipse IDE; you can use the "Eclipse IDE for Java Developers."

The first time you start Eclipse, you will be asked to specify a workspace, which is the directory where all your work will be stored. You can accept the default name, or provide one of your own. When startup is complete, the Eclipse window will be filled by a large "Welcome" screen that includes links to extensive documentation and tutorials. You can close this screen, by clicking the "X" next to the word "Welcome"; you can get back to it later by choosing "Welcome" from the "Help" menu.

The Eclipse GUI consists of one large window that is divided into several sections. Each section contains one or more views. For example, a view can be a text editor, it can be a place where a program can do I/O, or it can contain a list of all your projects. If there are several views in one section of the window, then there will be tabs at the top of the section to select the view that is displayed in that section. Each view displays a different type of information. The whole set of views is called a perspective. Eclipse uses different perspectives, that is different sets of views of different types of information, for different tasks. For compiling and running programs, the only perspective that you will need is the "Java Perspective," which is the default. As you become more experienced, you might want to the use the "Debug Perspective," which has features designed to help you find semantic errors in programs.

The Java Perspective includes a large area in the center of the window that contains text editor views. This is where you will create and edit your programs. To the left of this is the Package Explorer view, which will contain a list of your Java projects and source code files. To the right are some other views that I don't find very useful, and I suggest that you close them by clicking the small "X" next to the name of each one. Several other views that will be useful appear in a section of the window below the editing area. If you accidently close one of the important views, such as the Package Explorer, you can get it back by selecting it from the "Show View" submenu of the "Window" menu. You can also reset the whole window to its default contents by selecting "Reset Perspective" from the "Window" menu.

To do any work in Eclipse, you need a project. To start a Java project, go to the "New" submenu in the "File" menu, and select the "Java Project" command. In the window that pops up, it is only necessary to fill in a "Project Name" for the project and click the "Finish" button. The project name can be anything you like. The project should appear in the "Package Explorer" view. Click on the small triangle or plus sign next to the project name to see the contents of the project. Assuming that you use the default settings, there should be a directory named "src," which is where your Java source code files will go. It also contains the "JRE System Library"; this is the collection of standard built-in classes that come with Java.

To run the TextIO based examples from this textbook, you must add the source code file TextIO.java to your project. If you have downloaded the Web site of this book, you can find a copy of TextIO.java in the source directory. Alternatively, you can navigate to the file on-line and use the "Save As" command of your Web browser to save a copy of the file onto your computer. The easiest way to get TextIO into your project is to locate the source code file on your computer and drag the file icon onto the project name in the Eclipse window. If that doesn't work, you can try using copy-and-paste: Right-click the file icon (or control-click on Mac OS), select "Copy" from the pop-up menu, right-click the project's src folder in the Eclipse window, and select "Paste". (Be sure to paste it into the src folder, not into the project itself; files outside the source folder are not treated as Java source code files.) Another option is to add the file directly to the src folder inside your workspace directory. However, Eclipse will not automatically recognize a file added in this way; to make Eclipse find the file, right-click the project name in the Eclipse window and select "Refresh" from the pop-up menu. In any case, TextIO should appear under "src" in your project, inside a package named "default package". Once a file is in this list, you can open it by double-clicking it; it will appear in the editing area of the Eclipse window.

To run any of the Java programs from this textbook, copy the source code file into your Eclipse Java project in the same way that you copied TextIO.java. To run the program, right-click in the editor window, or on the file name in the Package Explorer view (or control-click in Mac OS). In the menu that pops up, go to the "Run As" submenu, and select "Java Application". The program will be executed. If the program writes to standard output, the output will appear in the "Console" view, in the area of the Eclipse window below the editing area. If the program uses TextIO for input, you will have to type the required input into the "Console" view -- click the "Console" view before you start typing, so that the characters that you type will be sent to the correct part of the window. (For an easier way to run a program, find and click the small "Run" button in Eclipse's tool bar.) Note that when you run a program in Eclipse, it is compiled automatically. There is no separate compilation step.

You can have more than one program in the same Eclipse project, or you can create additional projects to organize your work better. Remember to place a copy of TextIO.java in any project that requires it.

To create a new Java program in Eclipse, you must create a new Java class. To do that, right-click the Java project name in the "Project Explorer" view. Go to the "New" submenu of the popup menu, and select "Class". (Alternatively, there is a small icon in the toolbar at the top of the Eclipse window that you can click to create a new Java class.) In the window that opens, type in the name of the class that you want to create. The class name must be a legal Java identifier. Note that you want the name of the class, not the name of the source code file, so don't add ".java" at the end of the name. Examples in this book use the "default package," so you will also want to erase the contents of the box labeled "Package." (See the last section of this section for more information about packages.) Finally, click the "Finish" button to create the class. The class should appear inside the "src" folder, in the "default package," and it should automatically open in the editing area so that you can start typing in your program.

Eclipse has several features that aid you as you type your code. It will underline any syntax error with a jagged red line, and in some cases will place an error marker in the left border of the edit window. If you hover the mouse cursor over the error marker or over the error itself, a description of the error will appear. Note that you do not have to get rid of every error immediately as you type; some errors will go away as you type in more of the program. If an error marker displays a small "light bulb," Eclipse is offering to try to fix the error for you. Click the light bulb -- or simply hover your mouse over the actual error -- to get a list of possible fixes, then double click the fix that you want to apply. For example, if you use an undeclared variable in your program, Eclipse will offer to declare it for you. You can actually use this error-correcting feature to get Eclipse to write certain types of code for you! Unfortunately, you'll find that you won't understand a lot of the proposed fixes until you learn more about the Java language, and it is not a good idea to apply a fix that you don't understand -- often that will just make things worse in the end.

Eclipse will also look for spelling errors in comments and will underline them with jagged red lines. Hover your mouse over the error to get a list of possible correct spellings.

Another essential Eclipse feature is content assist. Content assist can be invoked by typing Control-Space. It will offer possible completions of whatever you are typing at the moment. For example, if you type part of an identifier and hit Control-Space, you will get a list of identifiers that start with the characters that you have typed; use the up and down arrow keys to select one of the items in the list, and press Return or Enter. (You can also click an item with the mouse to select it, or hit Escape to dismiss the list.) If there is only one possible completion when you hit Control-Space, it will be inserted automatically. By default, Content Assist will also pop up automatically, after a short delay, when you type a period or certain other characters. For example, if you type "TextIO." and pause for just a fraction of a second, you will get a list of all the subroutines in the TextIO class. Personally, I find this auto-activation annoying. You can disable it in the Eclipse Preferences. (Look under Java / Editor / Content Assist, and turn off the "Enable auto activation" option.) You can still call up Code Assist manually with Control-Space.

Once you have an error-free program, you can run it as described above. If you find a problem when you run it, it's very easy to go back to the editor, make changes, and run it again. Note that using Eclipse, there is no explicit "compile" command. The source code files in your project are automatically compiled, and are re-compiled whenever you modify them.

2.6.4  NetBeans

Another IDE for professional programming is NetBeans. It can be downloaded from netbeans.org. Alternatively, a bundle containing both NetBeans and the JDK is available on Oracle's Java download page.

Using NetBeans is very similar to using Eclipse. Even the layout of its window is very similar to the Eclipse window. Create a project in NetBeans with the "New Project" command. You will have to select the type of project in a pop-up window. You want to create a "Java Application." The project creation dialog will have a suggested name for the project, which you will want to change. It also has an option to create a main class for the project, which is selected by default. If you use that option, you should change the class name. For use with this book, the name should not be in a "package"; that is, it should not include a period.

A project will have a "Source Folder" where the source code files for the project are stored. You can drag TextIO.java and other files onto that folder, or you can copy-and-paste them from the file system. For running a file, you can right-click the file and select "Run File" from the pop-up menu. There is also a "Run" button in the NetBeans toolbar. There is no explicit compilation step. Input and ouput are done in an area below the edit window, just as in Eclipse.

When you are editing a file, NetBeans will mark errors as you type. (Remember, again, that many errors will go away on their own as you continue to type.) If NetBeans displays an error marker with a light bulb in the left-hand margin of the editor, you have to click the light bulb to get a list of possible automatic fixes for the error. NetBeans also has a "Code Completion" feature that is similar to Content Assist in Eclipse. Just press Control-Space as you are typing to get a list of possible completions.

2.6.5  BlueJ

Finally, I will mention BlueJ, an IDE that is designed specifically for people who are learning to program. It is much less complex than Eclipse or NetBeans, but it does have some features that make it useful for education. BlueJ can be downloaded from bluej.org.

In BlueJ, you can begin a project with the "New Project" command in the "Project" menu. A BlueJ project is simply a folder. When you create a project, you will have to select a folder name that does not already exist. The folder will be created and a window will be opened to show the contents of the folder. Files are shown as icons in the BlueJ window. You can drag .java files from the file system onto that window to add files to the project; they will be copied into the project folder as well as shown in the window. You can also copy files directly into the project folder, but BlueJ won't see them until the next time you open the project. For example, you can do this with TextIO.java and the sample programs from this book. When you restart BlueJ, it should show the last project you were working on, but you can open any project with a command from the "Project" menu.

There is a button in the project window for creating a new class. An icon for the class is added to the window, and a .java source code file is created in the project folder. The file is not automatically opened for editing. To edit a file, double-click its icon in the project window. An editor will be opened in a separate window. (A newly created class will contain some default code that you probably don't want; you can erase it and add a main() routine instead.) The BlueJ editor does not show errors as you type. Errors will be reported when you compile the program. Also, it does not offer automatic fixes for errors. It has a less capable version of Eclipse's Content Assist, which seems only to work for getting a list of available subroutines in a class or object; call up the list by hitting Control-Space after typing the period following the name of a class or object.

An editor window contains a button for compiling the program in the window. There is also a compile button in the project window, which compiles all the classes in the project.

To run a program, it must already be compiled. Right-click the icon of a compiled program. In the menu that pops up, you will see "void main(String[] args)". Select that option from the menu to run the program. Just click "OK" in the dialog box that pops up. A separate window will open for input/output.

One of the neatest features of BlueJ is that you can actually use it to run any subroutine, not just main. If a class contains other subroutines, you will see them in the list that you get by right-clicking its icon. A pop-up dialog allows you to enter any parameters required by the routine, and if the routine is a function, you will get another dialog box after the routine has been executed to tell you its return value. This allows easy testing of individual subroutines. Furthermore, you can also use BlueJ to create new objects from a class. An icon for the object will be added at the bottom of the project window, and you can right-click that icon to get a list of subroutines in the object. This will, of course, not be useful to you until we get to object-oriented programming in Chapter 5.

2.6.6  The Problem of Packages

Every class in Java is contained in something called a package. Classes that are not explicitly put into a package are in the "default" package. Almost all the examples in this textbook are in the default package, and I will not even discuss packages in any depth until Section 4.5. However, some IDEs force you to pay attention to packages.

In fact, the use of the default package is discouraged, according to official Java style guidelines. Nevertheless, I have chosen to use it, since it seems easier for beginning programmers to avoid the whole issue of packages, at least at first. If Eclipse or NetBeans tries to put a class into a package, you can delete the package name from the class-creation dialog to get it to use the default package instead. But if you do create a class in a package, the source code starts with a line that specifies which package the class is in. For example, if the class is in a package named test.pkg, then the first line of the source code will be

package test.pkg;

In an IDE, this will not cause any problem unless the program you are writing depends on TextIO. A class that is in a non-default package cannot use a class from the default package. To make TextIO available to such a class, you can move TextIO to a named, non-default package. This means that the source code file TextIO.java has to be modified to specify the package: A package statement like the one shown above must be added to the very beginning of the file, with the appropriate package name. (The IDE might do this for you, if you drag TextIO.java from the default package into a non-default package.) If you add TextIO to the same package as the class that uses it, then TextIO will be automatically available to that class. If TextIO is in a different named package, you have to add an "import" statement to the other class to make TextIO available to it. For example, if TextIO is in the package textio, add the statement

import textio.TextIO;

to the top of the other source code file, just after its own package declaration.

By the way, if you use packages in a command-line environment, other complications arise. For example, if a class is in a package named test.pkg, then the source code file must be in a subdirectory named "pkg" inside a directory named "test" that is in turn inside your main Java working directory. Nevertheless, when you compile or execute the program, you should be in the main directory, not in a subdirectory. When you compile the source code file, you have to include the name of the directory in the command: Use "javac test/pkg/ClassName.java" on Linux or Mac OS, or "javac test\pkg\ClassName.java" on Windows. The command for executing the program is then "java test.pkg.ClassName", with a period separating the package name from the class name. However, you will not need to worry about any of that when working with almost all of the examples in this book.

Programming Exercises for Chapter 2

Exercise 2.1:

Write a program that will print your initials to standard output in letters that are nine lines tall. Each big letter should be made up of a bunch of *'s. For example, if your initials were "DJE", then the output would look something like:

******           *************        **********
**    **                **            **
**     **               **            **
**      **              **            **
**      **              **            ********
**      **       **     **            **
**     **         **    **            **
**    **           **  **             **
*****               ****              **********

See the Solution

Exercise 2.2:

Write a program that simulates rolling a pair of dice. You can simulate rolling one die by choosing one of the integers 1, 2, 3, 4, 5, or 6 at random. The number you pick represents the number on the die after it is rolled. As pointed out in Section 2.5, the expression

(int)(Math.random()*6) + 1

does the computation to select a random integer between 1 and 6. You can assign this value to a variable to represent one of the dice that are being rolled. Do this twice and add the results together to get the total roll. Your program should report the number showing on each die as well as the total roll. For example:

The first die comes up 3
The second die comes up 5
Your total roll is 8

See the Solution

Exercise 2.3:

Write a program that asks the user's name, and then greets the user by name. Before outputting the user's name, convert it to upper case letters. For example, if the user's name is Fred, then the program should respond "Hello, FRED, nice to meet you!".

See the Solution

Exercise 2.4:

Write a program that helps the user count his change. The program should ask how many quarters the user has, then how many dimes, then how many nickels, then how many pennies. Then the program should tell the user how much money he has, expressed in dollars.

See the Solution

Exercise 2.5:

If you have N eggs, then you have N/12 dozen eggs, with N%12 eggs left over. (This is essentially the definition of the / and % operators for integers.) Write a program that asks the user how many eggs she has and then tells the user how many dozen eggs she has and how many extra eggs are left over.

A gross of eggs is equal to 144 eggs. Extend your program so that it will tell the user how many gross, how many dozen, and how many left over eggs she has. For example, if the user says that she has 1342 eggs, then your program would respond with

Your number of eggs is 9 gross, 3 dozen, and 10

since 1342 is equal to 9*144 + 3*12 + 10.

See the Solution

Exercise 2.6:

This exercise asks you to write a program that tests some of the built-in subroutines for working with Strings. The program should ask the user to enter their first name and their last name, separated by a space. Read the user's response using TextIO.getln(). Break the input string up into two strings, one containing the first name and one containing the last name. You can do that by using the indexOf() subroutine to find the position of the space, and then using substring() to extract each of the two names. Also output the number of characters in each name, and output the user's initials. (The initials are the first letter of the first name together with the first letter of the last name.) A sample run of the program should look something like this:

Please enter your first name and last name, separated by a space.
? Mary Smith
Your first name is Mary, which has 4 characters
Your last name is Smith, which has 5 characters
Your initials are MS

See the Solution

Exercise 2.7:

Suppose that a file named "testdata.txt" contains the following information: The first line of the file is the name of a student. Each of the next three lines contains an integer. The integers are the student's scores on three exams. Write a program that will read the information in the file and display (on standard output) a message that contains the name of the student and the student's average grade on the three exams. The average is obtained by adding up the individual exam grades and then dividing by the number of exams.

See the Solution

Quiz on Chapter 2

Question 1:

Briefly explain what is meant by the syntax and the semantics of a programming language. Give an example to illustrate the difference between a syntax error and a semantics error.

Question 2:

What does the computer do when it executes a variable declaration statement. Give an example.

Question 3:

What is a type, as this term relates to programming?

Question 4:

One of the primitive types in Java is boolean. What is the boolean type? Where are boolean values used? What are its possible values?

Question 5:

Give the meaning of each of the following Java operators:

a)    ++

b)    &&

c)    !=

Question 6:

Explain what is meant by an assignment statement, and give an example. What are assignment statements used for?

Question 7:

What is meant by precedence of operators?

Question 8:

What is a literal?

Question 9:

In Java, classes have two fundamentally different purposes. What are they?

Question 10:

What is the difference between the statement "x = TextIO.getDouble();" and the statement "x = TextIO.getlnDouble();"

Question 11:

Explain why the value of the expression 2 + 3 + "test" is the string "5test" while the value of the expression "test" + 2 + 3 is the string "test23". What is the value of "test" + 2 * 3 ?

Question 12:

Integrated Development Environments such as Eclipse often use syntax coloring, which assigns various colors to the characters in a program to reflect the syntax of the language. A student notices that Eclipse colors the word String differently from int, double, and boolean. The student asks why String should be a different color, since all these words are names of types. What's the answer to the student's question?

See the Answers

Chapter 3

Programming in the Small II: Control

The basic building blocks of programs -- variables, expressions, assignment statements, and subroutine call statements -- were covered in the previous chapter. Starting with this chapter, we look at how these building blocks can be put together to build complex programs with more interesting behavior.

Since we are still working on the level of "programming in the small" in this chapter, we are interested in the kind of complexity that can occur within a single subroutine. On this level, complexity is provided by control structures. The two types of control structures, loops and branches, can be used to repeat a sequence of statements over and over or to choose among two or more possible courses of action. Java includes several control structures of each type, and we will look at each of them in some detail.

Program complexity can be seen not just in control structures but also in data structures. A data structure is an organized collection of data, chunked together so that it can be treated as a unit. Section 3.8 in this chapter includes an introduction to one of the most common data structures: arrays.

The chapter will also begin the study of program design. Given a problem, how can you come up with a program to solve that problem? We'll look at a partial answer to this question in Section 3.2. Finally, Section 3.9 is a very brief first look at GUI programming.

Contents of Chapter 3:

- Section 1: Blocks, Loops, and Branches

- Section 2: Algorithm Development

- Section 3: The while and do..while Statements

- Section 4: The for Statement

- Section 5: The if Statement

- Section 6: The switch Statement

- Section 7: Introduction to Exceptions and try..catch

- Section 8: Introduction to Arrays

- Section 9: Introduction to GUI Programming

- Programming Exercises

- Quiz on This Chapter

Section 3.1

Blocks, Loops, and Branches

The ability of a computer to perform complex tasks is built on just a few ways of combining simple commands into control structures. In Java, there are just six such structures that are used to determine the normal flow of control in a program -- and, in fact, just three of them would be enough to write programs to perform any task. The six control structures are: the block, the while loop, the do..while loop, the for loop, the if statement, and the switch statement. Each of these structures is considered to be a single "statement," but a structured statement that can contain one or more other statements inside itself.

3.1.1  Blocks

The block is the simplest type of structured statement. Its purpose is simply to group a sequence of statements into a single statement. The format of a block is:

{
statements
}

That is, it consists of a sequence of statements enclosed between a pair of braces, "{" and "}". In fact, it is possible for a block to contain no statements at all; such a block is called an empty block, and can actually be useful at times. An empty block consists of nothing but an empty pair of braces. Block statements usually occur inside other statements, where their purpose is to group together several statements into a unit. However, a block can be legally used wherever a statement can occur. There is one place where a block is required: As you might have already noticed in the case of the main subroutine of a program, the definition of a subroutine is a block, since it is a sequence of statements enclosed inside a pair of braces.

I should probably note again at this point that Java is what is called a free-format language. There are no syntax rules about how the language has to be arranged on a page. So, for example, you could write an entire block on one line if you want. But as a matter of good programming style, you should lay out your program on the page in a way that will make its structure as clear as possible. In general, this means putting one statement per line and using indentation to indicate statements that are contained inside control structures. This is the format that I will generally use in my examples.

Here are two examples of blocks:

{
System.out.print("The answer is ");
System.out.println(ans);
}

{  // This block exchanges the values of x and y
int temp;      // A temporary variable for use in this block.
temp = x;      // Save a copy of the value of x in temp.
x = y;         // Copy the value of y into x.
y = temp;      // Copy the value of temp into y.
}

In the second example, a variable, temp, is declared inside the block. This is perfectly legal, and it is good style to declare a variable inside a block if that variable is used nowhere else but inside the block. A variable declared inside a block is completely inaccessible and invisible from outside that block. When the computer executes the variable declaration statement, it allocates memory to hold the value of the variable. When the block ends, that memory is discarded (that is, made available for reuse). The variable is said to be local to the block. There is a general concept called the "scope" of an identifier. The scope of an identifier is the part of the program in which that identifier is valid. The scope of a variable defined inside a block is limited to that block, and more specifically to the part of the block that comes after the declaration of the variable.

3.1.2  The Basic While Loop

The block statement by itself really doesn't affect the flow of control in a program. The five remaining control structures do. They can be divided into two classes: loop statements and branching statements. You really just need one control structure from each category in order to have a completely general-purpose programming language. More than that is just convenience. In this section, I'll introduce the while loop and the if statement. I'll give the full details of these statements and of the other three control structures in later sections.

A while loop is used to repeat a given statement over and over. Of course, it's not likely that you would want to keep repeating it forever. That would be an infinite loop, which is generally a bad thing. (There is an old story about computer pioneer Grace Murray Hopper, who read instructions on a bottle of shampoo telling her to "lather, rinse, repeat." As the story goes, she claims that she tried to follow the directions, but she ran out of shampoo. (In case you don't get it, this is a joke about the way that computers mindlessly follow instructions.))

To be more specific, a while loop will repeat a statement over and over, but only so long as a specified condition remains true. A while loop has the form:

while (boolean-expression)
statement

Since the statement can be, and usually is, a block, most while loops have the form:

while (boolean-expression) {
statements
}

Some programmers think that the braces should always be included as a matter of style, even when there is only one statement between them, but I don't always follow that advice myself.

The semantics of the while statement go like this: When the computer comes to a while statement, it evaluates the boolean-expression, which yields either true or false as its value. If the value is false, the computer skips over the rest of the while loop and proceeds to the next command in the program. If the value of the expression is true, the computer executes the statement or block of statements inside the loop. Then it returns to the beginning of the while loop and repeats the process. That is, it re-evaluates the boolean-expression, ends the loop if the value is false, and continues it if the value is true. This will continue over and over until the value of the expression is false when the computer evaluates it; if that never happens, then there will be an infinite loop.

Here is an example of a while loop that simply prints out the numbers 1, 2, 3, 4, 5:

int number;   // The number to be printed.
number = 1;   // Start with 1.
while ( number < 6 ) {  // Keep going as long as number is < 6.
System.out.println(number);
number = number + 1;  // Go on to the next number.
}
System.out.println("Done!");

The variable number is initialized with the value 1. So when the computer evaluates the expression "number < 6" for the first time, it is asking whether 1 is less than 6, which is true. The computer therefore proceeds to execute the two statements inside the loop. The first statement prints out "1". The second statement adds 1 to number and stores the result back into the variable number; the value of number has been changed to 2. The computer has reached the end of the loop, so it returns to the beginning and asks again whether number is less than 6. Once again this is true, so the computer executes the loop again, this time printing out 2 as the value of number and then changing the value of number to 3. It continues in this way until eventually number becomes equal to 6. At that point, the expression "number < 6" evaluates to false. So, the computer jumps past the end of the loop to the next statement and prints out the message "Done!". Note that when the loop ends, the value of number is 6, but the last value that was printed was 5.

By the way, you should remember that you'll never see a while loop standing by itself in a real program. It will always be inside a subroutine which is itself defined inside some class. As an example of a while loop used inside a complete program, here is a little program that computes the interest on an investment over several years. This is an improvement over examples from the previous chapter that just reported the results for one year:

/**
*  This class implements a simple program that will compute the amount of
*  interest that is earned on an investment over a period of 5 years.  The
*  initial amount of the investment and the interest rate are input by the
*  user.  The value of the investment at the end of each year is output.
*/

public class Interest3 {

public static void main(String[] args) {

double principal;  // The value of the investment.
double rate;       // The annual interest rate.

/* Get the initial investment and interest rate from the user. */

System.out.print("Enter the initial investment: ");
principal = TextIO.getlnDouble();

System.out.println();
System.out.println("Enter the annual interest rate.");
System.out.print("Enter a decimal, not a percentage: ");
rate = TextIO.getlnDouble();
System.out.println();

/* Simulate the investment for 5 years. */

int years;  // Counts the number of years that have passed.

years = 0;
while (years < 5) {
double interest;  // Interest for this year.
interest = principal * rate;
principal = principal + interest;     // Add it to principal.
years = years + 1;    // Count the current year.
System.out.print("The value of the investment after ");
System.out.print(years);
System.out.print(" years is $");
System.out.printf("%1.2f", principal);
System.out.println();
} // end of while loop

} // end of main()

} // end of class Interest3

You should study this program, and make sure that you understand what the computer does step-by-step as it executes the while loop.

3.1.3  The Basic If Statement

An if statement tells the computer to take one of two alternative courses of action, depending on whether the value of a given boolean-valued expression is true or false. It is an example of a "branching" or "decision" statement. An if statement has the form:

if ( boolean-expression )
statement1
else
statement2

When the computer executes an if statement, it evaluates the boolean expression. If the value is true, the computer executes the first statement and skips the statement that follows the "else". If the value of the expression is false, then the computer skips the first statement and executes the second one. Note that in any case, one and only one of the two statements inside the if statement is executed. The two statements represent alternative courses of action; the computer decides between these courses of action based on the value of the boolean expression.

In many cases, you want the computer to choose between doing something and not doing it. You can do this with an if statement that omits the else part:

if ( boolean-expression )
statement

To execute this statement, the computer evaluates the expression. If the value is true, the computer executes the statement that is contained inside the if statement; if the value is false, the computer skips over that statement. In either case, the computer then continues with whatever follows the if statement in the program.

Sometimes, novice programmers confuse while statements with simple if statements (with no else part), although their meanings are quite different. The statement in an if is executed at most once, while the statement in a while can be executed any number of times. It can be helpful to look at diagrams of the the flow of control for while and simple if statements:

In these diagrams, the arrows represent the flow of time as the statement is executed. Control enters the diagram at the top and leaves at the bottom. Similarly, a flow control diagram for an if..else statement makes it clear that exactly one of the two nested statements is executed:

Of course, either or both of the statements in an if statement can be a block, and again many programmers prefer to add the braces even when they contain just a single statement. So an if statement often looks like:

if ( boolean-expression ) {
statements
}
else {
statements
}

or:

if ( boolean-expression ) {
statements
}

As an example, here is an if statement that exchanges the value of two variables, x and y, but only if x is greater than y to begin with. After this if statement has been executed, we can be sure that the value of x is definitely less than or equal to the value of y:

if ( x > y ) {
int temp;      // A temporary variable for use in this block.
temp = x;      // Save a copy of the value of x in temp.
x = y;         // Copy the value of y into x.
y = temp;      // Copy the value of temp into y.
}

Finally, here is an example of an if statement that includes an else part. See if you can figure out what it does, and why it would be used:

if ( years > 1 ) {  // handle case for 2 or more years
System.out.print("The value of the investment after ");
System.out.print(years);
System.out.print(" years is $");
}
else {  // handle case for 1 year
System.out.print("The value of the investment after 1 year is $");
}  // end of if statement
System.out.printf("%1.2f", principal);  // this is done in any case

I'll have more to say about control structures later in this chapter. But you already know the essentials. If you never learned anything more about control structures, you would already know enough to perform any possible computing task. Simple looping and branching are all you really need!

3.1.4  Definite Assignment

I will finish this introduction to control structures with a somewhat technical issue that you might not fully understand the first time you encounter it. Consider the following two code segments, which seem to be entirely equivalent:

int y;                          int y;
if (x < 0) {                    if (x < 0) {
y = 1;                           y = 1;
}                               }
else {                          if (x >= 0) {
y = 2;                           y = 2;
}                               }
System.out.println(y);          System.out.println(y);

In the version on the left, y is assigned the value 1 if x < 0 and is assigned the value 2 otherwise, that is, if x >= 0. Exactly the same is true of the version on the right. However, there is a subtle difference. In fact, the Java compiler will report an error for the System.out.println statement in the code on the right, while the code on the left is perfectly fine!

The problem is that in the code on the right, the computer can't tell that the variable y has definitely been assigned a value. When an if statement has no else part, the statement inside the if might or might not be executed, depending on the value of the condition. The compiler can't tell whether it will be executed or not, since the condition will only be evaluated when the program is running. For the code on the right above, as far as the compiler is concerned, it is possible that neither statement, y = 1 or y = 2, will be evaluated, so it is possible that the output statement is trying to print an undefined value. The compiler considers this to be an error. The value of a variable can only be used if the compiler can verify that the variable will have been assigned a value at that point when the program is running. This is called definite assignment. (It doesn't matter that you can tell that y will always be assigned a value in this example. The question is whether the compiler can tell.)

Note that in the code on the left above, y is definitely assigned a value, since in an if..else statement, one of the two alternatives will be executed no matter what the value of the condition in the if. It is important that you understand that there is a difference between an if..else statement and a pair of plain if statements. Here is another pair of code segments that might seem to do the same thing, but don't. What's the value of x after each code segment is executed?

int x;                             int x;
x = -1;                            x = -1;
if (x < 0)                         if (x < 0)
x = 1;                             x = 1;
else                               if (x >= 0)
x = 2;                             x = 2;

After the code on the left is executed, x is 1; after the code on the right, x is 2.

Section 3.2

Algorithm Development

Programming is difficult (like many activities that are useful and worthwhile -- and like most of those activities, it can also be rewarding and a lot of fun). When you write a program, you have to tell the computer every small detail of what to do. And you have to get everything exactly right, since the computer will blindly follow your program exactly as written. How, then, do people write any but the most simple programs? It's not a big mystery, actually. It's a matter of learning to think in the right way.

A program is an expression of an idea. A programmer starts with a general idea of a task for the computer to perform. Presumably, the programmer has some idea of how to perform the task by hand, at least in general outline. The problem is to flesh out that outline into a complete, unambiguous, step-by-step procedure for carrying out the task. Such a procedure is called an "algorithm." (Technically, an algorithm is an unambiguous, step-by-step procedure that always terminates after a finite number of steps. We don't want to count procedures that might go on forever.) An algorithm is not the same as a program. A program is written in some particular programming language. An algorithm is more like the idea behind the program, but it's the idea of the steps the program will take to perform its task, not just the idea of the task itself. When describing an algorithm, the steps don't necessarily have to be specified in complete detail, as long as the steps are unambiguous and it's clear that carrying out the steps will accomplish the assigned task. An algorithm can be expressed in any language, including English. Of course, an algorithm can only be expressed as an actual program if all the details have been filled in.

So, where do algorithms come from? Usually, they have to be developed, often with a lot of thought and hard work. Skill at algorithm development is something that comes with practice, but there are techniques and guidelines that can help. I'll talk here about some techniques and guidelines that are relevant to "programming in the small," and I will return to the subject several times in later chapters.

3.2.1  Pseudocode and Stepwise Refinement

When programming in the small, you have a few basics to work with: variables, assignment statements, and input/output routines. You might also have some subroutines, objects, or other building blocks that have already been written by you or someone else. (Input/output routines fall into this class.) You can build sequences of these basic instructions, and you can also combine them into more complex control structures such as while loops and if statements.

Suppose you have a task in mind that you want the computer to perform. One way to proceed is to write a description of the task, and take that description as an outline of the algorithm you want to develop. Then you can refine and elaborate that description, gradually adding steps and detail, until you have a complete algorithm that can be translated directly into programming language. This method is called stepwise refinement, and it is a type of top-down design. As you proceed through the stages of stepwise refinement, you can write out descriptions of your algorithm in pseudocode -- informal instructions that imitate the structure of programming languages without the complete detail and perfect syntax of actual program code.

As an example, let's see how one might develop the program from the previous section, which computes the value of an investment over five years. The task that you want the program to perform is: "Compute and display the value of an investment for each of the next five years, where the initial investment and interest rate are to be specified by the user." You might then write -- or more likely just think -- that this can be expanded as:

Get the user's input
Compute the value of the investment after 1 year
Display the value
Compute the value after 2 years
Display the value
Compute the value after 3 years
Display the value
Compute the value after 4 years
Display the value
Compute the value after 5 years
Display the value

This is correct, but rather repetitive. And seeing that repetition, you might notice an opportunity to use a loop. A loop would take less typing. More important, it would be more general: Essentially the same loop will work no matter how many years you want to process. So, you might rewrite the above sequence of steps as:

Get the user's input
while there are more years to process:
Compute the value after the next year
Display the value

Following this algorithm would certainly solve the problem, but for a computer we'll have to be more explicit about how to "Get the user's input," how to "Compute the value after the next year," and what it means to say "there are more years to process." We can expand the step, "Get the user's input" into

Ask the user for the initial investment
Read the user's response
Ask the user for the interest rate
Read the user's response

To fill in the details of the step "Compute the value after the next year," you have to know how to do the computation yourself. (Maybe you need to ask your boss or professor for clarification?) Let's say you know that the value is computed by adding some interest to the previous value. Then we can refine the while loop to:

while there are more years to process:
Compute the interest
Add the interest to the value
Display the value

As for testing whether there are more years to process, the only way that we can do that is by counting the years ourselves. This displays a very common pattern, and you should expect to use something similar in a lot of programs: We have to start with zero years, add one each time we process a year, and stop when we reach the desired number of years. This is sometimes called a counting loop. So the while loop becomes:

years = 0
while years < 5:
years = years + 1
Compute the interest
Add the interest to the value
Display the value

We still have to know how to compute the interest. Let's say that the interest is to be computed by multiplying the interest rate by the current value of the investment. Putting this together with the part of the algorithm that gets the user's inputs, we have the complete algorithm:

Ask the user for the initial investment
Read the user's response
Ask the user for the interest rate
Read the user's response
years = 0
while years < 5:
years = years + 1
Compute interest = value * interest rate
Add the interest to the value
Display the value

Finally, we are at the point where we can translate pretty directly into proper programming-language syntax. We still have to choose names for the variables, decide exactly what we want to say to the user, and so forth. Having done this, we could express our algorithm in Java as:

double principal, rate, interest;  // declare the variables
int years;
System.out.print("Type initial investment: ");
principal = TextIO.getlnDouble();
System.out.print("Type interest rate: ");
rate = TextIO.getlnDouble();
years = 0;
while (years < 5) {
years = years + 1;
interest = principal * rate;
principal = principal + interest;
System.out.println(principal);
}

This still needs to be wrapped inside a complete program, it still needs to be commented, and it really needs to print out more information in a nicer format for the user. But it's essentially the same program as the one in the previous section. (Note that the pseudocode algorithm used indentation to show which statements are inside the loop. In Java, indentation is completely ignored by the computer, so you need a pair of braces to tell the computer which statements are in the loop. If you leave out the braces, the only statement inside the loop would be "years = years + 1;". The other statements would only be executed once, after the loop ends. The nasty thing is that the computer won't notice this error for you, like it would if you left out the parentheses around "(years < 5)". The parentheses are required by the syntax of the while statement. The braces are only required semantically. The computer can recognize syntax errors but not semantic errors.)

One thing you should have noticed here is that my original specification of the problem -- "Compute and display the value of an investment for each of the next five years" -- was far from being complete. Before you start writing a program, you should make sure you have a complete specification of exactly what the program is supposed to do. In particular, you need to know what information the program is going to input and output and what computation it is going to perform. Here is what a reasonably complete specification of the problem might look like in this example:

"Write a program that will compute and display the value of an investment for each of the next five years. Each year, interest is added to the value. The interest is computed by multiplying the current value by a fixed interest rate. Assume that the initial value and the rate of interest are to be input by the user when the program is run."

3.2.2  The 3N+1 Problem

Let's do another example, working this time with a program that you haven't already seen. The assignment here is an abstract mathematical problem that is one of my favorite programming exercises. This time, we'll start with a more complete specification of the task to be performed:

"Given a positive integer, N, define the '3N+1' sequence starting from N as follows: If N is an even number, then divide N by two; but if N is odd, then multiply N by 3 and add 1. Continue to generate numbers in this way until N becomes equal to 1. For example, starting from N = 3, which is odd, we multiply by 3 and add 1, giving N = 3*3+1 = 10. Then, since N is even, we divide by 2, giving N = 10/2 = 5. We continue in this way, stopping when we reach 1. The complete sequence is: 3, 10, 5, 16, 8, 4, 2, 1.

"Write a program that will read a positive integer from the user and will print out the 3N+1 sequence starting from that integer. The program should also count and print out the number of terms in the sequence."

A general outline of the algorithm for the program we want is:

Get a positive integer N from the user.
Compute, print, and count each number in the sequence.
Output the number of terms.

The bulk of the program is in the second step. We'll need a loop, since we want to keep computing numbers until we get 1. To put this in terms appropriate for a while loop, we need to know when to continue the loop rather than when to stop it: We want to continue as long as the number is not 1. So, we can expand our pseudocode algorithm to:

Get a positive integer N from the user;
while N is not 1:
Compute N = next term;
Output N;
Count this term;
Output the number of terms;

In order to compute the next term, the computer must take different actions depending on whether N is even or odd. We need an if statement to decide between the two cases:

Get a positive integer N from the user;
while N is not 1:
if N is even:
Compute N = N/2;
else
Compute N = 3 * N + 1;
Output N;
Count this term;
Output the number of terms;

We are almost there. The one problem that remains is counting. Counting means that you start with zero, and every time you have something to count, you add one. We need a variable to do the counting. The variable must be set to zero once, before the loop starts, and it must be incremented within the loop. (Again, this is a common pattern that you should expect to see over and over.) With the counter added, we get:

Get a positive integer N from the user;
Let counter = 0;
while N is not 1:
if N is even:
Compute N = N/2;
else
Compute N = 3 * N + 1;
Output N;
Add 1 to counter;
Output the counter;

We still have to worry about the very first step. How can we get a positive integer from the user? If we just read in a number, it's possible that the user might type in a negative number or zero. If you follow what happens when the value of N is negative or zero, you'll see that the program will go on forever, since the value of N will never become equal to 1. This is bad. In this case, the problem is probably no big deal, but in general you should try to write programs that are foolproof. One way to fix this is to keep reading in numbers until the user types in a positive number:

Ask user to input a positive number;
Let N be the user's response;
while N is not positive:
Print an error message;
Read another value for N;
Let counter = 0;
while N is not 1:
if N is even:
Compute N = N/2;
else
Compute N = 3 * N + 1;
Output N;
Add 1 to counter;
Output the counter;

The first while loop will end only when N is a positive number, as required. (A common beginning programmer's error is to use an if statement instead of a while statement here: "If N is not positive, ask the user to input another value." The problem arises if the second number input by the user is also non-positive. The if statement is only executed once, so the second input number is never tested, and the program proceeds into an infinite loop. With the while loop, after the second number is input, the computer jumps back to the beginning of the loop and tests whether the second number is positive. If not, it asks the user for a third number, and it will continue asking for numbers until the user enters an acceptable input. After the while loop ends, we can be absolutely sure that N is a positive number.)

Here is a Java program implementing this algorithm. It uses the operators <= to mean "is less than or equal to" and != to mean "is not equal to." To test whether N is even, it uses "N % 2 == 0". All the operators used here were discussed in Section 2.5.

/**
* This program prints out a 3N+1 sequence starting from a positive
* integer specified by the user.  It also counts the number of
* terms in the sequence, and prints out that number.
*/
public class ThreeN1 {

public static void main(String[] args) {

int N;       // for computing terms in the sequence
int counter; // for counting the terms

System.out.print("Starting point for sequence: ");
N = TextIO.getlnInt();
while (N <= 0) {
System.out.print(
"The starting point must be positive. Please try again: " );
N = TextIO.getlnInt();
}
// At this point, we know that N > 0

counter = 0;
while (N != 1) {
if (N % 2 == 0)
N = N / 2;
else
N = 3 * N + 1;
System.out.println(N);
counter = counter + 1;
}

System.out.println();
System.out.print("There were ");
System.out.print(counter);
System.out.println(" terms in the sequence.");

}  // end of main()

}  // end of class ThreeN1

Two final notes on this program: First, you might have noticed that the first term of the sequence -- the value of N input by the user -- is not printed or counted by this program. Is this an error? It's hard to say. Was the specification of the program careful enough to decide? This is the type of thing that might send you back to the boss/professor for clarification. The problem (if it is one!) can be fixed easily enough. Just replace the line "counter = 0" before the while loop with the two lines:

System.out.println(N);   // print out initial term
counter = 1;       // and count it

Second, there is the question of why this problem might be interesting. Well, it's interesting to mathematicians and computer scientists because of a simple question about the problem that they haven't been able to answer: Will the process of computing the 3N+1 sequence finish after a finite number of steps for all possible starting values of N? Although individual sequences are easy to compute, no one has been able to answer the general question. To put this another way, no one knows whether the process of computing 3N+1 sequences can properly be called an algorithm, since an algorithm is required to terminate after a finite number of steps! (Note: This discussion really applies to integers, not to values of type int! That is, it assumes that the value of N can take on arbitrarily large integer values, which is not true for a variable of type int in a Java program. When the value of N in the program becomes too large to be represented as a 32-bit int, the values output by the program are no longer mathematically correct. So the Java program does not compute the correct 3N+1 sequence if N becomes too large. See Exercise 8.2.)

3.2.3  Coding, Testing, Debugging

It would be nice if, having developed an algorithm for your program, you could relax, press a button, and get a perfectly working program. Unfortunately, the process of turning an algorithm into Java source code doesn't always go smoothly. And when you do get to the stage of a working program, it's often only working in the sense that it does something. Unfortunately not what you want it to do.

After program design comes coding: translating the design into a program written in Java or some other language. Usually, no matter how careful you are, a few syntax errors will creep in from somewhere, and the Java compiler will reject your program with some kind of error message. Unfortunately, while a compiler will always detect syntax errors, it's not very good about telling you exactly what's wrong. Sometimes, it's not even good about telling you where the real error is. A spelling error or missing "{" on line 45 might cause the compiler to choke on line 105. You can avoid lots of errors by making sure that you really understand the syntax rules of the language and by following some basic programming guidelines. For example, I never type a "{" without typing the matching "}". Then I go back and fill in the statements between the braces. A missing or extra brace can be one of the hardest errors to find in a large program. Always, always indent your program nicely. If you change the program, change the indentation to match. It's worth the trouble. Use a consistent naming scheme, so you don't have to struggle to remember whether you called that variable interestrate or interestRate. In general, when the compiler gives multiple error messages, don't try to fix the second error message from the compiler until you've fixed the first one. Once the compiler hits an error in your program, it can get confused, and the rest of the error messages might just be guesses. Maybe the best advice is: Take the time to understand the error before you try to fix it. Programming is not an experimental science.

When your program compiles without error, you are still not done. You have to test the program to make sure it works correctly. Remember that the goal is not to get the right output for the two sample inputs that the professor gave in class. The goal is a program that will work correctly for all reasonable inputs. Ideally, when faced with an unreasonable input, it should respond by gently chiding the user rather than by crashing. Test your program on a wide variety of inputs. Try to find a set of inputs that will test the full range of functionality that you've coded into your program. As you begin writing larger programs, write them in stages and test each stage along the way. You might even have to write some extra code to do the testing -- for example to call a subroutine that you've just written. You don't want to be faced, if you can avoid it, with 500 newly written lines of code that have an error in there somewhere.

The point of testing is to find bugs -- semantic errors that show up as incorrect behavior rather than as compilation errors. And the sad fact is that you will probably find them. Again, you can minimize bugs by careful design and careful coding, but no one has found a way to avoid them altogether. Once you've detected a bug, it's time for debugging. You have to track down the cause of the bug in the program's source code and eliminate it. Debugging is a skill that, like other aspects of programming, requires practice to master. So don't be afraid of bugs. Learn from them. One essential debugging skill is the ability to read source code -- the ability to put aside preconceptions about what you think it does and to follow it the way the computer does -- mechanically, step-by-step -- to see what it really does. This is hard. I can still remember the time I spent hours looking for a bug only to find that a line of code that I had looked at ten times had a "1" where it should have had an "i", or the time when I wrote a subroutine named WindowClosing which would have done exactly what I wanted except that the computer was looking for windowClosing (with a lower case "w"). Sometimes it can help to have someone who doesn't share your preconceptions look at your code.

Often, it's a problem just to find the part of the program that contains the error. Most programming environments come with a debugger, which is a program that can help you find bugs. Typically, your program can be run under the control of the debugger. The debugger allows you to set "breakpoints" in your program. A breakpoint is a point in the program where the debugger will pause the program so you can look at the values of the program's variables. The idea is to track down exactly when things start to go wrong during the program's execution. The debugger will also let you execute your program one line at a time, so that you can watch what happens in detail once you know the general area in the program where the bug is lurking.

I will confess that I only occasionally use debuggers myself. A more traditional approach to debugging is to insert debugging statements into your program. These are output statements that print out information about the state of the program. Typically, a debugging statement would say something like

System.out.println("At start of while loop, N = " + N);

You need to be able to tell from the output where in your program the output is coming from, and you want to know the value of important variables. Sometimes, you will find that the computer isn't even getting to a part of the program that you think it should be executing. Remember that the goal is to find the first point in the program where the state is not what you expect it to be. That's where the bug is.

And finally, remember the golden rule of debugging: If you are absolutely sure that everything in your program is right, and if it still doesn't work, then one of the things that you are absolutely sure of is wrong.

Section 3.3

The while and do..while Statements

Statements in Java can be either simple statements or compound statements. Simple statements, such as assignment statements and subroutine call statements, are the basic building blocks of a program. Compound statements, such as while loops and if statements, are used to organize simple statements into complex structures, which are called control structures because they control the order in which the statements are executed. The next five sections explore the details of control structures that are available in Java, starting with the while statement and the do..while statement in this section. At the same time, we'll look at examples of programming with each control structure and apply the techniques for designing algorithms that were introduced in the previous section.

3.3.1  The while Statement

The while statement was already introduced in Section 3.1. A while loop has the form

while ( boolean-expression )
statement

The statement can, of course, be a block statement consisting of several statements grouped together between a pair of braces. This statement is called the body of the loop. The body of the loop is repeated as long as the boolean-expression is true. This boolean expression is called the continuation condition, or more simply the test, of the loop. There are a few points that might need some clarification. What happens if the condition is false in the first place, before the body of the loop is executed even once? In that case, the body of the loop is never executed at all. The body of a while loop can be executed any number of times, including zero. What happens if the condition is true, but it becomes false somewhere in the middle of the loop body? Does the loop end as soon as this happens? It doesn't, because the computer continues executing the body of the loop until it gets to the end. Only then does it jump back to the beginning of the loop and test the condition, and only then can the loop end.

Let's look at a typical problem that can be solved using a while loop: finding the average of a set of positive integers entered by the user. The average is the sum of the integers, divided by the number of integers. The program will ask the user to enter one integer at a time. It will keep count of the number of integers entered, and it will keep a running total of all the numbers it has read so far. Here is a pseudocode algorithm for the program:

Let sum = 0     // The sum of the integers entered by the user.
Let count = 0   // The number of integers entered by the user.
while there are more integers to process:
Read an integer
Add it to the sum
Count it
Divide sum by count to get the average
Print out the average

But how can we test whether there are more integers to process? A typical solution is to tell the user to type in zero after all the data have been entered. This will work because we are assuming that all the data are positive numbers, so zero is not a legal data value. The zero is not itself part of the data to be averaged. It's just there to mark the end of the real data. A data value used in this way is sometimes called a sentinel value. So now the test in the while loop becomes "while the input integer is not zero". But there is another problem! The first time the test is evaluated, before the body of the loop has ever been executed, no integer has yet been read. There is no "input integer" yet, so testing whether the input integer is zero doesn't make sense. So, we have to do something before the while loop to make sure that the test makes sense. Setting things up so that the test in a while loop makes sense the first time it is executed is called priming the loop. In this case, we can simply read the first integer before the beginning of the loop. Here is a revised algorithm:

Let sum = 0
Let count = 0
Read an integer
while the integer is not zero:
Add the integer to the sum
Count it
Read an integer
Divide sum by count to get the average
Print out the average

Notice that I've rearranged the body of the loop. Since an integer is read before the loop, the loop has to begin by processing that integer. At the end of the loop, the computer reads a new integer. The computer then jumps back to the beginning of the loop and tests the integer that it has just read. Note that when the computer finally reads the sentinel value, the loop ends before the sentinel value is processed. It is not added to the sum, and it is not counted. This is the way it's supposed to work. The sentinel is not part of the data. The original algorithm, even if it could have been made to work without priming, was incorrect since it would have summed and counted all the integers, including the sentinel. (Since the sentinel is zero, the sum would still be correct, but the count would be off by one. Such so-called off-by-one errors are very common. Counting turns out to be harder than it looks!)

We can easily turn the algorithm into a complete program. Note that the program cannot use the statement "average = sum/count;" to compute the average. Since sum and count are both variables of type int, the value of sum/count is an integer. The average should be a real number. We've seen this problem before: we have to convert one of the int values to a double to force the computer to compute the quotient as a real number. This can be done by type-casting one of the variables to type double. The type cast "(double)sum" converts the value of sum to a real number, so in the program the average is computed as "average = ((double)sum) / count;". Another solution in this case would have been to declare sum to be a variable of type double in the first place.

One other issue is addressed by the program: If the user enters zero as the first input value, there are no data to process. We can test for this case by checking whether count is still equal to zero after the while loop. This might seem like a minor point, but a careful programmer should cover all the bases.

Here is the full source code for the program:

/**
* This program reads a sequence of positive integers input
* by the user, and it will print out the average of those
* integers.  The user is prompted to enter one integer at a
* time.  The user must enter a 0 to mark the end of the
* data.  (The zero is not counted as part of the data to
* be averaged.)  The program does not check whether the
* user's input is positive, so it will actually add up
* both positive and negative input values.
*/

public class ComputeAverage {

public static void main(String[] args) {

int inputNumber;   // One of the integers input by the user.
int sum;           // The sum of the positive integers.
int count;         // The number of positive integers.
double average;    // The average of the positive integers.

/* Initialize the summation and counting variables. */

sum = 0;
count = 0;

/* Read and process the user's input. */

System.out.print("Enter your first positive integer: ");
inputNumber = TextIO.getlnInt();

while (inputNumber != 0) {
sum += inputNumber;   // Add inputNumber to running sum.
count++;              // Count the input by adding 1 to count.
System.out.print("Enter your next positive integer, or 0 to end: ");
inputNumber = TextIO.getlnInt();
}

/* Display the result. */

if (count == 0) {
System.out.println("You didn't enter any data!");
}
else {
average = ((double)sum) / count;
System.out.println();
System.out.println("You entered " + count + " positive integers.");
System.out.printf("Their average is %1.3f.\n", average);
}

} // end main()

} // end class ComputeAverage

3.3.2  The do..while Statement

Sometimes it is more convenient to test the continuation condition at the end of a loop, instead of at the beginning, as is done in the while loop. The do..while statement is very similar to the while statement, except that the word "while," along with the condition that it tests, has been moved to the end. The word "do" is added to mark the beginning of the loop. A do..while statement has the form

do
statement
while ( boolean-expression );

or, since, as usual, the statement can be a block,

do {
statements
} while ( boolean-expression );

Note the semicolon, ';', at the very end. This semicolon is part of the statement, just as the semicolon at the end of an assignment statement or declaration is part of the statement. Omitting it is a syntax error. (More generally, every statement in Java ends either with a semicolon or a right brace, '}'.)

To execute a do loop, the computer first executes the body of the loop -- that is, the statement or statements inside the loop -- and then it evaluates the boolean expression. If the value of the expression is true, the computer returns to the beginning of the do loop and repeats the process; if the value is false, it ends the loop and continues with the next part of the program. Since the condition is not tested until the end of the loop, the body of a do loop is always executed at least once.

For example, consider the following pseudocode for a game-playing program. The do loop makes sense here instead of a while loop because with the do loop, you know there will be at least one game. Also, the test that is used at the end of the loop wouldn't even make sense at the beginning:

do {
Play a Game
Ask user if he wants to play another game
Read the user's response
} while ( the user's response is yes );

Let's convert this into proper Java code. Since I don't want to talk about game playing at the moment, let's say that we have a class named Checkers, and that the Checkers class contains a static member subroutine named playGame() that plays one game of checkers against the user. Then, the pseudocode "Play a game" can be expressed as the subroutine call statement "Checkers.playGame();". We need a variable to store the user's response. The TextIO class makes it convenient to use a boolean variable to store the answer to a yes/no question. The input function TextIO.getlnBoolean() allows the user to enter the value as "yes" or "no" (among other acceptable responses). "Yes" is considered to be true, and "no" is considered to be false. So, the algorithm can be coded as

boolean wantsToContinue;  // True if user wants to play again.
do {
Checkers.playGame();
System.out.print("Do you want to play again? ");
wantsToContinue = TextIO.getlnBoolean();
} while (wantsToContinue == true);

When the value of the boolean variable is set to false, it is a signal that the loop should end. When a boolean variable is used in this way -- as a signal that is set in one part of the program and tested in another part -- it is sometimes called a flag or flag variable (in the sense of a signal flag).

By the way, a more-than-usually-pedantic programmer would sneer at the test "while (wantsToContinue == true)". This test is exactly equivalent to "while (wantsToContinue)". Testing whether "wantsToContinue == true" is true amounts to the same thing as testing whether "wantsToContinue" is true. A little less offensive is an expression of the form "flag == false", where flag is a boolean variable. The value of "flag == false" is exactly the same as the value of "!flag", where ! is the boolean negation operator. So you can write "while (!flag)" instead of "while (flag == false)", and you can write "if (!flag)" instead of "if (flag == false)".

Although a do..while statement is sometimes more convenient than a while statement, having two kinds of loops does not make the language more powerful. Any problem that can be solved using do..while loops can also be solved using only while statements, and vice versa. In fact, if doSomething represents any block of program code, then

do {
doSomething
} while ( boolean-expression );

has exactly the same effect as

doSomething
while ( boolean-expression ) {
doSomething
}

Similarly,

while ( boolean-expression ) {
doSomething
}

can be replaced by

if ( boolean-expression ) {
do {
doSomething
} while ( boolean-expression );
}

without changing the meaning of the program in any way.

3.3.3  break and continue

The syntax of the while and do..while loops allows you to test the continuation condition at either the beginning of a loop or at the end. Sometimes, it is more natural to have the test in the middle of the loop, or to have several tests at different places in the same loop. Java provides a general method for breaking out of the middle of any loop. It's called the break statement, which takes the form

break;

When the computer executes a break statement in a loop, it will immediately jump out of the loop. It then continues on to whatever follows the loop in the program. Consider for example:

while (true) {  // looks like it will run forever!
System.out.print("Enter a positive number: ");
N = TextIO.getlnInt();
if (N > 0)   // the input value is OK, so jump out of loop
break;
System.out.println("Your answer must be > 0.");
}
// continue here after break

If the number entered by the user is greater than zero, the break statement will be executed and the computer will jump out of the loop. Otherwise, the computer will print out "Your answer must be > 0." and will jump back to the start of the loop to read another input value.

The first line of this loop, "while (true)" might look a bit strange, but it's perfectly legitimate. The condition in a while loop can be any boolean-valued expression. The computer evaluates this expression and checks whether the value is true or false. The boolean literal "true" is just a boolean expression that always evaluates to true. So "while (true)" can be used to write an infinite loop, or one that will be terminated by a break statement.

A break statement terminates the loop that immediately encloses the break statement. It is possible to have nested loops, where one loop statement is contained inside another. If you use a break statement inside a nested loop, it will only break out of that loop, not out of the loop that contains the nested loop. There is something called a labeled break statement that allows you to specify which loop you want to break. This is not very common, so I will go over it quickly. Labels work like this: You can put a label in front of any loop. A label consists of a simple identifier followed by a colon. For example, a while with a label might look like "mainloop: while...". Inside this loop you can use the labeled break statement "break mainloop;" to break out of the labeled loop. For example, here is a code segment that checks whether two strings, s1 and s2, have a character in common. If a common character is found, the value of the flag variable nothingInCommon is set to false, and a labeled break is used to end the processing at that point:

boolean nothingInCommon;
nothingInCommon = true;  // Assume s1 and s2 have no chars in common.
int i,j;  // Variables for iterating through the chars in s1 and s2.

i = 0;
bigloop: while (i < s1.length()) {
j = 0;
while (j < s2.length()) {
if (s1.charAt(i) == s2.charAt(j)) { // s1 and s2 have a common char.
nothingInCommon = false;
break bigloop;  // break out of BOTH loops
}
j++;  // Go on to the next char in s2.
}
i++;  //Go on to the next char in s1.
}

The continue statement is related to break, but less commonly used. A continue statement tells the computer to skip the rest of the current iteration of the loop. However, instead of jumping out of the loop altogether, it jumps back to the beginning of the loop and continues with the next iteration (including evaluating the loop's continuation condition to see whether any further iterations are required). As with break, when a continue is in a nested loop, it will continue the loop that directly contains it; a "labeled continue" can be used to continue the containing loop instead.

break and continue can be used in while loops and do..while loops. They can also be used in for loops, which are covered in the next section. In Section 3.6, we'll see that break can also be used to break out of a switch statement. A break can occur inside an if statement, but only if the if statement is nested inside a loop or inside a switch statement. In that case, it does not mean to break out of the if. Instead, it breaks out of the loop or switch statement that contains the if statement. The same consideration applies to continue statements inside ifs.

Section 3.4

The for Statement

We turn in this section to another type of loop, the for statement. Any for loop is equivalent to some while loop, so the language doesn't get any additional power by having for statements. But for a certain type of problem, a for loop can be easier to construct and easier to read than the corresponding while loop. It's quite possible that in real programs, for loops actually outnumber while loops.

3.4.1  For Loops

The for statement makes a common type of while loop easier to write. Many while loops have the general form:

initialization
while ( continuation-condition ) {
statements
update
}

For example, consider this example, copied from an example in Section 3.2:

years = 0;  // initialize the variable years
while ( years < 5 ) {   // condition for continuing loop

interest = principal * rate;    //
principal += interest;          // do three statements
System.out.println(principal);  //

years++;   // update the value of the variable, years
}

This loop can be written as the following equivalent for statement:

for ( years = 0;  years < 5;  years++ ) {
interest = principal * rate;
principal += interest;
System.out.println(principal);
}

The initialization, continuation condition, and updating have all been combined in the first line of the for loop. This keeps everything involved in the "control" of the loop in one place, which helps make the loop easier to read and understand. The for loop is executed in exactly the same way as the original code: The initialization part is executed once, before the loop begins. The continuation condition is executed before each execution of the loop, and the loop ends when this condition is false. The update part is executed at the end of each execution of the loop, just before jumping back to check the condition.

The formal syntax of the for statement is as follows:

for ( initialization; continuation-condition; update )
statement

or, using a block statement:

for ( initialization; continuation-condition; update ) {
statements
}

The continuation-condition must be a boolean-valued expression. The initialization is usually a declaration or an assignment statement, but it can be any expression that would be allowed as a statement in a program. The update can be any simple statement, but is usually an increment, a decrement, or an assignment statement. Any of the three parts can be empty. If the continuation condition is empty, it is treated as if it were "true," so the loop will be repeated forever or until it ends for some other reason, such as a break statement. (Some people like to begin an infinite loop with "for (;;)" instead of "while (true)".) Here's a flow control diagram for a for statement:

Usually, the initialization part of a for statement assigns a value to some variable, and the update changes the value of that variable with an assignment statement or with an increment or decrement operation. The value of the variable is tested in the continuation condition, and the loop ends when this condition evaluates to false. A variable used in this way is called a loop control variable. In the example given above, the loop control variable was years.

Certainly, the most common type of for loop is the counting loop, where a loop control variable takes on all integer values between some minimum and some maximum value. A counting loop has the form

for ( variable = min;  variable <= max; variable++ ) {
statements
}

where min and max are integer-valued expressions (usually constants). The variable takes on the values min, min+1, min+2, ..., max. The value of the loop control variable is often used in the body of the loop. The for loop at the beginning of this section is a counting loop in which the loop control variable, years, takes on the values 1, 2, 3, 4, 5. Here is an even simpler example, in which the numbers 1, 2, ..., 10 are displayed on standard output:

for ( N = 1 ;  N <= 10 ;  N++ )
System.out.println( N );

For various reasons, Java programmers like to start counting at 0 instead of 1, and they tend to use a "<" in the condition, rather than a "<=". The following variation of the above loop prints out the ten numbers 0, 1, 2, ..., 9:

for ( N = 0 ;  N < 10 ;  N++ )
System.out.println( N );

Using < instead of <= in the test, or vice versa, is a common source of off-by-one errors in programs. You should always stop and think, Do I want the final value to be processed or not?

It's easy to count down from 10 to 1 instead of counting up. Just start with 10, decrement the loop control variable instead of incrementing it, and continue as long as the variable is greater than or equal to one.

for ( N = 10 ;  N >= 1 ;  N-- )
System.out.println( N );

Now, in fact, the official syntax of a for statement actually allows both the initialization part and the update part to consist of several expressions, separated by commas. So we can even count up from 1 to 10 and count down from 10 to 1 at the same time!

for ( i=1, j=10;  i <= 10;  i++, j-- ) {
System.out.printf("%5d", i); // Output i in a 5-character wide column.
System.out.printf("%5d", j); // Output j in a 5-character column
System.out.println();       //     and end the line.
}

As a final introductory example, let's say that we want to use a for loop that prints out just the even numbers between 2 and 20, that is: 2, 4, 6, 8, 10, 12, 14, 16, 18, 20. There are several ways to do this. Just to show how even a very simple problem can be solved in many ways, here are four different solutions (three of which would get full credit):

(1)   // There are 10 numbers to print.
// Use a for loop to count 1, 2,
// ..., 10.  The numbers we want
// to print are 2*1, 2*2, ... 2*10.

for (N = 1; N <= 10; N++) {
System.out.println( 2*N );
}

(2)   // Use a for loop that counts
// 2, 4, ..., 20 directly by
// adding 2 to N each time through
// the loop.

for (N = 2; N <= 20; N = N + 2) {
System.out.println( N );
}

(3)   // Count off all the numbers
// 2, 3, 4, ..., 19, 20, but
// only print out the numbers
// that are even.

for (N = 2; N <= 20; N++) {
if ( N % 2 == 0 ) // is N even?
System.out.println( N );
}

(4)   // Irritate the professor with
// a solution that follows the
// letter of this silly assignment
// while making fun of it.

for (N = 1; N <= 1; N++) {
System.out.println("2 4 6 8 10 12 14 16 18 20");
}

Perhaps it is worth stressing one more time that a for statement, like any statement except for a variable declaration, never occurs on its own in a real program. A statement must be inside the main routine of a program or inside some other subroutine. And that subroutine must be defined inside a class. I should also remind you that every variable must be declared before it can be used, and that includes the loop control variable in a for statement. In all the examples that you have seen so far in this section, the loop control variables should be declared to be of type int. It is not required that a loop control variable be an integer. Here, for example, is a for loop in which the variable, ch, is of type char, using the fact that the ++ operator can be applied to characters as well as to numbers:

// Print out the alphabet on one line of output.
char ch;  // The loop control variable;
//       one of the letters to be printed.
for ( ch = 'A';  ch <= 'Z';  ch++ )
System.out.print(ch);
System.out.println();

3.4.2  Example: Counting Divisors

Let's look at a less trivial problem that can be solved with a for loop. If N and D are positive integers, we say that D is a divisor of N if the remainder when D is divided into N is zero. (Equivalently, we could say that N is an even multiple of D.) In terms of Java programming, D is a divisor of N if N % D is zero.

Let's write a program that inputs a positive integer, N, from the user and computes how many different divisors N has. The numbers that could possibly be divisors of N are 1, 2, ..., N. To compute the number of divisors of N, we can just test each possible divisor of N and count the ones that actually do divide N evenly. In pseudocode, the algorithm takes the form

Get a positive integer, N, from the user
Let divisorCount = 0
for each number, testDivisor, in the range from 1 to N:
if testDivisor is a divisor of N:
Count it by adding 1 to divisorCount
Output the count

This algorithm displays a common programming pattern that is used when some, but not all, of a sequence of items are to be processed. The general pattern is

for each item in the sequence:
if the item passes the test:
process it

The for loop in our divisor-counting algorithm can be translated into Java code as

for (testDivisor = 1; testDivisor <= N; testDivisor++) {
if ( N % testDivisor == 0 )
divisorCount++;
}

On a modern computer, this loop can be executed very quickly. It is not impossible to run it even for the largest legal int value, 2147483647. (If you wanted to run it for even larger values, you could use variables of type long rather than int.) However, it does take a significant amount of time for very large numbers. So when I implemented this algorithm, I decided to output a dot every time the computer has tested one million possible divisors. In the improved version of the program, there are two types of counting going on. We have to count the number of divisors and we also have to count the number of possible divisors that have been tested. So the program needs two counters. When the second counter reaches 1000000, the program outputs a '.' and resets the counter to zero so that we can start counting the next group of one million. Reverting to pseudocode, the algorithm now looks like

Get a positive integer, N, from the user
Let divisorCount = 0  // Number of divisors found.
Let numberTested = 0  // Number of possible divisors tested
//       since the last period was output.
for each number, testDivisor, in the range from 1 to N:
if testDivisor is a divisor of N:
Count it by adding 1 to divisorCount
Add 1 to numberTested
if numberTested is 1000000:
print out a '.'
Reset numberTested to 0
Output the count

Finally, we can translate the algorithm into a complete Java program:

/**
* This program reads a positive integer from the user.
* It counts how many divisors that number has, and
* then it prints the result.
*/

public class CountDivisors {

public static void main(String[] args) {

int N;  // A positive integer entered by the user.
// Divisors of this number will be counted.

int testDivisor;  // A number between 1 and N that is a
// possible divisor of N.

int divisorCount;  // Number of divisors of N that have been found.

int numberTested;  // Used to count how many possible divisors
// of N have been tested.  When the number
// reaches 1000000, a period is output and
// the value of numberTested is reset to zero.

/* Get a positive integer from the user. */

while (true) {
System.out.print("Enter a positive integer: ");
N = TextIO.getlnInt();
if (N > 0)
break;
System.out.println("That number is not positive.  Please try again.");
}

/* Count the divisors, printing a "." after every 1000000 tests. */

divisorCount = 0;
numberTested = 0;

for (testDivisor = 1; testDivisor <= N; testDivisor++) {
if ( N % testDivisor == 0 )
divisorCount++;
numberTested++;
if (numberTested == 1000000) {
System.out.print('.');
numberTested = 0;
}
}

/* Display the result. */

System.out.println();
System.out.println("The number of divisors of " + N
+ " is " + divisorCount);

} // end main()

} // end class CountDivisors

3.4.3  Nested for Loops

Control structures in Java are statements that contain other, simpler statements. In particular, control structures can contain control structures. You've already seen several examples of if statements inside loops, and one example of a while loop inside another while, but any combination of one control structure inside another is possible. We say that one structure is nested inside another. You can even have multiple levels of nesting, such as a while loop inside an if statement inside another while loop. The syntax of Java does not set a limit on the number of levels of nesting. As a practical matter, though, it's difficult to understand a program that has more than a few levels of nesting.

Nested for loops arise naturally in many algorithms, and it is important to understand how they work. Let's look at a couple of examples. First, consider the problem of printing out a multiplication table like this one:

1   2   3   4   5   6   7   8   9  10  11  12
2   4   6   8  10  12  14  16  18  20  22  24
3   6   9  12  15  18  21  24  27  30  33  36
4   8  12  16  20  24  28  32  36  40  44  48
5  10  15  20  25  30  35  40  45  50  55  60
6  12  18  24  30  36  42  48  54  60  66  72
7  14  21  28  35  42  49  56  63  70  77  84
8  16  24  32  40  48  56  64  72  80  88  96
9  18  27  36  45  54  63  72  81  90  99 108
10  20  30  40  50  60  70  80  90 100 110 120
11  22  33  44  55  66  77  88  99 110 121 132
12  24  36  48  60  72  84  96 108 120 132 144

The data in the table are arranged into 12 rows and 12 columns. The process of printing them out can be expressed in a pseudocode algorithm as

for each rowNumber = 1, 2, 3, ..., 12:
Print the first twelve multiples of rowNumber on one line
Output a carriage return

The first step in the for loop can itself be expressed as a for loop. We can expand "Print the first twelve multiples of rowNumber on one line" as:

for N = 1, 2, 3, ..., 12:
Print N * rowNumber

so a refined algorithm for printing the table has one for loop nested inside another:

for each rowNumber = 1, 2, 3, ..., 12:
for N = 1, 2, 3, ..., 12:
Print N * rowNumber
Output a carriage return

We want to print the output in neat columns, with each output number taking up four spaces. This can be done using formatted output with format specifier %4d. Assuming that rowNumber and N have been declared to be variables of type int, the algorithm can be expressed in Java as

for ( rowNumber = 1;  rowNumber <= 12;  rowNumber++ ) {
for ( N = 1;  N <= 12;  N++ ) {
// print in 4-character columns
System.out.printf( "%4d", N * rowNumber );  // No carriage return !
}
System.out.println();  // Add a carriage return at end of the line.
}

This section has been weighed down with lots of examples of numerical processing. For our next example, let's do some text processing. Consider the problem of finding which of the 26 letters of the alphabet occur in a given string. For example, the letters that occur in "Hello World" are D, E, H, L, O, R, and W. More specifically, we will write a program that will list all the letters contained in a string and will also count the number of different letters. The string will be input by the user. Let's start with a pseudocode algorithm for the program.

Ask the user to input a string
Read the response into a variable, str
Let count = 0  (for counting the number of different letters)
for each letter of the alphabet:
if the letter occurs in str:
Print the letter
Add 1 to count
Output the count

Since we want to process the entire line of text that is entered by the user, we'll use TextIO.getln() to read it. The line of the algorithm that reads "for each letter of the alphabet" can be expressed as "for (letter='A'; letter<='Z'; letter++)". But the if statement inside the for loop needs still more thought before we can write the program. How do we check whether the given letter, letter, occurs in str? One idea is to look at each character in the string in turn, and check whether that character is equal to letter. We can get the i-th character of str with the function call str.charAt(i), where i ranges from 0 to str.length() - 1.

One more difficulty: A letter such as 'A' can occur in str in either upper or lower case, 'A' or 'a'. We have to check for both of these. But we can avoid this difficulty by converting str to upper case before processing it. Then, we only have to check for the upper case letter. We can now flesh out the algorithm fully:

Ask the user to input a string
Read the response into a variable, str
Convert str to upper case
Let count = 0
for letter = 'A', 'B', ..., 'Z':
for i = 0, 1, ..., str.length()-1:
if letter == str.charAt(i):
Print letter
Add 1 to count
break  // jump out of the loop, to avoid counting letter twice
Output the count

Note the use of break in the nested for loop. It is required to avoid printing or counting a given letter more than once (in the case where it occurs more than once in the string). The break statement breaks out of the inner for loop, but not the outer for loop. Upon executing the break, the computer continues the outer loop with the next value of letter. You should try to figure out exactly what count would be at the end of this program, if the break statement were omitted. Here is the complete program:

/**
* This program reads a line of text entered by the user.
* It prints a list of the letters that occur in the text,
* and it reports how many different letters were found.
*/

public class ListLetters {

public static void main(String[] args) {

String str;  // Line of text entered by the user.
int count;   // Number of different letters found in str.
char letter; // A letter of the alphabet.

System.out.println("Please type in a line of text.");
str = TextIO.getln();

str = str.toUpperCase();

count = 0;
System.out.println("Your input contains the following letters:");
System.out.println();
System.out.print("   ");
for ( letter = 'A'; letter <= 'Z'; letter++ ) {
int i;  // Position of a character in str.
for ( i = 0; i < str.length(); i++ ) {
if ( letter == str.charAt(i) ) {
System.out.print(letter);
System.out.print(' ');
count++;
break;
}
}
}

System.out.println();
System.out.println();
System.out.println("There were " + count + " different letters.");

} // end main()

} // end class ListLetters

In fact, there is actually an easier way to determine whether a given letter occurs in a string, str. The built-in function str.indexOf(letter) will return -1 if letter does not occur in the string. It returns a number greater than or equal to zero if it does occur. So, we could check whether letter occurs in str simply by checking "if (str.indexOf(letter) >= 0)". If we used this technique in the above program, we wouldn't need a nested for loop. This gives you a preview of how subroutines can be used to deal with complexity.

Section 3.5

The if Statement

The first of the two branching statements in Java is the if statement, which you have already seen in Section 3.1. It takes the form

if (boolean-expression)
statement-1
else
statement-2

As usual, the statements inside an if statement can be blocks. The if statement represents a two-way branch. The else part of an if statement -- consisting of the word "else" and the statement that follows it -- can be omitted.

3.5.1  The Dangling else Problem

Now, an if statement is, in particular, a statement. This means that either statement-1 or statement-2 in the above if statement can itself be an if statement. A problem arises, however, if statement-1 is an if statement that has no else part. This special case is effectively forbidden by the syntax of Java. Suppose, for example, that you type

if ( x > 0 )
if (y > 0)
System.out.println("First case");
else
System.out.println("Second case");

Now, remember that the way you've indented this doesn't mean anything at all to the computer. You might think that the else part is the second half of your "if (x > 0)" statement, but the rule that the computer follows attaches the else to "if (y > 0)", which is closer. That is, the computer reads your statement as if it were formatted:

if ( x > 0 )
if (y > 0)
System.out.println("First case");
else
System.out.println("Second case");

You can force the computer to use the other interpretation by enclosing the nested if in a block:

if ( x > 0 ) {
if (y > 0)
System.out.println("First case");
}
else
System.out.println("Second case");

These two if statements have different meanings: In the case when x <= 0, the first statement doesn't print anything, but the second statement prints "Second case".

3.5.2  Multiway Branching

Much more interesting than this technicality is the case where statement-2, the else part of the if statement, is itself an if statement. The statement would look like this (perhaps without the final else part):

if (boolean-expression-1)
statement-1
else
if (boolean-expression-2)
statement-2
else
statement-3

However, since the computer doesn't care how a program is laid out on the page, this is almost always written in the format:

if (boolean-expression-1)
statement-1
else if (boolean-expression-2)
statement-2
else
statement-3

You should think of this as a single statement representing a three-way branch. When the computer executes this, one and only one of the three statements -- statement-1, statement-2, or statement-3 -- will be executed. The computer starts by evaluating boolean-expression-1. If it is true, the computer executes statement-1 and then jumps all the way to the end of the outer if statement, skipping the other two statements. If boolean-expression-1 is false, the computer skips statement-1 and executes the second, nested if statement. To do this, it tests the value of boolean-expression-2 and uses it to decide between statement-2 and statement-3.

Here is an example that will print out one of three different messages, depending on the value of a variable named temperature:

if (temperature < 50)
System.out.println("It's cold.");
else if (temperature < 80)
System.out.println("It's nice.");
else
System.out.println("It's hot.");

If temperature is, say, 42, the first test is true. The computer prints out the message "It's cold", and skips the rest -- without even evaluating the second condition. For a temperature of 75, the first test is false, so the computer goes on to the second test. This test is true, so the computer prints "It's nice" and skips the rest. If the temperature is 173, both of the tests evaluate to false, so the computer says "It's hot" (unless its circuits have been fried by the heat, that is).

You can go on stringing together "else-if's" to make multi-way branches with any number of cases:

if (test-1)
statement-1
else if (test-2)
statement-2
else if (test-3)
statement-3
.
. // (more cases)
.
else if (test-N)
statement-N
else
statement-(N+1)

The computer evaluates the tests, which are boolean expressions, one after the other until it comes to one that is true. It executes the associated statement and skips the rest. If none of the boolean expressions evaluate to true, then the statement in the else part is executed. This statement is called a multi-way branch because one and only one of the statements will be executed. The final else part can be omitted. In that case, if all the boolean expressions are false, none of the statements are executed. Of course, each of the statements can be a block, consisting of a number of statements enclosed between { and }. Admittedly, there is lot of syntax here; as you study and practice, you'll become comfortable with it. It might be useful to look at a flow control diagram for the general "if..else if" statement shown above:

3.5.3  If Statement Examples

As an example of using if statements, let's suppose that x, y, and z are variables of type int, and that each variable has already been assigned a value. Consider the problem of printing out the values of the three variables in increasing order. For example, if the values are 42, 17, and 20, then the output should be in the order 17, 20, 42.

One way to approach this is to ask, where does x belong in the list? It comes first if it's less than both y and z. It comes last if it's greater than both y and z. Otherwise, it comes in the middle. We can express this with a 3-way if statement, but we still have to worry about the order in which y and z should be printed. In pseudocode,

if (x < y && x < z) {
output x, followed by y and z in their correct order
}
else if (x > y && x > z) {
output y and z in their correct order, followed by x
}
else {
output x in between y and z in their correct order
}

Determining the relative order of y and z requires another if statement, so this becomes

if (x < y && x < z) {        // x comes first
if (y < z)
System.out.println( x + " " + y + " " + z );
else
System.out.println( x + " " + z + " " + y );
}
else if (x > y && x > z) {   // x comes last
if (y < z)
System.out.println( y + " " + z + " " + x );
else
System.out.println( z + " " + y + " " + x );
}
else {                       // x in the middle
if (y < z)
System.out.println( y + " " + x + " " + z);
else
System.out.println( z + " " + x + " " + y);
}

You might check that this code will work correctly even if some of the values are the same. If the values of two variables are the same, it doesn't matter which order you print them in.

Note, by the way, that even though you can say in English "if x is less than y and z," you can't say in Java "if (x < y && z)". The && operator can only be used between boolean values, so you have to make separate tests, x<y and x<z, and then combine the two tests with &&.

There is an alternative approach to this problem that begins by asking, "which order should x and y be printed in?" Once that's known, you only have to decide where to stick in z. This line of thought leads to different Java code:

if ( x < y ) {  // x comes before y
if ( z < x )   // z comes first
System.out.println( z + " " + x + " " + y);
else if ( z > y )   // z comes last
System.out.println( x + " " + y + " " + z);
else   // z is in the middle
System.out.println( x + " " + z + " " + y);
}
else {          // y comes before x
if ( z < y )   // z comes first
System.out.println( z + " " + y + " " + x);
else if ( z > x )  // z comes last
System.out.println( y + " " + x + " " + z);
else  // z is in the middle
System.out.println( y + " " + z + " " + x);
}

Once again, we see how the same problem can be solved in many different ways. The two approaches to this problem have not exhausted all the possibilities. For example, you might start by testing whether x is greater than y. If so, you could swap their values. Once you've done that, you know that x should be printed before y.

Finally, let's write a complete program that uses an if statement in an interesting way. I want a program that will convert measurements of length from one unit of measurement to another, such as miles to yards or inches to feet. So far, the problem is extremely under-specified. Let's say that the program will only deal with measurements in inches, feet, yards, and miles. It would be easy to extend it later to deal with other units. The user will type in a measurement in one of these units, such as "17 feet" or "2.73 miles". The output will show the length in terms of each of the four units of measure. (This is easier than asking the user which units to use in the output.) An outline of the process is

Read the user's input measurement and units of measure
Express the measurement in inches, feet, yards, and miles
Display the four results

The program can read both parts of the user's input from the same line by using TextIO.getDouble() to read the numerical measurement and TextIO.getlnWord() to read the unit of measure. The conversion into different units of measure can be simplified by first converting the user's input into inches. From there, the number of inches can easily be converted into feet, yards, and miles. Before converting into inches, we have to test the input to determine which unit of measure the user has specified:

Let measurement = TextIO.getDouble()
Let units = TextIO.getlnWord()
if the units are inches
Let inches = measurement
else if the units are feet
Let inches = measurement * 12         // 12 inches per foot
else if the units are yards
Let inches = measurement * 36         // 36 inches per yard
else if the units are miles
Let inches = measurement * 12 * 5280  // 5280 feet per mile
else
The units are illegal!
Print an error message and stop processing
Let feet = inches / 12.0
Let yards = inches / 36.0
Let miles = inches / (12.0 * 5280.0)
Display the results

Since units is a String, we can use units.equals("inches") to check whether the specified unit of measure is "inches". However, it would be nice to allow the units to be specified as "inch" or abbreviated to "in". To allow these three possibilities, we can check if (units.equals("inches") || units.equals("inch") || units.equals("in")). It would also be nice to allow upper case letters, as in "Inches" or "IN". We can do this by converting units to lower case before testing it or by substituting the function units.equalsIgnoreCase for units.equals.

In my final program, I decided to make things more interesting by allowing the user to repeat the process of entering a measurement and seeing the results of the conversion for each measurement. The program will end only when the user inputs 0. To program that, I just had to wrap the above algorithm inside a while loop, and make sure that the loop ends when the user inputs a 0. Here's the complete program:

/**
* This program will convert measurements expressed in inches,
* feet, yards, or miles into each of the possible units of
* measure.  The measurement is input by the user, followed by
* the unit of measure.  For example:  "17 feet", "1 inch", or
* "2.73 mi".  Abbreviations in, ft, yd, and mi are accepted.
* The program will continue to read and convert measurements
* until the user enters an input of 0.
*/

public class LengthConverter {

public static void main(String[] args) {

double measurement;  // Numerical measurement, input by user.
String units;        // The unit of measure for the input, also
//    specified by the user.

double inches, feet, yards, miles;  // Measurement expressed in
//   each possible unit of
//   measure.

System.out.println("Enter measurements in inches, feet, yards, or miles.");
System.out.println("For example:  1 inch    17 feet    2.73 miles");
System.out.println("You can use abbreviations:   in   ft  yd   mi");
System.out.println("I will convert your input into the other units");
System.out.println("of measure.");
System.out.println();

while (true) {

/* Get the user's input, and convert units to lower case. */

System.out.print("Enter your measurement, or 0 to end:  ");
measurement = TextIO.getDouble();
if (measurement == 0)
break;  // Terminate the while loop.
units = TextIO.getlnWord();
units = units.toLowerCase();  // convert units to lower case

/* Convert the input measurement to inches. */

if (units.equals("inch") || units.equals("inches")
|| units.equals("in")) {
inches = measurement;
}
else if (units.equals("foot") || units.equals("feet")
|| units.equals("ft")) {
inches = measurement * 12;
}
else if (units.equals("yard") || units.equals("yards")
|| units.equals("yd")) {
inches = measurement * 36;
}
else if (units.equals("mile") || units.equals("miles")
|| units.equals("mi")) {
inches = measurement * 12 * 5280;
}
else {
System.out.println("Sorry, but I don't understand \""
+ units + "\".");
continue;  // back to start of while loop
}

/* Convert measurement in inches to feet, yards, and miles. */

feet = inches / 12;
yards = inches / 36;
miles = inches / (12*5280);

/* Output measurement in terms of each unit of measure. */

System.out.println();
System.out.println("That's equivalent to:");
System.out.printf("%12.5g", inches);
System.out.println(" inches");
System.out.printf("%12.5g", feet);
System.out.println(" feet");
System.out.printf("%12.5g", yards);
System.out.println(" yards");
System.out.printf("%12.5g", miles);
System.out.println(" miles");
System.out.println();

} // end while

System.out.println();
System.out.println("OK!  Bye for now.");

} // end main()

} // end class LengthConverter

(Note that this program uses formatted output with the "g" format specifier. In this program, we have no control over how large or how small the numbers might be. It could easily make sense for the user to enter very large or very small measurements. The "g" format will print a real number in exponential form if it is very large or very small, and in the usual decimal form otherwise. Remember that in the format specification %12.5g, the 5 is the total number of significant digits that are to be printed, so we will always get the same number of significant digits in the output, no matter what the size of the number. If we had used an "f" format specifier such as %12.5f, the output would be in decimal form with 5 digits after the decimal point. This would print the number 0.000000000745482 as 0.00000, with no significant digits at all! With the "g" format specifier, the output would be 7.4549e-10.)

3.5.4  The Empty Statement

As a final note in this section, I will mention one more type of statement in Java: the empty statement. This is a statement that consists simply of a semicolon and which tells the computer to do nothing. The existence of the empty statement makes the following legal, even though you would not ordinarily see a semicolon after a } :

if (x < 0) {
x = -x;
};

The semicolon is legal after the }, but the computer considers it to be an empty statement, not part of the if statement. Occasionally, you might find yourself using the empty statement when what you mean is, in fact, "do nothing." For example, the rather contrived if statement

if ( done )
;  // Empty statement
else
System.out.println( "Not done yet.");

does nothing when the boolean variable done is true, and prints out "Not done yet" when it is false. You can't just leave out the semicolon in this example, since Java syntax requires an actual statement between the if and the else. I prefer, though, to use an empty block, consisting of { and } with nothing between, for such cases.

Occasionally, stray empty statements can cause annoying, hard-to-find errors in a program. For example, the following program segment prints out "Hello" just once, not ten times:

for (i = 0; i < 10; i++);
System.out.println("Hello");

Why? Because the ";" at the end of the first line is a statement, and it is this empty statement that is executed ten times. The System.out.println statement is not really inside the for statement at all, so it is executed just once, after the for loop has completed. The for loop just does nothing, ten times!

Section 3.6

The switch Statement

The second branching statement in Java is the switch statement, which is introduced in this section. The switch statement is used far less often than the if statement, but it is sometimes useful for expressing a certain type of multiway branch.

3.6.1  The Basic switch Statement

A switch statement allows you to test the value of an expression and, depending on that value, to jump directly to some location within the switch statement. Only expressions of certain types can be used. The value of the expression can be one of the primitive integer types int, short, or byte. It can be the primitive char type. It can be String. Or it can be an enum type (see Subsection 2.3.4 for an introduction to enums). In particular, note that the expression cannot be a double or float value.

The positions within a switch statement to which it can jump are marked with case labels that take the form: "case constant:". The constant here is a literal of the same type as the expression in the switch. A case label marks the position the computer jumps to when the expression evaluates to the given constant value. As the final case in a switch statement you can, optionally, use the label "default:", which provides a default jump point that is used when the value of the expression is not listed in any case label.

A switch statement, as it is most often used, has the form:

switch (expression) {
case constant-1:
statements-1
break;
case constant-2:
statements-2
break;
.
.   // (more cases)
.
case constant-N:
statements-N
break;
default:  // optional default case
statements-(N+1)
} // end of switch statement

This has exactly the same effect as the following multiway if statement, but the switch statement can be more efficient because the computer can evaluate one expression and jump directly to the correct case, whereas in the if statement, the computer must evaluate up to N expressions before it knows which set of statements to execute:

if (expression == constant-1) { // but use .equals for String!!
statements-1
}
else if (expression == constant-2) {
statements-2
}
else
.
.
.
else if (expression == constant-N) {
statements-N
}
else {
statements-(N+1)
}

The break statements in the switch are technically optional. The effect of a break is to make the computer jump past the end of the switch statement, skipping over all the remaining cases. If you leave out the break statement, the computer will just forge ahead after completing one case and will execute the statements associated with the next case label. This is rarely what you want, but it is legal. (I will note here -- although you won't understand it until you get to the next chapter -- that inside a subroutine, the break statement is sometimes replaced by a return statement, which terminates the subroutine as well as the switch.)

Note that you can leave out one of the groups of statements entirely (including the break). You then have two case labels in a row, containing two different constants. This just means that the computer will jump to the same place and perform the same action for each of the two constants.

Here is an example of a switch statement. This is not a useful example, but it should be easy for you to follow. Note, by the way, that the constants in the case labels don't have to be in any particular order, but they must all be different:

switch ( N ) {   // (Assume N is an integer variable.)
case 1:
System.out.println("The number is 1.");
break;
case 2:
case 4:
case 8:
System.out.println("The number is 2, 4, or 8.");
System.out.println("(That's a power of 2!)");
break;
case 3:
case 6:
case 9:
System.out.println("The number is 3, 6, or 9.");
System.out.println("(That's a multiple of 3!)");
break;
case 5:
System.out.println("The number is 5.");
break;
default:
System.out.println("The number is 7 or is outside the range 1 to 9.");
}

The switch statement is pretty primitive as control structures go, and it's easy to make mistakes when you use it. Java takes all its control structures directly from the older programming languages C and C++. The switch statement is certainly one place where the designers of Java should have introduced some improvements.

3.6.2  Menus and switch Statements

One application of switch statements is in processing menus. A menu is a list of options. The user selects one of the options. The computer has to respond to each possible choice in a different way. If the options are numbered 1, 2, ..., then the number of the chosen option can be used in a switch statement to select the proper response.

In a TextIO-based program, the menu can be presented as a numbered list of options, and the user can choose an option by typing in its number. Here is an example that could be used in a variation of the LengthConverter example from the previous section:

int optionNumber;   // Option number from menu, selected by user.
double measurement; // A numerical measurement, input by the user.
//    The unit of measurement depends on which
//    option the user has selected.
double inches;      // The same measurement, converted into inches.

/* Display menu and get user's selected option number. */

System.out.println("What unit of measurement does your input use?");
System.out.println();
System.out.println("         1.  inches");
System.out.println("         2.  feet");
System.out.println("         3.  yards");
System.out.println("         4.  miles");
System.out.println();
System.out.println("Enter the number of your choice: ");
optionNumber = TextIO.getlnInt();

/* Read user's measurement and convert to inches. */

switch ( optionNumber ) {
case 1:
System.out.println("Enter the number of inches: ");
measurement = TextIO.getlnDouble();
inches = measurement;
break;
case 2:
System.out.println("Enter the number of feet: ");
measurement = TextIO.getlnDouble();
inches = measurement * 12;
break;
case 3:
System.out.println("Enter the number of yards: ");
measurement = TextIO.getlnDouble();
inches = measurement * 36;
break;
case 4:
System.out.println("Enter the number of miles: ");
measurement = TextIO.getlnDouble();
inches = measurement * 12 * 5280;
break;
default:
System.out.println("Error!  Illegal option number!  I quit!");
System.exit(1);
} // end switch

/* Now go on to convert inches to feet, yards, and miles... */

This example could instead be written using a String in the switch statement:

String units;       // Unit of measurement, entered by user.
double measurement; // A numerical measurement, input by the user.
double inches;      // The same measurement, converted into inches.

/* Read the user's unit of measurement. */

System.out.println("What unit of measurement does your input use?");
System.out.print("Legal responses: inches, feet, yards, or miles : ");
units = TextIO.getln().toLowerCase();

/* Read user's measurement and convert to inches. */

System.out.print("Enter the number of " + units + ":  ");
measurement = TextIO.getlnDouble();

switch ( units ) {
case "inches":
inches = measurement;
break;
case "feet":
inches = measurement * 12;
break;
case "yards":
inches = measurement * 36;
break;
case "miles":
inches = measurement * 12 * 5280;
break;
default:
System.out.println("Wait a minute!  Illegal unit of measure!  I quit!");
System.exit(1);
} // end switch

3.6.3  Enums in switch Statements

The type of the expression in a switch can be an enumerated type. In that case, the constants in the case labels must be values from the enumerated type. For example, suppose that the type of the expression is the enumerated type Season defined by

enum Season { SPRING, SUMMER, FALL, WINTER }

and that the expression in a switch statement is an expression of type Season. The constants in the case label must be chosen from among the values Season.SPRING, Season.SUMMER, Season.FALL, or Season.WINTER. However, there is a quirk in the syntax: when an enum constant is used in a case label, only the simple name, such as "SPRING" is used, not the full name, such as "Season.SPRING". Of course, the computer already knows that the value in the case label must belong to the enumerated type, since it can tell that from the type of expression used, so there is really no need to specify the type name in the constant. For example, assuming that currentSeason is a variable of type Season, then we could have the switch statement:

switch ( currentSeason ) {
case WINTER:    // ( NOT Season.WINTER ! )
System.out.println("December, January, February");
break;
case SPRING:
System.out.println("March, April, May");
break;
case SUMMER:
System.out.println("June, July, August");
break;
case FALL:
System.out.println("September, October, November");
break;
}

3.6.4  Definite Assignment and switch Statements

As a somewhat more realistic example, the following switch statement makes a random choice among three possible alternatives. Recall that the value of the expression (int)(3*Math.random()) is one of the integers 0, 1, or 2, selected at random with equal probability, so the switch statement below will assign one of the values "Rock", "Paper", "Scissors" to computerMove, with probability 1/3 for each case:

switch ( (int)(3*Math.random()) ) {
case 0:
computerMove = "Rock";
break;
case 1:
computerMove = "Paper";
break;
case 2:
computerMove = "Scissors";
break;
}

Now, this switch statement is perfectly OK, but suppose that we use it in the following code segment:

String computerMove;
switch ( (int)(3*Math.random()) ) {
case 0:
computerMove = "Rock";
break;
case 1:
computerMove = "Paper";
break;
case 2:
computerMove = "Scissors";
break;
}
System.out.println("The computer's move is " + computerMove);  // ERROR!

Now there is a subtle error on the last line! The problem here is due to definite assignment, the idea that the Java compiler must be able to determine that a variable has definitely been assigned a value before its value is used. Definite assignment was introduced in Subsection 3.1.4. In this example, it's true that the three cases in the switch cover all the possibilities, but the compiler is not smart enough to figure that out; it just sees that there is an integer-valued expression in the switch but not all possible integer values are covered by the given cases.

A simple solution is to replace the final case in the switch statement with default. With a default case, all possible values of the expression in the switch are certainly covered, and the compiler knows that computerMove is definitely assigned a value:

String computerMove;
switch ( (int)(3*Math.random()) ) {
case 0:
computerMove = "Rock";
break;
case 1:
computerMove = "Paper";
break;
default:
computerMove = "Scissors";
break;
}
System.out.println("The computer's move is " + computerMove);  // OK!

Section 3.7

Introduction to Exceptions and try..catch

In addition to the control structures that determine the normal flow of control in a program, Java has a way to deal with "exceptional" cases that throw the flow of control off its normal track. When an error occurs during the execution of a program, the default behavior is to terminate the program and to print an error message. However, Java makes it possible to "catch" such errors and program a response different from simply letting the program crash. This is done with the try..catch statement. In this section, we will take a preliminary and incomplete look the try..catch statement, leaving out a lot of the rather complex syntax of this statement. Error handling is a complex topic, which we will return to in Chapter 8, and we will cover the full syntax of try..catch at that time.

3.7.1  Exceptions

The term exception is used to refer to the type of error that one might want to handle with a try..catch. An exception is an exception to the normal flow of control in the program. The term is used in preference to "error" because in some cases, an exception might not be considered to be an error at all. You can sometimes think of an exception as just another way to organize a program.

Exceptions in Java are represented as objects of type Exception. Actual exceptions are usually defined by subclasses of Exception. Different subclasses represent different types of exceptions. We will look at only two types of exception in this section: NumberFormatException and IllegalArgumentException.

A NumberFormatException can occur when an attempt is made to convert a string into a number. Such conversions are done by the functions Integer.parseInt and Double.parseDouble. (See Subsection 2.5.7.) Consider the function call Integer.parseInt(str) where str is a variable of type String. If the value of str is the string "42", then the function call will correctly convert the string into the int 42. However, if the value of str is, say, "fred", the function call will fail because "fred" is not a legal string representation of an int value. In this case, an exception of type NumberFormatException occurs. If nothing is done to handle the exception, the program will crash.

An IllegalArgumentException can occur when an illegal value is passed as a parameter to a subroutine. For example, if a subroutine requires that a parameter be greater than or equal to zero, an IllegalArgumentException might occur when a negative value is passed to the subroutine. How to respond to the illegal value is up to the person who wrote the subroutine, so we can't simply say that every illegal parameter value will result in an IllegalArgumentException. However, it is a common response.

3.7.2  try..catch

When an exception occurs, we say that the exception is "thrown". For example, we say that Integer.parseInt(str) throws an exception of type NumberFormatException when the value of str is illegal. When an exception is thrown, it is possible to "catch" the exception and prevent it from crashing the program. This is done with a try..catch statement. In simplified form, the syntax for a try..catch can be:

try {
statements-1
}
catch ( exception-class-name  variable-name ) {
statements-2
}

The exception-class-name could be NumberFormatException, IllegalArgumentException, or some other exception class. When the computer executes this try..catch statement, it executes the statements in the try part. If no exception occurs during the execution of statements-1, then the computer just skips over the catch part and proceeds with the rest of the program. However, if an exception of type exception-class-name occurs during the execution of statements-1, the computer immediately jumps from the point where the exception occurs to the catch part and executes statements-2, skipping any remaining statements in statements-1. Note that only one type of exception is caught; if some other type of exception occurs during the execution of statements-1, it will crash the program as usual.

During the execution of statements-2, the variable-name represents the exception object, so that you can, for example, print it out. The exception object contains information about the cause of the exception. This includes an error message, which will be displayed if you print out the exception object.

After the end of the catch part, the computer proceeds with the rest of the program; the exception has been caught and handled and does not crash the program.

By the way, note that the braces, { and }, are part of the syntax of the try..catch statement. They are required even if there is only one statement between the braces. This is different from the other statements we have seen, where the braces around a single statement are optional.

As an example, suppose that str is a variable of type String whose value might or might not represent a legal real number. Then we could say:

double x;
try {
x = Double.parseDouble(str);
System.out.println( "The number is " + x );
}
catch ( NumberFormatException e ) {
System.out.println( "Not a legal number." );
x = Double.NaN;
}

If an error is thrown by the call to Double.parseDouble(str), then the output statement in the try part is skipped, and the statement in the catch part is executed. (In this example, I set x to be the value Double.NaN when an exception occurs. Double.NaN is the special "not-a-number" value for type double.)

It's not always a good idea to catch exceptions and continue with the program. Often that can just lead to an even bigger mess later on, and it might be better just to let the exception crash the program at the point where it occurs. However, sometimes it's possible to recover from an error.

Suppose, for example, we want a program that will find the average of a sequence of real numbers entered by the user, and we want the user to signal the end of the sequence by entering a blank line. (This is similar to the sample program ComputeAverage.java from Section 3.3, but in that program the user entered a zero to signal end-of-input.) If we use TextIO.getlnInt() to read the user's input, we will have no way of detecting the blank line, since that function simply skips over blank lines. A solution is to use TextIO.getln() to read the user's input. This allows us to detect a blank input line, and we can convert non-blank inputs to numbers using Double.parseDouble. And we can use try..catch to avoid crashing the program when the user's input is not a legal number. Here's the program:

public class ComputeAverage2 {

public static void main(String[] args) {
String str;     // The user's input.
double number;  // The input converted into a number.
double total;   // The total of all numbers entered.
double avg;     // The average of the numbers.
int count;      // The number of numbers entered.
total = 0;
count = 0;
System.out.println("Enter your numbers, press return to end.");
while (true) {
System.out.print("? ");
str = TextIO.getln();
if (str.equals("")) {
break; // Exit the loop, since the input line was blank.
}
try {
number = Double.parseDouble(str);
// If an error occurs, the next 2 lines are skipped!
total = total + number;
count = count + 1;
}
catch (NumberFormatException e) {
System.out.println("Not a legal number!  Try again.");
}
}
avg = total/count;
System.out.printf("The average of %d numbers is %1.6g%n", count, avg);
}

}

3.7.3  Exceptions in TextIO

When TextIO reads a numeric value from the user, it makes sure that the user's response is legal, using a technique similar to the while loop and try..catch in the previous example. However, TextIO can read data from other sources besides the user. (See Subsection 2.4.4.) When it is reading from a file, there is no reasonable way for TextIO to recover from an illegal value in the input, so it responds by throwing an exception. To keep things simple, TextIO only throws exceptions of type IllegalArgumentException, no matter what type of error it encounters. For example, an exception will occur if an attempt is made to read from a file after all the data in the file has already been read. In TextIO, the exception is of type IllegalArgumentException. If you have a better response to file errors than to let the program crash, you can use a try..catch to catch exceptions of type IllegalArgumentException.

As an example, we will look at yet another number-averaging program. In this case, we will read the numbers from a file. Assume that the file contains nothing but real numbers, and we want a program that will read the numbers and find their sum and their average. Since it is unknown how many numbers are in the file, there is the question of when to stop reading. One approach is simply to try to keep reading indefinitely. When the end of the file is reached, an exception occurs. This exception is not really an error -- it's just a way of detecting the end of the data, so we can catch the exception and finish up the program. We can read the data in a while (true) loop and break out of the loop when an exception occurs. This is an example of the somewhat unusual technique of using an exception as part of the expected flow of control in a program.

To read from the file, we need to know the file's name. To make the program more general, we can let the user enter the file name, instead of hard-coding a fixed file name in the program. However, it is possible that the user will enter the name of a file that does not exist. When we use TextIO.readfile to open a file that does not exist, an exception of type IllegalArgumentException occurs. We can catch this exception and ask the user to enter a different file name. Here is a complete program that uses all these ideas:

/**
* This program reads numbers from a file.  It computes the sum and
* the average of the numbers that it reads.  The file should contain
* nothing but numbers of type double; if this is not the case, the
* output will be the sum and average of however many numbers were
* successfully read from the file.  The name of the file will be
* input by the user.
*/
public class AverageNumbersFromFile {

public static void main(String[] args) {

while (true) {
String fileName;  // The name of the file, to be input by the user.
System.out.print("Enter the name of the file: ");
fileName = TextIO.getln();
try {
TextIO.readFile( fileName );  // Try to open the file for input.
break;  // If that succeeds, break out of the loop.
}
catch ( IllegalArgumentException e ) {
System.out.println("Can't read from the file \"" + fileName + "\".");
System.out.println("Please try again.\n");
}
}

/* At this point, TextIO is reading from the file. */

double number;  // A number read from the data file.
double sum;     // The sum of all the numbers read so far.
int count;      // The number of numbers that were read.

sum = 0;
count = 0;

try {
while (true) { // Loop ends when an exception occurs.
number = TextIO.getDouble();
count++;  // This is skipped when the exception occurs
sum += number;
}
}
catch ( IllegalArgumentException e ) {
// We expect this to occur when the end-of-file is encountered.
// We don't consider this to be an error, so there is nothing to do
// in this catch clause.  Just proceed with the rest of the program.
}

// At this point, we've read the entire file.

System.out.println();
System.out.println("Number of data values read: " + count);
System.out.println("The sum of the data values: " + sum);
if ( count == 0 )
System.out.println("Can't compute an average of 0 values.");
else
System.out.println("The average of the values:  " + (sum/count));

}

}

Section 3.8

Introduction to Arrays

In previous sections of this chapter, we have already covered all of Java's control structures. But before moving on to the next chapter, we will take preliminary looks at two additional topics that are at least somewhat related to control structures.

This section is an introduction to arrays. Arrays are a basic and very commonly used data structure, and array processing is often an exercise in using control structures. The next section will introduce computer graphics and will allow you to apply what you know about control structures in another context.

3.8.1  Creating and Using Arrays

A data structure consists of a number of data items chunked together so that they can be treated as a unit. An array is a data structure in which the items are arranged as a numbered sequence, so that each individual item can be referred to by its position number. In Java -- but not in other programming languages -- all the items must be of the same type, and the numbering always starts at zero. You will need to learn several new terms to talk about arrays: The number of items in an array is called the length of the array. The type of the individual items in an array is called the base type of the array. And the position number of an item in an array is called the index of that item.

Suppose that you want to write a program that will process the names of, say, one thousand people. You will need a way to deal with all that data. Before you knew about arrays, you might have thought that the program would need a thousand variables to hold the thousand names, and if you wanted to print out all the names, you would need a thousand print statements. Clearly, that would be ridiculous! In reality, you can put all the names into an array. The array is a represented by a single variable, but it holds the entire list of names. The length of the array would be 1000, since there are 1000 individual names. The base type of the array would be String since the items in the array are strings. The first name would be at index 0 in the array, the second name at index 1, and so on, up to the thousandth name at index 999.

The base type of an array can be any Java type, but for now, we will stick to arrays whose base type is String or one of the eight primitive types. If the base type of an array is int, it is referred to as an "array of ints." An array with base type String is referred to as an "array of Strings." However, an array is not, properly speaking, a list of integers or strings or other values. It is better thought of as a list of variables of type int, or a list of variables of type String, or of some other type. As always, there is some potential for confusion between the two uses of a variable: as a name for a memory location and as a name for the value stored in that memory location. Each position in an array acts as a variable. Each position can hold a value of a specified type (the base type of the array), just as a variable can hold a value. The value can be changed at any time, just as the value of a variable can be changed. The items in an array -- really, the individual variables that make up the array -- are more often referred to as the elements of the array.

As I mentioned above, when you use an array in a program, you can use a variable to refer to array as a whole. But you often need to refer to the individual elements of the array. The name for an element of an array is based on the name for the array and the index number of the element. The syntax for referring to an element looks, for example, like this: namelist[7]. Here, namelist is the variable that names the array as a whole, and namelist[7] refers to the element at index 7 in that array. That is, to refer to an element of an array, you use the array name, followed by element index enclosed in square brackets. An element name of this form can be used like any other variable: You can assign a value to it, print it out, use it in an expression.

An array also contains a kind of variable representing its length. For example, you can refer to the length of the array namelist as namelist.length. However, you cannot assign a value to namelist.length, since the length of an array cannot be changed.

Before you can use a variable to refer to an array, that variable must be declared, and it must have a type. For an array of Strings, for example, the type for the array variable would be String[], and for an array of ints, it would be int[]. In general, an array type consists of the base type of the array followed by a pair of empty square brackets. Array types can be used to declare variables; for example,

String[] namelist;
int[] A;
double[] prices;

and variables declared in this way can refer to arrays. However, declaring a variable does not make the actual array. Like all variables, an array variable has to be assigned a value before it can be used. In this case, the value is an array. Arrays have to be created using a special syntax. (The syntax is related to the fact that arrays in Java are actually objects, but that doesn't need to concern us here.) Arrays are created with an operator named new. Here are some examples:

namelist = new String[1000];
A = new int[5];
prices = new double[100];

The general syntax is

array-variable = new base-type[array-length];

The length of the array can be given as either an integer or an integer-valued expression. For example, after the assignment statement "A = new int[5];", A is an array containing the five integer elements A[0], A[1], A[2], A[3], and A[4]. Also, A.length would have the value 5. It's useful to have a picture in mind:

When you create an array of int, each element of the array is automatically initialized to zero. Any array of numbers is filled with zeros when it is created. An array of boolean is filled with the value false. And an array of char is filled with the character that has Unicode code number zero. (For an array of String, the initial value is null, a special value used for objects that we won't encounter officially until Section 5.1.)

3.8.2  Arrays and For Loops

A lot of the real power of arrays comes from the fact that the index of an element can be given by an integer variable or even an integer-valued expression. For example, if list is an array and i is a variable of type int, then you can use list[i] and even list[2*i+1] as variable names. The meaning of list[i] depends on the value of i. This becomes especially useful when we want to process all the elements of an array, since that can be done with a for loop. For example, to print out all the items in an array, list, we can just write

int i;  // the array index
for (i = 0; i < list.length; i++) {
System.out.println( list[i] );
}

The first time through the loop, i is 0, and list[i] refers to list[0]. So, it is the value stored in the variable list[0] that is printed. The second time through the loop, i is 1, and the value stored in list[1] is printed. The loop ends after printing the value of list[4], when i becomes equal to 5 and the continuation condition "i < list.length" is no longer true. This is a typical example of using a loop to process an array.

Let's look at a few more examples. Suppose that A is an array of double, and we want to find the average of all the elements of the array. We can use a for loop to add up the numbers, and then divide by the length of the array to get the average:

double total;    // The sum of the numbers in the array.
double average;  // The average of the numbers.
int i;  // The array index.
total = 0;
for ( i = 0; i < A.length; i++ ) {
total = total + A[i];  // Add element number i to the total.
}
average = total / A.length;  // A.length is the number of items

Another typical problem is to find the largest number in the array A. The strategy is to go through the array, keeping track of the largest number found so far. We'll store the largest number found so far in a variable called max. As we look through the array, whenever we find a number larger than the current value of max, we change the value of max to that larger value. After the whole array has been processed, max is the largest item in the array overall. The only question is, what should the original value of max be? One possibility is to start with max equal to A[0], and then to look through the rest of the array, starting from A[1], for larger items:

double max;  // The largest number seen so far.
max = A[0];   // At first, the largest number seen is A[0].
int i;
for ( i = 1; i < A.length; i++ ) {
if (A[i] > max) {
max = A[i];
}
}
// at this point, max is the largest item in A

Sometimes, you only want to process some elements of the array. In that case, you can use an if statement inside the for loop to decide whether or not to process a given element. Let's look at the problem of averaging the elements of an array, but this time, suppose that we only want to average the non-zero elements. In this case, the number of items that we add up can be less than the length of the array, so we will need to keep a count of the number of items added to the sum:

double total;    // The sum of the non-zero numbers in the array.
int count;       // The number of non-zero numbers.
double average;  // The average of the non-zero numbers.
int i;
total = 0;
count = 0;
for ( i = 0; i < A.length; i++ ) {
if ( A[i] != 0 ) {
total = total + A[i];  // Add element to the total
count = count + 1;     //      and count it.
}
}
if (count == 0) {
System.out.println("There were no non-zero elements.");
}
else {
average = total / count;  // Divide by number of items
System.out.printf("Average of %d elements is %1.5g%n",
count, average);
}

3.8.3  Random Access

So far, my examples of array processing have used sequential access. That is, the elements of the array were processed one after the other in the sequence in which they occur in the array. But one of the big advantages of arrays is that they allow random access. That is, every element of the array is equally accessible at any given time.

As an example, let's look at a well-known problem called the birthday problem: Suppose that there are N people in a room. What's the chance that there are two people in the room who have the same birthday? (That is, they were born on the same day in the same month, but not necessarily in the same year.) Most people severely underestimate the probability. We will actually look at a different version of the question: Suppose you choose people at random and check their birthdays. How many people will you check before you find one who has the same birthday as someone you've already checked? Of course, the answer in a particular case depends on random factors, but we can simulate the experiment with a computer program and run the program several times to get an idea of how many people need to be checked on average.

To simulate the experiment, we need to keep track of each birthday that we find. There are 365 different possible birthdays. (We'll ignore leap years.) For each possible birthday, we need to keep track of whether or not we have already found a person who has that birthday. The answer to this question is a boolean value, true or false. To hold the data for all 365 possible birthdays, we can use an array of 365 boolean values:

boolean[] used;
used = new boolean[365];

For this problem, the days of the year are numbered from 0 to 364. The value of used[i] is true if someone has been selected whose birthday is day number i. Initially, all the values in the array are false. (Remember that this is done automatically when the array is created.) When we select someone whose birthday is day number i, we first check whether used[i] is true. If it is true, then this is the second person with that birthday. We are done. On the other hand, if used[i] is false, we set used[i] to be true to record the fact that we've encountered someone with that birthday, and we go on to the next person. Here is a program that carries out the simulated experiment (of course, in the program, there are no simulated people, only simulated birthdays):

/**
* Simulate choosing people at random and checking the day of the year they
* were born on.  If the birthday is the same as one that was seen previously,
* stop, and output the number of people who were checked.
*/
public class BirthdayProblem {

public static void main(String[] args) {

boolean[] used;  // For recording the possible birthdays
//   that have been seen so far.  A value
//   of true in used[i] means that a person
//   whose birthday is the i-th day of the
//   year has been found.

int count;       // The number of people who have been checked.

used = new boolean[365];  // Initially, all entries are false.

count = 0;

while (true) {
// Select a birthday at random, from 0 to 364.
// If the birthday has already been used, quit.
// Otherwise, record the birthday as used.

int birthday;  // The selected birthday.
birthday = (int)(Math.random()*365);
count++;

System.out.printf("Person %d has birthday number %d", count, birthday);
System.out.println();

if ( used[birthday] ) {
// This day was found before; it's a duplicate.  We are done.
break;
}

used[birthday] = true;

} // end while

System.out.println();
System.out.println("A duplicate birthday was found after "
+ count + " tries.");
}

} // end class BirthdayProblem

You should study the program to understand how it works and how it uses the array. Also, try it out! You will probably find that a duplicate birthday tends to occur sooner than you expect.

3.8.4  Partially Full Arrays

Consider an application where the number of items that we want to store in an array changes as the program runs. Since the size of the array can't be changed, a separate counter variable must be used to keep track of how many spaces in the array are in use. (Of course, every space in the array has to contain something; the question is, how many spaces contain useful or valid items?)

Consider, for example, a program that reads positive integers entered by the user and stores them for later processing. The program stops reading when the user inputs a number that is less than or equal to zero. The input numbers can be kept in an array, numbers, of type int[]. Let's say that no more than 100 numbers will be input. Then the size of the array can be fixed at 100. But the program must keep track of how many numbers have actually been read and stored in the array. For this, it can use an integer variable. Each time a number is stored in the array, we have to count it; that is, value of the counter variable must be incremented by one. One question is, when we add a new item to the array, where do we put it? Well, if the number of items is count, then they would be stored in the array in positions number 0, 1, ..., (count-1). The next open spot would be position number count, so that's where we should put the new item.

As a rather silly example, let's write a program that will read the numbers input by the user and then print them in the reverse of the order in which they were entered. Assume that an input value equal to zero marks the end of the data. (This is, at least, a processing task that requires that the numbers be saved in an array. Note that many types of processing, such as finding the sum or average or maximum of the numbers, can be done without saving the individual numbers.)

public class ReverseInputNumbers {

public static void main(String[] args) {

int[] numbers;  // An array for storing the input values.
int count;      // The number of numbers saved in the array.
int num;        // One of the numbers input by the user.
int i;          // for-loop variable.

numbers = new int[100];   // Space for 100 ints.
count = 0;                // No numbers have been saved yet.

System.out.println("Enter up to 100 positive integers; enter 0 to end.");

while (true) {   // Get the numbers and put them in the array.
System.out.print("? ");
num = TextIO.getlnInt();
if (num <= 0) {
// Zero marks the end of input; we have all the numbers.
break;
}
numbers[count] = num;  // Put num in position count.
count++;  // Count the number
}

System.out.println("\nYour numbers in reverse order are:\n");

for ( i = count - 1; i >= 0; i-- ) {
System.out.println( numbers[i] );
}

} // end main();

}  // end class ReverseInputNumbers

It is especially important to note how the variable count plays a dual role. It is the number of items that have been entered into the array. But it is also the index of the next available spot in the array.

When the time comes to print out the numbers in the array, the last occupied spot in the array is location count - 1, so the for loop prints out values starting from location count - 1 and going down to 0. This is also a nice example of processing the elements of an array in reverse order.

You might wonder what would happen in this program if the user tries to input more than 100 numbers. The result would be an error that would crash the program. When the user enters the 101-st number, the program tries to store that number in an array element number[100]. However, there is no such array element. There are only 100 items in the array, and the index of the last item is 99. The attempt to use number[100] generates an exception of type ArrayIndexOutOfBoundsException. Exceptions of this type are a common source of run-time errors in programs that use arrays.

3.8.5  Two-dimensional Arrays

The arrays that we have considered so far are "one-dimensional." This means that the array consists of a sequence of elements that can be thought of as being laid out along a line. It is also possible to have two-dimensional arrays, where the elements can be laid out in a rectangular grid. We consider them only briefly here, but will return to the topic in Section 7.5.

In a two-dimensional, or "2D," array, the elements can be arranged in rows and columns. Here, for example, is a 2D array of int that has five rows and seven columns:

This 5-by-7 grid contains a total of 35 elements. The rows in a 2D array are numbered 0, 1, 2, ..., up to the number of rows minus one. Similarly, the columns are numbered from zero up to the number of columns minus one. Each individual element in the array can be picked out by specifying its row number and its column number. (The illustration shown here is not what the array actually looks like in the computer's memory, but it does show the logical structure of the array.)

In Java, the syntax for two-dimensional arrays is similar to the syntax for one-dimensional arrays, except that an extra index is involved, since picking out an element requires both a row number and a column number. For example, if A is a 2D array of int, then A[3][2] would be the element in row 3, column 2. That would pick out the number 17 in the array shown above. The type for A would be given as int[][], with two pairs of empty brackets. To declare the array variable and create the array, you could say,

int[][]  A;
A  =  new int[5][7];

The second line creates a 2D array with 5 rows and 7 columns. Two-dimensional arrays are often processed using nested for loops. For example, the following code segment will print out the elements of A in neat columns:

int row, col;  // loop-control-variables for accessing rows and columns in A
for ( row = 0; row < 5; row++ ) {
for ( col = 0; col < 7; col++ ) {
System.out.printf( "%7d",  A[row][col] );
}
System.out.println();
}

The base type of a 2D array can be anything, so you can have arrays of type double[][], String[][], and so on.

There are some natural uses for 2D arrays. For example, a 2D array can be used to store the contents of the board in a game such as chess or checkers. And an example in Subsection 4.6.3 uses a 2D array to hold the colors of a grid of colored squares. But sometimes two-dimensional arrays are used in problems in which the grid is not so visually obvious. Consider a company that owns 25 stores. Suppose that the company has data about the profit earned at each store for each month in the year 2014. If the stores are numbered from 0 to 24, and if the twelve months from January 2014 through December 2014 are numbered from 0 to 11, then the profit data could be stored in an array, profit, created as follows:

double[][]  profit;
profit  =  new double[25][12];

profit[3][2] would be the amount of profit earned at store number 3 in March, and more generally, profit[storeNum][monthNum] would be the amount of profit earned in store number storeNum in month number monthNum (where the numbering, remember, starts from zero).

Let's assume that the profit array has already been filled with data. This data can be processed in a lot of interesting ways. For example, the total profit for the company -- for the whole year from all its stores -- can be calculated by adding up all the entries in the array:

double totalProfit;  // Company's total profit in 2014.
int store, month;  // variables for looping through the stores and the months
totalProfit = 0;
for ( store = 0; store < 25; store++ ) {
for ( month = 0; month < 12; month++ )
totalProfit += profit[store][month];
}

Sometimes it is necessary to process a single row or a single column of an array, not the entire array. For example, to compute the total profit earned by the company in December, that is, in month number 11, you could use the loop:

double decemberProfit;
int storeNum;
decemberProfit = 0.0;
for ( storeNum = 0; storeNum < 25; storeNum++ ) {
decemberProfit += profit[storeNum][11];
}

Two-dimensional arrays are sometimes useful, but they are much less common than one-dimensional arrays. Java actually allows arrays of even higher dimension, but they are only rarely encountered in practice.

Section 3.9

Introduction to GUI Programming

For the past two chapters, you've been learning the sort of programming that is done inside a single subroutine, "programming in the small." In the rest of this book, we'll be more concerned with the larger scale structure of programs, but the material that you've already learned will be an important foundation for everything to come. In this section, we see how techniques that you have learned so far can be applied in the context of graphical programming.

When you run a GUI program, it opens one or more windows on your computer screen. As a programmer, you can have complete control over what appears in the window and how the user can interact with it. For our first encounter, we look at one simple example: the ability of a program to display simple shapes like rectangles and lines in the window, with no user interaction. For now, the main point is to take a look at how programming-in-the-small can be used in other contexts besides text-based, command-line-style programs. You will see that a knowledge of programming-in-the-small applies to writing the guts of any subroutine, not just main().

3.9.1  Drawing Shapes

To understand computer graphics, you need to know a little about pixels and coordinate systems. The computer screen is made up of small squares called pixels, arranged in rows and columns, usually about 100 pixels per inch. The computer controls the color of the pixels, and drawing is done by changing the colors of individual pixels. Each pixel has a pair of integer coordinates, often called x and y, that specify the pixel's horizontal and vertical position. For a graphics context drawing to a rectangular area on the screen, the coordinates of the pixel in the upper left corner of the rectangle are (0,0). The x coordinate increases from the left to right, and the y coordinate increases from top to bottom. Shapes are specified using pixels. For example, a rectangle is specified by the x and y coordinates of its upper left corner and by its width and height measured in pixels. Here's a picture of a rectangular drawing area, showing the ranges of x and y coordinates. The "width" and "height" in this picture give the size of the drawing area, in pixels:

Assuming that the drawing area is 800-by-500 pixels, the rectangle in the upper left of the picture would have, approximately, width 200, height 150, and upper left corner at coordinates (50,50).

Drawing in Java is done using a graphics context. A graphics context is an object. As an object, it can include subroutines and data. Among the subroutines in a graphics context are routines for drawing basic shapes such as lines, rectangles, ovals, and text. (When text appears on the screen, the characters have to be drawn there by the computer, just like the computer draws any other shapes.) Among the data in a graphics context are the color and font that are currently selected for drawing. (A font determines the style and size of characters.) One other piece of data in a graphics context is the "drawing surface" on which the drawing is done. Generally, the drawing surface is a rectangle on the computer screen, although it can be other surfaces such as a page to be printed. Different graphics context objects can draw to different drawing surfaces. For us, the drawing surface will be the content area of a window, not including its border or title bar.

A graphics context is represented by a variable. The type for the variable is Graphics (just like the type for a string variable is String). The variable is often named g, but the name of the variable is of course up to the programmer. Here are a few of the subroutines that are available in a graphics context g:

- g.setColor(c), is called to set the color to be used for drawing. The parameter, c is an object belonging to a class named Color. There are about a dozen constants representing standard colors that can be used as the parameter in this subroutine. The standard colors include Color.BLACK, Color.WHITE, Color.LIGHT_GRAY, Color.RED, Color.GREEN, and Color.BLUE. (Later, we will see that it is also possible to create new colors.) For example, if you want to draw in red, you would say "g.setColor(Color.RED);". The specified color is used for all subsequent drawing operations up until the next time g.setColor() is called.

- g.drawLine(x1,y1,x2,y2) draws a line from the point with coordinates (x1,y1) to the point with coordinates (x2,y2).

- g.drawRect(x,y,w,h) draws the outline of a rectangle with vertical and horizontal sides. The parameters x, y, w, and h must be integers or integer-valued expressions. This subroutine draws the outline of the rectangle whose top-left corner is x pixels from the left edge of the drawing area and y pixels down from the top. The width of the rectangle is w pixels, and the height is h pixels. The color that is used is black, unless a different color has been set by calling g.setColor().

- g.fillRect(x,y,w,h) is similar to g.drawRect() except that it fills in the inside of the rectangle instead of drawing an outline.

- g.drawOval(x,y,w,h) draws the outline of an oval. The oval just fits inside the rectangle that would be drawn by g.drawRect(x,y,w,h). To get a circle, use the same values for w and for h.

- g.fillOval(x,y,w,h) is similar to g.drawOval() except that it fills in the inside of the oval instead of drawing an outline.

This is enough information to draw some pictures using Java graphics. To start with something simple, let's say that we want to draw a set of ten parallel lines, something like this:

Let's say that the lines are 200 pixels long and that the distance from each line to the next is 10 pixels, and let's put the start of the first line at the pixel with coordinates (100,50). To draw one line, we just have to call g.drawLine(x1,y1,x2,y2) with appropriate values for the parameters. Now, all the lines start at x-coordinate 100, so we can use the constant 100 as the value for x1. Since the lines are 200 pixels long, we can use the constant 300 as the value for x2. The y-coordinates of the lines are different, but we can see that both endpoints of a line have the same y-coordinates, so we can use a single variable as the value for y1 and for y2. Using y as the name of that variable, the command for drawing one of the lines becomes g.drawLine(100,y,300,y). The value of y is 50 for the top line and increases by 10 each time we move down from one line to the next. We just need to make sure that y takes on the correct sequence of values. We can use a for loop that counts from 1 to 10:

int y;   // y-coordinate for the line
int i;   // loop control variable
y = 50;  // y starts at 50 for the first line
for ( i = 1; i <= 10; i++ ) {
g.drawLine( 100, y, 300, y );
y = y + 10;  // increase y by 10 before drawing the next line.
}

Alternatively, we could use y itself as the loop control variable, noting that the value of y for the last line is 140:

int y;
for ( y = 50; y <= 140; y = y + 10 )
g.drawLine( 100, y, 300, y );

If we wanted to set the color of the lines, we could do that by calling g.setColor() before drawing them. If we just draw the lines without setting the color, they will be black.

For something a little more complicated, let's draw a large number of randomly colored, randomly positioned, filled circles. Since we only know a few colors, I will randomly select the color to be red, green, or blue. That can be done with a simple switch statement, similar to the ones in Subsection 3.6.4:

switch ( (int)(3*Math.random()) ) {
case 0:
g.setColor( Color.RED );
break;
case 1:
g.setColor( Color.GREEN );
break;
case 2:
g.setColor( Color.BLUE );
break;
}

I will choose the center points of the circles at random. Let's say that the width of the drawing area is given by a variable, width. Then we want a random value in the range 0 to width-1 for the horizontal position of the center. Similarly, the vertical position of the center will a random value in the range 0 to height-1. That leaves the size of the circle to be determined; I will make the radius of each circle equal to 50 pixels. We can draw the circle with a statement of the form g.fillOval(x,y,w,h). However, in this command, x and y are not the coordinates of the center of the circle; they are the upper left corner of a rectangle drawn around the circle. To get values for x and y, we have to move back from the center of the circle by 50 pixels, an amount equal to the radius of the circle. The parameters w and h give the width and height of the rectangle, which has to be twice the radius, or 100 pixels in this case. Taking all this into account, here is a code segment for drawing a random circle:

centerX = (int)(width*Math.random());
centerY = (int)(height*Math.random());
g.fillOval( centerX - 50, centerY - 50, 100, 100 );

This code comes after the color-setting code given above. In the end, I found that the picture looks better if I also draw a black outline around each filled circle, so I added this code at the end:

g.setColor( Color.BLACK );
g.drawOval( centerX - 50, centerY - 50, 100, 100 );

Finally, to get a large number of circles, I put all of the above code into a for loop that runs for 500 executions. Here's a typical drawing from the program, shown at reduced size:

3.9.2  Drawing in a Program

Now, as you know, you can't just have a bunch of Java code standing by itself. The code has to be inside a subroutine definition that is itself inside a class definition. In fact, for my circle-drawing program, the complete subroutine for drawing the picture looks like this:

public void drawFrame(Graphics g, int frameNumber, int width, int height) {

int centerX;     // The x-coord of the center of a disk.
int centerY;     // The y-coord of the center of a disk.
int colorChoice; // Used to select a random color.
int count;       // Loop control variable for counting disks.

for (count = 0; count < 500; count++) {

colorChoice = (int)(3*Math.random());
switch (colorChoice) {
case 0:
g.setColor(Color.RED);
break;
case 1:
g.setColor(Color.GREEN);
break;
case 2:
g.setColor(Color.BLUE);
break;
}

centerX = (int)(width*Math.random());
centerY = (int)(height*Math.random());

g.fillOval( centerX - 50, centerY - 50, 100, 100 );
g.setColor(Color.BLACK);
g.drawOval( centerX - 50, centerY - 50, 100, 100 );

}
}

This is the first subroutine definition that you have seen, other than main(), but you will learn all about defining subroutines in the next chapter. The first line of the definition makes available certain values that are used in the subroutine: the graphics context g and the width and height of the drawing area. (Ignore frameNumber for now.) These values come from outside the subroutine, but the subroutine can use them. The point here is that to draw something, you just have to fill in the inside of the subroutine, just as you write a program by filling in the inside of main().

The subroutine definition still has to go inside a class that defines the program. In this case, the class is named RandomCircles, and the complete program is available in the sample source code file RandomCircles.java. You can run that program to see the drawing.

There's a lot in the program that you won't understand. To make your own drawing, all you have to do is erase the inside of the drawFrame() routine in the source code and substitute your own drawing code. You don't need to understand the rest. The source code file SimpleAnimationStarter.java can be used as a basis for your own first graphics explorations. It's essentially the same as RandomCircles.java but with the drawing code omitted. You'll need it to do some of the exercises for this chapter.

(By the way, you might notice that the main() subroutine uses the word static in its definition, but drawFrame() does not. This has to do with the fact that the drawing area in this program is an object, and drawFrame is a subroutine in that object. The difference between static and non-static subroutines is important but not something that we need to worry about for the time being. It will become important for us in Chapter 5.)

3.9.3  Animation

The name "SimpleAnimationStarter" should give you a clue that we are looking at the possibility of more than just individual drawings here. A computer animation is simply a sequence of individual pictures, displayed quickly one after the other. If the change from each picture to the next is small, the user will perceive the sequence of images as a continuous animation. Each picture in the animation is called a frame. SimpleAnimationStarter.java is configured to draw fifty frames every second, although that can be changed. (In RandomCircles.java, it has been changed to one frame every three seconds, so that the program actually draws a new set of random circles every three seconds.) The frames in the animation are numbered 0, 1, 2, 3, ..., and the value of frameNumber in the drawFrame() subroutine tells you which frame you are drawing. The key to programming the animation is to base what you draw on the frameNumber.

As an example of animation, we look at drawing a set of nested rectangles. The rectangles will shrink towards the center of the drawing, giving an illusion of infinite motion. Here's one frame from the animation:

Consider how to draw a picture like this one. The rectangles can be drawn with a while loop, which draws the rectangles starting from the one on the outside and moving in. Think about what variables will be needed and how they change from one iteration of the while loop to the next. Each time through the loop, the rectangle that is drawn is smaller than the previous one and is moved down and over a bit. The difference between two rectangles is in their size and in the coordinates of the upper left corner. We need a variable to represent the size. The x and y-coordinates are the same, and they can be represented by the same variable. I call that variable inset, since it is the amount by which the edges of the rectangle are inset from the edges of the drawing area. The size decreases from one rectangle to the next, while the inset increases. The while loop ends when size becomes less than or equal to zero. In general outline, the algorithm for drawing one frame is

Set the drawing color to light gray  (using the g.setColor subroutine)
Fill in the entire picture (using the g.fillRect subroutine)
Set the drawing color to black
Set the amount of inset for the first rectangle
Set the rectangle width and height for the first rectangle
while the width and height are both greater than zero:
draw a rectangle (using the g.drawRect subroutine)
increase the inset (to move the next rectangle over and down)
decrease width the height (the make the next rectangle smaller)

In my program, each rectangle is 15 pixels away from the rectangle that surrounds it, so the inset is increased by 15 each time through the while loop. The rectangle shrinks by 15 pixels on the left and by 15 pixels on the right, so the width of the rectangle shrinks by 30 before drawing the next rectangle. The height also shrinks by 30 pixels each time through the loop.

The pseudocode is then easy to translate into Java, except that we need to know what initial values to use for the inset, width, and height of the first rectangle. To figure that out, we have to think about the fact that the picture is animated, so that what we draw will depend in some way on the frame number. From one frame to the next frame of the animation, the top-left corner of the outer rectangle moves over and down; that is, the inset for the outer rectangle increases from one frame to the next. We can make this happen by setting the inset for frame number 0 to 0, the inset for frame number 1 to 1, and so on. But that can't go on forever, or eventually all the rectangles would disappear. In fact, when the animation gets to frame 15, a new rectangle should appear at the outside of the drawing area -- but it's not really a "new rectangle," it's just that the inset for the outer rectangle goes back to zero. So, as the animation proceeds, the inset should go through the sequence of values 0, 1, 2, ..., 14 over and over. We can accomplish that very easily by setting

inset = frameNumber % 15;

Finally, note that the first rectangle fills the drawing area except for a border of size inset around the outside of the rectangle. This means that the the width of the rectangle is the width of the drawing area minus two times the inset, and similarly for the height. Here, then is the drawFrame() subroutine for the moving rectangle example:

public void drawFrame(Graphics g, int frameNumber, int width, int height) {

int inset; // Gap between edges of drawing area and the outer rectangle.

int rectWidth, rectHeight;   // The size of one of the rectangles.

g.setColor(Color.LIGHT_GRAY);
g.fillRect(0,0,width,height);  // Fill drawing area with light gray.

g.setColor(Color.BLACK);  // Draw the rectangles in black.

inset = frameNumber % 15;  // inset for the outer rectangle

rectWidth = width - 2*inset;   // drawing area width minus two insets
rectHeight = height - 2*inset; // drawing area height minus two insets

while (rectWidth >= 0 && rectHeight >= 0) {
g.drawRect(inset, inset, rectWidth, rectHeight);
inset += 15;       // rectangles are 15 pixels apart
rectWidth -= 30;
rectHeight -= 30;
}
}

You can find the full source code for the program is in the sample program MovingRects.java.

Programming Exercises for Chapter 3

Exercise 3.1:

How many times do you have to roll a pair of dice before they come up snake eyes? You could do the experiment by rolling the dice by hand. Write a computer program that simulates the experiment. The program should report the number of rolls that it makes before the dice come up snake eyes. (Note: "Snake eyes" means that both dice show a value of 1.) Exercise 2.2 explained how to simulate rolling a pair of dice.

See the Solution

Exercise 3.2:

Which integer between 1 and 10000 has the largest number of divisors, and how many divisors does it have? Write a program to find the answers and print out the results. It is possible that several integers in this range have the same, maximum number of divisors. Your program only has to print out one of them. An example in Subsection 3.4.2 discussed divisors. The source code for that example is CountDivisors.java.

You might need some hints about how to find a maximum value. The basic idea is to go through all the integers, keeping track of the largest number of divisors that you've seen so far. Also, keep track of the integer that had that number of divisors.

See the Solution

Exercise 3.3:

Write a program that will evaluate simple expressions such as 17 + 3 and 3.14159 * 4.7. The expressions are to be typed in by the user. The input always consists of a number, followed by an operator, followed by another number. The operators that are allowed are +, -, *, and /. You can read the numbers with TextIO.getDouble() and the operator with TextIO.getChar(). Your program should read an expression, print its value, read another expression, print its value, and so on. The program should end when the user enters 0 as the first number on the line.

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Exercise 3.4:

Write a program that reads one line of input text and breaks it up into words. The words should be output one per line. A word is defined to be a sequence of letters. Any characters in the input that are not letters should be discarded. For example, if the user inputs the line

He said, "That's not a good idea."

then the output of the program should be

He
said
That
s
not
a
good
idea

An improved version of the program would list "that's" as a single word. An apostrophe can be considered to be part of a word if there is a letter on each side of the apostrophe.

To test whether a character is a letter, you might use (ch >= 'a' && ch <= 'z') || (ch >= 'A' && ch <= 'Z'). However, this only works in English and similar languages. A better choice is to call the standard function Character.isLetter(ch), which returns a boolean value of true if ch is a letter and false if it is not. This works for any Unicode character.

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Exercise 3.5:

Suppose that a file contains information about sales figures for a company in various cities. Each line of the file contains a city name, followed by a colon (:) followed by the data for that city. The data is a number of type double. However, for some cities, no data was available. In these lines, the data is replaced by a comment explaining why the data is missing. For example, several lines from the file might look like:

San Francisco:  19887.32
Chicago:  no report received
New York: 298734.12

Write a program that will compute and print the total sales from all the cities together. The program should also report the number of cities for which data was not available. The name of the file is "sales.dat".

To complete this program, you'll need one fact about file input with TextIO that was not covered in Subsection 2.4.4. Since you don't know in advance how many lines there are in the file, you need a way to tell when you have gotten to the end of the file. When TextIO is reading from a file, the function TextIO.eof() can be used to test for end of file. This boolean-valued function returns true if the file has been entirely read and returns false if there is more data to read in the file. This means that you can read the lines of the file in a loop while (TextIO.eof() == false).... The loop will end when all the lines of the file have been read.

Suggestion: For each line, read and ignore characters up to the colon. Then read the rest of the line into a variable of type String. Try to convert the string into a number, and use try..catch to test whether the conversion succeeds.

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Exercise 3.6:

Exercise 3.2 asked you to find the number in the range 1 to 10000 that has the largest number of divisors. You only had to print out one such number. Revise the program so that it will print out all numbers that have the maximum number of divisors. Use an array as follows: As you count the divisors for each number, store each count in an array. Then at the end of the program, you can go through the array and print out all the numbers that have the maximum count. The output from the program should look something like this:

Among integers between 1 and 10000,
The maximum number of divisors was 64
Numbers with that many divisors include:
7560
9240

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Exercise 3.7:

An example in Subsection 3.8.3 tried to answer the question, How many random people do you have to select before you find a duplicate birthday? The source code for that program can be found in the file BirthdayProblem.java. Here are some related questions:

- How many random people do you have to select before you find three people who share the same birthday? (That is, all three people were born on the same day in the same month, but not necessarily in the same year.)

- Suppose you choose 365 people at random. How many different birthdays will they have? (The number could theoretically be anywhere from 1 to 365).

- How many different people do you have to check before you've found at least one person with a birthday on each of the 365 days of the year?

Write three programs to answer these questions. Each of your programs should simulate choosing people at random and checking their birthdays. (In each case, ignore the possibility of leap years.)

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Exercise 3.8:

Write a GUI program that draws a checkerboard. Base your solution on the sample program SimpleAnimationStarter.java, even though you are creating only a static picture rather than an animation. You will draw the checkerboard in the drawFrame() subroutine. You should read the comments in the file to discover other changes that you might need to make.

Assume that the size of the drawing area is 400-by-400 pixels. A checkerboard contains 8 rows and 8 columns of squares. If the size of the drawing area is 400, that means that each square can be 50-by-50 pixels. The squares are red and black (or whatever other colors you choose). Here is a tricky way to determine whether a given square should be red or black: The rows and columns can be thought of as numbered from 0 to 7. If the row number of the square and the column number of the square are either both even or both odd, then the square is red. Otherwise, it is black. Note that a square is just a rectangle in which the height is equal to the width, so you can use the subroutine g.fillRect() to draw the squares. Here is a reduced-size image of the checkerboard that you want to draw:

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Exercise 3.9:

Often, some element of an animation repeats over and over, every so many frames. Sometimes, the repetition is "cyclic," meaning that at the end it jumps back to the start. Sometimes the repetition is "oscillating," like a back-and-forth motion where the second half is the same as the first half played in reverse.

Write an animation that demonstrates both cyclic and oscillating motions at various speeds. For cyclic motion, you can use a square that moves across the drawing area, then jumps back to the start, and then repeats the same motion over and over. For oscillating motion, you can do something similar, but the square should move back and forth between the two edges of the drawing area; that is, it moves left-to-right during the first half of the animation and then backwards from right-to-left during the second half. To write the program, you can start with a copy of the sample program SimpleAnimationStarter.java, as in the previous exercise.

A cyclic motion has to repeat every N frames for some value of N. What you draw in some frame of the animation depends on the frameNumber. The frameNumber just keeps increasing forever. To implement cyclic motion, what you really want is a "cyclic frame number" that takes on the values 0, 1, 2, ..., (N-1), 0, 1, 2, ..., (N-1), 0, 1, 2, .... You can derive the value that you need from frameNumber simply by saying

cyclicFrameNumber = frameNumber % N;

Then, you just have to base what you draw on cyclicFrameNumber instead of on frameNumber. Similarly, for an oscillating animation, you need an "oscillation frame number" that takes on the values 0, 1, 2, ... (N-1), N, (N-1), (N-2), ... 2, 1, 0, 1, 2, and so on, repeating the back and forth motion forever. You can compute the value that you need with

oscilationFrameNumber = frameNumber % (2*N);
if (oscillationFrameNumber > N)
oscillationFrameNumber = (2*N) - oscillationFrameNumber;

Here is a screen shot from my version of the program. I use six squares. The top three do cyclic motion at various speeds, while the bottom three do oscillating motion. I drew black lines across the drawing area to separate the squares and to give them "channels" to move in.

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Quiz on Chapter 3

Question 1:

What is an algorithm?

Question 2:

Explain briefly what is meant by "pseudocode" and how is it useful in the development of algorithms.

Question 3:

What is a block statement? How are block statements used in Java programs?

Question 4:

What is the main difference between a while loop and a do..while loop?

Question 5:

What does it mean to prime a loop?

Question 6:

Explain what is meant by an animation and how a computer displays an animation.

Question 7:

Write a for loop that will print out all the multiples of 3 from 3 to 36, that is: 3 6 9 12 15 18 21 24 27 30 33 36.

Question 8:

Fill in the following main() routine so that it will ask the user to enter an integer, read the user's response, and tell the user whether the number entered is even or odd. (You can use TextIO.getInt() to read the integer. Recall that an integer n is even if n % 2 == 0.)

public static void main(String[] args) {

// Fill in the body of this subroutine!

}

Question 9:

Write a code segment that will print out two different random integers selected from the range 1 to 10. All possible outputs should have the same probability. Hint: You can easily select two random numbers, but you have to account for the fact that the two numbers that you pick might be the same.

Question 10:

Suppose that s1 and s2 are variables of type String, whose values are expected to be string representations of values of type int. Write a code segment that will compute and print the integer sum of those values, or will print an error message if the values cannot successfully be converted into integers. (Use a try..catch statement.)

Question 11:

Show the exact output that would be produced by the following main() routine:

public static void main(String[] args) {
int N;
N = 1;
while (N <= 32) {
N = 2 * N;
System.out.println(N);
}
}

Question 12:

Show the exact output produced by the following main() routine:

public static void main(String[] args) {
int x,y;
x = 5;
y = 1;
while (x > 0) {
x = x - 1;
y = y * x;
System.out.println(y);
}
}

Question 13:

What output is produced by the following program segment? Why? (Recall that name.charAt(i) is the i-th character in the string, name.)

String name;
int i;
boolean startWord;

name = "Richard M. Nixon";
startWord = true;
for (i = 0; i < name.length(); i++) {
if (startWord)
System.out.println(name.charAt(i));
if (name.charAt(i) == ' ')
startWord = true;
else
startWord = false;
}

Question 14:

Suppose that numbers is an array of type int[]. Write a code segment that will count and output the number of times that the number 42 occurs in the array.

Question 15:

Define the range of an array of numbers to be the maximum value in the array minus the minimum value. Suppose that raceTimes is an array of type double[]. Write a code segment that will find and print the range of raceTimes.

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Chapter 4

Programming in the Large I: Subroutines

One way to break up a complex program into manageable pieces is to use subroutines. A subroutine consists of the instructions for carrying out a certain task, grouped together and given a name. Elsewhere in the program, that name can be used as a stand-in for the whole set of instructions. As a computer executes a program, whenever it encounters a subroutine name, it executes all the instructions necessary to carry out the task associated with that subroutine.

Subroutines can be used over and over, at different places in the program. A subroutine can even be used inside another subroutine. This allows you to write simple subroutines and then use them to help write more complex subroutines, which can then be used in turn in other subroutines. In this way, very complex programs can be built up step-by-step, where each step in the construction is reasonably simple.

Subroutines in Java can be either static or non-static. This chapter covers static subroutines. Non-static subroutines, which are used in true object-oriented programming, will be covered in the next chapter.

Contents of Chapter 4:

- Section 1: Black Boxes

- Section 2: Static Subroutines and Static Variables

- Section 3: Parameters

- Section 4: Return Values

- Section 5: APIs, Packages, and Javadoc

- Section 6: More on Program Design

- Section 7: The Truth About Declarations

- Programming Exercises

- Quiz on This Chapter

Section 4.1

Black Boxes

A subroutine consists of instructions for performing some task, chunked together and given a name. "Chunking" allows you to deal with a potentially very complicated task as a single concept. Instead of worrying about the many, many steps that the computer might have to go though to perform that task, you just need to remember the name of the subroutine. Whenever you want your program to perform the task, you just call the subroutine. Subroutines are a major tool for dealing with complexity.

A subroutine is sometimes said to be a "black box" because you can't see what's "inside" it (or, to be more precise, you usually don't want to see inside it, because then you would have to deal with all the complexity that the subroutine is meant to hide). Of course, a black box that has no way of interacting with the rest of the world would be pretty useless. A black box needs some kind of interface with the rest of the world, which allows some interaction between what's inside the box and what's outside. A physical black box might have buttons on the outside that you can push, dials that you can set, and slots that can be used for passing information back and forth. Since we are trying to hide complexity, not create it, we have the first rule of black boxes:

The interface of a black box should be fairly straightforward, well-defined, and easy to understand.

Are there any examples of black boxes in the real world? Yes; in fact, you are surrounded by them. Your television, your car, your mobile phone, your refrigerator.... You can turn your television on and off, change channels, and set the volume by using elements of the television's interface -- dials, remote control, don't forget to plug in the power -- without understanding anything about how the thing actually works. The same goes for a mobile phone, although the interface in that case is a lot more complicated.

Now, a black box does have an inside -- the code in a subroutine that actually performs the task, or all the electronics inside your television set. The inside of a black box is called its implementation. The second rule of black boxes is that:

To use a black box, you shouldn't need to know anything about its implementation; all you need to know is its interface.

In fact, it should be possible to change the implementation, as long as the behavior of the box, as seen from the outside, remains unchanged. For example, when the insides of TV sets went from using vacuum tubes to using transistors, the users of the sets didn't need to know about it -- or even know what it means. Similarly, it should be possible to rewrite the inside of a subroutine, to use more efficient code, for example, without affecting the programs that use that subroutine.

Of course, to have a black box, someone must have designed and built the implementation in the first place. The black box idea works to the advantage of the implementor as well as the user of the black box. After all, the black box might be used in an unlimited number of different situations. The implementor of the black box doesn't need to know about any of that. The implementor just needs to make sure that the box performs its assigned task and interfaces correctly with the rest of the world. This is the third rule of black boxes:

The implementor of a black box should not need to know anything about the larger systems in which the box will be used.

In a way, a black box divides the world into two parts: the inside (implementation) and the outside. The interface is at the boundary, connecting those two parts.

By the way, you should not think of an interface as just the physical connection between the box and the rest of the world. The interface also includes a specification of what the box does and how it can be controlled by using the elements of the physical interface. It's not enough to say that a TV set has a power switch; you need to specify that the power switch is used to turn the TV on and off!

To put this in computer science terms, the interface of a subroutine has a semantic as well as a syntactic component. The syntactic part of the interface tells you just what you have to type in order to call the subroutine. The semantic component specifies exactly what task the subroutine will accomplish. To write a legal program, you need to know the syntactic specification of the subroutine. To understand the purpose of the subroutine and to use it effectively, you need to know the subroutine's semantic specification. I will refer to both parts of the interface -- syntactic and semantic -- collectively as the contract of the subroutine.

The contract of a subroutine says, essentially, "Here is what you have to do to use me, and here is what I will do for you, guaranteed." When you write a subroutine, the comments that you write for the subroutine should make the contract very clear. (I should admit that in practice, subroutines' contracts are often inadequately specified, much to the regret and annoyance of the programmers who have to use them.)

For the rest of this chapter, I turn from general ideas about black boxes and subroutines in general to the specifics of writing and using subroutines in Java. But keep the general ideas and principles in mind. They are the reasons that subroutines exist in the first place, and they are your guidelines for using them. This should be especially clear in Section 4.6, where I will discuss subroutines as a tool in program development.

You should keep in mind that subroutines are not the only example of black boxes in programming. For example, a class is also a black box. We'll see that a class can have a "public" part, representing its interface, and a "private" part that is entirely inside its hidden implementation. All the principles of black boxes apply to classes as well as to subroutines.

Section 4.2

Static Subroutines and Static Variables

Every subroutine in Java must be defined inside some class. This makes Java rather unusual among programming languages, since most languages allow free-floating, independent subroutines. One purpose of a class is to group together related subroutines and variables. Perhaps the designers of Java felt that everything must be related to something. As a less philosophical motivation, Java's designers wanted to place firm controls on the ways things are named, since a Java program potentially has access to a huge number of subroutines created by many different programmers. The fact that those subroutines are grouped into named classes (and classes are grouped into named "packages," as we will see later) helps control the confusion that might result from so many different names.

There is a basic distinction in Java between static and non-static subroutines. A class definition can contain the source code for both types of subroutine, but what's done with them when the program runs is very different. Static subroutines are easier to understand: In a running program, a static subroutine is a member of the class itself. Non-static subroutine definitions, on the other hand, are only there to be used when objects are created, and the subroutines themselves become members of the objects. Non-static subroutines only become relevant when you are working with objects. The distinction between static and non-static also applies to variables and to other things that can occur in class definitions. This chapter will deal with static subroutines and static variables almost exclusively. We'll turn to non-static stuff and to object-oriented programming in the next chapter.

A subroutine that is in a class or object is often called a method, and "method" is the term that most people prefer for subroutines in Java. I will start using the term "method" occasionally; however, I will continue to prefer the more general term "subroutine" in this chapter, at least for static subroutines. However, you should start thinking of the terms "method" and "subroutine" as being essentially synonymous as far as Java is concerned.

4.2.1  Subroutine Definitions

A subroutine must be defined somewhere. The definition has to include the name of the subroutine, enough information to make it possible to call the subroutine, and the code that will be executed each time the subroutine is called. A subroutine definition in Java takes the form:

modifiers  return-type  subroutine-name  ( parameter-list ) {
statements
}

It will take us a while -- most of the chapter -- to get through what all this means in detail. Of course, you've already seen examples of subroutines in previous chapters, such as the main() routine of a program and the drawFrame() routine of the animation programs in Section 3.9. So you are familiar with the general format.

The statements between the braces, { and }, in a subroutine definition make up the body of the subroutine. These statements are the inside, or implementation part, of the "black box," as discussed in the previous section. They are the instructions that the computer executes when the method is called. Subroutines can contain any of the statements discussed in Chapter 2 and Chapter 3.

The modifiers that can occur at the beginning of a subroutine definition are words that set certain characteristics of the subroutine, such as whether it is static or not. The modifiers that you've seen so far are "static" and "public". There are only about a half-dozen possible modifiers altogether.

If the subroutine is a function, whose job is to compute some value, then the return-type is used to specify the type of value that is returned by the function. It can be a type name such as String or int or even an array type such as double[]. We'll be looking at functions and return types in some detail in Section 4.4. If the subroutine is not a function, then the return-type is replaced by the special value void, which indicates that no value is returned. The term "void" is meant to indicate that the return value is empty or non-existent.

Finally, we come to the parameter-list of the method. Parameters are part of the interface of a subroutine. They represent information that is passed into the subroutine from outside, to be used by the subroutine's internal computations. For a concrete example, imagine a class named Television that includes a method named changeChannel(). The immediate question is: What channel should it change to? A parameter can be used to answer this question. Since the channel number is an integer, the type of the parameter would be int, and the declaration of the changeChannel() method might look like

public void changeChannel(int channelNum) { ... }

This declaration specifies that changeChannel() has a parameter named channelNum of type int. However, channelNum does not yet have any particular value. A value for channelNum is provided when the subroutine is called; for example: changeChannel(17);

The parameter list in a subroutine can be empty, or it can consist of one or more parameter declarations of the form type parameter-name. If there are several declarations, they are separated by commas. Note that each declaration can name only one parameter. For example, if you want two parameters of type double, you have to say "double x, double y", rather than "double x, y".

Parameters are covered in more detail in the next section.

Here are a few examples of subroutine definitions, leaving out the statements that define what the subroutines do:

public static void playGame() {
// "public" and "static" are modifiers; "void" is the
// return-type; "playGame" is the subroutine-name;
// the parameter-list is empty.
. . .  // Statements that define what playGame does go here.
}

int getNextN(int N) {
// There are no modifiers; "int" in the return-type;
// "getNextN" is the subroutine-name; the parameter-list
// includes one parameter whose name is "N" and whose
// type is "int".
. . .  // Statements that define what getNextN does go here.
}

static boolean lessThan(double x, double y) {
// "static" is a modifier; "boolean" is the
// return-type; "lessThan" is the subroutine-name;
// the parameter-list includes two parameters whose names are
// "x" and "y", and the type of each of these parameters
// is "double".
. . .  // Statements that define what lessThan does go here.
}

In the second example given here, getNextN is a non-static method, since its definition does not include the modifier "static" -- and so it's not an example that we should be looking at in this chapter! The other modifier shown in the examples is "public". This modifier indicates that the method can be called from anywhere in a program, even from outside the class where the method is defined. There is another modifier, "private", which indicates that the method can be called only from inside the same class. The modifiers public and private are called access specifiers. If no access specifier is given for a method, then by default, that method can be called from anywhere in the "package" that contains the class, but not from outside that package. (Packages were mentioned in Subsection 2.6.6, and you'll learn more about them later in this chapter, in Section 4.5.) There is one other access modifier, protected, which will only become relevant when we turn to object-oriented programming in Chapter 5.

Note, by the way, that the main() routine of a program follows the usual syntax rules for a subroutine. In

public static void main(String[] args) { ... }

the modifiers are public and static, the return type is void, the subroutine name is main, and the parameter list is "String[] args". In this case, the type for the parameter is the array type String[].

You've already had some experience with filling in the implementation of a subroutine. In this chapter, you'll learn all about writing your own complete subroutine definitions, including the interface part.

4.2.2  Calling Subroutines

When you define a subroutine, all you are doing is telling the computer that the subroutine exists and what it does. The subroutine doesn't actually get executed until it is called. (This is true even for the main() routine in a class -- even though you don't call it, it is called by the system when the system runs your program.) For example, the playGame() method given as an example above could be called using the following subroutine call statement:

playGame();

This statement could occur anywhere in the same class that includes the definition of playGame(), whether in a main() method or in some other subroutine. Since playGame() is a public method, it can also be called from other classes, but in that case, you have to tell the computer which class it comes from. Since playGame() is a static method, its full name includes the name of the class in which it is defined. Let's say, for example, that playGame() is defined in a class named Poker. Then to call playGame() from outside the Poker class, you would have to say

Poker.playGame();

The use of the class name here tells the computer which class to look in to find the method. It also lets you distinguish between Poker.playGame() and other potential playGame() methods defined in other classes, such as Roulette.playGame() or Blackjack.playGame().

More generally, a subroutine call statement for a static subroutine takes the form

subroutine-name(parameters);

if the subroutine that is being called is in the same class, or

class-name.subroutine-name(parameters);

if the subroutine is defined elsewhere, in a different class. (Non-static methods belong to objects rather than classes, and they are called using objects instead of class names. More on that later.) Note that the parameter list can be empty, as in the playGame() example, but the parentheses must be there even if there is nothing between them. The number of parameters that you provide when you call a subroutine must match the number listed in the parameter list in the subroutine definition, and the types of the parameters in the call statement must match the types in the subroutine definition.

4.2.3  Subroutines in Programs

It's time to give an example of what a complete program looks like, when it includes other subroutines in addition to the main() routine. Let's write a program that plays a guessing game with the user. The computer will choose a random number between 1 and 100, and the user will try to guess it. The computer tells the user whether the guess is high or low or correct. If the user gets the number after six guesses or fewer, the user wins the game. After each game, the user has the option of continuing with another game.

Since playing one game can be thought of as a single, coherent task, it makes sense to write a subroutine that will play one guessing game with the user. The main() routine will use a loop to call the playGame() subroutine over and over, as many times as the user wants to play. We approach the problem of designing the playGame() subroutine the same way we write a main() routine: Start with an outline of the algorithm and apply stepwise refinement. Here is a short pseudocode algorithm for a guessing game routine:

Pick a random number
while the game is not over:
Get the user's guess
Tell the user whether the guess is high, low, or correct.

The test for whether the game is over is complicated, since the game ends if either the user makes a correct guess or the number of guesses is six. As in many cases, the easiest thing to do is to use a "while (true)" loop and use break to end the loop whenever we find a reason to do so. Also, if we are going to end the game after six guesses, we'll have to keep track of the number of guesses that the user has made. Filling out the algorithm gives:

Let computersNumber be a random number between 1 and 100
Let guessCount = 0
while (true):
Get the user's guess
Count the guess by adding 1 to guess count
if the user's guess equals computersNumber:
Tell the user he won
break out of the loop
if the number of guesses is 6:
Tell the user he lost
break out of the loop
if the user's guess is less than computersNumber:
Tell the user the guess was low
else if the user's guess is higher than computersNumber:
Tell the user the guess was high

With variable declarations added and translated into Java, this becomes the definition of the playGame() routine. A random integer between 1 and 100 can be computed as (int)(100 * Math.random()) + 1. I've cleaned up the interaction with the user to make it flow better.

static void playGame() {
int computersNumber; // A random number picked by the computer.
int usersGuess;      // A number entered by user as a guess.
int guessCount;      // Number of guesses the user has made.
computersNumber = (int)(100 * Math.random()) + 1;
// The value assigned to computersNumber is a randomly
//    chosen integer between 1 and 100, inclusive.
guessCount = 0;
System.out.println();
System.out.print("What is your first guess? ");
while (true) {
usersGuess = TextIO.getInt();  // Get the user's guess.
guessCount++;
if (usersGuess == computersNumber) {
System.out.println("You got it in " + guessCount
+ " guesses!  My number was " + computersNumber);
break;  // The game is over; the user has won.
}
if (guessCount == 6) {
System.out.println("You didn't get the number in 6 guesses.");
System.out.println("You lose.  My number was " + computersNumber);
break;  // The game is over; the user has lost.
}
// If we get to this point, the game continues.
// Tell the user if the guess was too high or too low.
if (usersGuess < computersNumber)
System.out.print("That's too low.  Try again: ");
else if (usersGuess > computersNumber)
System.out.print("That's too high.  Try again: ");
}
System.out.println();
} // end of playGame()

Now, where exactly should you put this? It should be part of the same class as the main() routine, but not inside the main routine. It is not legal to have one subroutine physically nested inside another. The main() routine will call playGame(), but not contain its definition, only a call statement. You can put the definition of playGame() either before or after the main() routine. Java is not very picky about having the members of a class in any particular order.

It's pretty easy to write the main routine. You've done things like this before. Here's what the complete program looks like (except that a serious program needs more comments than I've included here).

public class GuessingGame {

public static void main(String[] args) {
System.out.println("Let's play a game.  I'll pick a number between");
System.out.println("1 and 100, and you try to guess it.");
boolean playAgain;
do {
playGame();  // call subroutine to play one game
System.out.print("Would you like to play again? ");
playAgain = TextIO.getlnBoolean();
} while (playAgain);
System.out.println("Thanks for playing.  Goodbye.");
} // end of main()

static void playGame() {
int computersNumber; // A random number picked by the computer.
int usersGuess;      // A number entered by user as a guess.
int guessCount;      // Number of guesses the user has made.
computersNumber = (int)(100 * Math.random()) + 1;
// The value assigned to computersNumber is a randomly
//    chosen integer between 1 and 100, inclusive.
guessCount = 0;
System.out.println();
System.out.print("What is your first guess? ");
while (true) {
usersGuess = TextIO.getInt();  // Get the user's guess.
guessCount++;
if (usersGuess == computersNumber) {
System.out.println("You got it in " + guessCount
+ " guesses!  My number was " + computersNumber);
break;  // The game is over; the user has won.
}
if (guessCount == 6) {
System.out.println("You didn't get the number in 6 guesses.");
System.out.println("You lose.  My number was " + computersNumber);
break;  // The game is over; the user has lost.
}
// If we get to this point, the game continues.
// Tell the user if the guess was too high or too low.
if (usersGuess < computersNumber)
System.out.print("That's too low.  Try again: ");
else if (usersGuess > computersNumber)
System.out.print("That's too high.  Try again: ");
}
System.out.println();
} // end of playGame()

} // end of class GuessingGame

Take some time to read the program carefully and figure out how it works. And try to convince yourself that even in this relatively simple case, breaking up the program into two methods makes the program easier to understand and probably made it easier to write each piece.

4.2.4  Member Variables

A class can include other things besides subroutines. In particular, it can also include variable declarations. Of course, you can declare variables inside subroutines. Those are called local variables. However, you can also have variables that are not part of any subroutine. To distinguish such variables from local variables, we call them member variables, since they are members of a class. Another term for them is global variable.

Just as with subroutines, member variables can be either static or non-static. In this chapter, we'll stick to static variables. A static member variable belongs to the class as a whole, and it exists as long as the class exists. Memory is allocated for the variable when the class is first loaded by the Java interpreter. Any assignment statement that assigns a value to the variable changes the content of that memory, no matter where that assignment statement is located in the program. Any time the variable is used in an expression, the value is fetched from that same memory, no matter where the expression is located in the program. This means that the value of a static member variable can be set in one subroutine and used in another subroutine. Static member variables are "shared" by all the static subroutines in the class. A local variable in a subroutine, on the other hand, exists only while that subroutine is being executed, and is completely inaccessible from outside that one subroutine.

The declaration of a member variable looks just like the declaration of a local variable except for two things: The member variable is declared outside any subroutine (although it still has to be inside a class), and the declaration can be marked with modifiers such as static, public, and private. Since we are only working with static member variables for now, every declaration of a member variable in this chapter will include the modifier static. They might also be marked as public or private. For example:

static String usersName;
public static int numberOfPlayers;
private static double velocity, time;

A static member variable that is not declared to be private can be accessed from outside the class where it is defined, as well as inside. When it is used in some other class, it must be referred to with a compound identifier of the form class-name.variable-name. For example, the System class contains the public static member variable named out, and you use this variable in your own classes by referring to System.out. Similarly, Math.PI is a public static member variable in the Math. If numberOfPlayers is a public static member variable in a class named Poker, then code in the Poker class would refer to it simply as numberOfPlayers, while code in another class would refer to it as Poker.numberOfPlayers.

As an example, let's add a couple static member variable to the GuessingGame class that we wrote earlier in this section. We add a variable named gamesPlayed to keep track of how many games the user has played and another variable named gamesWon to keep track of the number of games that the user has won. The variables are declared as static member variables:

static int gamesPlayed;
static int gamesWon;

In the playGame() routine, we always add 1 to gamesPlayed, and we add 1 to gamesWon if the user wins the game. At the end of the main() routine, we print out the values of both variables. It would be impossible to do the same thing with local variables, since both subroutines need to access the variables, and local variables exist in only one subroutine.

When you declare a local variable in a subroutine, you have to assign a value to that variable before you can do anything with it. Member variables, on the other hand are automatically initialized with a default value. The default values are the same as those that are used when initializing the elements of an array: For numeric variables, the default value is zero; for boolean variables, the default is false; for char variables, it's the character that has Unicode code number zero; and for objects, such as Strings, the default initial value is the special value null.

Since they are of type int, the static member variables gamesPlayed and gamesWon automatically get zero as their initial value. This happens to be the correct initial value for a variable that is being used as a counter. You can, of course, assign a value to a variable at the beginning of the main() routine if you are not satisfied with the default initial value, or if you want to emphasize that you are depending on the default.

Here's the revised version of GuessingGame.java. The changes from the above version are shown in red italic:

public class GuessingGame2 {

static int gamesPlayed;   // The number of games played.
static int gamesWon;      // The number of games won.

public static void main(String[] args) {
gamesPlayed = 0;
gamesWon = 0;  // This is actually redundant, since 0 is
//                 the default initial value.
System.out.println("Let's play a game.  I'll pick a number between");
System.out.println("1 and 100, and you try to guess it.");
boolean playAgain;
do {
playGame();  // call subroutine to play one game
System.out.print("Would you like to play again? ");
playAgain = TextIO.getlnBoolean();
} while (playAgain);
System.out.println();
System.out.println("You played " + gamesPlayed + " games,");
System.out.println("and you won " + gamesWon + " of those games.");
System.out.println("Thanks for playing.  Goodbye.");
} // end of main()

static void playGame() {
int computersNumber; // A random number picked by the computer.
int usersGuess;      // A number entered by user as a guess.
int guessCount;      // Number of guesses the user has made.
gamesPlayed++;  // Count this game.
computersNumber = (int)(100 * Math.random()) + 1;
// The value assigned to computersNumber is a randomly
//    chosen integer between 1 and 100, inclusive.
guessCount = 0;
System.out.println();
System.out.print("What is your first guess? ");
while (true) {
usersGuess = TextIO.getInt();  // Get the user's guess.
guessCount++;
if (usersGuess == computersNumber) {
System.out.println("You got it in " + guessCount
+ " guesses!  My number was " + computersNumber);
gamesWon++;  // Count this win.
break;       // The game is over; the user has won.
}
if (guessCount == 6) {
System.out.println("You didn't get the number in 6 guesses.");
System.out.println("You lose.  My number was " + computersNumber);
break;  // The game is over; the user has lost.
}
// If we get to this point, the game continues.
// Tell the user if the guess was too high or too low.
if (usersGuess < computersNumber)
System.out.print("That's too low.  Try again: ");
else if (usersGuess > computersNumber)
System.out.print("That's too high.  Try again: ");
}
System.out.println();
} // end of playGame()

} // end of class GuessingGame2

(By the way, notice that in my example programs, I didn't mark the static subroutines or variables as being public or private. You might wonder what it means to leave out both modifiers. Recall that global variables and subroutines with no access modifier can be used anywhere in the same package as the class where they are defined, but not in other packages. Classes that don't declare a package are in the default package. So, any class in the default package would have access to gamesPlayed, gamesWon, and playGame() -- and that includes pretty much every class in this book. In fact, it is considered to be good practice to make member variables and subroutines private, unless there is a reason for doing otherwise.)

Section 4.3

Parameters

If a subroutine is a black box, then a parameter is something that provides a mechanism for passing information from the outside world into the box. Parameters are part of the interface of a subroutine. They allow you to customize the behavior of a subroutine to adapt it to a particular situation.

As an analogy, consider a thermostat -- a black box whose task it is to keep your house at a certain temperature. The thermostat has a parameter, namely the dial that is used to set the desired temperature. The thermostat always performs the same task: maintaining a constant temperature. However, the exact task that it performs -- that is, which temperature it maintains -- is customized by the setting on its dial.

4.3.1  Using Parameters

As an example, let's go back to the "3N+1" problem that was discussed in Subsection 3.2.2. (Recall that a 3N+1 sequence is computed according to the rule, "if N is odd, multiply it by 3 and add 1; if N is even, divide it by 2; continue until N is equal to 1." For example, starting from N=3 we get the sequence: 3, 10, 5, 16, 8, 4, 2, 1.) Suppose that we want to write a subroutine to print out such sequences. The subroutine will always perform the same task: Print out a 3N+1 sequence. But the exact sequence it prints out depends on the starting value of N. So, the starting value of N would be a parameter to the subroutine. The subroutine can be written like this:

/**
* This subroutine prints a 3N+1 sequence to standard output, using
* startingValue as the initial value of N.  It also prints the number
* of terms in the sequence. The value of the parameter, startingValue,
* must  be a positive integer.
*/

static void print3NSequence(int startingValue) {

int N;      // One of the terms in the sequence.
int count;  // The number of terms.

N = startingValue;  // The first term is whatever value
//    is passed to the subroutine as
//    a parameter.

count = 1; // We have one term, the starting value, so far.

System.out.println("The 3N+1 sequence starting from " + N);
System.out.println();
System.out.println(N);  // print initial term of sequence

while (N > 1) {
if (N % 2 == 1)     // is N odd?
N = 3 * N + 1;
else
N = N / 2;
count++;   // count this term
System.out.println(N);  // print this term
}

System.out.println();
System.out.println("There were " + count + " terms in the sequence.");

}  // end print3NSequence

The parameter list of this subroutine, "(int startingValue)", specifies that the subroutine has one parameter, of type int. Within the body of the subroutine, the parameter name can be used in the same way as a variable name. But notice that there is nothing in the subroutine definition that gives a value to the parameter! The parameter gets its initial value from outside the subroutine. When the subroutine is called, a value must be provided for the parameter in the subroutine call statement. This value will be assigned to startingValue before the body of the subroutine is executed. For example, the subroutine could be called using the subroutine call statement "print3NSequence(17);". When the computer executes this statement, the computer first assigns the value 17 to startingValue and then executes the statements in the subroutine. This prints the 3N+1 sequence starting from 17. If K is a variable of type int, then the subroutine can be called by saying "print3NSequence(K);". When the computer executes this subroutine call statement, it takes the value of the variable K, assigns that value to startingValue, and then executes the body of the subroutine.

The class that contains print3NSequence can contain a main() routine (or other subroutines) that call print3NSequence. For example, here is a main() program that prints out 3N+1 sequences for various starting values specified by the user:

public static void main(String[] args) {
System.out.println("This program will print out 3N+1 sequences");
System.out.println("for starting values that you specify.");
System.out.println();
int K;  // Input from user; loop ends when K < 0.
do {
System.out.println("Enter a starting value.");
System.out.print("To end the program, enter 0: ");
K = TextIO.getInt();  // Get starting value from user.
if (K > 0)   // Print sequence, but only if K is > 0.
print3NSequence(K);
} while (K > 0);   // Continue only if K > 0.
} // end main

Remember that before you can use this program, the definitions of main and of print3NSequence must both be wrapped inside a class definition.

4.3.2  Formal and Actual Parameters

Note that the term "parameter" is used to refer to two different, but related, concepts. There are parameters that are used in the definitions of subroutines, such as startingValue in the above example. And there are parameters that are used in subroutine call statements, such as the K in the statement "print3NSequence(K);". Parameters in a subroutine definition are called formal parameters or dummy parameters. The parameters that are passed to a subroutine when it is called are called actual parameters or arguments. When a subroutine is called, the actual parameters in the subroutine call statement are evaluated and the values are assigned to the formal parameters in the subroutine's definition. Then the body of the subroutine is executed.

A formal parameter must be a name, that is, a simple identifier. A formal parameter is very much like a variable, and -- like a variable -- it has a specified type such as int, boolean, String, or double[]. An actual parameter is a value, and so it can be specified by any expression, provided that the expression computes a value of the correct type. The type of the actual parameter must be one that could legally be assigned to the formal parameter with an assignment statement. For example, if the formal parameter is of type double, then it would be legal to pass an int as the actual parameter since ints can legally be assigned to doubles. When you call a subroutine, you must provide one actual parameter for each formal parameter in the subroutine's definition. Consider, for example, a subroutine

static void doTask(int N, double x, boolean test) {
// statements to perform the task go here
}

This subroutine might be called with the statement

doTask(17, Math.sqrt(z+1), z >= 10);

When the computer executes this statement, it has essentially the same effect as the block of statements:

{
int N;       // Allocate memory locations for the formal parameters.
double x;
boolean test;
N = 17;              // Assign 17 to the first formal parameter, N.
x = Math.sqrt(z+1);  // Compute Math.sqrt(z+1), and assign it to
//    the second formal parameter, x.
test = (z >= 10);    // Evaluate "z >= 10" and assign the resulting
//     true/false value to the third formal
//     parameter, test.
// statements to perform the task go here
}

(There are a few technical differences between this and "doTask(17,Math.sqrt(z+1),z>=10);" -- besides the amount of typing -- because of questions about scope of variables and what happens when several variables or parameters have the same name.)

Beginning programming students often find parameters to be surprisingly confusing. Calling a subroutine that already exists is not a problem -- the idea of providing information to the subroutine in a parameter is clear enough. Writing the subroutine definition is another matter. A common beginner's mistake is to assign values to the formal parameters at the beginning of the subroutine, or to ask the user to input their values. This represents a fundamental misunderstanding. When the statements in the subroutine are executed, the formal parameters have already been assigned initial values! The computer automatically assigns values to the parameters before it starts executing the code inside the subroutine. The values come from the subroutine call statement. Remember that a subroutine is not independent. It is called by some other routine, and it is the subroutine call statement's responsibility to provide appropriate values for the parameters.

4.3.3  Overloading

In order to call a subroutine legally, you need to know its name, you need to know how many formal parameters it has, and you need to know the type of each parameter. This information is called the subroutine's signature. The signature of the subroutine doTask, used as an example above, can be expressed as: doTask(int,double,boolean). Note that the signature does not include the names of the parameters; in fact, if you just want to use the subroutine, you don't even need to know what the formal parameter names are, so the names are not part of the interface.

Java is somewhat unusual in that it allows two different subroutines in the same class to have the same name, provided that their signatures are different. When this happens, we say that the name of the subroutine is overloaded because it has several different meanings. The computer doesn't get the subroutines mixed up. It can tell which one you want to call by the number and types of the actual parameters that you provide in the subroutine call statement. You have already seen overloading used with System.out. This object includes many different methods named println, for example. These methods all have different signatures, such as:

println(int)                   println(double)
println(char)                  println(boolean)
println()

The computer knows which of these subroutines you want to use based on the type of the actual parameter that you provide. System.out.println(17) calls the subroutine with signature println(int), while System.out.println('A') calls the subroutine with signature println(char). Of course all these different subroutines are semantically related, which is why it is acceptable programming style to use the same name for them all. But as far as the computer is concerned, printing out an int is very different from printing out a char, which is different from printing out a boolean, and so forth -- so that each of these operations requires a different subroutine.

Note, by the way, that the signature does not include the subroutine's return type. It is illegal to have two subroutines in the same class that have the same signature but that have different return types. For example, it would be a syntax error for a class to contain two subroutines defined as:

int    getln() { ... }
double getln() { ... }

This is why in the TextIO class, the subroutines for reading different types are not all named getln(). In a given class, there can only be one routine that has the name getln with no parameters. So, the input routines in TextIO are distinguished by having different names, such as getlnInt() and getlnDouble().

4.3.4  Subroutine Examples

Let's do a few examples of writing small subroutines to perform assigned tasks. Of course, this is only one side of programming with subroutines. The task performed by a subroutine is always a subtask in a larger program. The art of designing those programs -- of deciding how to break them up into subtasks -- is the other side of programming with subroutines. We'll return to the question of program design in Section 4.6.

As a first example, let's write a subroutine to compute and print out all the divisors of a given positive integer. The integer will be a parameter to the subroutine. Remember that the syntax of any subroutine is:

modifiers  return-type  subroutine-name  ( parameter-list ) {
statements
}

Writing a subroutine always means filling out this format. In this case, the statement of the problem tells us that there is one parameter, of type int, and it tells us what the statements in the body of the subroutine should do. Since we are only working with static subroutines for now, we'll need to use static as a modifier. We could add an access modifier (public or private), but in the absence of any instructions, I'll leave it out. Since we are not told to return a value, the return type is void. Since no names are specified, we'll have to make up names for the formal parameter and for the subroutine itself. I'll use N for the parameter and printDivisors for the subroutine name. The subroutine will look like

static void printDivisors( int N ) {
statements
}

and all we have left to do is to write the statements that make up the body of the routine. This is not difficult. Just remember that you have to write the body assuming that N already has a value! The algorithm is: "For each possible divisor D in the range from 1 to N, if D evenly divides N, then print D." Written in Java, this becomes:

/**
* Print all the divisors of N.
* We assume that N is a positive integer.
*/
static void printDivisors( int N ) {
int D;   // One of the possible divisors of N.
System.out.println("The divisors of " + N + " are:");
for ( D = 1; D <= N; D++ ) {
if ( N % D == 0 )  // Does D evenly divide N?
System.out.println(D);
}
}

I've added a comment before the subroutine definition indicating the contract of the subroutine -- that is, what it does and what assumptions it makes. The contract includes the assumption that N is a positive integer. It is up to the caller of the subroutine to make sure that this assumption is satisfied.

As a second short example, consider the problem: Write a private subroutine named printRow. It should have a parameter ch of type char and a parameter N of type int. The subroutine should print out a line of text containing N copies of the character ch.

Here, we are told the name of the subroutine and the names of the two parameters, and we are told that the subroutine is private, so we don't have much choice about the first line of the subroutine definition. The task in this case is pretty simple, so the body of the subroutine is easy to write. The complete subroutine is given by

/**
* Write one line of output containing N copies of the
* character ch.  If N <= 0, an empty line is output.
*/
private static void printRow( char ch, int N ) {
int i;  // Loop-control variable for counting off the copies.
for ( i = 1; i <= N; i++ ) {
System.out.print( ch );
}
System.out.println();
}

Note that in this case, the contract makes no assumption about N, but it makes it clear what will happen in all cases, including the unexpected case that N < 0.

Finally, let's do an example that shows how one subroutine can build on another. Let's write a subroutine that takes a String as a parameter. For each character in the string, it should print a line of output containing 25 copies of that character. It should use the printRow() subroutine to produce the output.

Again, we get to choose a name for the subroutine and a name for the parameter. I'll call the subroutine printRowsFromString and the parameter str. The algorithm is pretty clear: For each position i in the string str, call printRow(str.charAt(i),25) to print one line of the output. So, we get:

/**
* For each character in str, write a line of output
* containing 25 copies of that character.
*/
private static void printRowsFromString( String str ) {
int i;  // Loop-control variable for counting off the chars.
for ( i = 0; i < str.length(); i++ ) {
printRow( str.charAt(i), 25 );
}
}

We could use printRowsFromString in a main() routine such as

public static void main(String[] args) {
String inputLine;  // Line of text input by user.
System.out.print("Enter a line of text: ");
inputLine = TextIO.getln();
System.out.println();
printRowsFromString( inputLine );
}

Of course, the three routines, main(), printRowsFromString(), and printRow(), would have to be collected together inside the same class. The program is rather useless, but it does demonstrate the use of subroutines. You'll find the program in the file RowsOfChars.java, if you want to take a look.

4.3.5  Array Parameters

It's possible for the type of a parameter to be an array type. This means that an entire array of values can be passed to the subroutine as a single parameter. For example, we might want a subroutine to print all the values in an integer array in a neat format, separated by commas and enclosed in a pair of square brackets. To tell it which array to print, the subroutine would have a parameter of type int[]:

static void printValuesInList( int[] list ) {
System.out.print('[');
int i;
for ( i = 0; i < list.length; i++ ) {
if ( i > 0 )
System.out.print(','); // No comma in front of list[0]
System.out.print(list[i]);
}
System.out.println(']');
}

To use this subroutine, you need an actual array. Here is a legal, though not very realistic, code segment that creates an array just to pass it as an argument to the subroutine:

int[] numbers;
numbers = new int[3];
numbers[0] = 42;
numbers[1] = 17;
numbers[2] = 256;
printValuesInList( numbers );

The output produced by the last statement would be [42,17,256].

4.3.6  Command-line Arguments

The main routine of a program has a parameter of type String[]. When the main routine is called, some actual array of String must be passed to main() as the value of the parameter. The system provides the actual parameter when it calls main(), so the values come from outside the program. Where do the strings in the array come from, and what do they mean? The strings in the array are command-line arguments from the command that was used to run the program. When using a command-line interface, the user types a command to tell the system to execute a program. The user can include extra input in this command, beyond the name of the program. This extra input becomes the command-line arguments. The system takes the command-line arguments, puts them into an array of strings, and passes that array to main().

For example, if the name of the program is myProg, then the user can type "java myProg" to execute the program. In this case, there are no command-line arguments. But if the user types the command

java myProg one two three

then the command-line arguments are the strings "one", "two", and "three". The system puts these strings into an array of Strings and passes that array as a parameter to the main() routine. Here, for example, is a short program that simply prints out any command line arguments entered by the user:

public class CLDemo {

public static void main(String[] args) {
System.out.println("You entered " + args.length
+ " command-line arguments");
if (args.length > 0) {
System.out.println("They were:");
int i;
for ( i = 0; i < args.length; i++ )
System.out.println("   " + args[i]);
}
} // end main()

} // end class CLDemo

Note that the parameter, args, can be an array of length zero. This just means that the user did not include any command-line arguments when running the program.

In practice, command-line arguments are often used to pass the names of files to a program. For example, consider the following program for making a copy of a text file. It does this by copying one line at a time from the original file to the copy, using TextIO. The function TextIO.eof() is a boolean-valued function that is true if the end of the file has been reached.

/**
*  Requires two command line arguments, which must be file names.  The
*  the first must be the name of an existing file.  The second is the name
*  of a file to be created by the program.  The contents of the first file
*  are copied into the second.  WARNING:  If the second file already
*  exists when the program is run, its previous contents will be lost!
*  This program only works for plain text files.
*/
public class CopyTextFile {

public static void main( String[] args ) {
if (args.length < 2 ) {
System.out.println("Two command-line arguments are required!");
System.exit(1);
}
TextIO.readFile( args[0] );   // Open the original file for reading.
TextIO.writeFile( args[1] );  // Open the copy file for writing.
int lineCount;  // Number of lines copied
lineCount = 0;
while ( TextIO.eof() == false ) {
// Read one line from the original file and write it to the copy.
String line;
line = TextIO.getln();
TextIO.putln(line);
lineCount++;
}
System.out.printf( "%d lines copied from %s to %s%n",
lineCount, args[0], args[1] );
}

}

Since most programs are run in a GUI environment these days, command-line arguments aren't as important as they used to be. But at least they provide a nice example of how array parameters can be used.

4.3.7  Throwing Exceptions

I have been talking about the "contract" of a subroutine. The contract says what the subroutine will do, provided that the caller of the subroutine provides acceptable values for the subroutine's parameters. The question arises, though, what should the subroutine do when the caller violates the contract by providing bad parameter values?

We've already seen that some subroutines respond to bad parameter values by throwing exceptions. (See Section 3.7.) For example, the contract of the built-in subroutine Double.parseDouble says that the parameter should be a string representation of a number of type double; if this is true, then the subroutine will convert the string into the equivalent numeric value. If the caller violates the contract by passing an invalid string as the actual parameter, the subroutine responds by throwing an exception of type NumberFormatException.

Many subroutines throw IllegalArgumentExceptions in response to bad parameter values. You might want to do the same in your own subroutines. This can be done with a throw statement. An exception is an object, and in order to throw an exception, you must create an exception object. You won't officially learn how to do this until Chapter 5, but for now, you can use the following syntax for a throw statement that throws an IllegalArgumentException:

throw  new  IllegalArgumentException( error-message );

where error-message is a string that describes the error that has been detected. (The word "new" in this statement is what creates the object.) To use this statement in a subroutine, you would check whether the values of the parameters are legal. If not, you would throw the exception. For example, consider the print3NSequence subroutine from the beginning of this section. The parameter of print3NSequence is supposed to be a positive integer. We can modify the subroutine definition to make it throw an exception when this condition is violated:

static void print3NSequence(int startingValue) {

if (startingValue <= 0)  // The contract is violated!
throw new IllegalArgumentException( "Starting value must be positive." );
.
.  // (The rest of the subroutine is the same as before.)
.

If the start value is bad, the computer executes the throw statement. This will immediately terminate the subroutine, without executing the rest of the body of the subroutine. Furthermore, the program as a whole will crash unless the exception is "caught" and handled elsewhere in the program by a try..catch statement, as discussed in Section 3.7. For this to work, the subroutine call would have to be in the "try" part of the statement.

4.3.8  Global and Local Variables

I'll finish this section on parameters by noting that we now have three different sorts of variables that can be used inside a subroutine: local variables declared in the subroutine, formal parameter names, and static member variables that are declared outside the subroutine.

Local variables have no connection to the outside world; they are purely part of the internal working of the subroutine.

Parameters are used to "drop" values into the subroutine when it is called, but once the subroutine starts executing, parameters act much like local variables. Changes made inside a subroutine to a formal parameter have no effect on the rest of the program (at least if the type of the parameter is one of the primitive types -- things are more complicated in the case of arrays and objects, as we'll see later).

Things are different when a subroutine uses a variable that is defined outside the subroutine. That variable exists independently of the subroutine, and it is accessible to other parts of the program as well. Such a variable is said to be global to the subroutine, as opposed to the local variables defined inside the subroutine. A global variable can be used in the entire class in which it is defined and, if it is not private, in other classes as well. Changes made to a global variable can have effects that extend outside the subroutine where the changes are made. You've seen how this works in the last example in the previous section, where the values of the global variables, gamesPlayed and gamesWon, are computed inside a subroutine and are used in the main() routine.

It's not always bad to use global variables in subroutines, but you should realize that the global variable then has to be considered part of the subroutine's interface. The subroutine uses the global variable to communicate with the rest of the program. This is a kind of sneaky, back-door communication that is less visible than communication done through parameters, and it risks violating the rule that the interface of a black box should be straightforward and easy to understand. So before you use a global variable in a subroutine, you should consider whether it's really necessary.

I don't advise you to take an absolute stand against using global variables inside subroutines. There is at least one good reason to do it: If you think of the class as a whole as being a kind of black box, it can be very reasonable to let the subroutines inside that box be a little sneaky about communicating with each other, if that will make the class as a whole look simpler from the outside.

Section 4.4

Return Values

A subroutine that returns a value is called a function. A given function can only return a value of a specified type, called the return type of the function. A function call generally occurs in a position where the computer is expecting to find a value, such as the right side of an assignment statement, as an actual parameter in a subroutine call, or in the middle of some larger expression. A boolean-valued function can even be used as the test condition in an if, while, for or do..while statement.

(It is also legal to use a function call as a stand-alone statement, just as if it were a regular subroutine. In this case, the computer ignores the value computed by the subroutine. Sometimes this makes sense. For example, the function TextIO.getln(), with a return type of String, reads and returns a line of input typed in by the user. Usually, the line that is returned is assigned to a variable to be used later in the program, as in the statement "name = TextIO.getln();". However, this function is also useful as a subroutine call statement "TextIO.getln();", which still reads all input up to and including the next carriage return. Since the return value is not assigned to a variable or used in an expression, it is simply discarded. So, the effect of the subroutine call is to read and discard some input. Sometimes, discarding unwanted input is exactly what you need to do.)

4.4.1  The return statement

You've already seen how functions such as Math.sqrt() and TextIO.getInt() can be used. What you haven't seen is how to write functions of your own. A function takes the same form as a regular subroutine, except that you have to specify the value that is to be returned by the subroutine. This is done with a return statement, which has the following syntax:

return  expression ;

Such a return statement can only occur inside the definition of a function, and the type of the expression must match the return type that was specified for the function. (More exactly, it must be legal to assign the expression to a variable whose type is specified by the return type.) When the computer executes this return statement, it evaluates the expression, terminates execution of the function, and uses the value of the expression as the returned value of the function.

For example, consider the function definition

static double pythagoras(double x, double y) {
// Computes the length of the hypotenuse of a right
// triangle, where the sides of the triangle are x and y.
return  Math.sqrt( x*x + y*y );
}

Suppose the computer executes the statement "totalLength = 17 + pythagoras(12,5);". When it gets to the term pythagoras(12,5), it assigns the actual parameters 12 and 5 to the formal parameters x and y in the function. In the body of the function, it evaluates Math.sqrt(12.0*12.0 + 5.0*5.0), which works out to 13.0. This value is "returned" by the function, so the 13.0 essentially replaces the function call in the assignment statement, which then has the same effect as the statement "totalLength = 17+13.0 ". The return value is added to 17, and the result, 30.0, is stored in the variable, totalLength.

Note that a return statement does not have to be the last statement in the function definition. At any point in the function where you know the value that you want to return, you can return it. Returning a value will end the function immediately, skipping any subsequent statements in the function. However, it must be the case that the function definitely does return some value, no matter what path the execution of the function takes through the code.

You can use a return statement inside an ordinary subroutine, one with declared return type "void". Since a void subroutine does not return a value, the return statement does not include an expression; it simply takes the form "return;". The effect of this statement is to terminate execution of the subroutine and return control back to the point in the program from which the subroutine was called. This can be convenient if you want to terminate execution somewhere in the middle of the subroutine, but return statements are optional in non-function subroutines. In a function, on the other hand, a return statement, with expression, is always required.

Note that a return inside a loop will end the loop as well as the subroutine that contains it. Similarly, a return in a switch statement breaks out of the switch statement as well as the subroutine. So, you will sometimes use return in contexts where you are used to seeing a break.

4.4.2  Function Examples

Here is a very simple function that could be used in a program to compute 3N+1 sequences. (The 3N+1 sequence problem is one we've looked at several times already, including in the previous section.) Given one term in a 3N+1 sequence, this function computes the next term of the sequence:

static int nextN(int currentN) {
if (currentN % 2 == 1)     // test if current N is odd
return 3*currentN + 1;  // if so, return this value
else
return currentN / 2;    // if not, return this instead
}

This function has two return statements. Exactly one of the two return statements is executed to give the value of the function. Some people prefer to use a single return statement at the very end of the function when possible. This allows the reader to find the return statement easily. You might choose to write nextN() like this, for example:

static int nextN(int currentN) {
int answer;  // answer will be the value returned
if (currentN % 2 == 1)    // test if current N is odd
answer = 3*currentN+1; // if so, this is the answer
else
answer = currentN / 2; // if not, this is the answer
return answer;   // (Don't forget to return the answer!)
}

Here is a subroutine that uses this nextN function. In this case, the improvement from the version of the subroutine in Section 4.3 is not great, but if nextN() were a long function that performed a complex computation, then it would make a lot of sense to hide that complexity inside a function:

static void print3NSequence(int startingValue) {

int N;       // One of the terms in the sequence.
int count;   // The number of terms found.

N = startingValue;   // Start the sequence with startingValue.
count = 1;

System.out.println("The 3N+1 sequence starting from " + N);
System.out.println();
System.out.println(N);  // print initial term of sequence

while (N > 1) {
N = nextN( N );   // Compute next term, using the function nextN.
count++;          // Count this term.
System.out.println(N);  // Print this term.
}

System.out.println();
System.out.println("There were " + count + " terms in the sequence.");

}

Here are a few more examples of functions. The first one computes a letter grade corresponding to a given numerical grade, on a typical grading scale:

/**
* Returns the letter grade corresponding to the numerical
* grade that is passed to this function as a parameter.
*/
static char letterGrade(int numGrade) {
if (numGrade >= 90)
return 'A';   // 90 or above gets an A
else if (numGrade >= 80)
return 'B';   // 80 to 89 gets a B
else if (numGrade >= 65)
return 'C';   // 65 to 79 gets a C
else if (numGrade >= 50)
return 'D';   // 50 to 64 gets a D
else
return 'F';   // anything else gets an F

}  // end of function letterGrade

The type of the return value of letterGrade() is char. Functions can return values of any type at all. Here's a function whose return value is of type boolean. It demonstrates some interesting programming points, so you should read the comments:

/**
* This function returns true if N is a prime number.  A prime number
* is an integer greater than 1 that is not divisible by any positive
* integer, except itself and 1.  If N has any divisor, D, in the range
* 1 < D < N, then it has a divisor in the range 2 to Math.sqrt(N), namely
* either D itself or N/D.  So we only test possible divisors from 2 to
* Math.sqrt(N).
*/
static boolean isPrime(int N) {

int divisor;  // A number we will test to see whether it evenly divides N.

if (N <= 1)
return false;  // No number <= 1 is a prime.

int maxToTry;  // The largest divisor that we need to test.

maxToTry = (int)Math.sqrt(N);
// We will try to divide N by numbers between 2 and maxToTry.
// If N is not evenly divisible by any of these numbers, then
// N is prime.  (Note that since Math.sqrt(N) is defined to
// return a value of type double, the value must be typecast
// to type int before it can be assigned to maxToTry.)

for (divisor = 2; divisor <= maxToTry; divisor++) {
if ( N % divisor == 0 )  // Test if divisor evenly divides N.
return false;         // If so, we know N is not prime.
// No need to continue testing!
}

// If we get to this point, N must be prime.  Otherwise,
// the function would already have been terminated by
// a return statement in the previous loop.

return true;  // Yes, N is prime.

}  // end of function isPrime

Finally, here is a function with return type String. This function has a String as parameter. The returned value is a reversed copy of the parameter. For example, the reverse of "Hello World" is "dlroW olleH". The algorithm for computing the reverse of a string, str, is to start with an empty string and then to append each character from str, starting from the last character of str and working backwards to the first:

static String reverse(String str) {
String copy;  // The reversed copy.
int i;        // One of the positions in str,
//       from str.length() - 1 down to 0.
copy = "";    // Start with an empty string.
for ( i = str.length() - 1;  i >= 0;  i-- ) {
// Append i-th char of str to copy.
copy = copy + str.charAt(i);
}
return copy;
}

A palindrome is a string that reads the same backwards and forwards, such as "radar". The reverse() function could be used to check whether a string, word, is a palindrome by testing "if (word.equals(reverse(word)))".

By the way, a typical beginner's error in writing functions is to print out the answer, instead of returning it. This represents a fundamental misunderstanding. The task of a function is to compute a value and return it to the point in the program where the function was called. That's where the value is used. Maybe it will be printed out. Maybe it will be assigned to a variable. Maybe it will be used in an expression. But it's not for the function to decide.

4.4.3  3N+1 Revisited

I'll finish this section with a complete new version of the 3N+1 program. This will give me a chance to show the function nextN(), which was defined above, used in a complete program. I'll also take the opportunity to improve the program by getting it to print the terms of the sequence in columns, with five terms on each line. This will make the output more presentable. The idea is this: Keep track of how many terms have been printed on the current line; when that number gets up to 5, start a new line of output. To make the terms line up into neat columns, I use formatted output.

/**
* A program that computes and displays several 3N+1 sequences.  Starting
* values for the sequences are input by the user.  Terms in the sequence
* are printed in columns, with five terms on each line of output.
* After a sequence has been displayed, the number of terms in that
* sequence is reported to the user.
*/
public class ThreeN2 {

public static void main(String[] args) {

System.out.println("This program will print out 3N+1 sequences");
System.out.println("for starting values that you specify.");
System.out.println();

int K;   // Starting point for sequence, specified by the user.
do {
System.out.println("Enter a starting value;");
System.out.print("To end the program, enter 0: ");
K = TextIO.getlnInt();  // get starting value from user
if (K > 0)              // print sequence, but only if K is > 0
print3NSequence(K);
} while (K > 0);           // continue only if K > 0

} // end main

/**
* print3NSequence prints a 3N+1 sequence to standard output, using
* startingValue as the initial value of N.  It also prints the number
* of terms in the sequence. The value of the parameter, startingValue,
* must be a positive integer.
*/
static void print3NSequence(int startingValue) {

int N;       // One of the terms in the sequence.
int count;   // The number of terms found.
int onLine;  // The number of terms that have been output
//     so far on the current line.

N = startingValue;   // Start the sequence with startingValue;
count = 1;           // We have one term so far.

System.out.println("The 3N+1 sequence starting from " + N);
System.out.println();
System.out.printf("%8d", N);  // Print initial term, using 8 characters.
onLine = 1;        // There's now 1 term on current output line.

while (N > 1) {
N = nextN(N);  // compute next term
count++;   // count this term
if (onLine == 5) {  // If current output line is full
System.out.println();  // ...then output a carriage return
onLine = 0;      // ...and note that there are no terms
//               on the new line.
}
System.out.printf("%8d", N);  // Print this term in an 8-char column.
onLine++;   // Add 1 to the number of terms on this line.
}

System.out.println();  // end current line of output
System.out.println();  // and then add a blank line
System.out.println("There were " + count + " terms in the sequence.");

}  // end of print3NSequence

/**
* nextN computes and returns the next term in a 3N+1 sequence,
* given that the current term is currentN.
*/
static int nextN(int currentN) {
if (currentN % 2 == 1)
return 3 * currentN + 1;
else
return currentN / 2;
}  // end of nextN()

} // end of class ThreeN2

You should read this program carefully and try to understand how it works.

Section 4.5

APIs, Packages, and Javadoc

As computers and their user interfaces have become easier to use, they have also become more complex for programmers to deal with. You can write programs for a simple console-style user interface using just a few subroutines that write output to the console and read the user's typed replies. A modern graphical user interface, with windows, buttons, scroll bars, menus, text-input boxes, and so on, might make things easier for the user, but it forces the programmer to cope with a hugely expanded array of possibilities. The programmer sees this increased complexity in the form of great numbers of subroutines that are provided for managing the user interface, as well as for other purposes.

4.5.1  Toolboxes

Someone who wanted to program for Macintosh computers -- and to produce programs that look and behave the way users expect them to -- had to deal with the Macintosh Toolbox, a collection of well over a thousand different subroutines. There are routines for opening and closing windows, for drawing geometric figures and text to windows, for adding buttons to windows, and for responding to mouse clicks on the window. There are other routines for creating menus and for reacting to user selections from menus. Aside from the user interface, there are routines for opening files and reading data from them, for communicating over a network, for sending output to a printer, for handling communication between programs, and in general for doing all the standard things that a computer has to do. Microsoft Windows provides its own set of subroutines for programmers to use, and they are quite a bit different from the subroutines used on the Mac. Linux has several different GUI toolboxes for the programmer to choose from.

The analogy of a "toolbox" is a good one to keep in mind. Every programming project involves a mixture of innovation and reuse of existing tools. A programmer is given a set of tools to work with, starting with the set of basic tools that are built into the language: things like variables, assignment statements, if statements, and loops. To these, the programmer can add existing toolboxes full of routines that have already been written for performing certain tasks. These tools, if they are well-designed, can be used as true black boxes: They can be called to perform their assigned tasks without worrying about the particular steps they go through to accomplish those tasks. The innovative part of programming is to take all these tools and apply them to some particular project or problem (word-processing, keeping track of bank accounts, processing image data from a space probe, Web browsing, computer games, ...). This is called applications programming.

A software toolbox is a kind of black box, and it presents a certain interface to the programmer. This interface is a specification of what routines are in the toolbox, what parameters they use, and what tasks they perform. This information constitutes the API, or Application Programming Interface, associated with the toolbox. The Macintosh API is a specification of all the routines available in the Macintosh Toolbox. A company that makes some hardware device -- say a card for connecting a computer to a network -- might publish an API for that device consisting of a list of routines that programmers can call in order to communicate with and control the device. Scientists who write a set of routines for doing some kind of complex computation -- such as solving "differential equations," say -- would provide an API to allow others to use those routines without understanding the details of the computations they perform.

The Java programming language is supplemented by a large, standard API. You've seen part of this API already, in the form of mathematical subroutines such as Math.sqrt(), the String data type and its associated routines, and the System.out.print() routines. The standard Java API includes routines for working with graphical user interfaces, for network communication, for reading and writing files, and more. It's tempting to think of these routines as being built into the Java language, but they are technically subroutines that have been written and made available for use in Java programs.

Java is platform-independent. That is, the same program can run on platforms as diverse as Mac OS, Windows, Linux, and others. The same Java API must work on all these platforms. But notice that it is the interface that is platform-independent; the implementation varies from one platform to another. A Java system on a particular computer includes implementations of all the standard API routines. A Java program includes only calls to those routines. When the Java interpreter executes a program and encounters a call to one of the standard routines, it will pull up and execute the implementation of that routine which is appropriate for the particular platform on which it is running. This is a very powerful idea. It means that you only need to learn one API to program for a wide variety of platforms.

4.5.2  Java's Standard Packages

Like all subroutines in Java, the routines in the standard API are grouped into classes. To provide larger-scale organization, classes in Java can be grouped into packages, which were introduced briefly in Subsection 2.6.6. You can have even higher levels of grouping, since packages can also contain other packages. In fact, the entire standard Java API is implemented in several packages. One of these, which is named "java", contains several non-GUI packages as well as the original AWT graphics user interface classes. Another package, "javax", was added in Java version 1.2 and contains the classes used by the Swing graphical user interface and other additions to the API.

A package can contain both classes and other packages. A package that is contained in another package is sometimes called a "sub-package." Both the java package and the javax package contain sub-packages. One of the sub-packages of java, for example, is called "awt". Since awt is contained within java, its full name is actually java.awt. This package contains classes that represent GUI components such as buttons and menus in the AWT. AWT is the older of the two Java GUI toolboxes and is no longer widely used. However, java.awt also contains a number of classes that form the foundation for all GUI programming, such as the Graphics class which provides routines for drawing on the screen, the Color class which represents colors, and the Font class which represents the fonts that are used to display characters on the screen. Since these classes are contained in the package java.awt, their full names are actually java.awt.Graphics, java.awt.Color, and java.awt.Font. (I hope that by now you've gotten the hang of how this naming thing works in Java.) Similarly, javax contains a sub-package named javax.swing, which includes such GUI classes as javax.swing.JButton, javax.swing.JMenu, and javax.swing.JFrame. The GUI classes in javax.swing, together with the foundational classes in java.awt, are all part of the API that makes it possible to program graphical user interfaces in Java.

The java package includes several other sub-packages, such as java.io, which provides facilities for input/output, java.net, which deals with network communication, and java.util, which provides a variety of "utility" classes. The most basic package is called java.lang. This package contains fundamental classes such as String, Math, Integer, and Double.

It might be helpful to look at a graphical representation of the levels of nesting in the java package, its sub-packages, the classes in those sub-packages, and the subroutines in those classes. This is not a complete picture, since it shows only a very few of the many items in each element:

The official documentation for the standard Java 7 API lists 209 different packages, including sub-packages, and it lists 4024 classes in these packages. Many of these are rather obscure or very specialized, but you might want to browse through the documentation to see what is available. As I write this, the documentation for the complete API can be found at

http://download.oracle.com/javase/7/docs/api/

Even an expert programmer won't be familiar with the entire API, or even a majority of it. In this book, you'll only encounter several dozen classes, and those will be sufficient for writing a wide variety of programs.

4.5.3  Using Classes from Packages

Let's say that you want to use the class java.awt.Color in a program that you are writing. Like any class, java.awt.Color is a type, which means that you can use it to declare variables and parameters and to specify the return type of a function. One way to do this is to use the full name of the class as the name of the type. For example, suppose that you want to declare a variable named rectColor of type java.awt.Color. You could say:

java.awt.Color  rectColor;

This is just an ordinary variable declaration of the form "type-name variable-name;". Of course, using the full name of every class can get tiresome, and you will hardly ever see full names like java.awt.Color used in a program. Java makes it possible to avoid using the full name of a class by importing the class. If you put

import java.awt.Color;

at the beginning of a Java source code file, then, in the rest of the file, you can abbreviate the full name java.awt.Color to just the simple name of the class, which is Color. Note that the import line comes at the start of a file (after the package statement, if there is one) and is not inside any class. Although it is sometimes referred to as a statement, it is more properly called an import directive since it is not a statement in the usual sense. The import directive "import java.awt.Color" would allow you to say

Color  rectColor;

to declare the variable. Note that the only effect of the import directive is to allow you to use simple class names instead of full "package.class" names. You aren't really importing anything substantial; if you leave out the import directive, you can still access the class -- you just have to use its full name. There is a shortcut for importing all the classes from a given package. You can import all the classes from java.awt by saying

import java.awt.*;

The "*" is a wildcard that matches every class in the package. (However, it does not match sub-packages; for example, you cannot import the entire contents of all the sub-packages of the java package by saying import java.*.)

Some programmers think that using a wildcard in an import statement is bad style, since it can make a large number of class names available that you are not going to use and might not even know about. They think it is better to explicitly import each individual class that you want to use. In my own programming, I often use wildcards to import all the classes from the most relevant packages, and use individual imports when I am using just one or two classes from a given package.

In fact, any Java program that uses a graphical user interface is likely to use many classes from the java.awt and javax.swing packages as well as from another package named java.awt.event, and I often begin such programs with

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

A program that works with networking might include the line "import java.net.*;", while one that reads or writes files might use "import java.io.*;". But when you start importing lots of packages in this way, you have to be careful about one thing: It's possible for two classes that are in different packages to have the same name. For example, both the java.awt package and the java.util package contain a class named List. If you import both java.awt.* and java.util.*, the simple name List will be ambiguous. If you try to declare a variable of type List, you will get a compiler error message about an ambiguous class name. You can still use both classes in your program: Use the full name of the class, either java.awt.List or java.util.List. Another solution, of course, is to use import to import the individual classes you need, instead of importing entire packages.

Because the package java.lang is so fundamental, all the classes in java.lang are automatically imported into every program. It's as if every program began with the statement "import java.lang.*;". This is why we have been able to use the class name String instead of java.lang.String, and Math.sqrt() instead of java.lang.Math.sqrt(). It would still, however, be perfectly legal to use the longer forms of the names.

Programmers can create new packages. Suppose that you want some classes that you are writing to be in a package named utilities. Then the source code file that defines those classes must begin with the line

package utilities;

This would come even before any import directive in that file. Furthermore, the source code file would be placed in a folder with the same name as the package, "utilities" in this example. And a class that is in a subpackage must be in a subfolder. For example, a class in a package named utilities.net would be in folder named "net" inside a folder named "utilities". A class that is in a package automatically has access to other classes in the same package; that is, a class doesn't have to import the package in which it is defined.

In projects that define large numbers of classes, it makes sense to organize those classes into packages. It also makes sense for programmers to create new packages as toolboxes that provide functionality and APIs for dealing with areas not covered in the standard Java API. (And in fact such "toolmaking" programmers often have more prestige than the applications programmers who use their tools.)

However, with just a couple of exceptions, I will not be creating packages in this textbook. For the purposes of this book, you need to know about packages mainly so that you will be able to import the standard packages. These packages are always available to the programs that you write. You might wonder where the standard classes are actually located. Again, that can depend to some extent on the version of Java that you are using, but in recent standard versions, they are stored in jar files in a subdirectory named lib inside the Java Runtime Environment installation directory. A jar (or "Java archive") file is a single file that can contain many classes. Most of the standard classes can be found in a jar file named rt.jar. In fact, Java programs are generally distributed in the form of jar files, instead of as individual class files.

Although we won't be creating packages explicitly, every class is actually part of a package. If a class is not specifically placed in a package, then it is put in something called the default package, which has no name. Almost all the examples that you see in this book are in the default package.

4.5.4  Javadoc

To use an API effectively, you need good documentation for it. The documentation for most Java APIs is prepared using a system called Javadoc. For example, this system is used to prepare the documentation for Java's standard packages. And almost everyone who creates a toolbox in Java publishes Javadoc documentation for it.

Javadoc documentation is prepared from special comments that are placed in the Java source code file. Recall that one type of Java comment begins with /* and ends with */. A Javadoc comment takes the same form, but it begins with /** rather than simply /*. You have already seen comments of this form in many of the examples in this book.

Note that a Javadoc comment must be placed just before the subroutine that it is commenting on. This rule is always followed. You can have Javadoc comments for subroutines, for member variables, and for classes. The Javadoc comment always immediately precedes the thing it is commenting on.

Like any comment, a Javadoc comment is ignored by the computer when the file is compiled. But there is a tool called javadoc that reads Java source code files, extracts any Javadoc comments that it finds, and creates a set of Web pages containing the comments in a nicely formatted, interlinked form. By default, javadoc will only collect information about public classes, subroutines, and member variables, but it allows the option of creating documentation for non-public things as well. If javadoc doesn't find any Javadoc comment for something, it will construct one, but the comment will contain only basic information such as the name and type of a member variable or the name, return type, and parameter list of a subroutine. This is syntactic information. To add information about semantics and pragmatics, you have to write a Javadoc comment.

As an example, you can look at the documentation Web page for TextIO. The documentation page was created by applying the javadoc tool to the source code file, TextIO.java. If you have downloaded the on-line version of this book, the documentation can be found in the TextIO_Javadoc directory, or you can find a link to it in the on-line version of this section.

In a Javadoc comment, the *'s at the start of each line are optional. The javadoc tool will remove them. In addition to normal text, the comment can contain certain special codes. For one thing, the comment can contain HTML mark-up commands. HTML is the language that is used to create web pages, and Javadoc comments are meant to be shown on web pages. The javadoc tool will copy any HTML commands in the comments to the web pages that it creates. The book will not teach you HTML, but as an example, you can add <p> to indicate the start of a new paragraph. (Generally, in the absence of HTML commands, blank lines and extra spaces in the comment are ignored. Furthermore, the characters & and < have special meaning in HTML and should not be used in Javadoc comments except with those meanings; they can be written as &amp; and &lt;.)

In addition to HTML commands, Javadoc comments can include doc tags, which are processed as commands by the javadoc tool. A doc tag has a name that begins with the character @. I will only discuss four tags: @author, @param, @return, and @throws. The @author tag can be used only for a class, and should be followed by the name of the author. The other three tags are used in Javadoc comments for a subroutine to provide information about its parameters, its return value, and the exceptions that it might throw. These tags must be placed at the end of the comment, after any description of the subroutine itself. The syntax for using them is:

@param  parameter-name   description-of-parameter

@return  description-of-return-value

@throws  exception-class-name   description-of-exception

The descriptions can extend over several lines. The description ends at the next doc tag or at the end of the comment. You can include a @param tag for every parameter of the subroutine and a @throws for as many types of exception as you want to document. You should have a @return tag only for a non-void subroutine. These tags do not have to be given in any particular order.

Here is an example that doesn't do anything exciting but that does use all three types of doc tag:

/**
* This subroutine computes the area of a rectangle, given its width
* and its height.  The length and the width should be positive numbers.
* @param width the length of one side of the rectangle
* @param height the length the second side of the rectangle
* @return the area of the rectangle
* @throws IllegalArgumentException if either the width or the height
*    is a negative number.
*/
public static double areaOfRectangle( double length, double width ) {
if ( width < 0  ||  height < 0 )
throw new IllegalArgumentException("Sides must have positive length.");
double area;
area = width * height;
return area;
}

I use Javadoc comments for many of my examples. I encourage you to use them in your own code, even if you don't plan to generate Web page documentation of your work, since it's a standard format that other Java programmers will be familiar with.

If you do want to create Web-page documentation, you need to run the javadoc tool. This tool is available as a command in the Java Development Kit that was discussed in Section 2.6. You can use javadoc in a command line interface similarly to the way that the javac and java commands are used. Javadoc can also be applied in the integrated development environments that were also discussed in Section 2.6. I won't go into any of the details here; consult the documentation for your programming environment.

4.5.5  Static Import

Before ending this section, I will mention an extension of the import directive. We have seen that import makes it possible to refer to a class such as java.awt.Color using its simple name, Color. But you still have to use compound names to refer to static member variables such as System.out and to static methods such as Math.sqrt.

There is another form of the import directive that can be used to import static members of a class in the same way that the ordinary import directive imports classes from a package. That form of the directive is called a static import, and it has syntax

import static package-name.class-name.static-member-name;

to import one static member name from a class, or

import static package-name.class-name.*;

to import all the public static members from a class. For example, if you preface a class definition with

import static java.lang.System.out;

then you can use the simple name out instead of the compound name System.out. This means you can say out.println instead of System.out.println. If you are going to work extensively with the Math class, you can preface your class definition with

import static java.lang.Math.*;

This would allow you to say sqrt instead of Math.sqrt, log instead of Math.log, PI instead of Math.PI, and so on.

Note that the static import directive requires a package-name, even for classes in the standard package java.lang. One consequence of this is that you can't do a static import from a class in the default package. In particular, it is not possible to do a static import from my TextIO class -- if you want to do that, you have to move TextIO into a package.

Section 4.6

More on Program Design

Understanding how programs work is one thing. Designing a program to perform some particular task is another thing altogether. In Section 3.2, I discussed how pseudocode and stepwise refinement can be used to methodically develop an algorithm. We can now see how subroutines can fit into the process.

Stepwise refinement is inherently a top-down process, but the process does have a "bottom," that is, a point at which you stop refining the pseudocode algorithm and translate what you have directly into proper program code. In the absence of subroutines, the process would not bottom out until you get down to the level of assignment statements and very primitive input/output operations. But if you have subroutines lying around to perform certain useful tasks, you can stop refining as soon as you've managed to express your algorithm in terms of those tasks.

This allows you to add a bottom-up element to the top-down approach of stepwise refinement. Given a problem, you might start by writing some subroutines that perform tasks relevant to the problem domain. The subroutines become a toolbox of ready-made tools that you can integrate into your algorithm as you develop it. (Alternatively, you might be able to buy or find a software toolbox written by someone else, containing subroutines that you can use in your project as black boxes.)

Subroutines can also be helpful even in a strict top-down approach. As you refine your algorithm, you are free at any point to take any sub-task in the algorithm and make it into a subroutine. Developing that subroutine then becomes a separate problem, which you can work on separately. Your main algorithm will merely call the subroutine. This, of course, is just a way of breaking your problem down into separate, smaller problems. It is still a top-down approach because the top-down analysis of the problem tells you what subroutines to write. In the bottom-up approach, you start by writing or obtaining subroutines that are relevant to the problem domain, and you build your solution to the problem on top of that foundation of subroutines.

4.6.1  Preconditions and Postconditions

When working with subroutines as building blocks, it is important to be clear about how a subroutine interacts with the rest of the program. This interaction is specified by the contract of the subroutine, as discussed in Section 4.1. A convenient way to express the contract of a subroutine is in terms of preconditions and postconditions.

A precondition of a subroutine is something that must be true when the subroutine is called, if the subroutine is to work correctly. For example, for the built-in function Math.sqrt(x), a precondition is that the parameter, x, is greater than or equal to zero, since it is not possible to take the square root of a negative number. In terms of a contract, a precondition represents an obligation of the caller of the subroutine. If you call a subroutine without meeting its precondition, then there is no reason to expect it to work properly. The program might crash or give incorrect results, but you can only blame yourself, not the subroutine.

A postcondition of a subroutine represents the other side of the contract. It is something that will be true after the subroutine has run (assuming that its preconditions were met -- and that there are no bugs in the subroutine). The postcondition of the function Math.sqrt() is that the square of the value that is returned by this function is equal to the parameter that is provided when the subroutine is called. Of course, this will only be true if the precondition -- that the parameter is greater than or equal to zero -- is met. A postcondition of the built-in subroutine System.out.print(x) is that the value of the parameter has been displayed on the screen.

Preconditions most often give restrictions on the acceptable values of parameters, as in the example of Math.sqrt(x). However, they can also refer to global variables that are used in the subroutine. Or if it only makes sense to call the subroutine at certain times, the precondition might refer to the state that the program must be in when the subroutine is called.

The postcondition of a subroutine, on the other hand, specifies the task that it performs. For a function, the postcondition should specify the value that the function returns.

Subroutines are sometimes described by comments that explicitly specify their preconditions and postconditions. When you are given a pre-written subroutine, a statement of its preconditions and postconditions tells you how to use it and what it does. When you are assigned to write a subroutine, the preconditions and postconditions give you an exact specification of what the subroutine is expected to do. I will use this approach in the example that constitutes the rest of this section. The comments are given in the form of Javadoc comments, but I will explicitly label the preconditions and postconditions. (Many computer scientists think that new doc tags @precondition and @postcondition should be added to the Javadoc system for explicit labeling of preconditions and postconditions, but that has not yet been done.)

4.6.2  A Design Example

Let's work through an example of program design using subroutines. In this example, we will use pre-written subroutines as building blocks and we will also design new subroutines that we need to complete the project. The API that I will use here is defined in Mosaic.java, which in turns depends on MosaicPanel.java. To compile and run a program that uses the API, the classes Mosaic and MosaicPanel must be available. That is, the files Mosaic.java and MosaicPanel.java, or the the corresponding compiled class files, must be in the same folder as the class that defines the program.

So, suppose that I have found an already-written class called Mosaic. This class allows a program to work with a window that displays little colored rectangles arranged in rows and columns. The window can be opened, closed, and otherwise manipulated with static member subroutines defined in the Mosaic class. In fact, the class defines a toolbox or API that can be used for working with such windows. Here are some of the available routines in the API, with Javadoc-style comments. (Remeber that a Javadoc comment comes before the thing that it is commenting on.)

/**
* Opens a "mosaic" window on the screen.
*
* Precondition:   The parameters rows, cols, w, and h are positive integers.
* Postcondition:  A window is open on the screen that can display rows and
*                   columns of colored rectangles.  Each rectangle is w pixels
*                   wide and h pixels high.  The number of rows is given by
*                   the first parameter and the number of columns by the
*                   second.  Initially, all rectangles are black.
*
* Note:  The rows are numbered from 0 to rows - 1, and the columns are
* numbered from 0 to cols - 1.
*/
public static void open(int rows, int cols, int w, int h)

/**
* Sets the color of one of the rectangles in the window.
*
* Precondition:   row and col are in the valid range of row and column numbers,
*                   and r, g, and b are in the range 0 to 255, inclusive.
* Postcondition:  The color of the rectangle in row number row and column
*                   number col has been set to the color specified by r, g,
*                   and b.  r gives the amount of red in the color with 0
*                   representing no red and 255 representing the maximum
*                   possible amount of red.  The larger the value of r, the
*                   more red in the color.  g and b work similarly for the
*                   green and blue color components.
*/
public static void setColor(int row, int col, int r, int g, int b)

/**
* Gets the red component of the color of one of the rectangles.
*
* Precondition:   row and col are in the valid range of row and column numbers.
* Postcondition:  The red component of the color of the specified rectangle is
*                   returned as an integer in the range 0 to 255 inclusive.
*/
public static int getRed(int row, int col)

/**
* Like getRed, but returns the green component of the color.
*/
public static int getGreen(int row, int col)

/**
* Like getRed, but returns the blue component of the color.
*/
public static int getBlue(int row, int col)

/**
* Tests whether the mosaic window is currently open.
*
* Precondition:   None.
* Postcondition:  The return value is true if the window is open when this
*                   function is called, and it is false if the window is
*                   closed.
*/
public static boolean isOpen()

/**
* Inserts a delay in the program (to regulate the speed at which the colors
* are changed, for example).
*
* Precondition:   milliseconds is a positive integer.
* Postcondition:  The program has paused for at least the specified number
*                   of milliseconds, where one second is equal to 1000
*                   milliseconds.
*/
public static void delay(int milliseconds)

Remember that these subroutines are members of the Mosaic class, so when they are called from outside Mosaic, the name of the class must be included as part of the name of the routine. For example, we'll have to use the name Mosaic.isOpen() rather than simply isOpen().

You'll notice that the comments on the subroutine don't specify what happens when the preconditions are not met. Although a subroutine is not really obligated by its contract to do anything particular in that case, it would be good to know what happens. For example, if the precondition, "row and col are in the valid range of row and column numbers," on the setColor() or getRed() routine is violated, an IllegalArgumentException will be thrown. Knowing that fact would allow you to write programs that catch and handle the exception, and it would be good to document it with a @throws doc tag in the Javadoc comment. Other questions remain about the behavior of the subroutines. For example, what happens if you call Mosaic.open() and there is already a mosaic window open on the screen? (In fact, the old one will be closed, and a new one will be created.) It's difficult to fully document the behavior of a piece of software -- sometimes, you just have to experiment or look at the full source code.

My idea for a program is to use the Mosaic class as the basis for a neat animation. I want to fill the window with randomly colored squares, and then randomly change the colors in a loop that continues as long as the window is open. "Randomly change the colors" could mean a lot of different things, but after thinking for a while, I decide it would be interesting to have a "disturbance" that wanders randomly around the window, changing the color of each square that it encounters. Here's a picture showing what the contents of the window might look like at one point in time:

With basic routines for manipulating the window as a foundation, I can turn to the specific problem at hand. A basic outline for my program is

Open a Mosaic window
Fill window with random colors
Move around, changing squares at random

Filling the window with random colors seems like a nice coherent task that I can work on separately, so let's decide to write a separate subroutine to do it. The third step can be expanded a bit more, into the steps: Start in the middle of the window, then keep moving to new squares and changing the color of those squares. This should continue as long as the mosaic window is still open. Thus we can refine the algorithm to:

Open a Mosaic window
Fill window with random colors
Set the current position to the middle square in the window
As long as the mosaic window is open:
Randomly change color of the square at the current position
Move current position up, down, left, or right, at random

I need to represent the current position in some way. That can be done with two int variables named currentRow and currentColumn that hold the row number and the column number of the square where the disturbance is currently located. I'll use 16 rows and 20 columns of squares in my mosaic, so setting the current position to be in the center means setting currentRow to 8 and currentColumn to 10. I already have a subroutine, Mosaic.open(), to open the window, and I have a function, Mosaic.isOpen(), to test whether the window is open. To keep the main routine simple, I decide that I will write two more subroutines of my own to carry out the two tasks in the while loop. The algorithm can then be written in Java as:

Mosaic.open(16,20,25,25)
fillWithRandomColors();
currentRow = 8;       // Middle row, halfway down the window.
currentColumn = 10;   // Middle column.
while ( Mosaic.isOpen() ) {
changeToRandomColor(currentRow, currentColumn);
randomMove();
}

With the proper wrapper, this is essentially the main() routine of my program. It turns out I have to make one small modification: To prevent the animation from running much, much too fast, the line "Mosaic.delay(1);" is added to the while loop.

The main() routine is taken care of, but to complete the program, I still have to write the subroutines fillWithRandomColors(), changeToRandomColor(int,int), and randomMove(). Writing each of these subroutines is a separate, small task. The fillWithRandomColors() routine is defined by the postcondition that "each of the rectangles in the mosaic has been changed to a random color." Pseudocode for an algorithm to accomplish this task can be given as:

For each row:
For each column:
set the square in that row and column to a random color

"For each row" and "for each column" can be implemented as for loops. We've already planned to write a subroutine changeToRandomColor that can be used to set the color. (The possibility of reusing subroutines in several places is one of the big payoffs of using them!) So, fillWithRandomColors() can be written in proper Java as:

static void fillWithRandomColors() {
int row, column;
for ( row = 0; row < 16; row++ )
for ( column = 0; column < 20; column++ )
changeToRandomColor(row,column);
}

Turning to the changeToRandomColor subroutine, we already have a method in the Mosaic class, Mosaic.setColor(), that can be used to change the color of a square. If we want a random color, we just have to choose random values for r, g, and b. According to the precondition of the Mosaic.setColor() subroutine, these random values must be integers in the range from 0 to 255. A formula for randomly selecting such an integer is "(int)(256*Math.random())". So the random color subroutine becomes:

static void changeToRandomColor(int rowNum, int colNum) {
int red = (int)(256*Math.random());
int green = (int)(256*Math.random());
int blue = (int)(256*Math.random());
Mosaic.setColor(rowNum,colNum,red,green,blue);
}

Finally, consider the randomMove subroutine, which is supposed to randomly move the disturbance up, down, left, or right. To make a random choice among four directions, we can choose a random integer in the range 0 to 3. If the integer is 0, move in one direction; if it is 1, move in another direction; and so on. The position of the disturbance is given by the variables currentRow and currentColumn. To "move up" means to subtract 1 from currentRow. This leaves open the question of what to do if currentRow becomes -1, which would put the disturbance above the window (which would violate a precondition of several of the Mosaic subroutines). Rather than let this happen, I decide to move the disturbance to the opposite edge of the grid by setting currentRow to 15. (Remember that the 16 rows are numbered from 0 to 15.) An alternative to jumping to the opposite edge would be to simply do nothing in this case. Moving the disturbance down, left, or right is handled similarly. If we use a switch statement to decide which direction to move, the code for randomMove becomes:

int directionNum;
directionNum = (int)(4*Math.random());
switch (directionNum) {
case 0:  // move up
currentRow--;
if (currentRow < 0)   // CurrentRow is outside the mosaic;
currentRow = 15;   // move it to the opposite edge.
break;
case 1:  // move right
currentColumn++;
if (currentColumn >= 20)
currentColumn = 0;
break;
case 2:  // move down
currentRow++;
if (currentRow >= 16)
currentRow = 0;
break;
case 3:  // move left
currentColumn--;
if (currentColumn < 0)
currentColumn = 19;
break;
}

4.6.3  The Program

Putting this all together, we get the following complete program. Note that I've added Javadoc-style comments for the class itself and for each of the subroutines. The variables currentRow and currentColumn are defined as static members of the class, rather than local variables, because each of them is used in several different subroutines. You can find a copy of the source code in RandomMosaicWalk.java. Remember that this program actually depends on two other files, Mosaic.java and MosaicPanel.java.

/**
* This program opens a window full of randomly colored squares.  A "disturbance"
* moves randomly around in the window, randomly changing the color of each
* square that it visits.  The program runs until the user closes the window.
*/
public class RandomMosaicWalk {

static int currentRow;    // Row currently containing the disturbance.
static int currentColumn; // Column currently containing disturbance.

/**
* The main program creates the window, fills it with random colors,
* and then moves the disturbance in a random walk around the window
* as long as the window is open.
*/
public static void main(String[] args) {
Mosaic.open(16,20,25,25);
fillWithRandomColors();
currentRow = 8;   // start at center of window
currentColumn = 10;
while (Mosaic.isOpen()) {
changeToRandomColor(currentRow, currentColumn);
randomMove();
Mosaic.delay(1);
}
}  // end main

/**
* Fills the window with randomly colored squares.
* Precondition:   The mosaic window is open.
* Postcondition:  Each square has been set to a random color.
*/
static void fillWithRandomColors() {
int row, column;
for ( row=0; row < 16; row++ ) {
for ( column=0; column < 20; column++ ) {
changeToRandomColor(row, column);
}
}
}  // end fillWithRandomColors

/**
* Changes one square to a new randomly selected color.
* Precondition:   The specified rowNum and colNum are in the valid range
*                 of row and column numbers.
* Postcondition:  The square in the specified row and column has
*                 been set to a random color.
* @param rowNum the row number of the square, counting rows down
*      from 0 at the top
* @param colNum the column number of the square, counting columns over
*      from 0 at the left
*/
static void changeToRandomColor(int rowNum, int colNum) {
int red = (int)(256*Math.random());    // Choose random levels in range
int green = (int)(256*Math.random());  //     0 to 255 for red, green,
int blue = (int)(256*Math.random());   //     and blue color components.
Mosaic.setColor(rowNum,colNum,red,green,blue);
}  // end changeToRandomColor

/**
* Move the disturbance.
* Precondition:   The global variables currentRow and currentColumn
*                 are within the legal range of row and column numbers.
* Postcondition:  currentRow or currentColumn is changed to one of the
*                 neighboring positions in the grid -- up, down, left, or
*                 right from the current position.  If this moves the
*                 position outside of the grid, then it is moved to the
*                 opposite edge of the grid.
*/
static void randomMove() {
int directionNum; // Randomly set to 0, 1, 2, or 3 to choose direction.
directionNum = (int)(4*Math.random());
switch (directionNum) {
case 0:  // move up
currentRow--;
if (currentRow < 0)
currentRow = 15;
break;
case 1:  // move right
currentColumn++;
if (currentColumn >= 20)
currentColumn = 0;
break;
case 2:  // move down
currentRow ++;
if (currentRow >= 16)
currentRow = 0;
break;
case 3:  // move left
currentColumn--;
if (currentColumn < 0)
currentColumn = 19;
break;
}
}  // end randomMove

} // end class RandomMosaicWalk

Section 4.7

The Truth About Declarations

Names are fundamental to programming, as I said a few chapters ago. There are a lot of details involved in declaring and using names. I have been avoiding some of those details. In this section, I'll reveal most of the truth (although still not the full truth) about declaring and using variables in Java. The material in the subsections "Initialization in Declarations" and "Named Constants" is particularly important, since I will be using it regularly from now on.

4.7.1  Initialization in Declarations

When a variable declaration is executed, memory is allocated for the variable. This memory must be initialized to contain some definite value before the variable can be used in an expression. In the case of a local variable, the declaration is often followed closely by an assignment statement that does the initialization. For example,

int count;    // Declare a variable named count.
count = 0;    // Give count its initial value.

However, the truth about declaration statements is that it is legal to include the initialization of the variable in the declaration statement. The two statements above can therefore be abbreviated as

int count = 0;  // Declare count and give it an initial value.

The computer still executes this statement in two steps: Declare the variable count, then assign the value 0 to the newly created variable. The initial value does not have to be a constant. It can be any expression. It is legal to initialize several variables in one declaration statement. For example,

char firstInitial = 'D', secondInitial = 'E';

int x, y = 1;   // OK, but only y has been initialized!

int N = 3, M = N+2;  // OK, N is initialized
//        before its value is used.

This feature is especially common in for loops, since it makes it possible to declare a loop control variable at the same point in the loop where it is initialized. Since the loop control variable generally has nothing to do with the rest of the program outside the loop, it's reasonable to have its declaration in the part of the program where it's actually used. For example:

for ( int i = 0;  i < 10;  i++ ) {
System.out.println(i);
}

You should remember that this is simply an abbreviation for the following, where I've added an extra pair of braces to show that i is considered to be local to the for statement and no longer exists after the for loop ends:

{
int i;
for ( i = 0;  i < 10;  i++ ) {
System.out.println(i);
}
}

A member variable can also be initialized at the point where it is declared, just as for a local variable. For example:

public class Bank {
private static double interestRate = 0.05;
private static int maxWithdrawal = 200;
.
.  // More variables and subroutines.
.
}

A static member variable is created as soon as the class is loaded by the Java interpreter, and the initialization is also done at that time. In the case of member variables, this is not simply an abbreviation for a declaration followed by an assignment statement. Declaration statements are the only type of statement that can occur outside of a subroutine. Assignment statements cannot, so the following is illegal:

public class Bank {
private static double interestRate;
interestRate = 0.05;  // ILLEGAL:
.                     //    Can't be outside a subroutine!:
.
.

Because of this, declarations of member variables often include initial values. In fact, as mentioned in Subsection 4.2.4, if no initial value is provided for a member variable, then a default initial value is used. For example, when declaring an integer member variable, count, "static int count;" is equivalent to "static int count = 0;".

Even array variables can be initialized. An array contains several elements, not just a single value. To initialize an array variable, you can provide a list of values, separated by commas, and enclosed between a pair of braces. For example:

int[] smallPrimes = { 2, 3, 5, 7, 11, 13, 17, 23, 29 };

In this statement, an array of int of length 9 is created and filled with the values in the list. The length of the array is determined by the number of items in the list.

Note that this syntax for initializing arrays cannot be used in assignment statements. It can only be used in a declaration statement at the time when the array variable is declared.

It is also possible to initialize an array variable with an array created using the new operator (which can also be used in assignment statements). For example:

String[] namelist = new String[100];

but in that case, of course, all the array elements will have their default value.

4.7.2  Named Constants

Sometimes, the value of a variable is not supposed to change after it is initialized. For example, in the above example where interestRate is initialized to the value 0.05, it's quite possible that 0.05 is meant to be the value throughout the entire program. In that case, the programmer is probably defining the variable, interestRate, to give a meaningful name to the otherwise meaningless number, 0.05. It's easier to understand what's going on when a program says "principal += principal*interestRate;" rather than "principal += principal*0.05;".

In Java, the modifier "final" can be applied to a variable declaration to ensure that the value stored in the variable cannot be changed after the variable has been initialized. For example, if the member variable interestRate is declared with

public final static double interestRate = 0.05;

then it would be impossible for the value of interestRate to change anywhere else in the program. Any assignment statement that tries to assign a value to interestRate will be rejected by the computer as a syntax error when the program is compiled. (A "final" modifier on a public interest rate makes a lot of sense -- a bank might want to publish its interest rate, but it certainly wouldn't want to let random people make changes to it!)

It is legal to apply the final modifier to local variables and even to formal parameters, but it is most useful for member variables. I will often refer to a static member variable that is declared to be final as a named constant, since its value remains constant for the whole time that the program is running. The readability of a program can be greatly enhanced by using named constants to give meaningful names to important quantities in the program. A recommended style rule for named constants is to give them names that consist entirely of upper case letters, with underscore characters to separate words if necessary. For example, the preferred style for the interest rate constant would be

public final static double INTEREST_RATE = 0.05;

This is the style that is generally used in Java's standard classes, which define many named constants. For example, we have already seen that the Math class contains a variable Math.PI. This variable is declared in the Math class as a "public final static" variable of type double. Similarly, the Color class contains named constants such as Color.RED and Color.YELLOW which are public final static variables of type Color. Many named constants are created just to give meaningful names to be used as parameters in subroutine calls. For example, the standard class named Font contains named constants Font.PLAIN, Font.BOLD, and Font.ITALIC. These constants are used for specifying different styles of text when calling various subroutines in the Font class.

Enumerated type constants (see Subsection 2.3.3) are also examples of named constants. The enumerated type definition

enum Alignment { LEFT, RIGHT, CENTER }

defines the constants Alignment.LEFT, Alignment.RIGHT, and Alignment.CENTER. Technically, Alignment is a class, and the three constants are public final static members of that class. Defining the enumerated type is similar to defining three constants of type, say, int:

public static final int ALIGNMENT_LEFT = 0;
public static final int ALIGNMENT_RIGHT = 1;
public static final int ALIGNMENT_CENTER = 2;

In fact, this is how things were generally done before the introduction of enumerated types, and it is what is done with the constants Font.PLAIN, Font.BOLD, and Font.ITALIC mentioned above. Using the integer constants, you could define a variable of type int and assign it the values ALIGNMENT_LEFT, ALIGNMENT_RIGHT, or ALIGNMENT_CENTER to represent different types of alignment. The only problem with this is that the computer has no way of knowing that you intend the value of the variable to represent an alignment, and it will not raise any objection if the value that is assigned to the variable is not one of the three valid alignment values. With the enumerated type, on the other hand, the only values that can be assigned to a variable of type Alignment are the constant values that are listed in the definition of the enumerated type. Any attempt to assign an invalid value to the variable is a syntax error which the computer will detect when the program is compiled. This extra safety is one of the major advantages of enumerated types.

Curiously enough, one of the major reasons to use named constants is that it's easy to change the value of a named constant. Of course, the value can't change while the program is running. But between runs of the program, it's easy to change the value in the source code and recompile the program. Consider the interest rate example. It's quite possible that the value of the interest rate is used many times throughout the program. Suppose that the bank changes the interest rate and the program has to be modified. If the literal number 0.05 were used throughout the program, the programmer would have to track down each place where the interest rate is used in the program and change the rate to the new value. (This is made even harder by the fact that the number 0.05 might occur in the program with other meanings besides the interest rate, as well as by the fact that someone might have, say, used 0.025 to represent half the interest rate.) On the other hand, if the named constant INTEREST_RATE is declared and used consistently throughout the program, then only the single line where the constant is initialized needs to be changed.

As an extended example, I will give a new version of the RandomMosaicWalk program from the previous section. This version uses named constants to represent the number of rows in the mosaic, the number of columns, and the size of each little square. The three constants are declared as final static member variables with the lines:

final static int ROWS = 20;        // Number of rows in mosaic.
final static int COLUMNS = 30;     // Number of columns in mosaic.
final static int SQUARE_SIZE = 15; // Size of each square in mosaic.

The rest of the program is carefully modified to use the named constants. For example, in the new version of the program, the Mosaic window is opened with the statement

Mosaic.open(ROWS, COLUMNS, SQUARE_SIZE, SQUARE_SIZE);

Sometimes, it's not easy to find all the places where a named constant needs to be used. If you don't use the named constant consistently, you've more or less defeated the purpose. It's always a good idea to run a program using several different values for any named constant, to test that it works properly in all cases.

Here is the complete new program, RandomMosaicWalk2, with all modifications from the previous version shown in red italic. I've left out some of the comments to save space.

public class RandomMosaicWalk2 {

final static int ROWS = 20;        // Number of rows in mosaic.
final static int COLUMNS = 30;     // Number of columns in mosaic.
final static int SQUARE_SIZE = 15; // Size of each square in mosaic.

static int currentRow;    // Row currently containing the disturbance.
static int currentColumn; // Column currently containing the disturbance.

public static void main(String[] args) {
Mosaic.open( ROWS, COLUMNS, SQUARE_SIZE, SQUARE_SIZE );
fillWithRandomColors();
currentRow = ROWS / 2;   // start at center of window
currentColumn = COLUMNS / 2;
while (Mosaic.isOpen()) {
changeToRandomColor(currentRow, currentColumn);
randomMove();
Mosaic.delay(1);
}
}  // end main

static void fillWithRandomColors() {
for (int row=0; row < ROWS; row++) {
for (int column=0; column < COLUMNS; column++) {
changeToRandomColor(row, column);
}
}
}  // end fillWithRandomColors

static void changeToRandomColor(int rowNum, int colNum) {
int red = (int)(256*Math.random());    // Choose random levels in range
int green = (int)(256*Math.random());  //     0 to 255 for red, green,
int blue = (int)(256*Math.random());   //     and blue color components.
Mosaic.setColor(rowNum,colNum,red,green,blue);
}  // end changeToRandomColor

static void randomMove() {
int directionNum; // Randomly set to 0, 1, 2, or 3 to choose direction.
directionNum = (int)(4*Math.random());
switch (directionNum) {
case 0:  // move up
currentRow--;
if (currentRow < 0)
currentRow = ROWS - 1;
break;
case 1:  // move right
currentColumn++;
if (currentColumn >= COLUMNS)
currentColumn = 0;
break;
case 2:  // move down
currentRow++;
if (currentRow >= ROWS)
currentRow = 0;
break;
case 3:  // move left
currentColumn--;
if (currentColumn < 0)
currentColumn = COLUMNS - 1;
break;
}
}  // end randomMove

} // end class RandomMosaicWalk2

4.7.3  Naming and Scope Rules

When a variable declaration is executed, memory is allocated for that variable. The variable name can be used in at least some part of the program source code to refer to that memory or to the data that is stored in the memory. The portion of the program source code where the variable is valid is called the scope of the variable. Similarly, we can refer to the scope of subroutine names and formal parameter names.

For static member subroutines, scope is straightforward. The scope of a static subroutine is the entire source code of the class in which it is defined. That is, it is possible to call the subroutine from any point in the class, including at a point in the source code before the point where the definition of the subroutine appears. It is even possible to call a subroutine from within itself. This is an example of something called "recursion," a fairly advanced topic that we will return to in Chapter 9. If the subroutine is not private, it can also be accessed from outside the class where it is defined, using its full name.

For a variable that is declared as a static member variable in a class, the situation is similar, but with one complication. It is legal to have a local variable or a formal parameter that has the same name as a member variable. In that case, within the scope of the local variable or parameter, the member variable is hidden. Consider, for example, a class named Game that has the form:

public class Game {

static int count;  // member variable

static void playGame() {
int count;  // local variable
.
.   // Some statements to define playGame()
.
}

.
.   // More variables and subroutines.
.

}  // end Game

In the statements that make up the body of the playGame() subroutine, the name "count" refers to the local variable. In the rest of the Game class, "count" refers to the member variable (unless hidden by other local variables or parameters named count). However, the member variable named count can also be referred to by the full name Game.count. Usually, the full name is only used outside the class where count is defined. However, there is no rule against using it inside the class. The full name, Game.count, can be used inside the playGame() subroutine to refer to the member variable instead of the local variable. So, the full scope rule is that the scope of a static member variable includes the entire class in which it is defined, but where the simple name of the member variable is hidden by a local variable or formal parameter name, the member variable must be referred to by its full name of the form className.variableName. (Scope rules for non-static members are similar to those for static members, except that, as we shall see, non-static members cannot be used in static subroutines.)

The scope of a formal parameter of a subroutine is the block that makes up the body of the subroutine. The scope of a local variable extends from the declaration statement that defines the variable to the end of the block in which the declaration occurs. As noted above, it is possible to declare a loop control variable of a for loop in the for statement, as in "for (int i=0; i < 10; i++)". The scope of such a declaration is considered as a special case: It is valid only within the for statement and does not extend to the remainder of the block that contains the for statement.

It is not legal to redefine the name of a formal parameter or local variable within its scope, even in a nested block. For example, this is not allowed:

void  badSub(int y) {
int x;
while (y > 0) {
int x;  // ERROR:  x is already defined.
.
.
.
}
}

In many languages, this would be legal; the declaration of x in the while loop would hide the original declaration. It is not legal in Java; however, once the block in which a variable is declared ends, its name does become available for reuse in Java. For example:

void goodSub(int y) {
while (y > 10) {
int x;
.
.
.
// The scope of x ends here.
}
while (y > 0) {
int x;  // OK: Previous declaration of x has expired.
.
.
.
}
}

You might wonder whether local variable names can hide subroutine names. This can't happen, for a reason that might be surprising. There is no rule that variables and subroutines have to have different names. The computer can always tell whether a name refers to a variable or to a subroutine, because a subroutine name is always followed by a left parenthesis. It's perfectly legal to have a variable called count and a subroutine called count in the same class. (This is one reason why I often write subroutine names with parentheses, as when I talk about the main() routine. It's a good idea to think of the parentheses as part of the name.) Even more is true: It's legal to reuse class names to name variables and subroutines. The syntax rules of Java guarantee that the computer can always tell when a name is being used as a class name. A class name is a type, and so it can be used to declare variables and formal parameters and to specify the return type of a function. This means that you could legally have a class called Insanity in which you declare a function

static  Insanity  Insanity( Insanity Insanity ) { ... }

The first Insanity is the return type of the function. The second is the function name, the third is the type of the formal parameter, and the fourth is the name of the formal parameter. However, please remember that not everything that is possible is a good idea!

Programming Exercises for Chapter 4

Exercise 4.1:

To "capitalize" a string means to change the first letter of each word in the string to upper case (if it is not already upper case). For example, a capitalized version of "Now is the time to act!" is "Now Is The Time To Act!". Write a subroutine named printCapitalized that will print a capitalized version of a string to standard output. The string to be printed should be a parameter to the subroutine. Test your subroutine with a main() routine that gets a line of input from the user and applies the subroutine to it.

Note that a letter is the first letter of a word if it is not immediately preceded in the string by another letter. Recall from Exercise 3.4 that there is a standard boolean-valued function Character.isLetter(char) that can be used to test whether its parameter is a letter. There is another standard char-valued function, Character.toUpperCase(char), that returns a capitalized version of the single character passed to it as a parameter. That is, if the parameter is a letter, it returns the upper-case version. If the parameter is not a letter, it just returns a copy of the parameter.

See the Solution

Exercise 4.2:

The hexadecimal digits are the ordinary, base-10 digits '0' through '9' plus the letters 'A' through 'F'. In the hexadecimal system, these digits represent the values 0 through 15, respectively. Write a function named hexValue that uses a switch statement to find the hexadecimal value of a given character. The character is a parameter to the function, and its hexadecimal value is the return value of the function. You should count lower case letters 'a' through 'f' as having the same value as the corresponding upper case letters. If the parameter is not one of the legal hexadecimal digits, return -1 as the value of the function.

A hexadecimal integer is a sequence of hexadecimal digits, such as 34A7, ff8, 174204, or FADE. If str is a string containing a hexadecimal integer, then the corresponding base-10 integer can be computed as follows:

value = 0;
for ( i = 0; i < str.length();  i++ )
value = value*16 + hexValue( str.charAt(i) );

Of course, this is not valid if str contains any characters that are not hexadecimal digits. Write a program that reads a string from the user. If all the characters in the string are hexadecimal digits, print out the corresponding base-10 value. If not, print out an error message.

See the Solution

Exercise 4.3:

Write a function that simulates rolling a pair of dice until the total on the dice comes up to be a given number. The number that you are rolling for is a parameter to the function. The number of times you have to roll the dice is the return value of the function. The parameter should be one of the possible totals: 2, 3, ..., 12. The function should throw an IllegalArgumentException if this is not the case. Use your function in a program that computes and prints the number of rolls it takes to get snake eyes. (Snake eyes means that the total showing on the dice is 2.)

See the Solution

Exercise 4.4:

This exercise builds on Exercise 4.3. Every time you roll the dice repeatedly, trying to get a given total, the number of rolls it takes can be different. The question naturally arises, what's the average number of rolls to get a given total? Write a function that performs the experiment of rolling to get a given total 10000 times. The desired total is a parameter to the subroutine. The average number of rolls is the return value. Each individual experiment should be done by calling the function you wrote for Exercise 4.3. Now, write a main program that will call your function once for each of the possible totals (2, 3, ..., 12). It should make a table of the results, something like:

Total On Dice     Average Number of Rolls
-------------     -----------------------
2               35.8382
3               18.0607
.                .
.                .

See the Solution

Exercise 4.5:

The sample program RandomMosaicWalk.java from Section 4.6 shows a "disturbance" that wanders around a grid of colored squares. When the disturbance visits a square, the color of that square is changed. Here's an idea for a variation on that program. In the new version, all the squares start out with the default color, black. Every time the disturbance visits a square, a small amount is added to the green component of the color of that square. The result will be a visually interesting effect, as the path followed by the disturbance gradually turns a brighter and brighter green.

Write a subroutine that will add 25 to the green component of one of the squares in the mosaic. (But don't let the green component go over 255, since that's the largest legal value for a color component.) The row and column numbers of the square should be given as parameters to the subroutine. Recall that you can discover the current green component of the square in row r and column c with the function call Mosaic.getGreen(r,c). Use your subroutine as a substitute for the changeToRandomColor() subroutine in the program RandomMosaicWalk2.java. (This is the improved version of the program from Section 4.7 that uses named constants for the number of rows, number of columns, and square size.) Set the number of rows and the number of columns to 80. Set the square size to 5.

By default, the rectangles in the mosaic have a "3D" appearance and a gray border that makes them look nicer in the random walk program. But for this program, you want to turn off that effect. To do so, call Mosaic.setUse3DEffect(false) in the main program.

Don't forget that you will need Mosaic.java and MosaicPanel.java to compile and run your program, since they define non-standard classes that are required by the program.

See the Solution

Exercise 4.6:

For this exercise, you will do something even more interesting with the Mosaic class that was discussed in Section 4.6. (Again, don't forget that you will need Mosaic.java and MosaicPanel.java to compile and run your program.)

The program that you write for this exercise should start by filling a mosaic with random colors. Then repeat the following until the user closes the mosaic window: Select one of the rectangles in the mosaic at random. Then select one of the neighboring rectangles -- above it, below it, to the left of it, or to the right of it. Copy the color of the originally selected rectangle to the selected neighbor, so that the two rectangles now have the same color.

As this process is repeated over and over, it becomes more and more likely that neighboring squares will have the same color. The result is to build up larger color patches. On the other hand, once the last square of a given color disappears, there is no way for that color to ever reappear. (Extinction is forever!) If you let the program run long enough, eventually the entire mosaic will be one uniform color.

See the Solution

Exercise 4.7:

Write a program that administers a basic addition quiz to the user. There should be ten questions. Each question is a simple addition problem such as 17 + 42, where the numbers in the problem are chosen at random (and are not too big). The program should ask the user all ten questions and get the user's answers. After asking all the questions, the user should print each question again, with the user's answer. If the user got the answer right, the program should say so; if not, the program should give the correct answer. At the end, tell the user their score on the quiz, where each correct answer counts for ten points.

The program should use three subroutines, one to create the quiz, one to administer the quiz, and one to grade the quiz. It can use three arrays, with three global variables of type int[], to refer to the arrays. The first array holds the first number from every question, the second holds the second number from every questions, and the third holds the user's answers.

See the Solution

Quiz on Chapter 4

Question 1:

A "black box" has an interface and an implementation. Explain what is meant by the terms interface and implementation.

Question 2:

A subroutine is said to have a contract. What is meant by the contract of a subroutine? When you want to use a subroutine, why is it important to understand its contract? The contract has both "syntactic" and "semantic" aspects. What is the syntactic aspect? What is the semantic aspect?

Question 3:

Briefly explain how subroutines can be useful in the top-down design of programs.

Question 4:

Discuss the concept of parameters. What are parameters for? What is the difference between formal parameters and actual parameters?

Question 5:

Give two different reasons for using named constants (declared with the final modifier).

Question 6:

What is an API? Give an example.

Question 7:

Write a subroutine named "stars" that will output a line of stars to standard output. (A star is the character "*".) The number of stars should be given as a parameter to the subroutine. Use a for loop. For example, the command "stars(20)" would output

********************

Question 8:

Write a main() routine that uses the subroutine that you wrote for Question 7 to output 10 lines of stars with 1 star in the first line, 2 stars in the second line, and so on, as shown below.

*
**
***
****
*****
******
*******
********
*********
**********

Question 9:

Write a function named countChars that has a String and a char as parameters. The function should count the number of times the character occurs in the string, and it should return the result as the value of the function.

Question 10:

Write a subroutine with three parameters of type int. The subroutine should determine which of its parameters is smallest. The value of the smallest parameter should be returned as the value of the subroutine.

Question 11:

Write a function that finds the average of the first N elements of an array of type double. The array and N are parameters to the subroutine.

Question 12:

Explain the purpose of the following function, and explain how it works:

static int[] stripZeros( int[] list ) {
int count = 0;
for (int i = 0; i < list.length; i++) {
if ( list[i] != 0 )
count++;
}
int[] newList;
newList = new int[count];
int j = 0;
for (int i = 0; i < list.length; i++) {
if ( list[i] != 0 ) {
newList[j] = list[i];
j++;
}
}
return newList;
}

See the Answers

Chapter 5

Programming in the Large II: Objects and Classes

Whereas a subroutine represents a single task, an object can encapsulate both data (in the form of instance variables) and a number of different tasks or "behaviors" related to that data (in the form of instance methods). Therefore objects provide another, more sophisticated type of structure that can be used to help manage the complexity of large programs.

The first four sections of this chapter introduce the basic things you need to know to work with objects and to define simple classes. The remaining sections cover more advanced topics; you might not understand them fully the first time through. In particular, Section 5.5 covers the most central ideas of object-oriented programming: inheritance and polymorphism. However, in this textbook, we will generally use these ideas in a limited form, by creating independent classes and building on existing classes rather than by designing entire hierarchies of classes from scratch.

Contents of Chapter 5:

- Section 1: Objects, Instance Methods, and Instance Variables

- Section 2: Constructors and Object Initialization

- Section 3: Programming with Objects

- Section 4: Programming Example: Card, Hand, Deck

- Section 5: Inheritance, Polymorphism, and Abstract Classes

- Section 6: this and super

- Section 7: Interfaces

- Section 8: Nested Classes

- Programming Exercises

- Quiz on This Chapter

Section 5.1

Objects, Instance Methods, and Instance Variables

Object-oriented programming (OOP) represents an attempt to make programs more closely model the way people think about and deal with the world. In the older styles of programming, a programmer who is faced with some problem must identify a computing task that needs to be performed in order to solve the problem. Programming then consists of finding a sequence of instructions that will accomplish that task. But at the heart of object-oriented programming, instead of tasks we find objects -- entities that have behaviors, that hold information, and that can interact with one another. Programming consists of designing a set of objects that somehow model the problem at hand. Software objects in the program can represent real or abstract entities in the problem domain. This is supposed to make the design of the program more natural and hence easier to get right and easier to understand.

To some extent, OOP is just a change in point of view. We can think of an object in standard programming terms as nothing more than a set of variables together with some subroutines for manipulating those variables. In fact, it is possible to use object-oriented techniques in any programming language. However, there is a big difference between a language that makes OOP possible and one that actively supports it. An object-oriented programming language such as Java includes a number of features that make it very different from a standard language. In order to make effective use of those features, you have to "orient" your thinking correctly.

As I have mentioned before, in the context of object-oriented programming, subroutines are often referred to as methods. Now that we are starting to use objects, I will be using the term "method" more often than "subroutine."

5.1.1  Objects, Classes, and Instances

Objects are closely related to classes. We have already been working with classes for several chapters, and we have seen that a class can contain variables and methods (that is, subroutines). If an object is also a collection of variables and methods, how do they differ from classes? And why does it require a different type of thinking to understand and use them effectively? In the one section where we worked with objects rather than classes, Section 3.9, it didn't seem to make much difference: We just left the word "static" out of the subroutine definitions!

I have said that classes "describe" objects, or more exactly that the non-static portions of classes describe objects. But it's probably not very clear what this means. The more usual terminology is to say that objects belong to classes, but this might not be much clearer. (There is a real shortage of English words to properly distinguish all the concepts involved. An object certainly doesn't "belong" to a class in the same way that a member variable "belongs" to a class.) From the point of view of programming, it is more exact to say that classes are used to create objects. A class is a kind of factory -- or blueprint -- for constructing objects. The non-static parts of the class specify, or describe, what variables and methods the objects will contain. This is part of the explanation of how objects differ from classes: Objects are created and destroyed as the program runs, and there can be many objects with the same structure, if they are created using the same class.

Consider a simple class whose job is to group together a few static member variables. For example, the following class could be used to store information about the person who is using the program:

class UserData {
static String name;
static int age;
}

In a program that uses this class, there is only one copy of each of the variables UserData.name and UserData.age. When the class is loaded into the computer, there is a section of memory devoted to the class, and that section of memory includes space for the values of the variables name and age. We can picture the class in memory as looking like this:

An important point is that the static member variables are part of the representation of the class in memory. Their full names, UserData.name and UserData.age, use the name of the class, since they are part of the class. When we use class UserData to represent the user of the program, there can only be one user, since we only have memory space to store data about one user. Note that the class, UserData, and the variables it contains exist as long as the program runs. (That is essentially what it means to be "static.") Now, consider a similar class that includes some non-static variables:

class PlayerData {
static int playerCount;
String name;
int age;
}

I've also included a static variable in the PlayerData class. Here, the static variable playerCount is stored as part of the representation of the class in memory. Its full name is PlayerData.playerCount, and there is only one of it, which exists as long as the program runs. However, the other two variables in the class definition are non-static. There is no such variable as PlayerData.name or PlayerData.age, since non-static variables do not become part of the class itself. But the PlayerData class can be used to create objects. There can be many objects created using the class, and each one will have its own variables called name and age. This is what it means for the non-static parts of the class to be a template for objects: Every object gets its own copy of the non-static part of the class. We can visualize the situation in the computer's memory after several object have been created like this:

Note that the static variable playerCount is part of the class, and there is only one copy. On the other hand, every object contains a name and an age. An object that is created from a class is called an instance of that class, and as the picture shows every object "knows" which class was used to create it. I've shown class PlayerData as containing something called a "constructor;" the constructor is a subroutine that creates objects.

Now there can be many "players" because we can make new objects to represent new players on demand. A program might use the PlayerData class to store information about multiple players in a game. Each player has a name and an age. When a player joins the game, a new PlayerData object can be created to represent that player. If a player leaves the game, the PlayerData object that represents that player can be destroyed. A system of objects in the program is being used to dynamically model what is happening in the game. You can't do this with static variables! "Dynamic" is the opposite of "static."

An object that is created using a class is said to be an instance of that class. We will sometimes say that the object belongs to the class. The variables that the object contains are called instance variables. The methods (that is, subroutines) that the object contains are called instance methods. For example, if the PlayerData class, as defined above, is used to create an object, then that object is an instance of the PlayerData class, and name and age are instance variables in the object.

My examples here don't include any methods, but methods work similarly to variables. Static methods are part of the class; non-static, or instance, methods become part of objects created from the class. It's not literally true that each object contains its own copy of the actual compiled code for an instance method. But logically an instance method is part of the object, and I will continue to say that the object "contains" the instance method.

Note that you should distinguish between the source code for the class, and the class itself (in memory). The source code determines both the class and the objects that are created from that class. The "static" definitions in the source code specify the things that are part of the class itself (in the computer's memory), whereas the non-static definitions in the source code specify things that will become part of every instance object that is created from the class. By the way, static member variables and static member subroutines in a class are sometimes called class variables and class methods, since they belong to the class itself, rather than to instances of that class.

As you can see, the static and the non-static portions of a class are very different things and serve very different purposes. Many classes contain only static members, or only non-static, and we will see only a few examples of classes that contain a mixture of the two.

5.1.2  Fundamentals of Objects

So far, I've been talking mostly in generalities, and I haven't given you much of an idea about what you have to put in a program if you want to work with objects. Let's look at a specific example to see how it works. Consider this extremely simplified version of a Student class, which could be used to store information about students taking a course:

public class Student {

public String name;  // Student's name.
public double test1, test2, test3;   // Grades on three tests.

public double getAverage() {  // compute average test grade
return (test1 + test2 + test3) / 3;
}

}  // end of class Student

None of the members of this class are declared to be static, so the class exists only for creating objects. This class definition says that any object that is an instance of the Student class will include instance variables named name, test1, test2, and test3, and it will include an instance method named getAverage(). The names and tests in different objects will generally have different values. When called for a particular student, the method getAverage() will compute an average using that student's test grades. Different students can have different averages. (Again, this is what it means to say that an instance method belongs to an individual object, not to the class.)

In Java, a class is a type, similar to the built-in types such as int and boolean. So, a class name can be used to specify the type of a variable in a declaration statement, or the type of a formal parameter, or the return type of a function. For example, a program could define a variable named std of type Student with the statement

Student std;

However, declaring a variable does not create an object! This is an important point, which is related to this Very Important Fact:

In Java, no variable can ever hold an object.
A variable can only hold a reference to an object.

You should think of objects as floating around independently in the computer's memory. In fact, there is a special portion of memory called the heap where objects live. Instead of holding an object itself, a variable holds the information necessary to find the object in memory. This information is called a reference or pointer to the object. In effect, a reference to an object is the address of the memory location where the object is stored. When you use a variable of object type, the computer uses the reference in the variable to find the actual object.

In a program, objects are created using an operator called new, which creates an object and returns a reference to that object. (In fact, the new operator calls a special subroutine called a "constructor" in the class.) For example, assuming that std is a variable of type Student, declared as above, the assignment statement

std = new Student();

would create a new object which is an instance of the class Student, and it would store a reference to that object in the variable std. The value of the variable is a reference, or pointer, to the object. The object itself is somewhere in the heap. It is not quite true, then, to say that the object is the "value of the variable std" (though sometimes it is hard to avoid using this terminology). It is certainly not at all true to say that the object is "stored in the variable std." The proper terminology is that "the variable std refers to or points to the object," and I will try to stick to that terminology as much as possible. If I ever say something like "std is an object," you should read it as meaning "std is a variable that refers to an object."

So, suppose that the variable std refers to an object that is an instance of class Student. That object contains instance variables name, test1, test2, and test3. These instance variables can be referred to as std.name, std.test1, std.test2, and std.test3. This follows the usual naming convention that when B is part of A, then the full name of B is A.B. For example, a program might include the lines

System.out.println("Hello, "  +  std.name  +  ".  Your test grades are:");
System.out.println(std.test1);
System.out.println(std.test2);
System.out.println(std.test3);

This would output the name and test grades from the object to which std refers. Similarly, std can be used to call the getAverage() instance method in the object by saying std.getAverage(). To print out the student's average, you could say:

System.out.println( "Your average is "  +  std.getAverage() );

More generally, you could use std.name any place where a variable of type String is legal. You can use it in expressions. You can assign a value to it. You can even use it to call subroutines from the String class. For example, std.name.length() is the number of characters in the student's name.

It is possible for a variable like std, whose type is given by a class, to refer to no object at all. We say in this case that std holds a null pointer or null reference. The null pointer is written in Java as "null". You can store a null reference in the variable std by saying

std = null;

null is an actual value that is stored in the variable, not a pointer to something else. It is not correct to say that the variable "points to null"; in fact, the variable is null. For example, you can test whether the value of std is null by testing

if (std == null) . . .

If the value of a variable is null, then it is, of course, illegal to refer to instance variables or instance methods through that variable -- since there is no object, and hence no instance variables to refer to! For example, if the value of the variable std is null, then it would be illegal to refer to std.test1. If your program attempts to use a null pointer illegally in this way, the result is an error called a null pointer exception. When this happens while the program is running, an exception of type NullPointerException is thrown.

Let's look at a sequence of statements that work with objects:

Student std, std1,       // Declare four variables of
std2, std3;    //   type Student.

std = new Student();     // Create a new object belonging
//   to the class Student, and
//   store a reference to that
//   object in the variable std.

std1 = new Student();    // Create a second Student object
//   and store a reference to
//   it in the variable std1.

std2 = std1;             // Copy the reference value in std1
//   into the variable std2.

std3 = null;             // Store a null reference in the
//   variable std3.

std.name = "John Smith";  // Set values of some instance variables.
std1.name = "Mary Jones";

// (Other instance variables have default
//    initial values of zero.)

After the computer executes these statements, the situation in the computer's memory looks like this:

In this picture, when a variable contains a reference to an object, the value of that variable is shown as an arrow pointing to the object. Note, by the way, that the Strings are objects! The variable std3, with a value of null, doesn't point anywhere. The arrows from std1 and std2 both point to the same object. This illustrates a Very Important Point:

When one object variable is assigned
to another, only a reference is copied.
The object referred to is not copied.

When the assignment "std2 = std1;" was executed, no new object was created. Instead, std2 was set to refer to the very same object that std1 refers to. This is to be expected, since the assignment statement just copies the value that is stored in std1 into std2, and that value is a pointer, not an object. But this has some consequences that might be surprising. For example, std1.name and std2.name are two different names for the same variable, namely the instance variable in the object that both std1 and std2 refer to. After the string "Mary Jones" is assigned to the variable std1.name, it is also true that the value of std2.name is "Mary Jones". There is a potential for a lot of confusion here, but you can help protect yourself from it if you keep telling yourself, "The object is not in the variable. The variable just holds a pointer to the object."

You can test objects for equality and inequality using the operators == and !=, but here again, the semantics are different from what you are used to. When you make a test "if (std1 == std2)", you are testing whether the values stored in std1 and std2 are the same. But the values that you are comparing are references to objects; they are not objects. So, you are testing whether std1 and std2 refer to the same object, that is, whether they point to the same location in memory. This is fine, if it's what you want to do. But sometimes, what you want to check is whether the instance variables in the objects have the same values. To do that, you would need to ask whether "std1.test1 == std2.test1 && std1.test2 == std2.test2 && std1.test3 == std2.test3 && std1.name.equals(std2.name)".

I've remarked previously that Strings are objects, and I've shown the strings "Mary Jones" and "John Smith" as objects in the above illustration. (Strings are special objects, treated by Java in a special way, and I haven't attempted to show the actual internal structure of the String objects.) Since strings are objects, a variable of type String can only hold a reference to a string, not the string itself. This explains why using the == operator to test strings for equality is not a good idea. Suppose that greeting is a variable of type String, and that it refers to the string "Hello". Then would the test greeting == "Hello" be true? Well, maybe, maybe not. The variable greeting and the String literal "Hello" each refer to a string that contains the characters H-e-l-l-o. But the strings could still be different objects, that just happen to contain the same characters; in that case, greeting == "Hello" would be false. The function greeting.equals("Hello") tests whether greeting and "Hello" contain the same characters, which is almost certainly the question you want to ask. The expression greeting == "Hello" tests whether greeting and "Hello" contain the same characters stored in the same memory location. (Of course, a String variable such as greeting can also contain the special value null, and it would make sense to use the == operator to test whether "greeting == null".)

The fact that variables hold references to objects, not objects themselves, has a couple of other consequences that you should be aware of. They follow logically, if you just keep in mind the basic fact that the object is not stored in the variable. The object is somewhere else; the variable points to it.

Suppose that a variable that refers to an object is declared to be final. This means that the value stored in the variable can never be changed, once the variable has been initialized. The value stored in the variable is a reference to the object. So the variable will continue to refer to the same object as long as the variable exists. However, this does not prevent the data in the object from changing. The variable is final, not the object. It's perfectly legal to say

final Student stu = new Student();

stu.name = "John Doe";  // Change data in the object;
// The value stored in stu is not changed!
// It still refers to the same object.

Next, suppose that obj is a variable that refers to an object. Let's consider what happens when obj is passed as an actual parameter to a subroutine. The value of obj is assigned to a formal parameter in the subroutine, and the subroutine is executed. The subroutine has no power to change the value stored in the variable, obj. It only has a copy of that value. However, the value is a reference to an object. Since the subroutine has a reference to the object, it can change the data stored in the object. After the subroutine ends, obj still points to the same object, but the data stored in the object might have changed. Suppose x is a variable of type int and stu is a variable of type Student. Compare:

void dontChange(int z) {                void change(Student s) {
z = 42;                                  s.name = "Fred";
}                                       }

The lines:                              The lines:

x = 17;                                 stu.name = "Jane";
dontChange(x);                          change(stu);
System.out.println(x);                  System.out.println(stu.name);

output the value 17.                    output the value "Fred".

The value of x is not                   The value of stu is not
changed by the subroutine,              changed, but stu.name is changed.
which is equivalent to                  This is equivalent to

z = x;                                  s = stu;
z = 42;                                 s.name = "Fred";

5.1.3  Getters and Setters

When writing new classes, it's a good idea to pay attention to the issue of access control. Recall that making a member of a class public makes it accessible from anywhere, including from other classes. On the other hand, a private member can only be used in the class where it is defined.

In the opinion of many programmers, almost all member variables should be declared private. This gives you complete control over what can be done with the variable. Even if the variable itself is private, you can allow other classes to find out what its value is by providing a public accessor method that returns the value of the variable. For example, if your class contains a private member variable, title, of type String, you can provide a method

public String getTitle() {
return title;
}

that returns the value of title. By convention, the name of an accessor method for a variable is obtained by capitalizing the name of variable and adding "get" in front of the name. So, for the variable title, we get an accessor method named "get" + "Title", or getTitle(). Because of this naming convention, accessor methods are more often referred to as getter methods. A getter method provides "read access" to a variable. (Sometimes for boolean variables, "is" is used in place of "get". For example, a getter for a boolean member variable named done might be called isDone().)

You might also want to allow "write access" to a private variable. That is, you might want to make it possible for other classes to specify a new value for the variable. This is done with a setter method. (If you don't like simple, Anglo-Saxon words, you can use the fancier term mutator method.) The name of a setter method should consist of "set" followed by a capitalized copy of the variable's name, and it should have a parameter with the same type as the variable. A setter method for the variable title could be written

public void setTitle( String newTitle ) {
title = newTitle;
}

It is actually very common to provide both a getter and a setter method for a private member variable. Since this allows other classes both to see and to change the value of the variable, you might wonder why not just make the variable public? The reason is that getters and setters are not restricted to simply reading and writing the variable's value. In fact, they can take any action at all. For example, a getter method might keep track of the number of times that the variable has been accessed:

public String getTitle() {
titleAccessCount++;  // Increment member variable titleAccessCount.
return title;
}

and a setter method might check that the value that is being assigned to the variable is legal:

public void setTitle( String newTitle ) {
if ( newTitle == null )   // Don't allow null strings as titles!
title = "(Untitled)";  // Use an appropriate default value instead.
else
title = newTitle;
}

Even if you can't think of any extra chores to do in a getter or setter method, you might change your mind in the future when you redesign and improve your class. If you've used a getter and setter from the beginning, you can make the modification to your class without affecting any of the classes that use your class. The private member variable is not part of the public interface of your class; only the public getter and setter methods are, and you are free to change their implementations without changing the public interface of your class. If you haven't used get and set from the beginning, you'll have to contact everyone who uses your class and tell them, "Sorry people, you'll have to track down every use that you've made of this variable and change your code to use my new get and set methods instead."

A couple of final notes: Some advanced aspects of Java rely on the naming convention for getter and setter methods, so it's a good idea to follow the convention rigorously. And though I've been talking about using getter and setter methods for a variable, you can define get and set methods even if there is no variable. A getter and/or setter method defines a property of the class, that might or might not correspond to a variable. For example, if a class includes a public void instance method with signature setValue(double), then the class has a "property" named value of type double, and it has this property whether or not the class has a member variable named value.

5.1.4  Arrays and Objects

As I noted in Subsection 3.8.1, arrays are objects. Like Strings they are special objects, with their own unique syntax. An array type such as int[] or String[] is actually a class, and arrays are created using a special version of the new operator. As in the case for other object variables, an array variable can never hold an actual array -- only a reference to an array object. The array object itself exists in the heap. It is possible for an array variable to hold the value null, which means there is no actual array.

For example, suppose that list is a variable of type int[]. If the value of list is null, then any attempt to access list.length or an array element list[i] would be an error and would cause an exception of type NullPointerException. If newlist is another variable of type int[], then the assignment statement

newlist = list;

only copies the reference value in list into newlist. If list is null, the result is that newlist will also be null. If list points to an array, the assignment statement does not make a copy of the array. It just sets newlist to refer to the same array as list. For example, the output of the following code segment

list = new int[3];
list[1] = 17;
newlist = list; // newlist points to the same array as list!
newlist[1] = 42;
System.out.println( list[1] );

would be 42, not 17, since list[1] and newlist[1] are just different names for the same element in the array. All this is very natural, once you understand that arrays are objects and array variables hold pointers to arrays.

This fact also comes into play when an array is passed as a parameter to a subroutine. The value that is copied into the subroutine is a pointer to the array. The array is not copied. Since the subroutine has a reference to the original array, any changes that it makes to elements of the array are being made to the original and will persist after the subroutine returns.

Arrays are objects. They can also hold objects. The base type of an array can be a class. We have already seen this when we used arrays of type String[], but any class can be used as the base type. For example, suppose Student is the class defined earlier in this section. Then we can have arrays of type Student[]. For an array of type Student[], each element of the array is a variable of type Student. To store information about 30 students, we could use an array

Student[] classlist;  // Declare a variable of type Student[].
classlist = new Student[30];  // The variable now points to an array.

The array has 30 elements, classlist[0], classlist[1], ... classlist[29]. When the array is created, it is filled with the default initial value, which for an object type is null. So, although we have 30 array elements of type Student, we don't yet have any actual Student objects! All we have is 30 nulls. If we want student objects, we have to create them:

Student[] classlist;
classlist = new Student[30];
for ( int i = 0; i < 30; i++ ) {
classlist[i] = new Student();
}

Once we have done this, each classlist[i] points to an object of type Student. If we want to talk about the name of student number 3, we can use classlist[3].name. The average for student number i can be computed by calling classlist[i].getAverage(). You can do anything with classlist[i] that you could do with any other variable of type Student.

Section 5.2

Constructors and Object Initialization

Object types in Java are very different from the primitive types. Simply declaring a variable whose type is given as a class does not automatically create an object of that class. Objects must be explicitly constructed. For the computer, the process of constructing an object means, first, finding some unused memory in the heap that can be used to hold the object and, second, filling in the object's instance variables. As a programmer, you don't care where in memory the object is stored, but you will usually want to exercise some control over what initial values are stored in a new object's instance variables. In many cases, you will also want to do more complicated initialization or bookkeeping every time an object is created.

5.2.1  Initializing Instance Variables

An instance variable can be assigned an initial value in its declaration, just like any other variable. For example, consider a class named PairOfDice. An object of this class will represent a pair of dice. It will contain two instance variables to represent the numbers showing on the dice and an instance method for rolling the dice:

public class PairOfDice {

public int die1 = 3;   // Number showing on the first die.
public int die2 = 4;   // Number showing on the second die.

public void roll() {
// Roll the dice by setting each of the dice to be
// a random number between 1 and 6.
die1 = (int)(Math.random()*6) + 1;
die2 = (int)(Math.random()*6) + 1;
}

} // end class PairOfDice

The instance variables die1 and die2 are initialized to the values 3 and 4 respectively. These initializations are executed whenever a PairOfDice object is constructed. It's important to understand when and how this happens. There can be many PairOfDice objects. Each time one is created, it gets its own instance variables, and the assignments "die1 = 3" and "die2 = 4" are executed to fill in the values of those variables. To make this clearer, consider a variation of the PairOfDice class:

public class PairOfDice {

public int die1 = (int)(Math.random()*6) + 1;
public int die2 = (int)(Math.random()*6) + 1;

public void roll() {
die1 = (int)(Math.random()*6) + 1;
die2 = (int)(Math.random()*6) + 1;
}

} // end class PairOfDice

Here, every time a new PairOfDice is created, the dice are initialized to random values, as if a new pair of dice were being thrown onto the gaming table. Since the initialization is executed for each new object, a set of random initial values will be computed for each new pair of dice. Different pairs of dice can have different initial values. For initialization of static member variables, of course, the situation is quite different. There is only one copy of a static variable, and initialization of that variable is executed just once, when the class is first loaded.

If you don't provide any initial value for an instance variable, a default initial value is provided automatically. Instance variables of numerical type (int, double, etc.) are automatically initialized to zero if you provide no other values; boolean variables are initialized to false; and char variables, to the Unicode character with code number zero. An instance variable can also be a variable of object type. For such variables, the default initial value is null. (In particular, since Strings are objects, the default initial value for String variables is null.)

5.2.2  Constructors

Objects are created with the operator, new. For example, a program that wants to use a PairOfDice object could say:

PairOfDice dice;   // Declare a variable of type PairOfDice.

dice = new PairOfDice();  // Construct a new object and store a
//   reference to it in the variable.

In this example, "new PairOfDice()" is an expression that allocates memory for the object, initializes the object's instance variables, and then returns a reference to the object. This reference is the value of the expression, and that value is stored by the assignment statement in the variable, dice, so that after the assignment statement is executed, dice refers to the newly created object. Part of this expression, "PairOfDice()", looks like a subroutine call, and that is no accident. It is, in fact, a call to a special type of subroutine called a constructor. This might puzzle you, since there is no such subroutine in the class definition. However, every class has at least one constructor. If the programmer doesn't write a constructor definition in a class, then the system will provide a default constructor for that class. This default constructor does nothing beyond the basics: allocate memory and initialize instance variables. If you want more than that to happen when an object is created, you can include one or more constructors in the class definition.

The definition of a constructor looks much like the definition of any other subroutine, with three exceptions. A constructor does not have any return type (not even void). The name of the constructor must be the same as the name of the class in which it is defined. And the only modifiers that can be used on a constructor definition are the access modifiers public, private, and protected. (In particular, a constructor can't be declared static.)

However, a constructor does have a subroutine body of the usual form, a block of statements. There are no restrictions on what statements can be used. And a constructor can have a list of formal parameters. In fact, the ability to include parameters is one of the main reasons for using constructors. The parameters can provide data to be used in the construction of the object. For example, a constructor for the PairOfDice class could provide the values that are initially showing on the dice. Here is what the class would look like in that case:

public class PairOfDice {

public int die1;   // Number showing on the first die.
public int die2;   // Number showing on the second die.

public PairOfDice(int val1, int val2) {
// Constructor.  Creates a pair of dice that
// are initially showing the values val1 and val2.
die1 = val1;  // Assign specified values
die2 = val2;  //           to the instance variables.
}

public void roll() {
// Roll the dice by setting each of the dice to be
// a random number between 1 and 6.
die1 = (int)(Math.random()*6) + 1;
die2 = (int)(Math.random()*6) + 1;
}

} // end class PairOfDice

The constructor is declared as "public PairOfDice(int val1, int val2) ...", with no return type and with the same name as the name of the class. This is how the Java compiler recognizes a constructor. The constructor has two parameters, and values for these parameters must be provided when the constructor is called. For example, the expression "new PairOfDice(3,4)" would create a PairOfDice object in which the values of the instance variables die1 and die2 are initially 3 and 4. Of course, in a program, the value returned by the constructor should be used in some way, as in

PairOfDice dice;            // Declare a variable of type PairOfDice.

dice = new PairOfDice(1,1); // Let dice refer to a new PairOfDice
//   object that initially shows 1, 1.

Now that we've added a constructor to the PairOfDice class, we can no longer create an object by saying "new PairOfDice()"! The system provides a default constructor for a class only if the class definition does not already include a constructor. In this version of PairOfDice, there is only one constructor in the class, and it requires two actual parameters. However, this is not a big problem, since we can add a second constructor to the class, one that has no parameters. In fact, you can have as many different constructors as you want, as long as their signatures are different, that is, as long as they have different numbers or types of formal parameters. In the PairOfDice class, we might have a constructor with no parameters which produces a pair of dice showing random numbers:

public class PairOfDice {

public int die1;   // Number showing on the first die.
public int die2;   // Number showing on the second die.

public PairOfDice() {
// Constructor.  Rolls the dice, so that they initially
// show some random values.
roll();  // Call the roll() method to roll the dice.
}

public PairOfDice(int val1, int val2) {
// Constructor.  Creates a pair of dice that
// are initially showing the values val1 and val2.
die1 = val1;  // Assign specified values
die2 = val2;  //            to the instance variables.
}

public void roll() {
// Roll the dice by setting each of the dice to be
// a random number between 1 and 6.
die1 = (int)(Math.random()*6) + 1;
die2 = (int)(Math.random()*6) + 1;
}

} // end class PairOfDice

Now we have the option of constructing a PairOfDice object either with "new PairOfDice()" or with "new PairOfDice(x,y)", where x and y are int-valued expressions.

This class, once it is written, can be used in any program that needs to work with one or more pairs of dice. None of those programs will ever have to use the obscure incantation "(int)(Math.random()*6)+1", because it's done inside the PairOfDice class. And the programmer, having once gotten the dice-rolling thing straight will never have to worry about it again. Here, for example, is a main program that uses the PairOfDice class to count how many times two pairs of dice are rolled before the two pairs come up showing the same value. This illustrates once again that you can create several instances of the same class:

public class RollTwoPairs {

public static void main(String[] args) {

PairOfDice firstDice;  // Refers to the first pair of dice.
firstDice = new PairOfDice();

PairOfDice secondDice; // Refers to the second pair of dice.
secondDice = new PairOfDice();

int countRolls;  // Counts how many times the two pairs of
//    dice have been rolled.

int total1;      // Total showing on first pair of dice.
int total2;      // Total showing on second pair of dice.

countRolls = 0;

do {  // Roll the two pairs of dice until totals are the same.

firstDice.roll();    // Roll the first pair of dice.
total1 = firstDice.die1 + firstDice.die2;   // Get total.
System.out.println("First pair comes up  " + total1);

secondDice.roll();    // Roll the second pair of dice.
total2 = secondDice.die1 + secondDice.die2;   // Get total.
System.out.println("Second pair comes up " + total2);

countRolls++;   // Count this roll.

System.out.println();  // Blank line.

} while (total1 != total2);

System.out.println("It took " + countRolls
+ " rolls until the totals were the same.");

} // end main()

} // end class RollTwoPairs

Constructors are subroutines, but they are subroutines of a special type. They are certainly not instance methods, since they don't belong to objects. Since they are responsible for creating objects, they exist before any objects have been created. They are more like static member subroutines, but they are not and cannot be declared to be static. In fact, according to the Java language specification, they are technically not members of the class at all! In particular, constructors are not referred to as "methods."

Unlike other subroutines, a constructor can only be called using the new operator, in an expression that has the form

new class-name ( parameter-list )

where the parameter-list is possibly empty. I call this an expression because it computes and returns a value, namely a reference to the object that is constructed. Most often, you will store the returned reference in a variable, but it is also legal to use a constructor call in other ways, for example as a parameter in a subroutine call or as part of a more complex expression. Of course, if you don't save the reference in a variable, you won't have any way of referring to the object that was just created.

A constructor call is more complicated than an ordinary subroutine or function call. It is helpful to understand the exact steps that the computer goes through to execute a constructor call:

1. First, the computer gets a block of unused memory in the heap, large enough to hold an object of the specified type.

2. It initializes the instance variables of the object. If the declaration of an instance variable specifies an initial value, then that value is computed and stored in the instance variable. Otherwise, the default initial value is used.

3. The actual parameters in the constructor, if any, are evaluated, and the values are assigned to the formal parameters of the constructor.

4. The statements in the body of the constructor, if any, are executed.

5. A reference to the object is returned as the value of the constructor call.

The end result of this is that you have a reference to a newly constructed object.

For another example, let's rewrite the Student class that was used in Section 1. I'll add a constructor, and I'll also take the opportunity to make the instance variable, name, private.

public class Student {

private String name;                 // Student's name.
public double test1, test2, test3;   // Grades on three tests.

Student(String theName) {
// Constructor for Student objects;
// provides a name for the Student.
// The name can't be null.
if ( theName == null )
throw new IllegalArgumentException("name can't be null");
name = theName;
}

public String getName() {
// Getter method for reading the value of the private
// instance variable, name.
return name;
}

public double getAverage() {
// Compute average test grade.
return (test1 + test2 + test3) / 3;
}

}  // end of class Student

An object of type Student contains information about some particular student. The constructor in this class has a parameter of type String, which specifies the name of that student. Objects of type Student can be created with statements such as:

std = new Student("John Smith");
std1 = new Student("Mary Jones");

In the original version of this class, the value of name had to be assigned by a program after it created the object of type Student. There was no guarantee that the programmer would always remember to set the name properly. In the new version of the class, there is no way to create a Student object except by calling the constructor, and that constructor automatically sets the name. Furthermore, the constructor makes it impossible to have a student object whose name is null. The programmer's life is made easier, and whole hordes of frustrating bugs are squashed before they even have a chance to be born.

Another type of guarantee is provided by the private modifier. Since the instance variable, name, is private, there is no way for any part of the program outside the Student class to get at the name directly. The program sets the value of name, indirectly, when it calls the constructor. I've provided a getter function, getName(), that can be used from outside the class to find out the name of the student. But I haven't provided any setter method or other way to change the name. Once a student object is created, it keeps the same name as long as it exists.

Note that it would be legal, and good style, to declare the variable name to be "final" in this class. An instance variable can be final provided it is either assigned a value in its declaration or is assigned a value in every constructor in the class. It is illegal to assign a value to a final instance variable, except inside a constructor.

Let's take this example a little farther to illustrate one more aspect of classes: What happens when you mix static and non-static in the same class? In that case, it's legal for an instance method in the class to use static member variables or call static member subroutines. An object knows what class it belongs to, and it can refer to static members of that class. But there it only one copy of the static member, in the class itself. Effectively, all the objects share one copy of the static member.

As an example, consider a version of the Student class to which I've added an ID for each student and a static member called nextUniqueID. Although there is an ID variable in each student object, there is only one nextUniqueID variable.

public class Student {

private String name;                 // Student's name.
public double test1, test2, test3;   // Grades on three tests.

private int ID;  // Unique ID number for this student.

private static int nextUniqueID = 0;
// keep track of next available unique ID number

Student(String theName) {
// Constructor for Student objects; provides a name for the Student,
// and assigns the student a unique ID number.
name = theName;
nextUniqueID++;
ID = nextUniqueID;
}

public String getName() {
// Getter method for reading the value of the private
// instance variable, name.
return name;
}

public int getID() {
// Getter method for reading the value of ID.
return ID;
}

public double getAverage() {
// Compute average test grade.
return (test1 + test2 + test3) / 3;
}

}  // end of class Student

Since nextUniqueID is a static variable, the initialization "nextUniqueID = 0" is done only once, when the class is first loaded. Whenever a Student object is constructed and the constructor says "nextUniqueID++;", it's always the same static member variable that is being incremented. When the very first Student object is created, nextUniqueID becomes 1. When the second object is created, nextUniqueID becomes 2. After the third object, it becomes 3. And so on. The constructor stores the new value of nextUniqueID in the ID variable of the object that is being created. Of course, ID is an instance variable, so every object has its own individual ID variable. The class is constructed so that each student will automatically get a different value for its ID variable. Furthermore, the ID variable is private, so there is no way for this variable to be tampered with after the object has been created. You are guaranteed, just by the way the class is designed, that every student object will have its own permanent, unique identification number. Which is kind of cool if you think about it.

(Unfortunately, if you think about it a bit more, it turns out that the guarantee isn't quite absolute. The guarantee is valid in programs that use a single thread. But, as a preview of the difficulties of parallel programming, I'll note that in multi-threaded programs, where several things can be going on at the same time, things can get a bit strange. In a multi-threaded program, it is possible that two threads are creating Student objects at exactly the same time, and it becomes possible for both objects to get the same ID number. We'll come back to this in Subsection 12.1.3, where you will learn how to fix the problem.)

5.2.3  Garbage Collection

So far, this section has been about creating objects. What about destroying them? In Java, the destruction of objects takes place automatically.

An object exists in the heap, and it can be accessed only through variables that hold references to the object. What should be done with an object if there are no variables that refer to it? Such things can happen. Consider the following two statements (though in reality, you'd never do anything like this in consecutive statements!):

Student std = new Student("John Smith");
std = null;

In the first line, a reference to a newly created Student object is stored in the variable std. But in the next line, the value of std is changed, and the reference to the Student object is gone. In fact, there are now no references whatsoever to that object, in any variable. So there is no way for the program ever to use the object again! It might as well not exist. In fact, the memory occupied by the object should be reclaimed to be used for another purpose.

Java uses a procedure called garbage collection to reclaim memory occupied by objects that are no longer accessible to a program. It is the responsibility of the system, not the programmer, to keep track of which objects are "garbage." In the above example, it was very easy to see that the Student object had become garbage. Usually, it's much harder. If an object has been used for a while, there might be several references to the object stored in several variables. The object doesn't become garbage until all those references have been dropped.

In many other programming languages, it's the programmer's responsibility to delete the garbage. Unfortunately, keeping track of memory usage is very error-prone, and many serious program bugs are caused by such errors. A programmer might accidently delete an object even though there are still references to that object. This is called a dangling pointer error, and it leads to problems when the program tries to access an object that is no longer there. Another type of error is a memory leak, where a programmer neglects to delete objects that are no longer in use. This can lead to filling memory with objects that are completely inaccessible, and the program might run out of memory even though, in fact, large amounts of memory are being wasted.

Because Java uses garbage collection, such errors are simply impossible. Garbage collection is an old idea and has been used in some programming languages since the 1960s. You might wonder why all languages don't use garbage collection. In the past, it was considered too slow and wasteful. However, research into garbage collection techniques combined with the incredible speed of modern computers have combined to make garbage collection feasible. Programmers should rejoice.

Section 5.3

Programming with Objects

There are several ways in which object-oriented concepts can be applied to the process of designing and writing programs. The broadest of these is object-oriented analysis and design which applies an object-oriented methodology to the earliest stages of program development, during which the overall design of a program is created. Here, the idea is to identify things in the problem domain that can be modeled as objects. On another level, object-oriented programming encourages programmers to produce generalized software components that can be used in a wide variety of programming projects.

Of course, for the most part, you will experience "generalized software components" by using the standard classes that come along with Java. We begin this section by looking at some built-in classes that are used for creating objects. At the end of the section, we will get back to generalities.

5.3.1  Some Built-in Classes

Although the focus of object-oriented programming is generally on the design and implementation of new classes, it's important not to forget that the designers of Java have already provided a large number of reusable classes. Some of these classes are meant to be extended to produce new classes, while others can be used directly to create useful objects. A true mastery of Java requires familiarity with a large number of built-in classes -- something that takes a lot of time and experience to develop. Let's take a moment to look at a few built-in classes that you might find useful.

A string can be built up from smaller pieces using the + operator, but this is not always efficient. If str is a String and ch is a character, then executing the command "str = str + ch;" involves creating a whole new string that is a copy of str, with the value of ch appended onto the end. Copying the string takes some time. Building up a long string letter by letter would require a surprising amount of processing. The class StringBuilder makes it possible to be efficient about building up a long string from a number of smaller pieces. To do this, you must make an object belonging to the StringBuilder class. For example:

StringBuilder builder = new StringBuilder();

(This statement both declares the variable builder and initializes it to refer to a newly created StringBuilder object. Combining declaration with initialization was covered in Subsection 4.7.1 and works for objects just as it does for primitive types.)

Like a String, a StringBuilder contains a sequence of characters. However, it is possible to add new characters onto the end of a StringBuilder without continually making copies of the data that it already contains. If x is a value of any type and builder is the variable defined above, then the command builder.append(x) will add x, converted into a string representation, onto the end of the data that was already in the builder. This can be done more efficiently than copying the data every time something is appended. A long string can be built up in a StringBuilder using a sequence of append() commands. When the string is complete, the function builder.toString() will return a copy of the string in the builder as an ordinary value of type String. The StringBuilder class is in the standard package java.lang, so you can use its simple name without importing it.

A number of useful classes are collected in the package java.util. For example, this package contains classes for working with collections of objects. We will study such collection classes extensively in Chapter 10. Another class in this package, java.util.Date, is used to represent times. When a Date object is constructed without parameters, the result represents the current date and time, so an easy way to display this information is:

System.out.println( new Date() );

Of course, since it is in the package java.util, in order to use the Date class in your program, you must make it available by importing it with one of the statements "import java.util.Date;" or "import java.util.*;" at the beginning of your program. (See Subsection 4.5.3 for a discussion of packages and import.)

I will also mention the class java.util.Random. An object belonging to this class is a source of random numbers (or, more precisely pseudorandom numbers). The standard function Math.random() uses one of these objects behind the scenes to generate its random numbers. An object of type Random can generate random integers, as well as random real numbers. If randGen is created with the command:

Random randGen = new Random();

and if N is a positive integer, then randGen.nextInt(N) generates a random integer in the range from 0 to N-1. For example, this makes it a little easier to roll a pair of dice. Instead of saying "die1 = (int)(6*Math.random())+1;", one can say "die1 = randGen.nextInt(6)+1;". (Since you also have to import the class java.util.Random and create the Random object, you might not agree that it is actually easier.) An object of type Random can also be used to generate so-called Gaussian distributed random real numbers.

Many of Java's standard classes are used in GUI programming. You will encounter a number of them in the Chapter 6. Here, I will mention only the class Color, from the package java.awt, so that I can use it in the next example. A Color object represents a color that can be used for drawing. In Section 3.9, you encountered color constants such as Color.RED. These constants are final, static member variables in the Color class, and their values are objects of type Color. It is also possible to create new color objects. Class Color has several constructors. One of them, which is called as new Color(r,g,b), takes three integer parameters to specify the red, green, and blue components of the color. The parameters must be in the range 0 to 255. Another constructor, new Color(r,g,b,t), adds a fourth integer parameter, which must also be in the range 0 to 255. The fourth parameter determines how transparent or opaque the color is. When you draw with a transparent color, the background shows through the color to some extent. A larger value of the parameter t gives a color that is less transparent and more opaque. (If you know hexadecimal color codes, there is a constructor for you. It takes one parameter of type int that encodes all the color components. For example, new Color(0x8B5413) creates a brown color.)

A Color object has only a few instance methods that you are likely to use. Mainly, there are functions like getRed() to get the individual color components of the color. There are no "setter" methods to change the color components. In fact, a Color is an immutable object, meaning that all of its instance variables are final and cannot be changed after the object is created. Strings are another example of immutable objects, and we will make some of our own later in this section.

The main point of all this, again, is that many problems have already been solved, and the solutions are available in Java's standard classes. If you are faced with a task that looks like it should be fairly common, it might be worth looking through a Java reference to see whether someone has already written a class that you can use.

5.3.2  The class "Object"

We have already seen that one of the major features of object-oriented programming is the ability to create subclasses of a class. The subclass inherits all the properties or behaviors of the class, but can modify and add to what it inherits. In Section 5.5, you'll learn how to create subclasses. What you don't know yet is that every class in Java (with just one exception) is a subclass of some other class. If you create a class and don't explicitly make it a subclass of some other class, then it automatically becomes a subclass of the special class named Object. (Object is the one class that is not a subclass of any other class.)

Class Object defines several instance methods that are inherited by every other class. These methods can be used with any object whatsoever. I will mention just one of them here. You will encounter more of them later in the book.

The instance method toString() in class Object returns a value of type String that is supposed to be a string representation of the object. You've already used this method implicitly, any time you've printed out an object or concatenated an object onto a string. When you use an object in a context that requires a string, the object is automatically converted to type String by calling its toString() method.

The version of toString that is defined in Object just returns the name of the class that the object belongs to, concatenated with a code number called the hash code of the object; this is not very useful. When you create a class, you can write a new toString() method for it, which will replace the inherited version. For example, we might add the following method to any of the PairOfDice classes from the previous section:

/**
* Return a String representation of a pair of dice, where die1
* and die2 are instance variables containing the numbers that are
* showing on the two dice.
*/
public String toString() {
if (die1 == die2)
return "double " + die1;
else
return die1 + " and " + die2;
}

If dice refers to a PairOfDice object, then dice.toString() will return strings such as "3 and 6", "5 and 1", and "double 2", depending on the numbers showing on the dice. This method would be used automatically to convert dice to type String in a statement such as

System.out.println( "The dice came up " + dice );

so this statement might output, "The dice came up 5 and 1" or "The dice came up double 2". You'll see another example of a toString() method in the next section.

5.3.3  Writing and Using a Class

As an example, we will write an animation program, based on the same animation framework that was used in Subsection 3.9.3. The animation shows a number of semi-transparent disk that grow in size as the animation plays. The disks have random colors and locations. When a disk gets too big, or sometimes just at random, the disk disappears and is replaced with a new disk at a random location. Here is a screenshot from the program:

A disk in this program can be represented as an object. A disk has properties -- color, location, and size -- that can be instance variables in the object. As for instance methods, we need to think about what we might want to do with a disk. An obvious candidate is that we need to be able to draw it, so we can include an instance method draw(g), where g is a graphics context that will be used to do the drawing. The class can also include one or more constructors. A constructor initializes the object. It's not always clear what data should be provided as parameters to the constructors. In this case, as an example, the constructor's parameters specify the location and size for the circle, but the constructor makes up a color using random values for the red, green, and blue components. Here's the complete class:

import java.awt.Color;  // import some standard GUI classes
import java.awt.Graphics;

/**
* A simple class that holds the size, color, and location of a colored disk,
* with a method for drawing the filled circle in a graphics context.  The
* circle is drawn as a filled oval, with a black outline.
*/
public class CircleInfo {

public int radius;    // The radius of the circle.
public int x,y;       // The location of the center of the circle.
public Color color;   // The color of the circle.

/**
* Create a CircleInfo with a given location and radius and with a
* randomly selected, semi-transparent color.
* @param centerX   The x coordinate of the center.
* @param centerY   The y coordinate of the center.
* @param rad       The radius of the circle.
*/
public CircleInfo( int centerX, int centerY, int rad ) {
x = centerX;
y = centerY;
radius = rad;
int red = (int)(255*Math.random());
int green = (int)(255*Math.random());
int blue = (int)(255*Math.random());
color = new Color(red,green,blue, 100);
}

/**
* Draw the disk in graphics context g, with a black outline.
*/
public void draw( Graphics g ) {
g.setColor( color );
g.fillOval( x - radius, y - radius, 2*radius, 2*radius );
g.setColor( Color.BLACK );
g.drawOval( x - radius, y - radius, 2*radius, 2*radius );
}
}

It would probably be better style to write getters and setters for the instance variables, but to keep things simple, I made them public.

The main program for my animation is a class GrowingCircleAnimation. The program uses 100 disks, each one represented by an object of type CircleInfo. To make that manageable, the program uses an array of objects. The array variable is a member variable in the class:

private CircleInfo[] circleData; // holds the data for all 100 circles

Note that it is not static. GUI programming generally uses objects rather than static variables and methods. Basically, this is because we can imagine having several GrowingCircleAnimations going on at the same time, each with its own array of disks. Each animation would be represented by an object, and each object will need to have its own circleData instance variable. If circleData were static, there would only be one array and all the animations would show exactly the same thing.

The array must be created and filled with data. The array is created using new CircleInfo[100], and then 100 objects of type CircleInfo are created to fill the array. The new objects are created with random locations and sizes. In the program, this is done before drawing the first frame of the animation. Here is the code, where width and height are the size of the drawing area:

circleData = new CircleInfo[100];  // create the array

for (int i = 0; i < circleData.length; i++) { // create the objects
circleData[i] = new CircleInfo(
(int)(width*Math.random()),
(int)(height*Math.random()),
(int)(100*Math.random()) );
}

In each frame, the radius of the disk is increased and the disk is drawn using the code

circleData[i].radius++;
circleData[i].draw(g);

These statements look complicated, so let's unpack them. Now, circleData[i] is an element of the array circleData. That means that it is a variable of type CircleInfo. This variable refers to an object of type CircleInfo, which contains a public instance variable named radius. This means that circleData[i].radius is the full name for that variable. Since it is a variable of type int, we can use the ++ operator to increment its value. So the effect of circleData[i].radius++ is to increase the radius of the circle by one. The second line of code is similar, but in that statement, circleData[i].draw is an instance method in the CircleInfo object. The statement circleData[i].draw(g) calls that instance method with parameter g.

The source code example GrowingCircleAnimation.java contains the full source code for the program, if you are interested. Since the program uses class CircleInfo, you will also need a copy of CircleInfo.java in order to compile and run the program.

5.3.4  Object-oriented Analysis and Design

Every programmer builds up a stock of techniques and expertise expressed as snippets of code that can be reused in new programs using the tried-and-true method of cut-and-paste: The old code is physically copied into the new program and then edited to customize it as necessary. The problem is that the editing is error-prone and time-consuming, and the whole enterprise is dependent on the programmer's ability to pull out that particular piece of code from last year's project that looks like it might be made to fit. (On the level of a corporation that wants to save money by not reinventing the wheel for each new project, just keeping track of all the old wheels becomes a major task.)

Well-designed classes are software components that can be reused without editing. A well-designed class is not carefully crafted to do a particular job in a particular program. Instead, it is crafted to model some particular type of object or a single coherent concept. Since objects and concepts can recur in many problems, a well-designed class is likely to be reusable without modification in a variety of projects.

Furthermore, in an object-oriented programming language, it is possible to make subclasses of an existing class. This makes classes even more reusable. If a class needs to be customized, a subclass can be created, and additions or modifications can be made in the subclass without making any changes to the original class. This can be done even if the programmer doesn't have access to the source code of the class and doesn't know any details of its internal, hidden implementation.

The PairOfDice class in the previous section is already an example of a generalized software component, although one that could certainly be improved. The class represents a single, coherent concept, "a pair of dice." The instance variables hold the data relevant to the state of the dice, that is, the number showing on each of the dice. The instance method represents the behavior of a pair of dice, that is, the ability to be rolled. This class would be reusable in many different programming projects.

On the other hand, the Student class from the previous section is not very reusable. It seems to be crafted to represent students in a particular course where the grade will be based on three tests. If there are more tests or quizzes or papers, it's useless. If there are two people in the class who have the same name, we are in trouble (one reason why numerical student ID's are often used). Admittedly, it's much more difficult to develop a general-purpose student class than a general-purpose pair-of-dice class. But this particular Student class is good mostly as an example in a programming textbook.

A large programming project goes through a number of stages, starting with specification of the problem to be solved, followed by analysis of the problem and design of a program to solve it. Then comes coding, in which the program's design is expressed in some actual programming language. This is followed by testing and debugging of the program. After that comes a long period of maintenance, which means fixing any new problems that are found in the program and modifying it to adapt it to changing requirements. Together, these stages form what is called the software life cycle. (In the real world, the ideal of consecutive stages is seldom if ever achieved. During the analysis stage, it might turn out that the specifications are incomplete or inconsistent. A problem found during testing requires at least a brief return to the coding stage. If the problem is serious enough, it might even require a new design. Maintenance usually involves redoing some of the work from previous stages....)

Large, complex programming projects are only likely to succeed if a careful, systematic approach is adopted during all stages of the software life cycle. The systematic approach to programming, using accepted principles of good design, is called software engineering. The software engineer tries to efficiently construct programs that verifiably meet their specifications and that are easy to modify if necessary. There is a wide range of "methodologies" that can be applied to help in the systematic design of programs. (Most of these methodologies seem to involve drawing little boxes to represent program components, with labeled arrows to represent relationships among the boxes.)

We have been discussing object orientation in programming languages, which is relevant to the coding stage of program development. But there are also object-oriented methodologies for analysis and design. The question in this stage of the software life cycle is, How can one discover or invent the overall structure of a program? As an example of a rather simple object-oriented approach to analysis and design, consider this advice: Write down a description of the problem. Underline all the nouns in that description. The nouns should be considered as candidates for becoming classes or objects in the program design. Similarly, underline all the verbs. These are candidates for methods. This is your starting point. Further analysis might uncover the need for more classes and methods, and it might reveal that subclassing can be used to take advantage of similarities among classes.

This is perhaps a bit simple-minded, but the idea is clear and the general approach can be effective: Analyze the problem to discover the concepts that are involved, and create classes to represent those concepts. The design should arise from the problem itself, and you should end up with a program whose structure reflects the structure of the problem in a natural way.

Section 5.4

Programming Example: Card, Hand, Deck

In this section, we look at some specific examples of object-oriented design in a domain that is simple enough that we have a chance of coming up with something reasonably reusable. Consider card games that are played with a standard deck of playing cards (a so-called "poker" deck, since it is used in the game of poker).

5.4.1  Designing the classes

In a typical card game, each player gets a hand of cards. The deck is shuffled and cards are dealt one at a time from the deck and added to the players' hands. In some games, cards can be removed from a hand, and new cards can be added. The game is won or lost depending on the value (ace, 2, ..., king) and suit (spades, diamonds, clubs, hearts) of the cards that a player receives. If we look for nouns in this description, there are several candidates for objects: game, player, hand, card, deck, value, and suit. Of these, the value and the suit of a card are simple values, and they might just be represented as instance variables in a Card object. In a complete program, the other five nouns might be represented by classes. But let's work on the ones that are most obviously reusable: card, hand, and deck.

If we look for verbs in the description of a card game, we see that we can shuffle a deck and deal a card from a deck. This gives use us two candidates for instance methods in a Deck class: shuffle() and dealCard(). Cards can be added to and removed from hands. This gives two candidates for instance methods in a Hand class: addCard() and removeCard(). Cards are relatively passive things, but we at least need to be able to determine their suits and values. We will discover more instance methods as we go along.

First, we'll design the deck class in detail. When a deck of cards is first created, it contains 52 cards in some standard order. The Deck class will need a constructor to create a new deck. The constructor needs no parameters because any new deck is the same as any other. There will be an instance method called shuffle() that will rearrange the 52 cards into a random order. The dealCard() instance method will get the next card from the deck. This will be a function with a return type of Card, since the caller needs to know what card is being dealt. It has no parameters -- when you deal the next card from the deck, you don't provide any information to the deck; you just get the next card, whatever it is. What will happen if there are no more cards in the deck when its dealCard() method is called? It should probably be considered an error to try to deal a card from an empty deck, so the deck can throw an exception in that case. But this raises another question: How will the rest of the program know whether the deck is empty? Of course, the program could keep track of how many cards it has used. But the deck itself should know how many cards it has left, so the program should just be able to ask the deck object. We can make this possible by specifying another instance method, cardsLeft(), that returns the number of cards remaining in the deck. This leads to a full specification of all the subroutines in the Deck class:

Constructor and instance methods in class Deck:

/**
* Constructor.  Create an unshuffled deck of cards.
*/
public Deck()

/**
* Put all the used cards back into the deck,
* and shuffle it into a random order.
*/
public void shuffle()

/**
* As cards are dealt from the deck, the number of
* cards left decreases.  This function returns the
* number of cards that are still left in the deck.
*/
public int cardsLeft()

/**
* Deals one card from the deck and returns it.
* @throws IllegalStateException if no more cards are left.
*/
public Card dealCard()

This is everything you need to know in order to use the Deck class. Of course, it doesn't tell us how to write the class. This has been an exercise in design, not in coding. You can look at the source code, Deck.java, if you want. It should not be a surprise that the class includes an array of Cards as an instance variable, but there are a few things you might not understand at this point. Of course, you can use the class in your programs as a black box, without understanding the implementation.

We can do a similar analysis for the Hand class. When a hand object is first created, it has no cards in it. An addCard() instance method will add a card to the hand. This method needs a parameter of type Card to specify which card is being added. For the removeCard() method, a parameter is needed to specify which card to remove. But should we specify the card itself ("Remove the ace of spades"), or should we specify the card by its position in the hand ("Remove the third card in the hand")? Actually, we don't have to decide, since we can allow for both options. We'll have two removeCard() instance methods, one with a parameter of type Card specifying the card to be removed and one with a parameter of type int specifying the position of the card in the hand. (Remember that you can have two methods in a class with the same name, provided they have different numbers or types of parameters.) Since a hand can contain a variable number of cards, it's convenient to be able to ask a hand object how many cards it contains. So, we need an instance method getCardCount() that returns the number of cards in the hand. When I play cards, I like to arrange the cards in my hand so that cards of the same value are next to each other. Since this is a generally useful thing to be able to do, we can provide instance methods for sorting the cards in the hand. Here is a full specification for a reusable Hand class:

Constructor and instance methods in class Hand:

/**
* Constructor. Create a Hand object that is initially empty.
*/
public Hand()

/**
* Discard all cards from the hand, making the hand empty.
*/
public void clear()

/**
* Add the card c to the hand.  c should be non-null.
* @throws NullPointerException if c is null.
*/
public void addCard(Card c)

/**
* If the specified card is in the hand, it is removed.
*/
public void removeCard(Card c)

/**
* Remove the card in the specified position from the
* hand.  Cards are numbered counting from zero.
* @throws IllegalArgumentException if the specified
*    position does not exist in the hand.
*/
public void removeCard(int position)

/**
* Return the number of cards in the hand.
*/
public int getCardCount()

/**
* Get the card from the hand in given position, where
* positions are numbered starting from 0.
* @throws IllegalArgumentException if the specified
*    position does not exist in the hand.
*/
public Card getCard(int position)

/**
* Sorts the cards in the hand so that cards of the same
* suit are grouped together, and within a suit the cards
* are sorted by value.
*/
public void sortBySuit()

/**
* Sorts the cards in the hand so that cards are sorted into
* order of increasing value.  Cards with the same value
* are sorted by suit. Note that aces are considered
* to have the lowest value.
*/
public void sortByValue()

Again, there are a few things in the implementation of the class that you won't understand at this point, but that doesn't stop you from using the class in your projects. The source code can be found in the file Hand.java

5.4.2  The Card Class

We will look at the design and implementation of a Card class in full detail. The class will have a constructor that specifies the value and suit of the card that is being created. There are four suits, which can be represented by the integers 0, 1, 2, and 3. It would be tough to remember which number represents which suit, so I've defined named constants in the Card class to represent the four possibilities. For example, Card.SPADES is a constant that represents the suit, "spades". (These constants are declared to be public final static ints. It might be better to use an enumerated type, but I will stick here to integer-valued constants.) The possible values of a card are the numbers 1, 2, ..., 13, with 1 standing for an ace, 11 for a jack, 12 for a queen, and 13 for a king. Again, I've defined some named constants to represent the values of aces and face cards. (When you read the Card class, you'll see that I've also added support for Jokers.)

A Card object can be constructed knowing the value and the suit of the card. For example, we can call the constructor with statements such as:

card1 = new Card( Card.ACE, Card.SPADES );  // Construct ace of spades.
card2 = new Card( 10, Card.DIAMONDS );   // Construct 10 of diamonds.
card3 = new Card( v, s );  // This is OK, as long as v and s
//               are integer expressions.

A Card object needs instance variables to represent its value and suit. I've made these private so that they cannot be changed from outside the class, and I've provided getter methods getSuit() and getValue() so that it will be possible to discover the suit and value from outside the class. The instance variables are initialized in the constructor, and are never changed after that. In fact, I've declared the instance variables suit and value to be final, since they are never changed after they are initialized. An instance variable can be declared final provided it is either given an initial value in its declaration or is initialized in every constructor in the class. Since all its instance variables are final, a Card is an immutable object.

Finally, I've added a few convenience methods to the class to make it easier to print out cards in a human-readable form. For example, I want to be able to print out the suit of a card as the word "Diamonds", rather than as the meaningless code number 2, which is used in the class to represent diamonds. Since this is something that I'll probably have to do in many programs, it makes sense to include support for it in the class. So, I've provided instance methods getSuitAsString() and getValueAsString() to return string representations of the suit and value of a card. Finally, I've defined the instance method toString() to return a string with both the value and suit, such as "Queen of Hearts". Recall that this method will be used automatically whenever a Card needs to be converted into a String, such as when the card is concatenated onto a string with the + operator. Thus, the statement

System.out.println( "Your card is the " + card );

is equivalent to

System.out.println( "Your card is the " + card.toString() );

If the card is the queen of hearts, either of these will print out "Your card is the Queen of Hearts".

Here is the complete Card class. It is general enough to be highly reusable, so the work that went into designing, writing, and testing it pays off handsomely in the long run.

/**
* An object of type Card represents a playing card from a
* standard Poker deck, including Jokers.  The card has a suit, which
* can be spades, hearts, diamonds, clubs, or joker.  A spade, heart,
* diamond, or club has one of the 13 values: ace, 2, 3, 4, 5, 6, 7,
* 8, 9, 10, jack, queen, or king.  Note that "ace" is considered to be
* the smallest value.  A joker can also have an associated value;
* this value can be anything and can be used to keep track of several
* different jokers.
*/
public class Card {

public final static int SPADES = 0;   // Codes for the 4 suits, plus Joker.
public final static int HEARTS = 1;
public final static int DIAMONDS = 2;
public final static int CLUBS = 3;
public final static int JOKER = 4;

public final static int ACE = 1;      // Codes for the non-numeric cards.
public final static int JACK = 11;    //   Cards 2 through 10 have their
public final static int QUEEN = 12;   //   numerical values for their codes.
public final static int KING = 13;

/**
* This card's suit, one of the constants SPADES, HEARTS, DIAMONDS,
* CLUBS, or JOKER.  The suit cannot be changed after the card is
* constructed.
*/
private final int suit;

/**
* The card's value.  For a normal card, this is one of the values
* 1 through 13, with 1 representing ACE.  For a JOKER, the value
* can be anything.  The value cannot be changed after the card
* is constructed.
*/
private final int value;

/**
* Creates a Joker, with 1 as the associated value.  (Note that
* "new Card()" is equivalent to "new Card(1,Card.JOKER)".)
*/
public Card() {
suit = JOKER;
value = 1;
}

/**
* Creates a card with a specified suit and value.
* @param theValue the value of the new card.  For a regular card (non-joker),
* the value must be in the range 1 through 13, with 1 representing an Ace.
* You can use the constants Card.ACE, Card.JACK, Card.QUEEN, and Card.KING.
* For a Joker, the value can be anything.
* @param theSuit the suit of the new card.  This must be one of the values
* Card.SPADES, Card.HEARTS, Card.DIAMONDS, Card.CLUBS, or Card.JOKER.
* @throws IllegalArgumentException if the parameter values are not in the
* permissible ranges
*/
public Card(int theValue, int theSuit) {
if (theSuit != SPADES && theSuit != HEARTS && theSuit != DIAMONDS &&
theSuit != CLUBS && theSuit != JOKER)
throw new IllegalArgumentException("Illegal playing card suit");
if (theSuit != JOKER && (theValue < 1 || theValue > 13))
throw new IllegalArgumentException("Illegal playing card value");
value = theValue;
suit = theSuit;
}

/**
* Returns the suit of this card.
* @returns the suit, which is one of the constants Card.SPADES,
* Card.HEARTS, Card.DIAMONDS, Card.CLUBS, or Card.JOKER
*/
public int getSuit() {
return suit;
}

/**
* Returns the value of this card.
* @return the value, which is one of the numbers 1 through 13, inclusive for
* a regular card, and which can be any value for a Joker.
*/
public int getValue() {
return value;
}

/**
* Returns a String representation of the card's suit.
* @return one of the strings "Spades", "Hearts", "Diamonds", "Clubs"
* or "Joker".
*/
public String getSuitAsString() {
switch ( suit ) {
case SPADES:   return "Spades";
case HEARTS:   return "Hearts";
case DIAMONDS: return "Diamonds";
case CLUBS:    return "Clubs";
default:       return "Joker";
}
}

/**
* Returns a String representation of the card's value.
* @return for a regular card, one of the strings "Ace", "2",
* "3", ..., "10", "Jack", "Queen", or "King".  For a Joker, the
* string is always numerical.
*/
public String getValueAsString() {
if (suit == JOKER)
return "" + value;
else {
switch ( value ) {
case 1:   return "Ace";
case 2:   return "2";
case 3:   return "3";
case 4:   return "4";
case 5:   return "5";
case 6:   return "6";
case 7:   return "7";
case 8:   return "8";
case 9:   return "9";
case 10:  return "10";
case 11:  return "Jack";
case 12:  return "Queen";
default:  return "King";
}
}
}

/**
* Returns a string representation of this card, including both
* its suit and its value (except that for a Joker with value 1,
* the return value is just "Joker").  Sample return values
* are: "Queen of Hearts", "10 of Diamonds", "Ace of Spades",
* "Joker", "Joker #2"
*/
public String toString() {
if (suit == JOKER) {
if (value == 1)
return "Joker";
else
return "Joker #" + value;
}
else
return getValueAsString() + " of " + getSuitAsString();
}

} // end class Card

5.4.3  Example: A Simple Card Game

I will finish this section by presenting a complete program that uses the Card and Deck classes. The program lets the user play a very simple card game called HighLow. A deck of cards is shuffled, and one card is dealt from the deck and shown to the user. The user predicts whether the next card from the deck will be higher or lower than the current card. If the user predicts correctly, then the next card from the deck becomes the current card, and the user makes another prediction. This continues until the user makes an incorrect prediction. The number of correct predictions is the user's score.

My program has a static method that plays one game of HighLow. This method has a return value that represents the user's score in the game. The main() routine lets the user play several games of HighLow. At the end, it reports the user's average score.

I won't go through the development of the algorithms used in this program, but I encourage you to read it carefully and make sure that you understand how it works. Note in particular that the subroutine that plays one game of HighLow returns the user's score in the game as its return value. This gets the score back to the main program, where it is needed. Here is the program:

/**
* This program lets the user play HighLow, a simple card game
* that is described in the output statements at the beginning of
* the main() routine.  After the user plays several games,
* the user's average score is reported.
*/
public class HighLow {

public static void main(String[] args) {

System.out.println("This program lets you play the simple card game,");
System.out.println("HighLow.  A card is dealt from a deck of cards.");
System.out.println("You have to predict whether the next card will be");
System.out.println("higher or lower.  Your score in the game is the");
System.out.println("number of correct predictions you make before");
System.out.println("you guess wrong.");
System.out.println();

int gamesPlayed = 0;     // Number of games user has played.
int sumOfScores = 0;     // The sum of all the scores from
//      all the games played.
double averageScore;     // Average score, computed by dividing
//      sumOfScores by gamesPlayed.
boolean playAgain;       // Record user's response when user is
//   asked whether he wants to play
//   another game.

do {
int scoreThisGame;        // Score for one game.
scoreThisGame = play();   // Play the game and get the score.
sumOfScores += scoreThisGame;
gamesPlayed++;
System.out.print("Play again? ");
playAgain = TextIO.getlnBoolean();
} while (playAgain);

averageScore = ((double)sumOfScores) / gamesPlayed;

System.out.println();
System.out.println("You played " + gamesPlayed + " games.");
System.out.printf("Your average score was %1.3f.\n", averageScore);

}  // end main()

/**
* Lets the user play one game of HighLow, and returns the
* user's score in that game.  The score is the number of
* correct guesses that the user makes.
*/
private static int play() {

Deck deck = new Deck();  // Get a new deck of cards, and
//   store a reference to it in
//   the variable, deck.

Card currentCard;  // The current card, which the user sees.

Card nextCard;   // The next card in the deck.  The user tries
//    to predict whether this is higher or lower
//    than the current card.

int correctGuesses ;  // The number of correct predictions the
//   user has made.  At the end of the game,
//   this will be the user's score.

char guess;   // The user's guess.  'H' if the user predicts that
//   the next card will be higher, 'L' if the user
//   predicts that it will be lower.

deck.shuffle();  // Shuffle the deck into a random order before
//    starting the game.

correctGuesses = 0;
currentCard = deck.dealCard();
System.out.println("The first card is the " + currentCard);

while (true) {  // Loop ends when user's prediction is wrong.

/* Get the user's prediction, 'H' or 'L' (or 'h' or 'l'). */

System.out.print("Will the next card be higher (H) or lower (L)?  ");
do {
guess = TextIO.getlnChar();
guess = Character.toUpperCase(guess);
if (guess != 'H' && guess != 'L')
System.out.print("Please respond with H or L:  ");
} while (guess != 'H' && guess != 'L');

/* Get the next card and show it to the user. */

nextCard = deck.dealCard();
System.out.println("The next card is " + nextCard);

/* Check the user's prediction. */

if (nextCard.getValue() == currentCard.getValue()) {
System.out.println("The value is the same as the previous card.");
System.out.println("You lose on ties.  Sorry!");
break;  // End the game.
}
else if (nextCard.getValue() > currentCard.getValue()) {
if (guess == 'H') {
System.out.println("Your prediction was correct.");
correctGuesses++;
}
else {
System.out.println("Your prediction was incorrect.");
break;  // End the game.
}
}
else {  // nextCard is lower
if (guess == 'L') {
System.out.println("Your prediction was correct.");
correctGuesses++;
}
else {
System.out.println("Your prediction was incorrect.");
break;  // End the game.
}
}

/* To set up for the next iteration of the loop, the nextCard
becomes the currentCard, since the currentCard has to be
the card that the user sees, and the nextCard will be
set to the next card in the deck after the user makes
his prediction.  */

currentCard = nextCard;
System.out.println();
System.out.println("The card is " + currentCard);

} // end of while loop

System.out.println();
System.out.println("The game is over.");
System.out.println("You made " + correctGuesses
+ " correct predictions.");
System.out.println();

return correctGuesses;

}  // end play()

} // end class HighLow

Section 5.5

Inheritance, Polymorphism, and Abstract Classes

A class represents a set of objects which share the same structure and behaviors. The class determines the structure of objects by specifying variables that are contained in each instance of the class, and it determines behavior by providing the instance methods that express the behavior of the objects. This is a powerful idea. However, something like this can be done in most programming languages. The central new idea in object-oriented programming -- the idea that really distinguishes it from traditional programming -- is to allow classes to express the similarities among objects that share some, but not all, of their structure and behavior. Such similarities can be expressed using inheritance and polymorphism.

5.5.1  Extending Existing Classes

The topics covered in later subsections of this section are relatively advanced aspects of object-oriented programming. Any programmer should know what is meant by subclass, inheritance, and polymorphism. However, it will probably be a while before you actually do anything with inheritance except for extending classes that already exist. In the first part of this section, we look at how that is done.

In day-to-day programming, especially for programmers who are just beginning to work with objects, subclassing is used mainly in one situation: There is an existing class that can be adapted with a few changes or additions. This is much more common than designing groups of classes and subclasses from scratch. The existing class can be extended to make a subclass. The syntax for this is

public class subclass-name extends existing-class-name {
.
.   // Changes and additions.
.
}

As an example, suppose you want to write a program that plays the card game, Blackjack. You can use the Card, Hand, and Deck classes developed in Section 5.4. However, a hand in the game of Blackjack is a little different from a hand of cards in general, since it must be possible to compute the "value" of a Blackjack hand according to the rules of the game. The rules are as follows: The value of a hand is obtained by adding up the values of the cards in the hand. The value of a numeric card such as a three or a ten is its numerical value. The value of a Jack, Queen, or King is 10. The value of an Ace can be either 1 or 11. An Ace should be counted as 11 unless doing so would put the total value of the hand over 21. Note that this means that the second, third, or fourth Ace in the hand will always be counted as 1.

One way to handle this is to extend the existing Hand class by adding a method that computes the Blackjack value of the hand. Here's the definition of such a class:

public class BlackjackHand extends Hand {

/**
* Computes and returns the value of this hand in the game
* of Blackjack.
*/
public int getBlackjackValue() {

int val;      // The value computed for the hand.
boolean ace;  // This will be set to true if the
//   hand contains an ace.
int cards;    // Number of cards in the hand.

val = 0;
ace = false;
cards = getCardCount();  // (method defined in class Hand.)

for ( int i = 0;  i < cards;  i++ ) {
// Add the value of the i-th card in the hand.
Card card;    // The i-th card;
int cardVal;  // The blackjack value of the i-th card.
card = getCard(i);
cardVal = card.getValue();  // The normal value, 1 to 13.
if (cardVal > 10) {
cardVal = 10;   // For a Jack, Queen, or King.
}
if (cardVal == 1) {
ace = true;     // There is at least one ace.
}
val = val + cardVal;
}

// Now, val is the value of the hand, counting any ace as 1.
// If there is an ace, and if changing its value from 1 to
// 11 would leave the score less than or equal to 21,
// then do so by adding the extra 10 points to val.

if ( ace == true  &&  val + 10 <= 21 )
val = val + 10;

return val;

}  // end getBlackjackValue()

} // end class BlackjackHand

Since BlackjackHand is a subclass of Hand, an object of type BlackjackHand contains all the instance variables and instance methods defined in Hand, plus the new instance method named getBlackjackValue(). For example, if bjh is a variable of type BlackjackHand, then the following are all legal: bjh.getCardCount(), bjh.removeCard(0), and bjh.getBlackjackValue(). The first two methods are defined in Hand, but are inherited by BlackjackHand.

Variables and methods from the Hand class are inherited by BlackjackHand, and they can be used in the definition of BlackjackHand just as if they were actually defined in that class -- except for any that are declared to be private, which prevents access even by subclasses. The statement "cards = getCardCount();" in the above definition of getBlackjackValue() calls the instance method getCardCount(), which was defined in Hand.

Extending existing classes is an easy way to build on previous work. We'll see that many standard classes have been written specifically to be used as the basis for making subclasses.

Access modifiers such as public and private are used to control access to members of a class. There is one more access modifier, protected, that comes into the picture when subclasses are taken into consideration. When protected is applied as an access modifier to a method or member variable in a class, that member can be used in subclasses -- direct or indirect -- of the class in which it is defined, but it cannot be used in non-subclasses. (There is an exception: A protected member can also be accessed by any class in the same package as the class that contains the protected member. Recall that using no access modifier makes a member accessible to classes in the same package, and nowhere else. Using the protected modifier is strictly more liberal than using no modifier at all: It allows access from classes in the same package and from subclasses that are not in the same package.)

When you declare a method or member variable to be protected, you are saying that it is part of the implementation of the class, rather than part of the public interface of the class. However, you are allowing subclasses to use and modify that part of the implementation.

For example, consider a PairOfDice class that has instance variables die1 and die2 to represent the numbers appearing on the two dice. We could make those variables private to make it impossible to change their values from outside the class, while still allowing read access through getter methods. However, if we think it possible that PairOfDice will be used to create subclasses, we might want to make it possible for subclasses to change the numbers on the dice. For example, a GraphicalDice subclass that draws the dice might want to change the numbers at other times besides when the dice are rolled. In that case, we could make die1 and die2 protected, which would allow the subclass to change their values without making them public to the rest of the world. (An even better idea would be to define protected setter methods for the variables. A setter method could, for example, ensure that the value that is being assigned to the variable is in the legal range 1 through 6.)

5.5.2  Inheritance and Class Hierarchy

The term inheritance refers to the fact that one class can inherit part or all of its structure and behavior from another class. The class that does the inheriting is said to be a subclass of the class from which it inherits. If class B is a subclass of class A, we also say that class A is a superclass of class B. (Sometimes the terms derived class and base class are used instead of subclass and superclass; this is the common terminology in C++.) A subclass can add to the structure and behavior that it inherits. It can also replace or modify inherited behavior (though not inherited structure). The relationship between subclass and superclass is sometimes shown by a diagram in which the subclass is shown below, and connected to, its superclass, as shown on the left below:

In Java, to create a class named "B" as a subclass of a class named "A", you would write

class B extends A {
.
.  // additions to, and modifications of,
.  // stuff inherited from class A
.
}

Several classes can be declared as subclasses of the same superclass. The subclasses, which might be referred to as "sibling classes," share some structures and behaviors -- namely, the ones they inherit from their common superclass. The superclass expresses these shared structures and behaviors. In the diagram shown on the right above, classes B, C, and D are sibling classes. Inheritance can also extend over several "generations" of classes. This is shown in the diagram, where class E is a subclass of class D which is itself a subclass of class A. In this case, class E is considered to be a subclass of class A, even though it is not a direct subclass. This whole set of classes forms a small class hierarchy.

5.5.3  Example: Vehicles

Let's look at an example. Suppose that a program has to deal with motor vehicles, including cars, trucks, and motorcycles. (This might be a program used by a Department of Motor Vehicles to keep track of registrations.) The program could use a class named Vehicle to represent all types of vehicles. Since cars, trucks, and motorcycles are types of vehicles, they would be represented by subclasses of the Vehicle class, as shown in this class hierarchy diagram:

The Vehicle class would include instance variables such as registrationNumber and owner and instance methods such as transferOwnership(). These are variables and methods common to all vehicles. The three subclasses of Vehicle -- Car, Truck, and Motorcycle -- could then be used to hold variables and methods specific to particular types of vehicles. The Car class might add an instance variable numberOfDoors, the Truck class might have numberOfAxles, and the Motorcycle class could have a boolean variable hasSidecar. (Well, it could in theory at least, even if it might give a chuckle to the people at the Department of Motor Vehicles.) The declarations of these classes in a Java program would look, in outline, like this (although they are likely to be defined in separate files and declared as public classes):

class Vehicle {
int registrationNumber;
Person owner;  // (Assuming that a Person class has been defined!)
void transferOwnership(Person newOwner) {
. . .
}
. . .
}

class Car extends Vehicle {
int numberOfDoors;
. . .
}

class Truck extends Vehicle {
int numberOfAxles;
. . .
}

class Motorcycle extends Vehicle {
boolean hasSidecar;
. . .
}

Suppose that myCar is a variable of type Car that has been declared and initialized with the statement

Car myCar = new Car();

Given this declaration, a program could refer to myCar.numberOfDoors, since numberOfDoors is an instance variable in the class Car. But since class Car extends class Vehicle, a car also has all the structure and behavior of a vehicle. This means that myCar.registrationNumber, myCar.owner, and myCar.transferOwnership() also exist.

Now, in the real world, cars, trucks, and motorcycles are in fact vehicles. The same is true in a program. That is, an object of type Car or Truck or Motorcycle is automatically an object of type Vehicle too. This brings us to the following Important Fact:

A variable that can hold a reference
to an object of class A can also hold a reference
to an object belonging to any subclass of A.

The practical effect of this in our example is that an object of type Car can be assigned to a variable of type Vehicle. That is, it would be legal to say

Vehicle myVehicle = myCar;

or even

Vehicle myVehicle = new Car();

After either of these statements, the variable myVehicle holds a reference to a Vehicle object that happens to be an instance of the subclass, Car. The object "remembers" that it is in fact a Car, and not just a Vehicle. Information about the actual class of an object is stored as part of that object. It is even possible to test whether a given object belongs to a given class, using the instanceof operator. The test:

if (myVehicle instanceof Car) ...

determines whether the object referred to by myVehicle is in fact a car.

On the other hand, the assignment statement

myCar = myVehicle;

would be illegal because myVehicle could potentially refer to other types of vehicles that are not cars. This is similar to a problem we saw previously in Subsection 2.5.6: The computer will not allow you to assign an int value to a variable of type short, because not every int is a short. Similarly, it will not allow you to assign a value of type Vehicle to a variable of type Car because not every vehicle is a car. As in the case of ints and shorts, the solution here is to use type-casting. If, for some reason, you happen to know that myVehicle does in fact refer to a Car, you can use the type cast (Car)myVehicle to tell the computer to treat myVehicle as if it were actually of type Car. So, you could say

myCar = (Car)myVehicle;

and you could even refer to ((Car)myVehicle).numberOfDoors. (The parentheses are necessary because of precedence. The "." has higher precedence than the type-cast, so (Car)myVehicle.numberOfDoors would be read as (Car)(myVehicle.numberOfDoors), an attempt to type-cast the int myVehicle.numberOfDoors into a Vehicle, which is impossible.)

As an example of how this could be used in a program, suppose that you want to print out relevant data about the Vehicle referred to by myVehicle. If it's a Car, you will want to print out the car's numberOfDoors, but you can't say myVehicle.numberOfDoors, since there is no numberOfDoors in the Vehicle class. But you could say:

System.out.println("Vehicle Data:");
System.out.println("Registration number:  "
+ myVehicle.registrationNumber);
if (myVehicle instanceof Car) {
System.out.println("Type of vehicle:  Car");
Car c;
c = (Car)myVehicle;  // Type-cast to get access to numberOfDoors!
System.out.println("Number of doors:  " + c.numberOfDoors);
}
else if (myVehicle instanceof Truck) {
System.out.println("Type of vehicle:  Truck");
Truck t;
t = (Truck)myVehicle;  // Type-cast to get access to numberOfAxles!
System.out.println("Number of axles:  " + t.numberOfAxles);
}
else if (myVehicle instanceof Motorcycle) {
System.out.println("Type of vehicle:  Motorcycle");
Motorcycle m;
m = (Motorcycle)myVehicle;  // Type-cast to get access to hasSidecar!
System.out.println("Has a sidecar:    " + m.hasSidecar);
}

Note that for object types, when the computer executes a program, it checks whether type-casts are valid. So, for example, if myVehicle refers to an object of type Truck, then the type cast (Car)myVehicle would be an error. When this happens, an exception of type ClassCastException is thrown. This check is done at run time, not compile time, because the actual type of the object referred to by myVehicle is not known when the program is compiled.

5.5.4  Polymorphism

As another example, consider a program that deals with shapes drawn on the screen. Let's say that the shapes include rectangles, ovals, and roundrects of various colors. (A "roundrect" is just a rectangle with rounded corners.)

Three classes, Rectangle, Oval, and RoundRect, could be used to represent the three types of shapes. These three classes would have a common superclass, Shape, to represent features that all three shapes have in common. The Shape class could include instance variables to represent the color, position, and size of a shape, and it could include instance methods for changing the values of those properties. Changing the color, for example, might involve changing the value of an instance variable, and then redrawing the shape in its new color:

class Shape {

Color color; // (must be imported from package java.awt)

void setColor(Color newColor) {
// Method to change the color of the shape.
color = newColor; // change value of instance variable
redraw(); // redraw shape, which will appear in new color
}

void redraw() {
// method for drawing the shape
? ? ?  // what commands should go here?
}

. . .          // more instance variables and methods

} // end of class Shape

Now, you might see a problem here with the method redraw(). The problem is that each different type of shape is drawn differently. The method setColor() can be called for any type of shape. How does the computer know which shape to draw when it executes the redraw()? Informally, we can answer the question like this: The computer executes redraw() by asking the shape to redraw itself. Every shape object knows what it has to do to redraw itself.

In practice, this means that each of the specific shape classes has its own redraw() method:

class Rectangle extends Shape {
void redraw() {
. . .  // commands for drawing a rectangle
}
. . . // possibly, more methods and variables
}

class Oval extends Shape {
void redraw() {
. . .  // commands for drawing an oval
}
. . . // possibly, more methods and variables
}

class RoundRect extends Shape {
void redraw() {
. . .  // commands for drawing a rounded rectangle
}
. . . // possibly, more methods and variables
}

Suppose that someShape is a variable of type Shape. Then it could refer to an object of any of the types Rectangle, Oval, or RoundRect. As a program executes, and the value of someShape changes, it could even refer to objects of different types at different times! Whenever the statement

someShape.redraw();

is executed, the redraw method that is actually called is the one appropriate for the type of object to which someShape actually refers. There may be no way of telling, from looking at the text of the program, what shape this statement will draw, since it depends on the value that someShape happens to have when the program is executed. Even more is true. Suppose the statement is in a loop and gets executed many times. If the value of someShape changes as the loop is executed, it is possible that the very same statement "someShape.redraw();" will call different methods and draw different shapes as it is executed over and over. We say that the redraw() method is polymorphic. A method is polymorphic if the action performed by the method depends on the actual type of the object to which the method is applied. Polymorphism is one of the major distinguishing features of object-oriented programming. This can be seen most vividly, perhaps, if we have an array of shapes. Suppose that shapelist is an a variable of type Shape[], and that the array has already been created and filled with data. Some of the elements in the array might be Rectangles, some might be Ovals, and some might be RoundRects. We can draw all the shapes in the array by saying

for (int i = 0; i < shapelist.length; i++ ) {
Shape shape = shapelist[i];
shape.redraw();
}

As the computer goes through this loop, the statement shape.redraw() will sometimes draw a rectangle, sometimes an oval, and sometimes a roundrect, depending on the type of object to which array element number i refers.

Perhaps this becomes more understandable if we change our terminology a bit: In object-oriented programming, calling a method is often referred to as sending a message to an object. The object responds to the message by executing the appropriate method. The statement "someShape.redraw();" is a message to the object referred to by someShape. Since that object knows what type of object it is, it knows how it should respond to the message. From this point of view, the computer always executes "someShape.redraw();" in the same way: by sending a message. The response to the message depends, naturally, on who receives it. From this point of view, objects are active entities that send and receive messages, and polymorphism is a natural, even necessary, part of this view. Polymorphism just means that different objects can respond to the same message in different ways.

One of the most beautiful things about polymorphism is that it lets code that you write do things that you didn't even conceive of, at the time you wrote it. Suppose that I decide to add beveled rectangles to the types of shapes my program can deal with. A beveled rectangle has a triangle cut off each corner:

To implement beveled rectangles, I can write a new subclass, BeveledRect, of class Shape and give it its own redraw() method. Automatically, code that I wrote previously -- such as the statement someShape.redraw() -- can now suddenly start drawing beveled rectangles, even though the beveled rectangle class didn't exist when I wrote the statement!

In the statement "someShape.redraw();", the redraw message is sent to the object someShape. Look back at the method in the Shape class for changing the color of a shape:

void setColor(Color newColor) {
color = newColor; // change value of instance variable
redraw(); // redraw shape, which will appear in new color
}

A redraw message is sent here, but which object is it sent to? Well, the setColor method is itself a message that was sent to some object. The answer is that the redraw message is sent to that same object, the one that received the setColor message. If that object is a rectangle, then it contains a redraw() method for drawing rectangles, and that is the one that is executed. If the object is an oval, then it is the redraw() method from the Oval class. This is what you should expect, but it means that the "redraw();" statement in the setColor() method does not necessarily call the redraw() method in the Shape class! The redraw() method that is executed could be in any subclass of Shape. This is just another case of polymorphism.

5.5.5  Abstract Classes

Whenever a Rectangle, Oval, or RoundRect object has to draw itself, it is the redraw() method in the appropriate class that is executed. This leaves open the question, What does the redraw() method in the Shape class do? How should it be defined?

The answer may be surprising: We should leave it blank! The fact is that the class Shape represents the abstract idea of a shape, and there is no way to draw such a thing. Only particular, concrete shapes like rectangles and ovals can be drawn. So, why should there even be a redraw() method in the Shape class? Well, it has to be there, or it would be illegal to call it in the setColor() method of the Shape class, and it would be illegal to write "someShape.redraw();". The compiler would complain that someShape is a variable of type Shape and there's no redraw() method in the Shape class.

Nevertheless the version of redraw() in the Shape class itself will never actually be called. In fact, if you think about it, there can never be any reason to construct an actual object of type Shape! You can have variables of type Shape, but the objects they refer to will always belong to one of the subclasses of Shape. We say that Shape is an abstract class. An abstract class is one that is not used to construct objects, but only as a basis for making subclasses. An abstract class exists only to express the common properties of all its subclasses. A class that is not abstract is said to be concrete. You can create objects belonging to a concrete class, but not to an abstract class. A variable whose type is given by an abstract class can only refer to objects that belong to concrete subclasses of the abstract class.

Similarly, we say that the redraw() method in class Shape is an abstract method, since it is never meant to be called. In fact, there is nothing for it to do -- any actual redrawing is done by redraw() methods in the subclasses of Shape. The redraw() method in Shape has to be there. But it is there only to tell the computer that all Shapes understand the redraw message. As an abstract method, it exists merely to specify the common interface of all the actual, concrete versions of redraw() in the subclasses. There is no reason for the abstract redraw() in class Shape to contain any code at all.

Shape and its redraw() method are semantically abstract. You can also tell the computer, syntactically, that they are abstract by adding the modifier "abstract" to their definitions. For an abstract method, the block of code that gives the implementation of an ordinary method is replaced by a semicolon. An implementation must then be provided for the abstract method in any concrete subclass of the abstract class. Here's what the Shape class would look like as an abstract class:

public abstract class Shape {

Color color;   // color of shape.

void setColor(Color newColor) {
// method to change the color of the shape
color = newColor; // change value of instance variable
redraw(); // redraw shape, which will appear in new color
}

abstract void redraw();
// abstract method -- must be defined in
// concrete subclasses

. . .  // more instance variables and methods

} // end of class Shape

Once you have declared the class to be abstract, it becomes illegal to try to create actual objects of type Shape, and the computer will report a syntax error if you try to do so.

Note, by the way, that the Vehicle class discussed above would probably also be an abstract class. There is no way to own a vehicle as such -- the actual vehicle has to be a car or a truck or a motorcycle, or some other "concrete" type of vehicle.

Recall from Subsection 5.3.2 that a class that is not explicitly declared to be a subclass of some other class is automatically made a subclass of the standard class Object. That is, a class declaration with no "extends" part such as

public class myClass { . . .

is exactly equivalent to

public class myClass extends Object { . . .

This means that class Object is at the top of a huge class hierarchy that includes every other class. (Semantically, Object is an abstract class, in fact the most abstract class of all. Curiously, however, it is not declared to be abstract syntactically, which means that you can create objects of type Object. However, there is not much that you can do with them.)

Since every class is a subclass of Object, a variable of type Object can refer to any object whatsoever, of any type. Similarly, an array of type Object[] can hold objects of any type.

The sample source code file ShapeDraw.java uses an abstract Shape class and an array of type Shape[] to hold a list of shapes. You might want to look at this file, even though you won't be able to understand all of it at this time. Even the definitions of the shape classes are somewhat different from those that I have described in this section. (For example, the draw() method has a parameter of type Graphics. This parameter is required because drawing in Java requires a graphics context.) I'll return to similar examples in later chapters when you know more about GUI programming. However, it would still be worthwhile to look at the definition of the Shape class and its subclasses in the source code. You might also check how an array is used to hold the list of shapes. Here is a screenshot from the program:

If you run the ShapeDraw program, you can click one of the buttons along the bottom to add a shape to the picture. The new shape will appear in the upper left corner of the drawing area. The color of the shape is given by the "pop-up menu" in the lower right. Once a shape is on the screen, you can drag it around with the mouse. A shape will maintain the same front-to-back order with respect to other shapes on the screen, even while you are dragging it. However, you can move a shape out in front of all the other shapes if you hold down the shift key as you click on it.

In the program, the only time when the actual class of a shape is used is when that shape is added to the screen. Once the shape has been created, it is manipulated entirely as an abstract shape. The routine that implements dragging, for example, works with variables of type Shape and makes no reference to any of its subclasses. As the shape is being dragged, the dragging routine just calls the shape's draw method each time the shape has to be drawn, so it doesn't have to know how to draw the shape or even what type of shape it is. The object is responsible for drawing itself. If I wanted to add a new type of shape to the program, I would define a new subclass of Shape, add another button, and program the button to add the correct type of shape to the screen. No other changes in the programming would be necessary.

Section 5.6

this and super

Although the basic ideas of object-oriented programming are reasonably simple and clear, they are subtle, and they take time to get used to. And unfortunately, beyond the basic ideas there are a lot of details. The rest of this chapter covers more of those annoying details. Remember that you don't need to master everything in this chapter the first time through. In this section, we'll look at two variables, this and super, that are automatically defined in any instance method.

5.6.1  The Special Variable this

What does it mean when you use a simple identifier such as amount or process() to refer to a variable or method? The answer depends on scope rules that tell where and how each declared variable and method can be accessed in a program. Inside the definition of a method, a simple variable name might refer to a local variable or parameter, if there is one "in scope," that is, one whose declaration is in effect at the point in the source code where the reference occurs. If not, it must refer to a member variable of the class in which the reference occurs. Similarly, a simple method name must refer to a method in the same class.

A static member of a class has a simple name that can only be used inside the class definition; for use outside the class, it has a full name of the form class-name.simple-name. For example, "Math.PI" is a static member variable with simple name "PI" in the class "Math". It's always legal to use the full name of a static member, even within the class where it's defined. Sometimes it's even necessary, as when the simple name of a static member variable is hidden by a local variable or parameter of the same name.

Instance variables and instance methods also have simple names. The simple name of such an instance member can be used in instance methods in the class where the instance member is defined (but not in static methods). Instance members also have full names -- but remember that an instance variable or instance method is actually contained in an object rather than in a class, and each object has its own version. A full name of an instance member starts with a reference to the object that contains the instance member. For example, if std is a variable that refers to an object of type Student, then std.test1 could be a full name for an instance variable named test1 that is contained in that object.

But when we are working inside a class and use a simple name to refer to an instance variable like test1, where is the object that contains the variable? The solution to this riddle is simple: Suppose that a reference to "test1" occurs in the definition of some instance method. The actual method that gets executed is part of some particular object of type Student. When that method gets executed, the occurrence of the name "test1" refers to the test1 variable in that same object. (This is why simple names of instance members cannot be used in static methods -- when a static method is executed, it is not part of an object, and hence there are no instance members in sight!)

This leaves open the question of full names for instance members inside the same class where they are defined. We need a way to refer to "the object that contains this method." Java defines a special variable named this for just this purpose. The variable this can be used in the source code of an instance method to refer to the object that contains the method. This intent of the name, "this," is to refer to "this object," the one right here that this very method is in. If var is an instance variable in the same object as the method, then "this.var" is a full name for that variable. If otherMethod() is an instance method in the same object, then this.otherMethod() could be used to call that method. Whenever the computer executes an instance method, it automatically sets the variable this to refer to the object that contains the method.

(Some object oriented languages use the name "self" instead of "this." Here, an object is seen as an entity that receives messages and responds by performing some action. From the point of view of that entity, an instance variable such as self.name refers to the entity's own name, something that is part of the entity itself. Calling an instance method such as self.redraw() is like saying "message to self: redraw!")

One common use of this is in constructors. For example:

public class Student {

private String name;  // Name of the student.

public Student(String name) {
// Constructor.  Create a student with specified name.
this.name = name;
}
.
.   // More variables and methods.
.
}

In the constructor, the instance variable called name is hidden by a formal parameter that is also called "name." However, the instance variable can still be referred to by its full name, which is this.name. In the assignment statement "this.name = name", the value of the formal parameter, name, is assigned to the instance variable, this.name. This is considered to be acceptable style: There is no need to dream up cute new names for formal parameters that are just used to initialize instance variables. You can use the same name for the parameter as for the instance variable.

There are other uses for this. Sometimes, when you are writing an instance method, you need to pass the object that contains the method to a subroutine, as an actual parameter. In that case, you can use this as the actual parameter. For example, if you wanted to print out a string representation of the object, you could say "System.out.println(this);". Or you could assign the value of this to another variable in an assignment statement. You can store it in an array. In fact, you can do anything with this that you could do with any other variable, except change its value. (Consider it to be a final variable.)

5.6.2  The Special Variable super

Java also defines another special variable, named "super", for use in the definitions of instance methods. The variable super is for use in a subclass. Like this, super refers to the object that contains the method. But it's forgetful. It forgets that the object belongs to the class you are writing, and it remembers only that it belongs to the superclass of that class. The point is that the class can contain additions and modifications to the superclass. super doesn't know about any of those additions and modifications; it can only be used to refer to methods and variables in the superclass.

Let's say that the class that you are writing contains an instance method named doSomething(). Consider the subroutine call statement super.doSomething(). Now, super doesn't know anything about the doSomething() method in the subclass. It only knows about things in the superclass, so it tries to execute a method named doSomething() from the superclass. If there is none -- if the doSomething() method was an addition rather than a modification -- you'll get a syntax error.

The reason super exists is so you can get access to things in the superclass that are hidden by things in the subclass. For example, super.var always refers to an instance variable named var in the superclass. This can be useful for the following reason: If a class contains an instance variable with the same name as an instance variable in its superclass, then an object of that class will actually contain two variables with the same name: one defined as part of the class itself and one defined as part of the superclass. The variable in the subclass does not replace the variable of the same name in the superclass; it merely hides it. The variable from the superclass can still be accessed, using super.

When a subclass contains an instance method that has the same signature as a method in its superclass, the method from the superclass is hidden in the same way. We say that the method in the subclass overrides the method from the superclass. Again, however, super can be used to access the method from the superclass.

The major use of super is to override a method with a new method that extends the behavior of the inherited method, instead of replacing that behavior entirely. The new method can use super to call the method from the superclass, and then it can add additional code to provide additional behavior. As an example, suppose you have a PairOfDice class that includes a roll() method. Suppose that you want a subclass, GraphicalDice, to represent a pair of dice drawn on the computer screen. The roll() method in the GraphicalDice class should do everything that the roll() method in the PairOfDice class does. We can express this with a call to super.roll(), which calls the method in the superclass. But in addition to that, the roll() method for a GraphicalDice object has to redraw the dice to show the new values. The GraphicalDice class might look something like this:

public class GraphicalDice extends PairOfDice {

public void roll() {
// Roll the dice, and redraw them.
super.roll();  // Call the roll method from PairOfDice.
redraw();      // Call a method to draw the dice.
}
.
.  // More stuff, including definition of redraw().
.
}

Note that this allows you to extend the behavior of the roll() method even if you don't know how the method is implemented in the superclass!

5.6.3  super and this As Constructors

Constructors are not inherited. That is, if you extend an existing class to make a subclass, the constructors in the superclass do not become part of the subclass. If you want constructors in the subclass, you have to define new ones from scratch. If you don't define any constructors in the subclass, then the computer will make up a default constructor, with no parameters, for you.

This could be a problem, if there is a constructor in the superclass that does a lot of necessary work. It looks like you might have to repeat all that work in the subclass! This could be a real problem if you don't have the source code to the superclass, and don't even know how it is implemented. It might look like an impossible problem, if the constructor in the superclass uses private member variables that you don't even have access to in the subclass!

Obviously, there has to be some fix for this, and there is. It involves the special variable, super. As the very first statement in a constructor, you can use super to call a constructor from the superclass. The notation for this is a bit ugly and misleading, and it can only be used in this one particular circumstance: It looks like you are calling super as a subroutine (even though super is not a subroutine and you can't call constructors the same way you call other subroutines anyway). As an example, assume that the PairOfDice class has a constructor that takes two integers as parameters. Consider a subclass:

public class GraphicalDice extends PairOfDice {

public GraphicalDice() {  // Constructor for this class.

super(3,4);  // Call the constructor from the
//   PairOfDice class, with parameters 3, 4.

initializeGraphics();  // Do some initialization specific
//   to the GraphicalDice class.
}
.
.  // More constructors, methods, variables...
.
}

The statement "super(3,4);" calls the constructor from the superclass. This call must be the first line of the constructor in the subclass. Note that if you don't explicitly call a constructor from the superclass in this way, then the default constructor from the superclass, the one with no parameters, will be called automatically. (And if no such constructor exists in the superclass, the compiler will consider it to be a syntax error.)

You can use the special variable this in exactly the same way to call another constructor in the same class. That is, the very first line of a constructor can look like a subroutine call with "this" as the name of the subroutine. The result is that the body of another constructor in the same class is executed. This can be very useful since it can save you from repeating the same code in several different constructors. As an example, consider MosaicPanel.java, which was used indirectly in Section 4.6. A MosaicPanel represents a grid of colored rectangles. It has a constructor with many parameters:

public MosaicPanel(int rows, int columns,
int preferredBlockWidth, int preferredBlockHeight,
Color borderColor, int borderWidth)

This constructor provides a lot of options and does a lot of initialization. I wanted to provide easier-to-use constructors with fewer options, but all the initialization still has to be done. The class also contains these constructors:

public MosaicPanel() {
this(42,42,16,16);
}

public MosaicPanel(int rows, int columns) {
this(rows,columns,16,16);
}

public MosaicPanel(int rows, int columns,
int preferredBlockWidth, int preferredBlockHeight) {
this(rows, columns, preferredBlockWidth, preferredBlockHeight, null, 0);
}

Each of these constructors exists just to call another constructor, while providing constant values for some of the parameters. For example, this(42,42,16,16) calls the last constructor listed here, while that constructor in turn calls the main, six-parameter constructor. That main constructor is eventually called in all cases, so that all the essential initialization gets done in every case.

Section 5.7

Interfaces

Some object-oriented programming languages, such as C++, allow a class to extend two or more superclasses. This is called multiple inheritance. In the illustration below, for example, class E is shown as having both class A and class B as direct superclasses, while class F has three direct superclasses.

Such multiple inheritance is not allowed in Java. The designers of Java wanted to keep the language reasonably simple, and felt that the benefits of multiple inheritance were not worth the cost in increased complexity. However, Java does have a feature that can be used to accomplish many of the same goals as multiple inheritance: interfaces.

5.7.1  Defining and Implementing Interfaces

We've encountered the term "interface" before, in connection with black boxes in general and subroutines in particular. The interface of a subroutine consists of the name of the subroutine, its return type, and the number and types of its parameters. This is the information you need to know if you want to call the subroutine. A subroutine also has an implementation: the block of code which defines it and which is executed when the subroutine is called.

In Java, interface is a reserved word with an additional, technical meaning. An "interface" in this sense consists of a set of instance method interfaces, without any associated implementations. (Actually, a Java interface can contain other things as well, as we'll see later.) A class can implement an interface by providing an implementation for each of the methods specified by the interface. Here is an example of a very simple Java interface:

public interface Drawable {
public void draw(Graphics g);
}

This looks much like a class definition, except that the implementation of the draw() method is omitted. A class that implements the interface Drawable must provide an implementation for this method. Of course, the class can also include other methods and variables. For example,

public class Line implements Drawable {
public void draw(Graphics g) {
. . . // do something -- presumably, draw a line
}
. . . // other methods and variables
}

Note that to implement an interface, a class must do more than simply provide an implementation for each method in the interface; it must also state that it implements the interface, using the reserved word implements as in this example: "public class Line implements Drawable". Any concrete class that implements the Drawable interface must define a draw() instance method. Any object created from such a class includes a draw() method. We say that an object implements an interface if it belongs to a class that implements the interface. For example, any object of type Line implements the Drawable interface.

While a class can extend only one other class, it can implement any number of interfaces. In fact, a class can both extend one other class and implement one or more interfaces. So, we can have things like

class FilledCircle extends Circle
implements Drawable, Fillable {
. . .
}

The point of all this is that, although interfaces are not classes, they are something very similar. An interface is very much like an abstract class, that is, a class that can never be used for constructing objects, but can be used as a basis for making subclasses. The subroutines in an interface are abstract methods, which must be implemented in any concrete class that implements the interface. You can compare the Drawable interface with the abstract class

public abstract class AbstractDrawable {
public abstract void draw(Graphics g);
}

The main difference is that a class that extends AbstractDrawable cannot extend any other class, while a class that implements Drawable can also extend some class, as well as implement other interfaces. Of course, an abstract class can contain non-abstract methods as well as abstract methods. An interface is like a "pure" abstract class, which contains only abstract methods.

Note that the methods declared in an interface must be public. In fact, since that is the only option, it is not necessary to specify the access modifier in the declaration.

In addition to method declarations, an interface can also include variable declarations. The variables must be "public static final" and effectively become public static final variables in every class that implements the interface. In fact, since the variables can only be public and static and final, specifying the modifiers is optional. For example,

public interface ConversionFactors {
int INCHES_PER_FOOT = 12;
int FEET_PER_YARD = 3;
int YARDS_PER_MILE = 1760;
}

This is a convenient way to define named constants that can be used in several classes. A class that implements ConversionFactors can use the constants defined in the interface as if they were defined in the class.

You are not likely to need to write your own interfaces until you get to the point of writing fairly complex programs. However, there are several interfaces that are used in important ways in Java's standard packages. You'll learn about some of these standard interfaces in the next few chapters, and you will write classes that implement them.

5.7.2  Interfaces as Types

As with abstract classes, even though you can't construct an object from an interface, you can declare a variable whose type is given by the interface. For example, if Drawable is the interface given above, and if Line and FilledCircle are classes that implement Drawable, as above, then you could say:

Drawable figure;  // Declare a variable of type Drawable.  It can
//    refer to any object that implements the
//    Drawable interface.

figure = new Line();  // figure now refers to an object of class Line
figure.draw(g);   // calls draw() method from class Line

figure = new FilledCircle();   // Now, figure refers to an object
//   of class FilledCircle.
figure.draw(g);   // calls draw() method from class FilledCircle

A variable of type Drawable can refer to any object of any class that implements the Drawable interface. A statement like figure.draw(g), above, is legal because figure is of type Drawable, and any Drawable object has a draw() method. So, whatever object figure refers to, that object must have a draw() method.

Note that a type is something that can be used to declare variables. A type can also be used to specify the type of a parameter in a subroutine, or the return type of a function. In Java, a type can be either a class, an interface, or one of the eight built-in primitive types. These are the only possibilities. Of these, however, only classes can be used to construct new objects.

An interface can also be the base type of an array. For example, we can use an array type Drawable[] to declare variables and create arrays. The elements of the array can refer to any objects that implement the Drawable interface:

Drawable[] listOfFigures;
listOfFigures = new Drawable[10];
listOfFigures[0] = new Line();
listOfFigures[1] = new FilledCircle();
listOfFigures[2] = new Line();
.
.
.

Every element of the array will then have a draw() method, so that we can say things like listOfFigures[i].draw(g).

5.7.3  Interfaces in Java 8

The newest version of Java, Java 8, makes a few useful additions to interfaces. The one that I will discuss here is default methods. Unlike the usual abstract methods in interfaces, a default method has an implementation. When a class implements the interface, it does not have to provide an implementation for the default method -- although it can do so if it wants to provide a different implementation. Essentially, default methods are inherited from interfaces in much the same way that ordinary methods are inherited from classes. This moves Java partway towards supporting multiple inheritance. It's not true multiple inheritance, however, since interfaces still cannot define instance variables.

A default method in an interface must be marked with the modifier default. It can optionally be marked public but, as for everything else in interfaces, default methods are automatically public and the public modifier can be omitted. Here is an example.:

public interface Readable { // represents a source of input

public char readChar();  // read the next character from the input

default public String readLine() { // read up to the next line feed
StringBuilder line = new StringBuilder();
char ch = readChar();
while (ch != '\n') {
line.append(ch);
ch = readChar();
}
return line.toString();
}

}

A concrete class that implements this interface must provide an implementation for readChar(). It will inherit a definition for readLine() from the interface, but can provide a new definition if necessary. Note that the default readLine() calls the abstract method readChar(), whose definition will only be provided in the implementing class. The reference to readChar() in the definition is polymorphic. The default implementation of readLine() is one that would make sense in almost any class that implements Readable. Here's a rather silly example of a class that implements Readable, including a main() routine that tests the class. Can you figure out what it does?

public class Stars implements Readable {

public char readChar() {
if (Math.random() > 0.02)
return '*';
else
return '\n';
}

public static void main(String[] args) {
Stars stars = new Stars();
for (int i = 0 ; i < 10; i++ ) {
String line = stars.readLine();
System.out.println( line );
}
}

}

Default methods provide Java with a capability similar to something called a "mixin" in other programming languages, namely the ability to mix functionality from another source into a class. Since a class can implement several interfaces, it is possible to mix in functionality from several different sources.

Section 5.8

Nested Classes

A class seems like it should be a pretty important thing. A class is a high-level building block of a program, representing a potentially complex idea and its associated data and behaviors. I've always felt a bit silly writing tiny little classes that exist only to group a few scraps of data together. However, such trivial classes are often useful and even essential. Fortunately, in Java, I can ease the embarrassment, because one class can be nested inside another class. My trivial little class doesn't have to stand on its own. It becomes part of a larger more respectable class. This is particularly useful when you want to create a little class specifically to support the work of a larger class. And, more seriously, there are other good reasons for nesting the definition of one class inside another class.

In Java, a nested class is any class whose definition is inside the definition of another class. (In fact, a class can even be nested inside a subroutine, which must, of course, itself be inside a class). Nested classes can be either named or anonymous. I will come back to the topic of anonymous classes later in this section. A named nested class, like most other things that occur in classes, can be either static or non-static.

5.8.1  Static Nested Classes

The definition of a static nested class looks just like the definition of any other class, except that it is nested inside another class and it has the modifier static as part of its declaration. A static nested class is part of the static structure of the containing class. It can be used inside that class to create objects in the usual way. If it is used outside the containing class, its name must indicate its membership in the containing class. That is, the full name of the static nested class consists of the name of the class in which it is nested, followed by a period, followed by the name of the nested class. This is similar to other static components of a class: A static nested class is part of the class itself in the same way that static member variables are parts of the class itself.

For example, suppose a class named WireFrameModel represents a set of lines in three-dimensional space. (Such models are used to represent three-dimensional objects in graphics programs.) Suppose that the WireFrameModel class contains a static nested class, Line, that represents a single line. Then, outside of the class WireFrameModel, the Line class would be referred to as WireFrameModel.Line. Of course, this just follows the normal naming convention for static members of a class. The definition of the WireFrameModel class with its nested Line class would look, in outline, like this:

public class WireFrameModel {

. . . // other members of the WireFrameModel class

static public class Line {
// Represents a line from the point (x1,y1,z1)
// to the point (x2,y2,z2) in 3-dimensional space.
double x1, y1, z1;
double x2, y2, z2;
} // end class Line

. . . // other members of the WireFrameModel class

} // end WireFrameModel

The full name of the nested class is WireFrameModel.Line. That name can be used, for example, to declare variables. Inside the WireFrameModel class, a Line object would be created with the constructor "new Line()". Outside the class, "new WireFrameModel.Line()" would be used.

A static nested class has full access to the static members of the containing class, even to the private members. Similarly, the containing class has full access to the members of the nested class, even if they are marked private. This can be another motivation for declaring a nested class, since it lets you give one class access to the private members of another class without making those members generally available to other classes. Note also that a nested class can itself be private, meaning that it can only be used inside the class in which it is nested.

When you compile the above class definition, two class files will be created. Even though the definition of Line is nested inside WireFrameModel, the compiled Line class is stored in a separate file. The name of the class file for Line will be WireFrameModel$Line.class.

5.8.2  Inner Classes

Non-static nested classes are referred to as inner classes. Inner classes are not, in practice, very different from static nested classes, but a non-static nested class is actually associated with an object rather than with the class in which its definition is nested. This can take some getting used to.

Any non-static member of a class is not really part of the class itself (although its source code is contained in the class definition). This is true for inner classes, just as it is for any other non-static part of a class. The non-static members of a class specify what will be contained in objects that are created from that class. The same is true -- at least logically -- for inner classes. It's as if each object that belongs to the containing class has its own copy of the nested class (although it does not literally contain a copy of the compiled code for the nested class). This copy has access to all the instance methods and instance variables of the object, even to those that are declared private. The two copies of the inner class in two different objects differ because the instance variables and methods they refer to are in different objects. In fact, the rule for deciding whether a nested class should be static or non-static is simple: If the nested class needs to use any instance variable or instance method from the containing class, make the nested class non-static. Otherwise, it might as well be static.

In most cases, an inner class is used only within the class where it is defined. When that is true, using the inner class is really not much different from using any other class. You can create variables and declare objects using the simple name of the inner class in the usual way.

From outside the containing class, however, an inner class has to be referred to using a name of the form variableName.NestedClassName, where variableName is a variable that refers to the object that contains the inner class. In order to create an object that belongs to an inner class, you must first have an object that belongs to the containing class. (When working inside the class, the object "this" is used implicitly.)

Looking at an example will help, and will hopefully convince you that inner classes are really very natural. Consider a class that represents poker games. This class might include a nested class to represent the players of the game. The structure of the PokerGame class could be:

public class PokerGame {  // Represents a game of poker.

class Player {  // Represents one of the players in this game.
.
.
.
} // end class Player

private Deck deck;      // A deck of cards for playing the game.
private int pot;        // The amount of money that has been bet.

.
.
.

} // end class PokerGame

If game is a variable of type PokerGame, then, conceptually, game contains its own copy of the Player class. In an instance method of a PokerGame object, a new Player object would be created by saying "new Player()", just as for any other class. (A Player object could be created outside the PokerGame class with an expression such as "game.new Player()". Again, however, this is rare.) The Player object will have access to the deck and pot instance variables in the PokerGame object. Each PokerGame object has its own deck and pot and Players. Players of that poker game use the deck and pot for that game; players of another poker game use the other game's deck and pot. That's the effect of making the Player class non-static. This is the most natural way for players to behave. A Player object represents a player of one particular poker game. If Player were an independent class or a static nested class, on the other hand, it would represent the general idea of a poker player, independent of a particular poker game.

5.8.3  Anonymous Inner Classes

In some cases, you might find yourself writing an inner class and then using that class in just a single line of your program. Is it worth creating such a class? Indeed, it can be, but for cases like this you have the option of using an anonymous inner class. An anonymous class is created with a variation of the new operator that has the form

new  superclass-or-interface ( parameter-list ) {
methods-and-variables
}

This constructor defines a new class, without giving it a name, and it simultaneously creates an object that belongs to that class. This form of the new operator can be used in any statement where a regular "new" could be used. The intention of this expression is to create: "a new object belonging to a class that is the same as superclass-or-interface but with these methods-and-variables added." The effect is to create a uniquely customized object, just at the point in the program where you need it. Note that it is possible to base an anonymous class on an interface, rather than a class. In this case, the anonymous class must implement the interface by defining all the methods that are declared in the interface. If an interface is used as a base, the parameter-list must be empty. Otherwise, it can contain parameters for a constructor in the superclass.

Anonymous classes are often used for handling events in graphical user interfaces, and we will encounter them several times in the chapters on GUI programming. For now, we will look at one not-very-plausible example. Consider the Drawable interface, which is defined earlier in this section. Suppose that we want a Drawable object that draws a filled, red, 100-pixel square. Rather than defining a new, separate class and then using that class to create the object, we can use an anonymous class to create the object in one statement:

Drawable redSquare = new Drawable() {
void draw(Graphics g) {
g.setColor(Color.RED);
g.fillRect(10,10,100,100);
}
};

Then redSquare refers to an object that implements Drawable and that draws a red square when its draw() method is called. By the way, the semicolon at the end of the statement is not part of the class definition; it's the semicolon that is required at the end of every declaration statement.

Anonymous classes are often used for actual parameters. For example, consider the following simple method, which draws a Drawable in two different graphics contexts:

void drawTwice( Graphics g1, Graphics g2, Drawable figure ) {
figure.draw(g1);
figure.draw(g2);
}

When calling this method, the third parameter can be created using an anonymous inner class. For example:

drawTwice( firstG, secondG, new Drawable() {
void draw(Graphics g) {
g.drawOval(10,10,100,100);
}
} );

When a Java class is compiled, each anonymous nested class will produce a separate class file. If the name of the main class is MainClass, for example, then the names of the class files for the anonymous nested classes will be MainClass$1.class, MainClass$2.class, MainClass$3.class, and so on.

5.8.4  Java 8 Lambda Expressions

The syntax for anonymous classes is cumbersome. In many cases, an anonymous class implements an interface that defines just one method. Java 8 introduces a new syntax that can be used in place of the anonymous class in that circumstance: the lambda expression. Here is what the previous subroutine call looks like using a lambda expression:

drawTwice( firstG, secondG, g -> g.drawOval(10,10,100,100) )

The lambda expression is g -> g.drawOval(10,10,100,100). Its meaning is, "the method that has a parameter g and executes the code g.drawOval(10,10,100,100)." The computer knows that g is of type Graphics because it is expecting a Drawable as the actual parameter, and the only method in the Drawable interface has a parameter of type Graphics. Lambda expressions can only be used in places where this kind of type inference can be made. The general syntax of a lambda expression is

formal-parameter-list  ->  method-body

where the method body can be a single expression, a single subroutine call, or a block of statements enclosed between { and }. When the body is a single expression or function call, the value of the expression is automatically used as the return value of the method that is being defined. The parameter list in the lambda expression does not have to specify the types of the parameters, although it can. Parentheses around the parameter list are optional if there is exactly one parameter and no type is specified for the parameter; this is the form seen in the example above. For a method with no parameters, the parameter list is just an empty set of parentheses. Here are a few more examples of lambda expressions:

() -> System.out.println("Hello World")

g -> { g.setColor(Color.RED); g.drawRect(10,10,100,100); }

(a, b) -> a + b

(int n) -> {
while (n > 0) {
System.out.println(n);
n = n/2;
}
} // lambda expressions ends here

As you can see, the syntax can still get pretty complicated. There is quite a lot more to say about lambda expressions, but my intention here is only to briefly introduce one of the most interesting new features in Java 8.

Programming Exercises for Chapter 5

Exercise 5.1:

In all versions of the PairOfDice class in Section 5.2, the instance variables die1 and die2 are declared to be public. They really should be private, so that they would be protected from being changed from outside the class. Write another version of the PairOfDice class in which the instance variables die1 and die2 are private. Your class will need "getter" methods that can be used to find out the values of die1 and die2. (The idea is to protect their values from being changed from outside the class, but still to allow the values to be read.) Include other improvements in the class, including at least a toString() method. Test your class with a short program that counts how many times a pair of dice is rolled, before the total of the two dice is equal to two.

See the Solution

Exercise 5.2:

A common programming task is computing statistics of a set of numbers. (A statistic is a number that summarizes some property of a set of data.) Common statistics include the mean (also known as the average) and the standard deviation (which tells how spread out the data are from the mean). I have written a little class called StatCalc that can be used to compute these statistics, as well as the sum of the items in the dataset and the number of items in the dataset. You can read the source code for this class in the file StatCalc.java. If calc is a variable of type StatCalc, then the following instance methods are available:

- calc.enter(item)  where item is a number, adds the item to the dataset.

- calc.getCount()  is a function that returns the number of items that have been added to the dataset.

- calc.getSum()  is a function that returns the sum of all the items that have been added to the dataset.

- calc.getMean()  is a function that returns the average of all the items.

- calc.getStandardDeviation()  is a function that returns the standard deviation of the items.

Typically, all the data are added one after the other by calling the enter() method over and over, as the data become available. After all the data have been entered, any of the other methods can be called to get statistical information about the data. The methods getMean() and getStandardDeviation() should only be called if the number of items is greater than zero.

Modify the current source code, StatCalc.java, to add instance methods getMax() and getMin(). The getMax() method should return the largest of all the items that have been added to the dataset, and getMin() should return the smallest. You will need to add two new instance variables to keep track of the largest and smallest items that have been seen so far.

Test your new class by using it in a program to compute statistics for a set of non-zero numbers entered by the user. Start by creating an object of type StatCalc:

StatCalc  calc;   // Object to be used to process the data.
calc = new StatCalc();

Read numbers from the user and add them to the dataset. Use 0 as a sentinel value (that is, stop reading numbers when the user enters 0). After all the user's non-zero numbers have been entered, print out each of the six statistics that are available from calc.

See the Solution

Exercise 5.3:

This problem uses the PairOfDice class from Exercise 5.1 and the StatCalc class from Exercise 5.2.

The program in Exercise 4.4 performs the experiment of counting how many times a pair of dice is rolled before a given total comes up. It repeats this experiment 10000 times and then reports the average number of rolls. It does this whole process for each possible total (2, 3, ..., 12).

Redo that exercise. But instead of just reporting the average number of rolls, you should also report the standard deviation and the maximum number of rolls. Use a PairOfDice object to represent the dice. Use a StatCalc object to compute the statistics. (You'll need a new StatCalc object for each possible total, 2, 3, ..., 12. You can use a new pair of dice if you want, but it's not required.)

See the Solution

Exercise 5.4:

The BlackjackHand class from Subsection 5.5.1 is an extension of the Hand class from Section 5.4. The instance methods in the Hand class are discussed in that section. In addition to those methods, BlackjackHand includes an instance method, getBlackjackValue(), which returns the value of the hand for the game of Blackjack. For this exercise, you will also need the Deck and Card classes from Section 5.4.

A Blackjack hand typically contains from two to six cards. Write a program to test the BlackjackHand class. You should create a BlackjackHand object and a Deck object. Pick a random number between 2 and 6. Deal that many cards from the deck and add them to the hand. Print out all the cards in the hand, and then print out the value computed for the hand by getBlackjackValue(). Repeat this as long as the user wants to continue.

In addition to TextIO.java, your program will depend on Card.java, Deck.java, Hand.java, and BlackjackHand.java.

See the Solution

Exercise 5.5:

Write a program that lets the user play Blackjack. The game will be a simplified version of Blackjack as it is played in a casino. The computer will act as the dealer. As in the previous exercise, your program will need the classes defined in Card.java, Deck.java, Hand.java, and BlackjackHand.java. (This is the longest and most complex program that has come up so far in the exercises.)

You should first write a subroutine in which the user plays one game. The subroutine should return a boolean value to indicate whether the user wins the game or not. Return true if the user wins, false if the dealer wins. The program needs an object of class Deck and two objects of type BlackjackHand, one for the dealer and one for the user. The general object in Blackjack is to get a hand of cards whose value is as close to 21 as possible, without going over. The game goes like this.

- First, two cards are dealt into each player's hand. If the dealer's hand has a value of 21 at this point, then the dealer wins. Otherwise, if the user has 21, then the user wins. (This is called a "Blackjack".) Note that the dealer wins on a tie, so if both players have Blackjack, then the dealer wins.

- Now, if the game has not ended, the user gets a chance to add some cards to her hand. In this phase, the user sees her own cards and sees one of the dealer's two cards. (In a casino, the dealer deals himself one card face up and one card face down. All the user's cards are dealt face up.) The user makes a decision whether to "Hit", which means to add another card to her hand, or to "Stand", which means to stop taking cards.

- If the user Hits, there is a possibility that the user will go over 21. In that case, the game is over and the user loses. If not, then the process continues. The user gets to decide again whether to Hit or Stand.

- If the user Stands, the game will end, but first the dealer gets a chance to draw cards. The dealer only follows rules, without any choice. The rule is that as long as the value of the dealer's hand is less than or equal to 16, the dealer Hits (that is, takes another card). The user should see all the dealer's cards at this point. Now, the winner can be determined: If the dealer has gone over 21, the user wins. Otherwise, if the dealer's total is greater than or equal to the user's total, then the dealer wins. Otherwise, the user wins.

Two notes on programming: At any point in the subroutine, as soon as you know who the winner is, you can say "return true;" or "return false;" to end the subroutine and return to the main program. To avoid having an overabundance of variables in your subroutine, remember that a function call such as userHand.getBlackjackValue() can be used anywhere that a number could be used, including in an output statement or in the condition of an if statement.

Write a main program that lets the user play several games of Blackjack. To make things interesting, give the user 100 dollars, and let the user make bets on the game. If the user loses, subtract the bet from the user's money. If the user wins, add an amount equal to the bet to the user's money. End the program when the user wants to quit or when she runs out of money.

See the Solution

Exercise 5.6:

Exercise 4.7 asked you to write a program that administers a 10-question addition quiz. Rewrite that program so that it uses the following class to represent addition questions:

public class AdditionQuestion {

private int a, b;  // The numbers in the problem.

public AdditionQuestion() { // constructor
a = (int)(Math.random() * 50 + 1);
b = (int)(Math.random() * 50);
}

public String getQuestion() {
return "What is " + a + " + " + b + " ?";
}

public int getCorrectAnswer() {
return a + b;
}

}

See the Solution

Exercise 5.7:

Rewrite the program from the previous exercise so that it administers a quiz with several different kinds of questions. In the previous exercise, you used a class to represent addition questions. For this exercise, you will use the following interface, or an equivalent abstract class, to represent the more general idea of a question that has an integer as its answer:

public interface IntQuestion {
public String getQuestion();
public int getCorrectAnswer();
}

You can make the AdditionQuestion class implement the interface simply by adding "implements IntQuestion" to its definition. Write a similar class to represent subtraction questions. When creating a subtraction problem, you should make sure that the answer is not negative.

For the new program, use an array of type IntQuestion[] to hold the quiz questions. Include some addition questions and some subtraction questions in the quiz. You can also add a couple non-math questions, including this one, created as an anonymous class:

IntQuestion bigQuestion = new IntQuestion() {
public String getQuestion() {
return "What is the answer to the ultimate question " +
" of life, the universe, and everything?";
}
public int getCorrectAnswer() {
return 42;
}
};

See the Solution

Quiz on Chapter 5

Question 1:

Object-oriented programming uses classes and objects. What are classes and what are objects? What is the relationship between classes and objects?

Question 2:

What are instance variables and instance methods?

Question 3:

Explain carefully what null means in Java, and why this special value is necessary.

Question 4:

What is a constructor? What is the purpose of a constructor in a class?

Question 5:

Suppose that Kumquat is the name of a class and that fruit is a variable of type Kumquat. What is the meaning of the statement "fruit = new Kumquat();"? That is, what does the computer do when it executes this statement? (Try to give a complete answer. The computer does several things.)

Question 6:

What is meant by the terms instance variable and instance method?

Question 7:

Explain what is meant by the terms subclass and superclass.

Question 8:

Modify the following class so that the two instance variables are private and there is a getter method and a setter method for each instance variable:

public class Player {
String name;
int score;
}

Question 9:

Explain why the class Player that is defined in the previous question has an instance method named toString(), even though no definition of this method appears in the definition of the class.

Question 10:

Explain the term polymorphism.

Question 11:

Java uses "garbage collection" for memory management. Explain what is meant here by garbage collection. What is the alternative to garbage collection?

Question 12:

What is an abstract class, and how can you recognize an abstract class in Java.

Question 13:

What is this?

Question 14:

For this problem, you should write a very simple but complete class. The class represents a counter that counts 0, 1, 2, 3, 4, .... The name of the class should be Counter. It has one private instance variable representing the value of the counter. It has two instance methods: increment() adds one to the counter value, and getValue() returns the current counter value. Write a complete definition for the class, Counter.

Question 15:

This problem uses the Counter class from the previous question. The following program segment is meant to simulate tossing a coin 100 times. It should use two Counter objects, headCount and tailCount, to count the number of heads and the number of tails. Fill in the blanks so that it will do so:

Counter headCount, tailCount;
tailCount = new Counter();
headCount = new Counter();
for ( int flip = 0;  flip < 100;  flip++ ) {
if (Math.random() < 0.5)    // There's a 50/50 chance that this is true.

______________________ ;   // Count a "head".

else

______________________ ;   // Count a "tail".
}

System.out.println("There were " + ___________________ + " heads.");

System.out.println("There were " + ___________________ + " tails.");

See the Answers

Chapter 6

Introduction to GUI Programming

Computer users today expect to interact with their computers using a graphical user interface (GUI). Java can be used to write GUI programs ranging from simple applets which run on a Web page to sophisticated stand-alone applications.

GUI programs differ from traditional "straight-through" programs that you have encountered in the first few chapters of this book. One big difference is that GUI programs are event-driven. That is, user actions such as clicking on a button or pressing a key on the keyboard generate events, and the program must respond to these events as they occur.

Event-driven programming builds on all the skills you have learned in the first five chapters of this text. You need to be able to write the methods that respond to events. Inside those methods, you are doing the kind of programming-in-the-small that was covered in Chapter 2 and Chapter 3. And of course, objects are everywhere in GUI programming. Events are objects. Colors and fonts are objects. GUI components such as buttons and menus are objects. Events are handled by instance methods contained in objects. In Java, GUI programming is object-oriented programming.

This chapter covers the basics of GUI programming. The discussion will continue in Chapter 13 with more details and with more advanced techniques.

Contents of Chapter 6:

- Section 1: The Basic GUI Application

- Section 2: Graphics and Painting

- Section 3: Mouse Events

- Section 4: Timers, KeyEvents, and State Machines

- Section 5: Basic Components

- Section 6: Basic Layout

- Section 7: Menus and Dialogs

- Programming Exercises

- Quiz on This Chapter

Section 6.1

The Basic GUI Application

The command-line programs that you have learned how to program would seem very alien to most computer users. The style of interaction where the user and the computer take turns typing strings of text seems like something out of the early days of computing, although it was only in the mid 1980s that home computers with graphical user interfaces started to become available. Today, most people interact with their computers exclusively through a GUI. A GUI program offers a much richer type of user interface, where the user uses a mouse and keyboard to interact with GUI components such as windows, menus, buttons, check boxes, text input boxes, scroll bars, and so on.

A GUI program still has a main() subroutine, but in general, that main routine just creates one or more GUI components and displays them on the computer screen. Once the GUI components have been created, they follow their own programming -- programming that tells them how to draw themselves on the screen and how to respond to events such as being clicked on by the user.

A GUI program doesn't have to be immensely complex. We can, for example, write a very simple GUI "Hello World" program that says "Hello" to the user, but does it by opening a window where the greeting is displayed:

import javax.swing.JOptionPane;

public class HelloWorldGUI1 {

public static void main(String[] args) {
JOptionPane.showMessageDialog( null, "Hello World!" );
}

}

When this program is run, a window appears on the screen that contains the message "Hello World!". The window also contains an "OK" button for the user to click after reading the message. When the user clicks this button, the window closes and the program ends. This program can be placed in a file named HelloWorldGUI1.java, compiled, and run using the java command on the command line just like any other Java program.

Now, this program is already doing some pretty fancy stuff. It creates a window, it draws the contents of that window, and it handles the event that is generated when the user clicks the button. The reason the program was so easy to write is that all the work is done by showMessageDialog(), a static method in the built-in class JOptionPane. (Note that the source code "imports" the class javax.swing.JOptionPane to make it possible to refer to the JOptionPane class using its simple name. See Subsection 4.5.3 for information about importing classes from Java's standard packages.)

If you want to display a message to the user in a GUI program, this is a good way to do it: Just use a standard class that already knows how to do the work! And in fact, JOptionPane is regularly used for just this purpose (but as part of a larger program, usually). Of course, if you want to do anything serious in a GUI program, there is a lot more to learn. To give you an idea of the types of things that are involved, we'll look at a short GUI program that does the same things as the previous program -- open a window containing a message and an OK button, and respond to a click on the button by ending the program -- but does it all by hand instead of by using the built-in JOptionPane class. Mind you, this is not a good way to write the program, but it will illustrate some important aspects of GUI programming in Java.

Here is the source code for the program. You are not expected to understand it yet. I will explain how it works below, but it will take the rest of the chapter before you will really understand completely. In this section, you will just get a brief overview of GUI programming.

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class HelloWorldGUI2 {

private static class HelloWorldDisplay extends JPanel {
public void paintComponent(Graphics g) {
super.paintComponent(g);
g.drawString( "Hello World!", 20, 30 );
}
}

private static class ButtonHandler implements ActionListener {
public void actionPerformed(ActionEvent e) {
System.exit(0);
}
}

public static void main(String[] args) {

HelloWorldDisplay displayPanel = new HelloWorldDisplay();
JButton okButton = new JButton("OK");
ButtonHandler listener = new ButtonHandler();
okButton.addActionListener(listener);

JPanel content = new JPanel();
content.setLayout(new BorderLayout());
content.add(displayPanel, BorderLayout.CENTER);
content.add(okButton, BorderLayout.SOUTH);

JFrame window = new JFrame("GUI Test");
window.setContentPane(content);
window.setSize(250,100);
window.setLocation(100,100);
window.setVisible(true);

}

}

6.1.1  JFrame and JPanel

In a Java GUI program, each GUI component in the interface is represented by an object in the program. One of the most fundamental types of component is the window. Windows have many behaviors. They can be opened and closed. They can be resized. They have "titles" that are displayed in the title bar above the window. And most important, they can contain other GUI components such as buttons and menus.

Java, of course, has a built-in class to represent windows. There are actually several different types of window, but the most common type is represented by the JFrame class (which is included in the package javax.swing). A JFrame is an independent window that can, for example, act as the main window of an application. One of the most important things to understand is that a JFrame object comes with many of the behaviors of windows already programmed in. In particular, it comes with the basic properties shared by all windows, such as a titlebar and the ability to be opened and closed. Since a JFrame comes with these behaviors, you don't have to program them yourself! This is, of course, one of the central ideas of object-oriented programming. What a JFrame doesn't come with, of course, is content, the stuff that is contained in the window. If you don't add any other content to a JFrame, it will just display a blank area -- or, if you don't set its size, it will be so tiny that it will be hard to find on the screen. You can add content either by creating a JFrame object and then adding the content to it or by creating a subclass of JFrame and adding the content in the constructor of that subclass.

The main program above declares a variable, window, of type JFrame and sets it to refer to a new window object with the statement:

JFrame window = new JFrame("GUI Test");

The parameter (the string "GUI test") in the constructor specifies the title that will be displayed in the titlebar of the window. This line creates the window object, but the window itself is not yet visible on the screen. Before making the window visible, some of its properties are set with these statements:

window.setContentPane(content);
window.setSize(250,100);
window.setLocation(100,100);

The first line here sets the content of the window. (The content itself was created earlier in the main program.) The second line says that the window will be 250 pixels wide and 100 pixels high. The third line says that the upper left corner of the window will be 100 pixels over from the left edge of the screen and 100 pixels down from the top. Once all this has been set up, the window is actually made visible on the screen with the command:

window.setVisible(true);

It might look as if the program ends at that point, and, in fact, the main() routine does end. However, the window is still on the screen and the program as a whole does not end until the user clicks the OK button. Once the window was opened, a new thread was created to manage the graphical user interface, and that thread continues to run even after main() has finished.

The content that is displayed in a JFrame is called its content pane. (In addition to its content pane, a JFrame can also have a menu bar, which is a separate thing that I will talk about later.) A basic JFrame already has a blank content pane; you can either add things to that pane or you can replace the basic content pane entirely. In my sample program, the line window.setContentPane(content) replaces the original blank content pane with a different component. (Remember that a "component" is just a visual element of a graphical user interface.) In this case, the new content is a component of type JPanel.

JPanel is another of the fundamental classes in Swing. The basic JPanel is, again, just a blank rectangle. There are two ways to make a useful JPanel: The first is to add other components to the panel; the second is to draw something in the panel. Both of these techniques are illustrated in the sample program. In fact, you will find two JPanels in the program: content, which is used to contain other components, and displayPanel, which is used as a drawing surface.

Let's look more closely at displayPanel. This variable is of type HelloWorldDisplay, which is a static nested class inside the HelloWorldGUI2 class. (Nested classes were introduced in Section 5.8.) This class defines just one instance method, paintComponent(), which overrides a method of the same name in the JPanel class:

private static class HelloWorldDisplay extends JPanel {
public void paintComponent(Graphics g) {
super.paintComponent(g);
g.drawString( "Hello World!", 20, 30 );
}
}

The paintComponent() method is called by the system when a component needs to be painted on the screen. In the JPanel class, the paintComponent method simply fills the panel with the panel's background color. The paintComponent() method in HelloWorldDisplay begins by calling super.paintComponent(g). This calls the version of paintComponent() that is defined in the superclass, JPanel; that is, it fills the panel with the background color. (See Subsection 5.6.2 for a discussion of the special variable super.) Then it calls g.drawString() to paint the string "Hello World!" onto the panel. The net result is that whenever a HelloWorldDisplay is shown on the screen, it displays the string "Hello World!".

We will often use JPanels in this way, as drawing surfaces. Usually, when we do this, we will define a class that is a subclass of JPanel and we will write a paintComponent method in that class to draw the desired content in the panel. The subclass of JPanel can be defined either as a separate class in its own file or as a nested class. In simple cases, I will tend to use a nested class for the convenience.

6.1.2  Components and Layout

Another way of using a JPanel is as a container to hold other components. Java has many classes that define GUI components. Except for top-level components like windows, components must be added to a container before they can appear on the screen. In the sample program, the variable named content refers to a JPanel that is used as a container. Two other components are added to that container. This is done in the statements:

content.add(displayPanel, BorderLayout.CENTER);
content.add(okButton, BorderLayout.SOUTH);

Here, content refers to an object of type JPanel; later in the program, this panel becomes the content pane of the window. The first component that is added to content is displayPanel which, as discussed above, displays the message, "Hello World!". The second is okButton which represents the button that the user clicks to close the window. The variable okButton is of type JButton, the Java class that represents push buttons.

The "BorderLayout" stuff in these statements has to do with how the two components are arranged in the container. When components are added to a container, there has to be some way of deciding how those components are arranged inside the container. This is called "laying out" the components in the container, and the most common technique for laying out components is to use a layout manager. A layout manager is an object that implements some policy for how to arrange the components in a container; different types of layout manager implement different policies. One type of layout manager is defined by the BorderLayout class. In the program, the statement

content.setLayout(new BorderLayout());

creates a new BorderLayout object and tells the content panel to use the new object as its layout manager. Essentially, this line determines how components that are added to the content panel will be arranged inside the panel. We will cover layout managers in much more detail later, but for now all you need to know is that adding okButton in the BorderLayout.SOUTH position puts the button at the bottom of the panel, and putting displayPanel in the BorderLayout.CENTER position makes it fill any space that is not taken up by the button.

This example shows a general technique for setting up a GUI: Create a container and assign a layout manager to it, create components and add them to the container, and use the container as the content pane of a window. A container is itself a component, so it is possible that some of the components that are added to the top-level container are themselves containers, with their own layout managers and components. This makes it possible to build up complex user interfaces in a hierarchical fashion, with containers inside containers inside containers...

6.1.3  Events and Listeners

The structure of containers and components sets up the physical appearance of a GUI, but it doesn't say anything about how the GUI behaves. That is, what can the user do to the GUI and how will it respond? GUIs are largely event-driven; that is, the program waits for events that are generated by the user's actions (or by some other cause). When an event occurs, the program responds by executing an event-handling method. In order to program the behavior of a GUI, you have to write event-handling methods to respond to the events that you are interested in.

The most common technique for handling events in Java is to use event listeners. A listener is an object that includes one or more event-handling methods. When an event is detected by another object, such as a button or menu, the listener object is notified and it responds by running the appropriate event-handling method. An event is detected or generated by an object. Another object, the listener, has the responsibility of responding to the event. The event itself is actually represented by a third object, which carries information about the type of event, when it occurred, and so on. This division of responsibilities makes it easier to organize large programs.

As an example, consider the OK button in the sample program. When the user clicks the button, an event is generated. This event is represented by an object belonging to the class ActionEvent. The event that is generated is associated with the button; we say that the button is the source of the event. The listener object in this case is an object belonging to the class ButtonHandler, which is defined as a nested class inside HelloWorldGUI2:

private static class ButtonHandler implements ActionListener {
public void actionPerformed(ActionEvent e) {
System.exit(0);
}
}

This class implements the ActionListener interface -- a requirement for listener objects that handle events from buttons. (Interfaces were introduced in Section 5.7.) The event-handling method is named actionPerformed, as specified by the ActionListener interface. This method contains the code that is executed when the user clicks the button; in this case, the code is simply a call to System.exit(), which will terminate the program.

There is one more ingredient that is necessary to get the event from the button to the listener object: The listener object must register itself with the button as an event listener. This is done with the statement:

okButton.addActionListener(listener);

This statement tells okButton that when the user clicks the button, the ActionEvent that is generated should be sent to listener. Without this statement, the button has no way of knowing that there is something that would like to listen for events from the button.

This example shows a general technique for programming the behavior of a GUI: Write classes that include event-handling methods. Create objects that belong to these classes and register them as listeners with the objects that will actually detect or generate the events. When an event occurs, the listener is notified, and the code that you wrote in one of its event-handling methods is executed. At first, this might seem like a very roundabout and complicated way to get things done, but as you gain experience with it, you will find that it is very flexible and that it goes together very well with object oriented programming.

This section has introduced some of the fundamentals of GUI programming. We will spend the rest of the chapter exploring them in more detail.

6.1.4  Some Java GUI History

The original GUI toolkit for Java was the AWT, the "Abstract Windowing Toolkit." It provided a common interface to the GUI components already built into various operating systems. At the very beginning, it used a simpler event model that did not require listener objects, but that model was abandoned in favor of listeners very quickly in Java 1.1.

When Java was first introduced, one of the important applications was applets. An applet is a GUI program that can run on a web page in a web browser. Applets were covered in previous versions of this textbook, but they have become much less widely used and have been dropped from this seventh edition of the book.

The Swing GUI toolkit was introduced in Java 1.2 as an improved alternative to the AWT, with a larger variety of sophisticated components and a more logical structure. Although Swing uses some aspects of the AWT, most of its components are written in Java rather than being based on operating system components. Swing has been the standard toolkit for writing GUI programs in Java for over ten years, and it is the toolkit that I cover in this book.

More recently, however, another GUI toolkit called JavaFX has been introduced. It uses many of the same core ideas as Swing, including components, layout, and events, but uses a different structure for its applications and a different set of classes. With Java 8, JavaFX becomes the preferred approach to writing GUI applications. However, I do not cover JavaFX in this book. JavaFX is compatible with Swing and can use Swing components, and Swing will continue to be supported in Java. (Indeed, the AWT is still supported!) And as I've said, JavaFX is built on the same core ideas as Swing.

Section 6.2

Graphics and Painting

Everything you see on a computer screen has to be drawn there, even the text. The Java API includes a range of classes and methods that are devoted to drawing. In this section, I'll look at some of the most basic of these. Some of this material was already covered in preliminary form in Section 3.9.

The physical structure of a GUI is built of components. The term component refers to a visual element in a GUI, including buttons, menus, text-input boxes, scroll bars, check boxes, and so on. In Java, GUI components are represented by objects belonging to subclasses of the class java.awt.Component. Most components in the Swing GUI toolkit -- although not top-level components like JFrame -- belong to subclasses of the class javax.swing.JComponent, which is itself a subclass of java.awt.Component. Every component is responsible for drawing itself. If you want to use a standard component, you only have to add it to your program. You don't have to worry about painting it on the screen. That will happen automatically, since it already knows how to draw itself.

Sometimes, however, you do want to draw on a component. You will have to do this whenever you want to display something that is not included among the standard, pre-defined component classes. When you want to do this, you have to define your own component class and provide a method in that class for drawing the component. I will always use a subclass of JPanel when I need a drawing surface of this kind, as I did for the HelloWorldDisplay class in the example HelloWorldGUI2.java in the previous section. A JPanel, like any JComponent, draws its content in the method

public void paintComponent(Graphics g)

To create a drawing surface, you should define a subclass of JPanel and provide a custom paintComponent() method. Create an object belonging to this class and use it in your program. When the time comes for your component to be drawn on the screen, the system will call its paintComponent() to do the drawing. That is, the code that you put into the paintComponent() method will be executed whenever the panel needs to be drawn on the screen; by writing this method, you determine the picture that will be displayed in the panel. Note that you are not likely to call a paintComponent() method any more than you are likely to call a main() routine. The system calls the method. You write the method to say what will happen when the system calls it.

Note that the paintComponent() method has a parameter of type Graphics. The Graphics object will be provided by the system when it calls your method. You need this object to do the actual drawing. To do any drawing at all in Java, you need a graphics context. A graphics context is an object belonging to the class java.awt.Graphics. Instance methods are provided in this class for drawing shapes, text, and images. Any given Graphics object can draw to only one location. In this chapter, that location will always be a GUI component belonging to some subclass of JPanel. The Graphics class is an abstract class, which means that it is impossible to create a graphics context directly, with a constructor. There are actually two ways to get a graphics context for drawing on a component: First of all, of course, when the paintComponent() method of a component is called by the system, the parameter to that method is a graphics context for drawing on the component. Second, every component has an instance method called getGraphics(). This method is a function that returns a graphics context that can be used for drawing on the component outside its paintComponent() method. The official line is that you should not do this, and I will almost always avoid it. But I have found it convenient to use getGraphics() in a few examples. (Note that if g is a graphics context created with getGraphics(), it is good form to call g.dispose() when finished using it. This releases any operating system resources that might be held by g.)

The paintComponent() method in the JPanel class simply fills the panel with the panel's background color. When defining a subclass of JPanel for use as a drawing surface, you will usually want to fill the panel with the background color before drawing other content onto the panel (although it is not necessary to do this if the drawing commands in the method cover the background of the component completely). This is traditionally done with a call to super.paintComponent(g), so most paintComponent() methods that you write will have the form:

public void paintComponent(g) {
super.paintComponent(g);
. . . // Draw the content of the component.
}

In general, a component should do all drawing operations in its paintComponent() method. What happens if, in the middle of some other method, you realize that the content of the component needs to be changed? You should not call paintComponent() directly to make the change. Instead, you have to inform the system that the component needs to be redrawn, and let the system do its job by calling paintComponent(). You do this by calling the component's repaint() method. The method

public void repaint();

is defined in the Component class, and so can be used with any component. You should call repaint() to inform the system that the component needs to be redrawn. It is important to understand that the repaint() method returns immediately, without doing any painting itself. The system will call the component's paintComponent() method later, as soon as it gets a chance to do so, after processing other pending events if there are any. It is even possible that many calls to repaint() will all be handled by one call to paintComponent(), if the calls to repaint() occur in a very short timespan.

Note that the system can also call paintComponent() for other reasons. It is called when the component first appears on the screen. It will also be called if the size of the component changes, which can happen when the user resizes the window that contains the component. This means that paintComponent() should be capable of redrawing the content of the component on demand. As you will see, however, some of our early examples will not be able to do this correctly.

This means that, to work properly, the paintComponent() method must be smart enough to correctly redraw the component at any time. To make this possible, a program should store data in its instance variables about the state of the component. These variables should contain all the information necessary to redraw the component completely. The paintComponent() method should use the data in these variables to decide what to draw. When the program wants to change the content of the component, it should not simply draw the new content. It should change the values of the relevant variables and call repaint(). When the system calls paintComponent(), that method will use the new values of the variables and will draw the component with the desired modifications. This might seem a roundabout way of doing things. Why not just draw the modifications directly? There are at least two reasons. First of all, it really does turn out to be easier to get things right if all drawing is done in one method. Second, even if you could directly draw the modifications, you would still have to save enough information about the modifications to enable paintComponent() to redraw the component correctly on demand.

You will see how all this works in practice as we work through examples in the rest of this chapter. For now, we will spend the rest of this section looking at how to get some actual drawing done.

6.2.1  Coordinates

The screen of a computer is a grid of little squares called pixels. The color of each pixel can be set individually, and drawing on the screen just means setting the colors of individual pixels.

A graphics context draws in a rectangle made up of pixels. A position in the rectangle is specified by a pair of integer coordinates, (x,y). The upper left corner has coordinates (0,0). The x coordinate increases from left to right, and the y coordinate increases from top to bottom. The illustration shows a 20-pixel by 12-pixel component (with very large pixels). A small line, rectangle, and oval are shown as they would be drawn by coloring individual pixels. (Note that, properly speaking, the coordinates don't belong to the pixels but to the grid lines between them.)

For any component, you can find out the size of the rectangle that it occupies by calling the instance methods getWidth() and getHeight(), which return the number of pixels in the horizontal and vertical directions, respectively. In general, it's not a good idea to assume that you know the size of a component, since the size is often set by a layout manager and can even change if the component is in a window and that window is resized by the user. This means that it's good form to check the size of a component before doing any drawing on that component. For example, you can use a paintComponent() method that looks like:

public void paintComponent(Graphics g) {
super.paintComponent(g);
int width =  getWidth();   // Find out the width of this component.
int height = getHeight();  // Find out its height.
. . .   // Draw the content of the component.
}

Of course, your drawing commands will have to take the size into account. That is, they will have to use (x,y) coordinates that are calculated based on the actual height and width of the component. (However, if you are sure that you know the size, using constants for the width and height can make the drawing easier.)

6.2.2  Colors

You will probably want to use some color when you draw. Java is designed to work with the RGB color system. An RGB color is specified by three numbers that give the level of red, green, and blue, respectively, in the color. A color in Java is an object of the class, java.awt.Color. You can construct a new color by specifying its red, blue, and green components. For example,

Color myColor = new Color(r,g,b);

There are two constructors that you can call in this way. In the one that I almost always use, r, g, and b are integers in the range 0 to 255. In the other, they are numbers of type float in the range 0.0F to 1.0F. (Recall that a literal of type float is written with an "F" to distinguish it from a double number.) Often, you can avoid constructing new colors altogether, since the Color class defines several named constants representing common colors: Color.WHITE, Color.BLACK, Color.RED, Color.GREEN, Color.BLUE, Color.CYAN, Color.MAGENTA, Color.YELLOW, Color.PINK, Color.ORANGE, Color.LIGHT_GRAY, Color.GRAY, and Color.DARK_GRAY. (There are older, alternative names for these constants that use lower case rather than upper case constants, such as Color.red instead of Color.RED, but the upper case versions are preferred because they follow the convention that constant names should be upper case.)

An alternative to RGB is the HSB color system. In the HSB system, a color is specified by three numbers called the hue, the saturation, and the brightness. The hue is the basic color, ranging from red through orange through all the other colors of the rainbow. The brightness is pretty much what it sounds like. A fully saturated color is a pure color tone. Decreasing the saturation is like mixing white or gray paint into the pure color. In Java, the hue, saturation and brightness are always specified by values of type float in the range from 0.0F to 1.0F. The Color class has a static member function named getHSBColor for creating HSB colors. To create the color with HSB values given by h, s, and b, you can say:

Color myColor = Color.getHSBColor(h,s,b);

For example, to make a color with a random hue that is as bright and as saturated as possible, you could use:

Color randomColor = Color.getHSBColor( (float)Math.random(), 1.0F, 1.0F );

The type cast is necessary because the value returned by Math.random() is of type double, and Color.getHSBColor() requires values of type float. (By the way, you might ask why RGB colors are created using a constructor while HSB colors are created using a static member function. The problem is that we would need two different constructors, both of them with three parameters of type float. Unfortunately, this is impossible. You can have two constructors only if the number of parameters or the parameter types differ.)

The RGB system and the HSB system are just different ways of describing the same set of colors. It is possible to translate between one system and the other. The best way to understand the color systems is to experiment with them. (The sample program SimpleColorChooser.java lets you do that. You won't understand the source code at this time, but you can run it to play with color selection or to find the RGB or HSB values for the color that want.)

One of the properties of a Graphics object is the current drawing color, which is used for all drawing of shapes and text. If g is a graphics context, you can change the current drawing color for g using the method g.setColor(c), where c is a Color. For example, if you want to draw in green, you would just say g.setColor(Color.GREEN) before doing the drawing. The graphics context continues to use the color until you explicitly change it with another setColor() command. If you want to know what the current drawing color is, you can call the function g.getColor(), which returns an object of type Color. This can be useful if you want to change to another drawing color temporarily and then restore the previous drawing color.

Every component has an associated foreground color and background color. Generally, the component is filled with the background color before anything else is drawn (although some components are "transparent," meaning that the background color is ignored). When a new graphics context is created for a component, the current drawing color is set to the foreground color. Note that the foreground color and background color are properties of the component, not of a graphics context.

The foreground and background colors of a component can be set by calling instance methods component.setForeground(color) and component.setBackground(color), which are defined in the Component class and therefore are available for use with any component. This can be useful even for standard components, if you want them to use colors that are different from the defaults.

6.2.3  Fonts

A font represents a particular size and style of text. The same character will appear different in different fonts. In Java, a font is characterized by a font name, a style, and a size. The available font names are system dependent, but you can always use the following four strings as font names: "Serif", "SansSerif", "Monospaced", and "Dialog". (A "serif" is a little decoration on a character, such as a short horizontal line at the bottom of the letter i. "SansSerif" means "without serifs." "Monospaced" means that all the characters in the font have the same width. The "Dialog" font is the one that is typically used in dialog boxes.)

The style of a font is specified using named constants that are defined in the Font class. You can specify the style as one of the four values:

- Font.PLAIN,

- Font.ITALIC,

- Font.BOLD, or

- Font.BOLD + Font.ITALIC.

The size of a font is an integer. Size typically ranges from about 9 to 36, although larger sizes can also be used. The size of a font is usually about equal to the height of the largest characters in the font, in pixels, but this is not an exact rule. The size of the default font is 12.

Java uses the class named java.awt.Font for representing fonts. You can construct a new font by specifying its font name, style, and size in a constructor:

Font plainFont = new Font("Serif", Font.PLAIN, 12);
Font bigBoldFont = new Font("SansSerif", Font.BOLD, 24);

Every graphics context has a current font, which is used for drawing text. You can change the current font with the setFont() method. For example, if g is a graphics context and bigBoldFont is a font, then the command g.setFont(bigBoldFont) will set the current font of g to bigBoldFont. The new font will be used for any text that is drawn after the setFont() command is given. You can find out the current font of g by calling the method g.getFont(), which returns an object of type Font.

Every component also has an associated font. It can be set with the instance method component.setFont(font), which is defined in the Component class. When a graphics context is created for drawing on a component, the graphic context's current font is set equal to the font of the component.

6.2.4  Shapes

The Graphics class includes a large number of instance methods for drawing various shapes, such as lines, rectangles, and ovals. The shapes are specified using the (x,y) coordinate system described above. They are drawn in the current drawing color of the graphics context. The current drawing color is set to the foreground color of the component when the graphics context is created, but it can be changed at any time using the setColor() method.

Some drawing methods were already listed in Subsection 3.9.1. Here, I describe those methods in more detail and add a few more. With all these commands, any drawing that is done outside the boundaries of the component is ignored. Note that all these methods are in the Graphics class, so they all must be called through an object of type Graphics. It is shown here as g, but of course the name of the graphics context is up to the programmer.

- g.drawString(String str, int x, int y) -- Draws the text given by the string str. The string is drawn using the current color and font of the graphics context. x specifies the x-coordinate of the left end of the string. y is the y-coordinate of the baseline of the string. The baseline is a horizontal line on which the characters rest. Some parts of the characters, such as the tail on a y or g, extend below the baseline.

- g.drawLine(int x1, int y1, int x2, int y2) -- Draws a line from the point (x1,y1) to the point (x2,y2). The line is drawn as if with a pen that extends one pixel to the right and one pixel down from the (x,y) point where the pen is located. For example, if g refers to an object of type Graphics, then the command g.drawLine(x,y,x,y), which corresponds to putting the pen down at a point, colors the single pixel with upper left corner at the point (x,y). Remember that coordinates really refer to the lines between the pixels.

- g.drawRect(int x, int y, int width, int height) -- Draws the outline of a rectangle. The upper left corner is at (x,y), and the width and height of the rectangle are as specified. If width equals height, then the rectangle is a square. If the width or the height is negative, then nothing is drawn. The rectangle is drawn with the same pen that is used for drawLine(). This means that the actual width of the rectangle as drawn is width+1, and similarly for the height. There is an extra pixel along the right edge and the bottom edge. For example, if you want to draw a rectangle around the edges of the component, you can say "g.drawRect(0, 0, getWidth()-1, getHeight()-1);". If you use "g.drawRect(0, 0, getWidth(), getHeight());", then the right and bottom edges of the rectangle will be drawn outside the component and will not appear on the screen.

- g.drawOval(int x, int y, int width, int height) -- Draws the outline of an oval. The oval is one that just fits inside the rectangle specified by x, y, width, and height. If width equals height, the oval is a circle.

- g.drawRoundRect(int x, int y, int width, int height, int xdiam, int ydiam) -- Draws the outline of a rectangle with rounded corners. The basic rectangle is specified by x, y, width, and height, but the corners are rounded. The degree of rounding is given by xdiam and ydiam. The corners are arcs of an ellipse with horizontal diameter xdiam and vertical diameter ydiam. A typical value for xdiam and ydiam is 16, but the value used should really depend on how big the rectangle is.

- g.draw3DRect(int x, int y, int width, int height, boolean raised) -- Draws the outline of a rectangle that is supposed to have a three-dimensional effect, as if it is raised from the screen or pushed into the screen. The basic rectangle is specified by x, y, width, and height. The raised parameter tells whether the rectangle seems to be raised from the screen or pushed into it. The 3D effect is achieved by using brighter and darker versions of the drawing color for different edges of the rectangle. The documentation recommends setting the drawing color equal to the background color before using this method. The effect won't work well for some colors.

- g.drawArc(int x, int y, int width, int height, int startAngle, int arcAngle) -- Draws part of the oval that just fits inside the rectangle specified by x, y, width, and height. The part drawn is an arc that extends arcAngle degrees from a starting angle at startAngle degrees. Angles are measured with 0 degrees at the 3 o'clock position (the positive direction of the horizontal axis). Positive angles are measured counterclockwise from zero, and negative angles are measured clockwise. To get an arc of a circle, make sure that width is equal to height.

- g.fillRect(int x, int y, int width, int height) -- Draws a filled-in rectangle. This fills in the interior of the rectangle that would be drawn by drawRect(x,y,width,height). The extra pixel along the bottom and right edges is not included. The width and height parameters give the exact width and height of the rectangle. For example, if you wanted to fill in the entire component, you could say "g.fillRect(0, 0, getWidth(), getHeight());"

- g.fillOval(int x, int y, int width, int height) -- Draws a filled-in oval.

- g.fillRoundRect(int x, int y, int width, int height, int xdiam, int ydiam) -- Draws a filled-in rounded rectangle.

- g.fill3DRect(int x, int y, int width, int height, boolean raised) -- Draws a filled-in three-dimensional rectangle.

- g.fillArc(int x, int y, int width, int height, int startAngle, int arcAngle) -- Draw a filled-in arc. This looks like a wedge of pie, whose crust is the arc that would be drawn by the drawArc method.

6.2.5  Graphics2D

All drawing in Java is done through an object of type Graphics. The Graphics class provides basic commands for such things as drawing shapes and text and for selecting a drawing color. These commands are adequate in many cases, but they fall far short of what's needed in a serious computer graphics program. Java has another class, Graphics2D, that provides a larger set of drawing operations. Graphics2D is a sub-class of Graphics, so all the methods from the Graphics class are also available in a Graphics2D.

The paintComponent() method of a JComponent gives you a graphics context of type Graphics that you can use for drawing on the component. In fact, the graphics context actually belongs to the sub-class Graphics2D, and can be type-cast to gain access to the advanced Graphics2D drawing methods:

public void paintComponent(Graphics g) {
super.paintComponent(g);
Graphics2D g2;
g2 = (Graphics2D)g;
.
. // Draw on the component using g2.
.
}

I mention Graphics2D here for completeness. I will cover some important aspects of Graphics2D in Section 13.2, but a full treatment is more than we will have time for in this book. However, there are two simple applications that I would like to start using now, without explaining how they work. If g2 is a variable of type Graphics2D, as in the paintComponent() method above, then the intimidating-looking command

g2.setRenderingHint( RenderingHints.KEY_ANTIALIASING,
RenderingHints.VALUE_ANTIALIAS_ON );

turns on antialiasing in the graphics context. Aliasing is a jagged appearance that can be seen when shapes are drawn using pixels. Antialiasing tries to reduce the jaggedness. It can make diagonal lines and the outlines of ovals look much nicer. It can also improve the appearance of text. Another useful command is

g2.setStroke( new BasicStroke(lineWidth) );

where lineWidth is an integer or a float. This command can be used to draw thicker lines. Lines drawn after the command will be lineWidth pixels wide. This also affects the thickness of the outlined shapes drawn by methods such as g.drawRect and g.drawOval().

6.2.6  An Example

Let's use some of the material covered in this section to write a subclass of JPanel for use as a drawing surface. All the drawing will be done in the paintComponent() method of the panel class. The panel will draw multiple copies of a message on a black background. Each copy of the message is in a random color. Five different fonts are used, with different sizes and styles. The message can be specified in the constructor; if the default constructor is used, the message is the string "Java!". The panel works OK no matter what its size.

There is one problem with the way this class works. When the panel's paintComponent() method is called, it chooses random colors, fonts, and locations for the messages. The information about which colors, fonts, and locations are used is not stored anywhere. The next time paintComponent() is called, it will make different random choices and will draw a different picture. If you resize a window containing the panel, the picture will be continually redrawn as the size of the window is changed! To avoid that, you would store enough information about the picture in instance variables to enable the paintComponent() method to draw the same picture each time it is called.

The source code for the panel class is shown below. I use an instance variable called message to hold the message that the panel will display. There are five instance variables of type Font that represent different sizes and styles of text. These variables are initialized in the constructor and are used in the paintComponent() method.

The paintComponent() method for the panel simply draws 25 copies of the message. For each copy, it chooses one of the five fonts at random, and it uses g.setFont() to select that font for drawing. It creates a random HSB color and uses g.setColor() to select that color for drawing. It then chooses random (x,y) coordinates for the location of the message. The x coordinate gives the horizontal position of the left end of the string. The formula used for the x coordinate is "-50 + (int)(Math.random() * (width+40))". This gives a random integer in the range from -50 to width-10. This makes it possible for the string to extend beyond the left edge or the right edge of the panel. Similarly, the formula for y allows the string to extend beyond the top and bottom.

Here is the complete source code for the RandomStringsPanel:

import java.awt.*;
import javax.swing.JPanel;

/**
* This panel displays 25 copies of a message.  The color and
* position of each message is selected at random.  The font
* of each message is randomly chosen from among five possible
* fonts.  The messages are displayed on a black background.
* Note:  The style of drawing used here is poor, because every
* time the paintComponent() method is called, new random values are
* used.  This means that a different picture will be drawn each time.
*/
public class RandomStringsPanel extends JPanel {

private String message;  // The message to be displayed.  This can be set in
// the constructor.  If no value is provided in the
// constructor, then the string "Java!" is used.

private Font font1, font2, font3, font4, font5;  // The five fonts.

/**
* Default constructor creates a panel that displays the message "Java!".
*/
public RandomStringsPanel() {
this(null);  // Call the other constructor, with parameter null.
}

/**
* Constructor creates a panel to display 25 copies of a specified message.
* @param messageString The message to be displayed.  If this is null,
* then the default message "Java!" is displayed.
*/
public RandomStringsPanel(String messageString) {

message = messageString;
if (message == null)
message = "Java!";

font1 = new Font("Serif", Font.BOLD, 14);
font2 = new Font("SansSerif", Font.BOLD + Font.ITALIC, 24);
font3 = new Font("Monospaced", Font.PLAIN, 30);
font4 = new Font("Dialog", Font.PLAIN, 36);
font5 = new Font("Serif", Font.ITALIC, 48);

setBackground(Color.BLACK);

}

/**
* The paintComponent method is responsible for drawing the content of the panel.
* It draws 25 copies of the message string, using a random color, font, and
* position for each string.
*/
public void paintComponent(Graphics g) {

super.paintComponent(g);  // Call the paintComponent method from the
// superclass, JPanel.  This simply fills the
// entire panel with the background color, black.

Graphics2D g2 = (Graphics2D)g;  // (To make the text smoother.)
g2.setRenderingHint( RenderingHints.KEY_ANTIALIASING,
RenderingHints.VALUE_ANTIALIAS_ON );

int width = getWidth();
int height = getHeight();

for (int i = 0; i < 25; i++) {

// Draw one string.  First, set the font to be one of the five
// available fonts, at random.

int fontNum = (int)(5*Math.random()) + 1;
switch (fontNum) {
case 1:
g.setFont(font1);
break;
case 2:
g.setFont(font2);
break;
case 3:
g.setFont(font3);
break;
case 4:
g.setFont(font4);
break;
case 5:
g.setFont(font5);
break;
} // end switch

// Set the color to a bright, saturated color, with random hue.

float hue = (float)Math.random();
g.setColor( Color.getHSBColor(hue, 1.0F, 1.0F) );

// Select the position of the string, at random.

int x,y;
x = -50 + (int)(Math.random()*(width+40));
y = (int)(Math.random()*(height+20));

// Draw the message.

g.drawString(message,x,y);

} // end for

} // end paintComponent()

}  // end class RandomStringsPanel

6.2.7  Where is main()?

The source code for the RandomStringsPanel class can be found in the example file RandomStringsPanel.java. You can compile that file, but you won't be able to run the compiled class. The problem is that the class doesn't have a main() routine. Only a class that has a main() routine can be run as a program.

Another problem is that a JPanel is not something that can stand on its own. It has to be placed into a container such as another panel or a window. In general, to make a complete program, we need a main() routine that will create a window of type JFrame. It can then create a panel and place the panel in the window. Here is a class with a main() routine that does this:

import javax.swing.JFrame;

public class RandomStrings {

public static void main(String[] args) {
JFrame window = new JFrame("Java!");
RandomStringsPanel content = new RandomStringsPanel();
window.setContentPane(content);
window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
window.setLocation(120,70);
window.setSize(350,250);
window.setVisible(true);
}

}

This class is defined by the file RandomStrings.java. You can compile and run the program, as long as the RandomStringsPanel class is also available.

The main routine is not logically a part of the panel class. It is just one way of using a panel. However, it's possible to include main() as part of the panel class, even if it doesn't logically belong there. This makes it possible to run the panel class as a program, and it has the advantage of keeping everything in one file. For an example, you can look at RandomStringsPanelWithMain.java, which is identical to the original class except for the addition of a main() routine. Although it might not be great style, I will usually take a similar approach in future examples.

I am not going to discuss the details of using JFrame here, but you can look ahead and find them in Subsection 6.7.3. You won't completely understand my main() routines until you read that section.

Section 6.3

Mouse Events

Events are central to programming for a graphical user interface. A GUI program doesn't have a main() routine that outlines what will happen when the program is run, in a step-by-step process from beginning to end. Instead, the program must be prepared to respond to various kinds of events that can happen at unpredictable times and in an order that the program doesn't control. The most basic kinds of events are generated by the mouse and keyboard. The user can press any key on the keyboard, move the mouse, or press a button on the mouse. The user can do any of these things at any time, and the computer has to respond appropriately.

In Java, events are represented by objects. When an event occurs, the system collects all the information relevant to the event and constructs an object to contain that information. Different types of events are represented by objects belonging to different classes. For example, when the user presses one of the buttons on a mouse, an object belonging to a class called MouseEvent is constructed. The object contains information such as the source of the event (that is, the component on which the user clicked), the (x,y) coordinates of the point in the component where the click occurred, the exact time of the click, and which button on the mouse was pressed. When the user presses a key on the keyboard, a KeyEvent is created. After the event object is constructed, it can be passed as a parameter to a designated method. By writing that method, the programmer says what should happen when the event occurs.

As a Java programmer, you get a fairly high-level view of events. There is a lot of processing that goes on between the time that the user presses a key or moves the mouse and the time that a subroutine in your program is called to respond to the event. Fortunately, you don't need to know much about that processing. But you should understand this much: Even though you didn't write it, there is a routine running somewhere that executes a loop of the form

while the program is still running:
Wait for the next event to occur
Call a subroutine to handle the event

This loop is called an event loop. Every GUI program has an event loop. In Java, you don't have to write the loop. It's part of "the system." If you write a GUI program in some other language, you might have to provide a main routine that runs the event loop.

In this section, we'll look at handling mouse events in Java, and we'll cover the framework for handling events in general. The next section will cover keyboard-related events and timer events. Java also has other types of events, which are produced by GUI components. These will be introduced in Section 6.5.

6.3.1  Event Handling

For an event to have any effect, a program must detect the event and react to it. In order to detect an event, the program must "listen" for it. Listening for events is something that is done by an object called an event listener. An event listener object must contain instance methods for handling the events for which it listens. For example, if an object is to serve as a listener for events of type MouseEvent, then it must contain the following method (among several others):

public void mousePressed(MouseEvent evt) { . . . }

The body of the method defines how the object responds when it is notified that a mouse button has been pressed. The parameter, evt, contains information about the event. This information can be used by the listener object to determine its response.

The methods that are required in a mouse event listener are specified in an interface named MouseListener. To be used as a listener for mouse events, an object must implement this MouseListener interface. Java interfaces were covered in Section 5.7. (To review briefly: An interface in Java is just a list of instance methods. A class can "implement" an interface by doing two things: First, the class must be declared to implement the interface, as in "class MouseHandler implements MouseListener" or "class MyPanel extends JPanel implements MouseListener"; and second, the class must include a definition for each instance method specified in the interface. An interface can be used as the type for a variable or formal parameter. We say that an object implements the MouseListener interface if it belongs to a class that implements the MouseListener interface. Note that it is not enough for the object to include the specified methods. It must also belong to a class that is specifically declared to implement the interface.)

Many events in Java are associated with GUI components. For example, when the user presses a button on the mouse, the associated component is the one that the user clicked on. Before a listener object can "hear" events associated with a given component, the listener object must be registered with the component. If a MouseListener object, mListener, needs to hear mouse events associated with a Component object, comp, the listener must be registered with the component by calling

comp.addMouseListener(mListener);

The addMouseListener() method is an instance method in class Component, and so can be used with any GUI component object. In our first few examples, we will listen for events on a JPanel that is being used as a drawing surface.

The event classes, such as MouseEvent, and the listener interfaces, such as MouseListener, are defined in the package java.awt.event. This means that if you want to work with events, you should either include the line "import java.awt.event.*;" at the beginning of your source code file or import the individual classes and interfaces.

Admittedly, there is a large number of details to tend to when you want to use events. To summarize, you must

1. Put the import specification "import java.awt.event.*;" (or individual imports) at the beginning of your source code;

2. Declare that some class implements the appropriate listener interface, such as MouseListener;

3. Provide definitions in that class for the methods specified by the interface;

4. Register an object that belongs to the listener class with the component that will generate the events by calling a method such as addMouseListener() in the component.

Any object can act as an event listener, provided that it implements the appropriate interface. A component can listen for the events that it itself generates. A panel can listen for events from components that are contained in the panel. A special class can be created just for the purpose of defining a listening object. Many people consider it to be good form to use anonymous inner classes to define listening objects (see Subsection 5.8.3), and named nested classes can also be appropriate. You will see all of these patterns in examples in this textbook.

6.3.2  MouseEvent and MouseListener

The MouseListener interface specifies these five instance methods:

public void mousePressed(MouseEvent evt);
public void mouseReleased(MouseEvent evt);
public void mouseClicked(MouseEvent evt);
public void mouseEntered(MouseEvent evt);
public void mouseExited(MouseEvent evt);

The mousePressed method is called as soon as the user presses down on one of the mouse buttons, and mouseReleased is called when the user releases a button. These are the two methods that are most commonly used, but any mouse listener object must define all five methods; you can leave the body of a method empty if you don't want to define a response. The mouseClicked method is called if the user presses a mouse button and then releases it, without moving the mouse. (When the user does this, all three routines -- mousePressed, mouseReleased, and mouseClicked -- will be called in that order.) In most cases, you should define mousePressed instead of mouseClicked. The mouseEntered and mouseExited methods are called when the mouse cursor enters or leaves the component. For example, if you want the component to change appearance whenever the user moves the mouse over the component, you could define these two methods.

As a first example, we will look at a small addition to the RandomStringsPanel example from the previous section. In the new version, the panel will repaint itself when the user clicks on it. In order for this to happen, a mouse listener should listen for mouse events on the panel, and when the listener detects a mousePressed event, it should respond by calling the repaint() method of the panel.

For the new version of the program, we need an object that implements the MouseListener interface. One way to create the object is to define a separate class, such as:

import java.awt.Component;
import java.awt.event.*;

/**
* An object of type RepaintOnClick is a MouseListener that
* will respond to a mousePressed event by calling the repaint()
* method of the source of the event.  That is, a RepaintOnClick
* object can be added as a mouse listener to any Component;
* when the user clicks that component, the component will be
* repainted.
*/
public class RepaintOnClick implements MouseListener {

public void mousePressed(MouseEvent evt) {
Component source = (Component)evt.getSource();
source.repaint();  // Call repaint() on the Component that was clicked.
}

public void mouseClicked(MouseEvent evt) { }
public void mouseReleased(MouseEvent evt) { }
public void mouseEntered(MouseEvent evt) { }
public void mouseExited(MouseEvent evt) { }

}

This class does three of the four things that we need to do in order to handle mouse events: First, it imports java.awt.event.* for easy access to event-related classes. Second, it is declared that the class "implements MouseListener". And third, it provides definitions for the five methods that are specified in the MouseListener interface. (Note that four of the methods have empty bodies, since we don't want to do anything in response to those events.)

We must do one more thing to set up the event handling for this example: We must register an event-handling object as a listener with the component that will generate the events. In this case, the mouse events that we are interested in will be generated by an object of type RandomStringsPanel. If panel is a variable that refers to the panel object, we can create a mouse listener object and register it with the panel with the statements:

RepaintOnClick listener = new RepaintOnClick();  // Create MouseListener object.
panel.addMouseListener(listener);  // Register MouseListener with the panel.

This could be done, for example, in the main() routine where the panel is created. Once the listener has been registered in this way, it will be notified of mouse events on the panel. When a mousePressed event occurs, the mousePressed() method in the listener will be called. The code in this method calls the repaint() method in the component that is the source of the event, that is, in the panel. The result is that the RandomStringsPanel is repainted with its strings in new random colors, fonts, and positions.

Although we have written the RepaintOnClick class for use with our RandomStringsPanel example, the event-handling class contains no reference at all to the RandomStringsPanel class. How can this be? The mousePressed() method in class RepaintOnClick looks at the source of the event, and calls its repaint() method. If we have registered the RepaintOnClick object as a listener on a RandomStringsPanel, then it is that panel that is repainted. But the listener object could be used with any type of component, and it would work in the same way.

Similarly, the RandomStringsPanel class contains no reference to the RepaintOnClick class -- in fact, RandomStringsPanel was written before we even knew anything about mouse events! The panel will send mouse events to any object that has registered with it as a mouse listener. It does not need to know anything about that object except that it is capable of receiving mouse events.

The relationship between an object that generates an event and an object that responds to that event is rather loose. The relationship is set up by registering one object to listen for events from the other object. This is something that can potentially be done from outside both objects. Each object can be developed independently, with no knowledge of the internal operation of the other object. This is the essence of modular design: Build a complex system out of modules that interact only in straightforward, easy to understand ways. Then each module is a separate design problem that can be tackled independently. Java's event-handling framework is designed to offer strong support for modular design.

To make this clearer, let's look at a new version of RandomStrings.java, the program from Subsection 6.2.7 that uses RandomStringsPanel. The new version is ClickableRandomStrings.java. For convenience, I have added RepaintOnClick as a static nested class, although it would work just as well as a separate class:

import java.awt.Component;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;
import javax.swing.JFrame;

/**
* Displays a window that shows 25 copies of the string "Java!" in
* random colors, fonts, and positions.  The content of the window
* is an object of type RandomStringsPanel.  When the user clicks
* the window, the content of the window is repainted, with the
* strings in newly selected random colors, fonts, and positions.
*/
public class ClickableRandomStrings {

public static void main(String[] args) {
JFrame window = new JFrame("Click Me to Redraw!");
RandomStringsPanel content = new RandomStringsPanel();
content.addMouseListener( new RepaintOnClick() );
window.setContentPane(content);
window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
window.setLocation(120,70);
window.setSize(350,250);
window.setVisible(true);
}

private static class RepaintOnClick implements MouseListener {

public void mousePressed(MouseEvent evt) {
Component source = (Component)evt.getSource();
source.repaint();
}

public void mouseClicked(MouseEvent evt) { }
public void mouseReleased(MouseEvent evt) { }
public void mouseEntered(MouseEvent evt) { }
public void mouseExited(MouseEvent evt) { }

}

} end class ClickableRandomStrings

6.3.3  MouseEvent Data

Often, when a mouse event occurs, you want to know the location of the mouse cursor. This information is available from the MouseEvent parameter to the event-handling method, which contains instance methods that return information about the event. If evt is the parameter, then you can find out the coordinates of the mouse cursor by calling evt.getX() and evt.getY(). These methods return integers which give the x and y coordinates where the mouse cursor was positioned at the time when the event occurred. The coordinates are expressed in the coordinate system of the component that generated the event, where the top left corner of the component is (0,0).

The user can hold down certain modifier keys while using the mouse. The possible modifier keys include: the Shift key, the Control key, the Alt key (called the Option key on the Mac), and the Meta key (called the Command or Apple key on the Mac). You might want to respond to a mouse event differently when the user is holding down a modifier key. The boolean-valued instance methods evt.isShiftDown(), evt.isControlDown(), evt.isAltDown(), and evt.isMetaDown() can be called to test whether the modifier keys are pressed.

You might also want to have different responses depending on whether the user presses the left mouse button, the middle mouse button, or the right mouse button. For events triggered by a mouse button, you can determine which button was pressed or released by calling evt.getButton(), which returns one of the integer constants MouseEvent.BUTTON1, MouseEvent.BUTTON2, or MouseEvent.BUTTON3 for the left, middle, and right buttons. For events such as mouseEntered and mouseExited that are not triggered by buttons, evt.getButton() returns MouseEvent.NOBUTTON.

Now, not every mouse has a middle button and a right button, and Java deals with that fact in a somewhat peculiar way. If the user clicks with the right mouse button, then evt.isMetaDown() will return true, even if the user was not holding down the Meta key. Similarly, if the user clicks with the middle mouse button, then evt.isAltDown() will return true, even if the user is not holding down the Alt/Option key. By using these functions, you can design an interface that will work even on computers that lack a middle or right mouse button. Note that there is a subtle difference between these functions and evt.getButton(): evt.getButton() really only applies to mousePressed, mouseReleased, and mouseClicked events, while evt.isMetaDown() and evt.isAltDown() are useful in any mouse event. I will often use them instead of evt.getButton().

As an example, consider a JPanel that does the following: Clicking on the panel with the left mouse button will place a red rectangle on the panel at the point where the mouse was clicked. Clicking with the right mouse button will place a blue oval on the panel. Holding down the Shift key while clicking will clear the panel by removing all the shapes that have been placed. You can try the sample program SimpleStamper.java. Here is what the panel looks like after some shapes have been added:

There are several ways to write this example. There could be a separate class to handle mouse events, as in the previous example. However, in this case, I decided to let the panel itself respond to mouse events. Any object can be a mouse listener, as long as it implements the MouseListener interface. In this case, the panel class implements the MouseListener interface, so the object that represents the main panel of the program can be the mouse listener for the program. The constructor for the panel class contains the statement

addMouseListener(this);

which is equivalent to saying this.addMouseListener(this). Now, the ordinary way to register a mouse listener is to say X.addMouseListener(Y) where Y is the listener and X is the component that will generate the mouse events. In the statement addMouseListener(this), both roles are played by this; that is, "this object" (the panel) is generating mouse events and is also listening for those events. Although this might seem a little strange, you should get used to seeing things like this. In a large program, however, it's usually a better idea to write a separate class to do the listening in order to have a more organized division of responsibilities.

The source code for the panel class is shown below. I have included a main() routine to allow the class to be run as a program, as discussed in Subsection 6.2.7. You should check how the instance methods in the MouseEvent object are used. You can also check for the Four Steps of Event Handling ("import java.awt.event.*", "implements MouseListener", definitions for the event-handling methods, and "addMouseListener"):

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

/**
* A simple demonstration of MouseEvents.  Shapes are drawn
* on a black background when the user clicks the panel.  If
* the user Shift-clicks, the panel is cleared.  If the user
* right-clicks the panel, a blue oval is drawn.  Otherwise,
* when the user clicks, a red rectangle is drawn.  The contents of
* the panel are not persistent.  For example, they might disappear
* if the panel is resized.
* This class has a main() routine to allow it to be run as an application.
*/
public class SimpleStamper extends JPanel implements MouseListener {

public static void main(String[] args) {
JFrame window = new JFrame("Simple Stamper");
SimpleStamper content = new SimpleStamper();
window.setContentPane(content);
window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
window.setLocation(120,70);
window.setSize(450,350);
window.setVisible(true);
}

// ----------------------------------------------------------------------

/**
* This constructor simply sets the background color of the panel to be black
* and sets the panel to listen for mouse events on itself.
*/
public SimpleStamper() {
setBackground(Color.BLACK);
addMouseListener(this);
}

/**
*  Since this panel has been set to listen for mouse events on itself,
*  this method will be called when the user clicks the mouse on the panel.
*  This method is part of the MouseListener interface.
*/
public void mousePressed(MouseEvent evt) {

if ( evt.isShiftDown() ) {
// The user was holding down the Shift key.  Just repaint the panel.
// Since this class does not define a paintComponent() method, the
// method from the superclass, JPanel, is called.  That method simply
// fills the panel with its background color, which is black.  The
// effect is to clear the panel.
repaint();
return;
}

int x = evt.getX();  // x-coordinate where user clicked.
int y = evt.getY();  // y-coordinate where user clicked.

Graphics g = getGraphics(); // Graphics context for drawing directly.
// NOTE: This is considered to be bad style!

if ( evt.isMetaDown() ) {
// User right-clicked at the point (x,y). Draw a blue oval centered
// at the point (x,y). (A black outline around the oval will make it
// more distinct when shapes overlap.)
g.setColor(Color.BLUE);  // Blue interior.
g.fillOval( x - 30, y - 15, 60, 30 );
g.setColor(Color.BLACK); // Black outline.
g.drawOval( x - 30, y - 15, 60, 30 );
}
else {
// User left-clicked (or middle-clicked) at (x,y).
// Draw a red rectangle centered at (x,y).
g.setColor(Color.RED);   // Red interior.
g.fillRect( x - 30, y - 15, 60, 30 );
g.setColor(Color.BLACK); // Black outline.
g.drawRect( x - 30, y - 15, 60, 30 );
}

g.dispose();  // We are finished with the graphics context, so dispose of it.

} // end mousePressed();

// The next four empty routines are required by the MouseListener interface.
// They don't do anything in this class, so their definitions are empty.

public void mouseEntered(MouseEvent evt) { }
public void mouseExited(MouseEvent evt) { }
public void mouseClicked(MouseEvent evt) { }
public void mouseReleased(MouseEvent evt) { }

} // end class SimpleStamper

Note, by the way, that this class violates the rule that all drawing should be done in a paintComponent() method. The rectangles and ovals are drawn directly in the mousePressed() routine. To make this possible, I need to obtain a graphics context by saying "g = getGraphics()". After using g for drawing, I call g.dispose() to inform the operating system that I will no longer be using g for drawing. I do not advise doing this type of direct drawing if it can be avoided, but you can see that it does work in this case.

6.3.4  MouseMotionListeners and Dragging

Whenever the mouse is moved, it generates events. The operating system of the computer detects these events and uses them to move the mouse cursor on the screen. It is also possible for a program to listen for these "mouse motion" events and respond to them. The most common reason to do so is to implement dragging. Dragging occurs when the user moves the mouse while holding down a mouse button.

The methods for responding to mouse motion events are defined in an interface named MouseMotionListener. This interface specifies two event-handling methods:

public void mouseDragged(MouseEvent evt);
public void mouseMoved(MouseEvent evt);

The mouseDragged method is called if the mouse is moved while a button on the mouse is pressed. If the mouse is moved while no mouse button is down, then mouseMoved is called instead. The parameter, evt, is an object of type MouseEvent, which contains the x and y coordinates of the mouse's location, as usual. As long as the user continues to move the mouse, one of these methods will be called over and over. (So many events are generated that it would be inefficient for a program to hear them all, if it doesn't want to do anything in response. This is why the mouse motion event-handlers are defined in a separate interface from the other mouse events: You can listen for the mouse events defined in MouseListener without automatically hearing all mouse motion events as well.)

If you want your program to respond to mouse motion events, you must create an object that implements the MouseMotionListener interface, and you must register that object to listen for events. The registration is done by calling a component's addMouseMotionListener() method. The object will then listen for mouseDragged and mouseMoved events associated with that component. In most cases, the listener object will also implement the MouseListener interface so that it can respond to the other mouse events as well.

(To get a better idea of how mouse events work, you should try the sample program SimpleTrackMouse.java. This program responds to any of the seven different kinds of mouse events by displaying the coordinates of the mouse, the type of event, and a list of the modifier keys that are down (Shift, Control, Meta, and Alt). You can experiment with the program to see what happens as you do various things with the mouse. I also encourage you to read the source code. You should now be familiar with all the techniques that it uses.)

It is interesting to look at what a program needs to do in order to respond to dragging operations. In general, the response involves three methods: mousePressed(), mouseDragged(), and mouseReleased(). The dragging gesture starts when the user presses a mouse button, it continues while the mouse is dragged, and it ends when the user releases the button. This means that the programming for the response to one dragging gesture must be spread out over the three methods! Furthermore, the mouseDragged() method can be called many times as the mouse moves. To keep track of what is going on between one method call and the next, you need to set up some instance variables. In many applications, for example, in order to process a mouseDragged event, you need to remember the previous coordinates of the mouse. You can store this information in two instance variables prevX and prevY of type int. It can also be useful to save the starting coordinates, where the original mousePressed event occurred, in instance variables. And I suggest having a boolean variable, dragging, which is set to true while a dragging gesture is being processed. This is necessary because in many applications, not every mousePressed event starts a dragging operation to which you want to respond. The mouseDragged and mouseReleased methods can use the value of dragging to check whether a drag operation is actually in progress. You might need other instance variables as well, but in general outline, a class that handles mouse dragging looks like this:

import java.awt.event.*;

public class MouseDragHandler implements MouseListener, MouseMotionListener {

private int startX, startY; // Point where the original mousePress occurred.
private int prevX, prevY;   // Most recently processed mouse coords.
private boolean dragging;   // Set to true when dragging is in process.
. . . // other instance variables for use in dragging

public void mousePressed(MouseEvent evt) {
if ( we-want-to-start-dragging ) {
dragging = true;
startX = evt.getX();  // Remember starting position.
startY = evt.getY();
prevX = startX;       // Remember most recent coords.
prevY = startY;
.
. // Other processing.
.
}
}

public void mouseDragged(MouseEvent evt) {
if ( dragging == false )  // First, check if we are
return;               //   processing a dragging gesture.
int x = evt.getX(); // Current position of Mouse.
int y = evt.getY();
.
.  // Process a mouse movement from (prevX, prevY) to (x,y).
.
prevX = x;  // Remember the current position for the next call.
prevY = y;
}

public void mouseReleased(MouseEvent evt) {
if ( dragging == false )  // First, check if we are
return;               //   processing a dragging gesture.
dragging = false;  // We are done dragging.
.
.  // Other processing and clean-up.
.
}

}

As an example, let's look at a typical use of dragging: allowing the user to sketch a curve by dragging the mouse. This example also shows many other features of graphics and mouse processing. In the program, you can draw a curve by dragging the mouse on a large white drawing area, and you can select a color for drawing by clicking on one of several colored rectangles to the right of the drawing area. The complete source code can be found in SimplePaint.java. Here is a picture of the program after some drawing has been done:

I will discuss a few aspects of the source code here, but I encourage you to read it carefully in its entirety. There are lots of informative comments in the source code.

The panel for this example is designed to work for any reasonable size, that is, unless the panel is too small. This means that coordinates are computed in terms of the actual width and height of the panel. (The width and height are obtained by calling getWidth() and getHeight().) This makes things quite a bit harder than they would be if we assumed some particular fixed size for the panel. Let's look at some of these computations in detail. For example, the large white drawing area extends from y = 3 to y = height - 3 vertically and from x = 3 to x = width - 56 horizontally. These numbers are needed in order to interpret the meaning of a mouse click. They take into account a gray border around the panel and the color palette along the right edge of the panel. The gray border is 3 pixels wide. The colored rectangles are 50 pixels wide. Together with the 3-pixel border around the panel and a 3-pixel divider between the drawing area and the colored rectangles, this adds up to put the right edge of the drawing area 56 pixels from the right edge of the panel.

A white square labeled "CLEAR" occupies a 50-by-50 pixel region beneath the colored rectangles on the right edge of the panel. Allowing for this square, we can figure out how much vertical space is available for the seven colored rectangles, and then divide that space by 7 to get the vertical space available for each rectangle. This quantity is represented by a variable, colorSpace. Out of this space, 3 pixels are used as spacing between the rectangles, so the height of each rectangle is colorSpace - 3. The top of the N-th rectangle is located (N*colorSpace + 3) pixels down from the top of the panel, assuming that we count the rectangles starting with zero. This is because there are N rectangles above the N-th rectangle, each of which uses colorSpace pixels. The extra 3 is for the border at the top of the panel. After all that, we can write down the command for drawing the N-th rectangle:

g.fillRect(width - 53, N*colorSpace + 3, 50, colorSpace - 3);

That was not easy! But it shows the kind of careful thinking and precision graphics that are sometimes necessary to get good results.

The mouse in this program is used to do three different things: Select a color, clear the drawing, and draw a curve. Only the third of these involves dragging, so not every mouse click will start a dragging operation. The mousePressed() method has to look at the (x,y) coordinates where the mouse was clicked and decide how to respond. If the user clicked on the CLEAR rectangle, the drawing area is cleared by calling repaint(). If the user clicked somewhere in the strip of colored rectangles, the corresponding color is selected for drawing. This involves computing which color the user clicked on, which is done by dividing the y coordinate by colorSpace. Finally, if the user clicked on the drawing area, a drag operation is initiated. In this case, a boolean variable, dragging, is set to true so that the mouseDragged and mouseReleased methods will know that a curve is being drawn. The code for this follows the general form given above. The actual drawing of the curve is done in the mouseDragged() method, which draws a line from the previous location of the mouse to its current location. Some effort is required to make sure that the line does not extend beyond the white drawing area of the panel. This is not automatic, since as far as the computer is concerned, the border and the color bar are part of the drawing surface. If the user drags the mouse outside the drawing area while drawing a line, the mouseDragged() routine changes the x and y coordinates to make them lie within the drawing area.

6.3.5  Anonymous Event Handlers and Adapter Classes

As I mentioned above, it is a fairly common practice to use anonymous inner classes to define listener objects. As discussed in Subsection 5.8.3, a special form of the new operator is used to create an object that belongs to an anonymous class. For example, a mouse listener object can be created with an expression of the form:

new MouseListener() {
public void mousePressed(MouseEvent evt) { . . . }
public void mouseReleased(MouseEvent evt) { . . . }
public void mouseClicked(MouseEvent evt) { . . . }
public void mouseEntered(MouseEvent evt) { . . . }
public void mouseExited(MouseEvent evt) { . . . }
}

This is all just one long expression that both defines an unnamed class and creates an object that belongs to that class. To use the object as a mouse listener, it can be passed as the parameter to some component's addMouseListener() method in a command of the form:

component.addMouseListener( new MouseListener() {
public void mousePressed(MouseEvent evt) { . . . }
public void mouseReleased(MouseEvent evt) { . . . }
public void mouseClicked(MouseEvent evt) { . . . }
public void mouseEntered(MouseEvent evt) { . . . }
public void mouseExited(MouseEvent evt) { . . . }
} );

Now, in a typical application, most of the method definitions in this class will be empty. A class that implements an interface must provide definitions for all the methods in that interface, even if the definitions are empty. To avoid the tedium of writing empty method definitions in cases like this, Java provides adapter classes. An adapter class implements a listener interface by providing empty definitions for all the methods in the interface. An adapter class is useful only as a basis for making subclasses. In the subclass, you can define just those methods that you actually want to use. For the remaining methods, the empty definitions that are provided by the adapter class will be used. The adapter class MouseAdapter implements both the MouseListener interface and the MouseMotionListener interface, so it can be used as a basis for creating a listener for any mouse event. As an example, if you want a mouse listener that only responds to mouse-pressed events, you can use a command of the form:

component.addMouseListener( new MouseAdapter() {
public void mousePressed(MouseEvent evt) { . . . }
} );

To see how this works in a real example, let's write another version of the ClickableRandomStrings program from Subsection 6.3.2. This version uses an anonymous class based on MouseAdapter to handle mouse events:

import java.awt.Component;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;
import javax.swing.JFrame;

public class ClickableRandomStrings2 {

public static void main(String[] args) {
JFrame window = new JFrame("Random Strings");
RandomStringsPanel content = new RandomStringsPanel();

content.addMouseListener( new MouseAdapter() {
// Register a mouse listener that is defined by an anonymous subclass
// of MouseAdapter.  This replaces the RepaintOnClick class that was
// used in the original version.
public void mousePressed(MouseEvent evt) {
Component source = (Component)evt.getSource();
source.repaint();
}
} );

window.setContentPane(content);
window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
window.setLocation(100,75);
window.setSize(300,240);
window.setVisible(true);
}

}

Anonymous inner classes can be used for other purposes besides event handling. For example, suppose that you want to define a subclass of JPanel to represent a drawing surface. The subclass will only be used once. It will redefine the paintComponent() method, but will make no other changes to JPanel. It might make sense to define the subclass as an anonymous inner class. You will see this pattern used in some future examples.

Section 6.4

Timers, KeyEvents, and State Machines

Not every event is generated by an action on the part of the user. Events can also be generated by objects as part of their regular programming, and these events can be monitored by other objects so that they can take appropriate actions when the events occur. One example of this is the class javax.swing.Timer. A Timer generates events at regular intervals. These events can be used to drive an animation or to perform some other task at regular intervals. We will begin this section with a look at timer events and animation. We will then look at another type of basic user-generated event: the KeyEvents that are generated when the user types on the keyboard. The example at the end of the section uses both a timer and keyboard events to implement a simple game and introduces the important idea of state machines.

6.4.1  Timers and Animation

An object belonging to the class javax.swing.Timer exists only to generate events. A Timer, by default, generates a sequence of events with a fixed delay between each event and the next. (It is also possible to set a Timer to emit a single event after a specified time delay; in that case, the timer is being used as an "alarm.") Each event belongs to the class ActionEvent. An object that is to listen for the events must implement the interface ActionListener, which defines just one method:

public void actionPerformed(ActionEvent evt)

To use a Timer, you must create an object that implements the ActionListener interface. That is, the object must belong to a class that is declared to "implement ActionListener", and that class must define the actionPerformed method. Then, if the object is set to listen for events from the timer, the code in the listener's actionPerformed method will be executed every time the timer generates an event.

Since there is no point to having a timer without having a listener to respond to its events, the action listener for a timer is specified as a parameter in the timer's constructor. The time delay between timer events is also specified in the constructor. If timer is a variable of type Timer, then the statement

timer = new Timer( millisDelay, listener );

creates a timer with a delay of millisDelay milliseconds between events (where 1000 milliseconds equal one second). Events from the timer are sent to the listener. (millisDelay must be of type int, and listener must be of type ActionListener.) The listener's actionPerfomed() will be executed every time the timer emits an event. Note that a timer is not guaranteed to deliver events at precisely regular intervals. If the computer is busy with some other task, an event might be delayed or even dropped altogether.

A timer does not automatically start generating events when the timer object is created. The start() method in the timer must be called to tell the timer to start running. The timer's stop() method can be used to turn the stream of events off. It can be restarted later by calling start() again.

One application of timers is computer animation. A computer animation is just a sequence of still images, presented to the user one after the other. If the time between images is short, and if the change from one image to another is not too great, then the user perceives continuous motion. The easiest way to do animation in Java is to use a Timer to drive the animation. Each time the timer generates an event, the next frame of the animation is computed and drawn on the screen -- the code that implements this goes in the actionPerformed method of an object that listens for events from the timer.

Our first example of using a timer is not exactly an animation, but it does display a new image for each timer event. The program shows randomly generated images that vaguely resemble works of abstract art. In fact, the program draws a new random image every time its paintComponent() method is called, and the response to a timer event is simply to call repaint(), which in turn triggers a call to paintComponent. The work of the program is done in a subclass of JPanel, which starts like this:

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class RandomArtPanel extends JPanel {

/**
* A RepaintAction object calls the repaint method of this panel each
* time its actionPerformed() method is called.  An object of this
* type is used as an action listener for a Timer that generates an
* ActionEvent every four seconds.  The result is that the panel is
* redrawn every four seconds.
*/
private class RepaintAction implements ActionListener {
public void actionPerformed(ActionEvent evt) {
repaint();  // Call the repaint() method in the panel class.
}
}

/**
* The constructor creates a timer with a delay time of four seconds
* (4000 milliseconds), and with a RepaintAction object as its
* ActionListener.  It also starts the timer running.
*/
public RandomArtPanel() {
RepaintAction action = new RepaintAction();
Timer timer = new Timer(4000, action);
timer.start();
}

/**
* The paintComponent() method fills the panel with a random shade of
* gray and then draws one of three types of random "art".  The type
* of art to be drawn is chosen at random.
*/
public void paintComponent(Graphics g) {
.
.   // The rest of the class is omitted
.

You can find the full source code for this class in the file RandomArt.java. I will only note that the very short RepaintAction class is a natural candidate to be replaced by an anonymous inner class. That can be done where the timer is created:

Timer timer = new timer(4000, new ActionListener() {
public void actionPerformed(ActionEvent evt) {
repaint();
}
});

Later in this section, we will use a timer to drive the animation in a simple computer game.

6.4.2  Keyboard Events

In Java, user actions become events in a program. These events are associated with GUI components. When the user presses a button on the mouse, the event that is generated is associated with the component that contains the mouse cursor. What about keyboard events? When the user presses a key, what component is associated with the key event that is generated?

A GUI uses the idea of input focus to determine the component associated with keyboard events. At any given time, exactly one interface element on the screen has the input focus, and that is where all keyboard events are directed. If the interface element happens to be a Java component, then the information about the keyboard event becomes a Java object of type KeyEvent, and it is delivered to any listener objects that are listening for KeyEvents associated with that component. The necessity of managing input focus adds an extra twist to working with keyboard events.

It's a good idea to give the user some visual feedback about which component has the input focus. For example, if the component is the typing area of a word-processor, the feedback is usually in the form of a blinking text cursor. Another possible visual clue is to draw a brightly colored border around the edge of a component when it has the input focus, as I do in the examples given later in this section.

If comp is any component, and you would like it to have the input focus, you can call requestFocusInWindow(), which should work as long as the window that contains the component is active and there is only one component that is requesting focus. In some cases, when there is only one component involved, it is enough to call this method once, just after opening the window, and the component will retain the focus for the rest of the program. (Note that there is also a requestFocus() method that might work even when the window is not active, but the newer method requestFocusInWindow() is preferred in most cases.)

In a typical user interface, the user can choose to give the focus to a component by clicking on that component with the mouse. And pressing the tab key will often move the focus from one component to another. This is handled automatically by the components involved, without any programming on your part. However, some components do not automatically request the input focus when the user clicks on them. To solve this problem, a program can register a mouse listener with the component to detect user clicks. In response to a user click, the mousePressed() method should call requestFocusInWindow() for the component. This is true, in particular, for JPanels that are used as drawing surfaces, since JPanel objects do not receive the input focus automatically.

As our first example of processing key events, we look at a simple program in which the user moves a square up, down, left, and right by pressing arrow keys. When the user hits the 'R', 'G', 'B', or 'K' key, the color of the square is set to red, green, blue, or black, respectively. Of course, none of these key events are delivered to the panel unless it has the input focus. The panel in the program changes its appearance when it has the input focus: When it does, a cyan-colored border is drawn around the panel; when it does not, a gray-colored border is drawn. The complete source code for this example can be found in the file KeyboardAndFocusDemo.java. I will discuss some aspects of it below. After reading this section, you should be able to understand the source code in its entirety. I suggest running the program to see how it works.

In Java, keyboard event objects belong to a class called KeyEvent. An object that needs to listen for KeyEvents must implement the interface named KeyListener. Furthermore, the object must be registered with a component by calling the component's addKeyListener() method. The registration is done with the command "component.addKeyListener(listener);" where listener is the object that is to listen for key events, and component is the object that will generate the key events (when it has the input focus). It is possible for component and listener to be the same object. All this is, of course, directly analogous to what you learned about mouse events in the previous section. The KeyListener interface defines the following methods, which must be included in any class that implements KeyListener:

public void keyPressed(KeyEvent evt);
public void keyReleased(KeyEvent evt);
public void keyTyped(KeyEvent evt);

Java makes a careful distinction between the keys that you press and the characters that you type. There are lots of keys on a keyboard: letter keys, number keys, modifier keys such as Control and Shift, arrow keys, page up and page down keys, keypad keys, function keys, and so on. In some cases, such as the shift key, pressing a key does not type a character. On the other hand, typing a character sometimes involves pressing several keys. For example, to type an uppercase 'A', you have to press the Shift key and then press the A key before releasing the Shift key. On my Mac OS computer, I can type an accented e, by holding down the Option key, pressing the E key, releasing the Option key, and pressing E again. Only one character was typed, but I had to perform three key-presses and I had to release a key at the right time. In Java, there are three types of KeyEvent. The types correspond to pressing a key, releasing a key, and typing a character. The keyPressed method is called when the user presses a key, the keyReleased method is called when the user releases a key, and the keyTyped method is called when the user types a character (whether that's done with one key press or several). Note that one user action, such as pressing the E key, can be responsible for two events, a keyPressed event and a keyTyped event. Typing an upper case 'A' can generate two keyPressed events, two keyReleased events, and one keyTyped event.

Usually, it is better to think in terms of two separate streams of events, one consisting of keyPressed and keyReleased events and the other consisting of keyTyped events. For some applications, you want to monitor the first stream; for other applications, you want to monitor the second one. Of course, the information in the keyTyped stream could be extracted from the keyPressed/keyReleased stream, but it would be difficult (and also system-dependent to some extent). Some user actions, such as pressing the Shift key, can only be detected as keyPressed events. I used to have a computer solitaire game that highlighted every card that could be moved, when I held down the Shift key. You can do something like that in Java by highlighting the cards when the Shift key is pressed and removing the highlight when the Shift key is released.

There is one more complication. Usually, when you hold down a key on the keyboard, that key will auto-repeat. This means that it will generate multiple keyPressed events with just one keyReleased at the end of the sequence. It can also generate multiple keyTyped events. For the most part, this will not affect your programming, but you should not expect every keyPressed event to have a corresponding keyReleased event.

Every key on the keyboard has an integer code number. (Actually, this is only true for keys that Java knows about. Many keyboards have extra keys that can't be used with Java.) When the keyPressed or keyReleased method is called, the parameter, evt, contains the code of the key that was pressed or released. The code can be obtained by calling the function evt.getKeyCode(). Rather than asking you to memorize a table of code numbers, Java provides a named constant for each key. These constants are defined in the KeyEvent class. For example the constant for the shift key is KeyEvent.VK_SHIFT. If you want to test whether the key that the user pressed is the Shift key, you could say "if (evt.getKeyCode() == KeyEvent.VK_SHIFT)". The key codes for the four arrow keys are KeyEvent.VK_LEFT, KeyEvent.VK_RIGHT, KeyEvent.VK_UP, and KeyEvent.VK_DOWN. Other keys have similar codes. (The "VK" stands for "Virtual Keyboard". In reality, different keyboards use different key codes, but Java translates the actual codes from the keyboard into its own "virtual" codes. Your program only sees these virtual key codes, so it will work with various keyboards on various platforms without modification.)

In the case of a keyTyped event, you want to know which character was typed. This information can be obtained from the parameter, evt, in the keyTyped method by calling the function evt.getKeyChar(). This function returns a value of type char representing the character that was typed.

In the KeyboardAndFocusDemo program, I use the keyPressed routine to respond when the user presses one of the arrow keys. The program includes instance variables, squareLeft and squareTop, that give the position of the upper left corner of the movable square. When the user presses one of the arrow keys, the keyPressed routine modifies the appropriate instance variable and calls repaint() to redraw the panel with the square in its new position. Note that the values of squareLeft and squareTop are restricted so that the square never moves outside the white area of the panel:

/**
* This is called each time the user presses a key while the panel has
* the input focus.  If the key pressed was one of the arrow keys,
* the square is moved (except that it is not allowed to move off the
* edge of the panel, allowing for a 3-pixel border).
*/
public void keyPressed(KeyEvent evt) {

int key = evt.getKeyCode();  // keyboard code for the pressed key

if (key == KeyEvent.VK_LEFT) {  // left-arrow key; move the square left
squareLeft -= 8;
if (squareLeft < 3)
squareLeft = 3;
repaint();
}
else if (key == KeyEvent.VK_RIGHT) {  // right-arrow key; move the square right
squareLeft += 8;
if (squareLeft > getWidth() - 3 - SQUARE_SIZE)
squareLeft = getWidth() - 3 - SQUARE_SIZE;
repaint();
}
else if (key == KeyEvent.VK_UP) {  // up-arrow key; move the square up
squareTop -= 8;
if (squareTop < 3)
squareTop = 3;
repaint();
}
else if (key == KeyEvent.VK_DOWN) {  // down-arrow key; move the square down
squareTop += 8;
if (squareTop > getHeight() - 3 - SQUARE_SIZE)
squareTop = getHeight() - 3 - SQUARE_SIZE;
repaint();
}

}  // end keyPressed()

Color changes -- which happen when the user types the characters 'R', 'G', 'B', and 'K', or the lower case equivalents -- are handled in the keyTyped method. I won't include it here, since it is so similar to the keyPressed method. Finally, to complete the KeyListener interface, the keyReleased method must be defined. In the sample program, the body of this method is empty since the program does nothing in response to keyReleased events.

6.4.3  Focus Events

If a component is to change its appearance when it has the input focus, it needs some way to know when it has the focus. In Java, objects are notified about changes of input focus by events of type FocusEvent. An object that wants to be notified of changes in focus can implement the FocusListener interface. This interface declares two methods:

public void focusGained(FocusEvent evt);
public void focusLost(FocusEvent evt);

Furthermore, the addFocusListener() method must be used to set up a listener for the focus events. When a component gets the input focus, it calls the focusGained() method of any registered with FocusListener. When it loses the focus, it calls the listener's focusLost() method.

In the sample KeyboardAndFocusDemo program, the response to a focus event is simply to redraw the panel. The paintComponent() method checks whether the panel has the input focus by calling the boolean-valued function hasFocus(), which is defined in the Component class, and it draws a different picture depending on whether or not the panel has the input focus. The net result is that the appearance of the panel changes when the panel gains or loses focus. The methods from the FocusListener interface are defined simply as:

public void focusGained(FocusEvent evt) {
// The panel now has the input focus.
repaint();  // will redraw with a new message and a cyan border
}

public void focusLost(FocusEvent evt) {
// The panel has now lost the input focus.
repaint();  // will redraw with a new message and a gray border
}

The other aspect of handling focus is to make sure that the panel actually gets the focus. In this case, I called requestFocusInWindow() for the panel in the program's main() routine, just after opening the window. This approach works because there is only one component in the window, and it should have focus as long as the window is active. If the user clicks over to another window while using the program, the window becomes inactive and the panel loses focus temporarily, but gets is back when the user clicks back to the program window.

There are still decisions to be made about the overall structure of the program. In this case, I decided to use a nested class named Listener to define an object that listens for both focus and key events. In the constructor for the panel, I create an object of type Listener and register it to listen for both key events and focus events from the panel. See the source code for full details.

6.4.4  State Machines

The information stored in an object's instance variables is said to represent the state of that object. When one of the object's methods is called, the action taken by the object can depend on its state. (Or, in the terminology we have been using, the definition of the method can look at the instance variables to decide what to do.) Furthermore, the state can change. (That is, the definition of the method can assign new values to the instance variables.) In computer science, there is the idea of a state machine, which is just something that has a state and can change state in response to events or inputs. The response of a state machine to an event depends on what state it's in when the event occurs. An object is a kind of state machine. Sometimes, this point of view can be very useful in designing classes.

The state machine point of view can be especially useful in the type of event-oriented programming that is required by graphical user interfaces. When designing a GUI program, you can ask yourself: What information about state do I need to keep track of? What events can change the state of the program? How will my response to a given event depend on the current state? Should the appearance of the GUI be changed to reflect a change in state? How should the paintComponent() method take the state into account? All this is an alternative to the top-down, step-wise-refinement style of program design, which does not apply to the overall design of an event-oriented program.

In the KeyboardAndFocusDemo program, shown above, the state of the program is recorded in the instance variables squareColor, squareLeft, and squareTop. These state variables are used in the paintComponent() method to decide how to draw the panel. Their values are changed in the two key-event-handling methods.

In the rest of this section, we'll look at another example, where the state plays an even bigger role. In this example, the user plays a simple arcade-style game by pressing the arrow keys. The program is defined in the source code file SubKiller.java. As usual, it would be a good idea to compile and run the program as well as read the full source code. Here is a picture:

The program shows a black "submarine" near the bottom of the panel. While the panel has the input focus, this submarine moves back and forth erratically near the bottom. Near the top, there is a blue "boat." You can move this boat back and forth by pressing the left and right arrow keys. Attached to the boat is a red "bomb" (or "depth charge"). You can drop the bomb by hitting the down arrow key. The objective is to blow up the submarine by hitting it with the bomb. If the bomb falls off the bottom of the screen, you get a new one. If the submarine explodes, a new sub is created and you get a new bomb. Try it! Make sure to hit the sub at least once, so you can see the explosion.

Let's think about how this game can be programmed. First of all, since we are doing object-oriented programming, I decided to represent the boat, the depth charge, and the submarine as objects. Each of these objects is defined by a separate nested class inside the main panel class, and each object has its own state which is represented by the instance variables in the corresponding class. I use variables boat, bomb, and sub in the panel class to refer to the boat, bomb, and submarine objects.

Now, what constitutes the "state" of the program? That is, what things change from time to time and affect the appearance or behavior of the program? Of course, the state includes the positions of the boat, submarine, and bomb, so those objects have instance variables to store the positions. Anything else, possibly less obvious? Well, sometimes the bomb is falling, and sometimes it's not. That is a difference in state. Since there are two possibilities, I represent this aspect of the state with a boolean variable in the bomb object, bomb.isFalling. Sometimes the submarine is moving left and sometimes it is moving right. The difference is represented by another boolean variable, sub.isMovingLeft. Sometimes, the sub is exploding. This is also part of the state, and it is represented by a boolean variable, sub.isExploding. However, the explosions require a little more thought. An explosion is something that takes place over a series of frames. While an explosion is in progress, the sub looks different in each frame, as the size of the explosion increases. Also, I need to know when the explosion is over so that I can go back to moving and drawing the sub as usual. So, I use an integer variable, sub.explosionFrameNumber to record how many frames have been drawn since the explosion started; the value of this variable is used only when an explosion is in progress.

How and when do the values of these state variables change? Some of them seem to change on their own: For example, as the sub moves left and right, the state variables that specify its position change. Of course, these variables are changing because of an animation, and that animation is driven by a timer. Each time an event is generated by the timer, some of the state variables have to change to get ready for the next frame of the animation. The changes are made by the action listener that listens for events from the timer. The boat, bomb, and sub objects each contain an updateForNextFrame() method that updates the state variables of the object to get ready for the next frame of the animation. The action listener for the timer calls these methods with the statements

boat.updateForNewFrame();
bomb.updateForNewFrame();
sub.updateForNewFrame();

The action listener also calls repaint(), so that the panel will be redrawn to reflect its new state. There are several state variables that change in these update methods, in addition to the position of the sub: If the bomb is falling, then its y-coordinate increases from one frame to the next. If the bomb hits the sub, then the isExploding variable of the sub changes to true, and the isFalling variable of the bomb becomes false. The isFalling variable also becomes false when the bomb falls off the bottom of the screen. If the sub is exploding, then its explosionFrameNumber increases from one frame to the next, and when it reaches a certain value, the explosion ends and isExploding is reset to false. At random times, the sub switches between moving to the left and moving to the right. Its direction of motion is recorded in the sub's isMovingLeft variable. The sub's updateForNewFrame() method includes these lines to change the value of isMovingLeft at random times:

if ( Math.random() < 0.04 )
isMovingLeft = ! isMovingLeft;

There is a 1 in 25 chance that Math.random() will be less than 0.04, so the statement "isMovingLeft = ! isMovingLeft" is executed in one in every twenty-five frames, on average. The effect of this statement is to reverse the value of isMovingLeft, from false to true or from true to false. That is, the direction of motion of the sub is reversed.

In addition to changes in state that take place from one frame to the next, a few state variables change when the user presses certain keys. In the program, this is checked in a method that responds to user keystrokes. If the user presses the left or right arrow key, the position of the boat is changed. If the user presses the down arrow key, the bomb changes from not-falling to falling. This is coded in the keyPressed()method of a KeyListener that is registered to listen for key events on the panel; that method reads as follows:

public void keyPressed(KeyEvent evt) {
int code = evt.getKeyCode();  // which key was pressed.
if (code == KeyEvent.VK_LEFT) {
// Move the boat left.  (If this moves the boat out of the frame, its
// position will be adjusted in the boat.updateForNewFrame() method.)
boat.centerX -= 15;
}
else if (code == KeyEvent.VK_RIGHT) {
// Move the boat right.  (If this moves boat out of the frame, its
// position will be adjusted in the boat.updateForNewFrame() method.)
boat.centerX += 15;
}
else if (code == KeyEvent.VK_DOWN) {
// Start the bomb falling, if it is not already falling.
if ( bomb.isFalling == false )
bomb.isFalling = true;
}
}

Note that it's not necessary to call repaint() in this method, since this panel shows an animation that is constantly being redrawn anyway. Any changes in the state will become visible to the user as soon as the next frame is drawn. At some point in the program, I have to make sure that the user does not move the boat off the screen. I could have done this in keyPressed(), but I choose to check for this in another routine, in the boat object.

The program uses four listeners, to respond to action events from the timer, key events from the user, focus events, and mouse events. In this program, the user must click the panel to start the game. The game is programmed to run as long as the panel has the input focus. In this example, the program does not automatically request the focus; the user has to do it. When the user clicks the panel, the mouse listener requests the input focus and the game begins. The timer runs only when the panel has the input focus; this is programmed by having the focus listener start the timer when the panel gains the input focus and stop the timer when the panel loses the input focus. All four listeners are created in the constructor of the SubKillerPanel class using anonymous inner classes. (See Subsection 6.3.5.)

I encourage you to read the source code in SubKiller.java. Although a few points are tricky, you should with some effort be able to read and understand the entire program. Try to understand the program in terms of state machines. Note how the state of each of the three objects in the program changes in response to events from the timer and from the user.

While it's not at all sophisticated as arcade games go, the SubKiller game does use some interesting programming. And it nicely illustrates how to apply state-machine thinking in event-oriented programming.

Section 6.5

Basic Components

In preceding sections, you've seen how to use a graphics context to draw on the screen and how to handle mouse events and keyboard events. In one sense, that's all there is to GUI programming. If you're willing to program all the drawing and handle all the mouse and keyboard events, you have nothing more to learn. However, you would either be doing a lot more work than you need to do, or you would be limiting yourself to very simple user interfaces. A typical user interface uses standard GUI components such as buttons, scroll bars, text-input boxes, and menus. These components have already been written for you, so you don't have to duplicate the work involved in developing them. They know how to draw themselves, and they can handle the details of processing the mouse and keyboard events that concern them.

Consider one of the simplest user interface components, a push button. The button has a border, and it displays some text. This text can be changed. Sometimes the button is disabled, so that clicking on it doesn't have any effect. When it is disabled, its appearance changes. When the user clicks on the push button, the button changes appearance while the mouse button is pressed and changes back when the mouse button is released. In fact, it's more complicated than that. If the user moves the mouse outside the push button before releasing the mouse button, the button changes to its regular appearance. To implement this, it is necessary to respond to mouse exit or mouse drag events. Furthermore, on many platforms, a button can receive the input focus. The button changes appearance when it has the focus. If the button has the focus and the user presses the space bar, the button is triggered. This means that the button must respond to keyboard and focus events as well.

Fortunately, you don't have to program any of this, provided you use an object belonging to the standard class javax.swing.JButton. A JButton object draws itself and processes mouse, keyboard, and focus events on its own. You only hear from the JButton when the user triggers it by clicking on it or pressing the space bar while the button has the input focus. When this happens, the JButton object creates an event object belonging to the class java.awt.event.ActionEvent. The event object is sent to any registered listeners to tell them that the button has been pushed. Your program gets only the information it needs -- the fact that a button was pushed.

The standard components that are defined as part of the Swing graphical user interface API are defined by subclasses of the class JComponent, which is itself a subclass of Component. (Note that this includes the JPanel class that we have already been working with extensively.) Many useful methods are defined in the Component and JComponent classes and so can be used with any Swing component. We begin by looking at a few of these methods. Suppose that comp is a variable that refers to some JComponent. Then the following methods can be used:

- comp.getWidth() and comp.getHeight() are functions that give the current size of the component, in pixels. One warning: When a component is first created, its size is zero. The size will be set later, probably by a layout manager. A common mistake is to check the size of a component before that size has been set, such as in a constructor.

- comp.setEnabled(true) and comp.setEnabled(false) can be used to enable and disable the component. When a component is disabled, its appearance might change, and the user cannot do anything with it. There is a boolean-valued function, comp.isEnabled() that you can call to discover whether the component is enabled.

- comp.setVisible(true) and comp.setVisible(false) can be called to hide or show the component.

- comp.setFont(font) sets the font that is used for text displayed on the component. See Subsection 6.2.3 for a discussion of fonts.

- comp.setBackground(color) and comp.setForeground(color) set the background and foreground colors for the component. See Subsection 6.2.2.

- comp.setOpaque(true) tells the component that the area occupied by the component should be filled with the component's background color before the content of the component is painted. By default, only JLabels are non-opaque. A non-opaque, or "transparent", component ignores its background color and simply paints its content over the content of its container. This usually means that it inherits the background color from its container.

- comp.setToolTipText(string) sets the specified string as a "tool tip" for the component. The tool tip is displayed if the mouse cursor is in the component and the mouse is not moved for a few seconds. The tool tip should give some information about the meaning of the component or how to use it.

- comp.setPreferredSize(size) sets the size at which the component should be displayed, if possible. The parameter is of type java.awt.Dimension, where an object of type Dimension has two public integer-valued instance variables, width and height. A call to this method usually looks something like "setPreferredSize( new Dimension(100,50) )". The preferred size is used as a hint by layout managers, but will not be respected in all cases. Standard components generally compute a correct preferred size automatically, but it can be useful to set it in some cases. For example, if you use a JPanel as a drawing surface, it is usually a good idea to set a preferred size for it, since its default preferred size is zero.

Note that using any component is a multi-step process. The component object must be created with a constructor. It must be added to a container. In many cases, a listener must be registered to respond to events from the component. And in some cases, a reference to the component must be saved in an instance variable so that the component can be manipulated by the program after it has been created. In this section, we will look at a few of the basic standard components that are available in Swing. In the next section we will consider the problem of laying out components in containers.

6.5.1  JButton

An object of class JButton is a push button that the user can click to trigger some action. You've already seen buttons used in Section 6.1, but we consider them in much more detail here. To use any component effectively, there are several aspects of the corresponding class that you should be familiar with. For JButton, as an example, I list these aspects explicitly:

- Constructors: The JButton class has a constructor that takes a string as a parameter. This string becomes the text displayed on the button. For example: stopGoButton = new JButton("Go"). This creates a button object that will display the text, "Go" (but remember that the button must still be added to a container before it can appear on the screen).

- Events: When the user clicks on a button, the button generates an event of type ActionEvent. This event is sent to any listener that has been registered with the button as an ActionListener.

- Listeners: An object that wants to handle events generated by buttons must implement the ActionListener interface. This interface defines just one method, "public void actionPerformed(ActionEvent evt)", which is called to notify the object of an action event.

- Registration of Listeners: In order to actually receive notification of an event from a button, an ActionListener must be registered with the button. This is done with the button's addActionListener() method. For example: stopGoButton.addActionListener( buttonHandler );

- Event methods: When actionPerformed(evt) is called by the button, the parameter, evt, contains information about the event. This information can be retrieved by calling methods in the ActionEvent class. In particular, evt.getActionCommand() returns a String giving the command associated with the button. By default, this command is the text that is displayed on the button, but it is possible to set it to some other string. The method evt.getSource() returns a reference to the object that produced the event, that is, to the JButton that was pressed. The return value is of type Object, not JButton, because other types of components can also produce ActionEvents.

- Component methods: Several useful methods are defined in the JButton class, in addition to the standard Component methods. For example, stopGoButton.setText("Stop") changes the text displayed on the button to "Stop". And stopGoButton.setActionCommand("sgb") changes the action command associated with this button for action events. The setEnabled() and setText() methods are particularly useful for giving the user information about what is going on in the program. A disabled button is better than a button that gives an obnoxious error message such as "Sorry, you can't click on me now!"

6.5.2  JLabel

JLabel is certainly the simplest type of component. An object of type JLabel exists just to display a line of text. The text cannot be edited by the user, although it can be changed by your program. The constructor for a JLabel specifies the text to be displayed:

JLabel message = new JLabel("Hello World!");

There is another constructor that specifies where in the label the text is located, if there is extra space. The possible alignments are given by the constants JLabel.LEFT, JLabel.CENTER, and JLabel.RIGHT. For example,

JLabel message = new JLabel("Hello World!", JLabel.CENTER);

creates a label whose text is centered in the available space. You can change the text displayed in a label by calling the label's setText() method:

message.setText("Goodbye World!");

Since the JLabel class is a subclass of JComponent, you can use methods such as setForeground() and setFont() with labels. If you want the background color to have any effect, you should call setOpaque(true) on the label, since otherwise the JLabel might not fill in its background. For example:

JLabel message = new JLabel("Hello World!", JLabel.CENTER);
message.setForeground(Color.RED);   // Display red text...
message.setBackground(Color.BLACK); //    on a black background...
message.setFont(new Font("Serif",Font.BOLD,18));  // in a big bold font.
message.setOpaque(true);  // Make sure background is filled in.

6.5.3  JCheckBox

A JCheckBox is a component that has two states: selected or unselected. The user can change the state of a check box by clicking on it. The state of a checkbox is represented by a boolean value that is true if the box is selected and is false if the box is unselected. A checkbox has a label, which is specified when the box is constructed:

JCheckBox showTime = new JCheckBox("Show Current Time");

Usually, it's the user who sets the state of a JCheckBox, but you can also set the state programmatically. The current state of a checkbox is set using its setSelected(boolean) method. For example, if you want the checkbox showTime to be checked, you would say "showTime.setSelected(true)". To uncheck the box, say "showTime.setSelected(false)". You can determine the current state of a checkbox by calling its isSelected() method, which returns a boolean value.

In many cases, you don't need to worry about events from checkboxes. Your program can just check the state whenever it needs to know it by calling the isSelected() method. However, a checkbox does generate an event when its state is changed by the user, and you can detect this event and respond to it if you want something to happen at the moment the state changes. When the state of a checkbox is changed by the user, it generates an event of type ActionEvent. If you want something to happen when the user changes the state, you must register an ActionListener with the checkbox by calling its addActionListener() method. (Note that if you change the state by calling the setSelected() method, no ActionEvent is generated. However, there is another method in the JCheckBox class, doClick(), which simulates a user click on the checkbox and does generate an ActionEvent.)

When handling an ActionEvent, you can call evt.getSource() in the actionPerformed() method to find out which object generated the event. (Of course, if you are only listening for events from one component, you don't have to do this.) The returned value is of type Object, but you can type-cast it to another type if you want. Once you know the object that generated the event, you can ask the object to tell you its current state. For example, if you know that the event had to come from one of two checkboxes, cb1 or cb2, then your actionPerformed() method might look like this:

public void actionPerformed(ActionEvent evt) {
Object source = evt.getSource();
if (source == cb1) {
boolean newState = cb1.isSelected();
... // respond to the change of state
}
else if (source == cb2) {
boolean newState = cb2.isSelected();
... // respond to the change of state
}
}

Alternatively, you can use evt.getActionCommand() to retrieve the action command associated with the source. For a JCheckBox, the action command is, by default, the label of the checkbox.

6.5.4  JTextField and JTextArea

The JTextField and JTextArea classes represent components that contain text that can be edited by the user. A JTextField holds a single line of text, while a JTextArea can hold multiple lines. It is also possible to set a JTextField or JTextArea to be read-only so that the user can read the text that it contains but cannot edit the text. Both classes are subclasses of an abstract class, JTextComponent, which defines their common properties.

JTextField and JTextArea have many methods in common. The instance method setText(), which takes a parameter of type String, can be used to change the text that is displayed in an input component. The contents of the component can be retrieved by calling its getText() instance method, which returns a value of type String. If you want to stop the user from modifying the text, you can call setEditable(false). Call the same method with a parameter of true to make the input component user-editable again.

The user can only type into a text component when it has the input focus. The user can give the input focus to a text component by clicking it with the mouse, but sometimes it is useful to give the input focus to a text field programmatically. You can do this by calling its requestFocusInWindow() method. For example, when I discover an error in the user's input, I usually call requestFocusInWindow() on the text field that contains the error. This helps the user see where the error occurred and lets the user start typing the correction immediately.

By default, there is no space between the text in a text component and the edge of the component, which usually doesn't look very good. You can use the setMargin() method of the component to add some blank space between the edge of the component and the text. This method takes a parameter of type java.awt.Insets which contains four integer instance variables that specify the margins on the top, left, bottom, and right edge of the component. For example,

textComponent.setMargin( new Insets(5,5,5,5) );

adds a five-pixel margin between the text in textComponent and each edge of the component.

The JTextField class has a constructor

public JTextField(int columns)

where columns is an integer that specifies the number of characters that should be visible in the text field. This is used to determine the preferred width of the text field. (Because characters can be of different sizes and because the preferred width is not always respected, the actual number of characters visible in the text field might not be equal to columns.) You don't have to specify the number of columns; for example, you might use the text field in a context where it will expand to fill whatever space is available. In that case, you can use the default constructor JTextField(), with no parameters. You can also use the following constructors, which specify the initial contents of the text field:

public JTextField(String contents);
public JTextField(String contents, int columns);

The constructors for a JTextArea are

public JTextArea()
public JTextArea(int rows, int columns)
public JTextArea(String contents)
public JTextArea(String contents, int rows, int columns)

The parameter rows specifies how many lines of text should be visible in the text area. This determines the preferred height of the text area, just as columns determines the preferred width. However, the text area can actually contain any number of lines; the text area can be scrolled to reveal lines that are not currently visible. It is common to use a JTextArea as the CENTER component of a BorderLayout. In that case, it is less useful to specify the number of lines and columns, since the TextArea will expand to fill all the space available in the center area of the container.

The JTextArea class adds a few useful methods to those inherited from JTextComponent. For example, the instance method append(moreText), where moreText is of type String, adds the specified text at the end of the current content of the text area. (When using append() or setText() to add text to a JTextArea, line breaks can be inserted in the text by using the newline character, '\n'.) And setLineWrap(wrap), where wrap is of type boolean, tells what should happen when a line of text is too long to be displayed in the text area. If wrap is true, then any line that is too long will be "wrapped" onto the next line; if wrap is false, the line will simply extend outside the text area, and the user will have to scroll the text area horizontally to see the entire line. The default value of wrap is false.

Since it might be necessary to scroll a text area to see all the text that it contains, you might expect a text area to come with scroll bars. Unfortunately, this does not happen automatically. To get scroll bars for a text area, you have to put the JTextArea inside another component, called a JScrollPane. This can be done as follows:

JTextArea inputArea = new JTextArea();
JScrollPane scroller = new JScrollPane( inputArea );

The scroll pane provides scroll bars that can be used to scroll the text in the text area. The scroll bars will appear only when needed, that is when the size of the text exceeds the size of the text area. Note that when you want to put the text area into a container, you should add the scroll pane, not the text area itself, to the container. See the program TextAreaDemo.java for a very short example of using a text area in a scroll pane.

When the user is typing in a JTextField and presses return, an ActionEvent is generated. If you want to respond to such events, you can register an ActionListener with the text field, using the text field's addActionListener() method. (Since a JTextArea can contain multiple lines of text, pressing return in a text area does not generate an event; it simply begins a new line of text.)

JTextField has a subclass, JPasswordField, which is identical except that it does not reveal the text that it contains. The characters in a JPasswordField are all displayed as asterisks (or some other fixed character). A password field is, obviously, designed to let the user enter a password without showing that password on the screen.

Text components are actually quite complex, and I have covered only their most basic properties here. I will return to the topic of text components in Chapter 13.

6.5.5  JSlider

A JSlider provides a way for the user to select an integer value from a range of possible values. The user does this by dragging a "knob" along a bar. A slider can, optionally, be decorated with tick marks and with labels. This picture, from the sample program SliderDemo.java, shows three sliders with different decorations and with different ranges of values:

Here, the second slider is decorated with tick marks, and the third one is decorated with labels. It's possible for a single slider to have both types of decorations.

The most commonly used constructor for JSliders specifies the start and end of the range of values for the slider and its initial value when it first appears on the screen:

public JSlider(int minimum, int maximum, int value)

If the parameters are omitted, the values 0, 100, and 50 are used. By default, a slider is horizontal, but you can make it vertical by calling its method setOrientation(JSlider.VERTICAL). The current value of a JSlider can be read at any time with its getValue() method, which returns a value of type int. If you want to change the value, you can do so with the method setValue(n), which takes a parameter of type int.

If you want to respond immediately when the user changes the value of a slider, you can register a listener with the slider. JSliders, unlike other components we have seen, do not generate ActionEvents. Instead, they generate events of type ChangeEvent. ChangeEvent and related classes are defined in the package javax.swing.event rather than java.awt.event, so if you want to use ChangeEvents, you should import javax.swing.event.* at the beginning of your program. You must also define some object to implement the ChangeListener interface, and you must register the change listener with the slider by calling its addChangeListener() method. A ChangeListener must provide a definition for the method:

public void stateChanged(ChangeEvent evt)

This method will be called whenever the value of the slider changes. Note that it will be called when you change the value with the setValue() method, as well as when the user changes the value. In the stateChanged() method, you can call evt.getSource() to find out which object generated the event. If you want to know whether the user generated the change event, call the slider's getValueIsAdjusting() method, which returns true if the user is dragging the knob on the slider.

Using tick marks on a slider is a two-step process: Specify the interval between the tick marks, and tell the slider that the tick marks should be displayed. There are actually two types of tick marks, "major" tick marks and "minor" tick marks. You can have one or the other or both. Major tick marks are a bit longer than minor tick marks. The method setMinorTickSpacing(i) indicates that there should be a minor tick mark every i units along the slider. The parameter is an integer. (The spacing is in terms of values on the slider, not pixels.) For the major tick marks, there is a similar command, setMajorTickSpacing(i). Calling these methods is not enough to make the tick marks appear. You also have to call setPaintTicks(true). For example, the second slider in the above illustration was created and configured using the commands:

slider2 = new JSlider();  // (Uses default min, max, and value.)
slider2.addChangeListener(this);
slider2.setMajorTickSpacing(25);
slider2.setMinorTickSpacing(5);
slider2.setPaintTicks(true);

Labels on a slider are handled similarly. You have to specify the labels and tell the slider to paint them. Specifying labels is a tricky business, but the JSlider class has a method to simplify it. You can create a set of labels and add them to a slider named sldr with the command:

sldr.setLabelTable( sldr.createStandardLabels(i) );

where i is an integer giving the spacing between the labels. To arrange for the labels to be displayed, call setPaintLabels(true). For example, the third slider in the above illustration was created and configured with the commands:

slider3 = new JSlider(2000,2100,2014);
slider3.addChangeListener(this);
slider3.setLabelTable( slider3.createStandardLabels(50) );
slider3.setPaintLabels(true);

Section 6.6

Basic Layout

Components are the fundamental building blocks of a graphical user interface. But you have to do more with components besides create them. Another aspect of GUI programming is laying out components on the screen, that is, deciding where they are drawn and how big they are. You have probably noticed that computing coordinates can be a difficult problem, especially if you don't assume a fixed size for the drawing area. Java has a solution for this, as well.

Components are the visible objects that make up a GUI. Some components are containers, which can hold other components. Containers in Java are objects that belong to some subclass of java.awt.Container. The content pane of a JFrame is an example of a container. The standard class JPanel, which we have mostly used as a drawing surface up until now, is another example of a container.

Because a JPanel object is a container, it can hold other components. Because a JPanel is itself a component, you can add a JPanel to another JPanel. This makes complex nesting of components possible. JPanels can be used to organize complicated user interfaces, as shown in this illustration:

In this picture, a large panel holds two smaller panels. Each of the two smaller panels in turn holds three components.

The components in a container must be "laid out," which means setting their sizes and positions. It's possible to program the layout yourself, but layout is ordinarily done by a layout manager. A layout manager is an object associated with a container that implements some policy for laying out the components in that container. Different types of layout manager implement different policies. In this section, we will cover the three most common types of layout manager, and then we will look at several programming examples that use components and layout.

Every container has a default layout manager and has an instance method, setLayout(), that takes a parameter of type LayoutManager and that is used to specify a different layout manager for the container. Components are added to a container by calling an instance method named add() in the container object. There are actually several versions of the add() method, with different parameter lists. Different versions of add() are appropriate for different layout managers, as we will see below.

6.6.1  Basic Layout Managers

Java has a variety of standard layout managers that can be used as parameters in the setLayout() method. They are defined by classes in the package java.awt. Here, we will look at just three of these layout manager classes: FlowLayout, BorderLayout, and GridLayout.

A FlowLayout simply lines up components in a row across the container. The size of each component is equal to that component's "preferred size." After laying out as many items as will fit in a row across the container, the layout manager will move on to the next row. The default layout for a JPanel is a FlowLayout; that is, a JPanel uses a FlowLayout unless you specify a different layout manager by calling the panel's setLayout() method.

The components in a given row can be either left-aligned, right-aligned, or centered within that row, and there can be horizontal and vertical gaps between components. If the default constructor, "new FlowLayout()", is used, then the components on each row will be centered and both the horizontal and the vertical gaps will be five pixels. The constructor

public FlowLayout(int align, int hgap, int vgap)

can be used to specify alternative alignment and gaps. The possible values of align are FlowLayout.LEFT, FlowLayout.RIGHT, and FlowLayout.CENTER.

Suppose that container is a container object that is using a FlowLayout as its layout manager. Then, a component, comp, can be added to the container with the statement

container.add(comp);

The FlowLayout will line up all the components that have been added to the container in this way. They will be lined up in the order in which they were added. For example, this picture shows five buttons in a panel that uses a FlowLayout:

Note that since the five buttons will not fit in a single row across the panel, they are arranged in two rows. In each row, the buttons are grouped together and are centered in the row. The buttons were added to the panel using the statements:

panel.add(button1);
panel.add(button2);
panel.add(button3);
panel.add(button4);
panel.add(button5);

When a container uses a layout manager, the layout manager is ordinarily responsible for computing the preferred size of the container (although a different preferred size could be set by calling the container's setPreferredSize method). A FlowLayout prefers to put its components in a single row, so the preferred width is the total of the preferred widths of all the components, plus the horizontal gaps between the components. The preferred height is the maximum preferred height of all the components.

A BorderLayout layout manager is designed to display one large, central component, with up to four smaller components arranged around the edges of the central component. If a container, cntr, is using a BorderLayout, then a component, comp, should be added to the container using a statement of the form

cntr.add( comp, borderLayoutPosition );

where borderLayoutPosition specifies what position the component should occupy in the layout and is given as one of the constants BorderLayout.CENTER, BorderLayout.NORTH, BorderLayout.SOUTH, BorderLayout.EAST, or BorderLayout.WEST. The meaning of the five positions is shown in this diagram:

Note that a border layout can contain fewer than five components, so that not all five of the possible positions need to be filled. It would be very unusual, however, to have no center component.

A BorderLayout sets the sizes of its components as follows: The NORTH and SOUTH components (if present) are shown at their preferred heights, but their width is set equal to the full width of the container. The EAST and WEST components are shown at their preferred widths, but their height is set to the height of the container, minus the space occupied by the NORTH and SOUTH components. Finally, the CENTER component takes up any remaining space. The preferred size of the CENTER component is ignored when the layout is done, but it is taken into account when the preferred size of the container as a whole is computed. You should make sure that the components that you put into a BorderLayout are suitable for the positions that they will occupy. A horizontal slider or text field, for example, would work well in the NORTH or SOUTH position, but wouldn't make much sense in the EAST or WEST position.

The default constructor, new BorderLayout(), leaves no space between components. If you would like to leave some space, you can specify horizontal and vertical gaps in the constructor of the BorderLayout object. For example, if you say

panel.setLayout(new BorderLayout(5,7));

then the layout manager will insert horizontal gaps of 5 pixels between components and vertical gaps of 7 pixels between components. The background color of the container will show through in these gaps. The default layout for the original content pane that comes with a JFrame is a BorderLayout with no horizontal or vertical gap.

Finally, we consider the GridLayout layout manager. A grid layout lays out components in a grid containing rows and columns of equal sized rectangles. This illustration shows how the components would be arranged in a grid layout with 4 rows and 3 columns:

If a container uses a GridLayout, the appropriate add method for the container takes a single parameter of type Component (for example: cntr.add(comp)). Components are added to the grid in the order shown; that is, each row is filled from left to right before going on the next row.

The constructor for a GridLayout takes the form "new GridLayout(R,C)", where R is the number of rows and C is the number of columns. If you want to leave horizontal gaps of H pixels between columns and vertical gaps of V pixels between rows, use "new GridLayout(R,C,H,V)" instead.

When you use a GridLayout, it's probably good form to add just enough components to fill the grid. However, this is not required. In fact, as long as you specify a non-zero value for the number of rows, then the number of columns is essentially ignored. The system will use just as many columns as are necessary to hold all the components that you add to the container. If you want to depend on this behavior, you should probably specify zero as the number of columns. You can also specify the number of rows as zero. In that case, you must give a non-zero number of columns. The system will use the specified number of columns, with just as many rows as necessary to hold the components that are added to the container.

Horizontal grids, with a single row, and vertical grids, with a single column, are very common. For example, suppose that button1, button2, and button3 are buttons and that you'd like to display them in a horizontal row in a panel. If you use a horizontal grid for the panel, then the buttons will completely fill that panel and will all be the same size. The panel can be created as follows:

JPanel buttonBar = new JPanel();
buttonBar.setLayout( new GridLayout(1,3) );
// (Note:  The "3" here is pretty much ignored, and
//  you could also say "new GridLayout(1,0)".
//  To leave gaps between the buttons, you could use
//  "new GridLayout(1,0,5,5)".)
buttonBar.add(button1);
buttonBar.add(button2);
buttonBar.add(button3);

You might find this button bar to be more attractive than the one that uses the default FlowLayout layout manager.

6.6.2  Borders

We have seen how to leave gaps between the components in a container, but what if you would like to leave a border around the outside of the container? This problem is not handled by layout managers. Instead, borders in Swing are represented by objects. A Border object can be added to any JComponent, not just to containers. Borders can be more than just empty space. The class javax.swing.BorderFactory contains a large number of static methods for creating border objects. For example, the function

BorderFactory.createLineBorder(Color.BLACK)

returns an object that represents a one-pixel wide black line around the outside of a component. If comp is a JComponent, a border can be added to comp using its setBorder() method. For example:

comp.setBorder( BorderFactory.createLineBorder(Color.BLACK) );

Once a border has been set for a JComponent, the border is drawn automatically, without any further effort on the part of the programmer. The border is drawn along the edges of the component, just inside its boundary. The layout manager of a JPanel or other container will take the space occupied by the border into account. The components that are added to the container will be displayed in the area inside the border. I don't recommend using a border on a JPanel that is being used as a drawing surface. However, if you do this, you should take the border into account. If you draw in the area occupied by the border, that part of your drawing will be covered by the border.

Here are some of the static methods that can be used to create borders:

- BorderFactory.createEmptyBorder(top,left,bottom,right) -- leaves an empty border around the edges of a component. Nothing is drawn in this space, so the background color of the component will appear in the area occupied by the border. The parameters are integers that give the width of the border along the top, left, bottom, and right edges of the component. This is actually very useful when used on a JPanel that contains other components. It puts some space between the components and the edge of the panel. It can also be useful on a JLabel, which otherwise would not have any space between the text and the edge of the label.

- BorderFactory.createLineBorder(color,thickness) -- draws a line around all four edges of a component. The first parameter is of type Color and specifies the color of the line. The second parameter is an integer that specifies the thickness of the border, in pixels. If the second parameter is omitted, a line of thickness 1 is drawn.

- BorderFactory.createMatteBorder(top,left,bottom,right,color) -- is similar to createLineBorder, except that you can specify individual thicknesses for the top, left, bottom, and right edges of the component.

- BorderFactory.createEtchedBorder() -- creates a border that looks like a groove etched around the boundary of the component. The effect is achieved using lighter and darker shades of the component's background color, and it does not work well with every background color.

- BorderFactory.createLoweredBevelBorder() -- gives a component a three-dimensional effect that makes it look like it is lowered into the computer screen. As with an EtchedBorder, this only works well for certain background colors.

- BorderFactory.createRaisedBevelBorder() -- similar to a LoweredBevelBorder, but the component looks like it is raised above the computer screen.

- BorderFactory.createTitledBorder(title) -- creates a border with a title. The title is a String, which is displayed in the upper left corner of the border.

There are many other methods in the BorderFactory class, most of them providing variations of the basic border styles given here. The following illustration shows six components with six different border styles. The text in each component is the command that created the border for that component:

(The source code for the program that produced this picture can be found in BorderDemo.java.)

6.6.3  SliderAndButtonDemo

Now that we have looked at components and layouts, it's time to put them together into some complete programs. We start with a simple demo that uses a JLabel, three JButtons, and a couple of JSliders, all laid out in a GridLayout, as shown in this picture:

The sliders in this program control the foreground and background color of the label, and the buttons control its font style. Writing this program is a matter of creating the components, laying them out, and programming listeners to respond to events from the sliders and buttons. My program is defined as a subclass of JPanel that implements ChangeListener and ActionListener, so that the panel itself can act as the listener for change events from the sliders and action events from the buttons. In the constructor, the six components are created and configured, a GridLayout is installed as the layout manager for the panel, and the components are added to the panel:

/* Create the display label, with properties to match the
values of the sliders and the setting of the combo box. */

displayLabel = new JLabel("Hello World!", JLabel.CENTER);
displayLabel.setOpaque(true);
displayLabel.setBackground( new Color(100,100,100) );
displayLabel.setForeground( Color.RED );
displayLabel.setFont( new Font("Serif", Font.BOLD, 30) );
displayLabel.setBorder(BorderFactory.createEmptyBorder(0,8,0,8));

/* Create the sliders, and set up the panel to listen for
ChangeEvents that are generated by the sliders. */

bgColorSlider = new JSlider(0,255,100);
bgColorSlider.addChangeListener(this);

fgColorSlider = new JSlider(0,100,0);
fgColorSlider.addChangeListener(this);

/* Create three buttons to control the font style, and set up the
panel to listen for ActionEvents from the buttons. */

JButton plainButton = new JButton("Plain Font");
plainButton.addActionListener(this);
JButton italicButton = new JButton("Italic Font");
italicButton.addActionListener(this);
JButton boldButton = new JButton("Bold Font");
boldButton.addActionListener(this);

/* Set the layout for the panel, and add the six components.
Use a GridLayout with 3 rows and 2 columns, and with
5 pixels between components. */

setLayout(new GridLayout(3,2,5,5));
add(displayLabel);
add(plainButton);
add(bgColorSlider);
add(italicButton);
add(fgColorSlider);
add(boldButton);

The class also defines the methods required by the ActionListener and ChangeListener interfaces. The actionPerformed() method is called when the user clicks one of the buttons. This method changes the font in the JLabel, where the font depends on which button was clicked. To determine which button was clicked, the method uses evt.getActionCommand(), which returns the text from the button:

public void actionPerformed(ActionEvent evt) {
String cmd = evt.getActionCommand();
if (cmd.equals("Plain Font")) {
displayLabel.setFont( new Font("Serif", Font.PLAIN, 30) );
}
else if (cmd.equals("Italic Font")) {
displayLabel.setFont( new Font("Serif", Font.ITALIC, 30) );
}
else if (cmd.equals("Bold Font")) {
displayLabel.setFont( new Font("Serif", Font.BOLD, 30) );
}
}

And the stateChanged() method, which is called when the user manipulates one of the sliders, uses the value on the slider to compute a new foreground or background color for the label. The method checks evt.getSource() to determine which slider was changed:

public void stateChanged(ChangeEvent evt) {
if (evt.getSource() == bgColorSlider) {
int bgVal = bgColorSlider.getValue();
displayLabel.setBackground( new Color(bgVal,bgVal,bgVal) );
// NOTE:  The background color is a shade of gray,
//        determined by the setting on the slider.
}
else {
float hue = fgColorSlider.getValue()/100.0f;
displayLabel.setForeground( Color.getHSBColor(hue, 1.0f, 1.0f) );
// Note:  The foreground color ranges through all the colors
// of the spectrum.
}
}

Note that the slider variables are global variables in the program because they are referenced in the stateChanged() method as well as in the constructor. On the other hand, the button variables are local variables in the constructor because that is the only place where they are used. The complete source code for this example is in the file SliderAndButtonDemo.java.

6.6.4  A Simple Calculator

As our next example, we look briefly at an example that uses nested subpanels to build a more complex user interface. The program has two JTextFields where the user can enter two numbers, four JButtons that the user can click to add, subtract, multiply, or divide the two numbers, and a JLabel that displays the result of the operation. Here is a picture from the program:

This example uses a panel with a GridLayout that has four rows and one column. In this case, the layout is created with the statement:

setLayout(new GridLayout(4,1,3,3));

which allows a 3-pixel gap between the rows where the gray background color of the panel is visible.

The first row of the grid layout actually contains two components, a JLabel displaying the text "x =" and a JTextField. A grid layout can only have one component in each position. In this case, the component in the first row is a JPanel, a subpanel that is nested inside the main panel. This subpanel in turn contains the label and text field. This can be programmed as follows:

xInput = new JTextField("0", 10); // Create a text field sized to hold 10 chars.
JPanel xPanel = new JPanel();     // Create the subpanel.
xPanel.add( new JLabel(" x = ")); // Add a label to the subpanel.
xPanel.add(xInput);               // Add the text field to the subpanel

add(xPanel);                      // Add the subpanel to the main panel.

The subpanel uses the default FlowLayout layout manager, so the label and text field are simply placed next to each other in the subpanel at their preferred size, and are centered in the subpanel.

Similarly, the third row of the grid layout is a subpanel that contains four buttons. In this case, the subpanel uses a GridLayout with one row and four columns, so that the buttons are all the same size and completely fill the subpanel.

One other point of interest in this example is the actionPerformed() method that responds when the user clicks one of the buttons. This method must retrieve the user's numbers from the text fields, perform the appropriate arithmetic operation on them (depending on which button was clicked), and set the text of the JLabel (named answer) to represent the result. However, the contents of the text fields can only be retrieved as strings, and these strings must be converted into numbers. If the conversion fails, the label is set to display an error message:

public void actionPerformed(ActionEvent evt) {

double x, y;  // The numbers from the input boxes.

try {
String xStr = xInput.getText();
x = Double.parseDouble(xStr);
}
catch (NumberFormatException e) {
// The string xStr is not a legal number.
answer.setText("Illegal data for x.");
xInput.requestFocusInWindow();
return;
}

try {
String yStr = yInput.getText();
y = Double.parseDouble(yStr);
}
catch (NumberFormatException e) {
// The string yStr is not a legal number.
answer.setText("Illegal data for y.");
yInput.requestFocusInWindow();
return;
}

/* Perform the operation based on the action command from the
button.  The action command is the text displayed on the button.
Note that division by zero produces an error message. */

String op = evt.getActionCommand();
if (op.equals("+"))
answer.setText( "x + y = " + (x+y) );
else if (op.equals("-"))
answer.setText( "x - y = " + (x-y) );
else if (op.equals("*"))
answer.setText( "x * y = " + (x*y) );
else if (op.equals("/")) {
if (y == 0)
answer.setText("Can't divide by zero!");
else
answer.setText( "x / y = " + (x/y) );
}

} // end actionPerformed()

The complete source code for this example can be found in SimpleCalc.java.

6.6.5  Using a null Layout

As mentioned above, it is possible to do without a layout manager altogether. For our next example, we'll look at a panel that does not use a layout manager. If you set the layout manager of a container to be null, by calling container.setLayout(null), then you assume complete responsibility for positioning and sizing the components in that container.

If comp is any component, then the statement

comp.setBounds(x, y, width, height);

puts the top left corner of the component at the point (x,y), measured in the coordinate system of the container that contains the component, and it sets the width and height of the component to the specified values. You should only set the bounds of a component if the container that contains it has a null layout manager. In a container that has a non-null layout manager, the layout manager is responsible for setting the bounds, and you should not interfere with its job.

Assuming that you have set the layout manager to null, you can call the setBounds() method any time you like. (You can even make a component that moves or changes size while the user is watching.) If you are writing a panel that has a known, fixed size, then you can set the bounds of each component in the panel's constructor. Note that you must also add the components to the panel, using the panel's add(component) instance method; otherwise, the component will not appear on the screen.

Our example contains four components: two buttons, a label, and a panel that displays a checkerboard pattern:

This is just an example of using a null layout; it doesn't do anything, except that clicking the buttons changes the text of the label. (We will use this example in Section 7.5 as a starting point for a checkers game.)

The panel in this program is defined by the class NullLayoutDemo, which is created as a subclass of JPanel. The four components are created and added to the panel in the constructor. Then the setBounds() method of each component is called to set the size and position of the component:

public NullLayoutDemo() {

setLayout(null);  // I will do the layout myself!

setBackground(new Color(0,120,0));  // A dark green background.

setBorder( BorderFactory.createEtchedBorder() );

setPreferredSize( new Dimension(350,240) );

/* Create the components and add them to the content pane.  If you
don't add them to a container, they won't appear, even if
you set their bounds! */

board = new Checkerboard();
// (Checkerboard is a subclass of JPanel, defined below as a static
//  nested class inside the main class.)
add(board);

newGameButton = new JButton("New Game");
newGameButton.addActionListener(this);
add(newGameButton);

resignButton = new JButton("Resign");
resignButton.addActionListener(this);
add(resignButton);

message = new JLabel("Click \"New Game\" to begin.");
message.setForeground( new Color(100,255,100) ); // Light green.
message.setFont(new Font("Serif", Font.BOLD, 14));
add(message);

/* Set the position and size of each component by calling
its setBounds() method. */

board.setBounds(20,20,164,164);
newGameButton.setBounds(210, 60, 120, 30);
resignButton.setBounds(210, 120, 120, 30);
message.setBounds(20, 200, 330, 30);

} // end constructor

It's fairly easy in this case to get a reasonable layout. It's much more difficult to do your own layout if you want to allow for changes of size. In that case, you have to respond to changes in the container's size by recomputing the sizes and positions of all the components that it contains. If you want to respond to changes in a container's size, you can register an appropriate listener with the container. Any component generates an event of type ComponentEvent when its size changes (and also when it is moved, hidden, or shown). You can register a ComponentListener with the container and respond to resize events by recomputing the sizes and positions of all the components in the container. Consult a Java reference for more information about ComponentEvents. However, my real advice is that if you want to allow for changes in the container's size, try to find a layout manager to do the work for you.

The complete source code for this example is in NullLayoutDemo.java.

6.6.6  A Little Card Game

For a final example, let's look at something a little more interesting as a program. The example is a simple card game in which you look at a playing card and try to predict whether the next card will be higher or lower in value. (Aces have the lowest value in this game.) You've seen a text-oriented version of the same game in Subsection 5.4.3. Section 5.4 also introduced Deck, Hand, and Card classes that are used by the program. In this GUI version of the game, you click on a button to make your prediction. If you predict wrong, you lose. If you make three correct predictions, you win. After completing one game, you can click the "New Game" button to start a new game. Here is what the program looks like in the middle of a game:

The complete source code for the panel can be found in the file HighLowGUI.java. I encourage you to compile and run it. Remember that you also need Card.java, Deck.java, and Hand.java, since they define classes that are used in the program.

The overall structure of the main panel in this example should be reasonably clear: It has three buttons in a subpanel at the bottom of the main panel and a large drawing surface that displays the cards and a message. (The cards and message are not components in this example; they are drawn using the graphics context in the panel's paintComponent() method.) The main panel uses a BorderLayout. The drawing surface occupies the CENTER position of the border layout. The subpanel that contains the buttons occupies the SOUTH position of the border layout, and the other three positions of the borderlayout are empty.

The drawing surface is defined by a nested class named CardPanel, which is subclass of JPanel. I have chosen to let the drawing surface object do most of the work of the game: It listens for events from the three buttons and responds by taking the appropriate actions. The main panel is defined by HighLowGUI itself, which is also a subclass of JPanel. The constructor of the HighLowGUI class creates all the other components, sets up event handling, and lays out the components:

public HighLowGUI() {   // The constructor.

setBackground( new Color(130,50,40) );

setLayout( new BorderLayout(3,3) );  // BorderLayout with 3-pixel gaps.

CardPanel board = new CardPanel();  // Where the cards are drawn.
add(board, BorderLayout.CENTER);

JPanel buttonPanel = new JPanel();  // The subpanel that holds the buttons.
buttonPanel.setBackground( new Color(220,200,180) );
add(buttonPanel, BorderLayout.SOUTH);

JButton higher = new JButton( "Higher" );
higher.addActionListener(board);   // The CardPanel listens for events.
buttonPanel.add(higher);

JButton lower = new JButton( "Lower" );
lower.addActionListener(board);
buttonPanel.add(lower);

JButton newGame = new JButton( "New Game" );
newGame.addActionListener(board);
buttonPanel.add(newGame);

setBorder(BorderFactory.createLineBorder( new Color(130,50,40), 3) );

}  // end constructor

The programming of the drawing surface class, CardPanel, is a nice example of thinking in terms of a state machine. (See Subsection 6.4.4.) It is important to think in terms of the states that the game can be in, how the state can change, and how the response to events can depend on the state. The approach that produced the original, text-oriented game in Subsection 5.4.3 is not appropriate here. Trying to think about the game in terms of a process that goes step-by-step from beginning to end is more likely to confuse you than to help you.

The state of the game includes the cards and the message. The cards are stored in an object of type Hand. The message is a String. These values are stored in instance variables. There is also another, less obvious aspect of the state: Sometimes a game is in progress, and the user is supposed to make a prediction about the next card. Sometimes we are between games, and the user is supposed to click the "New Game" button. It's a good idea to keep track of this basic difference in state. The CardPanel class uses a boolean instance variable named gameInProgress for this purpose.

The state of the game can change whenever the user clicks on a button. The CardPanel class implements the ActionListener interface and defines an actionPerformed() method to respond to the user's clicks. This method simply calls one of three other methods, doHigher(), doLower(), or newGame(), depending on which button was pressed. It's in these three event-handling methods that the action of the game takes place.

We don't want to let the user start a new game if a game is currently in progress. That would be cheating. So, the response in the newGame() method is different depending on whether the state variable gameInProgress is true or false. If a game is in progress, the message instance variable should be set to be an error message. If a game is not in progress, then all the state variables should be set to appropriate values for the beginning of a new game. In any case, the board must be repainted so that the user can see that the state has changed. The complete newGame() method is as follows:

/**
* Called by the CardPanel constructor, and called by actionPerformed() if
* the user clicks the "New Game" button.  Start a new game.
*/
void doNewGame() {
if (gameInProgress) {
// If the current game is not over, it is an error to try
// to start a new game.
message = "You still have to finish this game!";
repaint();
return;
}
deck = new Deck();   // Create the deck and hand to use for this game.
hand = new Hand();
deck.shuffle();
hand.addCard( deck.dealCard() );  // Deal the first card into the hand.
message = "Is the next card higher or lower?";
gameInProgress = true;
repaint();
} // end doNewGame()

The doHigher() and doLower() methods are almost identical to each other (and could probably have been combined into one method with a parameter, if I were more clever). Let's look at the doHigher() routine. This is called when the user clicks the "Higher" button. This only makes sense if a game is in progress, so the first thing doHigher() should do is check the value of the state variable gameInProgress. If the value is false, then doHigher() should just set up an error message. If a game is in progress, a new card should be added to the hand and the user's prediction should be tested. The user might win or lose at this time. If so, the value of the state variable gameInProgress must be set to false because the game is over. In any case, the board is repainted to show the new state. Here is the doHigher() method:

/**
* Called by actionPerformed() when user clicks the "Higher" button.
* Check the user's prediction.  Game ends if user guessed
* wrong or if the user has made three correct predictions.
*/
void doHigher() {
if (gameInProgress == false) {
// If the game has ended, it was an error to click "Higher",
// So set up an error message and abort processing.
message = "Click \"New Game\" to start a new game!";
repaint();
return;
}
hand.addCard( deck.dealCard() );     // Deal a card to the hand.
int cardCt = hand.getCardCount();
Card thisCard = hand.getCard( cardCt - 1 );  // Card just dealt.
Card prevCard = hand.getCard( cardCt - 2 );  // The previous card.
if ( thisCard.getValue() < prevCard.getValue() ) {
gameInProgress = false;
message = "Too bad! You lose.";
}
else if ( thisCard.getValue() == prevCard.getValue() ) {
gameInProgress = false;
message = "Too bad!  You lose on ties.";
}
else if ( cardCt == 4) {  // The hand is full, after three correct guesses.
gameInProgress = false;
message = "You win!  You made three correct guesses.";
}
else {
message = "Got it right!  Try for " + cardCt + ".";
}
repaint();
} // end doHigher()

The paintComponent() method of the CardPanel class uses the values in the state variables to decide what to show. It displays the string stored in the message variable. It draws each of the cards in the hand. There is one little tricky bit: If a game is in progress, it draws an extra face-down card, which is not in the hand, to represent the next card in the deck. Drawing the cards requires some care and computation. I wrote a method, "void drawCard(Graphics g, Card card, int x, int y)", which draws a card with its upper left corner at the point (x,y). The paintComponent() routine decides where to draw each card and calls this routine to do the drawing. You can check out all the details in the source code, HighLowGUI.java. (The playing cards used in this program are not very impressive. A version of the program with images that actually look like cards can be found in Subsection 13.1.3.)

Section 6.7

Menus and Dialogs

We have already encountered many of the basic aspects of GUI programming, but professional programs use many additional features. We will cover some of the advanced features of Java GUI programming in Chapter 13, but in this section we look briefly at a few more features that are essential for writing GUI programs. I will discuss these features in the context of a "MosaicDraw" program that is shown in this picture:

The source code for the program is in the file MosaicDraw.java. The program also requires MosaicPanel.java and MosaicDrawController.java. You will want to try it out!

As the user clicks-and-drags the mouse in the large drawing area of this program, it leaves a trail of little colored squares. There is some random variation in the color of the squares. (This is meant to make the picture look a little more like a real mosaic, which is a picture made out of small colored stones in which there would be some natural color variation.) There is a menu bar above the drawing area. The "Control" menu contains commands for filling and clearing the drawing area, along with a few options that affect the appearance of the picture. The "Color" menu lets the user select the color that will be used when the user draws. The "Tools" menu affects the behavior of the mouse. Using the default "Draw" tool, the mouse leaves a trail of single squares. Using the "Draw 3x3" tool, the mouse leaves a swath of colored squares that is three squares wide. There are also "Erase" tools, which let the user set squares back to their default black color.

The drawing area of the program is a panel that belongs to the MosaicPanel class, a subclass of JPanel that is defined in MosaicPanel.java. MosaicPanel is a highly reusable class for representing mosaics of colored rectangles. It was also used behind the scenes in the sample program in Subsection 4.6.3. The MosaicPanel class does not directly support drawing on the mosaic, but it does support setting the color of each individual square. The MosaicDraw program installs a mouse listener on the panel; the mouse listener responds to mousePressed and mouseDragged events on the panel by setting the color of the square that contains the mouse. This is a nice example of applying a listener to an object to do something that was not programmed into the object itself.

The file MosaicDraw.java is a simple class that contains only the main() routine for the program. Most of the programming for MosaicDraw can be found in MosaicDrawController.java. (It might have gone into the MosaicPanel class, if I had not decided to use that pre-existing class in unmodified form.) It is the MosaicDrawController class that creates a MosaicPanel object and adds a mouse listener to it. It also creates the menu bar that is shown at the top of the program, and it implements all the commands in the menu bar. It has an instance method getMosaicPanel() that returns a reference to the mosaic panel that it has created, and it has another instance method getMenuBar() that returns a menu bar for the program. These methods are used to obtain the panel and menu bar so that they can be added to the program's window.

I urge you to study MosaicDrawController.java and MosaicDraw.java. I will not be discussing all aspects of the code here, but you should be able to understand it all after reading this section. As for MosaicPanel.java, it uses some techniques that you would not understand at this point, but I encourage you to at least read the comments in this file to learn about the API for mosaic panels.

6.7.1  Menus and Menubars

MosaicDraw is the first example that we have seen that uses a menu bar. Fortunately, menus are very easy to use in Java. The items in a menu are represented by the class JMenuItem (this class and other menu-related classes are in package javax.swing). Menu items are used in almost exactly the same way as buttons. In fact, JMenuItem and JButton are both subclasses of a class, AbstractButton, that defines their common behavior. In particular, a JMenuItem is created using a constructor that specifies the text of the menu item, such as:

JMenuItem fillCommand = new JMenuItem("Fill");

You can add an ActionListener to a JMenuItem by calling the menu item's addActionListener() method. The actionPerformed() method of the action listener is called when the user selects the item from the menu. You can change the text of the item by calling its setText(String) method, and you can enable it and disable it using the setEnabled(boolean) method. All this works in exactly the same way as for a JButton.

The main difference between a menu item and a button, of course, is that a menu item is meant to appear in a menu rather than in a panel. A menu in Java is represented by the class JMenu. A JMenu has a name, which is specified in the constructor, and it has an add(JMenuItem) method that can be used to add a JMenuItem to the menu. For example, the "Tools" menu in the MosaicDraw program could be created as follows, where listener is a variable of type ActionListener:

JMenu toolsMenu = new JMenu("Tools");  // Create a menu with name "Tools"

JMenuItem drawCommand = new JMenuItem("Draw");   // Create a menu item.
drawCommand.addActionListener(listener);         // Add listener to menu item.
toolsMenu.add(drawCommand);                      // Add menu item to menu.

JMenuItem eraseCommand = new JMenuItem("Erase"); // Create a menu item.
eraseCommand.addActionListener(listener);        // Add listener to menu item.
toolsMenu.add(eraseCommand);                     // Add menu item to menu.
.
.  // Create and add other menu items.
.

Once a menu has been created, it must be added to a menu bar. A menu bar is represented by the class JMenuBar. A menu bar is just a container for menus. It does not have a name, and its constructor does not have any parameters. It has an add(JMenu) method that can be used to add menus to the menu bar. The name of the menu then appears in the menu bar. For example, the MosaicDraw program uses three menus, controlMenu, colorMenu, and toolsMenu. We could create a menu bar and add the menus to it with the statements:

JMenuBar menuBar = new JMenuBar();
menuBar.add(controlMenu);
menuBar.add(colorMenu);
menuBar.add(toolsMenu);

The final step in using menus is to use the menu bar in a window such as a JFrame. We have already seen that a frame has a "content pane." The menu bar is another component of the frame, not contained inside the content pane. The JFrame class has an instance method setMenuBar(JMenuBar) that can be used to set the menu bar. (There can only be one, so this is a "set" method rather than an "add" method.) In the MosaicDraw program, the menu bar is created by a MosaicDrawController object and can be obtained by calling that object's getMenuBar() method. The main() routine in MosaicDraw.java gets the menu bar from the controller and adds it to the window. Here is the basic code that is used (in somewhat modified form) to set up the interface:

MosaicDrawController controller = new MosaicDrawController();

MosaicPanel content = controller.getMosaicPanel();
window.setContentPane( content );  // Use panel from controller as content pane.

JMenuBar menuBar = controller.getMenuBar();
window.setJMenuBar( menuBar );    // Use the menu bar from the controller.

Using menus always follows the same general pattern: Create a menu bar. Create menus and add them to the menu bar. Create menu items and add them to the menus (and set up listening to handle action events from the menu items). Use the menu bar in a window by calling the window's setJMenuBar() method.

There are other kinds of menu items, defined by subclasses of JMenuItem, that can be added to menus. One of these is JCheckBoxMenuItem, which represents menu items that can be in one of two states, selected or not selected. A JCheckBoxMenuItem has the same functionality and is used in the same way as a JCheckBox (see Subsection 6.5.3). Three JCheckBoxMenuItems are used in the "Control" menu of the MosaicDraw program. One is used to turn the random color variation of the squares on and off. Another turns a symmetry feature on and off; when symmetry is turned on, the user's drawing is reflected horizontally and vertically to produce a symmetric pattern. And the third checkbox menu item shows and hides the "grouting" in the mosaic; the grouting is the gray lines that are drawn around each of the little squares in the mosaic. The menu item that corresponds to the "Use Randomness" option in the "Control" menu could be set up with the statements:

JMenuItem useRandomnessToggle = new JCheckBoxMenuItem("Use Randomness");
useRandomnessToggle.addActionListener(listener);  // Set up a listener.
useRandomnessToggle.setSelected(true);  // Randomness is initially turned on.
controlMenu.add(useRandomnessToggle);   // Add the menu item to the menu.

In my program, the "Use Randomness" JCheckBoxMenuItem corresponds to a boolean-valued instance variable named useRandomness in the MosaicDrawController class. This variable is part of the state of the controller object. Its value is tested whenever the user draws one of the squares, to decide whether or not to add a random variation to the color of the square. When the user selects the "Use Randomness" command from the menu, the state of the JCheckBoxMenuItem is reversed, from selected to not-selected or from not-selected to selected. The ActionListener for the menu item checks whether the menu item is selected or not, and it changes the value of useRandomness to match. Note that selecting the menu command does not have any immediate effect on the picture that is shown in the window. It just changes the state of the program so that future drawing operations on the part of the user will have a different effect. The "Use Symmetry" option in the "Control" menu works in much the same way. The "Show Grouting" option is a little different. Selecting the "Show Grouting" option does have an immediate effect: The picture is redrawn with or without the grouting, depending on the state of the menu item.

My program uses a single ActionListener to respond to all of the menu items in all the menus. This is not a particularly good design, but it is easy to implement for a small program like this one. The actionPerformed() method of the listener object uses the statement

String command = evt.getActionCommand();

to get the action command of the source of the event; this will be the text of the menu item. The listener tests the value of command to determine which menu item was selected by the user. If the menu item is a JCheckBoxMenuItem, the listener must check the state of the menu item. The menu item is the source of the event that is being processed. The listener can get its hands on the menu item object by calling evt.getSource(). Since the return value of getSource() is of type Object, the return value must be type-cast to the correct type. Here, for example, is the code that handles the "Use Randomness" command:

if (command.equals("Use Randomness")) {
// Set the value of useRandomness depending on the menu item's state.
JCheckBoxMenuItem toggle = (JCheckBoxMenuItem)evt.getSource();
useRandomness = toggle.isSelected();
}

(The actionPerformed() method uses a rather long if..then..else statement to check all the possible action commands. It might be more natural and efficient use a switch statement with command as the selector and all the possible action commands as cases.)

In addition to menu items, a menu can contain lines that separate the menu items into groups. In the MosaicDraw program, the "Control" menu contains such a separator. A JMenu has an instance method addSeparator() that can be used to add a separator to the menu. For example, the separator in the "Control" menu was created with the statement:

controlMenu.addSeparator();

A menu can also contain a submenu. The name of the submenu appears as an item in the main menu. When the user moves the mouse over the submenu name, the submenu pops up. (There is no example of this in the MosaicDraw program.) It is very easy to do this in Java: You can add one JMenu to another JMenu using a statement such as mainMenu.add(submenu), and it becomes a submenu.

6.7.2  Dialogs

One of the commands in the "Color" menu of the MosaicDraw program is "Custom Color...". When the user selects this command, a new window appears where the user can select a color. This window is an example of a dialog or dialog box. A dialog is a type of window that is generally used for short, single purpose interactions with the user. For example, a dialog box can be used to display a message to the user, to ask the user a question, to let the user select a file to be opened, or to let the user select a color. In Swing, a dialog box is represented by an object belonging to the class JDialog or to a subclass.

The JDialog class is very similar to JFrame and is used in much the same way. Like a frame, a dialog box is a separate window. Unlike a frame, however, a dialog is not completely independent. Every dialog is associated with a frame (or another dialog), which is called its parent window. The dialog box is dependent on its parent. For example, if the parent is closed, the dialog box will also be closed. It is possible to create a dialog box without specifying a parent, but in that case an invisible frame is created by the system to serve as the parent.

Dialog boxes can be either modal or modeless. When a modal dialog is created, its parent frame is blocked. That is, the user will not be able to interact with the parent until the dialog box is closed. Modeless dialog boxes do not block their parents in the same way, so they seem a lot more like independent windows. In practice, modal dialog boxes are easier to use and are much more common than modeless dialogs. All the examples we will look at are modal.

Aside from having a parent, a JDialog can be created and used in the same way as a JFrame. However, I will not give any examples here of using JDialog directly. Swing has many convenient methods for creating common types of dialog boxes. For example, the color choice dialog that appears when the user selects the "Custom Color" command in the MosaicDraw program belongs to the class JColorChooser, which is a subclass of JDialog. The JColorChooser class has a static method that makes color choice dialogs very easy to use:

Color JColorChooser.showDialog(Component parentComp,
String title, Color initialColor)

When you call this method, a dialog box appears that allows the user to select a color. The first parameter specifies the parent of the dialog; the parent window of the dialog will be the window (if any) that contains parentComp; this parameter can be null and it can itself be a frame or dialog object. The second parameter is a string that appears in the title bar of the dialog box. And the third parameter, initialColor, specifies the color that is selected when the color choice dialog first appears. The dialog has a sophisticated interface that allows the user to select a color. When the user presses an "OK" button, the dialog box closes and the selected color is returned as the value of the method. The user can also click a "Cancel" button or close the dialog box in some other way; in that case, null is returned as the value of the method. This is a modal dialog, and showDialog() does not return until the user dismisses the dialog box in some way. By using this predefined color chooser dialog, you can write one line of code that will let the user select an arbitrary color. Swing also has a JFileChooser class that makes it almost as easy to show a dialog box that lets the user select a file to be opened or saved.

The JOptionPane class includes a variety of methods for making simple dialog boxes that are variations on three basic types: a "message" dialog, a "confirm" dialog, and an "input" dialog. (The variations allow you to provide a title for the dialog box, to specify the icon that appears in the dialog, and to add other components to the dialog box. I will only cover the most basic forms here.)

A message dialog simply displays a message string to the user. The user (hopefully) reads the message and dismisses the dialog by clicking the "OK" button. A message dialog can be shown by calling the static method:

void JOptionPane.showMessageDialog(Component parentComp, String message)

The message can be more than one line long. Lines in the message should be separated by newline characters, \n. New lines will not be inserted automatically, even if the message is very long. For example, assuming that the special variable this refers to a Component:

JOptionPane.showMessageDialog( this, "This program is about to crash!\n"
+ "Sorry about that.");

An input dialog displays a question or request and lets the user type in a string as a response. You can show an input dialog by calling:

String JOptionPane.showInputDialog(Component parentComp, String question)

Again, parentComp can be null, and the question can include newline characters. The dialog box will contain an input box, an "OK" button, and a "Cancel" button. If the user clicks "Cancel", or closes the dialog box in some other way, then the return value of the method is null. If the user clicks "OK", then the return value is the string that was entered by the user. Note that the return value can be an empty string (which is not the same as a null value), if the user clicks "OK" without typing anything in the input box. If you want to use an input dialog to get a numerical value from the user, you will have to convert the return value into a number (see Subsection 3.7.2). As an example,

String name;
name = JOptionPanel.showInputDialog(null, "Hi!  What's your name?");
if (name == null)
JOptionPane.showMessageDialog(null, "Well, I'll call you Grumpy.");
else
JOptionPane.showMessageDialog(null, "Pleased to meet you, " + name);

Finally, a confirm dialog presents a question and three response buttons: "Yes", "No", and "Cancel". A confirm dialog can be shown by calling:

int JOptionPane.showConfirmDialog(Component parentComp, String question)

The return value tells you the user's response. It is one of the following constants:

- JOptionPane.YES_OPTION -- the user clicked the "Yes" button

- JOptionPane.NO_OPTION -- the user clicked the "No" button

- JOptionPane.CANCEL_OPTION -- the user clicked the "Cancel" button

- JOptionPane.CLOSE_OPTION -- the dialog was closed in some other way.

By the way, it is possible to omit the Cancel button from a confirm dialog by calling one of the other methods in the JOptionPane class. Just call:

JOptionPane.showConfirmDialog(
parent, question, title, JOptionPane.YES_NO_OPTION )

The final parameter is a constant which specifies that only a "Yes" button and a "No" button should be used. The third parameter is a string that will be displayed as the title of the dialog box window.

A small demo program, SimpleDialogDemo.java is available to demonstrate JColorChooser and several JOptionPane dialogs.

6.7.3  Fine Points of Frames

In previous sections, whenever I used a frame, I created a JFrame object in a main() routine and installed a panel as the content pane of that frame. This works fine, but a more object-oriented approach is to define a subclass of JFrame and to set up the contents of the frame in the constructor of that class. This is what I did in the case of the MosaicDraw program. MosaicDraw is defined as a subclass of JFrame. The definition of this class is very short, but it illustrates several new features of frames that I want to discuss:

public class MosaicDraw extends JFrame {

public static void main(String[] args) {
JFrame window = new MosaicDraw();
window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
window.setVisible(true);
}

public MosaicDraw() {
super("Mosaic Draw");
MosaicDrawController controller = new MosaicDrawController();
setContentPane( controller.getMosaicPanel() );
setJMenuBar( controller.getMenuBar() );
pack();
Dimension screensize = Toolkit.getDefaultToolkit().getScreenSize();
setLocation( (screensize.width - getWidth())/2,
(screensize.height - getHeight())/2 );
}

}

The constructor in this class begins with the statement super("Mosaic Draw"), which calls the constructor in the superclass, JFrame. The parameter specifies a title that will appear in the title bar of the window. The next three lines of the constructor set up the contents of the window; a MosaicDrawController is created, and the content pane and menu bar of the window are obtained from the controller. The next line is something new. If window is a variable of type JFrame (or JDialog), then the statement window.pack() will resize the window so that its size matches the preferred size of its contents. (In this case, of course, "pack()" is equivalent to "this.pack()"; that is, it refers to the window that is being created by the constructor.) The pack() method is usually the best way to set the size of a window. Note that it will only work correctly if every component in the window has a correct preferred size. This is only a problem in two cases: when a panel is used as a drawing surface and when a panel is used as a container with a null layout manager. In both these cases there is no way for the system to determine the correct preferred size automatically, and you should set a preferred size by hand. For example:

panel.setPreferredSize( new Dimension(400, 250) );

The last two lines in the constructor position the window so that it is exactly centered on the screen. The line

Dimension screensize = Toolkit.getDefaultToolkit().getScreenSize();

determines the size of the screen. The size of the screen is screensize.width pixels in the horizontal direction and screensize.height pixels in the vertical direction. The setLocation() method of the frame sets the position of the upper left corner of the frame on the screen. The expression "screensize.width - getWidth()" is the amount of horizontal space left on the screen after subtracting the width of the window. This is divided by 2 so that half of the empty space will be to the left of the window, leaving the other half of the space to the right of the window. Similarly, half of the extra vertical space is above the window, and half is below.

Note that the constructor has created the window and set its size and position, but that at the end of the constructor, the window is not yet visible on the screen. (More exactly, the constructor has created the window object, but the visual representation of that object on the screen has not yet been created.) To show the window on the screen, it will be necessary to call its instance method, window.setVisible(true).

In addition to the constructor, the MosaicDraw class includes a main() routine. This makes it possible to run MosaicDraw as a stand-alone application. (The main() routine, as a static method, has nothing to do with the function of a MosaicDraw object, and it could (and perhaps should) be in a separate class.) The main() routine creates a MosaicDraw and makes it visible on the screen. It also calls

window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

which means that the program will end when the user closes the window. Note that this is not done in the constructor because doing it there would make MosaicDraw less flexible. It is possible, for example, to write a program that lets the user open multiple MosaicDraw windows. In that case, we don't want to shut down the whole program just because the user has closed one of the windows. There are other possible values for the default close operation of a window:

- JFrame.DO_NOTHING_ON_CLOSE -- the user's attempts to close the window by clicking its close box will be ignored, except that it will generate a WindowEvent. A program can listen for this event and take any action it wants when the user attempts to close the window.

- JFrame.HIDE_ON_CLOSE -- when the user clicks its close box, the window will be hidden just as if window.setVisible(false) were called. The window can be made visible again by calling window.setVisible(true). This is the value that is used if you do not specify another value by calling setDefaultCloseOperation.

- JFrame.DISPOSE_ON_CLOSE -- the window is closed and any operating system resources used by the window are released. It is not possible to make the window visible again. (This is the proper way to permanently get rid of a window without ending the program. You can accomplish the same thing programmatically by calling the instance method window.dispose().)

6.7.4  Creating Jar Files

As the final topic for this chapter, we look again at jar files. Recall that a jar file is a "java archive" that can contain a number of class files. When creating a program that uses more than one class, it's usually a good idea to place all the classes that are required by the program into a jar file. If that is done, then a user will only need that one file to run the program. In fact, it is possible to make a so-called executable jar file. A user can run an executable jar file in much the same way as any other application, usually by double-clicking the icon of the jar file. (The user's computer must have a correct version of Java installed, and the computer must be configured correctly for this to work. The configuration is usually done automatically when Java is installed, at least on Windows and Mac OS.)

The question, then, is how to create a jar file. The answer depends on what programming environment you are using. The two basic types of programming environment -- command line and IDE -- were discussed in Section 2.6. Any IDE (Integrated Programming Environment) for Java should have a command for creating jar files. In the Eclipse IDE, for example, it can be done as follows: In the Package Explorer pane, select the programming project (or just all the individual source code files that you need). Right-click on the selection, and choose "Export" from the menu that pops up. In the window that appears, select "JAR file" and click "Next". In the window that appears next, enter a name for the jar file in the box labeled "JAR file". (Click the "Browse" button next to this box to select the file name using a file dialog box.) The name of the file should end with ".jar". If you are creating a regular jar file, not an executable one, you can hit "Finish" at this point, and the jar file will be created. To create an executable file, hit the "Next" button twice to get to the "Jar Manifest Specification" screen. At the bottom of this screen is an input box labeled "Main class". You have to enter the name of the class that contains the main() routine that will be run when the jar file is executed. If you hit the "Browse" button next to the "Main class" box, you can select the class from a list of classes that contain main() routines. Once you've selected the main class, you can click the "Finish" button to create the executable jar file. (Note that newer versions of Eclipse also have an option for exporting an executable Jar file in fewer steps.)

It is also possible to create jar files on the command line. The Java Development Kit includes a command-line program named jar that can be used to create jar files. If all your classes are in the default package (like most of the examples in this book), then the jar command is easy to use. To create a non-executable jar file on the command line, change to the directory that contains the class files that you want to include in the jar. Then give the command

jar  cf  JarFileName.jar  *.class

where JarFileName can be any name that you want to use for the jar file. The "*" in "*.class" is a wildcard that makes *.class match every class file in the current directory. This means that all the class files in the directory will be included in the jar file. If you want to include only certain class files, you can name them individually, separated by spaces. (Things get more complicated if your classes are not in the default package. In that case, the class files must be in subdirectories of the directory in which you issue the jar command. See Subsection 2.6.6.)

Making an executable jar file on the command line is more complicated. There has to be some way of specifying which class contains the main() routine. This is done by creating a manifest file. The manifest file can be a plain text file containing a single line of the form

Main-Class: ClassName

where ClassName should be replaced by the name of the class that contains the main() routine. For example, if the main() routine is in the class MosaicDraw, then the manifest file should read "Main-Class: MosaicDraw". You can give the manifest file any name you like. Put it in the same directory where you will issue the jar command, and use a command of the form

jar  cmf  ManifestFileName  JarFileName.jar  *.class

to create the jar file. (The jar command is capable of performing a variety of different operations. The first parameter to the command, such as "cf" or "cmf", tells it which operation to perform.)

By the way, if you have successfully created an executable jar file, you can run it on the command line using the command "java -jar". For example:

java  -jar  JarFileName.jar

Programming Exercises for Chapter 6

Exercise 6.1:

In the SimpleStamper example from Subsection 6.3.3, a rectangle or oval is drawn on the panel when the user clicks the mouse. Except, when the user shift-clicks, the panel is cleared instead. Modify this class so that the modified version will continue to draw figures as the user drags the mouse. That is, the mouse will leave a trail of figures as the user drags. However, if the user shift-clicks, the panel should simply be cleared and no figures should be drawn even if the user drags the mouse after shift-clicking. Here is a picture of my solution:

The source code for the original program is SimpleStamper.java. See the discussion of dragging in Subsection 6.3.4. (Note that the original version uses a black background, with a black border around each shape. That didn't work well with a lot of closely spaced shapes, so the new version uses a white background.)

If you want to make the problem a little more challenging, when drawing shapes during a drag operation, make sure that the shapes that are drawn are at least, say, 5 pixels apart. To implement this, you have to keep track of the position of the last shape that was drawn.

See the Solution

Exercise 6.2:

Write a program that shows a small red square and a small blue square. The user should be able to drag either square with the mouse. (You'll need an instance variable to remember which square the user is dragging.) The user can drag the square out of the panel if she wants; if she does this, there is no way to get it back.

Note that for this exercise, you should do all the drawing in the paintComponent() method (as indeed you should whenever possible).

See the Solution

Exercise 6.3:

Write a program that shows a pair of dice. When the user clicks on the panel in the program, the dice should be rolled (that is, the dice should be assigned newly computed random values). Each die should be drawn as a square showing from 1 to 6 dots. Since you have to draw two dice, its a good idea to write a subroutine, "void drawDie(Graphics g, int val, int x, int y)", to draw a die at the specified (x,y) coordinates. The second parameter, val, specifies the value that is showing on the die. Assume that the size of the panel is 100 by 100 pixels. Here is a picture of the panel that displays the dice:

See the Solution

Exercise 6.4:

In Exercise 6.3, you wrote a pair-of-dice panel where the dice are rolled when the user clicks on the panel. Now make a pair-of-dice program in which the user rolls the dice by clicking a button. The button should appear under the panel that shows the dice. Also make the following change: When the dice are rolled, instead of just showing the new value, show a short animation during which the values on the dice are changed in every frame. The animation is supposed to make the dice look more like they are actually rolling.

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Exercise 6.5:

In Exercise 3.8, you drew a checkerboard. For this exercise, write a program where the user can select a square by clicking on it. (Use a JPanel for the checkerboard.) Highlight the selected square by drawing a colored border around it. When the program starts, no square is selected. When the user clicks on a square that is not currently selected, it becomes selected (and the previously selected square, if any, is unselected). If the user clicks the square that is selected, it becomes unselected. Assume that the size of the panel is exactly 160 by 160 pixels, so that each square on the checkerboard is 20 by 20 pixels. Here is my checkerboard, with the square in row 3, column 3 selected:

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Exercise 6.6:

For this exercise, you should modify the SubKiller game from Subsection 6.4.4. You can start with the existing source code, from the file SubKiller.java. Modify the game so it keeps track of the number of hits and misses and displays these quantities. That is, every time the depth charge blows up the sub, the number of hits goes up by one. Every time the depth charge falls off the bottom of the screen without hitting the sub, the number of misses goes up by one. There is room at the top of the panel to display these numbers. To do this exercise, you only have to add a half-dozen lines to the source code. But you have to figure out what they are and where to add them. To do this, you'll have to read the source code closely enough to understand how it works.

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Exercise 6.7:

Exercise 5.2 involved a class, StatCalc.java, that could compute some statistics of a set of numbers. Write a GUI program that uses the StatCalc class to compute and display statistics of numbers entered by the user. The panel will have an instance variable of type StatCalc that does the computations. The panel should include a JTextField where the user enters a number. It should have four labels that display four statistics for the numbers that have been entered: the number of numbers, the sum, the mean, and the standard deviation. Every time the user enters a new number, the statistics displayed on the labels should change. The user enters a number by typing it into the JTextField and pressing return. There should be a "Clear" button that clears out all the data. This means creating a new StatCalc object and resetting the displays on the labels. My panel also has an "Enter" button that does the same thing as pressing the return key in the JTextField. (Recall that a JTextField generates an ActionEvent when the user presses return, so your panel should register itself to listen for ActionEvents from the JTextField as well as the buttons.) Here is a picture of my solution to this problem:

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Exercise 6.8:

Write a program that has a JTextArea where the user can enter some text. Then program should have a button such that when the user clicks on the button, the panel will count the number of lines in the user's input, the number of words in the user's input, and the number of characters in the user's input. This information should be displayed on three labels. Recall that if textInput is a JTextArea, then you can get the contents of the JTextArea by calling the function textInput.getText(). This function returns a String containing all the text from the text area. The number of characters is just the length of this String. Lines in the String are separated by the new line character, '\n', so the number of lines is just the number of new line characters in the String, plus one. Words are a little harder to count. Exercise 3.4 has some advice about finding the words in a String. Essentially, you want to count the number of characters that are first characters in words. Don't forget to put your JTextArea in a JScrollPane, and add the scroll pane to the container, not the text area. Scrollbars should appear when the user types more text than will fit in the available area. Here is a picture of my solution:

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Exercise 6.9:

A polygon is a geometric figure made up of a sequence of connected line segments. The points where the line segments meet are called the vertices of the polygon. The Graphics class includes commands for drawing and filling polygons. For these commands, the coordinates of the vertices of the polygon are stored in arrays. If g is a variable of type Graphics then

- g.drawPolygon(xCoords, yCoords, pointCt) will draw the outline of the polygon with vertices at the points (xCoords[0],yCoords[0]), (xCoords[1],yCoords[1]), ..., (xCoords[pointCt-1],yCoords[pointCt-1]). The third parameter, pointCt, is an int that specifies the number of vertices of the polygon. Its value should be 3 or greater. The first two parameters are arrays of type int[]. Note that the polygon automatically includes a line from the last point, (xCoords[pointCt-1],yCoords[pointCt-1]), back to the starting point (xCoords[0],yCoords[0]).

- g.fillPolygon(xCoords, yCoords, pointCt) fills the interior of the polygon with the current drawing color. The parameters have the same meaning as in the drawPolygon() method. Note that it is OK for the sides of the polygon to cross each other, but the interior of a polygon with self-intersections might not be exactly what you expect.

Write a program that lets the user draw polygons. As the user clicks a sequence of points, count them and store their x- and y-coordinates in two arrays. These points will be the vertices of the polygon. As the user is creating the polygon, you should just connect all the points with line segments. When the user clicks near the starting point, draw the complete polygon. Draw it with a red interior and a black border. Once the user has completed a polygon, the next click will clear the data and start a new polygon from scratch. All drawing should be done in the paintComponent() method.

Here is a picture of my solution after the user has drawn a fairly complex polygon:

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Exercise 6.10:

Write a GUI Blackjack program that lets the user play a game of Blackjack, with the computer as the dealer. The program should draw the user's cards and the dealer's cards, just as was done for the graphical HighLow card game in Subsection 6.6.6. You can use the source code for that game, HighLowGUI.java, for some ideas about how to write your Blackjack game. The structures of the HighLow panel and the Blackjack panel are very similar. You will certainly want to use the drawCard() method from the HighLow program.

You can find a description of the game of Blackjack in Exercise 5.5. Add the following rule to that description: If a player takes five cards without going over 21, that player wins immediately. This rule is used in some casinos. For your program, it means that you only have to allow room for five cards. You should assume that the panel is just wide enough to show five cards, and that it is tall enough show the user's hand and the dealer's hand.

Note that the design of a GUI Blackjack game is very different from the design of the text-oriented program that you wrote for Exercise 5.5. The user should play the game by clicking on "Hit" and "Stand" buttons. There should be a "New Game" button that can be used to start another game after one game ends. You have to decide what happens when each of these buttons is pressed. You don't have much chance of getting this right unless you think in terms of the states that the game can be in and how the state can change.

Your program will need the classes defined in Card.java, Hand.java, Deck.java, and BlackjackHand.java.

The next exercise has a picture of a blackjack game that you can use a guide, except that the version for this exercise does not allow betting.

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Exercise 6.11:

In the Blackjack game from Exercise 6.10, the user can click on the "Hit", "Stand", and "NewGame" buttons even when it doesn't make sense to do so. It would be better if the buttons were disabled at the appropriate times. The "New Game" button should be disabled when there is a game in progress. The "Hit" and "Stand" buttons should be disabled when there is not a game in progress. The instance variable gameInProgress tells whether or not a game is in progress, so you just have to make sure that the buttons are properly enabled and disabled whenever this variable changes value. I strongly advise writing a subroutine that can be called whenever it is necessary to set the value of the gameInProgress variable. Then the subroutine can take responsibility for enabling and disabling the buttons. Recall that if bttn is a variable of type JButton, then bttn.setEnabled(false) disables the button and bttn.setEnabled(true) enables the button.

As a second (and more difficult) improvement, make it possible for the user to place bets on the Blackjack game. When the program starts, give the user $100. Add a JTextField to the strip of controls along the bottom of the panel. The user can enter the bet in this JTextField. When the game begins, check the amount of the bet. You should do this when the game begins, not when it ends, because several errors can occur: The contents of the JTextField might not be a legal number, the bet that the user places might be more money than the user has, or the bet might be <= 0. You should detect these errors and show an error message instead of starting the game. The user's bet should be an integral number of dollars.

It would be a good idea to make the JTextField uneditable while the game is in progress. If betInput is the JTextField, you can make it editable and uneditable by the user with the commands betInput.setEditable(true) and betInput.setEditable(false).

In the paintComponent() method, you should include commands to display the amount of money that the user has left.

There is one other thing to think about: Ideally, the program should not start a new game when it is first created. The user should have a chance to set a bet amount before the game starts. So, in the constructor for the drawing surface class, you should not call doNewGame(). You might want to display a message such as "Welcome to Blackjack" before the first game starts.

Here is a picture of my program:

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Quiz on Chapter 6

Question 1:

Programs written for a graphical user interface have to deal with "events." Explain what is meant by the term event. Give at least two different examples of events, and discuss how a program might respond to those events.

Question 2:

Explain carefully what the repaint() method does.

Question 3:

Java has a standard class called JPanel. Discuss two ways in which JPanels can be used.

Question 4:

Draw the picture that will be produced by the following paintComponent() method:

public static void paintComponent(Graphics g) {
super.paintComponent(g);
for (int i=10; i <= 210; i = i + 50)
for (int j = 10; j <= 210; j = j + 50)
g.drawLine(i,10,j,60);
}

Question 5:

Suppose you would like a panel that displays a green square inside a red circle, as illustrated. Write a paintComponent() method for the panel class that will draw the image.

Question 6:

Java has a standard class called MouseEvent. What is the purpose of this class? What does an object of type MouseEvent do?

Question 7:

One of the main classes in Swing is the JComponent class. What is meant by a component? What are some examples?

Question 8:

What is the function of a LayoutManager in Java?

Question 9:

Consider the illustration of nested panels from the beginning of Section 6.6. What type of layout manager is being used for each of the three panels in that picture?

Question 10:

Explain how Timers are used to do animation.

Question 11:

What is a JCheckBox and how is it used?

Question 12:

How is the preferred size of a component set, and how is it used?

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Chapter 7

Arrays and ArrayLists

Computers get a lot of their power from working with data structures. A data structure is an organized collection of related data. An object is a data structure, but this type of data structure -- consisting of a fairly small number of named instance variables -- is just the beginning. In many cases, programmers build complicated data structures by hand, by linking objects together. We'll look at these custom-built data structures in Chapter 9. But there is one type of data structure that is so important and so basic that it is built into every programming language: the array.

You have already encountered arrays in Section 3.8 and Subsection 5.1.4. We continue the study of arrays in this chapter, including some new details of their use and some additional array-processing techniques. In particular, we will look at the important topic of algorithms for searching and sorting an array.

An array has a fixed size that can't be changed after the array is created. But in many cases, it is useful to have a data structure that can grow and shrink as necessary. In this chapter, we will look at a standard class, ArrayList, that represents such a data structure.

Contents of Chapter 7:

- Section 1: Array Details

- Section 2: Array Processing

- Section 3: ArrayList

- Section 4: Searching and Sorting

- Section 5: Two-dimensional Arrays

- Programming Exercises

- Quiz on This Chapter

Section 7.1

Array Details

Array basics have been discussed in previous chapters, but there are still details of Java syntax to be filled in, and there is a lot more to say about using arrays. This section looks at some of the syntactic details, with more information about array processing to come in the rest of the chapter.

To briefly review some of the basics.... An array is a numbered sequence of elements, and each element acts like a separate variable. All of the elements are of the same type, which is called the base type of the array. The array as a whole also has a type. If the base type is btype, then the array is of type btype[]. Each element in the array has an index, which is just its numerical position in the sequence of elements. If the array is A, then the i-th element of the array is A[i]. The number of elements in an array is called its length. The length of an array A is A.length. The length of an array can't be changed after the array is created. The elements of the array A are A[0], A[1], ..., A[A.length-1]. An attempt to refer to an array element with an index outside the range from zero to A.length-1 causes an ArrayIndexOutOfBoundsException.

Arrays in Java are objects, so an array variable can only refer to an array, it does not contain the array. The value of an array variable can also be null. In that case, it does not refer to any array, and an attempt to refer to an array element such as A[i] will cause a NullPointerException. Arrays are created using a special form of the new operator. For example,

int[] A = new int[10];

creates a new array with base type int and length 10, and it sets the variable A to refer to the newly created array.

7.1.1  For-each Loops

Arrays are often processed using for loops. A for loop makes it easy to process each element in an array from beginning to end. For example, if namelist is an array of Strings, then all the values in the list can be printed using

for (int i = 0; i < namelist.length; i++) {
System.out.println( namelist[i] );
}

This type of processing is so common that there is an alternative form of the for loop that makes it easier to write. The alternative is called a for-each loop. It is probably easiest to start with an example. Here is a for-each loop for printing all the values in an array of Strings:

for ( String name : namelist ) {
System.out.println( name );
}

The meaning of "for (String name : namelist)" is "for each string, name, in the array, namelist, do the following". The effect is that the variable name takes on each of the values in namelist in turn, and the body of the loop is executed for each of those values. Note that there is no array index in the loop. The loop control variable, name, represents one of the values in the array, not the index of one of the values.

The for-each loop is meant specifically for processing all the values in a data structure, and we will see in Chapter 10 that it applies to other data structures besides arrays. The for-each loop makes it possible to process the values without even knowing the details of how the data is structured. In the case of arrays, it lets you avoid the complications of using array indices.

A for-each loop will perform the same operation for each value that is stored in an array. If itemArray is an array of type BaseType[], then a for-each loop for itemArray has the form:

for ( BaseType item : itemArray ) {
.
.  // process the item
.
}

As usual, the braces are optional if there is only one statement inside the loop. In this loop, item is the loop control variable. It is declared as a variable of type BaseType, where BaseType is the base type of the array. (In a for-each loop, the loop control variable must be declared in the loop; it cannot be a variable that already exists outside the loop.) When this loop is executed, each value from the array is assigned to item in turn and the body of the loop is executed for each value. Thus, the above loop is exactly equivalent to:

for ( int index = 0; index < itemArray.length; index++ ) {
BaseType item;
item = itemArray[index];  // Get one of the values from the array
.
.  // process the item
.
}

For example, if A is an array of type int[], then we could print all the values from A with the for-each loop:

for ( int item : A )
System.out.println( item );

and we could add up all the positive integers in A with:

int sum = 0;   // This will be the sum of all the positive numbers in A
for ( int item : A ) {
if (item > 0)
sum = sum + item;
}

The for-each loop is not always appropriate. For example, there is no simple way to use it to process the items in just a part of an array, or to process the elements in reverse order. However, it does make the code a little simpler when you do want to process all the values, in order. since it eliminates any need to use array indices.

It's important to note that a for-each loop processes the values in the array, not the elements (where an element means the actual memory location that is part of the array). For example, consider the following incorrect attempt to fill an array of integers with 17's:

int[] intList = new int[10];
for ( int item : intList ) {   // INCORRECT! DOES NOT MODIFY THE ARRAY!
item = 17;
}

The assignment statement item = 17 assigns the value 17 to the loop control variable, item. However, this has nothing to do with the array. When the body of the loop is executed, the value from one of the elements of the array is copied into item. The statement item = 17 replaces that copied value but has no effect on the array element from which it was copied; the value in the array is not changed. The loop is equivalent to

int[] intList = new int[10];
for ( int i = 0; i < intList.length; i++ ) {
int item = intList[i];
item = 17;
}

which certainly does not change the value of any element in the array.

7.1.2  Variable Arity Methods

Before Java 5, every method in Java had a fixed arity. (The arity of a method is defined as the number of parameters in a call to the method.) In a fixed arity method, the number of parameters must be the same in every call to the method and must be the same as the number of formal parameters in the method's definition. Java 5 introduced variable arity methods. In a variable arity method, different calls to the method can have different numbers of parameters. For example, the formatted output method System.out.printf, which was introduced in Subsection 2.4.1, is a variable arity method. The first parameter of System.out.printf must be a String, but it can have any number of additional parameters, of any types.

Calling a variable arity method is no different from calling any other sort of method, but writing one requires some new syntax. As an example, consider a method that can compute the average of any number of values of type double. The definition of such a method could begin with:

public static double average( double...  numbers ) {

Here, the ... after the type name, double, is what makes this a variable arity method. It indicates that any number of values of type double can be provided when the subroutine is called, so that for example average(1,4,9,16), average(3.14,2.17), average(0.375), and even average() are all legal calls to this method. Note that actual parameters of type int can be passed to average. The integers will, as usual, be automatically converted to real numbers.

When the method is called, the values of all the actual parameters that correspond to the variable arity parameter are placed into an array, and it is this array that is actually passed to the method. That is, in the body of a method, a variable arity parameter of type T actually looks like an ordinary parameter of type T[]. The length of the array tells you how many actual parameters were provided in the method call. In the average example, the body of the method would see an array named numbers of type double[]. The number of actual parameters in the method call would be numbers.length, and the values of the actual parameters would be numbers[0], numbers[1], and so on. A complete definition of the method would be:

public static double average( double... numbers ) {
// Inside this method, numbers if of type double[].
double sum;      // The sum of all the actual parameters.
double average;  // The average of all the actual parameters.
sum = 0;
for (int i = 0; i < numbers.length; i++) {
sum = sum + numbers[i];  // Add one of the actual parameters to the sum.
}
average = sum / numbers.length;
return average;
}

By the way, it is possible to pass a single array to a variable arity method, instead of a list of individual values. For example, suppose that salesData is a variable of type double[]. Then it would be legal to call average(salesData), and this would compute the average of all the numbers in the array.

The formal parameter list in the definition of a variable-arity method can include more than one parameter, but the ... can only be applied to the very last formal parameter.

As an example, consider a method that can draw a polygon through any number of points. The points are given as values of type Point, where an object of type Point has two instance variables, x and y, of type int. In this case, the method has one ordinary parameter -- the graphics context that will be used to draw the polygon -- in addition to the variable arity parameter. Remember that inside the definition of the method, the parameter points becomes an array of Points:

public static void drawPolygon(Graphics g, Point... points) {
if (points.length > 1) {  // (Need at least 2 points to draw anything.)
for (int i = 0; i < points.length - 1; i++) {
// Draw a line from i-th point to (i+1)-th point
g.drawLine( points[i].x, points[i].y, points[i+1].x, points[i+1].y );
}
// Now, draw a line back to the starting point.
g.drawLine( points[points.length-1].x, points[points.length-1].y,
points[0].x, points[0].y );
}
}

When this method is called, the subroutine call statement must have one actual parameter of type Graphics, which can be followed by any number of actual parameters of type Point.

For a final example, let's look at a method that strings together all of the values in a list of strings into a single, long string. This example uses a for-each loop to process the array:

public static String concat( String... values ) {
StringBuffer buffer;  // Use a StringBuffer for more efficient concatenation.
buffer = new StringBuffer();  // Start with an empty buffer.
for ( String str : values ) { // A "for each" loop for processing the values.
buffer.append(str); // Add string to the buffer.
}
return buffer.toString(); // return the contents of the buffer
}

Given this method definition, concat("Hello", "World") would return the string "HelloWorld", and concat() would return an empty string. Since a variable arity method can also accept an array as actual parameter, we could also call concat(lines) where lines is of type String[]. This would concatenate all the elements of the array into a single string.

7.1.3  Array Literals

We have seen that it is possible to initialize an array variable with a list of values at the time it is declared. For example,

int[] squares = { 1, 4, 9, 16, 25, 36, 49 };

This initializes squares to refer to a newly created array that contains the seven values in the list

A list initializer of this form can be used only in a declaration statement, to give an initial value to a newly declared array variable. It cannot be used in an assignment statement to assign a value to a variable that already existed. However, there is another, similar notation for creating a new array that can be used in other places. The notation uses another form of the new operator to both create a new array object and fill it with values. (The rather odd syntax is similar to the syntax for anonymous inner classes, which were discussed in Subsection 5.8.3.) As an example, to assign a new value to an array variable, cubes, of type int[], you could use:

cubes = new int[] { 1, 8, 27, 64, 125, 216, 343 };

This is an assignment statement rather than a declaration, so the array initializer syntax, without "new int[]," would not be legal here. The general syntax for this form of the new operator is

new base-type [ ] { list-of-values }

This is actually an expression whose value is a reference to a newly created array object. In this sense, it is an "array literal," since it is something that you can type in a program to represent a value. This means that it can be used in any context where an object of type base-type [] is legal. For example, you could pass the newly created array as an actual parameter to a subroutine. Consider the following utility method for creating a menu from an array of strings:

/**
* Creates a JMenu.  The names for the JMenuItems in the menu are
* given as an array of strings.
* @param menuName  the name for the JMenu that is to be created.
* @param handler  the listener that will respond to items in the menu.
*    This ActionListener is added as a listener for each JMenuItem.
* @param itemNames  an array holding the text that appears in each
*     JMenuItem.  If a null value appears in the array, the corresponding
*     item in the menu will be a separator rather than a JMenuItem.
* @return  the menu that has been created and filled with items.
*/
public static JMenu createMenu(
String menuName, ActionListener handler, String[] itemNames ) {
JMenu menu = new JMenu(menuName);
for ( String itemName : itemNames ) {
if ( itemName == null ) {
menu.addSeparator();
}
else {
JMenuItem item = new JMenuItem(itemName);
item.addActionListener(handler);
menu.add(item);
}
}
return menu;
}

The third parameter in a call to createMenu is an array of strings. The array that is passed as an actual parameter could be created in place, using the new operator. For example, assuming that listener is of type ActionListener, we can use the following statement to create an entire File menu:

JMenu fileMenu = createMenu( "File", listener
new String[] { "New", "Open", "Close", null, "Quit" } );

This should convince you that being able to create and use an array "in place" in this way can be very convenient, in the same way that anonymous inner classes are convenient.

By the way, it is perfectly legal to use the "new BaseType[] { ... }" syntax instead of the array initializer syntax in the declaration of an array variable. For example, instead of saying:

int[] primes = { 2, 3, 5, 7, 11, 13, 17, 19 };

you can say, equivalently,

int[] primes = new int[] { 2, 3, 5, 7, 11, 17, 19 };

In fact, rather than use a special notation that works only in the context of declaration statements, I sometimes prefer to use the second form.

One final note: For historical reasons, an array declaration such as

int[] list;

can also be written as

int list[];

which is a syntax used in the languages C and C++. However, this alternative syntax does not really make much sense in the context of Java, and it is probably best avoided. After all, the intent is to declare a variable of a certain type, and the name of that type is "int[]". It makes sense to follow the "type-name variable-name;" syntax for such declarations.

Section 7.2

Array Processing

Most examples of array processing that we have looked at have actually been fairly straightforward: processing the elements of the array in order from beginning to end, or random access to an arbitrary element of the array. In this section and later in the chapter, you'll see some of the more interesting things that you can do with arrays.

7.2.1  Some Processing Examples

To begin, here's an example to remind you to be careful about avoiding array indices outside the legal range. Suppose that lines is an array of type String[], and we want to know whether lines contains any duplicate elements in consecutive locations. That is, we want to know whether lines[i].equals(lines[i+1]) for any index i. Here is a failed attempt to check that condition:

boolean dupp = false;  // Assume there are no duplicates
for ( int i = 0; i < list.length; i++ ) {
if ( lines[i].equals(lines[i+1]) ) {  // THERE IS AN ERROR HERE!
dupp = true;   // we have found a duplicate!
break;
}
}

This for loop looks like many others that we have written, so what's the problem? The error occurs when i takes on its final value in the loop, i equal to lines.length-1. In that case, i+1 is equal to lines.length. But the last element in the array has index lines.length-1, so lines.length is not a legal index. This means that the reference to lines[i+1] causes an ArrayIndexOutOfBoundsException. This is easy to fix; we just need to stop the loop before i+1 goes out of range:

boolean dupp = false;  // Assume there are no duplicates
for ( int i = 0; i < list.length - 1 ; i++ ) {
if ( lines[i].equals(lines[i+1]) ) {
dupp = true;   // we have found a duplicate!
break;
}
}

This type of error can be even more insidious when working with partially full arrays (see Subsection 3.8.4), where usually only part of the array is in use, and a counter is used to keep track of how many spaces in the array are used. With a partially full array, the problem is not looking beyond the end of the array, but looking beyond the part of the array that is in use. When your program tries to look beyond the end of an array, at least the program will crash to let you know that there is a problem. With a partially full array, the problem can go undetected.

For the next example, let's continue with partially full arrays. We have seen how to add an item to a partially full array, but suppose that we also want to be able to remove items? Suppose that you write a game program, and that players can join the game and leave the game as it progresses. As a good object-oriented programmer, you probably have a class named Player to represent the individual players in the game. A list of all players who are currently in the game could be stored in an array, playerList, of type Player[]. Since the number of players can change, you will follow the partially full array pattern, and you will need a variable, playerCt, to record the number of players currently in the game. Assuming that there will never be more than 10 players in the game, you could declare the variables as:

Player[] playerList = new Player[10];  // Up to 10 players.
int      playerCt = 0;  // At the start, there are no players.

After some players have joined the game, playerCt will be greater than 0, and the player objects representing the players will be stored in the array elements playerList[0], playerList[1], ..., playerList[playerCt-1]. Note that the array element playerList[playerCt] is not in use. The procedure for adding a new player, newPlayer, to the game is simple:

playerList[playerCt] = newPlayer; // Put new player in next
//     available spot.
playerCt++;  // And increment playerCt to count the new player.

But deleting a player from the game is a little harder, since you don't want to leave a "hole" in the array where the deleted player used to be. Suppose you want to delete the player at index k in playerList. The number of players goes down by one, so one fewer space is used in the array. If you are not worried about keeping the players in any particular order, then one way to do this is to move the player from the last occupied position in the array into position k and then to decrement the value of playerCt:

playerList[k] = playerList[playerCt - 1];
playerCt--;

The player previously in position k is no longer in the array, so we have deleted that player from the list. The player previously in position playerCt - 1 is now in the array twice. But it's only in the occupied or valid part of the array once, since playerCt has decreased by one. Remember that every element of the array has to hold some value, but only the values in positions 0 through playerCt - 1 will be looked at or processed in any way. (By the way, you should think about what happens if the player that is being deleted is in the last position in the list. The code does still work in this case. What exactly happens?)

Suppose that when deleting the player in position k, you'd like to keep the remaining players in the same order. (Maybe because they take turns in the order in which they are stored in the array.) To do this, all the players in positions k+1 and above must move down one position in the array. Player k+1 replaces player k, who is out of the game. Player k+2 fills the spot left open when player k+1 is moved. And so on. The code for this is

for (int i = k+1; i < playerCt; i++) {
playerList[i-1] = playerList[i];
}
playerCt--;

Here is an illustration of the two ways of deleting an item from a partially full array. Here, player "C" is being deleted:

This leaves open the question of what happens when a partially full array becomes full, but you still want to add more items to it? We can't change the size of the array -- but we can make a new, bigger array and copy the data from the old array into the new array. But what does it mean to copy an array in the first place?

Suppose that A and B are array variables, with the same base type, and that A already refers to an array. Suppose that we want B to refer to a copy of A. The first thing to note is that the assignment statement

B = A;

does not make a copy of A. Arrays are objects, and an array variable can only hold a pointer to an array. The assignment statement copies the pointer from A into B, and the result is that A and B now point to the same array. For example, A[0] and B[0] are just different names for exactly the same array element. To make B refer to a copy of A, we need to make an entirely new array and copy all the items from A into B. Let's say that A and B are of type double[]. Then to make a copy of A, we can say

double B;
B = new double[A.length];  // Make a new array with the same length as A.
for ( int i = 0; i < A.length; i++ ) {
B[i] = A[i];
}

To solve the problem of adding to a partially full array that has become full, we just need to make a new array that is bigger than the existing array. The usual choice is to make a new array twice as big as the old. We need to meet one more requirement: At the end, the variable that referred to the old array must now point to the new array. That variable is what gives us access to the data, and at the end that data is in the new array. Fortunately, a simple assignment statement will make the variable point to the correct array. Let's suppose that we are using playerList and playerCt to store the players in a game, as in the example above, and we want to add newPlayer to the game. Here is how we can do that even if the playerList array is full:

if ( playerCt == playerList.length ) {
// The number of players is already equal to the size of the array.
// The array is full.  Make a new array that has more space.
Player[] temp;   // A variable to point to the new array.
temp = new Player[ 2*playerList.length ];  // Twice as big as the old array.
for ( int i = 0; i < playerList.length; i++ ) {
temp[i] = playerList[i];  // Copy item from old array into new array.
}
playerList = temp;  // playerList now points to the new, bigger array.
}
// At this point, we know that there is room in the array for newPlayer.
playerList[playerCt] = newPlayer;
playerCt++;

After the new array has been created, there is no longer any variable that points to the old array, so it will be garbage collected.

7.2.2  Some Standard Array Methods

Copying an array seems like such a common method that you might expect Java to have a built-in method already defined to do it. In fact, Java comes with several standard array-processing methods. The methods are defined as static methods in a class named Arrays, which is in the package java.util. For example, for any array, list,

Arrays.copyOf( list, lengthOfCopy )

is a function that returns a new array whose length is given by lengthOfCopy, containing items copied from list. If lengthOfCopy is greater than list.length, then extra spaces in the new array will have their default value (zero for numeric arrays, null for object arrays, and so on). If lengthOfCopy is less than or equal to list.length, then only as many items are copied from list as will fit in the new array. So if A is any array, then

B = Arrays.copyOf( A, A.length );

sets B to refer to an exact copy of A, and

playerList = Arrays.copyOf( playerList, 2*playerList.length );

could be used to double the amount of space available in a partially full array with just one line of code. We can also use Arrays.copyOf to decrease the size of a partially full array. We might want to do that to avoid having a lot of excess, unused spaces. To implement this idea, the code for deleting player number k from the list of players might become

playerList[k] = playerList[playerCt-1];
playerCt--;
if ( playerCt < playerList.length/2 ) {
// More than half the spaces are empty. Cut the array size in half.
playerList = Arrays.copyOf( playerList, playerList.length/2 );
}

I should mention that class Arrays actually contains a bunch of copyOf methods, one for each of the primitive types and one for objects. I should also note that when an array of objects is copied, it is only pointers to objects that are copied into the new array. The contents of the objects are not copied. This is the usual rule for assignment of pointers.

The Arrays class contains other useful methods. I'll mention a few of them. As with Arrays.copyOf, there are actually multiple versions of all of these methods, for different array types.

- Arrays.fill( array, value ) -- Fill an entire array with a specified value. The type of value must be compatible with the base type of the array. For example, assuming that numlist is an array of type double[], then Arrays.fill(numlist,17) will set every element of numlist to have the value 17.

- Arrays.fill( array, fromIndex, toIndex, value ) -- Fills part of the array with value, starting at index number fromIndex and ending with index number toIndex-1. Note that toIndex itself is not included.

- Arrays.toString( array ) -- A function that returns a String containing all the values from array, separated by commas and enclosed between square brackets. The values in the array are converted into strings in the same way they would be if they were printed out.

- Arrays.sort( array ) -- Sorts the entire array. To sort an array means to rearrange the values in the array so that they are in increasing order. This method works for arrays of String and arrays of primitive type values (except for boolean, which would be kind of silly). But it does not work for all arrays, since it must be meaningful to compare any two values in the array, to see which is "smaller." We will discuss array-sorting algorithms in Section 7.4.

- Arrays.sort( array, fromIndex, toIndex ) -- Sorts just the elements from array[fromIndex] up to array[toIndex-1]

- Arrays.binarySearch( array, value ) -- Searches for value in the array. The array must already be sorted into increasing order. This is a function that returns an int. If the value is found in the array, the return value is the index of an element that contains that value. If the value does not occur in the array, the return value is -1. We will discuss the binary search algorithm in Section 7.4.

7.2.3  RandomStrings Revisited

One of the examples in Subsection 6.3.2 was a GUI program that shows multiple copies of a message in random positions, colors, and fonts. When the user clicks the program window, the positions, colors, and fonts are changed to new random values. Like several other examples from that chapter, the program had a flaw: It didn't have any way of storing the data that it would need to redraw itself. Arrays provide us with one possible solution to this problem. We can write a new version of RandomStrings that uses an array to store the position, font, and color of each string. When the panel is painted, this information is used to draw the strings, so that the panel will paint itself correctly whenever it has to be redrawn. When the user clicks, the array is filled with new random values and the panel is repainted using the new data. So, the only time that the picture will change is in response to a mouse click. The new version of the program is RandomStringsWithArray.java.

In the program, the number of copies of the message is given by a named constant, MESSAGE_COUNT. One way to store the position, color, and font of MESSAGE_COUNT strings would be to use four arrays:

int[] x = new int[MESSAGE_COUNT];
int[] y = new int[MESSAGE_COUNT];
Color[] color = new Color[MESSAGE_COUNT];
Font[] font = new Font[MESSAGE_COUNT];

These arrays would be filled with random values. In the paintComponent() method, the i-th copy of the string would be drawn at the point (x[i],y[i]). Its color would be given by color[i]. And it would be drawn in the font font[i]. This would be accomplished by the paintComponent() method

public void paintComponent(Graphics g) {
super.paintComponent(); // (Fill with background color.)
for (int i = 0; i < MESSAGE_COUNT; i++) {
g.setColor( color[i] );
g.setFont( font[i] );
g.drawString( message, x[i], y[i] );
}
}

This approach is said to use parallel arrays. The data for a given copy of the message is spread out across several arrays. If you think of the arrays as laid out in parallel columns -- array x in the first column, array y in the second, array color in the third, and array font in the fourth -- then the data for the i-th string can be found along the i-th row. There is nothing wrong with using parallel arrays in this simple example, but it does go against the object-oriented philosophy of keeping related data in one object. If we follow this rule, then we don't have to imagine the relationship among the data, because all the data for one copy of the message is physically in one place. So, when I wrote the program, I made a simple class to represent all the data that is needed for one copy of the message:

/**
* An object of this type holds the position, color, and font
* of one copy of the string.
*/
private static class StringData {
int x, y;     // The coordinates of the left end of baseline of string.
Color color;  // The color in which the string is drawn.
Font font;    // The font that is used to draw the string.
}

To store the data for multiple copies of the message, I use an array of type StringData[]. The array is declared as an instance variable, with the name stringData:

StringData[] stringData;

Of course, the value of stringData is null until an actual array is created and assigned to it. The array has to be created and filled with data. Furthermore, each element of the array is an object of type StringData which has to be created before it can be used. The following subroutine is used to create the array and fill it with random data:

private void createStringData() {
int width = getWidth();
int height = getHeight();
stringData = new StringData[MESSAGE_COUNT];
for (int i = 0; i < MESSAGE_COUNT; i++) {
// Create an object to represent the data for string number i,
// and fill it with random values.
stringData[i] = new StringData();
int fontIndex = (int)(Math.random() * 5);
stringData[i].font = fonts[fontIndex]; // one of 5 fonts, selected at random
float hue = (float)Math.random();
stringData[i].color = Color.getHSBColor(hue, 1.0F, 1.0F);  // random color
stringData[i].x = -50 + (int)(Math.random()*(width+40));  // random x-coord
stringData[i].y = (int)(Math.random()*(height+20));  // random y-coord
}
}

This method is called before the panel is painted for the first time. It is also called when the user clicks the panel with the mouse, so that a mouse click will cause new random data to be created. Those are the only times when the picture can change. For example, resizing the window will cause paintComponent() to be called, but since the data hasn't changed, paintComponent() will just redraw the same picture. Here is the code from paintComponent() that draws all the strings, using the data from the array:

for (int i = 0; i < MESSAGE_COUNT; i++) {
g.setColor( stringData[i].color );
g.setFont( stringData[i].font );
g.drawString( message, stringData[i].x, stringData[i].y );
}

Note that I could also have used a for-each loop here, which might be easier to understand:

for ( StringData data : stringData ) {
g.setColor( data.color );
g.setFont( data.font );
g.drawString( message, data.x, data.y );
}

In this loop, the loop control variable, data, holds a copy of one of the values from the array. That value is a reference to an object of type StringData, which has instance variables named color, font, x, and y. Once again, the use of a for-each loop has eliminated the need to work with array indices.

RandomStringsWithArray uses one other array of objects. The font for a given copy of the message is chosen at random from a set of five possible fonts. In the original version, there were five variables of type Font to represent the fonts. The variables were named font1, font2, font3, font4, and font5. To select one of these fonts at random, a switch statement can be used:

Font randomFont;  // One of the 5 fonts, chosen at random.
int rand;         // A random integer in the range 0 to 4.

fontNum = (int)(Math.random() * 5);
switch (fontNum) {
case 0:
randomFont = font1;
break;
case 1:
randomFont = font2;
break;
case 2:
randomFont = font3;
break;
case 3:
randomFont = font4;
break;
case 4:
randomFont = font5;
break;
}

In the new version of the program, the five fonts are stored in an array, which is named fonts. This array is declared as an instance variable of type Font[]

Font[] fonts;

The array is created in the constructor, and each element of the array is set to refer to a new Font object:

fonts = new Font[5];  // Create the array to hold the five fonts.

fonts[0] = new Font("Serif", Font.BOLD, 14);
fonts[1] = new Font("SansSerif", Font.BOLD + Font.ITALIC, 24);
fonts[2] = new Font("Monospaced", Font.PLAIN, 20);
fonts[3] = new Font("Dialog", Font.PLAIN, 30);
fonts[4] = new Font("Serif", Font.ITALIC, 36);

This makes it much easier to select one of the fonts at random. It can be done with the statements

Font randomFont;  // One of the 5 fonts, chosen at random.
int fontIndex;    // A random number in the range 0 to 4.
fontIndex = (int)(Math.random() * 5);
randomFont = fonts[ fontIndex ];

The switch statement has been replaced by a single line of code. In fact, the preceding four lines can be replaced by the single line:

Font randomFont = fonts[ (int)(Math.random() * 5) ];

This is a very typical application of arrays. Note that this example uses the random access property of arrays: We can pick an array index at random and go directly to the array element at that index.

Here is another example of the same sort of thing. Months are often stored as numbers 1, 2, 3, ..., 12. Sometimes, however, these numbers have to be translated into the names January, February, ..., December. The translation can be done very easily with an array. The array can be declared and initialized as

static String[] monthName = { "January", "February", "March",
"April",   "May",      "June",
"July",    "August",   "September",
"October", "November", "December" };

If mnth is a variable that holds one of the integers 1 through 12, then monthName[mnth-1] is the name of the corresponding month. We need the "-1" because months are numbered starting from 1, while array elements are numbered starting from 0. Simple array indexing does the translation for us!

7.2.4  Dynamic Arrays

Earlier, we discussed how a partially full array can be used to store a list of players in a game, allowing the list to grow and shrink over the course of the game. The list is "dynamic" in the sense that its size changes while the program is running. Dynamic lists are very common, and we might think about trying to write a class to represent the concept. By writing a class, we can avoid having to repeat the same code every time we want to use a similar data structure. We want something that is like an array, except that its size can change. Think about operations that we might want to perform on a dynamic array. Some essential and useful operations would include

- add an item to the end of the array

- remove the item at a specified position in the array

- get the value of one of the elements in the array

- set the value of one of the elements in the array

- get the number of items currently in the array

When we design our class, these operations will become instance methods in that class. The items in the dynamic array will actually be stored in a normal array, using the partially full array pattern. Using what we know, the class is not difficult to write. We do have to decide what to do when an attempt is made to access an array element that doesn't exist. It seems natural to throw an index-out-of-bounds exception in that case. Let's suppose that the items in the array will be of type int.

import java.util.Arrays;

/**
* Represents a list of int values that can grow and shrink.
*/
public class DynamicArrayOfInt {

private int[] items = new int[8];  // partially full array holding the ints
private int itemCt;

/**
* Return the item at a given index in the array.
* Throws ArrayIndexOutOfBoundsException if the index is not valid.
*/
public int get( int index ) {
if ( index < 0 || index >= itemCt )
throw new ArrayIndexOutOfBoundsException("Illegal index, " + index);
return items[index];
}

/**
* Set the value of the array element at a given index.
* Throws ArrayIndexOutOfBoundsException if the index is not valid.
*/
public void set( int index, int item ) {
if ( index < 0 || index >= itemCt )
throw new ArrayIndexOutOfBoundsException("Illegal index, " + index);
items[index] = item;
}

/**
* Returns the number of items currently in the array.
*/
public int size() {
return itemCt;
}

/**
* Adds a new item to the end of the array.  The size increases by one.
*/
public void add(int item) {
if (itemCt == items.length)
items = Arrays.copyOf( items, 2*items.length );
items[itemCt] = item;
itemCt++;
}

/**
* Removes the item at a given index in the array.  The size of the array
* decreases by one.  Items following the removed item are moved up one
* space in the array.
* Throws ArrayIndexOutOfBoundsException if the index is not valid.
*/
public void remove(int index) {
if ( index < 0 || index >= itemCt )
throw new ArrayIndexOutOfBoundsException("Illegal index, " + index);
for (int j = index+1; j < itemCt; j++)
items[j-1] = items[j];
itemCt--;
}

} // end class DynamicArrayOfInt

Everything here should be clear, except possibly why the original size of the items array is 8. In fact, the number 8 is arbitrary and has no effect on the functionality of the class. Any positive integer would work, but it doesn't make sense for the array to start off very big. The array will grow as needed if the number of items turns out to be large.

The example ReverseInputNumbers.java used a partially full array of int to print a list of input numbers in the reverse of the order in which they are input. In that case, an ordinary array of length 100 was used to hold the numbers. In any given run of the program, the size of the array could be much too large, or it could be too small, resulting in an exception. The program can now be written using a DynamicArrayOfInt, which will adapt itself to any number of inputs. For the program, see ReverseWithDynamicArray.java. It's a silly program, but the principle holds in any application where the amount of data cannot be predicted in advance: The size of a dynamic data structure can adapt itself to any amount of data.

This is a nice example, but there is a real problem with it. Suppose that we want to have a dynamic array of String. We can't use a DynamicArrayOfInt object to hold strings, so it looks like we need to write a whole new class, DynamicArrayOfString. If we want a dynamic array to store players in a game, we would need a class DynamicArrayOfPlayer. And so on. It looks like we have to write a dynamic array class for every possible type of data! That can't be right! In fact, Java has a solution to this problem, a standard class that implements dynamic arrays and can work with any type of data. The class is called ArrayList, and we'll see how it works in the next section.

Section 7.3

ArrayList

As we have just seen in Subsection 7.2.4, we can easily encode the dynamic array pattern into a class, but it looks like we need a different class for each data type. In fact, Java has a feature called "parameterized types" that makes it possible to avoid the multitude of classes, and Java has a single class named ArrayList that implements the dynamic array pattern for all data types.

7.3.1  ArrayList and Parameterized Types

Java has a standard type ArrayList<String> that represents dynamic arrays of Strings. Similarly, there is a type ArrayList<JButton> that can be used to represent dynamic arrays of JButtons. And if Player is a class representing players in a game, then the type ArrayList<Player> can be used to represent a dynamic array of Players.

It might look like we still have a multitude of classes here, but in fact there is only one class, the ArrayList class, defined in the package java.util. But ArrayList is a parameterized type. A parameterized type can take a type parameter, so that from the single class ArrayList, we get a multitude of types including ArrayList<String>, ArrayList<JButton>, and in fact ArrayList<T> for any object type T. The type parameter T must be an object type such as a class name or an interface name. It cannot be a primitive type. This means that, unfortunately, you can not have an ArrayList of int or an ArrayList of char.

Consider the type ArrayList<String>. As a type, it can be used to declare variables, such as

ArrayList<String> namelist;

It can also be used as the type of a formal parameter in a subroutine definition, or as the return type of a subroutine. It can be used with the new operator to create objects:

namelist = new ArrayList<String>();

The object created in this way is of type ArrayList<String> and represents a dynamic list of strings. It has instance methods such as namelist.add(str) for adding a String to the list, namelist.get(i) for getting the string at index i, and namelist.size() for getting the number of items currently in the list.

But we can also use ArrayList with other types. If Player is a class representing players in a game, we can create a list of players with

ArrayList<Player> playerList = new ArrayList<Player>();

Then to add a player, plr, to the game, we just have to say playerList.add(plr). And we can remove player number k with playerList.remove(k).

When you use a type such as ArrayList<T>, the compiler will ensure that only objects of type T can be added to the list. An attempt to add an object that is not of type T will be a syntax error, and the program will not compile. However, note that objects belonging to a subclass of T can be added to the list, since objects belonging to a subclass of T are still considered to be of type T. Thus, for example, a variable of type ArrayList<JComponent> can be used to hold objects of type JButton, JPanel, JTextField, or any other subclass of JComponent. (Of course, this is the same way arrays work: An object of type T[] can hold objects belonging to any subclass of T.) Similarly, if T is an interface, then any object that implements interface T can be added to the list.

An object of type ArrayList<T> has all of the instance methods that you would expect in a dynamic array implementation. Here are some of the most useful. Suppose that list is a variable of type ArrayList<T>. Then we have:

- list.size() -- This function returns the current size of the list, that is, the number of items currently in the list. The only valid positions in the list are numbers in the range 0 to list.size()-1. Note that the size can be zero. A call to the default constructor new ArrayList<T>() creates a list of size zero.

- list.add(obj) -- Adds an object onto the end of the list, increasing the size by 1. The parameter, obj, can refer to an object of type T, or it can be null.

- list.get(N) -- This function returns the value stored at position N in the list. The return type of this function is T. N must be an integer in the range 0 to list.size()-1. If N is outside this range, an error of type IndexOutOfBoundsException occurs. Calling this function is similar to referring to A[N] for an array, A, except that you can't use list.get(N) on the left side of an assignment statement.

- list.set(N, obj) -- Assigns the object, obj, to position N in the ArrayList, replacing the item previously stored at position N. The parameter obj must be of type T. The integer N must be in the range from 0 to list.size()-1. A call to this function is equivalent to the command A[N] = obj for an array A.

- list.remove(N) -- For an integer, N, this removes the N-th item in the ArrayList. N must be in the range 0 to list.size()-1. Any items in the list that come after the removed item are moved down one position. The size of the list decreases by 1.

- list.remove(obj) -- If the specified object occurs somewhere in the list, it is removed from the list. Any items in the list that come after the removed item are moved down one position. The size of the ArrayList decreases by 1. If obj occurs more than once in the list, only the first copy is removed.

- list.indexOf(obj) -- A function that searches for the object, obj, in the list. If the object is found in the list, then the position number where it is found is returned. If the object is not found, then -1 is returned.

For the last two methods listed here, obj is compared to an item in the list by calling obj.equals(item), unless obj is null. This means, for example, that strings are tested for equality by checking the contents of the strings, not their location in memory.

Java comes with several parameterized classes representing different data structures. Those classes make up the Java Collection Framework. Here we consider only ArrayList, but we will return to this important topic in much more detail in Chapter 10.

By the way, ArrayList can also be used as a non-parametrized type. This means that you can declare variables and create objects of type ArrayList such as

ArrayList list = new ArrayList();

The effect of this is similar to declaring list to be of type ArrayList<Object>. That is, list can hold any object that belongs to a subclass of Object. Since every class is a subclass of Object, this means that any object can be stored in list.

7.3.2  Wrapper Classes

As I have already noted, parameterized types don't work with the primitive types. There is no such thing as "ArrayList<int>". However, this limitation turns out not to be very limiting after all, because of the so-called wrapper classes such as Integer and Character.

We have already briefly encountered the classes Double and Integer in Section 2.5. These classes contain the static methods Double.parseDouble and Integer.parseInteger that are used to convert strings to numerical values, and constants such as Integer.MAX_VALUE and Double.NaN. We have also encountered the Character class in some examples, with the static method Character.isLetter, that can be used to test whether a given value of type char is a letter. There is a similar class for each of the other primitive types, Long, Short, Byte, Float, and Boolean. These classes are wrapper classes. Although they contain useful static members, they have another use as well: They are used for creating objects that represent primitive type values.

Remember that the primitive types are not classes, and values of primitive type are not objects. However, sometimes it's useful to treat a primitive value as if it were an object. This is true, for example, when you would like to store primitive type values in an ArrayList. You can't do that literally, but you can "wrap" the primitive type value in an object belonging to one of the wrapper classes.

For example, an object of type Double contains a single instance variable, of type double. The object is a "wrapper" for the double value. You can create an object that wraps the double value 6.0221415e23 with

Double d = new Double(6.0221415e23);

The value of d contains the same information as the value of type double, but it is an object. If you want to retrieve the double value that is wrapped in the object, you can call the function d.doubleValue(). Similarly, you can wrap an int in an object of type Integer, a boolean value in an object of type Boolean, and so on.

Furthermore, to make these classes even easier to use, there is automatic conversion between a primitive type and the corresponding wrapper class. For example, if you use a value of type int in a context that requires an object of type Integer, the int will automatically be wrapped in an Integer object. If you say

Integer answer = 42;

and the computer will silently read this as if it were

Integer answer = new Integer(42);

This is called autoboxing. It works in the other direction, too. For example, if d refers to an object of type Double, you can use d in a numerical expression such as 2*d. The double value inside d is automatically unboxed and multiplied by 2. Autoboxing and unboxing also apply to subroutine calls. For example, you can pass an actual parameter of type int to a subroutine that has a formal parameter of type Integer. In fact, autoboxing and unboxing make it possible in many circumstances to ignore the difference between primitive types and objects.

This is true in particular for parameterized types. Although there is no such thing as "ArrayList<int>", there is ArrayList<Integer>. An ArrayList<Integer> holds objects of type Integer, but any object of type Integer really just represents an int value in a rather thin wrapper. Suppose that we have an object of type ArrayList<Integer>:

ArrayList<Integer> integerList;
integerList = new ArrayList<Integer>();

Then we can, for example, add an object to integerList that represents the number 42:

integerList.add( new Integer(42) );

but because of autoboxing, we can actually say

integerList.add( 42 );

and the compiler will automatically wrap 42 in an object of type Integer before adding it to the list. Similarly, we can say

int num = integerList.get(3);

The value returned by integerList.get(3) is of type Integer but because of unboxing, the compiler will automatically convert the return value into an int, as if we had said

int num = integerList.get(3).intValue();

So, in effect, we can pretty much use integerList as if it were a dynamic array of int rather than a dynamic array of Integer. Of course, a similar statement holds for lists of other wrapper classes such as ArrayList<Double> and ArrayList<Character>. (There is one issue that sometimes causes problems: A list can hold null values, and a null does not correspond to any primitive type value. This means, for example, that the statement "int num = integerList.get(3);" can produce a null pointer exception in the case where integerList.get(3) returns null. Unless you are sure that all the values in your list are non-null, you need to take this possibility into account.)

7.3.3  Programming With ArrayList

As a simple first example, we can redo ReverseWithDynamicArray.java, from the previous section, using an ArrayList instead of a custom dynamic array class. In this case, we want to store integers in the list, so we should use ArrayList<Integer>. Here is the complete program:

import java.util.ArrayList;

/**
* Reads a list of non-zero numbers from the user, then prints
* out the input numbers in the reverse of the order in which
* the were entered.  There is no limit on the number of inputs.
*/
public class ReverseWithArrayList {

public static void main(String[] args) {
ArrayList<Integer> list;
list = new ArrayList<Integer>();
System.out.println("Enter some non-zero integers.  Enter 0 to end.");
while (true) {
System.out.print("? ");
int number = TextIO.getlnInt();
if (number == 0)
break;
list.add(number);
}
System.out.println();
System.out.println("Your numbers in reverse are:");
for (int i = list.size() - 1; i >= 0; i--) {
System.out.printf("%10d%n", list.get(i));
}
}

}

As illustrated in this example, ArrayLists are commonly processed using for loops, in much the same way that arrays are processed. for example, the following loop prints out all the items for a variable namelist of type ArrayList<String>:

for ( int i = 0; i < namelist.size(); i++ ) {
String item = namelist.get(i);
System.out.println(item);
}

You can also use for-each loops with ArrayLists, so this example could also be written

for ( String item : namelist ) {
System.out.println(item);
}

When working with wrapper classes, the loop control variable in the for-each loop can be a primitive type variable. This works because of unboxing. For example, if numbers is of type ArrayList<Double>, then the following loop can be used to add up all the values in the list:

double sum = 0;
for ( double num : numbers ) {
sum = sum + num;
}

This will work as long as none of the items in the list are null. If there is a possibility of null values, then you will want to use a loop control variable of type Double and test for nulls. For example, to add up all the non-null values in the list:

double sum;
for ( Double num : numbers ) {
if ( num != null ) {
sum = sum + num;  // Here, num is SAFELY unboxed to get a double.
}
}

For a more complete and useful example, we will look at the program SimplePaint2.java. This is a much improved version of SimplePaint.java from Subsection 6.3.4. In the new program, the user can sketch curves in a drawing area by clicking and dragging with the mouse. The curves can be of any color, and the user can select the drawing color using a menu. The background color of the drawing area can also be selected using a menu. And there is a "Control" menu that contains several commands: An "Undo" command, which removes the most recently drawn curve from the screen, a "Clear" command that removes all the curves, and a "Use Symmetry" checkbox that turns a symmetry feature on and off. Curves that are drawn by the user when the symmetry option is on are reflected horizontally and vertically to produce a symmetric pattern. (Symmetry is there just to look pretty.)

Unlike the original SimplePaint program, this new version uses a data structure to store information about the picture that has been drawn by the user. This data is used in the paintComponent() method to redraw the picture whenever necessary. The data structure is implemented using ArrayLists.

The main data for a curve consists of a list of the points on the curve. This data can be stored in an object of type ArrayList<Point>, where java.awt.Point is one of Java's standard classes. (A Point object contains two public integer variables x and y that represent the pixel coordinates of a point.) However, to redraw the curve, we also need to know its color, and we need to know whether the symmetry option should be applied to the curve. All the data that is needed to redraw the curve can be grouped into an object of type CurveData that is defined as

private static class CurveData {
Color color;  // The color of the curve.
boolean symmetric;  // Are horizontal and vertical reflections also drawn?
ArrayList<Point> points;  // The points on the curve.
}

However, a picture can contain many curves, not just one, so to store all the data necessary to redraw the entire picture, we need a list of objects of type CurveData. For this list, we can use a variable curves declared as

ArrayList<CurveData> curves = new ArrayList<CurveData>();

Here we have a list of objects, where each object contains a list of points as part of its data! Let's look at a few examples of processing this data structure. When the user clicks the mouse on the drawing surface, it's the start of a new curve, and a new CurveData object must be created and added to the list of curves. The instance variables in the new CurveData object must also be initialized. Here is the code from the mousePressed() routine that does this:

currentCurve = new CurveData();        // Create a new CurveData object.

currentCurve.color = currentColor;     // The color of the curve is taken from an
// instance variable that represents the
// currently selected drawing color.

currentCurve.symmetric = useSymmetry;  // The "symmetric" property of the curve
// is also copied from the current value
// of an instance variable, useSymmetry.

currentCurve.points = new ArrayList<Point>();  // Create a new point list object.

currentCurve.points.add( new Point(evt.getX(), evt.getY()) );
// The point where the user pressed the mouse is the first point on
// the curve.  A new Point object is created to hold the coordinates
// of that point and is added to the list of points for the curve.

curves.add(currentCurve);   // Add the CurveData object to the list of curves.

As the user drags the mouse, new points are added to currentCurve, and repaint() is called. When the picture is redrawn, the new point will be part of the picture.

The paintComponent() method has to use the data in curves to draw all the curves. The basic structure is a for-each loop that processes the data for each individual curve in turn. This has the form:

for ( CurveData curve : curves ) {
.
.  // Draw the curve represented by the object, curve, of type CurveData.
.
}

In the body of this loop, curve.points is a variable of type ArrayList<Point> that holds the list of points on the curve. The i-th point on the curve can be obtained by calling the get() method of this list: curve.points.get(i). This returns a value of type Point which contains instance variables named x and y. We can refer directly to the x-coordinate of the i-th point as:

curve.points.get(i).x

This might seem rather complicated, but it's a nice example of a complex name that specifies a path to a desired piece of data: Go to the object, curve. Inside curve, go to points. Inside points, get the i-th item. And from that item, get the instance variable named x. Here is the complete definition of the paintComponent() method:

public void paintComponent(Graphics g) {
super.paintComponent(g);
Graphics2D g2 = (Graphics2D)g;
g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
RenderingHints.VALUE_ANTIALIAS_ON);
for ( CurveData curve : curves) {
g.setColor(curve.color);
for (int i = 1; i < curve.points.size(); i++) {
// Draw a line segment from point number i-1 to point number i.
int x1 = curve.points.get(i-1).x;
int y1 = curve.points.get(i-1).y;
int x2 = curve.points.get(i).x;
int y2 = curve.points.get(i).y;
g.drawLine(x1,y1,x2,y2);
if (curve.symmetric) {
// Also draw the horizontal and vertical reflections
// of the line segment.
int w = getWidth();
int h = getHeight();
g.drawLine(w-x1,y1,w-x2,y2);
g.drawLine(x1,h-y1,x2,h-y2);
g.drawLine(w-x1,h-y1,w-x2,h-y2);
}
}
}
} // end paintComponent()

I encourage you to read the full source code, SimplePaint2.java, and to try out the program. In addition to serving as an example of using parameterized types, it also serves as another example of creating and using menus.

7.3.4  Vectors

Early versions of Java did not include ArrayList, but they did have a very similar class named java.util.Vector that serves much the same purpose. You can still see Vectors used in older code and in many of Java's standard classes, so it's worth knowing about them. Vector is a parameterized class, so that you can use types such as Vector<String> and Vector<Integer>, but you will often see it used without the type parameter, which is essentially equivalent to using Vector<Object>.

Using a Vector is similar to using an ArrayList, except that different names are used for some commonly used instance methods, and some instance methods in one class don't correspond to any instance method in the other class. Suppose that vec is a variable of type Vector<T>. Then we have instance methods:

- vec.size() -- a function that returns the number of elements currently in the vector.

- vec.elementAt(N) -- returns the N-th element of the vector, for an integer N. N must be in the range 0 to vec.size()-1. This is the same as get(N) for an ArrayList.

- vec.setElementAt(obj,N) -- sets the N-th element in the vector to be obj. N must be in the range 0 to vec.size()-1. This is the same as set(N,obj) for an ArrayList.

- vec.addElement(obj) -- adds the Object, obj, to the end of the vector. This is the same as the add() method of an ArrayList.

- vec.removeElement(obj) -- removes obj from the vector, if it occurs. Only the first occurrence is removed. This is the same as remove(obj) for an ArrayList.

- vec.removeElementAt(N) -- removes the N-th element, for an integer N. N must be in the range 0 to vec.size()-1. This is the same as remove(N) for an ArrayList.

- vec.setSize(N) -- sets the size of the vector to N. If there were more than N elements in vec, the extra elements are removed. If there were fewer than N elements, extra spaces are filled with null. The ArrayList class, unfortunately, does not have a corresponding setSize() method.

Section 7.4

Searching and Sorting

Two array processing techniques that are particularly common are searching and sorting. Searching here refers to finding an item in the array that meets some specified criterion. Sorting refers to rearranging all the items in the array into increasing or decreasing order (where the meaning of increasing and decreasing can depend on the context). We have seen in Subsection 7.2.2 that Java has some built-in methods for searching and sorting arrays. However, a computer science student should be familiar with the algorithms that are used in those methods. In this section, you will learn some algorithms for searching and sorting.

Sorting and searching are often discussed, in a theoretical sort of way, using an array of numbers as an example. In practical situations, though, more interesting types of data are usually involved. For example, the array might be a mailing list, and each element of the array might be an object containing a name and address. Given the name of a person, you might want to look up that person's address. This is an example of searching, since you want to find the object in the array that contains the given name. It would also be useful to be able to sort the array according to various criteria. One example of sorting would be ordering the elements of the array so that the names are in alphabetical order. Another example would be to order the elements of the array according to zip code before printing a set of mailing labels. (This kind of sorting can get you a cheaper postage rate on a large mailing.)

This example can be generalized to a more abstract situation in which we have an array that contains objects, and we want to search or sort the array based on the value of one of the instance variables in the objects. We can use some terminology here that originated in work with "databases," which are just large, organized collections of data. We refer to each of the objects in the array as a record. The instance variables in an object are then called fields of the record. In the mailing list example, each record would contain a name and address. The fields of the record might be the first name, last name, street address, state, city and zip code. For the purpose of searching or sorting, one of the fields is designated to be the key field. Searching then means finding a record in the array that has a specified value in its key field. Sorting means moving the records around in the array so that the key fields of the record are in increasing (or decreasing) order.

In this section, most of my examples follow the tradition of using arrays of numbers. But I'll also give a few examples using records and keys, to remind you of the more practical applications.

7.4.1  Searching

There is an obvious algorithm for searching for a particular item in an array: Look at each item in the array in turn, and check whether that item is the one you are looking for. If so, the search is finished. If you look at every item without finding the one you want, then you can be sure that the item is not in the array. It's easy to write a subroutine to implement this algorithm. Let's say the array that you want to search is an array of ints. Here is a method that will search the array for a specified integer. If the integer is found, the method returns the index of the location in the array where it is found. If the integer is not in the array, the method returns the value -1 as a signal that the integer could not be found:

/**
* Searches the array A for the integer N.  If N is not in the array,
* then -1 is returned.  If N is in the array, then the return value is
* the first integer i that satisfies A[i] == N.
*/
static int find(int[] A, int N) {

for (int index = 0; index < A.length; index++) {
if ( A[index] == N )
return index;  // N has been found at this index!
}

// If we get this far, then N has not been found
// anywhere in the array.  Return a value of -1.

return -1;

}

This method of searching an array by looking at each item in turn is called linear search. If nothing is known about the order of the items in the array, then there is really no better alternative algorithm. But if the elements in the array are known to be in increasing or decreasing order, then a much faster search algorithm can be used. An array in which the elements are in order is said to be sorted. Of course, it takes some work to sort an array, but if the array is to be searched many times, then the work done in sorting it can really pay off.

Binary search is a method for searching for a given item in a sorted array. Although the implementation is not trivial, the basic idea is simple: If you are searching for an item in a sorted list, then it is possible to eliminate half of the items in the list by inspecting a single item. For example, suppose that you are looking for the number 42 in a sorted array of 1000 integers. Let's assume that the array is sorted into increasing order. Suppose you check item number 500 in the array, and find that the item is 93. Since 42 is less than 93, and since the elements in the array are in increasing order, we can conclude that if 42 occurs in the array at all, then it must occur somewhere before location 500. All the locations numbered 500 or above contain values that are greater than or equal to 93. These locations can be eliminated as possible locations of the number 42.

The next obvious step is to check location 250. If the number at that location is, say, -21, then you can eliminate locations before 250 and limit further search to locations between 251 and 499. The next test will limit the search to about 125 locations, and the one after that to about 62. After just 10 steps, there is only one location left. This is a whole lot better than looking through every element in the array. If there were a million items, it would still take only 20 steps for binary search to search the array! (Mathematically, the number of steps is approximately equal to the logarithm, in the base 2, of the number of items in the array.)

In order to make binary search into a Java subroutine that searches an array A for an item N, we just have to keep track of the range of locations that could possibly contain N. At each step, as we eliminate possibilities, we reduce the size of this range. The basic operation is to look at the item in the middle of the range. If this item is greater than N, then the second half of the range can be eliminated. If it is less than N, then the first half of the range can be eliminated. If the number in the middle just happens to be N exactly, then the search is finished. If the size of the range decreases to zero, then the number N does not occur in the array. Here is a subroutine that returns the location of N in a sorted array A. If N cannot be found in the array, then a value of -1 is returned instead:

/**
* Searches the array A for the integer N.
* Precondition:  A must be sorted into increasing order.
* Postcondition: If N is in the array, then the return value, i,
*    satisfies A[i] == N.  If N is not in the array, then the
*    return value is -1.
*/
static int binarySearch(int[] A, int N) {

int lowestPossibleLoc = 0;
int highestPossibleLoc = A.length - 1;

while (highestPossibleLoc >= lowestPossibleLoc) {
int middle = (lowestPossibleLoc + highestPossibleLoc) / 2;
if (A[middle] == N) {
// N has been found at this index!
return middle;
}
else if (A[middle] > N) {
// eliminate locations >= middle
highestPossibleLoc = middle - 1;
}
else {
// eliminate locations <= middle
lowestPossibleLoc = middle + 1;
}
}

// At this point, highestPossibleLoc < LowestPossibleLoc,
// which means that N is known to be not in the array.  Return
// a -1 to indicate that N could not be found in the array.

return -1;

}

7.4.2  Association Lists

One particularly common application of searching is with association lists. The standard example of an association list is a dictionary. A dictionary associates definitions with words. Given a word, you can use the dictionary to look up its definition. We can think of the dictionary as being a list of pairs of the form (w,d), where w is a word and d is its definition. A general association list is a list of pairs (k,v), where k is some "key" value, and v is a value associated to that key. In general, we want to assume that no two pairs in the list have the same key. There are two basic operations on association lists: Given a key, k, find the value v associated with k, if any. And given a key, k, and a value v, add the pair (k,v) to the association list (replacing the pair, if any, that had the same key value). The two operations are usually called get and put.

Association lists are very widely used in computer science. For example, a compiler has to keep track of the location in memory associated with each variable. It can do this with an association list in which each key is a variable name and the associated value is the address of that variable in memory. Another example would be a mailing list, if we think of it as associating an address to each name on the list. As a related example, consider a phone directory that associates a phone number to each name. We'll look at a highly simplified version of this example. (This is not meant to be a realistic way to implement a phone directory.)

The items in the phone directory's association list could be objects belonging to the class:

class PhoneEntry {
String name;
String phoneNum;
}

The data for a phone directory consists of an array of type PhoneEntry[] and an integer variable to keep track of how many entries are actually stored in the directory. The technique of dynamic arrays (Subsection 7.2.4) can be used in order to avoid putting an arbitrary limit on the number of entries that the phone directory can hold. Using an ArrayList would be another possibility. A PhoneDirectory class should include instance methods that implement the "get" and "put" operations. Here is one possible simple definition of the class:

/**
* A PhoneDirectory holds a list of names with a phone number for
* each name.  It is possible to find the number associated with
* a given name, and to specify the phone number for a given name.
*/
public class PhoneDirectory {

/**
* An object of type PhoneEntry holds one name/number pair.
*/
private static class PhoneEntry {
String name;     // The name.
String number;   // The associated phone number.
}

private PhoneEntry[] data;  // Array that holds the name/number pairs.
private int dataCount;      // The number of pairs stored in the array.

/**
* Constructor creates an initially empty directory.
*/
public PhoneDirectory() {
data = new PhoneEntry[1];
dataCount = 0;
}

/**
* Looks for a name/number pair with a given name.  If found, the index
* of the pair in the data array is returned.  If no pair contains the
* given name, then the return value is -1.  This private method is
* used internally in getNumber() and putNumber().
*/
private int find( String name ) {
for (int i = 0; i < dataCount; i++) {
if (data[i].name.equals(name))
return i;  // The name has been found in position i.
}
return -1;  // The name does not exist in the array.
}

/**
* Finds the phone number, if any, for a given name.
* @return The phone number associated with the name; if the name does
*    not occur in the phone directory, then the return value is null.
*/
public String getNumber( String name ) {
int position = find(name);
if (position == -1)
return null;   // There is no phone entry for the given name.
else
return data[position].number;
}

/**
* Associates a given name with a given phone number.  If the name
* already exists in the phone directory, then the new number replaces
* the old one.  Otherwise, a new name/number pair is added.  The
* name and number should both be non-null.  An IllegalArgumentException
* is thrown if this is not the case.
*/
public void putNumber( String name, String number ) {
if (name == null || number == null)
throw new IllegalArgumentException("name and number cannot be null");
int i = find(name);
if (i >= 0) {
// The name already exists, in position i in the array.
// Just replace the old number at that position with the new.
data[i].number = number;
}
else {
// Add a new name/number pair to the array.  If the array is
// already full, first create a new, larger array.
if (dataCount == data.length) {
data = Arrays.copyOf( data, 2*data.length );
}
PhoneEntry newEntry = new PhoneEntry();  // Create a new pair.
newEntry.name = name;
newEntry.number = number;
data[dataCount] = newEntry;   // Add the new pair to the array.
dataCount++;
}
}

} // end class PhoneDirectory

The class defines a private instance method, find(), that uses linear search to find the position of a given name in the array of name/number pairs. The find() method is used both in the getNumber() method and in the putNumber() method. Note in particular that putNumber(name,number) has to check whether the name is in the phone directory. If so, it just changes the number in the existing entry; if not, it has to create a new phone entry and add it to the array.

This class could use a lot of improvement. For one thing, it would be nice to use binary search instead of simple linear search in the getNumber method. However, we could only do that if the list of PhoneEntries were sorted into alphabetical order according to name. In fact, it's really not all that hard to keep the list of entries in sorted order, as you'll see in the next subsection.

I will mention that association lists are also called "maps," and Java has a standard parameterized type name Map that implements association lists for keys and values of any type. You will encounter this class in Chapter 10.

7.4.3  Insertion Sort

We've seen that there are good reasons for sorting arrays. There are many algorithms available for doing so. One of the easiest to understand is the insertion sort algorithm. This technique is also applicable to the problem of keeping a list in sorted order as you add new items to the list. Let's consider that case first:

Suppose you have a sorted list and you want to add an item to that list. If you want to make sure that the modified list is still sorted, then the item must be inserted into the right location, with all the smaller items coming before it and all the bigger items after it. This will mean moving each of the bigger items up one space to make room for the new item.

/*
* Precondition:  itemsInArray is the number of items that are
*    stored in A.  These items must be in increasing order
*    (A[0] <= A[1] <= ... <= A[itemsInArray-1]).
*    The array size is at least one greater than itemsInArray.
* Postcondition:  The number of items has increased by one,
*    newItem has been added to the array, and all the items
*    in the array are still in increasing order.
* Note:  To complete the process of inserting an item in the
*    array, the variable that counts the number of items
*    in the array must be incremented, after calling this
*    subroutine.
*/
static void insert(int[] A, int itemsInArray, int newItem) {

int loc = itemsInArray - 1;  // Start at the end of the array.

/* Move items bigger than newItem up one space;
Stop when a smaller item is encountered or when the
beginning of the array (loc == 0) is reached. */

while (loc >= 0 && A[loc] > newItem) {
A[loc + 1] = A[loc];  // Bump item from A[loc] up to loc+1.
loc = loc - 1;        // Go on to next location.
}

A[loc + 1] = newItem;  // Put newItem in last vacated space.

}

Conceptually, this could be extended to a sorting method if we were to take all the items out of an unsorted array, and then insert them back into the array one-by-one, keeping the list in sorted order as we do so. Each insertion can be done using the insert routine given above. In the actual algorithm, we don't really take all the items from the array; we just remember what part of the array has been sorted:

static void insertionSort(int[] A) {
// Sort the array A into increasing order.

int itemsSorted; // Number of items that have been sorted so far.

for (itemsSorted = 1; itemsSorted < A.length; itemsSorted++) {
// Assume that items A[0], A[1], ... A[itemsSorted-1]
// have already been sorted.  Insert A[itemsSorted]
// into the sorted part of the list.

int temp = A[itemsSorted];  // The item to be inserted.
int loc = itemsSorted - 1;  // Start at end of list.

while (loc >= 0 && A[loc] > temp) {
A[loc + 1] = A[loc]; // Bump item from A[loc] up to loc+1.
loc = loc - 1;       // Go on to next location.
}

A[loc + 1] = temp; // Put temp in last vacated space.
}
}

Here is an illustration of one stage in insertion sort. It shows what happens during one execution of the for loop in the above method, when itemsSorted is 5:

7.4.4  Selection Sort

Another typical sorting method uses the idea of finding the biggest item in the list and moving it to the end -- which is where it belongs if the list is to be in increasing order. Once the biggest item is in its correct location, you can then apply the same idea to the remaining items. That is, find the next-biggest item, and move it into the next-to-last space, and so forth. This algorithm is called selection sort. It's easy to write:

static void selectionSort(int[] A) {
// Sort A into increasing order, using selection sort

for (int lastPlace = A.length-1; lastPlace > 0; lastPlace--) {
// Find the largest item among A[0], A[1], ...,
// A[lastPlace], and move it into position lastPlace
// by swapping it with the number that is currently
// in position lastPlace.

int maxLoc = 0;  // Location of largest item seen so far.

for (int j = 1; j <= lastPlace; j++) {
if (A[j] > A[maxLoc]) {
// Since A[j] is bigger than the maximum we've seen
// so far, j is the new location of the maximum value
// we've seen so far.
maxLoc = j;
}
}

int temp = A[maxLoc];  // Swap largest item with A[lastPlace].
A[maxLoc] = A[lastPlace];
A[lastPlace] = temp;

}  // end of for loop

}

A variation of selection sort is used in the Hand class that was introduced in Subsection 5.4.1. (By the way, you are finally in a position to fully understand the source code for Hand class class from that section; note that it uses ArrayList. The source file is Hand.java.)

In the Hand class, a hand of playing cards is represented by an ArrayList<Card>. The objects stored in the list are of type Card. A Card object contains instance methods getSuit() and getValue() that can be used to determine the suit and value of the card. In my sorting method, I actually create a new list and move the cards one-by-one from the old list to the new list. The cards are selected from the old list in increasing order. In the end, the new list becomes the hand and the old list is discarded. This is not the most efficient procedure. But hands of cards are so small that the inefficiency is negligible. Here is the code for sorting cards by suit:

/**
* Sorts the cards in the hand so that cards of the same suit are
* grouped together, and within a suit the cards are sorted by value.
* Note that aces are considered to have the lowest value, 1.
*/
public void sortBySuit() {
ArrayList<Card> newHand = new ArrayList<Card>();
while (hand.size() > 0) {
int pos = 0;  // Position of minimal card.
Card c = hand.get(0);  // Minimal card.
for (int i = 1; i < hand.size(); i++) {
Card c1 = hand.get(i);
if ( c1.getSuit() < c.getSuit() ||
(c1.getSuit() == c.getSuit() && c1.getValue() < c.getValue()) ) {
pos = i;
c = c1;
}
}
hand.remove(pos);
newHand.add(c);
}
hand = newHand;
}

This example illustrates the fact that comparing items in a list is not usually as simple as using the operator "<". In this case, we consider one card to be less than another if the suit of the first card is less than the suit of the second, and also if the suits are the same and the value of the second card is less than the value of the first. The second part of this test ensures that cards with the same suit will end up sorted by value.

Sorting a list of Strings raises a similar problem: the "<" operator is not defined for strings. However, the String class does define a compareTo method. If str1 and str2 are of type String, then

str1.compareTo(str2)

returns an int that is 0 when str1 is equal to str2, is less than 0 when str1 precedes str2, and is greater than 0 when str1 follows str2. The definition of "precedes" and "follows" for strings uses what is called lexicographic ordering, which is based on the Unicode values of the characters in the strings. Lexicographic ordering is not the same as alphabetical ordering, even for strings that consist entirely of letters (because in lexicographic ordering, all the upper case letters come before all the lower case letters). However, for words consisting strictly of the 26 lower case letters in the English alphabet, lexicographic and alphabetic ordering are the same. (The same holds true for uppercase letters.) Thus, if str1 and str2 are strings containing only letters from the English alphabet, then the test

str1.toLowerCase().compareTo(str2.toLowerCase()) < 0

is true if and only if str1 comes before str2 in alphabetical order.

Insertion sort and selection sort are suitable for sorting fairly small arrays (up to a few hundred elements, say). There are more complicated sorting algorithms that are much faster than insertion sort and selection sort for large arrays, to the same degree that binary search is faster than linear search. The standard method Arrays.sort uses these fast sorting algorithms. I'll discuss one such algorithm in Chapter 9.

7.4.5  Unsorting

I can't resist ending this section on sorting with a related problem that is much less common, but is a bit more fun. That is the problem of putting the elements of an array into a random order. The typical case of this problem is shuffling a deck of cards. A good algorithm for shuffling is similar to selection sort, except that instead of moving the biggest item to the end of the list, an item is selected at random and moved to the end of the list. Here is a subroutine to shuffle an array of ints:

/**
* Postcondition:  The items in A have been rearranged into a random order.
*/
static void shuffle(int[] A) {
for (int lastPlace = A.length-1; lastPlace > 0; lastPlace--) {
// Choose a random location from among 0,1,...,lastPlace.
int randLoc = (int)(Math.random()*(lastPlace+1));
// Swap items in locations randLoc and lastPlace.
int temp = A[randLoc];
A[randLoc] = A[lastPlace];
A[lastPlace] = temp;
}
}

Section 7.5

Two-dimensional Arrays

Two-dimensional arrays were introduced in Subsection 3.8.5, but we haven't done much with them since then. A 2D array has a type such as int[][] or String[][], with two pairs of square brackets. The elements of a 2D array are arranged in rows and columns, and the new operator for 2D arrays specifies both the number of rows and the number of columns. For example,

int[][] A;
A = new int[3][4];

This creates a 2D array of int that has 12 elements arranged in 3 rows and 4 columns. Although I haven't mentioned it, there are initializers for 2D arrays. For example, this statement creates the 4-by-3 array that is shown in the picture below:

int[][]  A  =  {  {  1,  0, 12, -1 },
{  7, -3,  2,  5 },
{ -5, -2,  2, -9 }
};

An array initializer for a 2D array contains the rows of A, separated by commas and enclosed between braces. Each row, in turn, is a list of values separated by commas and enclosed between braces. There are also 2D array literals with a similar syntax that can be used anywhere, not just in declarations. For example,

A  =  new int[][] {  {  1,  0, 12, -1 },
{  7, -3,  2,  5 },
{ -5, -2,  2, -9 }
};

All of this extends naturally to three-dimensional, four-dimensional, and even higher-dimensional arrays, but they are not used very often in practice.

7.5.1  The Truth About 2D Arrays

But before we go any farther, there is a little surprise. Java does not actually have two-dimensional arrays. In a true 2D array, all the elements of the array occupy a continuous block of memory, but that's not true in Java. The syntax for array types is a clue: For any type BaseType, we should be able to form the type BaseType[], meaning "array of BaseType." If we use int[] as the base type, the type that we get is "int[][] meaning "array of int[]" or "array of array of int." And in fact, that's what happens. The elements in a 2D array of type int[][] are variables of type int[]. And remember that a variable of type int[] can only hold a pointer to an array of int. So, a 2D array is really an array of pointers, where each pointer can refer to a one-dimensional array. Those one-dimensional arrays are the rows of the 2D array. A picture will help to explain this. Consider the 4-by-3 array A defined above.

For the most part, you can ignore the reality and keep the picture of a grid in mind. Sometimes, though, you will need to remember that each row in the grid is really an array in itself. These arrays can be referred to as A[0], A[1], and A[2]. Each row is in fact a value of type int[]. It could, for example, be passed to a subroutine that asks for a parameter of type int[].

Some of the consequences of this structure are a little subtle. For example, thinking of a 2D array, A, as an array of arrays, we see that A.length makes sense and is equal to the number of rows of A. If A has the usual shape for a 2D array, then the number of columns in A would be the same as the number of elements in the first row, that is, A[0].length. But there is no rule that says that all of the rows of A must have the same length (although an array created with new BaseType[rows][columns] will always have that form). Each row in a 2D array is a separate one-dimensional array, and each of those arrays can have a different length. In fact, it's even possible for a row to be null. For example, the statement

A = new int[3][];

with no number in the second set of brackets, creates an array of 3 elements where all the elements are null. There are places for three rows, but no actual rows have been created. You can then create the rows A[0], A[1], and A[2] individually.

As an example, consider a symmetric matrix. A symmetric matrix, M, is a two-dimensional array in which the number of rows is equal to the number of columns and satisfying M[i][j] equals M[j][i] for all i and j. Because of this equality, we only really need to store M[i][j] for i >= j. We can store the data in a "triangular matrix":

It's easy enough to make a triangular array, if we create each row separately. To create a 7-by-7 triangular array of double, we can use the code segment

double[][] matrix = new double[7][]; // rows have not yet been created!
for (int i = 0; i < 7; i++) {
matrix[i] = new double[i+1];  // Create row i with i + 1 elements.
}

We just have to remember that if we want to know the value of the matrix at (i,j), and if i < j, then we actually have to get the value of matrix[j][i] in the triangular matrix. And similarly for setting values. It's easy to write a class to represent symmetric matrices:

/**
* Represents symmetric n-by-n matrices of real numbers.
*/
public class SymmetricMatrix {

private double[][] matrix;  // A triangular matrix to hold the data.

/**
* Creates an n-by-n symmetric matrix in which all entries are 0.
*/
public SymmetricMatrix(int n) {
matrix = new double[n][];
for (int i = 0; i < n; i++)
matrix[i] = new double[i+1];
}

/**
* Returns the matrix entry at position (row,col).
*/
public double get( int row, int col ) {
if (row >= col)
return matrix[row][col];
else
return matrix[col][row];
}

/**
* Sets the value of the matrix entries at (row,col) and (col,row).
*/
public void set( int row, int col, double value ) {
if (row >= col)
matrix[row][col] = value;
else
matrix[col][row] = value;
}

/**
* Returns the number of rows and columns in the matrix.
*/
public int size() {
return matrix.length;  // The size is the number of rows.
}

} // end class SymmetricMatrix

This class is in the file SymmetricMatrix.java, and a small program to test it can be found in TestSymmetricMatrix.java.

By the way, the standard function Arrays.copyOf() can't make a full copy of a 2D array in a single step. To do that, you need to copy each row separately. To make a copy of a two-dimensional array of int, for example:

int[][] B = new int[A.length][];  // B has as many rows as A.
for (int i = 0; i < A.length; i++) {
B[i] = Arrays.copyOf(A[i], A[i].length)); // Copy row i.
}

7.5.2  Conway's Game Of Life

As an example of more typical 2D array processing, let's look at a very well-known example: John Conway's Game of Life, invented by mathematician John Horton Conway in 1970. This Game of Life is not really a game (although sometimes it's referred to as a "zero-person game" that plays itself). It's a "two-dimensional cellular automaton." This just means that it's a grid of cells whose content changes over time according to definite, deterministic rules. In Life, a cell can only have two possible contents: It can be "alive" or "dead." We will use a 2D array to represent the grid, with each element of the array representing the content of one cell in the grid. In the game, an initial grid is set up in which each cell is marked as either alive or dead. After that, the game "plays itself." The grid evolves through a series of time steps. The contents of the grid at each time step are completely determined by the contents at the previous time step, according to simple rules. Each cell in the grid looks at its eight neighbors (horizontal, vertical, and diagonal) and counts how many of its neighbors are alive. Then the state of the cell in the next step is determined by the rules:

- If the cell is alive in the current time step: If the cell has 2 or 3 living neighbors, then the cell remains alive in the next time step; otherwise, it dies. (A living cell dies of loneliness if it has 0 or 1 living neighbor, and of overcrowding if it has more than 3 living neighbors.)

- If the cell is dead the current time step: If the cell has 3 living neighbors, then the cell becomes alive in the next time step; otherwise, it remains dead. (Three living cells give birth to a new living cell.)

Here's a picture of part of a Life board, showing the same board before and after the rules have been applied. The rules are applied to every cell in the grid. The picture shows how they apply to four of the cells:

The Game of Life is interesting because it gives rise to many interesting and surprising patterns. (Look it up on Wikipedia.) Here, we are just interested in writing a program to simulate the game. The complete program can be found in the file Life.java. In the program, the life grid is shown as an array of squares. Dead squares are black, and living squares are white. (The program uses MosaicPanel.java from Section 4.6 to represent the grid, so you will also need that file to compile and run the program.) In the program, you can fill the life board randomly with dead and alive cells, or you can use the mouse to set up the game board. There is a "Step" button that will compute one time-step of the game, and a "Start" button that will run time steps as an animation.

We'll look at some of the array processing involved in implementing the Game of Life for this program. Since a cell can only be alive or dead, it is natural to use a two-dimensional array of boolean[][] to represent the states of all the cells. The array is named alive, and alive[r][c] is true when the cell in row r, column c is alive. The number of rows and the number of columns are equal and are given by a constant, GRID_SIZE. So, for example, to fill the Life grid with random values, the program uses simple nested for loops:

for (int r = 0; r < GRID_SIZE; r++) {
for (int c = 0; c < GRID_SIZE; c++) {
// Use a 25% probability that the cell is alive.
alive[r][c] = (Math.random() < 0.25);
}
}

Note that the expression (Math.random() < 0.25) is a true/false value that can be assigned to a boolean array element. The array is also used to set the color of the cells on the screen. Since the grid of cells is displayed on screen as a MosaicPanel, setting the colors is done using the MosaicPanel API. Note that the actual drawing is done in the paintComponent() method of the MosaicPanel (which has its own 2D array of type Color[][] to keep track of the colors of each cell). The Life program just has to set the colors in the mosaic. This is done in the program in a method named showBoard() that is called each time the board changes. Again, simple nested for loops are used to set the color of each square in the grid:

for (int r = 0; r < GRID_SIZE; r++) {
for (int c = 0; c < GRID_SIZE; c++) {
if (alive[r][c])
display.setColor(r,c,Color.WHITE);
else
display.setColor(r,c,null);  // Shows the background color, black.
}
}

Of course, the most interesting part of the program is computing the new state of the board by applying the rules to the current state. The rules apply to each individual cell, so again we can use nested for loops to work through all the cells on the board, but this time the processing is more complicated. Note first that we can't make changes to the values in the array as we work through it, since we will need to know the old state of a cell when processing its neighboring cells. In fact, the program uses a second array to hold the new board as it is being created. When the new board is finished, it can be substituted for the old board. The algorithm goes like this in pseudocode:

let newboard be a new boolean[][] array
for each row r:
for each column c:
Let N be the number of neighbors of cell (r,c) in the alive array
if ((N is 3) or (N is 2 and alive[r][c]))
newboard[r][c] = true;
else
newboard[r][c] = false;
alive = newboard

Note that at the end of the process, alive is pointing to a new array. This doesn't matter as long as the contents of the array represent the new state of the game. The old array will be garbage collected. The test for whether newboard[r][c] should be true or false might not be obvious, but it implements the rules correctly. We still need to work on counting the neighbors. Consider the cell in row r and column c. If it's not at an edge of the board, then it's clear where its neighbors are:

The row above row number r is row number r-1, and the row below is r+1. Similarly for the columns. We just have to look at the values of alive[r-1][c-1], alive[r-1][c], alive[r-1][c+1], alive[r][c-1], alive[r][c+1], alive[r+1][c-1], alive[r+1][c], and alive[r+1][c+1], and count the number that are true. (You should make sure that you understand how the array indexing works here.)

But there is a problem when the cell is along one of the edges of the grid. In that case, some of the array elements in the list don't exist, and an attempt to use them will cause an exception. To avoid the exception, we have to give special consideration to cells along the edges. One idea is that before referencing any array element, check that the array element actually exists. In that case, the code for neighbor counting becomes

if (r-1 >= 0 && c-1 >= 0 && alive[r-1][c-1])
N++; // A cell at position (r-1,c-1) exists and is alive.
if (r-1 >= 0 && alive[r-1][c])
N++; // A cell at position (r-1,c) exists and is alive.
if (r-1 >= 0 && c+1 <= GRID_SIZE && alive[r-1][c+1])
N++; // A cell at position (r-1,c+1) exists and is alive.
// and so on...

All the possible exceptions are avoided. But in my program, I actually do something that is common in 2D computer games -- I pretend that the left edge of the board is attached to the right edge and the top edge to the bottom edge. For example, for a cell in row 0, we say that the row "above" is actually the bottom row, row number GRID_SIZE-1. I use variables to represent the positions above, below, left, and right of a given cell. The code turns out to be simpler than the code shown above. Here is the complete method for computing the new board:

private void doFrame() { // Compute the new state of the Life board.
boolean[][] newboard = new boolean[GRID_SIZE][GRID_SIZE];
for ( int r = 0; r < GRID_SIZE; r++ ) {
int above, below; // rows considered above and below row number r
int left, right;  // columns considered left and right of column c
above = r > 0 ? r-1 : GRID_SIZE-1;  // (for "?:" see Subsection 2.5.5)
below = r < GRID_SIZE-1 ? r+1 : 0;
for ( int c = 0; c < GRID_SIZE; c++ ) {
left =  c > 0 ? c-1 : GRID_SIZE-1;
right = c < GRID_SIZE-1 ? c+1 : 0;
int n = 0; // number of alive cells in the 8 neighboring cells
if (alive[above][left])
n++;
if (alive[above][c])
n++;
if (alive[above][right])
n++;
if (alive[r][left])
n++;
if (alive[r][right])
n++;
if (alive[below][left])
n++;
if (alive[below][c])
n++;
if (alive[below][right])
n++;
if (n == 3 || (alive[r][c] && n == 2))
newboard[r][c] = true;
else
newboard[r][c] = false;
}
}
alive = newboard;
}

Again, I urge you to check out the source code, Life.java, and try the program. Don't forget that you will also need MosaicPanel.java.

7.5.3  Checkers

As a final example for this chapter, we'll look at a more substantial example of using a 2D array. This is the longest program that we have encountered so far, with 727 lines of code. The program lets two users play checkers against each other. The checkers game is played on an eight-by-eight board, which is based on an example from Subsection 6.6.5. The players are called "red" and "black," after the color of their checkers. I'm not going to explain the rules of checkers here; possibly you can learn them by trying out the program.

In the program, a player moves by clicking on the piece that they want to move, and then clicking on the empty square to which it is to be moved. As an aid to the players, the squares that the current player can legally click are highlighted. The square containing a piece that has been selected to be moved, if any, is surrounded by a white border. Other pieces that can legally be moved are surrounded by a cyan-colored border. If a piece has has already been selected, each empty square that it can legally move to is highlighted with a green border. The game enforces the rule that if the current player can jump one of the opponent's pieces, then the player must jump. When a player's piece becomes a king, by reaching the opposite end of the board, a big white "K" is drawn on the piece. Here is a picture of the program in the middle of a game. It is black's turn to move, and the pieces that black can legally move are highlighted with a cyan border. The next step is for black to click on one of those pieces.

I will only cover a part of the programming for this example. I encourage you to read the complete source code, Checkers.java. It's long and complex, but you should understand all the techniques that it uses with some study. The program is a good example of state-based, event-driven, object-oriented programming.

The data about the pieces on the board are stored in a two-dimensional array. Because of the complexity of the program, I wanted to divide it into several classes. In addition to the main class, there are several nested classes. One of these classes is CheckersData, which handles the data for the board. It is mainly this class that I want to talk about.

The CheckersData class has an instance variable named board of type int[][]. The value of board is set to "new int[8][8]", an 8-by-8 grid of integers. The values stored in the grid are defined as constants representing the possible contents of a square on a checkerboard:

static final int
EMPTY = 0,           // Value representing an empty square.
RED = 1,             // A regular red piece.
RED_KING = 2,        // A red king.
BLACK = 3,           // A regular black piece.
BLACK_KING = 4;      // A black king.

The constants RED and BLACK are also used in my program (or, perhaps, misused) to represent the two players in the game. When a game is started, the values in the array are set to represent the initial state of the board. The grid of values looks like

A regular black piece can only move "down" the grid. That is, the row number of the square it moves to must be greater than the row number of the square it comes from. A regular red piece can only move up the grid. Kings of either color, of course, can move in both directions.

One function of the CheckersData class is to take care of all the details of making moves on the board. An instance method named makeMove() is provided to do this. When a player moves a piece from one square to another, the values of two elements in the array are changed. But that's not all. If the move is a jump, then the piece that was jumped is removed from the board. (The method checks whether the move is a jump by checking if the square to which the piece is moving is two rows away from the square where it starts.) Furthermore, a RED piece that moves to row 0 or a BLACK piece that moves to row 7 becomes a king. Putting all that into a subroutine is good programming: the rest of the program doesn't have to worry about any of these details. It just calls this makeMove() method:

/**
* Make the move from (fromRow,fromCol) to (toRow,toCol).  It is
* ASSUMED that this move is legal!  If the move is a jump, the
* jumped piece is removed from the board.  If a piece moves
* to the last row on the opponent's side of the board, the
* piece becomes a king.
*/
void makeMove(int fromRow, int fromCol, int toRow, int toCol) {

board[toRow][toCol] = board[fromRow][fromCol]; // Move the piece.
board[fromRow][fromCol] = EMPTY; // The square it moved from is now empty.

if (fromRow - toRow == 2 || fromRow - toRow == -2) {
// The move is a jump.  Remove the jumped piece from the board.
int jumpRow = (fromRow + toRow) / 2; // Row of the jumped piece.
int jumpCol = (fromCol + toCol) / 2; // Column of the jumped piece.
board[jumpRow][jumpCol] = EMPTY;
}

if (toRow == 0 && board[toRow][toCol] == RED)
board[toRow][toCol] = RED_KING;  // Red piece becomes a king.
if (toRow == 7 && board[toRow][toCol] == BLACK)
board[toRow][toCol] = BLACK_KING;  // Black piece becomes a king.

}  // end makeMove()

An even more important function of the CheckersData class is to find legal moves on the board. In my program, a move in a Checkers game is represented by an object belonging to the following class:

/**
* A CheckersMove object represents a move in the game of
* Checkers.  It holds the row and column of the piece that is
* to be moved and the row and column of the square to which
* it is to be moved.  (This class makes no guarantee that
* the move is legal.)
*/
private static class CheckersMove {

int fromRow, fromCol;  // Position of piece to be moved.
int toRow, toCol;      // Square it is to move to.

CheckersMove(int r1, int c1, int r2, int c2) {
// Constructor.  Set the values of the instance variables.
fromRow = r1;
fromCol = c1;
toRow = r2;
toCol = c2;
}

boolean isJump() {
// Test whether this move is a jump.  It is assumed that
// the move is legal.  In a jump, the piece moves two
// rows.  (In a regular move, it only moves one row.)
return (fromRow - toRow == 2 || fromRow - toRow == -2);
}

}  // end class CheckersMove.

The CheckersData class has an instance method which finds all the legal moves that are currently available for a specified player. This method is a function that returns an array of type CheckersMove[]. The array contains all the legal moves, represented as CheckersMove objects. The specification for this method reads

/**
* Return an array containing all the legal CheckersMoves
* for the specified player on the current board.  If the player
* has no legal moves, null is returned.  The value of player
* should be one of the constants RED or BLACK; if not, null
* is returned.  If the returned value is non-null, it consists
* entirely of jump moves or entirely of regular moves, since
* if the player can jump, only jumps are legal moves.
*/
CheckersMove[] getLegalMoves(int player)

A brief pseudocode algorithm for the method is

Start with an empty list of moves
Find any legal jumps and add them to the list
if there are no jumps:
Find any other legal moves and add them to the list
if the list is empty:
return null
else:
return the list

Now, what is this "list"? We have to return the legal moves in an array. But since an array has a fixed size, we can't create the array until we know how many moves there are, and we don't know that until near the end of the method, after we've already made the list! A neat solution is to use an ArrayList instead of an array to hold the moves as we find them. In fact, I use an object defined by the parameterized type ArrayList<CheckersMove> so that the list is restricted to holding objects of type CheckersMove. As we add moves to the list, it will grow just as large as necessary. At the end of the method, we can create the array that we really want and copy the data into it:

Let "moves" be an empty ArrayList<CheckersMove>
Find any legal jumps and add them to moves
if moves.size() is 0:  // There are no legal jumps!
Find any other legal moves and add them to moves
if moves.size() is 0:  // There are no legal moves at all!
return null
else:
Let moveArray be an array of CheckersMoves of length moves.size()
Copy the contents of moves into moveArray
return moveArray

Now, how do we find the legal jumps or the legal moves? The information we need is in the board array, but it takes some work to extract it. We have to look through all the positions in the array and find the pieces that belong to the current player. For each piece, we have to check each square that it could conceivably move to, and check whether that would be a legal move. If we are looking for legal jumps, we want to look at squares that are two rows and two columns away from the piece. There are four squares to consider. Thus, the line in the algorithm that says "Find any legal jumps and add them to moves" expands to:

For each row of the board:
For each column of the board:
if one of the player's pieces is at this location:
if it is legal to jump to row + 2, column + 2
add this move to moves
if it is legal to jump to row - 2, column + 2
add this move to moves
if it is legal to jump to row + 2, column - 2
add this move to moves
if it is legal to jump to row - 2, column - 2
add this move to moves

The line that says "Find any other legal moves and add them to moves" expands to something similar, except that we have to look at the four squares that are one column and one row away from the piece. Testing whether a player can legally move from one given square to another given square is itself non-trivial. The square the player is moving to must actually be on the board, and it must be empty. Furthermore, regular red and black pieces can only move in one direction. I wrote the following utility method to check whether a player can make a given non-jump move:

/**
* This is called by the getLegalMoves() method to determine
* whether the player can legally move from (r1,c1) to (r2,c2).
* It is ASSUMED that (r1,c1) contains one of the player's
* pieces and that (r2,c2) is a neighboring square.
*/
private boolean canMove(int player, int r1, int c1, int r2, int c2) {

if (r2 < 0 || r2 >= 8 || c2 < 0 || c2 >= 8)
return false;  // (r2,c2) is off the board.

if (board[r2][c2] != EMPTY)
return false;  // (r2,c2) already contains a piece.

if (player == RED) {
if (board[r1][c1] == RED && r2 > r1)
return false;  // Regular red piece can only move down.
return true;  // The move is legal.
}
else {
if (board[r1][c1] == BLACK && r2 < r1)
return false;  // Regular black piece can only move up.
return true;  // The move is legal.
}

}  // end canMove()

This method is called by my getLegalMoves() method to check whether one of the possible moves that it has found is actually legal. I have a similar method that is called to check whether a jump is legal. In this case, I pass to the method the square containing the player's piece, the square that the player might move to, and the square between those two, which the player would be jumping over. The square that is being jumped must contain one of the opponent's pieces. This method has the specification:

/**
* This is called by other methods to check whether
* the player can legally jump from (r1,c1) to (r3,c3).
* It is assumed that the player has a piece at (r1,c1), that
* (r3,c3) is a position that is 2 rows and 2 columns distant
* from (r1,c1) and that (r2,c2) is the square between (r1,c1)
* and (r3,c3).
*/
private boolean canJump(int player, int r1, int c1,
int r2, int c2, int r3, int c3) { . . .

Given all this, you should be in a position to understand the complete getLegalMoves() method. It's a nice way to finish off this chapter, since it combines several topics that we've looked at: one-dimensional arrays, ArrayLists, and two-dimensional arrays:

CheckersMove[] getLegalMoves(int player) {

if (player != RED && player != BLACK)
return null;

int playerKing;  // The constant for a King belonging to the player.
if (player == RED)
playerKing = RED_KING;
else
playerKing = BLACK_KING;

ArrayList<CheckersMove> moves = new ArrayList<CheckersMove>();
// Moves will be stored in this list.

/*  First, check for any possible jumps.  Look at each square on
the board.  If that square contains one of the player's pieces,
look at a possible jump in each of the four directions from that
square.  If there is a legal jump in that direction, put it in
the moves ArrayList.
*/

for (int row = 0; row < 8; row++) {
for (int col = 0; col < 8; col++) {
if (board[row][col] == player || board[row][col] == playerKing) {
if (canJump(player, row, col, row+1, col+1, row+2, col+2))
moves.add(new CheckersMove(row, col, row+2, col+2));
if (canJump(player, row, col, row-1, col+1, row-2, col+2))
moves.add(new CheckersMove(row, col, row-2, col+2));
if (canJump(player, row, col, row+1, col-1, row+2, col-2))
moves.add(new CheckersMove(row, col, row+2, col-2));
if (canJump(player, row, col, row-1, col-1, row-2, col-2))
moves.add(new CheckersMove(row, col, row-2, col-2));
}
}
}

/*  If any jump moves were found, then the user must jump, so we
don't add any regular moves.  However, if no jumps were found,
check for any legal regular moves.  Look at each square on
the board.  If that square contains one of the player's pieces,
look at a possible move in each of the four directions from
that square.  If there is a legal move in that direction,
put it in the moves ArrayList.
*/

if (moves.size() == 0) {
for (int row = 0; row < 8; row++) {
for (int col = 0; col < 8; col++) {
if (board[row][col] == player || board[row][col] == playerKing) {
if (canMove(player,row,col,row+1,col+1))
moves.add(new CheckersMove(row,col,row+1,col+1));
if (canMove(player,row,col,row-1,col+1))
moves.add(new CheckersMove(row,col,row-1,col+1));
if (canMove(player,row,col,row+1,col-1))
moves.add(new CheckersMove(row,col,row+1,col-1));
if (canMove(player,row,col,row-1,col-1))
moves.add(new CheckersMove(row,col,row-1,col-1));
}
}
}
}

/* If no legal moves have been found, return null.  Otherwise, create
an array just big enough to hold all the legal moves, copy the
legal moves from the ArrayList into the array, and return the array.
*/

if (moves.size() == 0)
return null;
else {
CheckersMove[] moveArray = new CheckersMove[moves.size()];
for (int i = 0; i < moves.size(); i++)
moveArray[i] = moves.get(i);
return moveArray;
}

}  // end getLegalMoves

The checkers program is complex, and you can be sure that it didn't just fall together. It took a good deal of design work to decide what classes and objects would be used, what methods should be written, and what algorithms the methods should use. The complete source code is in the file Checkers.java. Take a look!

Programming Exercises for Chapter 7

Exercise 7.1:

Write a subroutine that creates an ArrayList containing several different random integers in the range from 1 up to some specified maximum. The number of integers and the maximum allowed value for the integers should be parameters to the subroutine. Write a main() routine to test your subroutine.

See the Solution

Exercise 7.2:

Suppose that M is a two-dimensional array that has R rows and C columns. The transpose of M is defined to be an array T that has C rows and R columns such that T[i][j] = M[j][i] for each i and j. Write a function that takes an array of type int[][] as a parameter, and returns the transpose of that array. (Assume that the parameter is a typical 2D array in which all the rows have the same length.) Also write a subroutine to print a 2D array of integers in neat rows and columns, and include a main() routine to test your work.

See the Solution

Exercise 7.3:

In Subsection 7.4.4, it is mentioned that the standard sorting method Arrays.sort() is much faster and efficient than selection sort. Write a program to test this claim. To be specific, your program should create a large array filled with random real numbers. It should use both Arrays.sort() and selectionSort() to sort the array, and it should time how long it takes to perform each sort. Furthermore, it should do the same thing for a large array of random Strings. To find the times, you can use System.currentTimeMillis() (see Subsection 2.3.1 and the example TimedComputation.java).

See the Solution

Exercise 7.4:

In Exercise 6.1, you wrote a program SimpleStamperWithDrag that allows the user to place red rectangles and blue ovals in a panel by clicking and dragging the mouse. However, that program does not store any information about what has been drawn, so the panel cannot repaint itself correctly. Revise the program to use an ArrayList to store data about the contents of the panel. All drawing should be done in a paintComponent() method.

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Exercise 7.5:

Write a program that will read a sequence of positive real numbers entered by the user and will print the same numbers in sorted order from smallest to largest. The user will input a zero to mark the end of the input. Assume that at most 100 positive numbers will be entered. Do not use any built-in function such as Arrays.sort(). Do the sorting yourself.

See the Solution

Exercise 7.6:

The sample program RandomArt.java from Subsection 6.4.1 shows a different random "artwork" every four seconds. There are three types of "art", one made from lines, one from circles, and one from filled squares. However, the program does not save the data for the picture that is shown on the screen. As a result, the picture cannot be redrawn when necessary. In fact, every time paintComponent() is called, a new picture is drawn.

Write a new version of RandomArt.java that saves the data needed to redraw its pictures. The paintComponent() method should simply use the data to draw the picture. New data should be recomputed only every four seconds, in response to an event from the timer that drives the program.

To make this interesting, write a separate class for each of the three different types of art. Also write an abstract class to serve as the common base class for the three classes. Since all three types of art use a random gray background, the background color can be defined in their superclass. The superclass also contains a draw() method that draws the picture; this is an abstract method because its implementation depends on the particular type of art that is being drawn. The abstract class can be defined as:

private abstract class ArtData {
Color backgroundColor;  // The background color for the art.
ArtData() {  // Constructor sets background color to be a random gray.
int x = (int)(256*Math.random());
backgroundColor = new Color( x, x, x );
}
abstract void draw(Graphics g);  // Draws this artwork.
}

Each of the three subclasses of ArtData must define its own draw() method. It must also define instance variables to hold the data necessary to draw the picture. I suggest that you should create random data for the picture in the constructor of the class, so that constructing the object will automatically create the data for the random artwork. (One problem with this is that you can't create the data until you know the size of the panel, so you can't create an ArtData object in the constructor of the panel. One solution is to create an ArtData object at the beginning of the paintComponent() method, if the object has not already been created.) In each of the three subclasses, you will need to use one or more arrays or ArrayLists to store the data.

See the Solution

Exercise 7.7:

Write a program that will read a text file selected by the user, and will make an alphabetical list of all the different words in that file. All words should be converted to lower case, and duplicates should be eliminated from the list. The list should be written to an output file selected by the user. As discussed in Subsection 2.4.4, you can use TextIO to read and write files. Use a variable of type ArrayList<String> to store the words. It is not easy to separate a file into words as you are reading it. You can use the following method:

/**
* Read the next word from TextIO, if there is one.  First, skip past
* any non-letters in the input.  If an end-of-file is encountered before
* a word is found, return null.  Otherwise, read and return the word.
* A word is defined as a sequence of letters.  Also, a word can include
* an apostrophe if the apostrophe is surrounded by letters on each side.
* @return the next word from TextIO, or null if an end-of-file is
*     encountered
*/
private static String readNextWord() {
char ch = TextIO.peek(); // Look at next character in input.
while (ch != TextIO.EOF && ! Character.isLetter(ch)) {
// Skip past non-letters.
TextIO.getAnyChar();  // Read the character.
ch = TextIO.peek();   // Look at the next character.
}
if (ch == TextIO.EOF) // Encountered end-of-file
return null;
// At this point, we know the next character is a letter, so read a word.
String word = "";  // This will be the word that is read.
while (true) {
word += TextIO.getAnyChar();  // Append the letter onto word.
ch = TextIO.peek();  // Look at next character.
if ( ch == '\'' ) {
// The next character is an apostrophe.  Read it, and
// if the following character is a letter, add both the
// apostrophe and the letter onto the word and continue
// reading the word.  If the character after the apostrophe
// is not a letter, the word is done, so break out of the loop.
TextIO.getAnyChar();   // Read the apostrophe.
ch = TextIO.peek();    // Look at char that follows apostrophe.
if (Character.isLetter(ch)) {
word += "\'" + TextIO.getAnyChar();
ch = TextIO.peek();  // Look at next char.
}
else
break;
}
if ( ! Character.isLetter(ch) ) {
// If the next character is not a letter, the word is
// finished, so break out of the loop.
break;
}
// If we haven't broken out of the loop, next char is a letter.
}
return word;  // Return the word that has been read.
}

Note that this method will return null when the file has been entirely read. You can use this as a signal to stop processing the input file.

See the Solution

Exercise 7.8:

The game of Go Moku (also known as Pente or Five Stones) is similar to Tic-Tac-Toe, except that it is played on a much larger board and the object is to get five squares in a row rather than three. Players take turns placing pieces on a board. A piece can be placed in any empty square. The first player to get five pieces in a row -- horizontally, vertically, or diagonally -- wins. If all squares are filled before either player wins, then the game is a draw. Write a program that lets two players play Go Moku against each other.

Your program will be simpler than the Checkers program from Subsection 7.5.3. Play alternates strictly between the two players, and there is no need to highlight the legal moves. You will only need two classes, a short panel class to set up the interface and a Board class to draw the board and do all the work of the game. Nevertheless, you will probably want to look at the source code for the checkers program, Checkers.java, for ideas about the general outline of the program.

The hardest part of the program is checking whether the move that a player makes is a winning move. To do this, you have to look in each of the four possible directions from the square where the user has placed a piece. You have to count how many pieces that player has in a row in that direction. If the number is five or more in any direction, then that player wins. As a hint, here is part of the code from my program. This code counts the number of pieces that the user has in a row in a specified direction. The direction is specified by two integers, dirX and dirY. The values of these variables are 0, 1, or -1, and at least one of them is non-zero. For example, to look in the horizontal direction, dirX is 1 and dirY is 0.

int ct = 1;  // Number of pieces in a row belonging to the player.

int r, c;    // A row and column to be examined

r = row + dirX;  // Look at square in specified direction.
c = col + dirY;
while ( r >= 0 && r < 13 && c >= 0 && c < 13
&& board[r][c] == player ) {
// Square is on the board, and it
// contains one of the players' pieces.
ct++;
r += dirX;  // Go on to next square in this direction.
c += dirY;
}

r = row - dirX;  // Now, look in the opposite direction.
c = col - dirY;
while ( r >= 0 && r < 13 && c >= 0 && c < 13
&& board[r][c] == player ) {
ct++;
r -= dirX;   // Go on to next square in this direction.
c -= dirY;
}

Here is a picture of my program, just after black has won the game.

See the Solution

Quiz on Chapter 7

Question 1:

What is meant by the basetype of an array?

Question 2:

What is the purpose of the following variable-arity method? What are the values of same(1,2,3), same(17,17,17,17), and same(2)? Why?

static double same( int... value ) {
for (int i = 1; i < value.length; i++) {
if ( value[i-1] != value[i] )
return false;
}
return true;
}

Question 3:

What does it mean to sort an array?

Question 4:

What is the main advantage of binary search over linear search? What is the main disadvantage?

Question 5:

What is meant by a dynamic array? What is the advantage of a dynamic array over a regular array?

Question 6:

What does it mean to say that ArrayList is a parameterized type?

Question 7:

Suppose that a variable strlst has been declared as

ArrayList<String> strlst = new ArrayList<String>();

Assume that the list is not empty and that all the items in the list are non-null. Write a code segment that will find and print the string in the list that comes first in lexicographic order.

Question 8:

Show the exact output produced by the following code segment.

char[][] pic = new char[6][6];
for (int i = 0; i < 6; i++)
for (int j = 0; j < 6; j++) {
if ( i == j  ||  i == 0  ||  i == 5 )
pic[i][j] = '*';
else
pic[i][j] = '.';
}
for (int i = 0; i < 6; i++) {
for (int j = 0; j < 6; j++)
System.out.print(pic[i][j]);
System.out.println();
}

Question 9:

Write a complete static method that finds the largest value in an array of ints. The method should have one parameter, which is an array of type int[]. The largest number in the array should be returned as the value of the method.

Question 10:

Suppose that temperature measurements were made on each day of 2014 in each of 100 cities. The measurements have been stored in an array

int[][]  temps  =  new  int[100][365];

where temps[c][d] holds the measurement for city number c on the d th day of the year. Write a code segment that will print out the average temperature, over the course of the whole year, for each city. The average temperature for a city can be obtained by adding up all 365 measurements for that city and dividing the answer by 365.0.

Question 11:

Suppose that a class, Employee, is defined as follows:

class Employee {
String lastName;
String firstName;
double hourlyWage;
int yearsWithCompany;
}

Suppose that data about 100 employees is already stored in an array:

Employee[] employeeData = new Employee[100];

Write a code segment that will output the first name, last name, and hourly wage of each employee who has been with the company for 20 years or more. Write two versions of the code segment, one that uses a regular for loop, and one that uses a for-each loop.

Question 12:

What is the purpose of the following subroutine? What is the meaning of the value that it returns, in terms of the value of its parameter?

static double[] sums( double[][] data ) {
double[] answers = new double[ data.length ];
for (int i = 0; i < data.length; i++) {
double sum = 0;
for (int j = 0; j < data[i].length; i++)
sum = sum + data[i][j];
answers[i] = sum;
}
return answers;
}

See the Answers

Chapter 8

Correctness, Robustness, Efficiency

In previous chapters, we have covered the fundamentals of programming. The chapters that follow this one will cover more advanced aspects of programming. The ideas that are presented will generally be more complex and the programs that use them a little more complicated. This relatively short chapter is a kind of turning point in which we look at the problem of getting such complex programs right.

Computer programs that fail are much too common. Programs are fragile. A tiny error can cause a program to misbehave or crash. Most of us are familiar with this from our own experience with computers. And we've all heard stories about software glitches that cause spacecraft to crash, telephone service to fail, and, in a few cases, people to die.

Programs don't have to be as bad as they are. It might well be impossible to guarantee that programs are problem-free, but careful programming and well-designed programming tools can help keep the problems to a minimum. This chapter will look at issues of correctness and robustness of programs. It also looks more closely at exceptions and the try..catch statement, and it introduces assertions, another of the tools that Java provides as an aid in writing correct programs.

We will also look at another issue that is important for programs in the real world: efficiency. Even a completely correct program is not very useful if it takes an unreasonable amount of time to run. The last section of this chapter introduces techniques for analyzing the run time efficiency of algorithms.

Contents of Chapter 8:

- Section 1: Introduction to Correctness and Robustness

- Section 2: Writing Correct Programs

- Section 3: Exceptions and try..catch

- Section 4: Assertions and Annotations

- Section 5: Analysis of Algorithms

- Programming Exercises

- Quiz on This Chapter

Section 8.1

Introduction to Correctness and Robustness

A program is correct if it accomplishes the task that it was designed to perform. It is robust if it can handle illegal inputs and other unexpected situations in a reasonable way. For example, consider a program that is designed to read some numbers from the user and then print the same numbers in sorted order. The program is correct if it works for any set of input numbers. It is robust if it can also deal with non-numeric input by, for example, printing an error message and ignoring the bad input. A non-robust program might crash or give nonsensical output in the same circumstance.

Every program should be correct. (A sorting program that doesn't sort correctly is pretty useless.) It's not the case that every program needs to be completely robust. It depends on who will use it and how it will be used. For example, a small utility program that you write for your own use doesn't have to be particularly robust.

The question of correctness is actually more subtle than it might appear. A programmer works from a specification of what the program is supposed to do. The programmer's work is correct if the program meets its specification. But does that mean that the program itself is correct? What if the specification is incorrect or incomplete? A correct program should be a correct implementation of a complete and correct specification. The question is whether the specification correctly expresses the intention and desires of the people for whom the program is being written. This is a question that lies largely outside the domain of computer science.

8.1.1  Horror Stories

Most computer users have personal experience with programs that don't work or that crash. In many cases, such problems are just annoyances, but even on a personal computer there can be more serious consequences, such as lost work or lost money. When computers are given more important tasks, the consequences of failure can be proportionately more serious.

About fifteen years ago, the failure of two multi-million dollar space missions to Mars was prominent in the news. Both failures were probably due to software problems, but in both cases the problem was not with an incorrect program as such. In September 1999, the Mars Climate Orbiter burned up in the Martian atmosphere because data that was expressed in English units of measurement (such as feet and pounds) was entered into a computer program that was designed to use metric units (such as centimeters and grams). A few months later, the Mars Polar Lander probably crashed because its software turned off its landing engines too soon. The program was supposed to detect the bump when the spacecraft landed and turn off the engines then. It has been determined that deployment of the landing gear might have jarred the spacecraft enough to activate the program, causing it to turn off the engines when the spacecraft was still in the air. The unpowered spacecraft would then have fallen to the Martian surface. A more robust system would have checked the altitude before turning off the engines!

There are many equally dramatic stories of problems caused by incorrect or poorly written software. Let's look at a few incidents recounted in the book Computer Ethics by Tom Forester and Perry Morrison. (This book covers various ethical issues in computing. It, or something like it, is essential reading for any student of computer science.)

- In 1985 and 1986, one person was killed and several were injured by excess radiation, while undergoing radiation treatments by a mis-programmed computerized radiation machine. In another case, over a ten-year period ending in 1992, almost 1,000 cancer patients received radiation dosages that were 30% less than prescribed because of a programming error.

- In 1985, a computer at the Bank of New York started destroying records of on-going security transactions because of an error in a program. It took less than 24 hours to fix the program, but by that time, the bank was out $5,000,000 in overnight interest payments on funds that it had to borrow to cover the problem.

- The programming of the inertial guidance system of the F-16 fighter plane would have turned the plane upside-down when it crossed the equator, if the problem had not been discovered in simulation. The Mariner 18 space probe was lost because of an error in one line of a program. The Gemini V space capsule missed its scheduled landing target by a hundred miles, because a programmer forgot to take into account the rotation of the Earth.

- In 1990, AT&T's long-distance telephone service was disrupted throughout the United States when a newly loaded computer program proved to contain a bug.

Of course, there have been more recent problems. For example, computer software error contributed to the Northeast Blackout of 2003, one of the largest power outages in history. In 2006, the Airbus A380 was delayed by software incompatibility problems, at a cost of perhaps billions of dollars. In 2007, a software problem grounded thousands of planes at the Los Angeles International Airport. On May 6, 2010, a flaw in an automatic trading program apparently resulted in a 1000-point drop in the Dow Jones Industrial Average.

These are just a few examples. Software problems are all too common. As programmers, we need to understand why that is true and what can be done about it.

8.1.2  Java to the Rescue

Part of the problem, according to the inventors of Java, can be traced to programming languages themselves. Java was designed to provide some protection against certain types of errors. How can a language feature help prevent errors? Let's look at a few examples.

Early programming languages did not require variables to be declared. In such languages, when a variable name is used in a program, the variable is created automatically. You might consider this more convenient than having to declare every variable explicitly, but there is an unfortunate consequence: An inadvertent spelling error might introduce an extra variable that you had no intention of creating. This type of error was responsible, according to one famous story, for yet another lost spacecraft. In the FORTRAN programming language, the command "DO 20 I = 1,5" is the first statement of a counting loop. Now, spaces are insignificant in FORTRAN, so this is equivalent to "DO20I=1,5". On the other hand, the command "DO20I=1.5", with a period instead of a comma, is an assignment statement that assigns the value 1.5 to the variable DO20I. Supposedly, the inadvertent substitution of a period for a comma in a statement of this type caused a rocket to blow up on take-off. Because FORTRAN doesn't require variables to be declared, the compiler would be happy to accept the statement "DO20I=1.5." It would just create a new variable named DO20I. If FORTRAN required variables to be declared, the compiler would have complained that the variable DO20I was undeclared.

While most programming languages today do require variables to be declared, there are other features in common programming languages that can cause problems. Java has eliminated some of these features. Some people complain that this makes Java less efficient and less powerful. While there is some justice in this criticism, the increase in security and robustness is probably worth the cost in most circumstances. The best defense against some types of errors is to design a programming language in which the errors are impossible. In other cases, where the error can't be completely eliminated, the language can be designed so that when the error does occur, it will automatically be detected. This will at least prevent the error from causing further harm, and it will alert the programmer that there is a bug that needs fixing. Let's look at a few cases where the designers of Java have taken these approaches.

An array is created with a certain number of locations, numbered from zero up to some specified maximum index. It is an error to try to use an array location that is outside of the specified range. In Java, any attempt to do so is detected automatically by the system. In some other languages, such as C and C++, it's up to the programmer to make sure that the index is within the legal range. Suppose that an array, A, has three locations, A[0], A[1], and A[2]. Then A[3], A[4], and so on refer to memory locations beyond the end of the array. In Java, an attempt to store data in A[3] will be detected. The program will be terminated (unless the error is "caught", as discussed in Section 3.7). In C or C++, the computer will just go ahead and store the data in memory that is not part of the array. Since there is no telling what that memory location is being used for, the result will be unpredictable. The consequences could be much more serious than a terminated program. (See, for example, the discussion of buffer overflow errors later in this section.)

Pointers are a notorious source of programming errors. In Java, a variable of object type holds either a pointer to an object or the special value null. Any attempt to use a null value as if it were a pointer to an actual object will be detected by the system. In some other languages, again, it's up to the programmer to avoid such null pointer errors. In my old Macintosh computer, a null pointer was actually implemented as if it were a pointer to memory location zero. A program could use a null pointer to change values stored in memory near location zero. Unfortunately, the Macintosh stored important system data in those locations. Changing that data could cause the whole system to crash, a consequence more severe than a single failed program.

Another type of pointer error occurs when a pointer value is pointing to an object of the wrong type or to a segment of memory that does not even hold a valid object at all. These types of errors are impossible in Java, which does not allow programmers to manipulate pointers directly. In other languages, it is possible to set a pointer to point, essentially, to any location in memory. If this is done incorrectly, then using the pointer can have unpredictable results.

Another type of error that cannot occur in Java is a memory leak. In Java, once there are no longer any pointers that refer to an object, that object is "garbage collected" so that the memory that it occupied can be reused. In other languages, it is the programmer's responsibility to return unused memory to the system. If the programmer fails to do this, unused memory can build up, leaving less memory for programs and data. There is a story that many common programs for older Windows computers had so many memory leaks that the computer would run out of memory after a few days of use and would have to be restarted.

Many programs have been found to suffer from buffer overflow errors. Buffer overflow errors often make the news because they are responsible for many network security problems. When one computer receives data from another computer over a network, that data is stored in a buffer. The buffer is just a segment of memory that has been allocated by a program to hold data that it expects to receive. A buffer overflow occurs when more data is received than will fit in the buffer. The question is, what happens then? If the error is detected by the program or by the networking software, then the only thing that has happened is a failed network data transmission. The real problem occurs when the software does not properly detect buffer overflows. In that case, the software continues to store data in memory even after the buffer is filled, and the extra data goes into some part of memory that was not allocated by the program as part of the buffer. That memory might be in use for some other purpose. It might contain important data. It might even contain part of the program itself. This is where the real security issues come in. Suppose that a buffer overflow causes part of a program to be replaced with extra data received over a network. When the computer goes to execute the part of the program that was replaced, it's actually executing data that was received from another computer. That data could be anything. It could be a program that crashes the computer or takes it over. A malicious programmer who finds a convenient buffer overflow error in networking software can try to exploit that error to trick other computers into executing his programs.

For software written completely in Java, buffer overflow errors are impossible. The language simply does not provide any way to store data into memory that has not been properly allocated. To do that, you would need a pointer that points to unallocated memory or you would have to refer to an array location that lies outside the range allocated for the array. As explained above, neither of these is possible in Java. (However, there could conceivably still be errors in Java's standard classes, since some of the methods in these classes are actually written in the C programming language rather than in Java.)

It's clear that language design can help prevent errors or detect them when they occur. Doing so involves restricting what a programmer is allowed to do. Or it requires tests, such as checking whether a pointer is null, that take some extra processing time. Some programmers feel that the sacrifice of power and efficiency is too high a price to pay for the extra security. In some applications, this is true. However, there are many situations where safety and security are primary considerations. Java is designed for such situations.

8.1.3  Problems Remain in Java

There is one area where the designers of Java chose not to detect errors automatically: numerical computations. In Java, a value of type int is represented as a 32-bit binary number. With 32 bits, it's possible to represent a little over four billion different values. The values of type int range from -2147483648 to 2147483647. What happens when the result of a computation lies outside this range? For example, what is 2147483647 + 1? And what is 2000000000 * 2? The mathematically correct result in each case cannot be represented as a value of type int. These are examples of integer overflow. In most cases, integer overflow should be considered an error. However, Java does not automatically detect such errors. For example, it will compute the value of 2147483647 + 1 to be the negative number, -2147483648. (What happens is that any extra bits beyond the 32-nd bit in the correct answer are discarded. Values greater than 2147483647 will "wrap around" to negative values. Mathematically speaking, the result is always "correct modulo 2^32.")

For example, consider the 3N+1 program, which was discussed in Subsection 3.2.2. Starting from a positive integer N, the program computes a certain sequence of integers:

while ( N != 1 ) {
if ( N % 2 == 0 )  // If N is even...
N = N / 2;
else
N = 3 * N + 1;
System.out.println(N);
}

But there is a problem here: If N is too large, then the value of 3*N+1 will not be mathematically correct because of integer overflow. The problem arises whenever 3*N+1 > 2147483647, that is when N > 2147483646/3. For a completely correct program, we should check for this possibility before computing 3*N+1:

while ( N != 1 ) {
if ( N % 2 == 0 )  // If N is even...
N = N / 2;
else {
if (N > 2147483646/3) {
System.out.println("Sorry, but the value of N has become");
System.out.println("too large for your computer!");
break;
}
N = 3 * N + 1;
}
System.out.println(N);
}

(Be sure you understand why we can't just test "if (3*N+1 > 2147483647)".) The problem here is not that the original algorithm for computing 3N+1 sequences was wrong. The problem is that it just can't be correctly implemented using 32-bit integers. Many programs ignore this type of problem. But integer overflow errors have been responsible for their share of serious computer failures, and a completely robust program should take the possibility of integer overflow into account. (The infamous "Y2K" bug at the start of the year 2000 was, in fact, just this sort of error.)

For numbers of type double, there are even more problems. There are still overflow errors, which occur when the result of a computation is outside the range of values that can be represented as a value of type double. This range extends up to about 1.7 times 10 to the power 308. Numbers beyond this range do not "wrap around" to negative values. Instead, they are represented by special values that have no real numerical equivalent. The special values Double.POSITIVE_INFINITY and Double.NEGATIVE_INFINITY represent numbers outside the range of legal values. For example, 20 * 1e308 is computed to be Double.POSITIVE_INFINITY. Another special value of type double, Double.NaN, represents an illegal or undefined result. ("NaN" stands for "Not a Number".) For example, the result of dividing zero by zero or taking the square root of a negative number is Double.NaN. You can test whether a number x is this special not-a-number value by calling the boolean-valued function Double.isNaN(x).

For real numbers, there is the added complication that most real numbers can only be represented approximately on a computer. A real number can have an infinite number of digits after the decimal point. A value of type double is only accurate to about 15 digits. The real number 1/3, for example, is the repeating decimal 0.333333333333..., and there is no way to represent it exactly using a finite number of digits. Computations with real numbers generally involve a loss of accuracy. In fact, if care is not exercised, the result of a large number of such computations might be completely wrong! There is a whole field of computer science, known as numerical analysis, which is devoted to studying algorithms that manipulate real numbers.

So you see that not all possible errors are avoided or detected automatically in Java. Furthermore, even when an error is detected automatically, the system's default response is to report the error and terminate the program. This is hardly robust behavior! So, a Java programmer still needs to learn techniques for avoiding and dealing with errors. These are the main topics of the next three sections.

Section 8.2

Writing Correct Programs

Correct programs don't just happen. It takes planning and attention to detail to avoid errors in programs. There are some techniques that programmers can use to increase the likelihood that their programs are correct.

8.2.1  Provably Correct Programs

In some cases, it is possible to prove that a program is correct. That is, it is possible to demonstrate mathematically that the sequence of computations represented by the program will always produce the correct result. Rigorous proof is difficult enough that in practice it can only be applied to fairly small programs. Furthermore, it depends on the fact that the "correct result" has been specified correctly and completely. As I've already pointed out, a program that correctly meets its specification is not useful if its specification was wrong. Nevertheless, even in everyday programming, we can apply some of the ideas and techniques that are used in proving that programs are correct.

The fundamental ideas are process and state. A state consists of all the information relevant to the execution of a program at a given moment during its execution. The state includes, for example, the values of all the variables in the program, the output that has been produced, any input that is waiting to be read, and a record of the position in the program where the computer is working. A process is the sequence of states that the computer goes through as it executes the program. From this point of view, the meaning of a statement in a program can be expressed in terms of the effect that the execution of that statement has on the computer's state. As a simple example, the meaning of the assignment statement "x = 7;" is that after this statement is executed, the value of the variable x will be 7. We can be absolutely sure of this fact, so it is something upon which we can build part of a mathematical proof.

In fact, it is often possible to look at a program and deduce that some fact must be true at a given point during the execution of a program. For example, consider the do loop:

do {
System.out.print("Enter a positive integer: ");
N = TextIO.getlnInt();
} while (N <= 0);

After this loop ends, we can be absolutely sure that the value of the variable N is greater than zero. The loop cannot end until this condition is satisfied. This fact is part of the meaning of the while loop. More generally, if a while loop uses the test "while (condition)", then after the loop ends, we can be sure that the condition is false. We can then use this fact to draw further deductions about what happens as the execution of the program continues. (With a loop, by the way, we also have to worry about the question of whether the loop will ever end. This is something that has to be verified separately.)

A fact that can be proven to be true after a given program segment has been executed is called a postcondition of that program segment. Postconditions are known facts upon which we can build further deductions about the behavior of the program. A postcondition of a program as a whole is simply a fact that can be proven to be true after the program has finished executing. A program can be proven to be correct by showing that the postconditions of the program meet the program's specification.

Consider the following program segment, where all the variables are of type double:

disc = B*B - 4*A*C;
x = (-B + Math.sqrt(disc)) / (2*A);

The quadratic formula (from high-school mathematics) assures us that the value assigned to x is a solution of the equation A*x^2 + B*x + C = 0, provided that the value of disc is greater than or equal to zero and the value of A is not zero. If we can assume or guarantee that B*B-4*A*C >= 0 and that A != 0, then the fact that x is a solution of the equation becomes a postcondition of the program segment. We say that the condition, B*B-4*A*C >= 0 is a precondition of the program segment. The condition that A != 0 is another precondition. A precondition is defined to be a condition that must be true at a given point in the execution of a program in order for the program to continue correctly. A precondition is something that you want to be true. It's something that you have to check or force to be true, if you want your program to be correct.

We've encountered preconditions and postconditions once before, in Subsection 4.6.1. That section introduced preconditions and postconditions as a way of specifying the contract of a subroutine. As the terms are being used here, a precondition of a subroutine is just a precondition of the code that makes up the definition of the subroutine, and the postcondition of a subroutine is a postcondition of the same code. In this section, we have generalized these terms to make them more useful in talking about program correctness.

Let's see how this works by considering a longer program segment:

do {
System.out.println("Enter A, B, and C.  B*B-4*A*C must be >= 0.");
System.out.print("A = ");
A = TextIO.getlnDouble();
System.out.print("B = ");
B = TextIO.getlnDouble();
System.out.print("C = ");
C = TextIO.getlnDouble();
if (A == 0 || B*B - 4*A*C < 0)
System.out.println("Your input is illegal.  Try again.");
} while (A == 0 || B*B - 4*A*C < 0);

disc = B*B - 4*A*C;
x = (-B + Math.sqrt(disc)) / (2*A);

After the loop ends, we can be sure that B*B-4*A*C >= 0 and that A != 0. The preconditions for the last two lines are fulfilled, so the postcondition that x is a solution of the equation A*x^2 + B*x + C = 0 is also valid. This program segment correctly and provably computes a solution to the equation. (Actually, because of problems with representing real numbers on computers, this is not 100% true. The algorithm is correct, but the program is not a perfect implementation of the algorithm. See the discussion in Subsection 8.1.3.)

Here is another variation, in which the precondition is checked by an if statement. In the first part of the if statement, where a solution is computed and printed, we know that the preconditions are fulfilled. In the other parts, we know that one of the preconditions fails to hold. In any case, the program is correct.

System.out.println("Enter your values for A, B, and C.");
System.out.print("A = ");
A = TextIO.getlnDouble();
System.out.print("B = ");
B = TextIO.getlnDouble();
System.out.print("C = ");
C = TextIO.getlnDouble();

if (A != 0 && B*B - 4*A*C >= 0) {
disc = B*B - 4*A*C;
x = (-B + Math.sqrt(disc)) / (2*A);
System.out.println("A solution of A*X*X + B*X + C = 0 is " + x);
}
else if (A == 0) {
System.out.println("The value of A cannot be zero.");
}
else {
System.out.println("Since B*B - 4*A*C is less than zero, the");
System.out.println("equation A*X*X + B*X + C = 0 has no solution.");
}

Whenever you write a program, it's a good idea to watch out for preconditions and think about how your program handles them. Often, a precondition can offer a clue about how to write the program.

For example, every array reference, such as A[i], has a precondition: The index must be within the range of legal indices for the array. For A[i], the precondition is that 0 <= i < A.length. The computer will check this condition when it evaluates A[i], and if the condition is not satisfied, the program will be terminated. In order to avoid this, you need to make sure that the index has a legal value. (There is actually another precondition, namely that A is not null, but let's leave that aside for the moment.) Consider the following code, which searches for the number x in the array A and sets the value of i to be the index of the array element that contains x:

i = 0;
while (A[i] != x) {
i++;
}

As this program segment stands, it has a precondition, namely that x is actually in the array. If this precondition is satisfied, then the loop will end when A[i] == x. That is, the value of i when the loop ends will be the position of x in the array. However, if x is not in the array, then the value of i will just keep increasing until it is equal to A.length. At that time, the reference to A[i] is illegal and the program will be terminated. To avoid this, we can add a test to make sure that the precondition for referring to A[i] is satisfied:

i = 0;
while (i < A.length && A[i] != x) {
i++;
}

Now, the loop will definitely end. After it ends, i will satisfy either i == A.length or A[i] == x. An if statement can be used after the loop to test which of these conditions caused the loop to end:

i = 0;
while (i < A.length && A[i] != x) {
i++;
}

if (i == A.length)
System.out.println("x is not in the array");
else
System.out.println("x is in position " + i);

8.2.2  Robust Handling of Input

One place where correctness and robustness are important -- and especially difficult -- is in the processing of input data, whether that data is typed in by the user, read from a file, or received over a network. Files and networking will be covered in Chapter 11, which will make essential use of material that will be covered in the next section of this chapter. For now, let's look at an example of processing user input.

Examples in this textbook use my TextIO class for reading input from the user. This class has built-in error handling. For example, the function TextIO.getDouble() is guaranteed to return a legal value of type double. If the user types an illegal value, then TextIO will ask the user to re-enter their response; your program never sees the illegal value. However, this approach can be clumsy and unsatisfactory, especially when the user is entering complex data. In the following example, I'll do my own error-checking.

Sometimes, it's useful to be able to look ahead at what's coming up in the input without actually reading it. For example, a program might need to know whether the next item in the input is a number or a word. For this purpose, the TextIO class includes the function TextIO.peek(). This function returns a char which is the next character in the user's input, but it does not actually read that character. If the next thing in the input is an end-of-line, then TextIO.peek() returns the new-line character, '\n'.

Often, what we really need to know is the next non-blank character in the user's input. Before we can test this, we need to skip past any spaces (and tabs). Here is a function that does this. It uses TextIO.peek() to look ahead, and it reads characters until the next character in the input is either an end-of-line or some non-blank character. (The function TextIO.getAnyChar() reads and returns the next character in the user's input, even if that character is a space. By contrast, the more common TextIO.getChar() would skip any blanks and then read and return the next non-blank character. We can't use TextIO.getChar() here since the object is to skip the blanks without reading the next non-blank character.)

/**
* Reads past any blanks and tabs in the input.
* Postcondition:  The next character in the input is an
*                 end-of-line or a non-blank character.
*/
static void skipBlanks() {
char ch;
ch = TextIO.peek();
while (ch == ' ' || ch == '\t') {
// Next character is a space or tab; read it
// and look at the character that follows it.
ch = TextIO.getAnyChar();
ch = TextIO.peek();
}
} // end skipBlanks()

(In fact, this operation is so common that it is built into TextIO. The method TextIO.skipBlanks() does essentially the same thing as the skipBlanks() method presented here.)

An example in Subsection 3.5.3 allowed the user to enter length measurements such as "3 miles" or "1 foot". It would then convert the measurement into inches, feet, yards, and miles. But people commonly use combined measurements such as "3 feet 7 inches". Let's improve the program so that it allows inputs of this form.

More specifically, the user will input lines containing one or more measurements such as "1 foot" or "3 miles 20 yards 2 feet". The legal units of measure are inch, foot, yard, and mile. The program will also recognize plurals (inches, feet, yards, miles) and abbreviations (in, ft, yd, mi). Let's write a subroutine that will read one line of input of this form and compute the equivalent number of inches. The main program uses the number of inches to compute the equivalent number of feet, yards, and miles. If there is any error in the input, the subroutine will print an error message and return the value -1. The subroutine assumes that the input line is not empty. The main program tests for this before calling the subroutine and uses an empty line as a signal for ending the program.

Ignoring the possibility of illegal inputs, a pseudocode algorithm for the subroutine is

inches = 0    // This will be the total number of inches
while there is more input on the line:
read the numerical measurement
read the units of measure
add the measurement to inches
return inches

We can test whether there is more input on the line by checking whether the next non-blank character is the end-of-line character. But this test has a precondition: Before we can test the next non-blank character, we have to skip over any blanks. So, the algorithm becomes

inches = 0
skipBlanks()
while TextIO.peek() is not '\n':
read the numerical measurement
read the unit of measure
add the measurement to inches
skipBlanks()
return inches

Note the call to skipBlanks() at the end of the while loop. This subroutine must be executed before the computer returns to the test at the beginning of the loop. More generally, if the test in a while loop has a precondition, then you have to make sure that this precondition holds at the end of the while loop, before the computer jumps back to re-evaluate the test, as well as before the start of the loop.

What about error checking? Before reading the numerical measurement, we have to make sure that there is really a number there to read. Before reading the unit of measure, we have to test that there is something there to read. (The number might have been the last thing on the line. An input such as "3", without a unit of measure, is not acceptable.) Also, we have to check that the unit of measure is one of the valid units: inches, feet, yards, or miles. Here is an algorithm that includes error-checking:

inches = 0
skipBlanks()

while TextIO.peek() is not '\n':

if the next character is not a digit:
report an error and return -1
Let measurement = TextIO.getDouble();

skipBlanks()    // Precondition for the next test!!
if the next character is end-of-line:
report an error and return -1
Let units = TextIO.getWord()

if the units are inches:
add measurement to inches
else if the units are feet:
add 12*measurement to inches
else if the units are yards:
add 36*measurement to inches
else if the units are miles:
add 12*5280*measurement to inches
else
report an error and return -1

skipBlanks()

return inches

As you can see, error-testing adds significantly to the complexity of the algorithm. Yet this is still a fairly simple example, and it doesn't even handle all the possible errors. For example, if the user enters a numerical measurement such as 1e400 that is outside the legal range of values of type double, then the program will fall back on the default error-handling in TextIO. Something even more interesting happens if the measurement is "1e308 miles". The number 1e308 is legal, but the corresponding number of inches is outside the legal range of values for type double. As mentioned in the previous section, the computer will get the value Double.POSITIVE_INFINITY when it does the computation. You might want to run the program and try this out.

Here is the subroutine written out in Java:

/**
* Reads the user's input measurement from one line of input.
* Precondition:   The input line is not empty.
* Postcondition:  If the user's input is legal, the measurement
*                 is converted to inches and returned.  If the
*                 input is not legal, the value -1 is returned.
*                 The end-of-line is NOT read by this routine.
*/
static double readMeasurement() {

double inches;  // Total number of inches in user's measurement.

double measurement;  // One measurement,
//   such as the 12 in "12 miles"
String units;        // The units specified for the measurement,
//   such as "miles"

char ch;  // Used to peek at next character in the user's input.

inches = 0;  // No inches have yet been read.

skipBlanks();
ch = TextIO.peek();

/* As long as there is more input on the line, read a measurement and
add the equivalent number of inches to the variable, inches.  If an
error is detected during the loop, end the subroutine immediately
by returning -1. */

while (ch != '\n') {

/* Get the next measurement and the units.  Before reading
anything, make sure that a legal value is there to read. */

if ( ! Character.isDigit(ch) ) {
System.out.println(
"Error:  Expected to find a number, but found " + ch);
return -1;
}
measurement = TextIO.getDouble();

skipBlanks();
if (TextIO.peek() == '\n') {
System.out.println(
"Error:  Missing unit of measure at end of line.");
return -1;
}
units = TextIO.getWord();
units = units.toLowerCase();

/* Convert the measurement to inches and add it to the total. */

if (units.equals("inch")
|| units.equals("inches") || units.equals("in")) {
inches += measurement;
}
else if (units.equals("foot")
|| units.equals("feet") || units.equals("ft")) {
inches += measurement * 12;
}
else if (units.equals("yard")
|| units.equals("yards") || units.equals("yd")) {
inches += measurement * 36;
}
else if (units.equals("mile")
|| units.equals("miles") || units.equals("mi")) {
inches += measurement * 12 * 5280;
}
else {
System.out.println("Error: \"" + units
+ "\" is not a legal unit of measure.");
return -1;
}

/* Look ahead to see whether the next thing on the line is
the end-of-line. */

skipBlanks();
ch = TextIO.peek();

}  // end while

return inches;

} // end readMeasurement()

The source code for the complete program can be found in the file LengthConverter2.java.

Section 8.3

Exceptions and try..catch

Getting a program to work under ideal circumstances is usually a lot easier than making the program robust. A robust program can survive unusual or "exceptional" circumstances without crashing. One approach to writing robust programs is to anticipate the problems that might arise and to include tests in the program for each possible problem. For example, a program will crash if it tries to use an array element A[i], when i is not within the declared range of indices for the array A. A robust program must anticipate the possibility of a bad index and guard against it. One way to do this is to write the program in a way that ensures (as a postcondition of the code that precedes the array reference) that the index is in the legal range. Another way is to test whether the index value is legal before using it in the array. This could be done with an if statement:

if (i < 0 || i >= A.length) {
...  // Do something to handle the out-of-range index, i
}
else {
...  // Process the array element, A[i]
}

There are some problems with this approach. It is difficult and sometimes impossible to anticipate all the possible things that might go wrong. It's not always clear what to do when an error is detected. Furthermore, trying to anticipate all the possible problems can turn what would otherwise be a straightforward program into a messy tangle of if statements.

8.3.1  Exceptions and Exception Classes

We have already seen in Section 3.7 that Java provides a neater, more structured alternative technique for dealing with errors that can occur while a program is running. The technique is referred to as exception handling. The word "exception" is meant to be more general than "error." It includes any circumstance that arises as the program is executed which is meant to be treated as an exception to the normal flow of control of the program. An exception might be an error, or it might just be a special case that you would rather not have clutter up your elegant algorithm.

When an exception occurs during the execution of a program, we say that the exception is thrown. When this happens, the normal flow of the program is thrown off-track, and the program is in danger of crashing. However, the crash can be avoided if the exception is caught and handled in some way. An exception can be thrown in one part of a program and caught in a different part. An exception that is not caught will generally cause the program to crash. (More exactly, the thread that throws the exception will crash. In a multithreaded program, it is possible for other threads to continue even after one crashes. We will cover threads in Chapter 12. In particular, GUI programs are multithreaded, and parts of the program might continue to function even while other parts are non-functional because of exceptions.)

By the way, since Java programs are executed by a Java interpreter, having a program crash simply means that it terminates abnormally and prematurely. It doesn't mean that the Java interpreter will crash. In effect, the interpreter catches any exceptions that are not caught by the program. The interpreter responds by terminating the program. In many other programming languages, a crashed program will sometimes crash the entire system and freeze the computer until it is restarted. With Java, such system crashes should be impossible -- which means that when they happen, you have the satisfaction of blaming the system rather than your own program.

Exceptions were introduced in Section 3.7, along with the try..catch statement, which is used to catch and handle exceptions. However, that section did not cover the complete syntax of try..catch or the full complexity of exceptions. In this section, we cover these topics in full detail.

When an exception occurs, the thing that is actually "thrown" is an object. This object can carry information (in its instance variables) from the point where the exception occurs to the point where it is caught and handled. This information always includes the subroutine call stack, which is a list of the subroutines that were being executed when the exception was thrown. (Since one subroutine can call another, several subroutines can be active at the same time.) Typically, an exception object also includes an error message describing what happened to cause the exception, and it can contain other data as well. All exception objects must belong to a subclass of the standard class java.lang.Throwable. In general, each different type of exception is represented by its own subclass of Throwable, and these subclasses are arranged in a fairly complex class hierarchy that shows the relationship among various types of exception. Throwable has two direct subclasses, Error and Exception. These two subclasses in turn have many other predefined subclasses. In addition, a programmer can create new exception classes to represent new types of exception.

Most of the subclasses of the class Error represent serious errors within the Java virtual machine that should ordinarily cause program termination because there is no reasonable way to handle them. In general, you should not try to catch and handle such errors. An example is a ClassFormatError, which occurs when the Java virtual machine finds some kind of illegal data in a file that is supposed to contain a compiled Java class. If that class was being loaded as part of the program, then there is really no way for the program to proceed.

On the other hand, subclasses of the class Exception represent exceptions that are meant to be caught. In many cases, these are exceptions that might naturally be called "errors," but they are errors in the program or in input data that a programmer can anticipate and possibly respond to in some reasonable way. (However, you should avoid the temptation of saying, "Well, I'll just put a thing here to catch all the errors that might occur, so my program won't crash." If you don't have a reasonable way to respond to the error, it's best just to let the program crash, because trying to go on will probably only lead to worse things down the road -- in the worst case, a program that gives an incorrect answer without giving you any indication that the answer might be wrong!)

The class Exception has its own subclass, RuntimeException. This class groups together many common exceptions, including all those that have been covered in previous sections. For example, IllegalArgumentException and NullPointerException are subclasses of RuntimeException. A RuntimeException generally indicates a bug in the program, which the programmer should fix. RuntimeExceptions and Errors share the property that a program can simply ignore the possibility that they might occur. ("Ignoring" here means that you are content to let your program crash if the exception occurs.) For example, a program does this every time it uses an array reference like A[i] without making arrangements to catch a possible ArrayIndexOutOfBoundsException. For all other exception classes besides Error, RuntimeException, and their subclasses, exception-handling is "mandatory" in a sense that I'll discuss below.

The following diagram is a class hierarchy showing the class Throwable and just a few of its subclasses. Classes that require mandatory exception-handling are shown in red:

The class Throwable includes several instance methods that can be used with any exception object. If e is of type Throwable (or one of its subclasses), then e.getMessage() is a function that returns a String that describes the exception. The function e.toString(), which is used by the system whenever it needs a string representation of the object, returns a String that contains the name of the class to which the exception belongs as well as the same string that would be returned by e.getMessage(). And the method e.printStackTrace() writes a stack trace to standard output that tells which subroutines were active when the exception occurred. A stack trace can be very useful when you are trying to determine the cause of the problem. (Note that if an exception is not caught by the program, then the default response to the exception prints the stack trace to standard output.)

8.3.2  The try Statement

To catch exceptions in a Java program, you need a try statement. We have been using such statements since Section 3.7, but the full syntax of the try statement is more complicated than what was presented there. The try statements that we have used so far had a syntax similar to the following example:

try {
double determinant = M[0][0]*M[1][1] - M[0][1]*M[1][0];
System.out.println("The determinant of M is " + determinant);
}
catch ( ArrayIndexOutOfBoundsException e ) {
System.out.println("M is the wrong size to have a determinant.");
e.printStackTrace();
}

Here, the computer tries to execute the block of statements following the word "try". If no exception occurs during the execution of this block, then the "catch" part of the statement is simply ignored. However, if an exception of type ArrayIndexOutOfBoundsException occurs, then the computer jumps immediately to the catch clause of the try statement. This block of statements is said to be an exception handler for ArrayIndexOutOfBoundsException. By handling the exception in this way, you prevent it from crashing the program. Before the body of the catch clause is executed, the object that represents the exception is assigned to the variable e, which is used in this example to print a stack trace.

However, the full syntax of the try statement has many options. It will take a while to go through them. For one thing, a try..catch statement can have more than one catch clause. This makes it possible to catch several different types of exception with one try statement. In the above example, in addition to the possible ArrayIndexOutOfBoundsException, there is a possible NullPointerException which will occur if the value of M is null. We can handle both possible exceptions by adding a second catch clause to the try statement:

try {
double determinant = M[0][0]*M[1][1] - M[0][1]*M[1][0];
System.out.println("The determinant of M is " + determinant);
}
catch ( ArrayIndexOutOfBoundsException e ) {
System.out.println("M is the wrong size to have a determinant.");
}
catch ( NullPointerException e ) {
System.out.print("Programming error!  M doesn't exist." + );
}

Here, the computer tries to execute the statements in the try clause. If no error occurs, both of the catch clauses are skipped. If an ArrayIndexOutOfBoundsException occurs, the computer executes the body of the first catch clause and skips the second one. If a NullPointerException occurs, it jumps to the second catch clause and executes that.

Note that both ArrayIndexOutOfBoundsException and NullPointerException are subclasses of RuntimeException. It's possible to catch all RuntimeExceptions with a single catch clause. For example:

try {
double determinant = M[0][0]*M[1][1] - M[0][1]*M[1][0];
System.out.println("The determinant of M is " + determinant);
}
catch ( RuntimeException err ) {
System.out.println("Sorry, an error has occurred.");
System.out.println("The error was: " + err);
}

The catch clause in this try statement will catch any exception belonging to class RuntimeException or to any of its subclasses. This shows why exception classes are organized into a class hierarchy. It allows you the option of casting your net narrowly to catch only a specific type of exception. Or you can cast your net widely to catch a wide class of exceptions. Because of subclassing, when there are multiple catch clauses in a try statement, it is possible that a given exception might match several of those catch clauses. For example, an exception of type NullPointerException would match catch clauses for NullPointerException, RuntimeException, Exception, or Throwable. In this case, only the first catch clause that matches the exception is executed.

Of course, catching RuntimeException would catch many more types of exception than the two that we are interested in. It is possible to combine several specific exception types in a single catch clause. For example,

try {
double determinant = M[0][0]*M[1][1] - M[0][1]*M[1][0];
System.out.println("The determinant of M is " + determinant);
}
catch ( NullPointerException | ArrayIndexOutOfBoundsException err ) {
System.out.println("Sorry, an error has occurred.");
System.out.println("The error was: " + err);
}

Here, the two exception types are combined with a "|", the vertical line character that is also used in the boolean or operator. This example will catch errors of type NullPointerException or ArrayIndexOutOfBoundsException, and no other types.

The example I've been using here is not realistic, because you are not very likely to use exception-handling to guard against null pointers and bad array indices. This is a case where careful programming is better than exception handling: Just be sure that your program assigns a reasonable, non-null value to the array M. You would certainly resent it if the designers of Java forced you to set up a try..catch statement every time you wanted to use an array! This is why handling of potential RuntimeExceptions is not mandatory. There are just too many things that might go wrong! (This also shows that exception-handling does not solve the problem of program robustness. It just gives you a tool that will in many cases let you approach the problem in a more organized way.)

I have still not completely specified the syntax of the try statement. The next variation is the possibility of a finally clause at the end of a try statement. With this addition, syntax of the try statement can be described as:

try {
statements
}
optional-catch-clauses
optional-finally-clause

Note that the catch clauses are also listed as optional. The try statement can include zero or more catch clauses and, optionally, a finally clause. The try statement must include one or the other. That is, a try statement can have either a finally clause, or one or more catch clauses, or both. The syntax for a catch clause is

catch ( exception-class-names variable-name ) {
statements
}

where exception-class-names can be a single exception class or several classes separated by "|". The syntax for a finally clause is

finally {
statements
}

The semantics of the finally clause is that the block of statements in the finally clause is guaranteed to be executed as the last step in the execution of the try statement, whether or not any exception occurs and whether or not any exception that does occur is caught and handled. The finally clause is meant for doing essential cleanup that under no circumstances should be omitted. One example of this type of cleanup is closing a network connection. Although you don't yet know enough about networking to look at the actual programming in this case, we can consider some pseudocode:

try {
open a network connection
communicate over the connection
}
catch ( IOException e ) {
report the error
}
finally {
if the connection was successfully opened
close the connection
}

The finally clause ensures that the network connection will definitely be closed, whether or not an error occurs during the communication. The pseudocode in this example follows a general pattern that can be used to robustly obtain a resource, use the resource, and then release the resource.

The pattern of obtaining a resource, then using the resource, and then releasing the resource is very common. Note that the resource can only be released if no error occurred while obtaining it. And, if it was successfully obtained, then it should be closed whether or not an error occurs while using it. This pattern is so common that it leads to one last option in the try statement syntax. With this option, you only need code to obtain the resource, and you don't need to worry about releasing it. That will happen automatically at the end of the try statement.

In order for this to work, the resource must be represented by an object that implements an interface named AutoCloseable, which defines a single method named close(), with no parameters. Standard Java classes that represent things like files and network connections already implement AutoClosable. So does the Scanner class, which was introduced in Subsection 2.4.6. In that section, I showed how to use a Scanner to read from System.in. Although I didn't do it in that section, it's considered good form to close a Scanner after using it. Here is an example that uses the resource pattern in a try statement to make sure that the Scanner is closed automatically:

try( Scanner in = new Scanner(System.in) ) {
// Use the Scanner to read from standard input
}
catch (Exception e) {
// ... some error occurred while using the Scanner
}

The statement that allocates the resource goes in parentheses after the word "try". The statement must have the form of a variable declaration that includes an initialization of the variable. The variable is local to the try statement. (You can actually declare several variables in the parentheses, separated by semicolons.) In this example, we can be sure that in.close() will definitely be called by the system at the end of the try statement, as long as the Scanner was successfully initialized.

This is all getting quite complicated, and I won't continue the discussion here. The sample program TryStatementDemo.java demonstrates a try statement with all its options, and it includes a lot of comments to help you understand what can happen when you run the program.

8.3.3  Throwing Exceptions

There are times when it makes sense for a program to deliberately throw an exception. This is the case when the program discovers some sort of exceptional or error condition, but there is no reasonable way to handle the error at the point where the problem is discovered. The program can throw an exception in the hope that some other part of the program will catch and handle the exception. This can be done with a throw statement. You have already seen an example of this in Subsection 4.3.8. In this section, we cover the throw statement more fully. The syntax of the throw statement is:

throw  exception-object ;

The exception-object must be an object belonging to one of the subclasses of Throwable. Usually, it will in fact belong to one of the subclasses of Exception. In most cases, it will be a newly constructed object created with the new operator. For example:

throw new ArithmeticException("Division by zero");

The parameter in the constructor becomes the error message in the exception object; if e refers to the object, the error message can be retrieved by calling e.getMessage(). (You might find this example a bit odd, because you might expect the system itself to throw an ArithmeticException when an attempt is made to divide by zero. So why should a programmer bother to throw the exception? Recall that if the numbers that are being divided are of type int, then division by zero will indeed throw an ArithmeticException. However, no arithmetic operations with floating-point numbers will ever produce an exception. Instead, the special value Double.NaN is used to represent the result of an illegal operation. In some situations, you might prefer to throw an ArithmeticException when a real number is divided by zero.)

An exception can be thrown either by the system or by a throw statement. The exception is processed in exactly the same way in either case. Suppose that the exception is thrown inside a try statement. If that try statement has a catch clause that handles that type of exception, then the computer jumps to the catch clause and executes it. The exception has been handled. After handling the exception, the computer executes the finally clause of the try statement, if there is one. It then continues normally with the rest of the program, which follows the try statement. If the exception is not immediately caught and handled, the processing of the exception will continue.

When an exception is thrown during the execution of a subroutine and the exception is not handled in the same subroutine, then that subroutine is terminated (after the execution of any pending finally clauses). Then the routine that called that subroutine gets a chance to handle the exception. That is, if the subroutine was called inside a try statement that has an appropriate catch clause, then that catch clause will be executed and the program will continue on normally from there. Again, if the second routine does not handle the exception, then it also is terminated and the routine that called it (if any) gets the next shot at the exception. The exception will crash the program only if it passes up through the entire chain of subroutine calls without being handled. (In fact, even this is not quite true: In a multithreaded program, only the thread in which the exception occurred is terminated.)

A subroutine that might generate an exception can announce this fact by adding a clause "throws exception-class-name" to the header of the routine. For example:

/**
* Returns the larger of the two roots of the quadratic equation
* A*x*x + B*x + C = 0, provided it has any roots.  If A == 0 or
* if the discriminant, B*B - 4*A*C, is negative, then an exception
* of type IllegalArgumentException is thrown.
*/
static public double root( double A, double B, double C )
throws IllegalArgumentException {
if (A == 0) {
throw new IllegalArgumentException("A can't be zero.");
}
else {
double disc = B*B - 4*A*C;
if (disc < 0)
throw new IllegalArgumentException("Discriminant < zero.");
return  (-B + Math.sqrt(disc)) / (2*A);
}
}

As discussed in the previous section, the computation in this subroutine has the preconditions that A != 0 and B*B-4*A*C >= 0. The subroutine throws an exception of type IllegalArgumentException when either of these preconditions is violated. When an illegal condition is found in a subroutine, throwing an exception is often a reasonable response. If the program that called the subroutine knows some good way to handle the error, it can catch the exception. If not, the program will crash -- and the programmer will know that the program needs to be fixed.

A throws clause in a subroutine heading can declare several different types of exception, separated by commas. For example:

void processArray(int[] A) throws NullPointerException,
ArrayIndexOutOfBoundsException { ...

8.3.4  Mandatory Exception Handling

In the preceding example, declaring that the subroutine root() can throw an IllegalArgumentException is just a courtesy to potential readers of this routine. This is because handling of IllegalArgumentExceptions is not "mandatory." A routine can throw an IllegalArgumentException without announcing the possibility. And a program that calls that routine is free either to catch or to ignore the exception, just as a programmer can choose either to catch or to ignore an exception of type NullPointerException.

For those exception classes that require mandatory handling, the situation is different. If a subroutine can throw such an exception, that fact must be announced in a throws clause in the routine definition. Failing to do so is a syntax error that will be reported by the compiler. Exceptions that require mandatory handling are called checked exceptions. The compiler will check that such exceptions are handled by the program.

Suppose that some statement in the body of a subroutine can generate a checked exception, one that requires mandatory handling. The statement could be a throw statement, which throws the exception directly, or it could be a call to a subroutine that can throw the exception. In either case, the exception must be handled. This can be done in one of two ways: The first way is to place the statement in a try statement that has a catch clause that handles the exception; in this case, the exception is handled within the subroutine, so that no caller of the subroutine can ever see the exception. The second way is to declare that the subroutine can throw the exception. This is done by adding a "throws" clause to the subroutine heading, which alerts any callers to the possibility that the exception might be generated when the subroutine is executed. The caller will, in turn, be forced either to handle the exception in a try statement or to declare the exception in a throws clause in its own header.

Exception-handling is mandatory for any exception class that is not a subclass of either Error or RuntimeException. These checked exceptions generally represent conditions that are outside the control of the programmer. For example, they might represent bad input or an illegal action taken by the user. There is no way to avoid such errors, so a robust program has to be prepared to handle them. The design of Java makes it impossible for programmers to ignore the possibility of such errors.

Among the checked exceptions are several that can occur when using Java's input/output routines. This means that you can't even use these routines unless you understand something about exception-handling. Chapter 11 deals with input/output and uses checked exceptions extensively.

8.3.5  Programming with Exceptions

Exceptions can be used to help write robust programs. They provide an organized and structured approach to robustness. Without exceptions, a program can become cluttered with if statements that test for various possible error conditions. With exceptions, it becomes possible to write a clean implementation of an algorithm that will handle all the normal cases. The exceptional cases can be handled elsewhere, in a catch clause of a try statement.

When a program encounters an exceptional condition and has no way of handling it immediately, the program can throw an exception. In some cases, it makes sense to throw an exception belonging to one of Java's predefined classes, such as IllegalArgumentException or IOException. However, if there is no standard class that adequately represents the exceptional condition, the programmer can define a new exception class. The new class must extend the standard class Throwable or one of its subclasses. In general, if the programmer does not want to require mandatory exception handling, the new class will extend RuntimeException (or one of its subclasses). To create a new checked exception class, which does require mandatory handling, the programmer can extend one of the other subclasses of Exception or can extend Exception itself.

Here, for example, is a class that extends Exception, and therefore requires mandatory exception handling when it is used:

public class ParseError extends Exception {
public ParseError(String message) {
// Create a ParseError object containing
// the given message as its error message.
super(message);
}
}

The class contains only a constructor that makes it possible to create a ParseError object containing a given error message. (The statement "super(message)" calls a constructor in the superclass, Exception. See Subsection 5.6.3.) Of course the class inherits the getMessage() and printStackTrace() routines from its superclass. If e refers to an object of type ParseError, then the function call e.getMessage() will retrieve the error message that was specified in the constructor. But the main point of the ParseError class is simply to exist. When an object of type ParseError is thrown, it indicates that a certain type of error has occurred. (Parsing, by the way, refers to figuring out the syntax of a string. A ParseError would indicate, presumably, that some string that is being processed by the program does not have the expected form.)

A throw statement can be used in a program to throw an error of type ParseError. The constructor for the ParseError object must specify an error message. For example:

throw new ParseError("Encountered an illegal negative number.");

or

throw new ParseError("The word '" + word
+ "' is not a valid file name.");

Since ParseError is defined as a subclass of Exception, it is a checked exception. If the throw statement does not occur in a try statement that catches the error, then the subroutine that contains the throw must declare that it can throw a ParseError by adding the clause "throws ParseError" to the subroutine heading. For example,

void getUserData() throws ParseError {
. . .
}

This would not be required if ParseError were defined as a subclass of RuntimeException instead of Exception, since in that case ParseErrors would not be checked exceptions.

A routine that wants to handle ParseErrors can use a try statement with a catch clause that catches ParseErrors. For example:

try {
getUserData();
processUserData();
}
catch (ParseError pe) {
. . .  // Handle the error
}

Note that since ParseError is a subclass of Exception, a catch clause of the form "catch (Exception e)" would also catch ParseErrors, along with any other object of type Exception.

Sometimes, it's useful to store extra data in an exception object. For example,

class ShipDestroyed extends RuntimeException {
Ship ship;  // Which ship was destroyed.
int where_x, where_y;  // Location where ship was destroyed.
ShipDestroyed(String message, Ship s, int x, int y) {
// Constructor creates a ShipDestroyed object
// carrying an error message plus the information
// that the ship s was destroyed at location (x,y)
// on the screen.
super(message);
ship = s;
where_x = x;
where_y = y;
}
}

Here, a ShipDestroyed object contains an error message and some information about a ship that was destroyed. This could be used, for example, in a statement:

if ( userShip.isHit() )
throw new ShipDestroyed("You've been hit!", userShip, xPos, yPos);

Note that the condition represented by a ShipDestroyed object might not even be considered an error. It could be just an expected interruption to the normal flow of a game. Exceptions can sometimes be used to handle such interruptions neatly.

The ability to throw exceptions is particularly useful in writing general-purpose methods and classes that are meant to be used in more than one program. In this case, the person writing the method or class often has no reasonable way of handling the error, since that person has no way of knowing exactly how the method or class will be used. In such circumstances, a novice programmer is often tempted to print an error message and forge ahead, but this is almost never satisfactory since it can lead to unpredictable results down the line. Printing an error message and terminating the program is almost as bad, since it gives the program no chance to handle the error.

The program that calls the method or uses the class needs to know that the error has occurred. In languages that do not support exceptions, the only alternative is to return some special value or to set the value of some variable to indicate that an error has occurred. For example, the readMeasurement() function in Subsection 8.2.2 returns the value -1 if the user's input is illegal. However, this only does any good if the main program bothers to test the return value. It is very easy to be lazy about checking for special return values every time a subroutine is called. And in this case, using -1 as a signal that an error has occurred makes it impossible to allow negative measurements. Exceptions are a cleaner way for a subroutine to react when it encounters an error.

It is easy to modify the readMeasurement() function to use exceptions instead of a special return value to signal an error. My modified subroutine throws a ParseError when the user's input is illegal, where ParseError is the subclass of Exception that was defined above. (Arguably, it might be reasonable to avoid defining a new class by using the standard exception class IllegalArgumentException instead.) The changes from the original version are shown in red italic:

/**
* Reads the user's input measurement from one line of input.
* Precondition:   The input line is not empty.
* Postcondition:  If the user's input is legal, the measurement
*                 is converted to inches and returned.
* @throws ParseError if the user's input is not legal.
*/
static double readMeasurement() throws ParseError {

double inches;  // Total number of inches in user's measurement.

double measurement;  // One measurement,
//   such as the 12 in "12 miles."
String units;        // The units specified for the measurement,
//   such as "miles."

char ch;  // Used to peek at next character in the user's input.

inches = 0;  // No inches have yet been read.

skipBlanks();
ch = TextIO.peek();

/* As long as there is more input on the line, read a measurement and
add the equivalent number of inches to the variable, inches.  If an
error is detected during the loop, end the subroutine immediately
by throwing a ParseError. */

while (ch != '\n') {

/* Get the next measurement and the units.  Before reading
anything, make sure that a legal value is there to read. */

if ( ! Character.isDigit(ch) ) {
throw new ParseError("Expected to find a number, but found " + ch);
}
measurement = TextIO.getDouble();

skipBlanks();
if (TextIO.peek() == '\n') {
throw new ParseError("Missing unit of measure at end of line.");
}
units = TextIO.getWord();
units = units.toLowerCase();

/* Convert the measurement to inches and add it to the total. */

if (units.equals("inch")
|| units.equals("inches") || units.equals("in")) {
inches += measurement;
}
else if (units.equals("foot")
|| units.equals("feet") || units.equals("ft")) {
inches += measurement * 12;
}
else if (units.equals("yard")
|| units.equals("yards") || units.equals("yd")) {
inches += measurement * 36;
}
else if (units.equals("mile")
|| units.equals("miles") || units.equals("mi")) {
inches += measurement * 12 * 5280;
}
else {
throw new ParseError("\"" + units
+ "\" is not a legal unit of measure.");
}

/* Look ahead to see whether the next thing on the line is
the end-of-line. */

skipBlanks();
ch = TextIO.peek();

}  // end while

return inches;

} // end readMeasurement()

In the main program, this subroutine is called in a try statement of the form

try {
inches = readMeasurement();
}
catch (ParseError e) {
. . .  // Handle the error.
}

The complete program can be found in the file LengthConverter3.java. From the user's point of view, this program has exactly the same behavior as the program LengthConverter2 from the previous section. Internally, however, the programs are significantly different, since LengthConverter3 uses exception handling.

Section 8.4

Assertions and Annotations

In this short section, we look briefly at two features of Java that are not covered or used elsewhere in this textbook, assertions and annotations. They are included here for completeness, but they are mostly meant for more advanced programming.

8.4.1  Assertions

Recall that a precondition is a condition that must be true at a certain point in a program, for the execution of the program to continue correctly from that point. In the case where there is a chance that the precondition might not be satisfied -- for example, if it depends on input from the user -- then it's a good idea to insert an if statement to test it. But then the question arises, What should be done if the precondition does not hold? One option is to throw an exception. This will terminate the program, unless the exception is caught and handled elsewhere in the program.

In many cases, of course, instead of using an if statement to test whether a precondition holds, a programmer tries to write the program in a way that will guarantee that the precondition holds. In that case, the test should not be necessary, and the if statement can be avoided. The problem is that programmers are not perfect. In spite of the programmer's intention, the program might contain a bug that screws up the precondition. So maybe it's a good idea to check the precondition after all -- at least during the debugging phase of program development.

Similarly, a postcondition is a condition that is true at a certain point in the program as a consequence of the code that has been executed before that point. Assuming that the code is correctly written, a postcondition is guaranteed to be true, but here again testing whether a desired postcondition is actually true is a way of checking for a bug that might have screwed up the postcondition. This is something that might be desirable during debugging.

The programming languages C and C++ have always had a facility for adding what are called assertions to a program. These assertions take the form "assert( condition )", where condition is a boolean-valued expression. This condition expresses a precondition or postcondition that should hold at that point in the program. When the computer encounters an assertion during the execution of the program, it evaluates the condition. If the condition is false, the program is terminated. Otherwise, the program continues normally. This allows the programmer's belief that the condition is true to be tested; if it is not true, that indicates that the part of the program that preceded the assertion contained a bug. One nice thing about assertions in C and C++ is that they can be "turned off" at compile time. That is, if the program is compiled in one way, then the assertions are included in the compiled code. If the program is compiled in another way, the assertions are not included. During debugging, the first type of compilation is used, with assertions turned on. The release version of the program is compiled with assertions turned off. The release version will be more efficient, because the computer won't have to evaluate all the assertions.

Although early versions of Java did not have assertions, an assertion facility similar to the one in C/C++ has been available in Java since version 1.4. As with the C/C++ version, Java assertions can be turned on during debugging and turned off during normal execution. In Java, however, assertions are turned on and off at run time rather than at compile time. An assertion in the Java source code is always included in the compiled class file. When the program is run in the normal way, these assertions are ignored; since the condition in the assertion is not evaluated in this case, there is little or no performance penalty for having the assertions in the program. When the program is being debugged, it can be run with assertions enabled, as discussed below, and then the assertions can be a great help in locating and identifying bugs.

An assertion statement in Java takes one of the following two forms:

assert condition ;

or

assert condition : error-message ;

where condition is a boolean-valued expression and error-message is a string or an expression of type String. The word "assert" is a reserved word in Java, which cannot be used as an identifier. An assertion statement can be used anyplace in Java where a statement is legal.

If a program is run with assertions disabled, an assertion statement is equivalent to an empty statement and has no effect. When assertions are enabled and an assertion statement is encountered in the program, the condition in the assertion is evaluated. If the value is true, the program proceeds normally. If the value of the condition is false, then an exception of type java.lang.AssertionError is thrown, and the program will crash (unless the error is caught by a try statement). If the assert statement includes an error-message, then the error message string becomes the message in the AssertionError.

So, the statement "assert condition : error-message;" is similar to

if ( condition == false )
throw new AssertionError( error-message );

except that the if statement is executed whenever the program is run, and the assert statement is executed only when the program is run with assertions enabled.

The question is, when to use assertions instead of exceptions? The general rule is to use assertions to test conditions that should definitely be true, if the program is written correctly. Assertions are useful for testing a program to see whether or not it is correct and for finding the errors in an incorrect program. After testing and debugging, when the program is used in the normal way, the assertions in the program will be ignored. However, if a problem turns up later, the assertions are still there in the program to be used to help locate the error. If someone writes to you to say that your program doesn't work when he does such-and-such, you can run the program with assertions enabled, do such-and-such, and hope that the assertions in the program will help you locate the point in the program where it goes wrong.

Consider, for example, the root() method from Subsection 8.3.3 that calculates a root of a quadratic equation. If you believe that your program will always call this method with legal arguments, then it would make sense to write the method using assertions instead of exceptions:

/**
* Returns the larger of the two roots of the quadratic equation
* A*x*x + B*x + C = 0, provided it has any roots.
* Precondition: A != 0 and B*B - 4*A*C >= 0.
*/
static public double root( double A, double B, double C )  {
assert A != 0 : "Leading coefficient of quadratic equation cannot be zero.";
double disc = B*B - 4*A*C;
assert disc >= 0 : "Discriminant of quadratic equation cannot be negative.";
return  (-B + Math.sqrt(disc)) / (2*A);
}

The assertions are not checked when the program is run in the normal way. If you are correct in your belief that the method is never called with illegal arguments, then checking the conditions in the assertions would be unnecessary. If your belief is not correct, the problem should turn up during testing or debugging, when the program is run with the assertions enabled.

If the root() method is part of a software library that you expect other people to use, then the situation is less clear. Oracle's Java documentation advises that assertions should not be used for checking the contract of public methods: If the caller of a method violates the contract by passing illegal parameters, then an exception should be thrown. This will enforce the contract whether or not assertions are enabled. (However, while it's true that Java programmers expect the contract of a method to be enforced with exceptions, there are reasonable arguments for using assertions instead, in some cases.) One might say that assertions are for you, to help you in debugging your code, while exceptions are for people who use your code, to alert them that they are misusing it.

On the other hand, it never hurts to use an assertion to check a postcondition of a method. A postcondition is something that is supposed to be true after the method has executed, and it can be tested with an assert statement at the end of the method. If the postcondition is false, there is a bug in the method itself, and that is something that needs to be found during the development of the method.

To have any effect, assertions must be enabled when the program is run. How to do this depends on what programming environment you are using. (See Section 2.6 for a discussion of programming environments.) In the usual command line environment, assertions are enabled by adding the option -enableassertions to the java command that is used to run the program. For example, if the class that contains the main program is RootFinder, then the command

java -enableassertions RootFinder

will run the program with assertions enabled. The -enableassertions option can be abbreviated to -ea, so the command can alternatively be written as

java -ea RootFinder

In fact, it is possible to enable assertions in just part of a program. An option of the form "-ea:class-name " enables only the assertions in the specified class. Note that there are no spaces between the -ea, the ":", and the name of the class. To enable all the assertions in a package and in its sub-packages, you can use an option of the form "-ea:package-name...". To enable assertions in the "default package" (that is, classes that are not specified to belong to a package, like almost all the classes in this book), use "-ea:...". For example, to run a Java program named "MegaPaint" with assertions enabled for every class in the packages named "paintutils" and "drawing", you would use the command:

java  -ea:paintutils...  -ea:drawing...  MegaPaint

If you are using the Eclipse integrated development environment, you can specify the -ea option by creating a run configuration. Right-click the name of the main program class in the Package Explorer pane, and select "Run As" from the pop-up menu and then "Run..." from the submenu. This will open a dialog box where you can manage run configurations. The name of the project and of the main class will be already be filled in. Click the "Arguments" tab, and enter -ea in the box under "VM Arguments". The contents of this box are added to the java command that is used to run the program. You can enter other options in this box, including more complicated enableassertions options such as -ea:paintutils.... When you click the "Run" button, the options will be applied. Furthermore, they will be applied whenever you run the program, unless you change the run configuration or add a new configuration. Note that it is possible to make two run configurations for the same class, one with assertions enabled and one with assertions disabled.

8.4.2  Annotations

The term "annotation" refers to notes added to or written alongside a main text, to help you understand or appreciate the text. An annotation might be a note that you make to yourself in the margin of a book. It might be a footnote added to an old novel by an editor to explain the historical context of some event. The annotation is metadata or "metatext," that is, text written about the main text rather than as part of the main text itself.

Comments on a program are actually a kind of annotation. Since they are ignored by the compiler, they have no effect on the meaning of the program. They are there to explain that meaning to a human reader. It is possible, of course, for another computer program (not the compiler) to process comments. That's what done in the case of Javadoc comments, which are processed by a program that uses them to create API documentation. But comments are only one type of metadata that might be added to programs.

In Java 5.0, a new feature called annotations was added to the Java language to make it easier to create new kinds of metadata for Java programs. This has made it possible for programmers to devise new ways of annotating programs, and to write programs that can read and use their annotations.

Java annotations have no direct effect on the program that they annotate. But they do have many potential uses. Some annotations are used to make the programmer's intent more explicit. Such annotations might be checked by a compiler to make sure that the code is consistent with the programmer's intention. For example, @Override is a standard annotation that can be used to annotate method definitions. It means that the method is intended to override (that is replace) a method with the same signature that was defined in some superclass. A compiler can check that the superclass method actually exists; if not, it can inform the programmer. An annotation used in this way is an aid to writing correct programs, since the programmer can be warned about a potential error in advance, instead of having to hunt it down later as a bug.

To annotate a method definition with the @Override annotation, simply place it in front of the definition. Syntactically, annotations are modifiers that are used in much the same way as built-in modifiers like "public" and "final." For example,

@Override public void WindowClosed(WindowEvent evt) { ... }

If there is no "WindowClosed(WindowEvent)" method in any superclass, then the compiler can issue an error. In fact, this example is based on a hard-to-find bug that I once introduced when trying to override a method named "windowClosed" with a method that I called "WindowClosed" (with an upper case "W"). If the @Override annotation had existed at that time -- and if I had used it -- the compiler could have rejected my code and saved me the trouble of tracking down the bug.

(Annotations are a fairly advanced feature, and I might not have mentioned them in this textbook, except that the @Override annotation can show up in code generated by Eclipse and other integrated development environments.)

There are two other standard annotations. One is @Deprecated, which can be used to mark deprecated classes, methods, and variables. (A deprecated item is one that is considered to be obsolete, but is still part of the Java language for backwards compatibility for old code.) Use of this annotation would allow a compiler to generate warnings when the deprecated item is used.

The other standard annotation is @SurpressWarnings, which can be used by a compiler to turn off warning messages that would ordinarily be generated when a class or method is compiled. @SuppressWarnings is an example of an annotation that has a parameter. The parameter tells what class of warnings are to be suppressed. For example, when a class or method is annotated with

@SuppressWarnings("deprecation")

then no warnings about the use of deprecated items will be emitted when the class or method is compiled. There are other types of warning that can be suppressed; unfortunately the list of warnings and their names is not standardized and will vary from one compiler to another.

Note, by the way, that the syntax for annotation parameters -- especially for an annotation that accepts multiple parameters -- is not the same as the syntax for method parameters. I won't cover the annotation syntax here.

Programmers can define new annotations for use in their code. Such annotations are ignored by standard compilers and programming tools, but it's possible to write programs that can understand the annotations and check for their presence in source code. It is even possible to create annotations that will be retained at run-time and become part of the running program. In that case, a program can check for annotations in the actual compiled code that is being executed, and take actions that depend on the presence of the annotation or the values of its parameters.

Annotations can help programmers to write correct programs. To use an example from the Java documentation, they can help with the creation of "boilerplate" code -- that is, code that has a very standardized format and that can be generated mechanically. Often, boilerplate code is generated based on other code. Doing that by hand is a tedious and error-prone process. A simple example might be code to save certain aspects of a program's state to a file and to restore it later. The code for reading and writing the values of all the relevant state variables is highly repetitious. Instead of writing that code by hand, a programmer could use an annotation to mark the variables that are part of the state that is to be saved. A program could then be used to check for the annotations and generate the save-and-restore code. In fact, it would even be possible to do without that code altogether, if the program checks for the presence of the annotation at run time to decide which variables to save and restore.

Section 8.5

Analysis of Algorithms

This chapter has concentrated mostly on correctness of programs. In practice, another issue is also important: efficiency. When analyzing a program in terms of efficiency, we want to look at questions such as, "How long does it take for the program to run?" and "Is there another approach that will get the answer more quickly?" Efficiency will always be less important than correctness; if you don't care whether a program works correctly, you can make it run very quickly indeed, but no one will think it's much of an achievement! On the other hand, a program that gives a correct answer after ten thousand years isn't very useful either, so efficiency is often an important issue.

The term "efficiency" can refer to efficient use of almost any resource, including time, computer memory, disk space, or network bandwidth. In this section, however, we will deal exclusively with time efficiency, and the major question that we want to ask about a program is, how long does it take to perform its task?

It really makes little sense to classify an individual program as being "efficient" or "inefficient." It makes more sense to compare two (correct) programs that perform the same task and ask which one of the two is "more efficient," that is, which one performs the task more quickly. However, even here there are difficulties. The running time of a program is not well-defined. The run time can be different depending on the number and speed of the processors in the computer on which it is run and, in the case of Java, on the design of the Java Virtual Machine which is used to interpret the program. It can depend on details of the compiler which is used to translate the program from high-level language to machine language. Furthermore, the run time of a program depends on the size of the problem which the program has to solve. It takes a sorting program longer to sort 10000 items than it takes it to sort 100 items. When the run times of two programs are compared, it often happens that Program A solves small problems faster than Program B, while Program B solves large problems faster than Program A, so that it is simply not the case that one program is faster than the other in all cases.

In spite of these difficulties, there is a field of computer science dedicated to analyzing the efficiency of programs. The field is known as Analysis of Algorithms. The focus is on algorithms, rather than on programs as such, to avoid having to deal with multiple implementations of the same algorithm written in different languages, compiled with different compilers, and running on different computers. Analysis of Algorithms is a mathematical field that abstracts away from these down-and-dirty details. Still, even though it is a theoretical field, every working programmer should be aware of some of its techniques and results. This section is a very brief introduction to some of those techniques and results. Because this is not a mathematics book, the treatment will be rather informal.

One of the main techniques of analysis of algorithms is asymptotic analysis. The term "asymptotic" here means basically "the tendency in the long run." An asymptotic analysis of an algorithm's run time looks at the question of how the run time depends on the size of the problem. The analysis is asymptotic because it only considers what happens to the run time as the size of the problem increases without limit; it is not concerned with what happens for problems of small size or, in fact, for problems of any fixed finite size. Only what happens in the long run, as the problem size increases without limit, is important. Showing that Algorithm A is asymptotically faster than Algorithm B doesn't necessarily mean that Algorithm A will run faster than Algorithm B for problems of size 10 or size 1000 or even size 1000000 -- it only means that if you keep increasing the problem size, you will eventually come to a point where Algorithm A is faster than Algorithm B. An asymptotic analysis is only a first approximation, but in practice it often gives important and useful information.

Central to asymptotic analysis is Big-Oh notation. Using this notation, we might say, for example, that an algorithm has a running time that is O(n^2) or O(n) or O(log(n)). These notations are read "Big-Oh of n squared," "Big-Oh of n," and "Big-Oh of log n" (where log is a logarithm function). More generally, we can refer to O(f(n)) ("Big-Oh of f of n"), where f(n) is some function that assigns a positive real number to every positive integer n. The "n" in this notation refers to the size of the problem. Before you can even begin an asymptotic analysis, you need some way to measure problem size. Usually, this is not a big issue. For example, if the problem is to sort a list of items, then the problem size can be taken to be the number of items in the list. When the input to an algorithm is an integer, as in the case of an algorithm that checks whether a given positive integer is prime, the usual measure of the size of a problem is the number of bits in the input integer rather than the integer itself. More generally, the number of bits in the input to a problem is often a good measure of the size of the problem.

To say that the running time of an algorithm is O(f(n)) means that for large values of the problem size, n, the running time of the algorithm is no bigger than some constant times f(n). (More rigorously, there is a number C and a positive integer M such that whenever n is greater than M, the run time is less than or equal to C*f(n).) The constant takes into account details such as the speed of the computer on which the algorithm is run; if you use a slower computer, you might have to use a bigger constant in the formula, but changing the constant won't change the basic fact that the run time is O(f(n)). The constant also makes it unnecessary to say whether we are measuring time in seconds, years, CPU cycles, or any other unit of measure; a change from one unit of measure to another is just multiplication by a constant. Note also that O(f(n)) doesn't depend at all on what happens for small problem sizes, only on what happens in the long run as the problem size increases without limit.

To look at a simple example, consider the problem of adding up all the numbers in an array. The problem size, n, is the length of the array. Using A as the name of the array, the algorithm can be expressed in Java as:

total = 0;
for (int i = 0; i < n; i++)
total = total + A[i];

This algorithm performs the same operation, total = total + A[i], n times. The total time spent on this operation is a*n, where a is the time it takes to perform the operation once. Now, this is not the only thing that is done in the algorithm. The value of i is incremented and is compared to n each time through the loop. This adds an additional time of b*n to the run time, for some constant b. Furthermore, i and total both have to be initialized to zero; this adds some constant amount c to the running time. The exact running time would then be (a+b)*n+c, where the constants a, b, and c depend on factors such as how the code is compiled and what computer it is run on. Using the fact that c is less than or equal to c*n for any positive integer n, we can say that the run time is less than or equal to (a+b+c)*n. That is, the run time is less than or equal to a constant times n. By definition, this means that the run time for this algorithm is O(n).

If this explanation is too mathematical for you, we can just note that for large values of n, the c in the formula (a+b)*n+c is insignificant compared to the other term, (a+b)*n. We say that c is a "lower order term." When doing asymptotic analysis, lower order terms can be discarded. A rough, but correct, asymptotic analysis of the algorithm would go something like this: Each iteration of the for loop takes a certain constant amount of time. There are n iterations of the loop, so the total run time is a constant times n, plus lower order terms (to account for the initialization). Disregarding lower order terms, we see that the run time is O(n).

Note that to say that an algorithm has run time O(f(n)) is to say that its run time is no bigger than some constant times f(n) (for large values of n). O(f(n)) puts an upper limit on the run time. However, the run time could be smaller, even much smaller. For example, if the run time is O(n), it would also be correct to say that the run time is O(n^2) or even O(n^10). If the run time is less than a constant times n, then it is certainly less than the same constant times n^2 or n^10.

Of course, sometimes it's useful to have a lower limit on the run time. That is, we want to be able to say that the run time is greater than or equal to some constant times f(n) (for large values of n). The notation for this is Ω(f(n)), read "Omega of f of n." "Omega" is the name of a letter in the Greek alphabet, and Ω is the upper case version of that letter. (To be technical, saying that the run time of an algorithm is Ω(f(n)) means that there is a positive number C and a positive integer M such that whenever n is greater than M, the run time is greater than or equal to C*f(n).) O(f(n)) tells you something about the maximum amount of time that you might have to wait for an algorithm to finish; Ω(f(n)) tells you something about the minimum time.

The algorithm for adding up the numbers in an array has a run time that is Ω(n) as well as O(n). When an algorithm has a run time that is both Ω(f(n)) and O(f(n)), its run time is said to be Θ(f(n)), read "Theta of f of n." (Theta is another letter from the Greek alphabet.) To say that the run time of an algorithm is Θ(f(n)) means that for large values of n, the run time is between a*f(n) and b*f(n), where a and b are constants (with b greater than a, and both greater than 0).

Let's look at another example. Consider the algorithm that can be expressed in Java in the following method:

/**
* Sorts the n array elements A[0], A[1], ..., A[n-1] into increasing order.
*/
public static simpleBubbleSort( int[] A, int n ) {
for (int i = 0; i < n; i++) {
// Do n passes through the array...
for (int j = 0; j < n-1; j++) {
if ( A[j] > A[j+1] ) {
// A[j] and A[j+1] are out of order, so swap them
int temp = A[j];
A[j] = A[j+1];
A[j+1] = temp;
}
}
}
}

Here, the parameter n represents the problem size. The outer for loop in the method is executed n times. Each time the outer for loop is executed, the inner for loop is executed n-1 times, so the if statement is executed n*(n-1) times. This is n^2-n, but since lower order terms are not significant in an asymptotic analysis, it's good enough to say that the if statement is executed about n^2 times. In particular, the test A[j] > A[j+1] is executed about n^2 times, and this fact by itself is enough to say that the run time of the algorithm is Ω(n^2), that is, the run time is at least some constant times n^2. Furthermore, if we look at other operations -- the assignment statements, incrementing i and j, etc. -- none of them are executed more than n^2 times, so the run time is also O(n^2), that is, the run time is no more than some constant times n^2. Since it is both Ω(n^2) and O(n^2), the run time of the simpleBubbleSort algorithm is Θ(n^2).

You should be aware that some people use the notation O(f(n)) as if it meant Θ(f(n)). That is, when they say that the run time of an algorithm is O(f(n)), they mean to say that the run time is about equal to a constant times f(n). For that, they should use Θ(f(n)). Properly speaking, O(f(n)) means that the run time is less than a constant times f(n), possibly much less.

So far, my analysis has ignored an important detail. We have looked at how run time depends on the problem size, but in fact the run time usually depends not just on the size of the problem but on the specific data that has to be processed. For example, the run time of a sorting algorithm can depend on the initial order of the items that are to be sorted, and not just on the number of items.

To account for this dependency, we can consider either the worst case run time analysis or the average case run time analysis of an algorithm. For a worst case run time analysis, we consider all possible problems of size n and look at the longest possible run time for all such problems. For an average case analysis, we consider all possible problems of size n and look at the average of the run times for all such problems. Usually, the average case analysis assumes that all problems of size n are equally likely to be encountered, although this is not always realistic -- or even possible in the case where there is an infinite number of different problems of a given size.

In many cases, the average and the worst case run times are the same to within a constant multiple. This means that as far as asymptotic analysis is concerned, they are the same. That is, if the average case run time is O(f(n)) or Θ(f(n)), then so is the worst case. However, later in the book, we will encounter a few cases where the average and worst case asymptotic analyses differ.

So, what do you really have to know about analysis of algorithms to read the rest of this book? We will not do any rigorous mathematical analysis, but you should be able to follow informal discussion of simple cases such as the examples that we have looked at in this section. Most important, though, you should have a feeling for exactly what it means to say that the running time of an algorithm is O(f(n)) or Θ(f(n)) for some common functions f(n). The main point is that these notations do not tell you anything about the actual numerical value of the running time of the algorithm for any particular case. They do not tell you anything at all about the running time for small values of n. What they do tell you is something about the rate of growth of the running time as the size of the problem increases.

Suppose you compare two algorithms that solve the same problem. The run time of one algorithm is Θ(n^2), while the run time of the second algorithm is Θ(n^3). What does this tell you? If you want to know which algorithm will be faster for some particular problem of size, say, 100, nothing is certain. As far as you can tell just from the asymptotic analysis, either algorithm could be faster for that particular case -- or in any particular case. But what you can say for sure is that if you look at larger and larger problems, you will come to a point where the Θ(n^2) algorithm is faster than the Θ(n^3) algorithm. Furthermore, as you continue to increase the problem size, the relative advantage of the Θ(n^2) algorithm will continue to grow. There will be values of n for which the Θ(n^2) algorithm is a thousand times faster, a million times faster, a billion times faster, and so on. This is because for any positive constants a and b, the function a*n^3 grows faster than the function b*n^2 as n gets larger. (Mathematically, the limit of the ratio of a*n^3 to b*n^2 is infinite as n approaches infinity.)

This means that for "large" problems, a Θ(n^2) algorithm will definitely be faster than a Θ(n^3) algorithm. You just don't know -- based on the asymptotic analysis alone -- exactly how large "large" has to be. In practice, in fact, it is likely that the Θ(n^2) algorithm will be faster even for fairly small values of n, and absent other information you would generally prefer a Θ(n^2) algorithm to a Θ(n^3) algorithm.

So, to understand and apply asymptotic analysis, it is essential to have some idea of the rates of growth of some common functions. For the power functions n, n^2, n^3, n^4, ..., the larger the exponent, the greater the rate of growth of the function. Exponential functions such as 2n and 10n, where the n is in the exponent, have a growth rate that is faster than that of any power function. In fact, exponential functions grow so quickly that an algorithm whose run time grows exponentially is almost certainly impractical even for relatively modest values of n, because the running time is just too long. Another function that often turns up in asymptotic analysis is the logarithm function, log(n). There are actually many different logarithm functions, but the one that is usually used in computer science is the so-called logarithm to the base two, which is defined by the fact that log(2x) = x for any number x. (Usually, this function is written log_2(n), but I will leave out the subscript 2, since I will only use the base-two logarithm in this book.) The logarithm function grows very slowly. The growth rate of log(n) is much smaller than the growth rate of n. The growth rate of n*log(n) is a little larger than the growth rate of n, but much smaller than the growth rate of n^2. The following table should help you understand the differences among the rates of grows of various functions:

The reason that log(n) shows up so often is because of its association with multiplying and dividing by two: Suppose you start with the number n and divide it by 2, then divide by 2 again, and so on, until you get a number that is less than or equal to 1. Then the number of divisions is equal (to the nearest integer) to log(n).

As an example, consider the binary search algorithm from Subsection 7.4.1. This algorithm searches for an item in a sorted array. The problem size, n, can be taken to be the length of the array. Each step in the binary search algorithm divides the number of items still under consideration by 2, and the algorithm stops when the number of items under consideration is less than or equal to 1 (or sooner). It follows that the number of steps for an array of length n is at most log(n). This means that the worst-case run time for binary search is Θ(log(n)). (The average case run time is also Θ(log(n)).) By comparison, the linear search algorithm, which was also presented in Subsection 7.4.1 has a run time that is Θ(n). The Θ notation gives us a quantitative way to express and to understand the fact that binary search is "much faster" than linear search.

In binary search, each step of the algorithm divides the problem size by 2. It often happens that some operation in an algorithm (not necessarily a single step) divides the problem size by 2. Whenever that happens, the logarithm function is likely to show up in an asymptotic analysis of the run time of the algorithm.

Analysis of Algorithms is a large, fascinating field. We will only use a few of the most basic ideas from this field, but even those can be very helpful for understanding the differences among algorithms.

Programming Exercises for Chapter 8

Exercise 8.1:

Write a program that uses the following subroutine, from Subsection 8.3.3, to solve equations specified by the user.

/**
* Returns the larger of the two roots of the quadratic equation
* A*x*x + B*x + C = 0, provided it has any roots.  If A == 0 or
* if the discriminant, B*B - 4*A*C, is negative, then an exception
* of type IllegalArgumentException is thrown.
*/
static public double root( double A, double B, double C )
throws IllegalArgumentException {
if (A == 0) {
throw new IllegalArgumentException("A can't be zero.");
}
else {
double disc = B*B - 4*A*C;
if (disc < 0)
throw new IllegalArgumentException("Discriminant < zero.");
return  (-B + Math.sqrt(disc)) / (2*A);
}
}

Your program should allow the user to specify values for A, B, and C. It should call the subroutine to compute a solution of the equation. If no error occurs, it should print the root. However, if an error occurs, your program should catch that error and print an error message. After processing one equation, the program should ask whether the user wants to enter another equation. The program should continue until the user answers no.

See the Solution

Exercise 8.2:

As discussed in Section 8.1, values of type int are limited to 32 bits. Integers that are too large to be represented in 32 bits cannot be stored in an int variable. Java has a standard class, java.math.BigInteger, that addresses this problem. An object of type BigInteger is an integer that can be arbitrarily large. (The maximum size is limited only by the amount of memory available to the Java Virtual Machine.) Since BigIntegers are objects, they must be manipulated using instance methods from the BigInteger class. For example, you can't add two BigIntegers with the + operator. Instead, if N and M are variables that refer to BigIntegers, you can compute the sum of N and M with the function call N.add(M). The value returned by this function is a new BigInteger object that is equal to the sum of N and M.

The BigInteger class has a constructor new BigInteger(str), where str is a string. The string must represent an integer, such as "3" or "39849823783783283733". If the string does not represent a legal integer, then the constructor throws a NumberFormatException.

There are many instance methods in the BigInteger class. Here are a few that you will find useful for this exercise. Assume that N and M are variables of type BigInteger.

- N.add(M) -- a function that returns a BigInteger representing the sum of N and M.

- N.multiply(M) -- a function that returns a BigInteger representing the result of multiplying N times M.

- N.divide(M) -- a function that returns a BigInteger representing the result of dividing N by M, discarding the remainder.

- N.signum() -- a function that returns an ordinary int. The returned value represents the sign of the integer N. The returned value is 1 if N is greater than zero. It is -1 if N is less than zero. And it is 0 if N is zero.

- N.equals(M) -- a function that returns a boolean value that is true if N and M have the same integer value.

- N.toString() -- a function that returns a String representing the value of N.

- N.testBit(k) -- a function that returns a boolean value. The parameter k is an integer. The return value is true if the k-th bit in N is 1, and it is false if the k-th bit is 0. Bits are numbered from right to left, starting with 0. Testing "if (N.testBit(0))" is an easy way to check whether N is even or odd. N.testBit(0) is true if and only if N is an odd number.

For this exercise, you should write a program that prints 3N+1 sequences with starting values specified by the user. In this version of the program, you should use BigIntegers to represent the terms in the sequence. You can read the user's input into a String with the TextIO.getln() function. Use the input value to create the BigInteger object that represents the starting point of the 3N+1 sequence. Don't forget to catch and handle the NumberFormatException that will occur if the user's input is not a legal integer! You should also check that the input number is greater than zero.

If the user's input is legal, print out the 3N+1 sequence. Count the number of terms in the sequence, and print the count at the end of the sequence. Exit the program when the user inputs an empty line.

See the Solution

Exercise 8.3:

A Roman numeral represents an integer using letters. Examples are XVII to represent 17, MCMLIII for 1953, and MMMCCCIII for 3303. By contrast, ordinary numbers such as 17 or 1953 are called Arabic numerals. The following table shows the Arabic equivalent of all the single-letter Roman numerals:

M    1000            X   10
D     500            V    5
C     100            I    1
L      50

When letters are strung together, the values of the letters are just added up, with the following exception. When a letter of smaller value is followed by a letter of larger value, the smaller value is subtracted from the larger value. For example, IV represents 5 - 1, or 4. And MCMXCV is interpreted as M + CM + XC + V, or 1000 + (1000 - 100) + (100 - 10) + 5, which is 1995. In standard Roman numerals, no more than three consecutive copies of the same letter are used. Following these rules, every number between 1 and 3999 can be represented as a Roman numeral made up of the following one- and two-letter combinations:

M    1000            X   10
CM    900            IX   9
D     500            V    5
CD    400            IV   4
C     100            I    1
XC     90
L      50
XL     40

Write a class to represent Roman numerals. The class should have two constructors. One constructs a Roman numeral from a string such as "XVII" or "MCMXCV". It should throw a NumberFormatException if the string is not a legal Roman numeral. The other constructor constructs a Roman numeral from an int. It should throw a NumberFormatException if the int is outside the range 1 to 3999.

In addition, the class should have two instance methods. The method toString() returns the string that represents the Roman numeral. The method toInt() returns the value of the Roman numeral as an int.

At some point in your class, you will have to convert an int into the string that represents the corresponding Roman numeral. One way to approach this is to gradually "move" value from the Arabic numeral to the Roman numeral. Here is the beginning of a routine that will do this, where number is the int that is to be converted:

String roman = "";
int N = number;
while (N >= 1000) {
// Move 1000 from N to roman.
roman += "M";
N -= 1000;
}
while (N >= 900) {
// Move 900 from N to roman.
roman += "CM";
N -= 900;
}
.
.  // Continue with other values from the above table.
.

(You can save yourself a lot of typing in this routine if you use arrays in a clever way to represent the data in the above table.)

Once you've written your class, use it in a main program that will read both Arabic numerals and Roman numerals entered by the user. If the user enters an Arabic numeral, print the corresponding Roman numeral. If the user enters a Roman numeral, print the corresponding Arabic numeral. (You can tell the difference by using TextIO.peek() to peek at the first character in the user's input (see Subsection 8.2.2). If the first character is a digit, then the user's input is an Arabic numeral. Otherwise, it's a Roman numeral.) The program should end when the user inputs an empty line.

See the Solution

Exercise 8.4:

The source code file Expr.java defines a class, Expr, that can be used to represent mathematical expressions involving the variable x. The expression can use the operators +, -, *, /, and ^ (where ^ represents the operation of raising a number to a power). It can use mathematical functions such as sin, cos, abs, and ln. See the source code file for full details. The Expr class uses some advanced techniques which have not yet been covered in this textbook. However, the interface is easy to understand. It contains only a constructor and two public methods.

The constructor new Expr(def) creates an Expr object defined by a given expression. The parameter, def, is a string that contains the definition. For example, new Expr("x^2") or new Expr("sin(x)+3*x"). If the parameter in the constructor call does not represent a legal expression, then the constructor throws an IllegalArgumentException. The message in the exception describes the error.

If func is a variable of type Expr and num is of type double, then func.value(num) is a function that returns the value of the expression when the number num is substituted for the variable x in the expression. For example, if Expr represents the expression 3*x+1, then func.value(5) is 3*5+1, or 16. If the expression is undefined for the specified value of x, then the special value Double.NaN is returned; no exception is thrown.

Finally, func.toString() returns the definition of the expression. This is just the string that was used in the constructor that created the expression object.

For this exercise, you should write a program that lets the user enter an expression. If the expression contains an error, print an error message. Otherwise, let the user enter some numerical values for the variable x. Print the value of the expression for each number that the user enters. However, if the expression is undefined for the specified value of x, print a message to that effect. You can use the boolean-valued function Double.isNaN(val) to check whether a number, val, is Double.NaN.

The user should be able to enter as many values of x as desired. After that, the user should be able to enter a new expression.

See the Solution

Exercise 8.5:

This exercise uses the class Expr, which was described in Exercise 8.4 and which is defined in the source code file Expr.java. For this exercise, you should write a GUI program that can graph a function, f(x), whose definition is entered by the user. The program should have a text-input box where the user can enter an expression involving the variable x, such as x^2 or sin(x-3)/x. This expression is the definition of the function. When the user presses return in the text input box, the program should use the contents of the text input box to construct an object of type Expr. If an error is found in the definition, then the program should display an error message. Otherwise, it should display a graph of the function. (Recall: A JTextField generates an ActionEvent when the user presses return.)

The program will need a JPanel for displaying the graph. To keep things simple, this panel should represent a fixed region in the xy-plane, defined by -5 <= x <= 5 and -5 <= y <= 5. To draw the graph, compute a large number of points and connect them with line segments. (This method does not handle discontinuous functions properly; doing so is very hard, so you shouldn't try to do it for this exercise.) My program divides the interval -5 <= x <= 5 into 300 subintervals and uses the 301 endpoints of these subintervals for drawing the graph. Note that the function might be undefined at one of these x-values. In that case, you have to skip that point.

A point on the graph has the form (x,y) where y is obtained by evaluating the user's expression at the given value of x. You will have to convert these real numbers to the integer coordinates of the corresponding pixel on the canvas. The formulas for the conversion are:

a  =  (int)( (x + 5)/10 * width );
b  =  (int)( (5 - y)/10 * height );

where a and b are the horizontal and vertical coordinates of the pixel, and width and height are the width and height of the panel.

See the Solution

Quiz on Chapter 8

Question 1:

What does it mean to say that a program is robust?

Question 2:

Why do programming languages require that variables be declared before they are used? What does this have to do with correctness and robustness?

Question 3:

What is a precondition? Give an example.

Question 4:

Explain how preconditions can be used as an aid in writing correct programs.

Question 5:

Java has a predefined class called Throwable. What does this class represent? Why does it exist?

Question 6:

Write a method that prints out a 3N+1 sequence starting from a given integer, N. The starting value should be a parameter to the method. If the parameter is less than or equal to zero, throw an IllegalArgumentException. If the number in the sequence becomes too large to be represented as a value of type int, throw an ArithmeticException.

Question 7:

Rewrite the method from the previous question, using assert statements instead of exceptions to check for errors. What is the difference between the two versions of the method when the program is run?

Question 8:

Some classes of exceptions are checked exceptions that require mandatory exception handling. Explain what this means.

Question 9:

Consider a subroutine processData() that has the header

static void processData() throws IOException

Write a try..catch statement that calls this subroutine and prints an error message if an IOException occurs.

Question 10:

Why should a subroutine throw an exception when it encounters an error? Why not just terminate the program?

Question 11:

Suppose that you have a choice of two algorithms that perform the same task. One has average-case run time that is Θ(n^2) while the run time of the second algorithm has an average-case run time that is Θ(n*log(n)). Suppose that you need to process an input of size n = 100. Which algorithm would you choose? Can you be certain that you are choosing the fastest algorithm for the input that you intend to process.

Question 12:

Analyze the run time of the following algorithm. That is, find a function f(n) such that the run time of the algorithm is O(f(n)) or, better, Θ(f(n)). Assume that A is an array of integers, and use the length of the array as the input size, n.

int total = 0;
for (int i = 0; i < A.length; i++) {
if (A[i] > 0)
total = total + A[i];
}

See the Answers

Chapter 9

Linked Data Structures and Recursion

In this chapter, we look at two advanced programming techniques, recursion and linked data structures, and some of their applications. Both of these techniques are related to the seemingly paradoxical idea of defining something in terms of itself. This turns out to be a remarkably powerful idea.

A subroutine is said to be recursive if it calls itself, either directly or indirectly. What this means is that the subroutine is used in its own definition. Recursion can often be used to solve complex problems by reducing them to simpler problems of the same type.

A reference to one object can be stored in an instance variable of another object. The objects are then said to be "linked." Complex data structures can be built by linking objects together. An especially interesting case occurs when an object contains a link to another object that belongs to the same class. In that case, the class is used in its own definition. Several important types of data structures are built using classes of this kind.

Contents of Chapter 9:

- Section 1: Recursion

- Section 2: Linked Data Structures

- Section 3: Stacks, Queues, and ADTs

- Section 4: Binary Trees

- Section 5: A Simple Recursive Descent Parser

- Programming Exercises

- Quiz on This Chapter

Section 9.1

Recursion

At one time or another, you've probably been told that you can't define something in terms of itself. Nevertheless, if it's done right, defining something at least partially in terms of itself can be a very powerful technique. A recursive definition is one that uses the concept or thing that is being defined as part of the definition. For example: An "ancestor" is either a parent or an ancestor of a parent. A "sentence" can be, among other things, two sentences joined by a conjunction such as "and." A "directory" is a part of a disk drive that can hold files and directories. In mathematics, a "set" is a collection of elements, which can themselves be sets. A "statement" in Java can be a while statement, which is made up of the word "while", a boolean-valued condition, and a statement.

Recursive definitions can describe very complex situations with just a few words. A definition of the term "ancestor" without using recursion might go something like "a parent, or a grandparent, or a great-grandparent, or a great-great-grandparent, and so on." But saying "and so on" is not very rigorous. (I've often thought that recursion is really just a rigorous way of saying "and so on.") You run into the same problem if you try to define a "directory" as "a file that is a list of files, where some of the files can be lists of files, where some of those files can be lists of files, and so on." Trying to describe what a Java statement can look like, without using recursion in the definition, would be difficult and probably pretty comical.

Recursion can be used as a programming technique. A recursive subroutine (or recursive method) is one that calls itself, either directly or indirectly. To say that a subroutine calls itself directly means that its definition contains a subroutine call statement that calls the subroutine that is being defined. To say that a subroutine calls itself indirectly means that it calls a second subroutine which in turn calls the first subroutine (either directly or indirectly). A recursive subroutine can define a complex task in just a few lines of code. In the rest of this section, we'll look at a variety of examples, and we'll see other examples in the rest of the book.

9.1.1  Recursive Binary Search

Let's start with an example that you've seen before: the binary search algorithm from Subsection 7.4.1. Binary search is used to find a specified value in a sorted list of items (or, if it does not occur in the list, to determine that fact). The idea is to test the element in the middle of the list. If that element is equal to the specified value, you are done. If the specified value is less than the middle element of the list, then you should search for the value in the first half of the list. Otherwise, you should search for the value in the second half of the list. The method used to search for the value in the first or second half of the list is binary search. That is, you look at the middle element in the half of the list that is still under consideration, and either you've found the value you are looking for, or you have to apply binary search to one half of the remaining elements. And so on! This is a recursive description, and we can write a recursive subroutine to implement it.

Before we can do that, though, there are two considerations that we need to take into account. Each of these illustrates an important general fact about recursive subroutines. First of all, the binary search algorithm begins by looking at the "middle element of the list." But what if the list is empty? If there are no elements in the list, then it is impossible to look at the middle element. In the terminology of Subsection 8.2.1, having a non-empty list is a "precondition" for looking at the middle element, and this is a clue that we have to modify the algorithm to take this precondition into account. What should we do if we find ourselves searching for a specified value in an empty list? The answer is easy: If the list is empty, we can be sure that the value does not occur in the list, so we can give the answer without any further work. An empty list is a base case for the binary search algorithm. A base case for a recursive algorithm is a case that is handled directly, rather than by applying the algorithm recursively. The binary search algorithm actually has another type of base case: If we find the element we are looking for in the middle of the list, we are done. There is no need for further recursion.

The second consideration has to do with the parameters to the subroutine. The problem is phrased in terms of searching for a value in a list. In the original, non-recursive binary search subroutine, the list was given as an array. However, in the recursive approach, we have to be able to apply the subroutine recursively to just a part of the original list. Where the original subroutine was designed to search an entire array, the recursive subroutine must be able to search part of an array. The parameters to the subroutine must tell it what part of the array to search. This illustrates a general fact that in order to solve a problem recursively, it is often necessary to generalize the problem slightly.

Here is a recursive binary search algorithm that searches for a given value in part of an array of integers:

/**
* Search in the array A in positions numbered loIndex to hiIndex,
* inclusive, for the specified value.   If the value is found, return
* the index in the array where it occurs. If the value is not found,
* return -1.  Precondition:  The array must be sorted into increasing
* order.
*/
static int binarySearch(int[] A, int loIndex, int hiIndex, int value) {

if (loIndex > hiIndex) {
// The starting position comes after the final index,
// so there are actually no elements in the specified
// range.  The value does not occur in this empty list!
return -1;
}

else {
// Look at the middle position in the list.  If the
// value occurs at that position, return that position.
// Otherwise, search recursively in either the first
// half or the second half of the list.
int middle = (loIndex + hiIndex) / 2;
if (value == A[middle])
return middle;
else if (value < A[middle])
return binarySearch(A, loIndex, middle - 1, value);
else   // value must be > A[middle]
return binarySearch(A, middle + 1, hiIndex, value);
}

} // end binarySearch()

In this routine, the parameters loIndex and hiIndex specify the part of the array that is to be searched. To search an entire array, it is only necessary to call binarySearch(A, 0, A.length - 1, value). In the two base cases -- when there are no elements in the specified range of indices and when the value is found in the middle of the range -- the subroutine can return an answer immediately, without using recursion. In the other cases, it uses a recursive call to compute the answer and returns that answer.

Most people find it difficult at first to convince themselves that recursion actually works. The key is to note two things that must be true for recursion to work properly: There must be one or more base cases, which can be handled without using recursion. And when recursion is applied during the solution of a problem, it must be applied to a problem that is in some sense smaller -- that is, closer to the base cases -- than the original problem. The idea is that if you can solve small problems and if you can reduce big problems to smaller problems, then you can solve problems of any size. Ultimately, of course, the big problems have to be reduced, possibly in many, many steps, to the very smallest problems (the base cases). Doing so might involve an immense amount of detailed bookkeeping. But the computer does that bookkeeping, not you! As a programmer, you lay out the big picture: the base cases and the reduction of big problems to smaller problems. The computer takes care of the details involved in reducing a big problem, in many steps, all the way down to base cases. Trying to think through this reduction in detail is likely to drive you crazy, and will probably make you think that recursion is hard. Whereas in fact, recursion is an elegant and powerful method that is often the simplest approach to solving a complex problem.

A common error in writing recursive subroutines is to violate one of the two rules: There must be one or more base cases, and when the subroutine is applied recursively, it must be applied to a problem that is smaller than the original problem. If these rules are violated, the result can be an infinite recursion, where the subroutine keeps calling itself over and over, without ever reaching a base case. Infinite recursion is similar to an infinite loop. However, since each recursive call to the subroutine uses up some of the computer's memory, a program that is stuck in an infinite recursion will run out of memory and crash before long. In Java, the program will crash with an exception of type StackOverflowError.

9.1.2  Towers of Hanoi

We have been studying an algorithm, binary search, that can easily be implemented with a while loop, instead of with recursion. Next, we turn to a problem that is easy to solve with recursion but difficult to solve without it. This is a standard example known as "The Towers of Hanoi." The problem involves a stack of various-sized disks, piled up on a base in order of decreasing size. The object is to move the stack from one base to another, subject to two rules: Only one disk can be moved at a time, and no disk can ever be placed on top of a smaller disk. There is a third base that can be used as a "spare." The starting situation for a stack of ten disks is shown in the top half of the following picture. The situation after a number of moves have been made is shown in the bottom half of the picture. (These illustrations are from a sample program from Chapter 12, TowersOfHanoiGUI.java, which displays an animation of the step-by-step solution of the problem; however, that program uses some techniques that you haven't learned yet.)

The problem is to move ten disks from Stack 0 to Stack 1, subject to the rules given above. Stack 2 can be used as a spare location. Can we reduce this to smaller problems of the same type, possibly generalizing the problem a bit to make this possible? It seems natural to consider the size of the problem to be the number of disks to be moved. If there are N disks in Stack 0, we know that we will eventually have to move the bottom disk from Stack 0 to Stack 1. But before we can do that, according to the rules, the first N-1 disks must be on Stack 2. Once we've moved the N-th disk to Stack 1, we must move the other N-1 disks from Stack 2 to Stack 1 to complete the solution. But moving N-1 disks is the same type of problem as moving N disks, except that it's a smaller version of the problem. This is exactly what we need to do recursion! The problem has to be generalized a bit, because the smaller problems involve moving disks from Stack 0 to Stack 2 or from Stack 2 to Stack 1, instead of from Stack 0 to Stack 1. In the recursive subroutine that solves the problem, the stacks that serve as the source and destination of the disks have to be specified. It's also convenient to specify the stack that is to be used as a spare, even though we could figure that out from the other two parameters. The base case is when there is only one disk to be moved. The solution in this case is trivial: Just move the disk in one step. Here is a version of the subroutine that will print out step-by-step instructions for solving the problem:

/**
* Solve the problem of moving the number of disks specified
* by the first parameter from the stack specified by the
* second parameter to the stack specified by the third
* parameter.  The stack specified by the fourth parameter
* is available for use as a spare.  Stacks are specified by
* number: 0, 1, or 2.
*/
static void towersOfHanoi(int disks, int from, int to, int spare) {
if (disks == 1) {
// There is only one disk to be moved.  Just move it.
System.out.printf("Move disk 1 from stack %d to stack %d%n",
from, to);
}
else {
// Move all but one disk to the spare stack, then
// move the bottom disk, then put all the other
// disks on top of it.
towersOfHanoi(disks-1, from, spare, to);
System.out.printf("Move disk %d from stack %d to stack %d%n",
disks, from, to);
towersOfHanoi(disks-1, spare, to, from);
}
}

This subroutine just expresses the natural recursive solution. The recursion works because each recursive call involves a smaller number of disks, and the problem is trivial to solve in the base case, when there is only one disk. To solve the "top level" problem of moving N disks from Stack 0 to Stack 1, it should be called with the command TowersOfHanoi(N,0,1,2). The subroutine is demonstrated by the sample program TowersOfHanoi.java.

Here, for example, is the output from the program when it is run with the number of disks set equal to 4:

Move disk 1 from stack 0 to stack 2
Move disk 2 from stack 0 to stack 1
Move disk 1 from stack 2 to stack 1
Move disk 3 from stack 0 to stack 2
Move disk 1 from stack 1 to stack 0
Move disk 2 from stack 1 to stack 2
Move disk 1 from stack 0 to stack 2
Move disk 4 from stack 0 to stack 1
Move disk 1 from stack 2 to stack 1
Move disk 2 from stack 2 to stack 0
Move disk 1 from stack 1 to stack 0
Move disk 3 from stack 2 to stack 1
Move disk 1 from stack 0 to stack 2
Move disk 2 from stack 0 to stack 1
Move disk 1 from stack 2 to stack 1

The output of this program shows you a mass of detail that you don't really want to think about! The difficulty of following the details contrasts sharply with the simplicity and elegance of the recursive solution. Of course, you really want to leave the details to the computer. (You might think about what happens when the precondition that the number of disks is positive is violated. The result is an example of infinite recursion.)

There is, by the way, a story that explains the name of this problem. According to this story, on the first day of creation, a group of monks in an isolated tower near Hanoi were given a stack of 64 disks and were assigned the task of moving one disk every day, according to the rules of the Towers of Hanoi problem. On the day that they complete their task of moving all the disks from one stack to another, the universe will come to an end. But don't worry. The number of steps required to solve the problem for N disks is 2N - 1, and 2^64 - 1 days is over 50,000,000,000,000 years. We have a long way to go.

(In the terminology of Section 8.5, the Towers of Hanoi algorithm has a run time that is Θ(2n), where n is the number of disks that have to be moved. Since the exponential function 2n grows so quickly, the Towers of Hanoi problem can be solved in practice only for a small number of disks.)

By the way, in addition to the graphical Towers of Hanoi program, mentioned above, there are two more demo programs that you might want to look at. Each program provides a visual demonstration of a recursive algorithm. In Maze.java, recursion is used to solve a maze. In LittlePentominos.java, it is used to solve a well-known kind of puzzle. (LittlePentominos.java also requires the file MosaicPanel.java.) It would be useful to run the programs and watch them for a while, but the source code uses some techniques that won't be covered until Chapter 12.

The Maze program first creates a random maze. It then tries to solve the maze by finding a path through the maze from the upper left corner to the lower right corner. This problem is actually very similar to a "blob-counting" problem that is considered later in this section. The recursive maze-solving routine starts from a given square, and it visits each neighboring square and calls itself recursively from there. The recursion ends if the routine finds itself at the lower right corner of the maze. When it can't find a solution from a square, it "backs up" out of that square and tries somewhere else. This common technique is referred to as recursive backtracking.

The LittlePentominos program is an implementation of a classic puzzle. A pentomino is a connected figure made up of five equal-sized squares. There are exactly twelve figures that can be made in this way, not counting all the possible rotations and reflections of the basic figures. The problem is to place the twelve pentominos on an 8-by-8 board in which four of the squares have already been marked as filled. The recursive solution looks at a board that has already been partially filled with pentominos. The subroutine looks at each remaining piece in turn. It tries to place that piece in the next available place on the board. If the piece fits, it calls itself recursively to try to fill in the rest of the solution. If that fails, then the subroutine goes on to the next piece -- another example of recursive backtracking. A generalized version of the pentominos program with many more features can be found at http://math.hws.edu/xJava/PentominosSolver/.

9.1.3  A Recursive Sorting Algorithm

Turning next to an application that is perhaps more practical, we'll look at a recursive algorithm for sorting an array. The selection sort and insertion sort algorithms, which were covered in Section 7.4, are fairly simple, but they are rather slow when applied to large arrays. Faster sorting algorithms are available. One of these is Quicksort, a recursive algorithm which turns out to be the fastest sorting algorithm in most situations.

The Quicksort algorithm is based on a simple but clever idea: Given a list of items, select any item from the list. This item is called the pivot. (In practice, I'll just use the first item in the list.) Move all the items that are smaller than the pivot to the beginning of the list, and move all the items that are larger than the pivot to the end of the list. Now, put the pivot between the two groups of items. This puts the pivot in the position that it will occupy in the final, completely sorted array. It will not have to be moved again. We'll refer to this procedure as QuicksortStep.

QuicksortStep is not recursive. It is used as a subroutine by Quicksort. The speed of Quicksort depends on having a fast implementation of QuicksortStep. Since it's not the main point of this discussion, I present one without much comment.

/**
* Apply QuicksortStep to the list of items in locations lo through hi
* in the array A.  The value returned by this routine is the final
* position of the pivot item in the array.
*/
static int quicksortStep(int[] A, int lo, int hi) {

int pivot = A[lo];  // Get the pivot value.

// The numbers hi and lo mark the endpoints of a range
// of numbers that have not yet been tested.  Decrease hi
// and increase lo until they become equal, moving numbers
// bigger than pivot so that they lie above hi and moving
// numbers less than the pivot so that they lie below lo.
// When we begin, A[lo] is an available space, since its
// value has been moved into the local variable, pivot.

while (hi > lo) {

while (hi > lo && A[hi] >= pivot) {
// Move hi down past numbers greater than pivot.
// These numbers do not have to be moved.
hi--;
}

if (hi == lo)
break;

// The number A[hi] is less than pivot.  Move it into
// the available space at A[lo], leaving an available
// space at A[hi].

A[lo] = A[hi];
lo++;

while (hi > lo && A[lo] <= pivot) {
// Move lo up past numbers less than pivot.
// These numbers do not have to be moved.
lo++;
}

if (hi == lo)
break;

// The number A[lo] is greater than pivot.  Move it into
// the available space at A[hi], leaving an available
// space at A[lo].

A[hi] = A[lo];
hi--;

} // end while

// At this point, lo has become equal to hi, and there is
// an available space at that position.  This position lies
// between numbers less than pivot and numbers greater than
// pivot.  Put pivot in this space and return its location.

A[lo] = pivot;
return lo;

}  // end QuicksortStep

With this subroutine in hand, Quicksort is easy. The Quicksort algorithm for sorting a list consists of applying QuicksortStep to the list, then applying Quicksort recursively to the items that lie to the left of the new position of the pivot and to the items that lie to the right of that position. Of course, we need base cases. If the list has only one item, or no items, then the list is already as sorted as it can ever be, so Quicksort doesn't have to do anything in these cases.

/**
* Apply quicksort to put the array elements between
* position lo and position hi into increasing order.
*/
static void quicksort(int[] A, int lo, int hi) {
if (hi <= lo) {
// The list has length one or zero.  Nothing needs
// to be done, so just return from the subroutine.
return;
}
else {
// Apply quicksortStep and get the new pivot position.
// Then apply quicksort to sort the items that
// precede the pivot and the items that follow it.
int pivotPosition = quicksortStep(A, lo, hi);
quicksort(A, lo, pivotPosition - 1);
quicksort(A, pivotPosition + 1, hi);
}
}

As usual, we had to generalize the problem. The original problem was to sort an array, but the recursive algorithm is set up to sort a specified part of an array. To sort an entire array, A, using the quickSort() subroutine, you would call quicksort(A, 0, A.length - 1).

Quicksort is an interesting example from the point of view of the analysis of algorithms (Section 8.5), because its average case run time differs greatly from its worst case run time. Here is a very informal analysis, starting with the average case: Note that an application of quicksortStep divides a problem into two sub-problems. On the average, the subproblems will be of approximately the same size. A problem of size n is divided into two problems that are roughly of size n/2; these are then divided into four problems that are roughly of size n/4; and so on. Since the problem size is divided by 2 on each level, there will be approximately log(n) levels of subdivision. The amount of processing on each level is proportional to n. (On the top level, each element in the array is looked at and possibly moved. On the second level, where there are two subproblems, every element but one in the array is part of one of those two subproblems and must be looked at and possibly moved, so there is a total of about n steps in both subproblems combined. Similarly, on the third level, there are four subproblems and a total of about n steps in the four subproblems on that level....) With a total of n steps on each level and approximately log(n) levels in the average case, the average case run time for Quicksort is Θ(n*log(n)). This analysis assumes that quicksortStep divides a problem into two approximately equal parts. However, in the worst case, each application of quicksortStep divides a problem of size n into a problem of size 0 and a problem of size n-1. This happens when the pivot element ends up at the beginning or end of the array. In this worst case, there are n levels of subproblems, and the worst-case run time is Θ(n^2). The worst case is very rare -- it depends on the items in the array being arranged in a very special way, so the average performance of Quicksort can be very good even though it is not so good in certain rare cases. There are sorting algorithms that have both an average case and a worst case run time of Θ(n*log(n)). One example is MergeSort, which you can look up if you are interested.

9.1.4  Blob Counting

Next, we will look at counting the number of squares in a group of connected squares. I call a group of squares a "blob," and the sample program that we will consider is Blobs.java. The program displays a grid of small white and gray squares. Here is a screenshot from the program, showing the grid of squares along with some controls:

The gray squares are considered to be "filled" and the white squares are "empty." For the purposes of this example, we define a "blob" to consist of a filled square and all the filled squares that can be reached from it by moving up, down, left, and right through other filled squares. If the user clicks on any filled square in the program, the computer will count the squares in the blob that contains the clicked square, and it will change the color of those squares to red. The program has several controls. There is a "New Blobs" button; clicking this button will create a new random pattern in the grid. A pop-up menu specifies the approximate percentage of squares that will be filled in the new pattern. The more filled squares, the larger the blobs. And a button labeled "Count the Blobs" will tell you how many different blobs there are in the pattern.

Recursion is used in this program to count the number of squares in a blob. Without recursion, this would be a very difficult thing to implement. Recursion makes it relatively easy, but it still requires a new technique, which is also useful in a number of other applications.

The data for the grid of squares is stored in a two dimensional array of boolean values,

boolean[][]  filled;

The value of filled[r][c] is true if the square in row r and in column c of the grid is filled. The number of rows in the grid is stored in an instance variable named rows, and the number of columns is stored in columns. The program uses a recursive instance method named getBlobSize() to count the number of squares in the blob that contains the square in a given row r and column c. If there is no filled square at position (r,c), then the answer is zero. Otherwise, getBlobSize() has to count all the filled squares that can be reached from the square at position (r,c). The idea is to use getBlobSize() recursively to get the number of filled squares that can be reached from each of the neighboring positions: (r+1,c), (r-1,c), (r,c+1), and (r,c-1). Add up these numbers, and add one to count the square at (r,c) itself, and you get the total number of filled squares that can be reached from (r,c). Here is an implementation of this algorithm, as stated. Unfortunately, it has a serious flaw: It leads to an infinite recursion!

int getBlobSize(int r, int c) {  // BUGGY, INCORRECT VERSION!!
// This INCORRECT method tries to count all the filled
// squares that can be reached from position (r,c) in the grid.
if (r < 0 || r >= rows || c < 0 || c >= columns) {
// This position is not in the grid, so there is
// no blob at this position.  Return a blob size of zero.
return 0;
}
if (filled[r][c] == false) {
// This square is not part of a blob, so return zero.
return 0;
}
int size = 1;  // Count the square at this position, then count the
//   the blobs that are connected to this square
//   horizontally or vertically.
size += getBlobSize(r-1,c);
size += getBlobSize(r+1,c);
size += getBlobSize(r,c-1);
size += getBlobSize(r,c+1);
return size;
}  // end INCORRECT getBlobSize()

Unfortunately, this routine will count the same square more than once. In fact, it will try to count each square infinitely often! Think of yourself standing at position (r,c) and trying to follow these instructions. The first instruction tells you to move up one row. You do that, and then you apply the same procedure. As one of the steps in that procedure, you have to move down one row and apply the same procedure yet again. But that puts you back at position (r,c)! From there, you move up one row, and from there you move down one row.... Back and forth forever! We have to make sure that a square is only counted and processed once, so we don't end up going around in circles. The solution is to leave a trail of breadcrumbs -- or on the computer a trail of boolean values -- to mark the squares that you've already visited. Once a square is marked as visited, it won't be processed again. The remaining, unvisited squares are reduced in number, so definite progress has been made in reducing the size of the problem. Infinite recursion is avoided!

A second boolean array, visited[r][c], is used to keep track of which squares have already been visited and processed. It is assumed that all the values in this array are set to false before getBlobSize() is called. As getBlobSize() encounters unvisited squares, it marks them as visited by setting the corresponding entry in the visited array to true. When getBlobSize() encounters a square that it has already visited, it doesn't count it or process it further. The technique of "marking" items as they are encountered is one that is used over and over in the programming of recursive algorithms. Here is the corrected version of getBlobSize(), with changes shown in red italic:

/**
* Counts the squares in the blob at position (r,c) in the
* grid.  Squares are only counted if they are filled and
* unvisited.  If this routine is called for a position that
* has been visited, the return value will be zero.
*/
int getBlobSize(int r, int c) {
if (r < 0 || r >= rows || c < 0 || c >= columns) {
// This position is not in the grid, so there is
// no blob at this position.  Return a blob size of zero.
return 0;
}
if (filled[r][c] == false || visited[r][c] == true) {
// This square is not part of a blob, or else it has
// already been counted, so return zero.
return 0;
}
visited[r][c] = true;   // Mark the square as visited so that
//    we won't count it again during the
//    following recursive calls.
int size = 1;  // Count the square at this position, then count the
//   the blobs that are connected to this square
//   horizontally or vertically.
size += getBlobSize(r-1,c);
size += getBlobSize(r+1,c);
size += getBlobSize(r,c-1);
size += getBlobSize(r,c+1);
return size;
}  // end getBlobSize()

In the program, this method is used to determine the size of a blob when the user clicks on a square. After getBlobSize() has performed its task, all the squares in the blob are still marked as visited. The paintComponent() method draws visited squares in red, which makes the blob visible. The getBlobSize() method is also used for counting blobs. This is done by the following method, which includes comments to explain how it works:

/**
* When the user clicks the "Count the Blobs" button, find the
* number of blobs in the grid and report the number in the
* message label.
*/
void countBlobs() {

int count = 0; // Number of blobs.

/* First clear out the visited array. The getBlobSize() method
will mark every filled square that it finds by setting the
corresponding element of the array to true.  Once a square
has been marked as visited, it will stay marked until all the
blobs have been counted.  This will prevent the same blob from
being counted more than once. */

for (int r = 0; r < rows; r++)
for (int c = 0; c < columns; c++)
visited[r][c] = false;

/* For each position in the grid, call getBlobSize() to get the
size of the blob at that position.  If the size is not zero,
count a blob.  Note that if we come to a position that was part
of a previously counted blob, getBlobSize() will return 0 and
the blob will not be counted again. */

for (int r = 0; r < rows; r++)
for (int c = 0; c < columns; c++) {
if (getBlobSize(r,c) > 0)
count++;
}

repaint();  // Note that all the filled squares will be red,
//   since they have all now been visited.

message.setText("The number of blobs is " + count);

} // end countBlobs()

Section 9.2

Linked Data Structures

Every useful object contains instance variables. When the type of an instance variable is given by a class or interface name, the variable can hold a reference to another object. Such a reference is also called a pointer, and we say that the variable points to the object. (Of course, any variable that can contain a reference to an object can also contain the special value null, which points to nowhere.) When one object contains an instance variable that points to another object, we think of the objects as being "linked" by the pointer. Data structures of great complexity can be constructed by linking objects together.

9.2.1  Recursive Linking

Something interesting happens when an object contains an instance variable that can refer to another object of the same type. In that case, the definition of the object's class is recursive. Such recursion arises naturally in many cases. For example, consider a class designed to represent employees at a company. Suppose that every employee except the boss has a supervisor, who is another employee of the company. Then the Employee class would naturally contain an instance variable of type Employee that points to the employee's supervisor:

/**
* An object of type Employee holds data about one employee.
*/
public class Employee {

String name;          // Name of the employee.

Employee supervisor;  // The employee's supervisor.

.
.  // (Other instance variables and methods.)
.

} // end class Employee

If emp is a variable of type Employee, then emp.supervisor is another variable of type Employee. If emp refers to the boss, then the value of emp.supervisor should be null to indicate the fact that the boss has no supervisor. If we wanted to print out the name of the employee's supervisor, for example, we could use the following Java statement:

if ( emp.supervisor == null) {
System.out.println( emp.name + " is the boss and has no supervisor!" );
}
else {
System.out.print( "The supervisor of " + emp.name + " is " );
System.out.println( emp.supervisor.name );
}

Now, suppose that we want to know how many levels of supervisors there are between a given employee and the boss. We just have to follow the chain of command through a series of supervisor links, and count how many steps it takes to get to the boss:

if ( emp.supervisor == null ) {
System.out.println( emp.name + " is the boss!" );
}
else {
Employee runner;  // For "running" up the chain of command.
runner = emp.supervisor;
if ( runner.supervisor == null) {
System.out.println( emp.name  + " reports directly to the boss." );
}
else {
int count = 0;
while ( runner.supervisor != null ) {
count++;  // Count the supervisor on this level.
runner = runner.supervisor; // Move up to the next level.
}
System.out.println( "There are " + count
+ " supervisors between " + emp.name
+ " and the boss." );
}
}

As the while loop is executed, runner points in turn to the original employee (emp), then to emp's supervisor, then to the supervisor of emp's supervisor, and so on. The count variable is incremented each time runner "visits" a new employee. The loop ends when runner.supervisor is null, which indicates that runner has reached the boss. At that point, count has counted the number of steps between emp and the boss.

In this example, the supervisor variable is quite natural and useful. In fact, data structures that are built by linking objects together are so useful that they are a major topic of study in computer science. We'll be looking at a few typical examples. In this section and the next, we'll be looking at linked lists. A linked list consists of a chain of objects of the same type, linked together by pointers from one object to the next. This is much like the chain of supervisors between emp and the boss in the above example. It's also possible to have more complex situations, in which one object can contain links to several other objects. We'll look at an example of this in Section 9.4.

9.2.2  Linked Lists

For most of the examples in the rest of this section, linked lists will be constructed out of objects belonging to the class Node which is defined as follows:

class Node {
String item;
Node next;
}

The term node is often used to refer to one of the objects in a linked data structure. Objects of type Node can be chained together as shown in the top part of the above illustration. Each node holds a String and a pointer to the next node in the list (if any). The last node in such a list can always be identified by the fact that the instance variable next in the last node holds the value null instead of a pointer to another node. The purpose of the chain of nodes is to represent a list of strings. The first string in the list is stored in the first node, the second string is stored in the second node, and so on. The pointers and the node objects are used to build the structure, but the data that we want to represent is the list of strings. Of course, we could just as easily represent a list of integers or a list of JButtons or a list of any other type of data by changing the type of the item that is stored in each node.

Although the Nodes in this example are very simple, we can use them to illustrate the common operations on linked lists. Typical operations include deleting nodes from the list, inserting new nodes into the list, and searching for a specified String among the items in the list. We will look at subroutines to perform all of these operations, among others.

For a linked list to be used in a program, that program needs a variable that refers to the first node in the list. It only needs a pointer to the first node since all the other nodes in the list can be accessed by starting at the first node and following links along the list from one node to the next. In my examples, I will always use a variable named head, of type Node, that points to the first node in the linked list. When the list is empty, the value of head is null.

9.2.3  Basic Linked List Processing

It is very common to want to process all the items in a linked list in some way. The common pattern is to start at the head of the list, then move from each node to the next by following the pointer in the node, stopping when the null that marks the end of the list is reached. If head is a variable of type Node that points to the first node in the list, then the general form of the code for processing all the items in a linked list is:

Node runner;    // A pointer that will be used to traverse the list.
runner = head;  // Start with runner pointing to the head of the list.
while ( runner != null ) {     // Continue until null is encountered.
process( runner.item );     // Do something with the item in the current node.
runner = runner.next;       // Move on to the next node in the list.
}

Our only access to the list is through the variable head, so we start by getting a copy of the value in head with the assignment statement runner = head. We need a copy of head because we are going to change the value of runner. We can't change the value of head, or we would lose our only access to the list! The variable runner will point to each node of the list in turn. When runner points to one of the nodes in the list, runner.next is a pointer to the next node in the list, so the assignment statement runner = runner.next moves the pointer along the list from each node to the next. We know that we've reached the end of the list when runner becomes equal to null. Note that our list-processing code works even for an empty list, since for an empty list the value of head is null and the body of the while loop is not executed at all. As an example, we can print all the strings in a list of Strings by saying:

Node runner = head;
while ( runner != null ) {
System.out.println( runner.item );
runner = runner.next;
}

The while loop can, by the way, be rewritten as a for loop. Remember that even though the loop control variable in a for loop is often numerical, that is not a requirement. Here is a for loop that is equivalent to the above while loop:

for ( Node runner = head; runner != null; runner = runner.next ) {
System.out.println( runner.item );
}

Similarly, we can traverse a list of integers to add up all the numbers in the list. A linked list of integers can be constructed using the class

public class IntNode {
int item;       // One of the integers in the list.
IntNode next;   // Pointer to the next node in the list.
}

If head is a variable of type IntNode that points to a linked list of integers, we can find the sum of the integers in the list using:

int sum = 0;
IntNode runner = head;
while ( runner != null ) {
sum = sum + runner.item;   // Add current item to the sum.
runner = runner.next;
}
System.out.println("The sum of the list of items is " + sum);

It is also possible to use recursion to process a linked list. Recursion is rarely the natural way to process a list, since it's so easy to use a loop to traverse the list. However, understanding how to apply recursion to lists can help with understanding the recursive processing of more complex data structures. A non-empty linked list can be thought of as consisting of two parts: the head of the list, which is just the first node in the list, and the tail of the list, which consists of the remainder of the list after the head. Note that the tail is itself a linked list and that it is shorter than the original list (by one node). This is a natural setup for recursion, where the problem of processing a list can be divided into processing the head and recursively processing the tail. The base case occurs in the case of an empty list (or sometimes in the case of a list of length one). For example, here is a recursive algorithm for adding up the numbers in a linked list of integers:

if the list is empty then
return 0 (since there are no numbers to be added up)
otherwise
let listsum = the number in the head node
let tailsum be the sum of the numbers in the tail list (recursively)
add tailsum to listsum
return listsum

One remaining question is, how do we get the tail of a non-empty linked list? If head is a variable that points to the head node of the list, then head.next is a variable that points to the second node of the list -- and that node is in fact the first node of the tail. So, we can view head.next as a pointer to the tail of the list. One special case is when the original list consists of a single node. In that case, the tail of the list is empty, and head.next is null. Since an empty list is represented by a null pointer, head.next represents the tail of the list even in this special case. This allows us to write a recursive list-summing function in Java as

/**
*  Compute the sum of all the integers in a linked list of integers.
*  @param head a pointer to the first node in the linked list
*/
public static int addItemsInList( IntNode head ) {
if ( head == null ) {
// Base case:  The list is empty,  so the sum is zero.
return 0;
}
else {
// Recursive case:  The list is non-empty.  Find the sum of
// the tail list, and add that to the item in the head node.
// (Note that this case could be written simply as
//     return head.item + addItemsInList( head.next );)
int listsum = head.item;
int tailsum = addItemsInList( head.next );
listsum = listsum + tailsum;
return listsum;
}
}

I will finish by presenting a list-processing problem that is easy to solve with recursion, but quite tricky to solve without it. The problem is to print out all the strings in a linked list of strings in the reverse of the order in which they occur in the list. Note that when we do this, the item in the head of a list is printed out after all the items in the tail of the list. This leads to the following recursive routine. You should convince yourself that it works, and you should think about trying to do the same thing without using recursion:

public static void printReversed( Node head ) {
if ( head == null ) {
// Base case:  The list is empty, and there is nothing to print.
return;
}
else {
// Recursive case:  The list is non-empty.
printReversed( head.next );  // Print strings from tail, in reverse order.
System.out.println( head.item );  // Then print string from head node.
}
}

In the rest of this section, we'll look at a few more advanced operations on a linked list of strings. The subroutines that we consider are instance methods in a class that I wrote named StringList. An object of type StringList represents a linked list of strings. The class has a private instance variable named head of type Node that points to the first node in the list, or is null if the list is empty. Instance methods in class StringList access head as a global variable. The source code for StringList is in the file StringList.java, and it is used in a sample program named ListDemo.java, so you can take a look at the code in context if you want.

One of the methods in the StringList class searches the list, looking for a specified string. If the string that we are looking for is searchItem, then we have to compare searchItem to each item in the list. This is an example of basic list traversal and processing. However, in this case, we can stop processing if we find the item that we are looking for.

/**
* Searches the list for a specified item.
* @param searchItem the item that is to be searched for
* @return true if searchItem is one of the items in the list or false if
*    searchItem does not occur in the list.
*/
public boolean find(String searchItem) {

Node runner;    // A pointer for traversing the list.

runner = head;  // Start by looking at the head of the list.
//   (head is an instance variable! )

while ( runner != null ) {
// Go through the list looking at the string in each
// node.  If the string is the one we are looking for,
// return true, since the string has been found in the list.
if ( runner.item.equals(searchItem) )
return true;
runner = runner.next;  // Move on to the next node.
}

// At this point, we have looked at all the items in the list
// without finding searchItem.  Return false to indicate that
// the item does not exist in the list.

return false;

} // end find()

It is possible that the list is empty, that is, that the value of head is null. We should be careful that this case is handled properly. In the above code, if head is null, then the body of the while loop is never executed at all, so no nodes are processed and the return value is false. This is exactly what we want when the list is empty, since the searchItem can't occur in an empty list.

9.2.4  Inserting into a Linked List

The problem of inserting a new item into a linked list is more difficult, at least in the case where the item is inserted into the middle of the list. (In fact, it's probably the most difficult operation on linked data structures that you'll encounter in this chapter.) In the StringList class, the items in the nodes of the linked list are kept in increasing order. When a new item is inserted into the list, it must be inserted at the correct position according to this ordering. This means that, usually, we will have to insert the new item somewhere in the middle of the list, between two existing nodes. To do this, it's convenient to have two variables of type Node, which refer to the existing nodes that will lie on either side of the new node. In the following illustration, these variables are previous and runner. Another variable, newNode, refers to the new node. In order to do the insertion, the link from previous to runner must be "broken," and new links from previous to newNode and from newNode to runner must be added:

Once we have previous and runner pointing to the right nodes, the command "previous.next = newNode;" can be used to make previous.next point to the new node. And the command "newNode.next = runner" will set newNode.next to point to the correct place. However, before we can use these commands, we need to set up runner and previous as shown in the illustration. The idea is to start at the first node of the list, and then move along the list past all the items that are less than the new item. While doing this, we have to be aware of the danger of "falling off the end of the list." That is, we can't continue if runner reaches the end of the list and becomes null. If insertItem is the item that is to be inserted, and if we assume that it does, in fact, belong somewhere in the middle of the list, then the following code would correctly position previous and runner:

Node runner, previous;
previous = head;     // Start at the beginning of the list.
runner = head.next;
while ( runner != null && runner.item.compareTo(insertItem) < 0 ) {
previous = runner;  // "previous = previous.next" would also work
runner = runner.next;
}

(This uses the compareTo() instance method from the String class to test whether the item in the node is less than the item that is being inserted. See Subsection 2.3.3.)

This is fine, except that the assumption that the new node is inserted into the middle of the list is not always valid. It might be that insertItem is less than the first item of the list. In that case, the new node must be inserted at the head of the list. This can be done with the instructions

newNode.next = head;   // Make newNode.next point to the old head.
head = newNode;        // Make newNode the new head of the list.

It is also possible that the list is empty. In that case, newNode will become the first and only node in the list. This can be accomplished simply by setting head = newNode. The following insert() method from the StringList class covers all of these possibilities:

/**
* Insert a specified item into the list, keeping the list in order.
* @param insertItem the item that is to be inserted.
*/
public void insert(String insertItem) {

Node newNode;          // A Node to contain the new item.
newNode = new Node();
newNode.item = insertItem;  // (N.B.  newNode.next is null.)

if ( head == null ) {
// The new item is the first (and only) one in the list.
// Set head to point to it.
head = newNode;
}
else if ( head.item.compareTo(insertItem) >= 0 ) {
// The new item is less than the first item in the list,
// so it has to be inserted at the head of the list.
newNode.next = head;
head = newNode;
}
else {
// The new item belongs somewhere after the first item
// in the list.  Search for its proper position and insert it.
Node runner;     // A node for traversing the list.
Node previous;   // Always points to the node preceding runner.
runner = head.next;   // Start by looking at the SECOND position.
previous = head;
while ( runner != null && runner.item.compareTo(insertItem) < 0 ) {
// Move previous and runner along the list until runner
// falls off the end or hits a list element that is
// greater than or equal to insertItem.  When this
// loop ends, previous indicates the position where
// insertItem must be inserted.
previous = runner;
runner = runner.next;
}
newNode.next = runner;     // Insert newNode after previous.
previous.next = newNode;
}

}  // end insert()

If you were paying close attention to the above discussion, you might have noticed that there is one special case which is not mentioned. What happens if the new node has to be inserted at the end of the list? This will happen if all the items in the list are less than the new item. In fact, this case is already handled correctly by the subroutine, in the last part of the if statement. If insertItem is greater than all the items in the list, then the while loop will end when runner has traversed the entire list and become null. However, when that happens, previous will be left pointing to the last node in the list. Setting previous.next = newNode adds newNode onto the end of the list. Since runner is null, the command newNode.next = runner sets newNode.next to null, which is exactly what is needed to mark the end of the list.

9.2.5  Deleting from a Linked List

The delete operation is similar to insert, although a little simpler. There are still special cases to consider. When the first node in the list is to be deleted, then the value of head has to be changed to point to what was previously the second node in the list. Since head.next refers to the second node in the list, this can be done by setting head = head.next. (Once again, you should check that this works when head.next is null, that is, when there is no second node in the list. In that case, the list becomes empty.)

If the node that is being deleted is in the middle of the list, then we can set up previous and runner with runner pointing to the node that is to be deleted and with previous pointing to the node that precedes that node in the list. Once that is done, the command "previous.next = runner.next;" will delete the node. The deleted node will be garbage collected. I encourage you to draw a picture for yourself to illustrate this operation. Here is the complete code for the delete() method:

/**
* Delete a specified item from the list, if that item is present.
* If multiple copies of the item are present in the list, only
* the one that comes first in the list is deleted.
* @param deleteItem the item to be deleted
* @return true if the item was found and deleted, or false if the item
*    was not in the list.
*/
public boolean delete(String deleteItem) {

if ( head == null ) {
// The list is empty, so it certainly doesn't contain deleteString.
return false;
}
else if ( head.item.equals(deleteItem) ) {
// The string is the first item of the list.  Remove it.
head = head.next;
return true;
}
else {
// The string, if it occurs at all, is somewhere beyond the
// first element of the list.  Search the list.
Node runner;     // A node for traversing the list.
Node previous;   // Always points to the node preceding runner.
runner = head.next;   // Start by looking at the SECOND list node.
previous = head;
while ( runner != null && runner.item.compareTo(deleteItem) < 0 ) {
// Move previous and runner along the list until runner
// falls off the end or hits a list element that is
// greater than or equal to deleteItem.  When this
// loop ends, runner indicates the position where
// deleteItem must be, if it is in the list.
previous = runner;
runner = runner.next;
}
if ( runner != null && runner.item.equals(deleteItem) ) {
// Runner points to the node that is to be deleted.
// Remove it by changing the pointer in the previous node.
previous.next = runner.next;
return true;
}
else {
// The item does not exist in the list.
return false;
}
}

} // end delete()

Section 9.3

Stacks, Queues, and ADTs

A linked list is a particular type of data structure, made up of objects linked together by pointers. In the previous section, we used a linked list to store an ordered list of Strings, and we implemented insert, delete, and find operations on that list. However, we could easily have stored the list of Strings in an array or ArrayList, instead of in a linked list. We could still have implemented the same operations on the list. The implementations of these operations would have been different, but their interfaces and logical behavior would still be the same.

The term abstract data type, or ADT, refers to a set of possible values and a set of operations on those values, without any specification of how the values are to be represented or how the operations are to be implemented. An "ordered list of strings" can be defined as an abstract data type. Any sequence of Strings that is arranged in increasing order is a possible value of this data type. The operations on the data type include inserting a new string, deleting a string, and finding a string in the list. There are often several different ways to implement the same abstract data type. For example, the "ordered list of strings" ADT can be implemented as a linked list or as an array. A program that only depends on the abstract definition of the ADT can use either implementation, interchangeably. In particular, the implementation of the ADT can be changed without affecting the program as a whole. This can make the program easier to debug and maintain, so ADTs are an important tool in software engineering.

In this section, we'll look at two common abstract data types, stacks and queues. Both stacks and queues are often implemented as linked lists, but that is not the only possible implementation. You should think of the rest of this section partly as a discussion of stacks and queues and partly as a case study in ADTs.

9.3.1  Stacks

A stack consists of a sequence of items, which should be thought of as piled one on top of the other like a physical stack of boxes or cafeteria trays. Only the top item on the stack is accessible at any given time. It can be removed from the stack with an operation called pop. An item lower down on the stack can only be removed after all the items on top of it have been popped off the stack. A new item can be added to the top of the stack with an operation called push. We can make a stack of any type of items. If, for example, the items are values of type int, then the push and pop operations can be implemented as instance methods

- void push (int newItem) -- Add newItem to top of stack.

- int pop() -- Remove the top int from the stack and return it.

It is an error to try to pop an item from an empty stack, so it is important to be able to tell whether a stack is empty. We need another stack operation to do the test, implemented as an instance method

- boolean isEmpty() -- Returns true if the stack is empty.

This defines a "stack of ints" as an abstract data type. This ADT can be implemented in several ways, but however it is implemented, its behavior must correspond to the abstract mental image of a stack.

In the linked list implementation of a stack, the top of the stack is actually the node at the head of the list. It is easy to add and remove nodes at the front of a linked list -- much easier than inserting and deleting nodes in the middle of the list. Here is a class that implements the "stack of ints" ADT using a linked list. (It uses a static nested class to represent the nodes of the linked list, but that is part of the private implementation of the ADT.)

public class StackOfInts {

/**
* An object of type Node holds one of the items in the linked list
* that represents the stack.
*/
private static class Node {
int item;
Node next;
}

private Node top;  // Pointer to the Node that is at the top of
//   of the stack.  If top == null, then the
//   stack is empty.

/**
* Add N to the top of the stack.
*/
public void push( int N ) {
Node newTop;         // A Node to hold the new item.
newTop = new Node();
newTop.item = N;     // Store N in the new Node.
newTop.next = top;   // The new Node points to the old top.
top = newTop;        // The new item is now on top.
}

/**
* Remove the top item from the stack, and return it.
* Throws an IllegalStateException if the stack is empty when
* this method is called.
*/
public int pop() {
if ( top == null )
throw new IllegalStateException("Can't pop from an empty stack.");
int topItem = top.item;  // The item that is being popped.
top = top.next;          // The previous second item is now on top.
return topItem;
}

/**
* Returns true if the stack is empty.  Returns false
* if there are one or more items on the stack.
*/
public boolean isEmpty() {
return (top == null);
}

} // end class StackOfInts

You should make sure that you understand how the push and pop operations operate on the linked list. Drawing some pictures might help. Note that the linked list is part of the private implementation of the StackOfInts class. A program that uses this class doesn't even need to know that a linked list is being used.

Now, it's pretty easy to implement a stack as an array instead of as a linked list. Since the number of items on the stack varies with time, a counter is needed to keep track of how many spaces in the array are actually in use. If this counter is called top, then the items on the stack are stored in positions 0, 1, ..., top-1 in the array. The item in position 0 is on the bottom of the stack, and the item in position top-1 is on the top of the stack. Pushing an item onto the stack is easy: Put the item in position top and add 1 to the value of top. If we don't want to put a limit on the number of items that the stack can hold, we can use the dynamic array techniques from Subsection 7.2.4. Note that the typical picture of the array would show the stack "upside down," with the bottom of the stack at the top of the array. This doesn't matter. The array is just an implementation of the abstract idea of a stack, and as long as the stack operations work the way they are supposed to, we are OK. Here is a second implementation of the StackOfInts class, using a dynamic array:

public class StackOfInts {  // (alternate version, using an array)

private int[] items = new int[10];  // Holds the items on the stack.

private int top = 0;  // The number of items currently on the stack.

/**
* Add N to the top of the stack.
*/
public void push( int N ) {
if (top == items.length) {
// The array is full, so make a new, larger array and
// copy the current stack items into it.  (Note that
// java.util.Arrays must be imported.)
items = Arrays.copyOf( items, 2*items.length );
}
items[top] = N;  // Put N in next available spot.
top++;           // Number of items goes up by one.
}

/**
* Remove the top item from the stack, and return it.
* Throws an IllegalStateException if the stack is empty when
* this method is called.
*/
public int pop() {
if ( top == 0 )
throw new IllegalStateException("Can't pop from an empty stack.");
int topItem = items[top - 1];  // Top item in the stack.
top--;    // Number of items on the stack goes down by one.
return topItem;
}

/**
* Returns true if the stack is empty.  Returns false
* if there are one or more items on the stack.
*/
public boolean isEmpty() {
return (top == 0);
}

} // end class StackOfInts

Once again, the implementation of the stack (as an array) is private to the class. The two versions of the StackOfInts class can be used interchangeably, since their public interfaces are identical.

It's interesting to look at the run time analysis of stack operations. (See Section 8.5). We can measure the size of the problem by the number of items that are on the stack. For the linked list implementation of a stack, the worst case run time both for the push and for the pop operation is Θ(1). This just means that the run time is less than some constant, independent of the number of items on the stack. This is easy to see if you look at the code. The operations are implemented with a few simple assignment statements, and the number of items on the stack has no effect.

For the array implementation, on the other hand, a special case occurs in the push operation when the array is full. In that case, a new array is created and all the stack items are copied into the new array. This takes an amount of time that is proportional to the number of items on the stack. So, although the run time for push is usually Θ(1), the worst case run time is Θ(n), where n is the number of items on the stack. (However, the worst case occurs only rarely, and there is a natural sense in which the average case run time for the array implementation is still Θ(1).)

9.3.2  Queues

Queues are similar to stacks in that a queue consists of a sequence of items, and there are restrictions about how items can be added to and removed from the list. However, a queue has two ends, called the front and the back of the queue. Items are always added to the queue at the back and removed from the queue at the front. The operations of adding and removing items are called enqueue and dequeue. An item that is added to the back of the queue will remain on the queue until all the items in front of it have been removed. This should sound familiar. A queue is like a "line" or "queue" of customers waiting for service. Customers are serviced in the order in which they arrive on the queue.

A queue can hold items of any type. For a queue of ints, the enqueue and dequeue operations can be implemented as instance methods in a "QueueOfInts" class. We also need an instance method for checking whether the queue is empty:

- void enqueue(int N) -- Add N to the back of the queue.

- int dequeue() -- Remove the item at the front and return it.

- boolean isEmpty() -- Return true if the queue is empty.

A queue can be implemented as a linked list or as an array. An efficient array implementation is trickier than the array implementation of a stack, so I won't give it here. In the linked list implementation, the first item of the list is at the front of the queue. Dequeueing an item from the front of the queue is just like popping an item off a stack. The back of the queue is at the end of the list. Enqueueing an item involves setting a pointer in the last node of the current list to point to a new node that contains the item. To do this, we'll need a command like "tail.next = newNode;", where tail is a pointer to the last node in the list. If head is a pointer to the first node of the list, it would always be possible to get a pointer to the last node of the list by saying:

Node tail;    // This will point to the last node in the list.
tail = head;  // Start at the first node.
while (tail.next != null) {
tail = tail.next;  // Move to next node.
}
// At this point, tail.next is null, so tail points to
// the last node in the list.

However, it would be very inefficient to do this over and over every time an item is enqueued. For the sake of efficiency, we'll keep a pointer to the last node in an instance variable. This complicates the class somewhat; we have to be careful to update the value of this variable whenever a new node is added to the end of the list. Given all this, writing the QueueOfInts class is not all that difficult:

public class QueueOfInts {

/**
* An object of type Node holds one of the items
* in the linked list that represents the queue.
*/
private static class Node {
int item;
Node next;
}

private Node head = null;  // Points to first Node in the queue.
// The queue is empty when head is null.

private Node tail = null;  // Points to last Node in the queue.

/**
* Add N to the back of the queue.
*/
public void enqueue( int N ) {
Node newTail = new Node();  // A Node to hold the new item.
newTail.item = N;
if (head == null) {
// The queue was empty.  The new Node becomes
// the only node in the list.  Since it is both
// the first and last node, both head and tail
// point to it.
head = newTail;
tail = newTail;
}
else {
// The new node becomes the new tail of the list.
// (The head of the list is unaffected.)
tail.next = newTail;
tail = newTail;
}
}

/**
* Remove and return the front item in the queue.
* Throws an IllegalStateException if the queue is empty.
*/
public int dequeue() {
if ( head == null)
throw new IllegalStateException("Can't dequeue from an empty queue.");
int firstItem = head.item;
head = head.next;  // The previous second item is now first.
// If we have just removed the last item,
// then head is null.
if (head == null) {
// The queue has become empty.  The Node that was
// deleted was the tail as well as the head of the
// list, so now there is no tail.  (Actually, the
// class would work fine without this step.)
tail = null;
}
return firstItem;
}

/**
* Return true if the queue is empty.
*/
boolean isEmpty() {
return (head == null);
}

} // end class QueueOfInts

Queues are typically used in a computer (as in real life) when only one item can be processed at a time, but several items can be waiting for processing. For example:

- In a Java program that has multiple threads, the threads that want processing time on the CPU are kept in a queue. When a new thread is started, it is added to the back of the queue. A thread is removed from the front of the queue, given some processing time, and then -- if it has not terminated -- is sent to the back of the queue to wait for another turn.

- Events such as keystrokes and mouse clicks are stored in a queue called the "event queue". A program removes events from the event queue and processes them. It's possible for several more events to occur while one event is being processed, but since the events are stored in a queue, they will always be processed in the order in which they occurred.

- A web server is a program that receives requests from web browsers for "pages." It is easy for new requests to arrive while the web server is still fulfilling a previous request. Requests that arrive while the web server is busy are placed into a queue to await processing. Using a queue ensures that requests will be processed in the order in which they were received.

Queues are said to implement a FIFO policy: First In, First Out. Or, as it is more commonly expressed, first come, first served. Stacks, on the other hand implement a LIFO policy: Last In, First Out. The item that comes out of the stack is the last one that was put in. Just like queues, stacks can be used to hold items that are waiting for processing (although in applications where queues are typically used, a stack would be considered "unfair").

To get a better handle on the difference between stacks and queues, consider the sample program DepthBreadth.java. I suggest that you try out the program. The program shows a grid of squares. Initially, all the squares are white. When you click on a white square, the program will gradually mark all the squares in the grid, starting from the one where you click. To understand how the program does this, think of yourself in the place of the program. When the user clicks a square, you are handed an index card. The location of the square -- its row and column -- is written on the card. You put the card in a pile, which then contains just that one card. Then, you repeat the following: If the pile is empty, you are done. Otherwise, remove an index card from the pile. The index card specifies a square. Look at each horizontal and vertical neighbor of that square. If the neighbor has not already been encountered, write its location on a new index card and put the card in the pile. You are done when there are no more index cards waiting in the pile to be processed.

In the program, while a square is in the pile, waiting to be processed, it is colored red; that is, red squares have been encountered but not yet processed. When a square is taken from the pile and processed, its color changes to gray. Once a square has been colored gray, its color won't change again. Eventually, all the squares have been processed, and the procedure ends. At that time, every square is gray. In the index card analogy, the pile of cards has been emptied.

The program can use your choice of three methods: Stack, Queue, and Random. In each case, the same general procedure is used. The only difference is how the "pile of index cards" is managed. For a stack, cards are added and removed at the top of the pile. For a queue, cards are added to the bottom of the pile and removed from the top. In the random case, the card to be processed is picked at random from among all the cards in the pile. The order of processing is very different in these three cases. Here are three pictures from the program, using the three different processing methods. In each case, the process was started by selecting a square near the middle of the grid. A stack is used for the picture on the left, a queue for the picture in the middle, and random selection for the picture on the right:

The patterns that are produced are very different. When using a stack, the program explores out as far as possible before it starts backtracking to look at previously encountered squares. With a queue, squares are processed roughly in the order of their distance from the starting point. When random selection is used, the result is an irregular blob, but it is a connected blob since a square can only be encountered if it is next to a previously encountered square.

You should experiment with the program to see how it all works. Try to understand how stacks and queues are being used. Try starting from one of the corner squares. While the process is going on, you can click on other white squares, and they will be added to the list of encountered squares. When you do this with a stack, you should notice that the square you click is processed immediately, and all the red squares that were already waiting for processing have to wait. On the other hand, if you do this with a queue, the square that you click will wait its turn until all the squares that were already in the pile have been processed. Again, the source code for the program is DepthBreadth.java.

Queues seem very natural because they occur so often in real life, but there are times when stacks are appropriate and even essential. For example, consider what happens when a routine calls a subroutine. The first routine is suspended while the subroutine is executed, and it will continue only when the subroutine returns. Now, suppose that the subroutine calls a second subroutine, and the second subroutine calls a third, and so on. Each subroutine is suspended while the subsequent subroutines are executed. The computer has to keep track of all the subroutines that are suspended. It does this with a stack.

When a subroutine is called, an activation record is created for that subroutine. The activation record contains information relevant to the execution of the subroutine, such as its local variables and parameters. The activation record for the subroutine is placed on a stack. It will be removed from the stack and destroyed when the subroutine returns. If the subroutine calls another subroutine, the activation record of the second subroutine is pushed onto the stack, on top of the activation record of the first subroutine. The stack can continue to grow as more subroutines are called, and it shrinks as those subroutines return.

9.3.3  Postfix Expressions

As another example, stacks can be used to evaluate postfix expressions. An ordinary mathematical expression such as 2+(15-12)*17 is called an infix expression. In an infix expression, an operator comes in between its two operands, as in "2 + 2". In a postfix expression, an operator comes after its two operands, as in "2 2 +". The infix expression "2+(15-12)*17" would be written in postfix form as "2 15 12 - 17 * +". The "-" operator in this expression applies to the two operands that precede it, namely "15" and "12". The "*" operator applies to the two operands that precede it, namely "15 12 -" and "17". And the "+" operator applies to "2" and "15 12 - 17 *". These are the same computations that are done in the original infix expression.

Now, suppose that we want to process the expression "2 15 12 - 17 * +", from left to right and find its value. The first item we encounter is the 2, but what can we do with it? At this point, we don't know what operator, if any, will be applied to the 2 or what the other operand might be. We have to remember the 2 for later processing. We do this by pushing it onto a stack. Moving on to the next item, we see a 15, which is pushed onto the stack on top of the 2. Then the 12 is added to the stack. Now, we come to the operator, "-". This operation applies to the two operands that preceded it in the expression. We have saved those two operands on the stack. So, to process the "-" operator, we pop two numbers from the stack, 12 and 15, and compute 15 - 12 to get the answer 3. This 3 must be remembered to be used in later processing, so we push it onto the stack, on top of the 2 that is still waiting there. The next item in the expression is a 17, which is processed by pushing it onto the stack, on top of the 3. To process the next item, "*", we pop two numbers from the stack. The numbers are 17 and the 3 that represents the value of "15 12 -". These numbers are multiplied, and the result, 51 is pushed onto the stack. The next item in the expression is a "+" operator, which is processed by popping 51 and 2 from the stack, adding them, and pushing the result, 53, onto the stack. Finally, we've come to the end of the expression. The number on the stack is the value of the entire expression, so all we have to do is pop the answer from the stack, and we are done! The value of the expression is 53.

Although it's easier for people to work with infix expressions, postfix expressions have some advantages. For one thing, postfix expressions don't require parentheses or precedence rules. The order in which operators are applied is determined entirely by the order in which they occur in the expression. This allows the algorithm for evaluating postfix expressions to be fairly straightforward:

Start with an empty stack
for each item in the expression:
if the item is a number:
Push the number onto the stack
else if the item is an operator:
Pop the operands from the stack  // Can generate an error
Apply the operator to the operands
Push the result onto the stack
else
There is an error in the expression
Pop a number from the stack  // Can generate an error
if the stack is not empty:
There is an error in the expression
else:
The last number that was popped is the value of the expression

Errors in an expression can be detected easily. For example, in the expression "2 3 + *", there are not enough operands for the "*" operation. This will be detected in the algorithm when an attempt is made to pop the second operand for "*" from the stack, since the stack will be empty. The opposite problem occurs in "2 3 4 +". There are not enough operators for all the numbers. This will be detected when the 2 is left still sitting in the stack at the end of the algorithm.

This algorithm is demonstrated in the sample program PostfixEval.java. This program lets you type in postfix expressions made up of non-negative real numbers and the operators "+", "-", "*", "/", and "^". The "^" represents exponentiation. That is, "2 3 ^" is evaluated as 2 3. The program prints out a message as it processes each item in the expression. The stack class that is used in the program is defined in the file StackOfDouble.java. The StackOfDouble class is identical to the first StackOfInts class, given above, except that it has been modified to store values of type double instead of values of type int.

The only interesting aspect of this program is the method that implements the postfix evaluation algorithm. It is a direct implementation of the pseudocode algorithm given above:

/**
*  Read one line of input and process it as a postfix expression.
*  If the input is not a legal postfix expression, then an error
*  message is displayed.  Otherwise, the value of the expression
*  is displayed.  It is assumed that the first character on
*  the input line is a non-blank.
*/
private static void readAndEvaluate() {

StackOfDouble stack;  // For evaluating the expression.

stack = new StackOfDouble();  // Make a new, empty stack.

System.out.println();

while (TextIO.peek() != '\n') {

if ( Character.isDigit(TextIO.peek()) ) {
// The next item in input is a number.  Read it and
// save it on the stack.
double num = TextIO.getDouble();
stack.push(num);
System.out.println("   Pushed constant " + num);
}
else {
// Since the next item is not a number, the only thing
// it can legally be is an operator.  Get the operator
// and perform the operation.
char op;  // The operator, which must be +, -, *, /, or ^.
double x,y;     // The operands, from the stack, for the operation.
double answer;  // The result, to be pushed onto the stack.
op = TextIO.getChar();
if (op != '+' && op != '-' && op != '*' && op != '/' && op != '^') {
// The character is not one of the acceptable operations.
System.out.println("\nIllegal operator found in input: " + op);
return;
}
if (stack.isEmpty()) {
System.out.println("   Stack is empty while trying to evaluate " + op);
System.out.println("\nNot enough numbers in expression!");
return;
}
y = stack.pop();
if (stack.isEmpty()) {
System.out.println("   Stack is empty while trying to evaluate " + op);
System.out.println("\nNot enough numbers in expression!");
return;
}
x = stack.pop();
switch (op) {
case '+':
answer = x + y;
break;
case '-':
answer = x - y;
break;
case '*':
answer = x * y;
break;
case '/':
answer = x / y;
break;
default:
answer = Math.pow(x,y);  // (op must be '^'.)
}
stack.push(answer);
System.out.println("   Evaluated " + op + " and pushed " + answer);
}

TextIO.skipBlanks();

}  // end while

// If we get to this point, the input has been read successfully.
// If the expression was legal, then the value of the expression is
// on the stack, and it is the only thing on the stack.

if (stack.isEmpty()) {  // Impossible if the input is really non-empty.
System.out.println("No expression provided.");
return;
}

double value = stack.pop();  // Value of the expression.
System.out.println("   Popped " + value + " at end of expression.");

if (stack.isEmpty() == false) {
System.out.println("   Stack is not empty.");
System.out.println("\nNot enough operators for all the numbers!");
return;
}

System.out.println("\nValue = " + value);

} // end readAndEvaluate()

Postfix expressions are often used internally by computers. In fact, the Java virtual machine is a "stack machine" which uses the stack-based approach to expression evaluation that we have been discussing. The algorithm can easily be extended to handle variables, as well as constants. When a variable is encountered in the expression, the value of the variable is pushed onto the stack. It also works for operators with more or fewer than two operands. As many operands as are needed are popped from the stack and the result is pushed back onto the stack. For example, the unary minus operator, which is used in the expression "-x", has a single operand. We will continue to look at expressions and expression evaluation in the next two sections.

Section 9.4

Binary Trees

We have seen in the two previous sections how objects can be linked into lists. When an object contains two pointers to objects of the same type, structures can be created that are much more complicated than linked lists. In this section, we'll look at one of the most basic and useful structures of this type: binary trees. Each of the objects in a binary tree contains two pointers, typically called left and right. In addition to these pointers, of course, the nodes can contain other types of data. For example, a binary tree of integers would be made up of objects of the following type:

class TreeNode {
int item;        // The data in this node.
TreeNode left;   // Pointer to the left subtree.
TreeNode right;  // Pointer to the right subtree.
}

The left and right pointers in a TreeNode can be null or can point to other objects of type TreeNode. A node that points to another node is said to be the parent of that node, and the node it points to is called a child. In the picture below, for example, node 3 is the parent of node 6, and nodes 4 and 5 are children of node 2. Not every linked structure made up of tree nodes is a binary tree. A binary tree must have the following properties: There is exactly one node in the tree which has no parent. This node is called the root of the tree. Every other node in the tree has exactly one parent. Finally, there can be no loops in a binary tree. That is, it is not possible to follow a chain of pointers starting at some node and arriving back at the same node.

A node that has no children is called a leaf. A leaf node can be recognized by the fact that both the left and right pointers in the node are null. In the standard picture of a binary tree, the root node is shown at the top and the leaf nodes at the bottom -- which doesn't show much respect for the analogy to real trees. But at least you can see the branching, tree-like structure that gives a binary tree its name.

9.4.1  Tree Traversal

Consider any node in a binary tree. Look at that node together with all its descendants (that is, its children, the children of its children, and so on). This set of nodes forms a binary tree, which is called a subtree of the original tree. For example, in the picture, nodes 2, 4, and 5 form a subtree. This subtree is called the left subtree of the root. Similarly, nodes 3 and 6 make up the right subtree of the root. We can consider any non-empty binary tree to be made up of a root node, a left subtree, and a right subtree. Either or both of the subtrees can be empty. This is a recursive definition, matching the recursive definition of the TreeNode class. So it should not be a surprise that recursive subroutines are often used to process trees.

Consider the problem of counting the nodes in a binary tree. As an exercise, you might try to come up with a non-recursive algorithm to do the counting, but you shouldn't expect to find one easily. The heart of the problem is keeping track of which nodes remain to be counted. It's not so easy to do this, and in fact it's not even possible without an auxiliary data structure such as a stack or queue. With recursion, however, the algorithm is almost trivial. Either the tree is empty or it consists of a root and two subtrees. If the tree is empty, the number of nodes is zero. (This is the base case of the recursion.) Otherwise, use recursion to count the nodes in each subtree. Add the results from the subtrees together, and add one to count the root. This gives the total number of nodes in the tree. Written out in Java:

/**
* Count the nodes in the binary tree to which root points, and
* return the answer.  If root is null, the answer is zero.
*/
static int countNodes( TreeNode root ) {
if ( root == null )
return 0;  // The tree is empty.  It contains no nodes.
else {
int count = 1;   // Start by counting the root.
count += countNodes(root.left);  // Add the number of nodes
//     in the left subtree.
count += countNodes(root.right); // Add the number of nodes
//    in the right subtree.
return count;  // Return the total.
}
} // end countNodes()

Or, consider the problem of printing the items in a binary tree. If the tree is empty, there is nothing to do. If the tree is non-empty, then it consists of a root and two subtrees. Print the item in the root and use recursion to print the items in the subtrees. Here is a subroutine that prints all the items on one line of output:

/**
* Print all the items in the tree to which root points.
* The item in the root is printed first, followed by the
* items in the left subtree and then the items in the
* right subtree.
*/
static void preorderPrint( TreeNode root ) {
if ( root != null ) {  // (Otherwise, there's nothing to print.)
System.out.print( root.item + " " );  // Print the root item.
preorderPrint( root.left );   // Print items in left subtree.
preorderPrint( root.right );  // Print items in right subtree.
}
} // end preorderPrint()

This routine is called "preorderPrint" because it uses a preorder traversal of the tree. In a preorder traversal, the root node of the tree is processed first, then the left subtree is traversed, then the right subtree. In a postorder traversal, the left subtree is traversed, then the right subtree, and then the root node is processed. And in an inorder traversal, the left subtree is traversed first, then the root node is processed, then the right subtree is traversed. Printing subroutines that use postorder and inorder traversal differ from preorderPrint only in the placement of the statement that outputs the root item:

/**
* Print all the items in the tree to which root points.
* The item in the left subtree is printed first, followed
* by the items in the right subtree and then the item
* in the root node.
*/
static void postorderPrint( TreeNode root ) {
if ( root != null ) {  // (Otherwise, there's nothing to print.)
postorderPrint( root.left );   // Print items in left subtree.
postorderPrint( root.right );  // Print items in right subtree.
System.out.print( root.item + " " );  // Print the root item.
}
} // end postorderPrint()

/**
* Print all the items in the tree to which root points.
* The item in the left subtree is printed first, followed
* by the item in the root node and then the items
* in the right subtree.
*/
static void inorderPrint( TreeNode root ) {
if ( root != null ) {  // (Otherwise, there's nothing to print.)
inorderPrint( root.left );   // Print items in left subtree.
System.out.print( root.item + " " );  // Print the root item.
inorderPrint( root.right );  // Print items in right subtree.
}
} // end inorderPrint()

Each of these subroutines can be applied to the binary tree shown in the illustration at the beginning of this section. The order in which the items are printed differs in each case:

preorderPrint outputs:   1  2  4  5  3  6

postorderPrint outputs:  4  5  2  6  3  1

inorderPrint outputs:    4  2  5  1  3  6

In preorderPrint, for example, the item at the root of the tree, 1, is output before anything else. But the preorder printing also applies to each of the subtrees of the root. The root item of the left subtree, 2, is printed before the other items in that subtree, 4 and 5. As for the right subtree of the root, 3 is output before 6. A preorder traversal applies at all levels in the tree. The other two traversal orders can be analyzed similarly.

9.4.2  Binary Sort Trees

One of the examples in Section 9.2 was a linked list of strings, in which the strings were kept in increasing order. While a linked list works well for a small number of strings, it becomes inefficient for a large number of items. When inserting an item into the list, searching for that item's position requires looking at, on average, half the items in the list. Finding an item in the list requires a similar amount of time. If the strings are stored in a sorted array instead of in a linked list, then searching becomes more efficient because binary search can be used. However, inserting a new item into the array is still inefficient since it means moving, on average, half of the items in the array to make a space for the new item. A binary tree can be used to store an ordered list of strings, or other items, in a way that makes both searching and insertion efficient. A binary tree used in this way is called a binary sort tree.

A binary sort tree is a binary tree with the following property: For every node in the tree, the item in that node is greater than or equal to every item in the left subtree of that node, and it is less than or equal to all the items in the right subtree of that node. Here for example is a binary sort tree containing items of type String. (In this picture, I haven't bothered to draw all the pointer variables. Non-null pointers are shown as arrows.)

Binary sort trees have this useful property: An inorder traversal of the tree will process the items in increasing order. In fact, this is really just another way of expressing the definition. For example, if an inorder traversal is used to print the items in the tree shown above, then the items will be in alphabetical order. The definition of an inorder traversal guarantees that all the items in the left subtree of "judy" are printed before "judy", and all the items in the right subtree of "judy" are printed after "judy". But the binary sort tree property guarantees that the items in the left subtree of "judy" are precisely those that precede "judy" in alphabetical order, and all the items in the right subtree follow "judy" in alphabetical order. So, we know that "judy" is output in its proper alphabetical position. But the same argument applies to the subtrees. "Bill" will be output after "alice" and before "fred" and its descendents. "Fred" will be output after "dave" and before "jane" and "joe". And so on.

Suppose that we want to search for a given item in a binary search tree. Compare that item to the root item of the tree. If they are equal, we're done. If the item we are looking for is less than the root item, then we need to search the left subtree of the root -- the right subtree can be eliminated because it only contains items that are greater than or equal to the root. Similarly, if the item we are looking for is greater than the item in the root, then we only need to look in the right subtree. In either case, the same procedure can then be applied to search the subtree. Inserting a new item is similar: Start by searching the tree for the position where the new item belongs. When that position is found, create a new node and attach it to the tree at that position.

Searching and inserting are efficient operations on a binary search tree, provided that the tree is close to being balanced. A binary tree is balanced if for each node, the left subtree of that node contains approximately the same number of nodes as the right subtree. In a perfectly balanced tree, the two numbers differ by at most one. Not all binary trees are balanced, but if the tree is created by inserting items in a random order, there is a high probability that the tree is approximately balanced. (If the order of insertion is not random, however, it's quite possible for the tree to be very unbalanced.) During a search of any binary sort tree, every comparison eliminates one of two subtrees from further consideration. If the tree is balanced, that means cutting the number of items still under consideration in half. This is exactly the same as the binary search algorithm, and the result is a similarly efficient algorithm.

In terms of asymptotic analysis (Section 8.5), searching, inserting, and deleting in a binary search tree have average case run time Θ(log(n)). The problem size, n, is the number of items in the tree, and the average is taken over all the different orders in which the items could have been inserted into the tree. As long as the actual insertion order is random, the actual run time can be expected to be close to the average. However, the worst case run time for binary search tree operations is Θ(n), which is much worse than Θ(log(n)). The worst case occurs for particular insertion orders. For example, if the items are inserted into the tree in order of increasing size, then every item that is inserted moves always to the right as it moves down the tree. The result is a "tree" that looks more like a linked list, since it consists of a linear string of nodes strung together by their right child pointers. Operations on such a tree have the same performance as operations on a linked list. Now, there are data structures that are similar to simple binary sort trees, except that insertion and deletion of nodes are implemented in a way that will always keep the tree balanced, or almost balanced. For these data structures, searching, inserting, and deleting have both average case and worst case run times that are Θ(log(n)). Here, however, we will look at only the simple versions of inserting and searching.

The sample program SortTreeDemo.java is a demonstration of binary sort trees. The program includes subroutines that implement inorder traversal, searching, and insertion. We'll look at the latter two subroutines below. The main() routine tests the subroutines by letting you type in strings to be inserted into the tree.

In SortTreeDemo, nodes in the binary tree are represented using the following static nested class, including a simple constructor that makes creating nodes easier:

/**
* An object of type TreeNode represents one node in a binary tree of strings.
*/
private static class TreeNode {
String item;      // The data in this node.
TreeNode left;    // Pointer to left subtree.
TreeNode right;   // Pointer to right subtree.
TreeNode(String str) {
// Constructor.  Make a node containing str.
// Note that left and right pointers are null.
item = str;
}
}  // end class TreeNode

A static member variable of type TreeNode points to the binary sort tree that is used by the program:

private static TreeNode root;  // Pointer to the root node in the tree.
// When the tree is empty, root is null.

A recursive subroutine named treeContains is used to search for a given item in the tree. This routine implements the search algorithm for binary trees that was outlined above:

/**
* Return true if item is one of the items in the binary
* sort tree to which root points.   Return false if not.
*/
static boolean treeContains( TreeNode root, String item ) {
if ( root == null ) {
// Tree is empty, so it certainly doesn't contain item.
return false;
}
else if ( item.equals(root.item) ) {
// Yes, the item has been found in the root node.
return true;
}
else if ( item.compareTo(root.item) < 0 ) {
// If the item occurs, it must be in the left subtree.
return treeContains( root.left, item );
}
else {
// If the item occurs, it must be in the right subtree.
return treeContains( root.right, item );
}
}  // end treeContains()

When this routine is called in the main() routine, the first parameter is the static member variable root, which points to the root of the entire binary sort tree.

It's worth noting that recursion is not really essential in this case. A simple, non-recursive algorithm for searching a binary sort tree follows the rule: Start at the root and move down the tree until you find the item or reach a null pointer. Since the search follows a single path down the tree, it can be implemented as a while loop. Here is a non-recursive version of the search routine:

private static boolean treeContainsNR( TreeNode root, String item ) {
TreeNode runner;  // For "running" down the tree.
runner = root;    // Start at the root node.
while (true) {
if (runner == null) {
// We've fallen off the tree without finding item.
return false;
}
else if ( item.equals(runner.item) ) {
// We've found the item.
return true;
}
else if ( item.compareTo(runner.item) < 0 ) {
// If the item occurs, it must be in the left subtree,
// So, advance the runner down one level to the left.
runner = runner.left;
}
else {
// If the item occurs, it must be in the right subtree.
// So, advance the runner down one level to the right.
runner = runner.right;
}
}  // end while
} // end treeContainsNR();

The subroutine for inserting a new item into the tree turns out to be more similar to the non-recursive search routine than to the recursive. The insertion routine has to handle the case where the tree is empty. In that case, the value of root must be changed to point to a node that contains the new item:

root = new TreeNode( newItem );

But this means, effectively, that the root can't be passed as a parameter to the subroutine, because it is impossible for a subroutine to change the value stored in an actual parameter. (I should note that this is something that is possible in other languages.) Recursion uses parameters in an essential way. There are ways to work around the problem, but the easiest thing is just to use a non-recursive insertion routine that accesses the static member variable root directly. One difference between inserting an item and searching for an item is that we have to be careful not to fall off the tree. That is, we have to stop searching just before runner becomes null. When we get to an empty spot in the tree, that's where we have to insert the new node:

/**
* Add the item to the binary sort tree to which the global variable
* "root" refers.  (Note that root can't be passed as  a parameter to
* this routine because the value of root might change, and a change
* in the value of a formal parameter does not change the actual parameter.)
*/
private static void treeInsert(String newItem) {
if ( root == null ) {
// The tree is empty.  Set root to point to a new node containing
// the new item.  This becomes the only node in the tree.
root = new TreeNode( newItem );
return;
}
TreeNode runner;  // Runs down the tree to find a place for newItem.
runner = root;   // Start at the root.
while (true) {
if ( newItem.compareTo(runner.item) < 0 ) {
// Since the new item is less than the item in runner,
// it belongs in the left subtree of runner.  If there
// is an open space at runner.left, add a new node there.
// Otherwise, advance runner down one level to the left.
if ( runner.left == null ) {
runner.left = new TreeNode( newItem );
return;  // New item has been added to the tree.
}
else
runner = runner.left;
}
else {
// Since the new item is greater than or equal to the item in
// runner, it belongs in the right subtree of runner.  If there
// is an open space at runner.right, add a new node there.
// Otherwise, advance runner down one level to the right.
if ( runner.right == null ) {
runner.right = new TreeNode( newItem );
return;  // New item has been added to the tree.
}
else
runner = runner.right;
}
} // end while
}  // end treeInsert()

9.4.3  Expression Trees

Another application of trees is to store mathematical expressions such as 15*(x+y) or sqrt(42)+7 in a convenient form. Let's stick for the moment to expressions made up of numbers and the operators +, -, *, and /. Consider the expression 3*((7+1)/4)+(17-5). This expression is made up of two subexpressions, 3*((7+1)/4) and (17-5), combined with the operator "+". When the expression is represented as a binary tree, the root node holds the operator +, while the subtrees of the root node represent the subexpressions 3*((7+1)/4) and (17-5). Every node in the tree holds either a number or an operator. A node that holds a number is a leaf node of the tree. A node that holds an operator has two subtrees representing the operands to which the operator applies. The tree is shown in the illustration below. I will refer to a tree of this type as an expression tree.

Given an expression tree, it's easy to find the value of the expression that it represents. Each node in the tree has an associated value. If the node is a leaf node, then its value is simply the number that the node contains. If the node contains an operator, then the associated value is computed by first finding the values of its child nodes and then applying the operator to those values. The process is shown by the upward-directed arrows in the illustration. The value computed for the root node is the value of the expression as a whole. There are other uses for expression trees. For example, a postorder traversal of the tree will output the postfix form of the expression.

An expression tree contains two types of nodes: nodes that contain numbers and nodes that contain operators. Furthermore, we might want to add other types of nodes to make the trees more useful, such as nodes that contain variables. If we want to work with expression trees in Java, how can we deal with this variety of nodes? One way -- which will be frowned upon by object-oriented purists -- is to include an instance variable in each node object to record which type of node it is:

enum NodeType { NUMBER, OPERATOR }   // Possible kinds of node.

class ExpNode {  // A node in an expression tree.

NodeType kind;  // Which type of node is this?
double number;  // The value in a node of type NUMBER.
char op;        // The operator in a node of type OPERATOR.
ExpNode left;   // Pointers to subtrees,
ExpNode right;  //     in a node of type OPERATOR.

ExpNode( double val ) {
// Constructor for making a node of type NUMBER.
kind = NodeType.NUMBER;
number = val;
}

ExpNode( char op, ExpNode left, ExpNode right ) {
// Constructor for making a node of type OPERATOR.
kind = NodeType.OPERATOR;
this.op = op;
this.left = left;
this.right = right;
}

} // end class ExpNode

Given this definition, the following recursive subroutine will find the value of an expression tree:

static double getValue( ExpNode node ) {
// Return the value of the expression represented by
// the tree to which node refers.  Node must be non-null.
if ( node.kind == NodeType.NUMBER ) {
// The value of a NUMBER node is the number it holds.
return node.number;
}
else {  // The kind must be OPERATOR.
// Get the values of the operands and combine them
//    using the operator.
double leftVal = getValue( node.left );
double rightVal = getValue( node.right );
switch ( node.op ) {
case '+':  return leftVal + rightVal;
case '-':  return leftVal - rightVal;
case '*':  return leftVal * rightVal;
case '/':  return leftVal / rightVal;
default:   return Double.NaN;  // Bad operator.
}
}
} // end getValue()

Although this approach works, a more object-oriented approach is to note that since there are two types of nodes, there should be two classes to represent them, ConstNode and BinOpNode. To represent the general idea of a node in an expression tree, we need another class, ExpNode. Both ConstNode and BinOpNode will be subclasses of ExpNode. Since any actual node will be either a ConstNode or a BinOpNode, ExpNode should be an abstract class. (See Subsection 5.5.5.) Since one of the things we want to do with nodes is find their values, each class should have an instance method for finding the value:

abstract class ExpNode {
// Represents a node of any type in an expression tree.

abstract double value();  // Return the value of this node.

} // end class ExpNode

class ConstNode extends ExpNode {
// Represents a node that holds a number.

double number;  // The number in the node.

ConstNode( double val ) {
// Constructor.  Create a node to hold val.
number = val;
}

double value() {
// The value is just the number that the node holds.
return number;
}

} // end class ConstNode

class BinOpNode extends ExpNode {
// Represents a node that holds an operator.

char op;        // The operator.
ExpNode left;   // The left operand.
ExpNode right;  // The right operand.

BinOpNode( char op, ExpNode left, ExpNode right ) {
// Constructor.  Create a node to hold the given data.
this.op = op;
this.left = left;
this.right = right;
}

double value() {
// To get the value, compute the value of the left and
// right operands, and combine them with the operator.
double leftVal = left.value();
double rightVal = right.value();
switch ( op ) {
case '+':  return leftVal + rightVal;
case '-':  return leftVal - rightVal;
case '*':  return leftVal * rightVal;
case '/':  return leftVal / rightVal;
default:   return Double.NaN;  // Bad operator.
}
}

} // end class BinOpNode

Note that the left and right operands of a BinOpNode are of type ExpNode, not BinOpNode. This allows the operand to be either a ConstNode or another BinOpNode -- or any other type of ExpNode that we might eventually create. Since every ExpNode has a value() method, we can call left.value() to compute the value of the left operand. If left is in fact a ConstNode, this will call the value() method in the ConstNode class. If it is in fact a BinOpNode, then left.value() will call the value() method in the BinOpNode class. Each node knows how to compute its own value.

Although it might seem more complicated at first, the object-oriented approach has some real advantages. For one thing, it doesn't waste memory. In the original ExpNode class, only some of the instance variables in each node were actually used, and we needed an extra instance variable to keep track of the type of node. More important, though, is the fact that new types of nodes can be added more cleanly, since it can be done by creating a new subclass of ExpNode rather than by modifying an existing class.

We'll return to the topic of expression trees in the next section, where we'll see how to create an expression tree to represent a given expression.

Section 9.5

A Simple Recursive Descent Parser

I have always been fascinated by language -- both natural languages like English and the artificial languages that are used by computers. There are many difficult questions about how languages can convey information, how they are structured, and how they can be processed. Natural and artificial languages are similar enough that the study of programming languages, which are pretty well understood, can give some insight into the much more complex and difficult natural languages. And programming languages raise more than enough interesting issues to make them worth studying in their own right. How can it be, after all, that computers can be made to "understand" even the relatively simple languages that are used to write programs? Computers can only directly use instructions expressed in very simple machine language. Higher level languages must be translated into machine language. But the translation is done by a compiler, which is just a program. How could such a translation program be written?

9.5.1  Backus-Naur Form

Natural and artificial languages are similar in that they have a structure known as grammar or syntax. Syntax can be expressed by a set of rules that describe what it means to be a legal sentence or program. For programming languages, syntax rules are often expressed in BNF (Backus-Naur Form), a system that was developed by computer scientists John Backus and Peter Naur in the late 1950s. Interestingly, an equivalent system was developed independently at about the same time by linguist Noam Chomsky to describe the grammar of natural language. BNF cannot express all possible syntax rules. For example, it can't express the fact that a variable must be defined before it is used. Furthermore, it says nothing about the meaning or semantics of the language. The problem of specifying the semantics of a language -- even of an artificial programming language -- is one that is still far from being completely solved. However, BNF does express the basic structure of the language, and it plays a central role in the design of compilers.

In English, terms such as "noun", "transitive verb," and "prepositional phrase" are syntactic categories that describe building blocks of sentences. Similarly, "statement", "number," and "while loop" are syntactic categories that describe building blocks of Java programs. In BNF, a syntactic category is written as a word enclosed between "<" and ">". For example: <noun>, <verb-phrase>, or <while-loop>. A rule in BNF specifies the structure of an item in a given syntactic category, in terms of other syntactic categories and/or basic symbols of the language. For example, one BNF rule for the English language might be

<sentence>  ::=  <noun-phrase> <verb-phrase>

The symbol "::=" is read "can be", so this rule says that a <sentence> can be a <noun-phrase> followed by a <verb-phrase>. (The term is "can be" rather than "is" because there might be other rules that specify other possible forms for a sentence.) This rule can be thought of as a recipe for a sentence: If you want to make a sentence, make a noun-phrase and follow it by a verb-phrase. Noun-phrase and verb-phrase must, in turn, be defined by other BNF rules.

In BNF, a choice between alternatives is represented by the symbol "|", which is read "or". For example, the rule

<verb-phrase>  ::=  <intransitive-verb>  |
( <transitive-verb> <noun-phrase> )

says that a <verb-phrase> can be an <intransitive-verb>, or a <transitive-verb> followed by a <noun-phrase>. Note also that parentheses can be used for grouping. To express the fact that an item is optional, it can be enclosed between "[" and "]". An optional item that can be repeated any number of times is enclosed between "[" and "]...". And a symbol that is an actual part of the language that is being described is enclosed in quotes. For example,

<noun-phrase>  ::=  <common-noun> [ "that" <verb-phrase> ]  |
<common-noun> [ <prepositional-phrase> ]...

says that a <noun-phrase> can be a <common-noun>, optionally followed by the literal word "that" and a <verb-phrase>, or it can be a <common-noun> followed by zero or more <prepositional-phrase>'s. Obviously, we can describe very complex structures in this way. The real power comes from the fact that BNF rules can be recursive. In fact, the two preceding rules, taken together, are recursive. A <noun-phrase> is defined partly in terms of <verb-phrase>, while <verb-phrase> is defined partly in terms of <noun-phrase>. For example, a <noun-phrase> might be "the rat that ate the cheese", since "ate the cheese" is a <verb-phrase>. But then we can, recursively, make the more complex <noun-phrase> "the cat that caught the rat that ate the cheese" out of the <common-noun> "the cat", the word "that" and the <verb-phrase> "caught the rat that ate the cheese". Building from there, we can make the <noun-phrase> "the dog that chased the cat that caught the rat that ate the cheese". The recursive structure of language is one of the most fundamental properties of language, and the ability of BNF to express this recursive structure is what makes it so useful.

BNF can be used to describe the syntax of a programming language such as Java in a formal and precise way. For example, a <while-loop> can be defined as

<while-loop>  ::=  "while" "(" <condition> ")" <statement>

This says that a <while-loop> consists of the word "while", followed by a left parenthesis, followed by a <condition>, followed by a right parenthesis, followed by a <statement>. Of course, it still remains to define what is meant by a condition and by a statement. Since a statement can be, among other things, a while loop, we can already see the recursive structure of the Java language. The exact specification of an if statement, which is hard to express clearly in words, can be given as

<if-statement>  ::=
"if" "(" <condition> ")" <statement>
[ "else" "if" "(" <condition> ")" <statement> ]...
[ "else" <statement> ]

This rule makes it clear that the "else" part is optional and that there can be, optionally, one or more "else if" parts.

9.5.2  Recursive Descent Parsing

In the rest of this section, I will show how a BNF grammar for a language can be used as a guide for constructing a parser. A parser is a program that determines the grammatical structure of a phrase in the language. This is the first step in determining the meaning of the phrase -- which for a programming language means translating it into machine language. Although we will look at only a simple example, I hope it will be enough to convince you that compilers can in fact be written and understood by mortals and to give you some idea of how that can be done.

The parsing method that we will use is called recursive descent parsing. It is not the only possible parsing method, or the most efficient, but it is the one most suited for writing compilers by hand (rather than with the help of so called "parser generator" programs). In a recursive descent parser, every rule of the BNF grammar is the model for a subroutine. Not every BNF grammar is suitable for recursive descent parsing. The grammar must satisfy a certain property. Essentially, while parsing a phrase, it must be possible to tell what syntactic category is coming up next just by looking at the next item in the input. Many grammars are designed with this property in mind.

I should also mention that many variations of BNF are in use. The one that I've described here is one that is well-suited for recursive descent parsing.

When we try to parse a phrase that contains a syntax error, we need some way to respond to the error. A convenient way of doing this is to throw an exception. I'll use an exception class called ParseError, defined as follows:

/**
* An object of type ParseError represents a syntax error found in
* the user's input.
*/
private static class ParseError extends Exception {
ParseError(String message) {
super(message);
}
} // end nested class ParseError

Another general point is that our BNF rules don't say anything about spaces between items, but in reality we want to be able to insert spaces between items at will. To allow for this, I'll always call the routine TextIO.skipBlanks() before trying to look ahead to see what's coming up next in input. TextIO.skipBlanks() skips past any whitespace, such as spaces and tabs, in the input, and stops when the next character in the input is either a non-blank character or the end-of-line character.

Let's start with a very simple example. A "fully parenthesized expression" can be specified in BNF by the rules

<expression>  ::=  <number>  |
"(" <expression> <operator> <expression> ")"

<operator>  ::=  "+" | "-" | "*" | "/"

where <number> refers to any non-negative real number. An example of a fully parenthesized expression is "(((34-17)*8)+(2*7))". Since every operator corresponds to a pair of parentheses, there is no ambiguity about the order in which the operators are to be applied. Suppose we want a program that will read and evaluate such expressions. We'll read the expressions from standard input, using TextIO. To apply recursive descent parsing, we need a subroutine for each rule in the grammar. Corresponding to the rule for <operator>, we get a subroutine that reads an operator. The operator can be a choice of any of four things. Any other input will be an error.

/**
* If the next character in input is one of the legal operators,
* read it and return it.  Otherwise, throw a ParseError.
*/
static char getOperator() throws ParseError {
TextIO.skipBlanks();
char op = TextIO.peek(); // look ahead at the next char, without reading it
if ( op == '+' || op == '-' || op == '*' || op == '/' ) {
TextIO.getAnyChar();  // read the operator, to remove it from the input
return op;
}
else if (op == '\n')
throw new ParseError("Missing operator at end of line.");
else
throw new ParseError("Missing operator.  Found \"" +
op + "\" instead of +, -, *, or /.");
} // end getOperator()

I've tried to give a reasonable error message, depending on whether the next character is an end-of-line or something else. I use TextIO.peek() to look ahead at the next character before I read it, and I call TextIO.skipBlanks() before testing TextIO.peek() in order to ignore any blanks that separate items. I will follow this same pattern in every case.

When we come to the subroutine for <expression>, things are a little more interesting. The rule says that an expression can be either a number or an expression enclosed in parentheses. We can tell which it is by looking ahead at the next character. If the character is a digit, we have to read a number. If the character is a "(", we have to read the "(", followed by an expression, followed by an operator, followed by another expression, followed by a ")". If the next character is anything else, there is an error. Note that we need recursion to read the nested expressions. The routine doesn't just read the expression. It also computes and returns its value. This requires semantical information that is not specified in the BNF rule.

/**
* Read an expression from the current line of input and return its value.
* @throws ParseError if the input contains a syntax error
*/
private static double expressionValue() throws ParseError {
TextIO.skipBlanks();
if ( Character.isDigit(TextIO.peek()) ) {
// The next item in input is a number, so the expression
// must consist of just that number.  Read and return
// the number.
return TextIO.getDouble();
}
else if ( TextIO.peek() == '(' ) {
// The expression must be of the form
//         "(" <expression> <operator> <expression> ")"
// Read all these items, perform the operation, and
// return the result.
TextIO.getAnyChar();  // Read the "("
double leftVal = expressionValue();  // Read and evaluate first operand.
char op = getOperator();             // Read the operator.
double rightVal = expressionValue(); // Read and evaluate second operand.
TextIO.skipBlanks();
if ( TextIO.peek() != ')' ) {
// According to the rule, there must be a ")" here.
// Since it's missing, throw a ParseError.
throw new ParseError("Missing right parenthesis.");
}
TextIO.getAnyChar();  // Read the ")"
switch (op) {   //  Apply the operator and return the result.
case '+':  return leftVal + rightVal;
case '-':  return leftVal - rightVal;
case '*':  return leftVal * rightVal;
case '/':  return leftVal / rightVal;
default:   return 0;  // Can't occur since op is one of the above.
// (But Java syntax requires a return value.)
}
}
else {  // No other character can legally start an expression.
throw new ParseError("Encountered unexpected character, \"" +
TextIO.peek() + "\" in input.");
}
} // end expressionValue()

I hope that you can see how this routine corresponds to the BNF rule. Where the rule uses "|" to give a choice between alternatives, there is an if statement in the routine to determine which choice to take. Where the rule contains a sequence of items, "(" <expression> <operator> <expression> ")", there is a sequence of statements in the subroutine to read each item in turn.

When expressionValue() is called to evaluate the expression (((34-17)*8)+(2*7)), it sees the "(" at the beginning of the input, so the else part of the if statement is executed. The "(" is read. Then the first recursive call to expressionValue() reads and evaluates the subexpression ((34-17)*8), the call to getOperator() reads the "+" operator, and the second recursive call to expressionValue() reads and evaluates the second subexpression (2*7). Finally, the ")" at the end of the expression is read. Of course, reading the first subexpression, ((34-17)*8), involves further recursive calls to the expressionValue() routine, but it's better not to think too deeply about that! Rely on the recursion to handle the details.

You'll find a complete program that uses these routines in the file SimpleParser1.java.

Fully parenthesized expressions aren't very natural for people to use. But with ordinary expressions, we have to worry about the question of operator precedence, which tells us, for example, that the "*" in the expression "5+3*7" is applied before the "+". The complex expression "3*6+8*(7+1)/4-24" should be seen as made up of three "terms", 3*6, 8*(7+1)/4, and 24, combined with "+" and "-" operators. A term, on the other hand, can be made up of several factors combined with "*" and "/" operators. For example, 8*(7+1)/4 contains the factors 8, (7+1) and 4. This example also shows that a factor can be either a number or an expression in parentheses. To complicate things a bit more, we allow for leading minus signs in expressions, as in "-(3+4)" or "-7". (Since a <number> is a positive number, this is the only way we can get negative numbers. It's done this way to avoid "3 * -7", for example.) This structure can be expressed by the BNF rules

<expression>  ::=  [ "-" ] <term> [ ( "+" | "-" ) <term> ]...
<term>  ::=  <factor> [ ( "*" | "/" ) <factor> ]...
<factor>  ::=  <number>  |  "(" <expression> ")"

The first rule uses the "[ ]..." notation, which says that the items that it encloses can occur zero, one, two, or more times. The rule means that an <expression> can begin, optionally, with a "-". Then there must be a <term> which can optionally be followed by one of the operators "+" or "-" and another <term>, optionally followed by another operator and <term>, and so on. In a subroutine that reads and evaluates expressions, this repetition is handled by a while loop. An if statement is used at the beginning of the loop to test whether a leading minus sign is present:

/**
* Read an expression from the current line of input and return its value.
* @throws ParseError if the input contains a syntax error
*/
private static double expressionValue() throws ParseError {
TextIO.skipBlanks();
boolean negative;  // True if there is a leading minus sign.
negative = false;
if (TextIO.peek() == '-') {
TextIO.getAnyChar();  // Read the minus sign.
negative = true;
}
double val;  // Value of the expression.
val = termValue();
if (negative)
val = -val;
TextIO.skipBlanks();
while ( TextIO.peek() == '+' || TextIO.peek() == '-' ) {
// Read the next term and add it to or subtract it from
// the value of previous terms in the expression.
char op = TextIO.getAnyChar();  // Read the operator.
double nextVal = termValue();
if (op == '+')
val += nextVal;
else
val -= nextVal;
TextIO.skipBlanks();
}
return val;
} // end expressionValue()

The subroutine for <term> is very similar to this, and the subroutine for <factor> is similar to the example given above for fully parenthesized expressions. A complete program that reads and evaluates expressions based on the above BNF rules can be found in the file SimpleParser2.java.

9.5.3  Building an Expression Tree

Now, so far, we've only evaluated expressions. What does that have to do with translating programs into machine language? Well, instead of actually evaluating the expression, it would be almost as easy to generate the machine language instructions that are needed to evaluate the expression. If we are working with a "stack machine," these instructions would be stack operations such as "push a number" or "apply a + operation". The program SimpleParser3.java can both evaluate the expression and print a list of stack machine operations for evaluating the expression.

It's quite a jump from this program to a recursive descent parser that can read a program written in Java and generate the equivalent machine language code -- but the conceptual leap is not huge.

The SimpleParser3 program doesn't actually generate the stack operations directly as it parses an expression. Instead, it builds an expression tree, as discussed in Subsection 9.4.3, to represent the expression. The expression tree is then used to find the value and to generate the stack operations. The tree is made up of nodes belonging to classes ConstNode and BinOpNode that are similar to those given in Subsection 9.4.3. Another class, UnaryMinusNode, has been introduced to represent the unary minus operation. I've added a method, printStackCommands(), to each class. This method is responsible for printing out the stack operations that are necessary to evaluate an expression. Here for example is the new BinOpNode class from SimpleParser3.java:

private static class BinOpNode extends ExpNode {
char op;        // The operator.
ExpNode left;   // The expression for its left operand.
ExpNode right;  // The expression for its right operand.
BinOpNode(char op, ExpNode left, ExpNode right) {
// Construct a BinOpNode containing the specified data.
assert op == '+' || op == '-' || op == '*' || op == '/';
assert left != null && right != null;
this.op = op;
this.left = left;
this.right = right;
}
double value() {
// The value is obtained by evaluating the left and right
// operands and combining the values with the operator.
double x = left.value();
double y = right.value();
switch (op) {
case '+':
return x + y;
case '-':
return x - y;
case '*':
return x * y;
case '/':
return x / y;
default:
return Double.NaN;  // Bad operator! Should not be possible.
}
}
void  printStackCommands() {
// To evaluate the expression on a stack machine, first do
// whatever is necessary to evaluate the left operand, leaving
// the answer on the stack.  Then do the same thing for the
// second operand.  Then apply the operator (which means popping
// the operands, applying the operator, and pushing the result).
left.printStackCommands();
right.printStackCommands();
System.out.println("  Operator " + op);
}
}

It's also interesting to look at the new parsing subroutines. Instead of computing a value, each subroutine builds an expression tree. For example, the subroutine corresponding to the rule for <expression> becomes

static ExpNode expressionTree() throws ParseError {
// Read an expression from the current line of input and
// return an expression tree representing the expression.
TextIO.skipBlanks();
boolean negative;  // True if there is a leading minus sign.
negative = false;
if (TextIO.peek() == '-') {
TextIO.getAnyChar();
negative = true;
}
ExpNode exp;   // The expression tree for the expression.
exp = termTree();  // Start with a tree for first term.
if (negative) {
// Build the tree that corresponds to applying a
// unary minus operator to the term we've
// just read.
exp = new UnaryMinusNode(exp);
}
TextIO.skipBlanks();
while ( TextIO.peek() == '+' || TextIO.peek() == '-' ) {
// Read the next term and combine it with the
// previous terms into a bigger expression tree.
char op = TextIO.getAnyChar();
ExpNode nextTerm = termTree();
// Create a tree that applies the binary operator
// to the previous tree and the term we just read.
exp = new BinOpNode(op, exp, nextTerm);
TextIO.skipBlanks();
}
return exp;
} // end expressionTree()

In some real compilers, the parser creates a tree to represent the program that is being parsed. This tree is called a parse tree. Parse trees are somewhat different in form from expression trees, but the purpose is the same. Once you have the tree, there are a number of things you can do with it. For one thing, it can be used to generate machine language code. But there are also techniques for examining the tree and detecting certain types of programming errors, such as an attempt to reference a local variable before it has been assigned a value. (The Java compiler, of course, will reject the program if it contains such an error.) It's also possible to manipulate the tree to optimize the program. In optimization, the tree is transformed to make the program more efficient before the code is generated.

And so we are back where we started in Chapter 1, looking at programming languages, compilers, and machine language. But looking at them, I hope, with a lot more understanding and a much wider perspective.

Programming Exercises for Chapter 9

Exercise 9.1:

In many textbooks, the first examples of recursion are the mathematical functions factorial and fibonacci. These functions are defined for non-negative integers using the following recursive formulas:

factorial(0)  =  1
factorial(N)  =  N*factorial(N-1)   for N > 0

fibonacci(0)  =  1
fibonacci(1)  =  1
fibonacci(N)  =  fibonacci(N-1) + fibonacci(N-2)   for N > 1

Write recursive functions to compute factorial(N) and fibonacci(N) for a given non-negative integer N, and write a main() routine to test your functions. Consider using the BigInteger class (see Exercise 8.2)

(In fact, factorial and fibonacci are really not very good examples of recursion, since the most natural way to compute them is to use simple for loops. Furthermore, fibonacci is a particularly bad example, since the natural recursive approach to computing this function is extremely inefficient.)

See the Solution

Exercise 9.2:

Exercise 7.7 asked you to read a file, make an alphabetical list of all the words that occur in the file, and write the list to another file. In that exercise, you were asked to use an ArrayList<String> to store the words. Write a new version of the same program that stores the words in a binary sort tree instead of in an arraylist. You can use the binary sort tree routines from SortTreeDemo.java, which was discussed in Subsection 9.4.2.

See the Solution

Exercise 9.3:

Suppose that linked lists of integers are made from objects belonging to the class

class ListNode {
int item;       // An item in the list.
ListNode next;  // Pointer to the next node in the list.
}

Write a subroutine that will make a copy of a list, with the order of the items of the list reversed. The subroutine should have a parameter of type ListNode, and it should return a value of type ListNode. The original list should not be modified.

You should also write a main() routine to test your subroutine.

See the Solution

Exercise 9.4:

Subsection 9.4.1 explains how to use recursion to print out the items in a binary tree in various orders. That section also notes that a non-recursive subroutine can be used to print the items, provided that a stack or queue is used as an auxiliary data structure. Assuming that a queue is used, here is an algorithm for such a subroutine:

Add the root node to an empty queue
while the queue is not empty:
Get a node from the queue
Print the item in the node
if node.left is not null:
add it to the queue
if node.right is not null:
add it to the queue

Write a subroutine that implements this algorithm, and write a program to test the subroutine. Note that you will need a queue of TreeNodes, so you will need to write a class to represent such queues.

(Note that the order in which items are printed by this algorithm is different from all three of the orders considered in Subsection 9.4.1.)

See the Solution

Exercise 9.5:

In Subsection 9.4.2, I say that "if the [binary sort] tree is created by inserting items in a random order, there is a high probability that the tree is approximately balanced." For this exercise, you will do an experiment to test whether that is true.

The depth of a node in a binary tree is the length of the path from the root of the tree to that node. That is, the root has depth 0, its children have depth 1, its grandchildren have depth 2, and so on. In a balanced tree, all the leaves in the tree are about the same depth. For example, in a perfectly balanced tree with 1023 nodes, all the leaves are at depth 9. In an approximately balanced tree with 1023 nodes, the average depth of all the leaves should be not too much bigger than 9.

On the other hand, even if the tree is approximately balanced, there might be a few leaves that have much larger depth than the average, so we might also want to look at the maximum depth among all the leaves in a tree.

For this exercise, you should create a random binary sort tree with 1023 nodes. The items in the tree can be real numbers, and you can create the tree by generating 1023 random real numbers and inserting them into the tree, using the usual treeInsert() method for binary sort trees. Once you have the tree, you should compute and output the average depth of all the leaves in the tree and the maximum depth of all the leaves. To do this, you will need three recursive subroutines: one to count the leaves, one to find the sum of the depths of all the leaves, and one to find the maximum depth. The latter two subroutines should have an int-valued parameter, depth, that tells how deep in the tree you've gone. When you call this routine from the main program, the depth parameter is 0; when you call the routine recursively, the parameter increases by 1.

See the Solution

Exercise 9.6:

The parsing programs in Section 9.5 work with expressions made up of numbers and operators. We can make things a little more interesting by allowing the variable "x" to occur. This would allow expression such as "3*(x-1)*(x+1)", for example. Make a new version of the sample program SimpleParser3.java that can work with such expressions. In your program, the main() routine can't simply print the value of the expression, since the value of the expression now depends on the value of x. Instead, it should print the value of the expression for x=0, x=1, x=2, and x=3.

The original program will have to be modified in several other ways. Currently, the program uses classes ConstNode, BinOpNode, and UnaryMinusNode to represent nodes in an expression tree. Since expressions can now include x, you will need a new class, VariableNode, to represent an occurrence of x in the expression.

In the original program, each of the node classes has an instance method, "double value()", which returns the value of the node. But in your program, the value can depend on x, so you should replace this method with one of the form "double value(double xValue)", where the parameter xValue is the value of x.

Finally, the parsing subroutines in your program will have to take into account the fact that expressions can contain x. There is just one small change in the BNF rules for the expressions: A <factor> is allowed to be the variable x:

<factor>  ::=  <number>  |  <x-variable>  |  "(" <expression> ")"

where <x-variable> can be either a lower case or an upper case "X". This change in the BNF requires a change in the factorTree() subroutine.

See the Solution

Exercise 9.7:

This exercise builds on the previous exercise, Exercise 9.6. To understand it, you should have some background in Calculus. The derivative of an expression that involves the variable x can be defined by a few recursive rules:

- The derivative of a constant is 0.

- The derivative of x is 1.

- If A is an expression, let dA be the derivative of A. Then the derivative of -A is -dA.

- If A and B are expressions, let dA be the derivative of A and let dB be the derivative of B. Then the derivative of A+B is dA+dB.

- The derivative of A-B is dA-dB.

- The derivative of A*B is A*dB + B*dA.

- The derivative of A/B is (B*dA - A*dB) / (B*B).

For this exercise, you should modify your program from the previous exercise so that it can compute the derivative of an expression. You can do this by adding a derivative-computing method to each of the node classes. First, add another abstract method to the ExpNode class:

abstract ExpNode derivative();

Then implement this method in each of the four subclasses of ExpNode. All the information that you need is in the rules given above. In your main program, instead of printing the stack operations for the original expression, you should print out the stack operations that define the derivative. Note that the formula that you get for the derivative can be much more complicated than it needs to be. For example, the derivative of 3*x+1 will be computed as (3*1+0*x)+0. This is correct, even though it's kind of ugly, and it would be nice for it to be simplified. However, simplifying expressions is not easy.

As an alternative to printing out stack operations, you might want to print the derivative as a fully parenthesized expression. You can do this by adding a printInfix() routine to each node class. It would be nice to leave out unnecessary parentheses, but again, the problem of deciding which parentheses can be left out without altering the meaning of the expression is a fairly difficult one, which I don't advise you to attempt.

(There is one curious thing that happens here: If you apply the rules, as given, to an expression tree, the result is no longer a tree, since the same subexpression can occur at multiple points in the derivative. For example, if you build a node to represent B*B by saying "new BinOpNode('*',B,B)", then the left and right children of the new node are actually the same node! This is not allowed in a tree. However, the difference is harmless in this case since, like a tree, the structure that you get has no loops in it. Loops, on the other hand, would be a disaster in most of the recursive tree-processing subroutines that we have written, since it would lead to infinite recursion. The type of structure that is built by the derivative functions is technically referred to as a directed acyclic graph.)

See the Solution

Quiz on Chapter 9

Question 1:

Explain what is meant by a recursive subroutine.

Question 2:

Consider the following subroutine:

static void printStuff(int level) {
if (level == 0) {
System.out.print("*");
}
else {
System.out.print("[");
printStuff(level - 1);
System.out.print(",");
printStuff(level - 1);
System.out.print("]");
}
}

Show the output that would be produced by the subroutine calls printStuff(0), printStuff(1), printStuff(2), and printStuff(3).

Question 3:

Suppose that a linked list is formed from objects that belong to the class

class ListNode {
int item;       // An item in the list.
ListNode next;  // Pointer to next item in the list.
}

Write a subroutine that will count the number of zeros that occur in a given linked list of ints. The subroutine should have a parameter of type ListNode and should return a value of type int.

Question 4:

What are the three operations on a stack?

Question 5:

What is the basic difference between a stack and a queue?

Question 6:

What is an activation record? What role does a stack of activation records play in a computer?

Question 7:

Suppose that a binary tree of integers is formed from objects belonging to the class

class TreeNode {
int item;       // One item in the tree.
TreeNode left;  // Pointer to the left subtree.
TreeNode right; // Pointer to the right subtree.
}

Write a recursive subroutine that will find the sum of all the nodes in the tree. Your subroutine should have a parameter of type TreeNode, and it should return a value of type int.

Question 8:

What is a postorder traversal of a binary tree?

Question 9:

Suppose that a <multilist> is defined by the BNF rule

<multilist>  ::=  <word>  |  "(" [ <multilist> ]... ")"

where a <word> can be any sequence of letters. Give five different <multilist>'s that can be generated by this rule. (This rule, by the way, is almost the entire syntax of the programming language LISP! LISP is known for its simple syntax and its elegant and powerful semantics.)

Question 10:

Explain what is meant by parsing a computer program.

See the Answers

Chapter 10

Generic Programming and Collection Classes

How to avoid reinventing the wheel? Many data structures and algorithms, such as those from Chapter 9, have been studied, programmed, and re-programmed by generations of computer science students. This is a valuable learning experience. Unfortunately, they have also been programmed and re-programmed by generations of working computer professionals, taking up time that could be devoted to new, more creative work. A programmer who needs a list or a binary tree shouldn't have to re-code these data structures from scratch. They are well-understood and have been programmed thousands of times before. The problem is how to make pre-written, robust data structures available to programmers. In this chapter, we'll look at Java's attempt to address this problem.

You have already seen part of the solution in Section 7.3. That section introduced parameterized types and the ArrayList class. Parameterized types make it possible for the same class to work with many different kinds of data. This idea -- that the same code can be used for a variety of data types -- is called generic programming.

Contents of Chapter 10:

- Section 1: Generic Programming

- Section 2: Lists and Sets

- Section 3: Maps

- Section 4: Programming with the Java Collection Framework

- Section 5: Writing Generic Classes and Methods

- Programming Exercises

- Quiz on This Chapter

Section 10.1

Generic Programming

Generic programming refers to writing code that will work for many types of data. We encountered the alternative to generic programming in Subsection 7.2.4, where we looked at dynamic arrays of integers. The source code presented there for working with dynamic arrays of integers works only for data of type int. But the source code for dynamic arrays of double, String, JButton, or any other type would be almost identical, except for the substitution of one type name for another. It seems silly to write essentially the same code over and over. Java's approach to this problem is parameterized types. As we saw in Section 7.3, the parameterized class ArrayList implements dynamic arrays. Since it is parameterized, there are types such as ArrayList<String> to represent dynamic arrays of String, ArrayList<JButton> for dynamic arrays of buttons, and more generally ArrayList<T> for any object type T. ArrayList is just one class, but the source code works for many different types. This is generic programming.

The ArrayList class is just one of several standard classes that are used for generic programming in Java. We will spend the next few sections looking at these classes and how they are used, and we'll see that there are also generic methods and generic interfaces. All the classes and interfaces discussed in these sections are defined in the package java.util, and you will need an import statement at the beginning of your program to get access to them. (Before you start putting "import java.util.*" at the beginning of every program, you should know that some things in java.util have names that are the same as things in other packages. For example, both java.util.List and java.awt.List exist, so it is often better to import the individual classes that you need.)

In the final section of this chapter, we will see that it is possible to define new generic classes, interfaces, and methods. Until then, we will stick to using the generics that are predefined in Java's standard library.

It is no easy task to design a library for generic programming. Java's solution has many nice features but is certainly not the only possible approach. It is almost certainly not the best, and has a few features that in my opinion can only be called bizarre, but in the context of the overall design of Java, it might be close to optimal. To get some perspective on generic programming in general, it might be useful to look very briefly at some other approaches to generic programming.

10.1.1  Generic Programming in Smalltalk

Smalltalk was one of the very first object-oriented programming languages. It is still used today, although its use is not very common. It never achieved anything like the popularity of Java or C++, but it is the source of many ideas used in these languages. In Smalltalk, essentially all programming is generic, because of two basic properties of the language.

First of all, variables in Smalltalk are typeless. A data value has a type, such as integer or string, but variables do not have types. Any variable can hold data of any type. Parameters are also typeless, so a subroutine can be applied to parameter values of any type. Similarly, a data structure can hold data values of any type. For example, once you've defined a binary tree data structure in SmallTalk, you can use it for binary trees of integers or strings or dates or data of any other type. There is simply no need to write new code for each data type.

Secondly, all data values are objects, and all operations on objects are defined by methods in a class. This is true even for types that are "primitive" in Java, such as integers. When the "+" operator is used to add two integers, the operation is performed by calling a method in the integer class. When you define a new class, you can define a "+" operator, and you will then be able to add objects belonging to that class by saying "a + b" just as if you were adding numbers. Now, suppose that you write a subroutine that uses the "+" operator to add up the items in a list. The subroutine can be applied to a list of integers, but it can also be applied, automatically, to any other data type for which "+" is defined. Similarly, a subroutine that uses the "<" operator to sort a list can be applied to lists containing any type of data for which "<" is defined. There is no need to write a different sorting subroutine for each type of data.

Put these two features together and you have a language where data structures and algorithms will work for any type of data for which they make sense, that is, for which the appropriate operations are defined. This is real generic programming. This might sound pretty good, and you might be asking yourself why all programming languages don't work this way. This type of freedom makes it easier to write programs, but unfortunately it makes it harder to write programs that are correct and robust (see Chapter 8). Once you have a data structure that can contain data of any type, it becomes hard to ensure that it only holds the type of data that you want it to hold. If you have a subroutine that can sort any type of data, it's hard to ensure that it will only be applied to data for which the "<" operator is defined. More particularly, there is no way for a compiler to ensure these things. The problem will only show up at run time when an attempt is made to apply some operation to a data type for which it is not defined, and the program will crash.

10.1.2  Generic Programming in C++

Unlike Smalltalk, C++ is a very strongly typed language, even more so than Java. Every variable has a type, and can only hold data values of that type. This means that the kind of generic programming that is used in Smalltalk is impossible in C++. Nevertheless, C++ has a powerful and flexible system of generic programming. It is made possible by a language feature known as templates. In C++, instead of writing a different sorting subroutine for each type of data, you can write a single subroutine template. The template is not a subroutine; it's more like a factory for making subroutines. We can look at an example, since the syntax of C++ is very similar to Java's:

template<class ItemType>
void sort( ItemType A[], int count ) {
// Sort items in the array, A, into increasing order.
// The items in positions 0, 1, 2, ..., (count-1) are sorted.
// The algorithm that is used here is selection sort.
for (int i = count-1; i > 0; i--) {
int position_of_max = 0;
for (int j = 1; j <= i ; j++)
if ( A[j] > A[position_of_max] )
position_of_max = j;
ItemType temp = A[i];
A[i] = A[position_of_max];
A[position_of_max] = temp;
}
}

This piece of code defines a subroutine template. If you remove the first line, "template<class ItemType>", and substitute the word "int" for the word "ItemType" in the rest of the template, you get a subroutine for sorting arrays of ints. (Even though it says "class ItemType", you can actually substitute any type for ItemType, including the primitive types.) If you substitute "string" for "ItemType", you get a subroutine for sorting arrays of strings. This is pretty much what the compiler does with the template. If your program says "sort(list,10)" where list is an array of ints, the compiler uses the template to generate a subroutine for sorting arrays of ints. If you say "sort(cards,10)" where cards is an array of objects of type Card, then the compiler generates a subroutine for sorting arrays of Cards. At least, it tries to. The template uses the ">" operator to compare values. If this operator is defined for values of type Card, then the compiler will successfully use the template to generate a subroutine for sorting cards. If ">" is not defined for Cards, then the compiler will fail -- but this will happen at compile time, not, as in Smalltalk, at run time where it would make the program crash. (By the way, in C++, it is possible to write definitions of operators like > for any type, so that it is possible that > might work for values of type Card.)

In addition to subroutine templates, C++ also has templates for making classes. If you write a template for a binary tree class, you can use it to generate classes for binary trees of ints, binary trees of strings, binary trees of dates, and so on -- all from one template. Modern C++ comes with a large number of pre-written templates called the Standard Template Library or STL. The STL is quite complex. Many people would say that it's much too complex. But it is also one of the most interesting features of C++.

10.1.3  Generic Programming in Java

Java's generic programming features have gone through several stages of development. Early versions of Java did not have parameterized types, but they did have classes to represent common data structures. Those classes were designed to work with Objects; that is, they could hold objects of any type, and there was no way to restrict the types of objects that could be stored in a given data structure. For example, ArrayList was not originally a parameterized type, so that any ArrayList could hold any type of object. This means that if list was an ArrayList, then list.get(i) would return a value of type Object. If the programmer was actually using the list to store Strings, the value returned by list.get(i) would have to be type-cast to treat it as a string:

String item = (String)list.get(i);

This is still a kind of generic programming, since one class can work for any kind of object, but it was closer in spirit to Smalltalk than it was to C++, since there is no way to do type checks at compile time. Unfortunately, as in Smalltalk, the result is a category of errors that show up only at run time, rather than at compile time. If a programmer assumes that all the items in a data structure are strings and tries to process those items as strings, a run-time error will occur if other types of data have inadvertently been added to the data structure. In Java, the error will most likely occur when the program retrieves an Object from the data structure and tries to type-cast it to type String. If the object is not actually of type String, the illegal type-cast will throw an error of type ClassCastException.

Java 5.0 introduced parameterized types, which made it possible to create generic data structures that can be type-checked at compile time rather than at run time. For example, if list is of type ArrayList<String>, then the compiler will only allow objects of type String to be added to list. Furthermore, the return type of list.get(i) is String, so type-casting is not necessary. Java's parameterized classes are similar to template classes in C++ (although the implementation is very different), and their introduction moves Java's generic programming model closer to C++ and farther from Smalltalk. In this chapter, I will use the parameterized types exclusively, but you should remember that their use is not mandatory. It is still legal to use a parameterized class as a non-parameterized type, such as a plain ArrayList. In that case, any type of object can be stored in the data structure.

Note that there is a significant difference between parameterized classes in Java and template classes in C++. A template class in C++ is not really a class at all -- it's a kind of factory for generating classes. Every time the template is used with a new type, a new compiled class is created. With a Java parameterized class, there is only one compiled class file. For example, there is only one compiled class file, ArrayList.class, for the parameterized class ArrayList. The parameterized types ArrayList<String> and ArrayList<Integer> both use the same compiled class file, as does the plain ArrayList type. The type parameter -- String or Integer -- just tells the compiler to limit the type of object that can be stored in the data structure. The type parameter has no effect at run time and is not even known at run time. The type information is said to be "erased" at run time. This type erasure introduces a certain amount of weirdness. For example, you can't test "if (list instanceof ArrayList<String>)" because the instanceof operator is evaluated at run time, and at run time only the plain ArrayList exists. Similarly, you can't type-cast to the type ArrayList<String>. Even worse, you can't create an array that has base type ArrayList<String> by using the new operator, as in "new ArrayList<String>[N]". This is because the new operator is evaluated at run time, and at run time there is no such thing as "ArrayList<String>"; only the non-parameterized type ArrayList exists at run time. (However, although you can't have an array of ArrayList<String>, you can have an ArrayList of ArrayList<String> -- with the type written as ArrayList<ArrayList<String>> -- which is just as good or better.)

Fortunately, most programmers don't have to deal with such problems, since they turn up only in fairly advanced programming. Most people who use parameterized types will not encounter the problems, and they will get the benefits of type-safe generic programming with little difficulty.

10.1.4  The Java Collection Framework

As I've said, Java comes with a number of parameterized classes that implement common data structures. This collection of data structure classes is referred to as the Java Collection Framework, or JFC. We will spend the next few sections learning about the JFC.

The generic data structures in the Java Collection Framework can be divided into two categories: collections and maps. A collection is more or less what it sounds like: a collection of objects. A map associates objects in one set with objects in another set in the way that a dictionary associates definitions with words or a phone book associates phone numbers with names. A map is similar to what I called an "association list" in Subsection 7.4.2. In Java, collections and maps are represented by the parameterized interfaces Collection<T> and Map<T,S>. Here, "T" and "S" stand for any type except for the primitive types. Map<T,S> is an example of a parameterized type that has two type parameters, T and S; we will not deal further with this possibility until we look at maps more closely in Section 10.3. In this section and the next, we look at collections only.

There are two types of collections: lists and sets. A list is a collection in which the objects are arranged in a linear sequence. A list has a first item, a second item, and so on. For any item in the list, except the last, there is an item that directly follows it. For collections that are "sets," the defining property is that no object can occur more than once in a set; the elements of a set are not necessarily thought of as being in any particular order. The ideas of lists and sets are represented as parameterized interfaces List<T> and Set<T>. These are sub-interfaces of Collection<T>. That is, any object that implements the interface List<T> or Set<T> automatically implements Collection<T> as well. The interface Collection<T> specifies general operations that can be applied to any collection at all. List<T> and Set<T> add additional operations that are appropriate for lists and sets respectively.

Of course, any actual object that is a collection, list, or set must belong to a concrete class that implements the corresponding interface. For example, the class ArrayList<T> implements the interface List<T> and therefore also implements Collection<T>. This means that all the methods that are defined in the list and collection interfaces can be used with an ArrayList. We will look at various classes that implement the list and set interfaces in the next section. But before we do that, we'll look briefly at some of the general operations that are available for all collections.

The interface Collection<T> specifies methods for performing some basic operations on any collection of objects. Since "collection" is a very general concept, operations that can be applied to all collections are also very general. They are generic operations in the sense that they can be applied to various types of collections containing various types of objects. Suppose that coll is an object that implements the interface Collection<T> (for some specific non-primitive type T). Then the following operations, which are specified in the interface Collection<T>, are defined for coll:

- coll.size() -- returns an int that gives the number of objects in the collection.

- coll.isEmpty() -- returns a boolean value which is true if the size of the collection is 0.

- coll.clear() -- removes all objects from the collection.

- coll.add(tobject) -- adds tobject to the collection. The parameter must be of type T; if not, a syntax error occurs at compile time. (Remember that if T is a class, this includes objects belonging to a subclass of T, and if T is an interface, it includes any object that implements T.) The add() method returns a boolean value which tells you whether the operation actually modified the collection. For example, adding an object to a Set has no effect if that object was already in the set.

- coll.contains(object) -- returns a boolean value that is true if object is in the collection. Note that object is not required to be of type T, since it makes sense to check whether object is in the collection, no matter what type object has. (For testing equality, null is considered to be equal to itself. The criterion for testing non-null objects for equality can differ from one kind of collection to another; see Subsection 10.1.6, below.)

- coll.remove(object) -- removes object from the collection, if it occurs in the collection, and returns a boolean value that tells you whether the object was found. Again, object is not required to be of type T. The test for equality is the same test that is used by contains().

- coll.containsAll(coll2) -- returns a boolean value that is true if every object in coll2 is also in coll. The parameter can be any collection.

- coll.addAll(coll2) -- adds all the objects in coll2 to coll. The parameter, coll2, can be any collection of type Collection<T>. However, it can also be more general. For example, if T is a class and S is a sub-class of T, then coll2 can be of type Collection<S>. This makes sense because any object of type S is automatically of type T and so can legally be added to coll.

- coll.removeAll(coll2) -- removes every object from coll that also occurs in the collection coll2. coll2 can be any collection.

- coll.retainAll(coll2) -- removes every object from coll that does not occur in the collection coll2. It "retains" only the objects that do occur in coll2. coll2 can be any collection.

- coll.toArray() -- returns an array of type Object[] that contains all the items in the collection. Note that the return type is Object[], not T[]! However, there is another version of this method that takes an array of type T[] as a parameter: the method coll.toArray(tarray) returns an array of type T[] containing all the items in the collection. If the array parameter tarray is large enough to hold the entire collection, then the items are stored in tarray and tarray is also the return value of the collection. If tarray is not large enough, then a new array is created to hold the items; in that case tarray serves only to specify the type of the array. For example, coll.toArray(new String[0]) can be used if coll is a collection of Strings and will return a new array of type String[].

Since these methods are part of the Collection<T> interface, they must be defined for every object that implements that interface. There is a problem with this, however. For example, the size of some collections cannot be changed after they are created. Methods that add or remove objects don't make sense for these collections. While it is still legal to call the methods, an exception will be thrown when the call is evaluated at run time. The type of the exception is UnsupportedOperationException. Furthermore, since Collection<T> is only an interface, not a concrete class, the actual implementation of the method is left to the classes that implement the interface. This means that the semantics of the methods, as described above, are not guaranteed to be valid for all collection objects; they are valid, however, for classes in the Java Collection Framework.

There is also the question of efficiency. Even when an operation is defined for several types of collections, it might not be equally efficient in all cases. Even a method as simple as size() can vary greatly in efficiency. For some collections, computing the size() might involve counting the items in the collection. The number of steps in this process is equal to the number of items. Other collections might have instance variables to keep track of the size, so evaluating size() just means returning the value of a variable. In this case, the computation takes only one step, no matter how many items there are. When working with collections, it's good to have some idea of how efficient operations are and to choose a collection for which the operations that you need can be implemented most efficiently. We'll see specific examples of this in the next two sections.

10.1.5  Iterators and for-each Loops

The interface Collection<T> defines a few basic generic algorithms, but suppose you want to write your own generic algorithms. Suppose, for example, you want to do something as simple as printing out every item in a collection. To do this in a generic way, you need some way of going through an arbitrary collection, accessing each item in turn. We have seen how to do this for specific data structures: For an array, you can use a for loop to iterate through all the array indices. For a linked list, you can use a while loop in which you advance a pointer along the list. For a binary tree, you can use a recursive subroutine to do an inorder traversal. Collections can be represented in any of these forms and many others besides. With such a variety of traversal mechanisms, how can we even hope to come up with a single generic method that will work for collections that are stored in wildly different forms? This problem is solved by iterators. An iterator is an object that can be used to traverse a collection. Different types of collections have iterators that are implemented in different ways, but all iterators are used in the same way. An algorithm that uses an iterator to traverse a collection is generic, because the same technique can be applied to any type of collection. Iterators can seem rather strange to someone who is encountering generic programming for the first time, but you should understand that they solve a difficult problem in an elegant way.

The interface Collection<T> defines a method that can be used to obtain an iterator for any collection. If coll is a collection, then coll.iterator() returns an iterator that can be used to traverse the collection. You should think of the iterator as a kind of generalized pointer that starts at the beginning of the collection and can move along the collection from one item to the next. Iterators are defined by a parameterized interface named Iterator<T>. If coll implements the interface Collection<T> for some specific type T, then coll.iterator() returns an iterator of type Iterator<T>, with the same type T as its type parameter. The interface Iterator<T> defines just three methods. If iter refers to an object that implements Iterator<T>, then we have:

- iter.next() -- returns the next item, and advances the iterator. The return value is of type T. This method lets you look at one of the items in the collection. Note that there is no way to look at an item without advancing the iterator past that item. If this method is called when no items remain, it will throw a NoSuchElementException.

- iter.hasNext() -- returns a boolean value telling you whether there are more items to be processed. In general, you should test this before calling iter.next().

- iter.remove() -- if you call this after calling iter.next(), it will remove the item that you just saw from the collection. Note that this method has no parameter. It removes the item that was most recently returned by iter.next(). This might produce an UnsupportedOperationException, if the collection does not support removal of items.

Using iterators, we can write code for printing all the items in any collection. Suppose, for example, that coll is of type Collection<String>. In that case, the value returned by coll.iterator() is of type Iterator<String>, and we can say:

Iterator<String> iter;          // Declare the iterator variable.
iter = coll.iterator();         // Get an iterator for the collection.
while ( iter.hasNext() ) {
String item = iter.next();   // Get the next item.
System.out.println(item);
}

The same general form will work for other types of processing. For example, the following code will remove all null values from any collection of type Collection<JButton> (as long as that collection supports removal of values):

Iterator<JButton> iter = coll.iterator():
while ( iter.hasNext() ) {
JButton item = iter.next();
if (item == null)
iter.remove();
}

(Note, by the way, that when Collection<T>, Iterator<T>, or any other parameterized type is used in actual code, they are always used with actual types such as String or JButton in place of the "formal type parameter" T. An iterator of type Iterator<String> is used to iterate through a collection of Strings; an iterator of type Iterator<JButton> is used to iterate through a collection of JButtons; and so on.)

An iterator is often used to apply the same operation to all the elements in a collection. In many cases, it's possible to avoid the use of iterators for this purpose by using a for-each loop. The for-each loop was discussed in Subsection 7.1.1 for use with arrays and in Subsection 7.3.3 for use with ArrayLists. But in fact, a for-each loop can be used to iterate through any collection. For a collection coll of type Collection<T>, a for-each loop takes the form:

for ( T x : coll ) { // "for each object x, of type T, in coll"
//  process x
}

Here, x is the loop control variable. Each object in coll will be assigned to x in turn, and the body of the loop will be executed for each object. Since objects in coll are of type T, x is declared to be of type T. For example, if namelist is of type Collection<String>, we can print out all the names in the collection with:

for ( String name : namelist ) {
System.out.println( name );
}

This for-each loop could, of course, be written as a while loop using an iterator, but the for-each loop is much easier to follow.

10.1.6  Equality and Comparison

There are several methods in the Collection interface that test objects for equality. For example, the methods coll.contains(object) and coll.remove(object) look for an item in the collection that is equal to object. However, equality is not such a simple matter. The obvious technique for testing equality -- using the == operator -- does not usually give a reasonable answer when applied to objects. The == operator tests whether two objects are identical in the sense that they share the same location in memory. Usually, however, we want to consider two objects to be equal if they represent the same value, which is a very different thing. Two values of type String should be considered equal if they contain the same sequence of characters. The question of whether those characters are stored in the same location in memory is irrelevant. Two values of type Date should be considered equal if they represent the same time.

The Object class defines the boolean-valued method equals(Object) for testing whether one object is equal to another. This method is used by many, but not by all, collection classes for deciding whether two objects are to be considered the same. In the Object class, obj1.equals(obj2) is defined to be the same as obj1 == obj2. However, for many sub-classes of Object, this definition is not reasonable, and it should be overridden. The String class, for example, overrides equals() so that for a String str, str.equals(obj) if obj is also a String and obj contains the same sequence of characters as str.

If you write your own class, you might want to define an equals() method in that class to get the correct behavior when objects are tested for equality. For example, a Card class that will work correctly when used in collections could be defined as:

public class Card {  // Class to represent playing cards.

private int suit;  // Number from 0 to 3 that codes for the suit --
// spades, diamonds, clubs or hearts.
private int value; // Number from 1 to 13 that represents the value.

public boolean equals(Object obj) {
try {
Card other = (Card)obj;  // Type-cast obj to a Card.
if (suit == other.suit && value == other.value) {
// The other card has the same suit and value as
// this card, so they should be considered equal.
return true;
}
else
return false;
}
catch (Exception e) {
// This will catch the NullPointerException that occurs if obj
// is null and the ClassCastException that occurs if obj is
// not of type Card.  In these cases, obj is not equal to
// this Card, so return false.
return false;
}
}

.
. // other methods and constructors
.
}

Without the equals() method in this class, methods such as contains() and remove() from the interface Collection<Card> will not work as expected.

A similar concern arises when items in a collection are sorted. Sorting refers to arranging a sequence of items in ascending order, according to some criterion. The problem is that there is no natural notion of ascending order for arbitrary objects. Before objects can be sorted, some method must be defined for comparing them. Objects that are meant to be compared should implement the interface java.lang.Comparable. In fact, Comparable is defined as a parameterized interface, Comparable<T>, which represents the ability to be compared to an object of type T. The interface Comparable<T> defines one method:

public int compareTo( T obj )

The value returned by obj1.compareTo(obj2) should be negative if and only if obj1 comes before obj2, when the objects are arranged in ascending order. It should be positive if and only if obj1 comes after obj2. A return value of zero means that the objects are considered to be the same for the purposes of this comparison. This does not necessarily mean that the objects are equal in the sense that obj1.equals(obj2) is true. For example, if the objects are of type Address, representing mailing addresses, it might be useful to sort the objects by zip code. Two Addresses are considered the same for the purposes of the sort if they have the same zip code -- but clearly that would not mean that they are the same address. (Some classes in the JFC use compareTo() rather than equals() to test objects for equality.)

The String class implements the interface Comparable<String> and defines compareTo in a reasonable way. In this case, the return value of compareTo is zero if and only if the two strings that are being compared are equal. If you define your own class and want to be able to sort objects belonging to that class, you should do the same. For example:

/**
* Represents a full name consisting of a first name and a last name.
*/
public class FullName implements Comparable<FullName> {

private String firstName, lastName;  // Non-null first and last names.

public FullName(String first, String last) {  // Constructor.
if (first == null || last == null)
throw new IllegalArgumentException("Names must be non-null.");
firstName = first;
lastName = last;
}

public boolean equals(Object obj) {
try {
FullName other = (FullName)obj;  // Type-cast obj to type FullName
return firstName.equals(other.firstName)
&& lastName.equals(other.lastName);
}
catch (Exception e) {
return false;  // if obj is null or is not of type FullName
}
}

public int compareTo( FullName other ) {
if ( lastName.compareTo(other.lastName) < 0 ) {
// If lastName comes before the last name of
// the other object, then this FullName comes
// before the other FullName.  Return a negative
// value to indicate this.
return -1;
}
else if ( lastName.compareTo(other.lastName) > 0 ) {
// If lastName comes after the last name of
// the other object, then this FullName comes
// after the other FullName.  Return a positive
// value to indicate this.
return 1;
}
else {
// Last names are the same, so base the comparison on
// the first names, using compareTo from class String.
return firstName.compareTo(other.firstName);
}
}

.
. // other methods
.
}

(I find it a little odd that the class here is declared as "class FullName implements Comparable<FullName>", with "FullName" repeated as a type parameter in the name of the interface. However, it does make sense. It means that we are going to compare objects that belong to the class FullName to other objects of the same type. Even though this is the only reasonable thing to do, that fact is not obvious to the Java compiler -- and the type parameter in Comparable<FullName> is there for the compiler.)

There is another way to allow for comparison of objects in Java, and that is to provide a separate object that is capable of making the comparison. The object must implement the interface Comparator<T>, where T is the type of the objects that are to be compared. The interface Comparator<T> defines the method:

public int compare( T obj1, T obj2 )

This method compares two objects of type T and returns a value that is negative, or positive, or zero, depending on whether obj1 comes before obj2, or comes after obj2, or is considered to be the same as obj2 for the purposes of this comparison. Comparators are useful for comparing objects that do not implement the Comparable interface and for defining several different orderings on the same collection of objects.

In the next two sections, we'll see how Comparable and Comparator are used in the context of collections and maps.

10.1.7  Generics and Wrapper Classes

As noted in Section 7.3 the case of ArrayList, Java's generic programming does not apply to the primitive types, since generic data structures can only hold objects, while values of primitive type are not objects. However, the "wrapper classes" that were introduced in Subsection 7.3.2 make it possible to get around this restriction to a great extent.

Recall that each primitive type has an associated wrapper class: class Integer for type int, class Boolean for type boolean, class Character for type char, and so on.

An object of type Integer contains a value of type int. The object serves as a "wrapper" for the primitive type value, which allows it to be used in contexts where objects are required, such as in generic data structures. For example, a list of Integers can be stored in a variable of type ArrayList<Integer>, and interfaces such as Collection<Integer> and Set<Integer> are defined. Furthermore, class Integer defines equals(), compareTo(), and toString() methods that do what you would expect (that is, that compare and write out the corresponding primitive type values in the usual way). Similar remarks apply for all the wrapper classes.

Recall also that Java does automatic conversions between a primitive type and the corresponding wrapper type. (These conversions, which are called autoboxing and unboxing, were also introduced in Subsection 7.3.3.) This means that once you have created a generic data structure to hold objects belonging to one of the wrapper classes, you can use the data structure pretty much as if it actually contained primitive type values. For example, if numbers is a variable of type Collection<Integer>, it is legal to call numbers.add(17) or numbers.remove(42). You can't literally add the primitive type value 17 to numbers, but Java will automatically convert the 17 to the corresponding wrapper object, new Integer(17), and the wrapper object will be added to the collection. (The creation of the object does add some time and memory overhead to the operation, and you should keep that in mind in situations where efficiency is important. An array of int is more efficient than an ArrayList<Integer>.)

Section 10.2

Lists and Sets

In the previous section, we looked at the general properties of collection classes in Java. In this section, we look at some specific collection classes and how to use them. These classes can be divided into two main categories: lists and sets. A list consists of a sequence of items arranged in a linear order. A list has a definite order, but is not necessarily sorted into ascending order. A set is a collection that has no duplicate entries. The elements of a set might or might not be arranged into some definite order. I will also briefly discuss a third category of collection known as a "priority queue."

10.2.1  ArrayList and LinkedList

There are two obvious ways to represent a list: as a dynamic array and as a linked list. We've encountered these already in Section 7.3 and Section 9.2. Both of these options are available in generic form as the collection classes java.util.ArrayList and java.util.LinkedList. These classes are part of the Java Collection Framework. Each implements the interface List<T>, and therefore the interface Collection<T>. An object of type ArrayList<T> represents an ordered sequence of objects of type T, stored in an array that will grow in size whenever necessary as new items are added. An object of type LinkedList<T> also represents an ordered sequence of objects of type T, but the objects are stored in nodes that are linked together with pointers.

Both list classes support the basic list operations that are defined in the interface List<T>, and an abstract data type is defined by its operations, not by its representation. So why two classes? Why not a single List class with a single representation? The problem is that there is no single representation of lists for which all list operations are efficient. For some operations, linked lists are more efficient than arrays. For others, arrays are more efficient. In a particular application of lists, it's likely that only a few operations will be used frequently. You want to choose the representation for which the frequently used operations will be as efficient as possible.

Broadly speaking, the LinkedList class is more efficient in applications where items will often be added or removed at the beginning of the list or in the middle of the list. In an array, these operations require moving a large number of items up or down one position in the array, to make a space for a new item or to fill in the hole left by the removal of an item. In terms of asymptotic analysis (Section 8.5), adding an element at the beginning or in the middle of an array has run time Θ(n), where n is the number of items in the array. In a linked list, nodes can be added or removed at any position by changing a few pointer values, an operation that has run time Θ(1). That is, the operation takes only some constant amount of time, independent of how many items are in the list.

On the other hand, the ArrayList class is more efficient when random access to items is required. Random access means accessing the k-th item in the list, for any integer k. Random access is used when you get or change the value stored at a specified position in the list. This is trivial for an array, with run time Θ(1). But for a linked list it means starting at the beginning of the list and moving from node to node along the list for k steps, an operation that has run time Θ(k).

Operations that can be done efficiently for both types of lists include sorting and adding an item at the end of the list.

All lists implement the methods from interface Collection<T> that were discussed in Subsection 10.1.4. These methods include size(), isEmpty(), add(T), remove(Object), and clear(). The add(T) method adds the object at the end of the list. The remove(Object) method involves first finding the object, which uses linear search and is not very efficient for any list since it involves going through the items in the list from beginning to end until the object is found. The interface List<T> adds some methods for accessing list items according to their numerical positions in the list. Suppose that list is an object of type List<T>. Then we have the methods:

- list.get(index) -- returns the object of type T that is at position index in the list, where index is an integer. Items are numbered 0, 1, 2, ..., list.size()-1. The parameter must be in this range, or an IndexOutOfBoundsException is thrown.

- list.set(index,obj) -- stores the object obj at position number index in the list, replacing the object that was there previously. The object obj must be of type T. This does not change the number of elements in the list or move any of the other elements.

- list.add(index,obj) -- inserts an object obj into the list at position number index, where obj must be of type T. The number of items in the list increases by one, and items that come after position index move down one position to make room for the new item. The value of index must be in the range 0 to list.size(), inclusive. If index is equal to list.size(), then obj is added at the end of the list.

- list.remove(index) -- removes the object at position number index, and returns that object as the return value of the method. Items after this position move up one space in the list to fill the hole, and the size of the list decreases by one. The value of index must be in the range 0 to list.size()-1.

- list.indexOf(obj) -- returns an int that gives the position of obj in the list, if it occurs. If it does not occur, the return value is -1. The object obj can be of any type, not just of type T. If obj occurs more than once in the list, the index of the first occurrence is returned.

These methods are defined both in class ArrayList<T> and in class LinkedList<T>, although some of them -- get and set -- are only efficient for ArrayLists. The class LinkedList<T> adds a few additional methods, which are not defined for an ArrayList. If linkedlist is an object of type LinkedList<T>, then we have

- linkedlist.getFirst() -- returns the object of type T that is the first item in the list. The list is not modified. If the list is empty when the method is called, an exception of type NoSuchElementException is thrown (the same is true for the next three methods as well).

- linkedlist.getLast() -- returns the object of type T that is the last item in the list. The list is not modified.

- linkedlist.removeFirst() -- removes the first item from the list, and returns that object of type T as its return value. The functions linkedlist.remove() and linkedlist.pop() are also defined, with the same meaning as removeFirst().

- linkedlist.removeLast() -- removes the last item from the list, and returns that object of type T as its return value.

- linkedlist.addFirst(obj) -- adds the obj, which must be of type T, to the beginning of the list. The function linkedlist.push(obj) has the same meaning.

- linkedlist.addLast(obj) -- adds the object obj, which must be of type T, to the end of the list. This is exactly the same as linkedlist.add(obj) but is defined to keep the naming consistent.

There is some redundancy here, apparently to make it easy to use a LinkedList as if it were a stack or a queue. (See Section 9.3.) For example, we can use a LinkedList as a stack by using the methods named push() and pop(), or as a queue by using add() and remove() to implement the enqueue and dequeue operations.

If list is an object of type List<T>, then the method list.iterator(), defined in the interface Collection<T>, returns an Iterator that can be used to traverse the list from beginning to end. However, for Lists, there is a special type of Iterator, called a ListIterator, which offers additional capabilities. ListIterator<T> is an interface that extends the interface Iterator<T>. The method list.listIterator() returns an object of type ListIterator<T>.

A ListIterator has the usual Iterator methods, hasNext(), next(), and remove(), but it also has methods hasPrevious(), previous(), add(obj), and set(obj) that make it possible to move backwards in the list, to add an item at the current position of the iterator, and to replace one of the items in the list. To understand how these work, it's best to think of an iterator as pointing to a position between two list elements, or at the beginning or end of the list. In this diagram, the items in a list are represented by squares, and arrows indicate the possible positions of an iterator:

If iter is of type ListIterator<T>, then iter.next() moves the iterator one space to the right along the list and returns the item that the iterator passes as it moves. The method iter.previous() moves the iterator one space to the left along the list and returns the item that it passes. The method iter.remove() removes an item from the list; the item that is removed is the item that the iterator passed most recently in a call to either iter.next() or iter.previous(). The method iter.set(obj) works similarly; it replaces the item that would be removed by iter.remove(). There is also a method iter.add(obj) that adds the specified object to the list at the current position of the iterator (where obj must be of type T). This can be between two existing items or at the beginning of the list or at the end of the list.

(By the way, the lists that are used in class LinkedList<T> are doubly linked lists. That is, each node in the list contains two pointers -- one to the next node in the list and one to the previous node. This makes it possible to efficiently implement both the next() and previous() methods of a ListIterator. Also, to make the addLast() and getLast() methods of a LinkedList efficient, the class LinkedList<T> includes a "tail pointer" that points to the last node in the list.)

As an example of using a ListIterator, suppose that we want to maintain a list of items that is always sorted into increasing order. When adding an item to the list, we can use a ListIterator to find the position in the list where the item should be added. Once the position has been found, we use the same list iterator to place the item in that position. The idea is to start at the beginning of the list and to move the iterator forward past all the items that are smaller than the item that is being inserted. At that point, the iterator's add() method can be used to insert the item. To be more definite, suppose that stringList is a variable of type List<String>. Assume that the strings that are already in the list are stored in ascending order and that newItem is a string that we would like to insert into the list. The following code will place newItem in the list in its correct position, so that the modified list is still in ascending order:

ListIterator<String> iter = stringList.listIterator();

// Move the iterator so that it points to the position where
// newItem should be inserted into the list.  If newItem is
// bigger than all the items in the list, then the while loop
// will end when iter.hasNext() becomes false, that is, when
// the iterator has reached the end of the list.

while (iter.hasNext()) {
String item = iter.next();
if (newItem.compareTo(item) <= 0) {
// newItem should come BEFORE item in the list.
// Move the iterator back one space so that
// it points to the correct insertion point,
// and end the loop.
iter.previous();
break;
}
}

iter.add(newItem);

Here, stringList might be of type ArrayList<String> or of type LinkedList<String>. The algorithm that is used to insert newItem into the list will be about equally efficient for both types of lists, and it will even work for other classes that implement the interface List<String>. You would probably find it easier to design an insertion algorithm that uses array-like indexing with the methods get(index) and add(index,obj). However, that algorithm would be horribly inefficient for LinkedLists because random access is so inefficient for linked lists. (By the way, the insertion algorithm works when the list is empty. It might be useful for you to think about why this is true.)

10.2.2  Sorting

Sorting a list is a fairly common operation, and there should really be a sorting method in the List interface. There is not, presumably because it only makes sense to sort lists of certain types of objects. However, methods for sorting lists are available as static methods in the class java.util.Collections. This class contains a variety of static utility methods for working with collections. The methods are generic; that is, they will work for collections of objects of various types. (You have already seen similar methods for arrays in the Arrays class.) Suppose that list is of type List<T>. The command

Collections.sort(list);

can be used to sort the list into ascending order. The items in the list should implement the interface Comparable<T> (see Subsection 10.1.6). The method Collections.sort() will work, for example, for lists of String and for lists of any of the wrapper classes such as Integer and Double. There is also a sorting method that takes a Comparator as its second argument:

Collections.sort(list,comparator);

In this method, the comparator will be used to compare the items in the list. As mentioned in the previous section, a Comparator is an object that defines a compare() method that can be used to compare two objects. We'll see an example of using a Comparator in Section 10.4.

The sorting method that is used by Collections.sort() is the so-called "merge sort" algorithm, which has both worst-case and average-case run times that are Θ(n*log(n)) for a list of size n. Although the average run time for MergeSort is a little slower than that of QuickSort, its worst-case performance is much better than QuickSort's. (QuickSort was covered in Subsection 9.1.3.) MergeSort also has a nice property called "stability" that we will encounter at the end of Subsection 10.4.3.

The Collections class has at least two other useful methods for modifying lists. Collections.shuffle(list) will rearrange the elements of the list into a random order. Collections.reverse(list) will reverse the order of the elements, so that the last element is moved to the beginning of the list, the next-to-last element to the second position, and so on.

Since an efficient sorting method is provided for Lists, there is no need to write one yourself.

10.2.3  TreeSet and HashSet

A set is a collection of objects in which no object occurs more than once. Sets implement all the methods in the interface Collection<T>, but do so in a way that ensures that no element occurs twice in the set. For example, if set is an object of type Set<T>, then set.add(obj) will have no effect on the set if obj is already an element of the set. Java has two classes that implement the interface Set<T>: java.util.TreeSet and java.util.HashSet.

In addition to being a Set, a TreeSet has the property that the elements of the set are arranged into ascending sorted order. An Iterator (or a for-each loop) for a TreeSet will always visit the elements of the set in ascending order.

A TreeSet cannot hold arbitrary objects, since there must be a way to determine the sorted order of the objects it contains. Ordinarily, this means that the objects in a set of type TreeSet<T> should implement the interface Comparable<T> and that obj1.compareTo(obj2) should be defined in a reasonable way for any two objects obj1 and obj2 in the set. Alternatively, an object of type Comparator<T> can be provided as a parameter to the constructor when the TreeSet is created. In that case, the compare() method of the Comparator will be used to compare objects that are added to the set.

A TreeSet does not use the equals() method to test whether two objects are the same. Instead, it uses the compareTo() (or compare()) method. This can be a problem. Recall from Subsection 10.1.6 that compareTo() can consider two objects to be the same for the purpose of the comparison even though the objects are not equal. For a TreeSet, this means that only one of those objects can be in the set. For example, if the TreeSet contains mailing addresses and if the compareTo() method for addresses just compares their zip codes, then the set can contain only one address in each zip code. Clearly, this is not right! But that only means that you have to be aware of the semantics of TreeSets, and you need to make sure that compareTo() is defined in a reasonable way for objects that you put into a TreeSet. This will be true, by the way, for Strings, Integers, and many other built-in types, since the compareTo() method for these types considers two objects to be the same only if they are actually equal.

In the implementation of a TreeSet, the elements are stored in something similar to a binary sort tree. (See Subsection 9.4.2.) However, the data structure that is used is balanced in the sense that all the leaves of the tree are at about the same distance from the root of the tree. This ensures that all the basic operations -- inserting, deleting, and searching -- are efficient, with worst-case run time Θ(log(n)), where n is the number of items in the set.

The fact that a TreeSet sorts its elements and removes duplicates makes it very useful in some applications. Exercise 7.7 asked you to write a program that would read a file and output an alphabetical list of all the words that occurred in the file, with duplicates removed. The words were to be stored in an ArrayList, so it was up to you to make sure that the list was sorted and contained no duplicates. The same task can be programmed much more easily using a TreeSet instead of a list. A TreeSet automatically eliminates duplicates, and an iterator for the set will automatically visit the items in the set in sorted order. An algorithm for the program, using a TreeSet, would be:

TreeSet<String> words = new TreeSet<String>();

while there is more data in the input file:
Let word = the next word from the file
Convert word to lower case
words.add(word)   // Adds the word only if not already present.

for ( String w : words ) // for each String w in words
Output w  // words are output in sorted order

If you would like to see a complete, working program, you can find it in the file WordListWithTreeSet.java.

As another example, suppose that coll is any Collection of Strings. (This would also work for any other type for which compareTo() is properly defined.) We can use a TreeSet to sort the items of coll and remove the duplicates simply by saying:

TreeSet<String> set = new TreeSet<String>();
set.addAll(coll);

The second statement adds all the elements of the collection to the set. Since it's a Set, duplicates are ignored. Since it's a TreeSet, the elements of the set are sorted. If you would like to have the data in some other type of data structure, it's easy to copy the data from the set. For example, to place the answer in an ArrayList, you could say:

TreeSet<String> set = new TreeSet<String>();
set.addAll(coll);
ArrayList<String> list = new ArrayList<String>();
list.addAll(set);

Now, in fact, every one of Java's collection classes has a constructor that takes a Collection as an argument. All the items in that Collection are added to the new collection when it is created. So, if coll is of type Collection<String>, then "new TreeSet<String>(coll)" creates a TreeSet that contains the same elements as coll, but with duplicates removed and in sorted order. This means that we can abbreviate the four lines in the above example to the single command:

ArrayList<String> list = new ArrayList<String>( new TreeSet<String>(coll) );

This makes a sorted list of the elements of coll with no duplicates. Although the repeated type parameter, "<String>", makes it a bit ugly to look at, this is still a nice example of the power of generic programming.

A HashSet stores its elements in a hash table, a type of data structure that I will discuss in the next section. The operations of finding, adding, and removing elements are implemented very efficiently in hash tables, even more so than for TreeSets. The elements of a HashSet are not stored in any particular order, and so do not need to implement the Comparable interface. (They do, however, need to define a proper "hash code," as we'll see in the next section.)

The equals() method is used to determine whether two objects in a HashSet are to be considered the same. An Iterator for a HashSet will visit its elements in what seems to be a completely arbitrary order, and it's possible for the order to change completely when a new element is added. Use a HashSet instead of a TreeSet when the elements it contains are not comparable, or when the order is not important, or when the small advantage in efficiency is important.

A note about the mathematics of sets: In mathematical set theory, the items in a set are called members or elements of that set. Important operations include adding an element to a set, removing an element from a set, and testing whether a given entity is an element of a set. Operations that can be performed on two sets include union, intersection, and set difference. All these operations are defined in Java for objects of type Set, but with different names. Suppose that A and B are Sets. Then:

- A.add(x) adds the element x to the set A.

- A.remove(x) removes the element x from the set A.

- A.contains(x) tests whether x is an element of the set A.

- A.addAll(B) computes the union of A and B.

- A.retainAll(B) computes the intersection of A and B.

- A.removeAll(B) computes the set difference, A - B.

There are of course, differences between mathematical sets and sets in Java. Most important, perhaps, sets in Java must be finite, while in mathematics, most of the fun in set theory comes from working with infinity. In mathematics, a set can contain arbitrary elements, while in Java, a set of type Set<T> can only contain elements of type T. The operation A.addAll(B) acts by modifying the value of A, while in mathematics the operation A union B computes a new set, without changing the value of A or B. See Exercise 10.2 for an example of mathematical set operations in Java.

10.2.4  EnumSet

Enumerated types (or "enums") were introduced in Subsection 2.3.4. Suppose that E is an enumerated type. Since E is a class, it is possible to create objects of type TreeSet<E> and HashSet<E>. However, because enums are so simple, trees and hash tables are not the most efficient implementation for sets of enumerated type values. Java provides the class java.util.EnumSet as an alternative way to create such sets.

Sets of enumerated type values are created using static methods in the class EnumSet. For example, if e1, e2, and e3 are values belonging to the enumerated type E, then the method

EnumSet.of( e1, e2, e3 )

creates and returns a set of type EnumSet<E> that contains exactly the elements e1, e2, and e3. The set implements the interface Set<E>, so all the usual set and collection operations are available. The implementation of these operations is very efficient. The implementation uses what is called a bit vector. A bit is a quantity that has only two possible values, zero and one. A set of type EnumSet<E> is represented by a bit vector that contains one bit for each enum constant in the enumerated type E; the bit corresponding to the enum constant e is 1 if e is a member of the set and is 0 if e is not a member of the set. The bit vectors for two sets of type EnumSet<E> can be very easily combined to represent such operations as the union and intersection of two sets. The bit vector representation is feasible for EnumSets, but not for other sets in Java, because an enumerated type contains only a small finite number of enum constants. (Java actually has a class named BitSet that uses bit vectors to represent finite sets of non-negative integers, but this class is not part of the Java Collection Framework and does not implement the Set interface.)

The function EnumSet.of() can be used with any positive number of parameters. All the parameters must be values of the same enumerated type. Null values are not allowed. An EnumSet cannot contain the value null -- any attempt to add null to an EnumSet will result in a NullPointerException.

There is also a function EnumSet.range(e1,e2) that returns an EnumSet consisting of the enum constants between e1 and e2, inclusive. The ordering of enum constants is the same as the order in which they are listed in the definition of the enum. In EnumSet.range(e1,e2), e1 and e2 must belong to the same enumerated type, and e1 must be less than or equal to e2.

If E is an enum, then EnumSet.allOf(E.class) is a set that contains all values of type E. EnumSet.noneOf(E.class) is an empty set, a set of type EnumSet<E> that contains no elements at all. Note that in EnumSet.allOf(E.class) and EnumSet.noneOf(E.class), the odd-looking parameter represents the enumerated type class itself. If eset is a set of type EnumSet<E>, then EnumSet.complementOf(eset) is a set that contains all the enum constants of E that are not in eset.

As an example, consider a program that keeps schedules of events. The program must keep track of repeating events that happen on specified days of the week. For example, an event might take place only on weekdays, or only on Wednesdays and Fridays. In other words, associated with the event is the set of days of the week on which it takes place. This information can be represented using the enumerated type

enum Day { SUNDAY, MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY }

The days of the week on which an event takes place would then be a value of type EnumSet<Day>. An object of type RepeatingEvent would have an instance variable of type EnumSet<Day> to hold this information. An event that takes place on Wednesdays and Fridays would have the associated set

EnumSet.of( Day.WEDNESDAY, Day.FRIDAY )

We could define some common sets of Days as

EnumSet<Day> weekday = EnumSet.range( Day.MONDAY, Day.FRIDAY );
EnumSet<Day> weekend = EnumSet.complementOf( weekday );
EnumSet<Day> everyday = EnumSet.allOf( Day.class );

EnumSets are often used to specify sets of "options" that are to be applied during some type of processing. For example, a program that draws characters in fancy fonts might have various options that can be applied. Let's say that the options are bold, italic, underlined, strikethrough, and boxed. Note that we are assuming that options can be combined in arbitrary ways. For example, you can have italic, boxed, underlined characters. This just means that we need to keep track of a set of options. If the options are represented by the enumerated type

enum FontOption { BOLD, ITALIC, UNDERLINED, STRIKETHROUGH, BOXED }

then a set of options is represented by a value of type EnumSet<FontOption>. Suppose that options is a variable of this type that represents the set of options that are currently being applied by the program. Then we can do things like:

- options = EnumSet.noneOf( FontOption.class ) -- Turn off all options.

- options = EnumSet.of( FontOption.BOLD ) -- Use bold, with no other options.

- options.add( FontOption.BOLD ) -- Add bold to any options that are already on.

- options.remove( FontOption.UNDERLINED ) -- Turn underlining off (if it's on).

This is a nice, safe way to work with sets of options. Applications like this are one of the major reasons that enumerated types were introduced.

10.2.5  Priority Queues

A priority queue is an ADT that represents a collection of items, where each item has an assigned "priority" that allows any two items to be compared. Operations on a priority queue include add, which adds an item to the collection, and remove, which removes and returns an item from the collection that has the minimum priority among items currently in the collection. (Maximum priority would also be possible, but in Java's version, the remove operation removes a minimum priority item.)

A simple implementation of priority queue could be achieved by using a linked list to store the items in the queue in order of increasing priority. In that case, remove would simply remove and return the first item in the list. However, add would have to insert the new item into its correct position in the list, an operation with average run time Θ(n), where n is the number of items in the list. In fact, priority queues can be implemented so that both add and remove have run time Θ(log(n)), which is much more efficient. (The efficient implementation uses something called a "heap," which is not to be confused with the heap where objects are created. I will not discuss the implementation here.)

The parameterized class PriorityQueue<T> implements a priority queue of objects of type T. This class implements the interface Collection<T>. So, if pq is a PriorityQueue, then it has all the methods defined in that interface. But the essential priority queue operations are

- pq.add(obj) -- adds obj to the priority queue, where obj must be an object of type T

- pq.remove() -- removes and returns an item of minimal priority. The return value is an object of type T. Throws an exception if the queue is empty.

- pq.isEmpty() -- tests whether the priority queue is empty.

You've probably noticed that I haven't yet mentioned how the priority of items in the priority queue are determined. The situation is much like sorting: We need to be able to compare any two items in the queue. As with sorting, there are two solutions. If the items implement the Comparable interface, then they can be compared using the compareTo() method from that interface. Alternatively, a Comparator object can be provided as a parameter to the PriorityQueue constructor. In that case, the Comparator's compare method will be used to compare items.

Classes such as String, Integer, and Date that implement Comparable can be used in a priority queue. For example, a PriorityQueue<String> can be used to sort strings into lexicographic order: Just add all the strings to the priority queue, then remove them one-by-one. Since items are removed from a queue in order of priority, they will be removed in lexicographic order. Earlier, I showed how to use a TreeSet to sort and remove duplicates from a collection. A PriorityQueue can be used in a similar way to sort a collection without removing duplicates. For example, if coll is of type Collection<String>, then the following code segment will print all the items from coll in order, including duplicates:

PriorityQueue<String> pq = new PriorityQueue<String>();
pq.addAll( coll );
while ( ! pq.isEmpty() ) {
System.out.println( pq.remove() );
}

(Note, by the way, that we can't use an iterator or a for-each loop to print the items in this example, since interators and for-each loops do not traverse a priority queue in ascending order.)

The sample program WordListWithPriorityQueue.java makes a sorted list of words from a file without removing duplicates, using a priority queue to hold the words. It is a minor modification of WordListWithTreeSet.java.

Although priority queues can be used for sorting, they have other natural applications. For example, consider the problem of scheduling "jobs" to be executed on a computer, where each job is assigned a priority and jobs with lower priority should always be executed before jobs with higher priority. Jobs can be placed into a priority queue as they are created. When the computer removes jobs from the queue for execution, they will be removed in order of increasing priority.

Section 10.3

Maps

An array of N elements can be thought of as a way of associating some item with each of the integers 0, 1, ..., N-1. If i is one of these integers, it's possible to get the item associated with i, and it's possible to put a new item in the i-th position. These "get" and "put" operations define what it means to be an array.

A map is a kind of generalized array. Like an array, a map is defined by "get" and "put" operations. But in a map, these operations are defined not for integers 0, 1, ..., N-1, but for arbitrary objects of some specified type T. Associated to these objects of type T are objects of some possibly different type S.

In fact, some programming languages use the term associative array instead of "map" and use the same notation for associative arrays as for regular arrays. In those languages, for example, you might see the notation A["fred"] used to indicate the item associated to the string "fred" in an associative array A. Java does not use array notation for maps, unfortunately, but the idea is the same: A map is like an array, but the indices for a map are objects, not integers. In a map, an object that serves as an "index" is called a key. The object that is associated with a key is called a value. Note that a key can have at most one associated value, but the same value can be associated to several different keys. A map can be considered to be a set of "associations," where each association is a key/value pair.

10.3.1  The Map Interface

In Java, maps are defined by the interface java.util.Map, which includes put and get methods as well as other general methods for working with maps. The map interface, Map<K,V>, is parameterized by two types. The first type parameter, K, specifies the type of objects that are possible keys in the map; the second type parameter, V, specifies the type of objects that are possible values in the map. For example, a map of type Map<Date,JButton> would associate values of type JButton to keys of type Date. For a map of type Map<String,String>, both the keys and the values are of type String.

Suppose that map is a variable of type Map<K,V> for some specific types K and V. Then the following are some of the methods that are defined for map:

- map.get(key) -- returns the object of type V that is associated by the map to the key. If the map does not associate any value with key, then the return value is null. Note that it's also possible for the return value to be null when the map explicitly associates the value null with the key. Referring to "map.get(key)" is similar to referring to "A[key]" for an array A. (But note that there is nothing like an IndexOutOfBoundsException for maps.)

- map.put(key,value) -- Associates the specified value with the specified key, where key must be of type K and value must be of type V. If the map already associated some other value with the key, then the new value replaces the old one. This is similar to the command "A[key] = value" for an array.

- map.putAll(map2) -- if map2 is another map of type Map<K,V>, this copies all the associations from map2 into map.

- map.remove(key) -- if map associates a value to the specified key, that association is removed from the map.

- map.containsKey(key) -- returns a boolean value that is true if the map associates some value to the specified key.

- map.containsValue(value) -- returns a boolean value that is true if the map associates the specified value to some key.

- map.size() -- returns an int that gives the number of key/value associations in the map.

- map.isEmpty() -- returns a boolean value that is true if the map is empty, that is if it contains no associations.

- map.clear() -- removes all associations from the map, leaving it empty.

The put and get methods are certainly the most commonly used of the methods in the Map interface. In many applications, these are the only methods that are needed, and in such cases a map is really no more difficult to use than a standard array.

Java includes two classes that implement the interface Map<K,V>: TreeMap<K,V> and HashMap<K,V>. In a TreeMap, the key/value associations are stored in a sorted tree, in which they are sorted according to their keys. For this to work, it must be possible to compare the keys to one another. This means either that the keys must implement the interface Comparable<K>, or that a Comparator must be provided for comparing keys. (The Comparator can be provided as a parameter to the TreeMap constructor.) Note that in a TreeMap, as in a TreeSet, the compareTo() (or compare()) method is used to decide whether two keys are to be considered the same. This can have undesirable consequences if the compareTo() method does not agree with the usual notion of equality, and you should keep this in mind when using TreeMaps.

A HashMap does not store associations in any particular order, so the keys that can be used in a HashMap do not have to be comparable. However, the key class should have reasonable definitions for the equals() method and for a hashCode() method that is discussed later in this section; most of Java's standard classes define these methods correctly. Most operations are a little faster on HashMaps than they are on TreeMaps. In general, you should use a HashMap unless you have some particular need for the ordering property of a TreeMap. In particular, if you are only using the put and get operations, you can safely use a HashMap.

Let's consider an example where maps would be useful. In Subsection 7.4.2, I presented a simple PhoneDirectory class that associated phone numbers with names. That class defined operations addEntry(name,number) and getNumber(name), where both name and number are given as Strings. In fact, the phone directory is acting just like a map, with the addEntry method playing the role of the put operation and getNumber playing the role of get. In a real programming application, there would be no need to define a new class; we could simply use a map of type Map<String,String>. A directory could be defined as

Map<String,String> directory = new TreeMap<String,String>();

(using TreeMap so that the entries are kept in sorted order by name). Then directory.put(name,number) would record a phone number in the directory and directory.get(name) would retrieve the phone number associated with a given name.

10.3.2  Views, SubSets, and SubMaps

A Map is not a Collection, and maps do not implement all the operations defined on collections. In particular, there are no iterators for maps. Sometimes, though, it's useful to be able to iterate through all the associations in a map. Java makes this possible in a roundabout but clever way. If map is a variable of type Map<K,V>, then the method

map.keySet()

returns the set of all objects that occur as keys for associations in the map. The value returned by this method is an object that implements the interface Set<K>. The elements of this set are the map's keys. The obvious way to implement the keySet() method would be to create a new set object, add all the keys from the map, and return that set. But that's not how it's done. The value returned by map.keySet() is not an independent object. It is what is called a view of the actual objects that are stored in the map. This "view" of the map implements the Set<K> interface, but it does it in such a way that the methods defined in the interface refer directly to keys in the map. For example, if you remove a key from the view, that key -- along with its associated value -- is actually removed from the map. It's not legal to add an object to the view, since it doesn't make sense to add a key to a map without specifying the value that should be associated to the key. Since map.keySet() does not create a new set, it's very efficient, even for very large maps.

One of the things that you can do with a Set is get an Iterator for it and use the iterator to visit each of the elements of the set in turn. We can use an iterator for the key set of a map to traverse the map. For example, if map is of type Map<String,Double>, we could write:

Set<String> keys = map.keySet();     // The set of keys in the map.
Iterator<String> keyIter = keys.iterator();
System.out.println("The map contains the following associations:");
while (keyIter.hasNext()) {
String key = keyIter.next();  // Get the next key.
Double value = map.get(key);  // Get the value for that key.
System.out.println( "   (" + key + "," + value + ")" );
}

Or we could do the same thing more easily, avoiding the explicit use of an iterator, with a for-each loop:

System.out.println("The map contains the following associations:");
for ( String key : map.keySet() ) {  // "for each key in the map's key set"
Double value = map.get(key);
System.out.println( "   (" + key + "," + value + ")" );
}

If the map is a TreeMap, then the key set of the map is a sorted set, and the iterator will visit the keys in ascending order. For a HashMap, the keys are visited in an arbitrary, unpredictable order.

The Map interface defines two other views. If map is a variable of type Map<K,V>, then the method:

map.values()

returns an object of type Collection<V> that contains all the values from the associations that are stored in the map. The return value is a Collection rather than a Set because it can contain duplicate elements (since a map can associate the same value to any number of keys). The method:

map.entrySet()

returns a set that contains all the associations from the map. The elements in the set are objects of type Map.Entry<K,V>. Map.Entry<K,V> is defined as a static nested interface inside the interface Map<K,V>, so its full name contains a period. However, the name can be used in the same way as any other type name. (The return type of the method map.entrySet() is written as Set<Map.Entry<K,V>>. The type parameter in this case is itself a parameterized type. Although this might look confusing, it's just Java's way of saying that the elements of the set are of type Map.Entry<K,V>.) The information in the set returned by map.entrySet() is actually no different from the information in the map itself, but the set provides a different view of this information, with different operations. Each Map.Entry object contains one key/value pair, and defines methods getKey() and getValue() for retrieving the key and the value. There is also a method, setValue(value), for setting the value; calling this method for a Map.Entry object will modify the map itself, just as if the map's put method were called. As an example, we can use the entry set of a map to print all the key/value pairs in the map. This is more efficient than using the key set to print the same information, as I did in the above example, since we don't have to use the get() method to look up the value associated with each key. Suppose again that map is of type Map<String,Double>. Then we can write:

Set<Map.Entry<String,Double>> entries = map.entrySet();
Iterator<Map.Entry<String,Double>> entryIter = entries.iterator();
System.out.println("The map contains the following associations:");
while (entryIter.hasNext()) {
Map.Entry<String,Double> entry = entryIter.next();
String key = entry.getKey();  // Get the key from the entry.
Double value = entry.getValue();  // Get the value.
System.out.println( "   (" + key + "," + value + ")" );
}

or, using a for-each loop:

System.out.println("The map contains the following associations:");
for ( Map.Entry<String,Double> entry : map.entrySet() ) {
System.out.println( "   (" + entry.getKey() + "," + entry.getValue() + ")" );
}

Maps are not the only place in Java's generic programming framework where views are used. For example, the interface List<T> defines a sublist as a view of a part of a list. If list implements the interface List<T>, then the method

list.subList( fromIndex, toIndex )

where fromIndex and toIndex are integers, returns a view of the part of the list consisting of the list elements in positions between fromIndex and toIndex (including fromIndex but excluding toIndex). This view lets you operate on the sublist using any of the operations defined for lists, but the sublist is not an independent list. Changes made to the sublist are actually made to the original list.

Similarly, it is possible to obtain views that represent certain subsets of a sorted set. If set is of type TreeSet<T>, then set.subSet(fromElement,toElement) returns a Set<T> that contains all the elements of set that are between fromElement and toElement (including fromElement and excluding toElement). The parameters fromElement and toElement must be objects of type T. For example, if words is a set of type TreeSet<String> in which all the elements are strings of lower case letters, then words.subSet("m","n") contains all the elements of words that begin with the letter 'm'. This subset is a view of part of the original set. That is, creating the subset does not involve copying elements. And changes made to the subset, such as adding or removing elements, are actually made to the original set. The view set.headSet(toElement) consists of all elements from the set which are strictly less than toElement, and set.tailSet(fromElement) is a view that contains all elements from the set that are greater than or equal to fromElement.

The class TreeMap<K,V> defines three submap views. A submap is similar to a subset. A submap is a Map that contains a subset of the keys from the original Map, along with their associated values. If map is a variable of type TreeMap<K,V>, and if fromKey and toKey are of type K, then map.subMap(fromKey,toKey) returns a view that contains all key/value pairs from map whose keys are between fromKey and toKey (including fromKey and excluding toKey). There are also views map.headMap(toKey) and map.tailMap(fromKey) which are defined analogously to headSet and tailSet. Suppose, for example, that blackBook is a map of type TreeMap<String,String> in which the keys are names and the values are phone numbers. We can print out all the entries from blackBook where the name begins with "M" as follows:

Map<String,String> ems = blackBook.subMap("M","N");
// This submap contains entries for which the key is greater
// than or equal to "M" and strictly less than "N".

if (ems.isEmpty()) {
System.out.println("No entries beginning with M.");
}
else {
System.out.println("Entries beginning with M:");
for ( Map.Entry<String,String> entry : ems.entrySet() )
System.out.println( "   " + entry.getKey() + ": " + entry.getValue() );
}

Subsets and submaps are probably best thought of as generalized search operations that make it possible to find all the items in a range of values, rather than just to find a single value. Suppose, for example that a database of scheduled events is stored in a map of type TreeMap<Date,Event> in which the keys are the times of the events, and suppose you want a listing of all events that are scheduled for some time on July 4, 2014. Just make a submap containing all keys in the range from 12:00 AM, July 4, 2014 to 12:00 AM, July 5, 2014, and output all the entries from that submap. This type of search, which is known as a subrange query is quite common.

10.3.3  Hash Tables and Hash Codes

HashSets and HashMaps are implemented using a data structure known as a hash table. You don't need to understand hash tables to use HashSets or HashMaps, but any computer programmer should be familiar with hash tables and how they work.

Hash tables are an elegant solution to the search problem. A hash table, like a HashMap, stores key/value pairs. Given a key, you have to search the table for the corresponding key/value pair. When a hash table is used to implement a set, there are no values, and the only question is whether or not the key occurs in the set. You still have to search for the key to check whether it is there or not.

In most search algorithms, in order to find the item you are interested in, you have to look through a bunch of other items that don't interest you. To find something in an unsorted list, you have to go through the items one-by-one until you come to the one you are looking for. In a binary sort tree, you have to start at the root and move down the tree until you find the item you want. When you search for a key/value pair in a hash table, you can go directly to the location that contains the item you want. You don't have to look through any other items. (This is not quite true, but it's close.) The location of the key/value pair is computed from the key: You just look at the key, and then you go directly to the location where it is stored.

How can this work? If the keys were integers in the range 0 to 99, we could store the key/value pairs in an array, A, of 100 elements. The key/value pair with key K would be stored in A[K]. The key takes us directly to the location of the key/value pair. The problem is that there are usually far too many different possible keys for us to be able to use an array with one location for each possible key. For example, if the key can be any value of type int, then we would need an array with over four billion locations -- quite a waste of space if we are only going to store, say, a few thousand items! If the key can be a string of any length, then the number of possible keys is infinite, and using an array with one location for each possible key is simply impossible.

Nevertheless, hash tables store their data in an array, and the array index where a key is stored is based on the key. The index is not equal to the key, but it is computed from the key. The array index for a key is called the hash code for that key. A function that computes a hash code, given a key, is called a hash function. To find a key in a hash table, you just have to compute the hash code of the key and go directly to the array location given by that hash code. If the hash code is 17, look in array location number 17.

Now, since there are fewer array locations than there are possible keys, it's possible that we might try to store two or more keys in the same array location. This is called a collision. A collision is not an error. We can't reject a key just because another key happened to have the same hash code. A hash table must be able to handle collisions in some reasonable way. In the type of hash table that is used in Java, each array location actually holds a linked list of key/value pairs (possibly an empty list). When two items have the same hash code, they are in the same linked list. The structure of the hash table looks something like this:

In this diagram, there are two items with hash code 0, no items with hash code 1, one item with hash code 2, and so on. In a properly designed hash table, most of the linked lists are of length zero or one, and the average length of the lists is less than one. Although the hash code of a key doesn't necessarily take you directly to that key, there are probably no more than one or two other items that you have to look through before finding the key you want. For this to work properly, the number of items in the hash table should be somewhat less than the number of locations in the array. In Java's implementation, whenever the number of items exceeds 75% of the array size, the array is replaced by a larger one and all the items in the old array are inserted into the new one. (This is why adding one new item will sometimes cause the ordering of all the items in the hash table to change completely.)

There is still the question of where hash codes come from. Every object in Java has a hash code. The Object class defines the method hashCode(), which returns a value of type int. When an object, obj, is stored in a hash table that has N locations, a hash code in the range 0 to N-1 is needed. This hash code is computed as Math.abs(obj.hashCode()) % N, the remainder when the absolute value of obj.hashCode() is divided by N. (The Math.abs is necessary because obj.hashCode() can be a negative integer, and we need a non-negative number to use as an array index.)

For hashing to work properly, two objects that are equal according to the equals() method must have the same hash code. In the Object class, this condition is satisfied because both equals() and hashCode() are based on the address of the memory location where the object is stored. However, as noted in Subsection 10.1.6, many classes redefine the equals() method. If a class redefines the equals() method, and if objects of that class will be used as keys in hash tables, then the class must also redefine the hashCode() method. For example, in the String class, the equals() method is redefined so that two objects of type String are considered to be equal if they contain the same sequence of characters. The hashCode() method is also redefined in the String class, so that the hash code of a string is computed from the characters in that string rather than from its location in memory. For Java's standard classes, you can expect equals() and hashCode() to be correctly defined. However, you might need to define these methods in classes that you write yourself.

Writing a good hash function is something of an art. In order to work well, the hash function must spread the possible keys fairly evenly over the hash table. Otherwise, the items in a table can be concentrated in a subset of the available locations, and the linked lists at those locations can grow to large size; that would destroy the efficiency that is the major reason for hash tables to exist in the first place. However, I won't cover techniques for creating good hash functions in this book.

Section 10.4

Programming with the Java Collection Framework

In this section, we'll look at some programming examples that use classes from the Java Collection Framework. The Collection Framework is easy to use, especially compared to the difficulty of programming new data structures from scratch.

10.4.1  Symbol Tables

We begin with a straightforward but important application of maps. When a compiler reads the source code of a program, it encounters definitions of variables, subroutines, and classes. The names of these things can be used later in the program. The compiler has to remember the definition of each name, so that it can recognize the name and apply the definition when the name is encountered later in the program. This is a natural application for a Map. The name can be used as a key in the map. The value associated to the key is the definition of the name, encoded somehow as an object. A map that is used in this way is called a symbol table.

In a compiler, the values in a symbol table can be quite complicated, since the compiler has to deal with names for various sorts of things, and it needs a different type of information for each different type of name. We will keep things simple by looking at a symbol table in another context. Suppose that we want a program that can evaluate expressions entered by the user, and suppose that the expressions can contain variables, in addition to operators, numbers, and parentheses. For this to make sense, we need some way of assigning values to variables. When a variable is used in an expression, we need to retrieve the variable's value. A symbol table can be used to store the data that we need. The keys for the symbol table are variable names. The value associated with a key is the value of that variable, which is of type double. The symbol table will be an object of type Map<String,Double>. (Remember that primitive types such as double can't be used as type parameters; a wrapper class such as Double must be used instead. See Subsection 10.1.7.)

To demonstrate the idea, we'll use a rather simple-minded program in which the user types commands such as:

let x = 3 + 12
print 2 + 2
print 10*x +17
let rate = 0.06
print 1000*(1+rate)

The program is an interpreter for a very simple language. The only two commands that the program understands are "print" and "let". When a "print" command is executed, the computer evaluates the expression and displays the value. If the expression contains a variable, the computer has to look up the value of that variable in the symbol table. A "let" command is used to give a value to a variable. The computer has to store the value of the variable in the symbol table. (Note: The "variables" I am talking about here are not variables in the Java program. The Java program is executing a sort of program typed in by the user. I am talking about variables in the user's program. The user gets to make up variable names, so there is no way for the Java program to know in advance what the variables will be.)

In Subsection 9.5.2, we saw how to write a program, SimpleParser2.java, that can evaluate expressions that do not contain variables. Here, I will discuss another example program, SimpleInterpreter.java, that is based on the older program. I will only talk about the parts that are relevant to the symbol table.

The program uses a HashMap as the symbol table. A TreeMap could also be used, but since the program does not need to access the variables in alphabetical order, we don't need to have the keys stored in sorted order. The symbol table in the program is represented by a variable named symbolTable of type HashMap<String,Double>. At the beginning of the program, the symbol table object is created with the command:

symbolTable = new HashMap<String,Double>();

This creates a map that initially contains no key/value associations. To execute a "let" command, the program uses the symbol table's put() method to associate a value with the variable name. Suppose that the name of the variable is given by a String, varName, and the value of the variable is stored in a variable val of type double. The following command would then set the value associated with the variable in the symbol table:

symbolTable.put( varName, val );

In the program SimpleInterpreter.java, you'll find this in the method named doLetCommand(). The actual value that is stored in the symbol table is an object of type Double. We can use the double value val in the call to put because Java does an automatic conversion of type double to Double when necessary. The double value is "wrapped" in an object of type Double, so that, in effect, the above statement is equivalent to

symbolTable.put( varName, new Double(val) );

Just for fun, I decided to pre-define two variables named "pi" and "e" whose values are the usual mathematical constants π and e. In Java, the values of these constants are given by Math.PI and Math.E. To make these variables available to the user of the program, they are added to the symbol table with the commands:

symbolTable.put( "pi", Math.PI );
symbolTable.put( "e", Math.E );

When the program encounters a variable while evaluating an expression, the symbol table's get() method is used to retrieve its value. The function symbolTable.get(varName) returns a value of type Double. It is possible that the return value is null; this will happen if no value has ever been assigned to varName in the symbol table. It's important to check this possibility. It indicates that the user is trying to use a variable that the user has not defined. The program considers this to be an error, so the processing looks something like this:

Double val = symbolTable.get(varName);
if (val == null) {
... // Throw an exception:  Undefined variable.
}
// The value associated to varName is val.doubleValue()

You will find this code, more or less, in a method named primaryValue() in SimpleInterpreter.java.

As you can see from this example, Maps are very useful and are really quite easy to use.

10.4.2  Sets Inside a Map

The objects in a collection or map can be of any type. They can even be collections. Here's an example where it's natural to store sets as the value objects in a map.

Consider the problem of making an index for a book. An index consists of a list of terms that appear in the book. Next to each term is a list of the pages on which that term appears. To represent an index in a program, we need a data structure that can hold a list of terms, along with a list of pages for each term. Adding new data should be easy and efficient. When it's time to print the index, it should be easy to access the terms in alphabetical order. There are many ways this could be done, but I'd like to use Java's generic data structures and let them do as much of the work as possible.

We can think of an index as a Map that associates a list of page references to each term. The terms are keys, and the value associated with a given key is the list of page references for that term. A Map can be either a TreeMap or a HashMap, but only a TreeMap will make it easy to access the terms in sorted order. The value associated with a term is a list of page references. How can we represent such a value? If you think about it, you see that it's not really a list in the sense of Java's generic classes. If you look in any index, you'll see that a list of page references has no duplicates, so it's really a set rather than a list. Furthermore, the page references for a given term are always printed in increasing order, so we want a sorted set. This means that we should use a TreeSet to represent each list of page references. The values that we really want to put in this set are of type int, but once again we have to deal with the fact that generic data structures can only hold objects, so we must use the wrapper class, Integer, for the objects in the set.

To summarize, an index will be represented by a TreeMap. The keys for the map will be terms, which are of type String. The values in the map will be TreeSets that contain Integers that are the page numbers of every page on which a term appears. The parameterized type that we should use for the sets is TreeSet<Integer>. For the TreeMap that represents the index as a whole, the key type is String and the value type is TreeSet<Integer>. This means that the index has type

TreeMap< String, TreeSet<Integer> >

This is just the usual TreeMap<K,V> with K=String and V=TreeSet<Integer>. A type name as complicated as this one can look intimidating (especially, I think, when used in a constructor with the new operator), but if you think about the data structure that we want to represent, it makes sense. Given a little time and practice, you can get used to types like this one.

To make an index, we need to start with an empty TreeMap and look through the book, inserting every reference that we want to be in the index into the map. We then need to print out the data from the map. Let's leave aside the question of how we find the references to put in the index, and just look at how the TreeMap is used. It can be created with the commands:

TreeMap<String,TreeSet<Integer>>  index;         // Declare the variable.
index = new TreeMap<String,TreeSet<Integer>>();  // Create the map object.

Now, suppose that we find a reference to some term (of type String) on some pageNum (of type int). We need to insert this information into the index. To do this, we should look up the term in the index, using index.get(term). The return value is either null or is the set of page references that we have previously found for the term. If the return value is null, then this is the first page reference for the term, so we should add the term to the index, with a new set that contains the page reference we've just found. If the return value is non-null, we already have a set of page references, and we should just add the new page reference to the set. Here is a subroutine that does this:

/**
* Add a page reference to the index.
*/
void addReference(String term, int pageNum) {
TreeSet<Integer> references; // The set of page references that we
//    have so far for the term.
references = index.get(term);
if (references == null){
// This is the first reference that we have
// found for the term.  Make a new set containing
// the page number and add it to the index, with
// the term as the key.
TreeSet<Integer> firstRef = new TreeSet<Integer>();
firstRef.add( pageNum );  // pageNum is "autoboxed" to give an Integer!
index.put(term,firstRef);
}
else {
// references is the set of page references
// that we have found previously for the term.
// Add the new page number to that set.  This
// set is already associated to term in the index.
references.add( pageNum ); // pageNum is "autoboxed" to give an Integer!
}
}

The only other thing we need to do with the index is print it out. We want to iterate through the index and print out each term, together with the set of page references for that term. We could use an Iterator to iterate through the index, but it's much easier to do it with a for-each loop. The loop will iterate through the entry set of the map (see Subsection 10.3.2). Each "entry" is a key/value pair from the map; the key is a term and the value is the associated set of page references. Inside the for-each loop, we will have to print out a set of Integers, which can also be done with a for-each loop. So, here we have an example of nested for-each loops. (You might try to do the same thing entirely with iterators; doing so should give you some appreciation for the for-each loop!) Here is a subroutine that will print the index:

/**
* Print each entry in the index.
*/
void printIndex() {

for ( Map.Entry<String,TreeSet<Integer>>  entry :  index.entrySet() ) {

String term = entry.getKey();
TreeSet<Integer> pageSet = entry.getValue();

System.out.print( term + " " );
for ( int page : pageSet ) {
System.out.print( page + " " );
}
System.out.println();

}

}

The hardest thing here is the name of the type Map.Entry<String,TreeSet<Integer>>! Remember that the entries in a map of type Map<K,V> have type Map.Entry<K,V>, so the type parameters in Map.Entry<String,TreeSet<Integer>> are simply copied from the declaration of index. Another thing to note is that I used a loop control variable, page, of type int to iterate through the elements of pageSet, which is of type TreeSet<Integer>. You might have expected page to be of type Integer, not int, and in fact Integer would have worked just as well here. However, int does work, because of automatic type conversion: It's legal to assign a value of type Integer to a variable of type int. (To be honest, I was sort of surprised that this worked when I first tried it!)

This is not a lot of code, considering the complexity of the operations. I have not written a complete indexing program, but Exercise 10.5 presents a problem that is almost identical to the indexing problem.

By the way, in this example, I would prefer to print each list of page references with the integers separated by commas. In the printIndex() method given above, they are separated by spaces. There is an extra space after the last page reference in the list, but it does no harm since it's invisible in the printout. An extra comma at the end of the list would be annoying. The lists should be in a form such as "17,42,105" and not "17,42,105,". The problem is, how to leave that last comma out. Unfortunately, this is not so easy to do with a for-each loop. It might be fun to look at a few ways to solve this problem. One alternative is to use an iterator:

Iterator<Integer>  iter = pageSet.iterator();
int firstPage = iter.next();  // In this program, we know the set has at least
// one element.
System.out.print(firstPage);
while ( iter.hasNext() ) {
int nextPage = iter.next();
System.out.print("," + nextPage);
}

Another possibility is to use the fact that the TreeSet class defines a method first() that returns the first item in the set, that is, the one that is smallest in terms of the ordering that is used to compare items in the set. (It also defines the method last().) We can solve our problem using this method and a for-each loop:

int firstPage = pageSet.first();  // Find out the first page number in the set.
for ( int page : pageSet ) {
if ( page != firstPage )
System.out.print(","); // Output comma only if this is not the first page.
System.out.print(page);
}

Finally, here is an elegant solution using a subset view of the tree. (See Subsection 10.3.2.) Actually, this solution might be a bit extreme:

int firstPage = pageSet.first();  // Get first item, which we know exists.
System.out.print(firstPage);      // Print first item, with no comma.
for ( int page : pageSet.tailSet( firstPage+1 ) ) // Process remaining items.
System.out.print( "," + page );

10.4.3  Using a Comparator

There is a potential problem with our solution to the indexing problem. If the terms in the index can contain both upper case and lower case letters, then the terms will not be in alphabetical order! The ordering on String is not alphabetical. It is based on the Unicode codes of the characters in the string. The codes for all the upper case letters are less than the codes for the lower case letters. So, for example, terms beginning with "Z" come before terms beginning with "a". If the terms are restricted to use lower case letters only (or upper case only), then the ordering would be alphabetical. But suppose that we allow both upper and lower case, and that we insist on alphabetical order. In that case, our index can't use the usual ordering for Strings. Fortunately, it's possible to specify a different method to be used for comparing the keys of a map. This is a typical use for a Comparator.

Recall that an object that implements the interface Comparator<T> defines a method for comparing two objects of type T:

public int compare( T obj1, T obj2 )

This method should return an integer that is negative, zero, or positive, depending on whether obj1 is less than, equal to, or greater than obj2. We need an object of type Comparator<String> that will compare two Strings based on alphabetical order. The easiest way to do this is to convert the Strings to lower case and use the default comparison on the lower case Strings. The following class defines such a comparator:

/**
* Represents a Comparator that can be used for comparing two
* strings based on alphabetical order.
*/
class AlphabeticalOrder implements Comparator<String> {
public int compare(String str1, String str2) {
String s1 = str1.toLowerCase();  // Convert to lower case.
String s2 = str2.toLowerCase();
return s1.compareTo(s2);  // Compare lower-case Strings.
}
}

To solve our indexing problem, we just need to tell our index to use an object of type AlphabeticalOrder for comparing keys. This is done by providing a Comparator object as a parameter to the constructor. We just have to create the index in our example with the command:

index = new TreeMap<String,TreeSet<Integer>>( new AlphabeticalOrder() );

This does work. However, I've been concealing one technicality. Suppose, for example, that the indexing program calls addReference("aardvark",56) and that it later calls addReference("Aardvark",102). The words "aardvark" and "Aardvark" differ only in that one of them begins with an upper case letter; when converted to lower case, they are the same. When we insert them into the index, do they count as two different terms or as one term? The answer depends on the way that a TreeMap tests objects for equality. In fact, TreeMaps and TreeSets always use a Comparator object or a compareTo method to test for equality. They do not use the equals() method for this purpose. The Comparator that is used for the TreeMap in this example returns the value zero when it is used to compare "aardvark" and "Aardvark", so the TreeMap considers them to be the same. Page references to "aardvark" and "Aardvark" are combined into a single list, and when the index is printed it will contain only the first version of the word that was encountered by the program. This is probably acceptable behavior in this example. If not, some other technique must be used to sort the terms into alphabetical order.

10.4.4  Word Counting

The final example in this section also deals with storing information about words. The problem here is to make a list of all the words that occur in a file, along with the number of times that each word occurs. The file will be selected by the user. The output of the program will consist of two lists. Each list contains all the words from the file, along with the number of times that the word occurred. One list is sorted alphabetically, and the other is sorted according to the number of occurrences, with the most common words at the top and the least common at the bottom. The problem here is a generalization of Exercise 7.7, which asked you to make an alphabetical list of all the words in a file, without counting the number of occurrences.

My word counting program can be found in the file WordCount.java. As the program reads an input file, it must keep track of how many times it encounters each word. We could simply throw all the words, with duplicates, into a list and count them later. But that would require a lot of extra storage space and would not be very efficient. A better method is to keep a counter for each word. The first time the word is encountered, the counter is initialized to 1. On subsequent encounters, the counter is incremented. To keep track of the data for one word, the program uses a simple class that holds a word and the counter for that word. The class is a static nested class:

/**
* Represents the data we need about a word:  the word and
* the number of times it has been encountered.
*/
private static class WordData {
String word;
int count;
WordData(String w) {
// Constructor for creating a WordData object when
// we encounter a new word.
word = w;
count = 1;  // The initial value of count is 1.
}
} // end class WordData

The program has to store all the WordData objects in some sort of data structure. We want to be able to add new words efficiently. Given a word, we need to check whether a WordData object already exists for that word, and if it does, we need to find that object so that we can increment its counter. A Map can be used to implement these operations. Given a word, we want to look up a WordData object in the Map. This means that the word is the key, and the WordData object is the value. (It might seem strange that the key is also one of the instance variables in the value object, but in fact this is probably the most common situation: The value object contains all the information about some entity, and the key is one of those pieces of information; the partial information in the key is used to retrieve the full information in the value object.) After reading the file, we want to output the words in alphabetical order, so we should use a TreeMap rather than a HashMap. This program converts all words to lower case so that the default ordering on Strings will put the words in alphabetical order. The data is stored in a variable named words of type TreeMap<String,WordData>. The variable is declared and the map object is created with the statement:

TreeMap<String,WordData> words = new TreeMap<String,WordData>();

When the program reads a word from a file, it calls words.get(word) to find out if that word is already in the map. If the return value is null, then this is the first time the word has been encountered, so a new WordData object is created and inserted into the map with the command words.put(word, new WordData(word)). If words.get(word) is not null, then its value is the WordData object for this word, and the program only has to increment the counter in that object. The program uses a method readNextWord(), which was given in Exercise 7.7, to read one word from the file. This method returns null when the end of the file is encountered. Here is the complete code segment that reads the file and collects the data:

String word = readNextWord();
while (word != null) {
word = word.toLowerCase();  // convert word to lower case
WordData data = words.get(word);
if (data == null)
words.put( word, new WordData(word) );
else
data.count++;
word = readNextWord();
}

After reading the words and printing them out in alphabetical order, the program has to sort the words by frequency and print them again. To do the sorting using a generic algorithm, I defined a simple Comparator class for comparing two word objects according to their frequency counts. The class implements the interface Comparator<WordData>, since it will be used to compare two objects of type WordData:

/**
* A comparator class for comparing objects of type WordData according to
* their counts.  This is used for sorting the list of words by frequency.
*/
private static class CountCompare implements Comparator<WordData> {
public int compare(WordData data1, WordData data2) {
return data2.count - data1.count;
// The return value is positive if data1.count < data2.count.
// I.E., data1 comes after data2 in the ordering if there
// were FEWER occurrences of data1.word than of data2.word.
// The words are sorted according to decreasing counts.
}
} // end class CountCompare

Given this class, we can sort the WordData objects according to frequency by first copying them into a list and then using the generic method Collections.sort(list,comparator). The WordData objects that we need are the values in the map, words. Recall that words.values() returns a Collection that contains all the values from the map. The constructor for the ArrayList class lets you specify a collection to be copied into the list when it is created. So, we can use the following commands to create a list of type ArrayList<WordData> containing the word data and then sort that list according to frequency:

ArrayList<WordData> wordsByFrequency = new ArrayList<WordData>( words.values() );
Collections.sort( wordsByFrequency, new CountCompare() );

You should notice that these two lines replace a lot of code! It requires some practice to think in terms of generic data structures and algorithms, but the payoff is significant in terms of saved time and effort.

The only remaining problem is to print the data. We have to print the data from all the WordData objects twice, first in alphabetical order and then sorted according to frequency count. The data is in alphabetical order in the TreeMap, or more precisely, in the values of the TreeMap. We can use a for-each loop to print the data in the collection words.values(), and the words will appear in alphabetical order. Another for-each loop can be used to print the data in the list wordsByFrequency, and the words will be printed in order of decreasing frequency. Here is the code that does it:

TextIO.putln("List of words in alphabetical order"
+ " (with counts in parentheses):\n");
for ( WordData data : words.values() )
TextIO.putln("   " + data.word + " (" + data.count + ")");

TextIO.putln("\n\nList of words by frequency of occurrence:\n");
for ( WordData data : wordsByFrequency )
TextIO.putln("   " + data.word + " (" + data.count + ")");

You can find the complete word-counting program in the file WordCount.java. Note that for reading and writing files, it uses the file I/O capabilities of TextIO.java, which were discussed in Subsection 2.4.4.

By the way, if you run the WordCount program on a reasonably large file and take a look at the output, it will illustrate something about the Collections.sort() method. The second list of words in the output is ordered by frequency, but if you look at a group of words that all have the same frequency, you will see that the words in that group are in alphabetical order. The method Collections.sort() was applied to sort the words by frequency, but before it was applied, the words were already in alphabetical order. When Collections.sort() rearranged the words, it did not change the ordering of words that have the same frequency, so they were still in alphabetical order within the group of words with that frequency. This is because the algorithm used by Collections.sort() is a stable sorting algorithm. A sorting algorithm is said to be stable if it satisfies the following condition: When the algorithm is used to sort a list according to some property of the items in the list, then the sort does not change the relative order of items that have the same value of that property. That is, if item B comes after item A in the list before the sort, and if both items have the same value for the property that is being used as the basis for sorting, then item B will still come after item A after the sorting has been done. Neither SelectionSort nor QuickSort are stable sorting algorithms. Insertion sort is stable, but is not very fast. Merge sort, the sorting algorithm used by Collections.sort(), is both stable and fast.

I hope that the programming examples in this section have convinced you of the usefulness of the Java Collection Framework!

Section 10.5

Writing Generic Classes and Methods

So far in this chapter, you have learned about using the generic classes and methods that are part of the Java Collection Framework. Now, it's time to learn how to write new generic classes and methods from scratch. Generic programming produces highly general and reusable code -- it's very useful for people who write reusable software libraries to know how to do generic programming, since it enables them to write code that can be used in many different situations. Not every programmer needs to write reusable software libraries, but every programmer should know at least a little about how to do it. In fact, just to read the Javadoc documentation for Java's standard generic classes, you need to know some of the syntax that is introduced in this section.

I will not cover every detail of generic programming in Java in this section, but the material presented here should be sufficient to cover the most common cases.

10.5.1  Simple Generic Classes

Let's start with an example that illustrates the motivation for generic programming. In Subsection 10.2.1, I remarked that it would be easy to use a LinkedList to implement a queue. (Queues were introduced in Subsection 9.3.2.) To ensure that the only operations that are performed on the list are the queue operations enqueue, dequeue, and isEmpty, we can create a new class that contains the linked list as a private instance variable. To implement queues of strings, for example, we can define the class:

class QueueOfStrings {
private LinkedList<String> items = new LinkedList<String>();
public void enqueue(String item) {
items.addLast(item);
}
public String dequeue() {
return items.removeFirst();
}
public boolean isEmpty() {
return (items.size() == 0);
}
}

This is a fine and useful class. But, if this is how we write queue classes, and if we want queues of Integers or Doubles or JButtons or any other type, then we will have to write a different class for each type. The code for all of these classes will be almost identical, which seems like a lot of redundant programming. To avoid the redundancy, we can write a generic Queue class that can be used to define queues of any type of object.

The syntax for writing the generic class is straightforward: We replace the specific type String with a type parameter such as T, and we add the type parameter to the name of the class:

class Queue<T> {
private LinkedList<T> items = new LinkedList<T>();
public void enqueue(T item) {
items.addLast(item);
}
public T dequeue() {
return items.removeFirst();
}
public boolean isEmpty() {
return (items.size() == 0);
}
}

Note that within the class, the type parameter T is used just like any regular type name. It's used to declare the return type for dequeue, as the type of the formal parameter item in enqueue, and even as the actual type parameter in LinkedList<T>. Given this class definition, we can use parameterized types such as Queue<String> and Queue<Integer> and Queue<JButton>. That is, the Queue class is used in exactly the same way as built-in generic classes like LinkedList and HashSet.

Note that you don't have to use "T" as the name of the type parameter in the definition of the generic class. Type parameters are like formal parameters in subroutines. You can make up any name you like in the definition of the class. The name in the definition will be replaced by an actual type name when the class is used to declare variables or create objects. If you prefer to use a more meaningful name for the type parameter, you might define the Queue class as:

class Queue<ItemType> {
private LinkedList<ItemType> items = new LinkedList<ItemType>();
public void enqueue(ItemType item) {
items.addLast(item);
}
public ItemType dequeue() {
return items.removeFirst();
}
public boolean isEmpty() {
return (items.size() == 0);
}
}

Changing the name from "T" to "ItemType" has absolutely no effect on the meaning of the class definition or on the way that Queue is used.

Generic interfaces can be defined in a similar way. It's also easy to define generic classes and interfaces that have two or more type parameters, as is done with the standard interface Map<K,V>. A typical example is the definition of a "Pair" that contains two objects, possibly of different types. A simple version of such a class can be defined as:

class Pair<T,S> {
public T first;
public S second;
public Pair( T a, S b ) {  // Constructor.
first = a;
second = b;
}
}

This class can be used to declare variables and create objects such as:

Pair<String,Color> colorName = new Pair<String,Color>("Red", Color.RED);
Pair<Double,Double> coordinates = new Pair<Double,Double>(17.3,42.8);

Note that in the definition of the constructor in this class, the name "Pair" does not have type parameters. You might have expected "Pair<T,S>". However, the name of the class is "Pair", not "Pair<T,S>", and within the definition of the class, "T" and "S" are used as if they are the names of specific, actual types. Note in any case that type parameters are never added to the names of methods or constructors, only to the names of classes and interfaces.

10.5.2  Simple Generic Methods

In addition to generic classes, Java also has generic methods. An example is the method Collections.sort(), which can sort collections of objects of any type. To see how to write generic methods, let's start with a non-generic method for counting the number of times that a given string occurs in an array of strings:

/**
* Returns the number of times that itemToCount occurs in list.  Items in the
* list are tested for equality using itemToCount.equals(), except in the
* special case where itemToCount is null.
*/
public static int countOccurrences(String[] list, String itemToCount) {
int count = 0;
if (itemToCount == null) {
for ( String listItem : list )
if (listItem == null)
count++;
}
else {
for ( String listItem : list )
if (itemToCount.equals(listItem))
count++;
}
return count;
}

Once again, we have some code that works for type String, and we can imagine writing almost identical code to work with other types of objects. By writing a generic method, we get to write a single method definition that will work for objects of any type. We need to replace the specific type String in the definition of the method with the name of a type parameter, such as T. However, if that's the only change we make, the compiler will think that "T" is the name of an actual type, and it will mark it as an undeclared identifier. We need some way of telling the compiler that "T" is a type parameter. That's what the "<T>" does in the definition of the generic class "class Queue<T> { ...". For a generic method, the "<T>" goes just before the name of the return type of the method:

public static <T> int countOccurrences(T[] list, T itemToCount) {
int count = 0;
if (itemToCount == null) {
for ( T listItem : list )
if (listItem == null)
count++;
}
else {
for ( T listItem : list )
if (itemToCount.equals(listItem))
count++;
}
return count;
}

The "<T>" marks the method as being generic and specifies the name of the type parameter that will be used in the definition. Of course, the name of the type parameter doesn't have to be "T"; it can be anything. (The "<T>" looks a little strange in that position, I know, but it had to go somewhere and that's just where the designers of Java decided to put it.)

Given the generic method definition, we can apply it to objects of any type. If wordList is a variable of type String[] and word is a variable of type String, then

int ct = countOccurrences( wordList, word );

will count the number of times that word occurs in wordList. If palette is a variable of type Color[] and color is a variable of type Color, then

int ct = countOccurrences( palette, color );

will count the number of times that color occurs in palette. If numbers is a variable of type Integer[], then

int ct = countOccurrences( numbers, 17 );

will count the number of times that 17 occurs in numbers. This last example uses autoboxing; the 17 is automatically converted to a value of type Integer. Note that, since generic programming in Java applies only to objects, we cannot use countOccurrences to count the number of occurrences of 17 in an array of type int[].

A generic method can have one or more type parameters, such as the "T" in countOccurrences. Note that when a generic method is used, as in the function call "countOccurrences(wordlist, word)", there is no explicit mention of the type that is substituted for the type parameter. The compiler deduces the type from the types of the actual parameters in the method call. Since wordlist is of type String[], the compiler can tell that in "countOccurrences(wordlist, word)", the type that replaces T is String. This contrasts with the use of generic classes, as in "new Queue<String>()", where the type parameter is specified explicitly.

The countOccurrences method operates on an array. We could also write a similar method to count occurrences of an object in any collection:

public static <T> int countOccurrences(Collection<T> collection, T itemToCount) {
int count = 0;
if (itemToCount == null) {
for ( T item : collection )
if (item == null)
count++;
}
else {
for ( T item : collection )
if (itemToCount.equals(item))
count++;
}
return count;
}

Since Collection<T> is itself a generic type, this method is very general. It can operate on an ArrayList of Integers, a TreeSet of Strings, a LinkedList of JButtons, ....

10.5.3  Type Wildcards

There is a limitation on the sort of generic classes and methods that we have looked at so far: The type parameter in our examples, usually named T, can be any type at all. This is OK in many cases, but it means that the only things that you can do with T are things that can be done with every type, and the only things that you can do with objects of type T are things that you can do with every object. With the techniques that we have covered so far, you can't, for example, write a generic method that compares objects with the compareTo() method, since that method is not defined for all objects. The compareTo() method is defined in the Comparable interface. What we need is a way of specifying that a generic class or method only applies to objects of type Comparable and not to arbitrary objects. With that restriction, we should be free to use compareTo() in the definition of the generic class or method.

There are two different but related syntaxes for putting restrictions on the types that are used in generic programming. One of these is bounded type parameters, which are used as formal type parameters in generic class and method definitions; a bounded type parameter would be used in place of the simple type parameter T in "class GenericClass<T> ..." or in "public static <T> void genericMethod(...". The second syntax is wildcard types, which are used as type parameters in the declarations of variables and of formal parameters in method definitions; a wildcard type could be used in place of the type parameter String in the declaration statement "List<String> list;" or in the formal parameter list "void concat(Collection<String> c)". We will look at wildcard types first, and we will return to the topic of bounded types later in this section.

Let's start with a simple example in which a wildcard type is useful. Suppose that Shape is a class that defines a method public void draw(), and suppose that Shape has subclasses such as Rect and Oval. Suppose that we want a method that can draw all the shapes in a collection of Shapes. We might try:

public static void drawAll(Collection<Shape> shapes) {
for ( Shape s : shapes )
s.draw();
}

This method works fine if we apply it to a variable of type Collection<Shape>, or ArrayList<Shape>, or any other collection class with type parameter Shape. Suppose, however, that you have a list of Rects stored in a variable named rectangles of type Collection<Rect>. Since Rects are Shapes, you might expect to be able to call drawAll(rectangles). Unfortunately, this will not work; a collection of Rects is not considered to be a collection of Shapes! The variable rectangles cannot be assigned to the formal parameter shapes. The solution is to replace the type parameter "Shape" in the declaration of shapes with the wildcard type "? extends Shape":

public static void drawAll(Collection<? extends Shape> shapes) {
for ( Shape s : shapes )
s.draw();
}

The wildcard type "? extends Shape" means roughly "any type that is either equal to Shape or that is a subclass of Shape". When the parameter shapes is declared to be of type Collection<? extends Shape>, it becomes possible to call the drawAll method with an actual parameter of type Collection<Rect> since Rect is a subclass of Shape and therefore matches the wildcard. We could also pass actual parameters to drawAll of type ArrayList<Rect> or Set<Oval> or List<Oval>. And we can still pass variables of type Collection<Shape> or ArrayList<Shape>, since the class Shape itself matches "? extends Shape". We have greatly increased the usefulness of the method by using the wildcard type.

(Although it is not essential, you might be interested in knowing why Java does not allow a collection of Rects to be used as a collection of Shapes, even though every Rect is considered to be a Shape. Consider the rather silly but legal method that adds an oval to a list of shapes:

static void addOval(List<Shape> shapes, Oval oval) {
shapes.add(oval);
}

Suppose that rectangles is of type List<Rect>. It's illegal to call addOval(rectangles,oval), because of the rule that a list of Rects is not a list of Shapes. If we dropped that rule, then addOval(rectangles,oval) would be legal, and it would add an Oval to a list of Rects. This would be bad: Since Oval is not a subclass of Rect, an Oval is not a Rect, and a list of Rects should never be able to contain an Oval. The method call addOval(rectangles,oval) does not make sense and should be illegal, so the rule that a collection of Rects is not a collection of Shapes is a good rule.)

As another example, consider the method addAll() from the interface Collection<T>. In my description of this method in Subsection 10.1.4, I say that for a collection, coll, of type Collection<T>, coll.addAll(coll2) "adds all the objects in coll2 to coll. The parameter, coll2, can be any collection of type Collection<T>. However, it can also be more general. For example, if T is a class and S is a sub-class of T, then coll2 can be of type Collection<S>. This makes sense because any object of type S is automatically of type T and so can legally be added to coll." If you think for a moment, you'll see that what I'm describing here, a little awkwardly, is a use of wildcard types: We don't want to require coll2 to be a collection of objects of type T; we want to allow collections of any subclass of T. To be more specific, let's look at how a similar addAll() method could be added to the generic Queue class that was defined earlier in this section:

class Queue<T> {
private LinkedList<T> items = new LinkedList<T>();
public void enqueue(T item) {
items.addLast(item);
}
public T dequeue() {
return items.removeFirst();
}
public boolean isEmpty() {
return (items.size() == 0);
}
public void addAll(Collection<? extends T> collection) {
// Add all the items from the collection to the end of the queue
for ( T item : collection )
enqueue(item);
}
}

Here, T is a type parameter in the generic class definition. We are combining wildcard types with generic classes. Inside the generic class definition, "T" is used as if it is a specific, though unknown, type. The wildcard type "? extends T" means some type that is equal to or extends that specific type. When we create a queue of type Queue<Shape>, "T" refers to "Shape", and the wildcard type "? extends T" in the class definition means "? extends Shape", meaning that the addAll method of the queue can be applied to collections of Rects and Ovals as well as to collections of Shapes.

The for-each loop in the definition of addAll iterates through the collection using a variable, item, of type T. Now, collection can be of type Collection<S>, where S is a subclass of T. Since item is of type T, not S, do we have a problem here? No, no problem. As long as S is a subclass of T, a value of type S can be assigned to a variable of type T. The restriction on the wildcard type makes everything work nicely.

The addAll method adds all the items from a collection to the queue. Suppose that we wanted to do the opposite: Add all the items that are currently on the queue to a given collection. An instance method defined as

public void addAllTo(Collection<T> collection)

would only work for collections whose base type is exactly the same as T. This is too restrictive. We need some sort of wildcard. However, "? extends T" won't work. Suppose we try it:

public void addAllTo(Collection<? extends T> collection) {
// Remove all items currently on the queue and add them to collection
while ( ! isEmpty() ) {
T item = dequeue();  // Remove an item from the queue.
collection.add( item );  // Add it to the collection.  ILLEGAL!!
}
}

The problem is that we can't add an item of type T to a collection that might only be able to hold items belonging to some subclass, S, of T. The containment is going in the wrong direction: An item of type T is not necessarily of type S. For example, if we have a queue of type Queue<Shape>, it doesn't make sense to add items from the queue to a collection of type Collection<Rect>, since not every Shape is a Rect. On the other hand, if we have a Queue<Rect>, it would make sense to add items from that queue to a Collection<Shape> or indeed to any collection Collection<S> where S is a superclass of Rect.

To express this type of relationship, we need a new kind of type wildcard: "? super T". This wildcard means, roughly, "either T itself or any class that is a superclass of T." For example, Collection<? super Rect> would match the types Collection<Shape>, ArrayList<Object>, and Set<Rect>. This is what we need for our addAllTo method. With this change, our complete generic queue class becomes:

class Queue<T> {
private LinkedList<T> items = new LinkedList<T>();
public void enqueue(T item) {
items.addLast(item);
}
public T dequeue() {
return items.removeFirst();
}
public boolean isEmpty() {
return (items.size() == 0);
}
public void addAll(Collection<? extends T> collection) {
// Add all the items from the collection to the end of the queue
for ( T item : collection )
enqueue(item);
}
public void addAllTo(Collection<? super T> collection) {
// Remove all items currently on the queue and add them to collection
while ( ! isEmpty() ) {
T item = dequeue();  // Remove an item from the queue.
collection.add( item );  // Add it to the collection.
}
}
}

In a wildcard type such as "? extends T", T can be an interface instead of a class. Note that the term "extends" (not "implements") is used in the wildcard type, even if T is an interface. For example, we will see that Runnable is an interface that defines the method public void run(). (Runnable objects are usually associated with threads; see Chapter 12.) Here is a method that runs all the objects in a collection of Runnables by executing the run() method from each runnable object:

public static runAll( Collection<? extends Runnable> runnables ) {
for ( Runnable runnable : runnables ) {
runnable.run();
}
}

Wildcard types are used only as type parameters in parameterized types, such as Collection<? extends Runnable>. The place where a wildcard type is most likely to occur, by far, is in a formal parameter list, where the wildcard type is used in the declaration of the type of a formal parameter. However, they can also be used in a few other places. For example, they can be used in the type specification in a variable declaration statement.

One final remark: The wildcard type "<?>" is equivalent to "<? extends Object>". That is, it matches any possible type. For example, the removeAll() method in the generic interface Collections<T> is declared as

public boolean removeAll( Collection<?> c ) { ...

This just means that the removeAll method can be applied to any collection of any type of object.

10.5.4  Bounded Types

Wildcard types don't solve all of our problems. They allow us to generalize method definitions so that they can work with collections of objects of various types, rather than just a single type. However, they do not allow us to restrict the types that are allowed as type parameters in a generic class or method definition. Bounded types exist for this purpose.

We start with a small, not very realistic example. Suppose that you would like to create groups of GUI components using a generic class named ComponentGroup. For example, the parameterized type ComponentGroup<JButton> would represent a group of JButtons, while ComponentGroup<JPanel> would represent a group of JPanels. The class will include methods that can be called to apply certain operations to all components in the group at once. For example, there will be an instance method of the form

public void repaintAll() {
.
.  // Call the repaint() method of every component in the group.
.
}

The problem is that the repaint() method is defined in a JComponent object, but not for objects of arbitrary type. It wouldn't make sense to allow types such as ComponentGroup<String> or ComponentGroup<Integer>, since Strings and Integers don't have repaint() methods. We need some way to restrict the type parameter T in ComponentGroup<T> so that only JComponent and subclasses of JComponent are allowed as actual type parameters. We can do this by using the bounded type "T extends JComponent" instead of a plain "T" in the definition of the class:

public class ComponentGroup<T extends JComponent> {
private ArrayList<T> components; // For storing the components in this group.
public void repaintAll() {
for ( JComponent c : components )
if (c != null)
c.repaint();
}
public void setAllEnabled( boolean enable ) {
for ( JComponent c : components )
if (c != null)
c.setEnabled(enable);
}
}
public void add( T c ) {  // Add a value c, of type T, to the group.
components.add(c);
}
.
.  // Additional methods and constructors.
.
}

The restriction "extends JComponent" on T makes it illegal to create the parameterized types ComponentGroup<String> and ComponentGroup<Integer>, since the actual type parameter that replaces "T" is required to be either JComponent itself or a subclass of JComponent. With this restriction, we know -- and, more important, the compiler knows -- that the objects in the group are of type JComponent and the operations c.repaint() and c.setEnabled() are defined for any c in the group.

In general, a bounded type parameter "T extends SomeType" means roughly "a type, T, that is either equal to SomeType or is a subclass of SomeType", and the upshot is that any object of type T is also of type SomeType, and any operation that is defined for objects of type SomeType is defined for objects of type T. The type SomeType doesn't have to be the name of a class. It can be any name that represents an actual object type. For example, it can be an interface or even a parameterized type.

Bounded types and wildcard types are clearly related. They are, however, used in very different ways. A bounded type can be used only as a formal type parameter in the definition of a generic method, class, or interface. A wildcard type is used most often to declare the type of a formal parameter in a method and cannot be used as a formal type parameter. One other difference, by the way, is that, in contrast to wildcard types, bounded type parameters can only use "extends", never "super".

Bounded type parameters can be used when declaring generic methods. For example, as an alternative to the generic ComponentGroup class, one could write a free-standing generic static method that can repaint any collection of JComponents as follows:

public static <T extends JComponent> void repaintAll(Collection<T> comps) {
for ( JComponent c : comps )
if (c != null)
c.repaint();
}

Using "<T extends JComponent>" as the formal type parameter means that the method can only be called for collections whose base type is JComponent or some subclass of JComponent. Thus, it is legal to call repaintAll(coll) where coll is of type List<JPanel> but not where coll is of type Set<String>.

Note that we don't really need a generic type parameter in this case. We can write an equivalent method using a wildcard type:

public static void repaintAll(Collection<? extends JComponent> comps) {
for ( JComponent c : comps )
if (c != null)
c.repaint();
}

In this situation, the version that uses the wildcard type is to be preferred, since the implementation is simpler. However, there are some situations where a generic method with a bounded type parameter cannot be rewritten using a wildcard type. Note that a generic type parameter gives a name, such as T, to the unknown type, while a wildcard type does not give a name to the unknown type. The name makes it possible to refer to the unknown type in the body of the method that is being defined. If a generic method definition uses the generic type name more than once or uses it outside the formal parameter list of the method, then the generic type cannot be replaced with a wildcard type.

Let's look at a generic method in which a bounded type parameter is essential. In Subsection 10.2.1, I presented a code segment for inserting a string into a sorted list of strings, in such a way that the modified list is still in sorted order. Here is the same code, but this time in the form of a method definition (and without the comments):

static void sortedInsert(List<String> sortedList, String newItem) {
ListIterator<String> iter = sortedList.listIterator();
while (iter.hasNext()) {
String item = iter.next();
if (newItem.compareTo(item) <= 0) {
iter.previous();
break;
}
}
iter.add(newItem);
}

This method works fine for lists of strings, but it would be nice to have a generic method that can be applied to lists of other types of objects. The problem, of course, is that the code assumes that the compareTo() method is defined for objects in the list, so the method can only work for lists of objects that implement the Comparable interface. We can't simply use a wildcard type to enforce this restriction. Suppose we try to do it, by replacing List<String> with List<? extends Comparable>:

static void sortedInsert(List<? extends Comparable> sortedList, ???? newItem) {
ListIterator<????> iter = stringList.listIterator();
...

We immediately run into a problem, because we have no name for the unknown type represented by the wildcard. We need a name for that type because the type of newItem and of iter should be the same as the type of the items in the list. The problem is solved if we write a generic method with a bounded type parameter, since then we have a name for the unknown type, and we can write a valid generic method:

static <T extends Comparable> void sortedInsert(List<T> sortedList, T newItem) {
ListIterator<T> iter = sortedList.listIterator();
while (iter.hasNext()) {
T item = iter.next();
if (newItem.compareTo(item) <= 0) {
iter.previous();
break;
}
}
iter.add(newItem);
}

There is still one technicality to cover in this example. Comparable is itself a parameterized type, but I have used it here without a type parameter. This is legal but the compiler might give you a warning about using a "raw type." In fact, the objects in the list should implement the parameterized interface Comparable<T>, since they are being compared to items of type T. This just means that instead of using Comparable as the type bound, we should use Comparable<T>:

static <T extends Comparable<T>> void sortedInsert(List<T> sortedList, ...

With this example, I will leave the topic of generic types and generic programming. In this chapter, I have occasionally used terms such as "strange" and "weird" to talk about generic programming in Java. I will confess that I have some affection for the more simple-minded generic programming style of Smalltalk. Nevertheless, I recognize the power and increased robustness of generics in Java. I hope that I have convinced you that using the Java Collection Framework is reasonably natural and straightforward, and that using it can save you a lot of time and effort compared to repeatedly recoding the same data structures and algorithms from scratch. Things become more technical when you start writing new generic classes and methods of your own, and the syntax is (as I've said) a little strange. But with some practice, you'll get used to the syntax and will find that it's not that difficult after all.

Programming Exercises for Chapter 10

Exercise 10.1:

Rewrite the PhoneDirectory class from Subsection 7.4.2 so that it uses a TreeMap to store directory entries, instead of an array. (Doing this was suggested in Subsection 10.3.1.)

See the Solution

Exercise 10.2:

In mathematics, several operations are defined on sets. The union of two sets A and B is a set that contains all the elements that are in A together with all the elements that are in B. The intersection of A and B is the set that contains elements that are in both A and B. The difference of A and B is the set that contains all the elements of A except for those elements that are also in B.

Suppose that A and B are variables of type set in Java. The mathematical operations on A and B can be computed using methods from the Set interface. In particular: A.addAll(B) computes the union of A and B; A.retainAll(B) computes the intersection of A and B; and A.removeAll(B) computes the difference of A and B. (These operations change the contents of the set A, while the mathematical operations create a new set without changing A, but that difference is not relevant to this exercise.)

For this exercise, you should write a program that can be used as a "set calculator" for simple operations on sets of non-negative integers. (Negative integers are not allowed.) A set of such integers will be represented as a list of integers, separated by commas and, optionally, spaces and enclosed in square brackets. For example: [1,2,3] or [17, 42, 9, 53, 108]. The characters +, *, and - will be used for the union, intersection, and difference operations. The user of the program will type in lines of input containing two sets, separated by an operator. The program should perform the operation and print the resulting set. Here are some examples:

Input                                 Output
-------------------------           -------------------
[1, 2, 3] + [3,  5,  7]             [1, 2, 3, 5, 7]
[10,9,8,7] * [2,4,6,8]              [8]
[ 5, 10, 15, 20 ] - [ 0, 10, 20 ]   [5, 15]

To represent sets of non-negative integers, use sets of type TreeSet<Integer>. Read the user's input, create two TreeSets, and use the appropriate TreeSet method to perform the requested operation on the two sets. Your program should be able to read and process any number of lines of input. If a line contains a syntax error, your program should not crash. It should report the error and move on to the next line of input. (Note: To print out a Set, A, of Integers, you can just say System.out.println(A). I've chosen the syntax for sets to be the same as that used by the system for outputting a set.)

See the Solution

Exercise 10.3:

The fact that Java has a HashMap class means that no Java programmer has to write an implementation of hash tables from scratch -- unless, of course, that programmer is a computer science student.

For this exercise, you should write a hash table in which both the keys and the values are of type String. (This is not an exercise in generic programming; do not try to write a generic class.) Write an implementation of hash tables from scratch. Define the following methods: get(key), put(key,value), remove(key), containsKey(key), and size(). Remember that every object, obj, has a method obj.hashCode() that can be used for computing a hash code for the object, so at least you don't have to define your own hash function. Do not use any of Java's built-in generic types; create your own linked lists using nodes as covered in Subsection 9.2.2. However, you do not have to worry about increasing the size of the table when it becomes too full.

You should also write a short program to test your solution.

See the Solution

Exercise 10.4:

A predicate is a boolean-valued function with one parameter. Some languages use predicates in generic programming. Java 7 doesn't, but this exercise looks at how predicates might work.

In Java, we could implement "predicate objects" by defining a generic interface:

public interface Predicate<T> {
public boolean test( T obj );
}

The idea is that an object that implements this interface knows how to "test" objects of type T in some way. Define a class that contains the following generic static methods for working with predicate objects. The name of the class should be Predicates, in analogy with the standard class Collections that provides various static methods for working with collections.

public static <T> void remove(Collection<T> coll, Predicate<T> pred)
// Remove every object, obj, from coll for which
// pred.test(obj) is true.

public static <T> void retain(Collection<T> coll, Predicate<T> pred)
// Remove every object, obj, from coll for which
// pred.test(obj) is false.  (That is, retain the
// objects for which the predicate is true.)

public static <T> List<T> collect(Collection<T> coll, Predicate<T> pred)
// Return a List that contains all the objects, obj,
// from the collection, coll, such that pred.test(obj)
// is true.

public static <T> int find(ArrayList<T> list, Predicate<T> pred)
// Return the index of the first item in list
// for which the predicate is true, if any.
// If there is no such item, return -1.

(In C++, methods similar to these are included as a standard part of the generic programming framework. And Java 8 has a similar predicate interface, and the Collections class has a removeIf() method that uses a predicate.)

See the Solution

Exercise 10.5:

An example in Subsection 10.4.2 concerns the problem of making an index for a book. A related problem is making a concordance for a document. A concordance lists every word that occurs in the document, and for each word it gives the line number of every line in the document where the word occurs. All the subroutines for creating an index that were presented in Subsection 10.4.2 can also be used to create a concordance. The only real difference is that the integers in a concordance are line numbers rather than page numbers.

Write a program that can create a concordance. The document should be read from an input file, and the concordance data should be written to an output file. You can use the indexing subroutines from Subsection 10.4.2, modified to write the data to TextIO instead of to System.out. (You will need to make these subroutines static.) The input and output files should be selected by the user when the program is run. The sample program WordCount.java, from Subsection 10.4.4, can be used as a model of how to use files. That program also has a useful subroutine that reads one word from input.

As you read the file, you want to take each word that you encounter and add it to the concordance along with the current line number. Keeping track of the line numbers is one of the trickiest parts of the problem. In an input file, the end of each line in the file is marked by the newline character, '\n'. Every time you encounter this character, you have to add one to the line number. WordCount.java ignores ends of lines. Because you need to find and count the end-of-line characters, your program cannot process the input file in exactly the same way as does WordCount.java. Also, you will need to detect the end of the file. The function TextIO.peek(), which is used to look ahead at the next character in the input, returns the value TextIO.EOF at end-of-file, after all the characters in the file have been read.

Because it is so common, don't include the word "the" in your concordance. Also, do not include words that have length less than 3.

See the Solution

Exercise 10.6:

The sample program SimpleInterpreter.java from Subsection 10.4.1 can carry out commands of the form "let variable = expression" or "print expression". That program can handle expressions that contain variables, numbers, operators, and parentheses. Extend the program so that it can also handle the standard mathematical functions sin, cos, tan, abs, sqrt, and log. For example, the program should be able to evaluate an expression such as sin(3*x-7)+log(sqrt(y)), assuming that the variables x and y have been given values. Note that the name of a function must be followed by an expression that is enclosed in parentheses.

In the original program, a symbol table holds a value for each variable that has been defined. In your program, you should add another type of symbol to the table to represent standard functions. You can use the following nested enumerated type and class for this purpose:

private enum Functions { SIN, COS, TAN, ABS, SQRT, LOG }

/**
* An object of this class represents one of the standard functions.
*/
private static class StandardFunction {

/**
* Tells which function this is.
*/
Functions functionCode;

/**
* Constructor creates an object to represent one of
* the standard functions
* @param code which function is represented.
*/
StandardFunction(Functions code) {
functionCode = code;
}

/**
* Finds the value of this function for the specified
* parameter value, x.
*/
double evaluate(double x) {
switch(functionCode) {
case SIN:
return Math.sin(x);
case COS:
return Math.cos(x);
case TAN:
return Math.tan(x);
case ABS:
return Math.abs(x);
case SQRT:
return Math.sqrt(x);
default:
return Math.log(x);
}
}

} // end class StandardFunction

Add a symbol to the symbol table to represent each function. The key is the name of the function and the value is an object of type StandardFunction that represents the function. For example:

symbolTable.put("sin", new StandardFunction(Function.SIN));

In SimpleInterpreter.java, the symbol table is a map of type HashMap<String,Double>. It's not legal to use a StandardFunction as the value in such a map, so you will have to change the type of the map. The map has to hold two different types of objects. The easy way to make this possible is to create a map of type HashMap<String,Object>. (A better way is to create a general type to represent objects that can be values in the symbol table, and to define two subclasses of that class, one to represent variables and one to represent standard functions, but for this exercise, you should do it the easy way.)

In your parser, when you encounter a word, you have to be able to tell whether it's a variable or a standard function. Look up the word in the symbol table. If the associated object is non-null and is of type Double, then the word is a variable. If it is of type StandardFunction, then the word is a function. Remember that you can test the type of an object using the instanceof operator. For example: if (obj instanceof Double)

See the Solution

Quiz on Chapter 10

Question 1:

What is meant by generic programming and what is the alternative?

Question 2:

Why can't you make an object of type LinkedList<int>? What should you do instead?

Question 3:

What is an iterator and why are iterators necessary for generic programming?

Question 4:

Suppose that integers is a variable of type Collection<Integer>. Write a code segment that uses an iterator to compute the sum of all the integer values in the collection. Write a second code segment that does the same thing using a for-each loop.

Question 5:

Interfaces such as List, Set, and Map define abstract data types. Explain what this means.

Question 6:

What is the fundamental property that distinguishes Sets from other types of Collections?

Question 7:

What is the essential difference in functionality between a TreeMap and a HashMap?

Question 8:

Explain what is meant by a hash code.

Question 9:

Modify the following Date class so that it implements the interface Comparable<Date>. The ordering on objects of type Date should be the natural, chronological ordering.

class Date {
int month;  // Month number in range 1 to 12.
int day;    // Day number in range 1 to 31.
int year;   // Year number.
Date(int m, int d, int y) {
month = m;
day = d;
year = y;
}
}

Question 10:

Suppose that syllabus is a variable of type TreeMap<Date,String>, where Date is the class from the preceding exercise. Write a code segment that will write out the value string for every key that is in the month of December, 2014.

Question 11:

Write a generic class Stack<T> that can be used to represent stacks of objects of type T. The class should include methods push(), pop(), and isEmpty(). Inside the class, use an ArrayList to hold the items on the stack.

Question 12:

Write a generic method, using a generic type parameter <T>, that replaces every occurrence in an ArrayList<T> of a specified item with a specified replacement item. The list and the two items are parameters to the method. Both items are of type T. Take into account the fact that the item that is being replaced might be null. For a non-null item, use equals() to do the comparison.

See the Answers

Chapter 11

Advanced Input/Output: Streams, Files, and Networking

Computer programs are only useful if they interact with the rest of the world in some way. This interaction is referred to as input/output, or I/O. Up until now, this book has concentrated on just one type of interaction: interaction with the user, through either a graphical user interface or a command-line interface. But the user is only one possible source of information and only one possible destination for information. We have already encountered one other type of input/output, since TextIO can read data from files and write data to files. However, Java has an input/output framework that provides much more power and flexibility than does TextIO, and that covers other kinds of I/O in addition to files. Most importantly, it supports communication over network connections. In Java, the most common type of input/output involving files and networks is based on streams, which are objects that support I/O commands that are similar to those that you have already used. In fact, standard output (System.out) and standard input (System.in) are examples of streams.

Working with files and networks requires familiarity with exceptions, which were covered in Section 8.3. Many of the subroutines that are used can throw checked exceptions, which require mandatory exception handling. This generally means calling the subroutine in a try..catch statement that can deal with the exception if one occurs. Effective network communication also requires the use of threads, which will be covered in Chapter 12. We will look at the basic networking API in this chapter, but we will return to the topic of threads and networking in Section 12.4.

Contents of Chapter 11:

- Section 1: Streams, Readers, and Writers

- Section 2: Files

- Section 3: Programming With Files

- Section 4: Networking

- Section 5: A Brief Introduction to XML

- Programming Exercises

- Quiz on This Chapter

Section 11.1

Streams, Readers, and Writers

Without the ability to interact with the rest of the world, a program would be useless. The interaction of a program with the rest of the world is referred to as input/output or I/O. Historically, one of the hardest parts of programming language design has been coming up with good facilities for doing input and output. A computer can be connected to many different types of input and output devices. If a programming language had to deal with each type of device as a special case, the complexity would be overwhelming. One of the major achievements in the history of programming has been to come up with good abstractions for representing I/O devices. In Java, the main I/O abstractions are called streams. Other I/O abstractions, such as "files" and "channels" also exist, but in this section we will look only at streams. Every stream represents either a source of input or a destination to which output can be sent.

11.1.1  Character and Byte Streams

When dealing with input/output, you have to keep in mind that there are two broad categories of data: machine-formatted data and human-readable text. Machine-formatted data is represented in binary form, the same way that data is represented inside the computer, that is, as strings of zeros and ones. Human-readable data is in the form of characters. When you read a number such as 3.141592654, you are reading a sequence of characters and interpreting them as a number. The same number would be represented in the computer as a bit-string that you would find unrecognizable.

To deal with the two broad categories of data representation, Java has two broad categories of streams: byte streams for machine-formatted data and character streams for human-readable data. There are many predefined classes that represent streams of each type.

An object that outputs data to a byte stream belongs to one of the subclasses of the abstract class OutputStream. Objects that read data from a byte stream belong to subclasses of the abstract class InputStream. If you write numbers to an OutputStream, you won't be able to read the resulting data yourself. But the data can be read back into the computer with an InputStream. The writing and reading of the data will be very efficient, since there is no translation involved: the bits that are used to represent the data inside the computer are simply copied to and from the streams.

For reading and writing human-readable character data, the main classes are the abstract classes Reader and Writer. All character stream classes are subclasses of one of these. If a number is to be written to a Writer stream, the computer must translate it into a human-readable sequence of characters that represents that number. Reading a number from a Reader stream into a numeric variable also involves a translation, from a character sequence into the appropriate bit string. (Even if the data you are working with consists of characters in the first place, such as words from a text editor, there might still be some translation. Characters are stored in the computer as 16-bit Unicode values. For people who use Western alphabets, character data is generally stored in files in ASCII code, which uses only 8 bits per character. The Reader and Writer classes take care of this translation, and can also handle non-western alphabets in countries that use them.)

Byte streams can be useful for direct machine-to-machine communication, and they can sometimes be useful for storing data in files, especially when large amounts of data need to be stored efficiently, such as in large databases. However, binary data is fragile in the sense that its meaning is not self-evident. When faced with a long series of zeros and ones, you have to know what information it is meant to represent and how that information is encoded before you will be able to interpret it. Of course, the same is true to some extent for character data, since characters, like any other kind of data, have to be coded as binary numbers to be stored or processed by a computer. But the binary encoding of character data has been standardized and is well understood, and data expressed in character form can be made meaningful to human readers. The current trend seems to be towards increased use of character data, represented in a way that will make its meaning as self-evident as possible. We'll look at one way this is done in Section 11.5.

I should note that the original version of Java did not have character streams, and that for ASCII-encoded character data, byte streams are largely interchangeable with character streams. In fact, the standard input and output streams, System.in and System.out, are byte streams rather than character streams. However, you should use Readers and Writers rather than InputStreams and OutputStreams when working with character data, even when working with the standard ASCII character set.

The standard stream classes discussed in this section are defined in the package java.io, along with several supporting classes. You must import the classes from this package if you want to use them in your program. That means either importing individual classes or putting the directive "import java.io.*;" at the beginning of your source file. Streams are necessary for working with files and for doing communication over a network. They can also be used for communication between two concurrently running threads, and there are stream classes for reading and writing data stored in the computer's memory.

The beauty of the stream abstraction is that it is as easy to write data to a file or to send data over a network as it is to print information on the screen.

The basic I/O classes Reader, Writer, InputStream, and OutputStream provide only very primitive I/O operations. For example, the InputStream class declares an abstract instance method

public int read() throws IOException

for reading one byte of data, as a number in the range 0 to 255, from an input stream. If the end of the input stream is encountered, the read() method will return the value -1 instead. If some error occurs during the input attempt, an exception of type IOException is thrown. Since IOException is a checked exception, this means that you can't use the read() method except inside a try statement or in a subroutine that is itself declared with a "throws IOException" clause. (Checked exceptions and mandatory exception handling were covered in Subsection 8.3.3.)

The InputStream class also defines methods for reading multiple bytes of data in one step into an array of bytes. However, InputStream provides no convenient methods for reading other types of data, such as int or double, from a stream. This is not a problem because you'll never use an object of type InputStream itself. Instead, you'll use subclasses of InputStream that add more convenient input methods to InputStream's rather primitive capabilities. Similarly, the OutputStream class defines a primitive output method for writing one byte of data to an output stream. The method is defined as:

public void write(int b) throws IOException

The parameter is of type int rather than byte, but the parameter value is type-cast to type byte before it is written; this effectively discards all but the eight low order bits of b. Again, in practice, you will almost always use higher-level output operations defined in some subclass of OutputStream.

The Reader and Writer classes provide the analogous low-level read and write methods. As in the byte stream classes, the parameter of the write(c) method in Writer and the return value of the read() method in Reader are of type int, but in these character-oriented classes, the I/O operations read and write characters rather than bytes. The return value of read() is -1 if the end of the input stream has been reached. Otherwise, the return value must be type-cast to type char to obtain the character that was read. In practice, you will ordinarily use higher level I/O operations provided by sub-classes of Reader and Writer, as discussed below.

11.1.2  PrintWriter

One of the neat things about Java's I/O package is that it lets you add capabilities to a stream by "wrapping" it in another stream object that provides those capabilities. The wrapper object is also a stream, so you can read from or write to it -- but you can do so using fancier operations than those available for basic streams.

For example, PrintWriter is a subclass of Writer that provides convenient methods for outputting human-readable character representations of all of Java's basic data types. If you have an object belonging to the Writer class, or any of its subclasses, and you would like to use PrintWriter methods to output data to that Writer, all you have to do is wrap the Writer in a PrintWriter object. You do this by constructing a new PrintWriter object, using the Writer as input to the constructor. For example, if charSink is of type Writer, then you could say

PrintWriter printableCharSink = new PrintWriter(charSink);

In fact, the parameter to the constructor can also be an OutputStream or a File, and the constructor will build a PrintWriter that can write to that output destination. (Files are covered in the next section.) When you output data to the PrintWriter printableCharSink, using the high-level output methods in PrintWriter, that data will go to exactly the same place as data written directly to charSink. You've just provided a better interface to the same output destination. For example, this allows you to use PrintWriter methods to send data to a file or over a network connection.

For the record, if out is a variable of type PrintWriter, then the following methods are defined:

- out.print(x) -- prints the value of x, represented in the form of a string of characters, to the output stream; x can be an expression of any type, including both primitive types and object types. An object is converted to string form using its toString() method. A null value is represented by the string "null".

- out.println() -- outputs an end-of-line to the output stream.

- out.println(x) -- outputs the value of x, followed by an end-of-line; this is equivalent to out.print(x) followed by out.println().

- out.printf(formatString, x1, x2, ...) -- does formatted output of x1, x2, ... to the output stream. The first parameter is a string that specifies the format of the output. There can be any number of additional parameters, of any type, but the types of the parameters must match the formatting directives in the format string. Formatted output for the standard output stream, System.out, was introduced in Subsection 2.4.1, and out.printf has the same functionality.

- out.flush() -- ensures that characters that have been written with the above methods are actually sent to the output destination. In some cases, notably when writing to a file or to the network, it might be necessary to call this method to force the output to actually appear at the destination.

Note that none of these methods will ever throw an IOException. Instead, the PrintWriter class includes the method

public boolean checkError()

which will return true if any error has been encountered while writing to the stream. The PrintWriter class catches any IOExceptions internally, and sets the value of an internal error flag if one occurs. The checkError() method can be used to check the error flag. This allows you to use PrintWriter methods without worrying about catching exceptions. On the other hand, to write a fully robust program, you should call checkError() to test for possible errors whenever you use a PrintWriter.

11.1.3  Data Streams

When you use a PrintWriter to output data to a stream, the data is converted into the sequence of characters that represents the data in human-readable form. Suppose you want to output the data in byte-oriented, machine-formatted form? The java.io package includes a byte-stream class, DataOutputStream that can be used for writing data values to streams in internal, binary-number format. DataOutputStream bears the same relationship to OutputStream that PrintWriter bears to Writer. That is, whereas OutputStream only has methods for outputting bytes, DataOutputStream has methods writeDouble(double x) for outputting values of type double, writeInt(int x) for outputting values of type int, and so on. Furthermore, you can wrap any OutputStream in a DataOutputStream so that you can use the higher level output methods on it. For example, if byteSink is of type OutputStream, you could say

DataOutputStream dataSink = new DataOutputStream(byteSink);

to wrap byteSink in a DataOutputStream.

For input of machine-readable data, such as that created by writing to a DataOutputStream, java.io provides the class DataInputStream. You can wrap any InputStream in a DataInputStream object to provide it with the ability to read data of various types from the byte-stream. The methods in the DataInputStream for reading binary data are called readDouble(), readInt(), and so on. Data written by a DataOutputStream is guaranteed to be in a format that can be read by a DataInputStream. This is true even if the data stream is created on one type of computer and read on another type of computer. The cross-platform compatibility of binary data is a major aspect of Java's platform independence.

In some circumstances, you might need to read character data from an InputStream or write character data to an OutputStream. This is not a problem, since characters, like all data, are ultimately represented as binary numbers. However, for character data, it is convenient to use Reader and Writer instead of InputStream and OutputStream. To make this possible, you can wrap a byte stream in a character stream. If byteSource is a variable of type InputStream and byteSink is of type OutputStream, then the statements

Reader charSource = new InputStreamReader( byteSource );
Writer charSink   = new OutputStreamWriter( byteSink );

create character streams that can be used to read character data from and write character data to the byte streams. In particular, the standard input stream System.in, which is of type InputStream for historical reasons, can be wrapped in a Reader to make it easier to read character data from standard input:

Reader charIn = new InputStreamReader( System.in );

As another application, the input and output streams that are associated with a network connection are byte streams rather than character streams, but the byte streams can be wrapped in character streams to make it easy to send and receive character data over the network. We will encounter network I/O in Section 11.4.

There are various ways for characters to be encoded as binary data. A particular encoding is known as a charset or character set. Charsets have standardized names such as "UTF-16," "UTF-8," and "ISO-8859-1." In UTF-16, characters are encoded as 16-bit UNICODE values; this is the character set that is used internally by Java. UTF-8 is a way of encoding UNICODE characters using 8 bits for common ASCII characters and longer codes for other characters. ISO-8859-1, also known as "Latin-1," is an 8-bit encoding that includes ASCII characters as well as certain accented characters that are used in several European languages. Readers and Writers use the default charset for the computer on which they are running, unless you specify a different one. This can be done, for example, in a constructor such as

Writer charSink = new OutputStreamWriter( byteSink, "ISO-8859-1" );

Certainly, the existence of a variety of charset encodings has made text processing more complicated -- unfortunate for us English-speakers but essential for people who use non-Western character sets. Ordinarily, you don't have to worry about this, but it's a good idea to be aware that different charsets exist in case you run into textual data encoded in a non-default way.

11.1.4  Reading Text

Much I/O is done in the form of human-readable characters. In view of this, it is surprising that Java does not provide a standard character input class that can read character data in a manner that is reasonably symmetrical with the character output capabilities of PrintWriter. (The Scanner class, introduced briefly in Subsection 2.4.6 and covered in more detail below, comes pretty close.) There is one basic case that is easily handled by the standard class BufferedReader, which has a method

public String readLine() throws IOException

that reads one line of text from its input source. If the end of the stream has been reached, the return value is null. When a line of text is read, the end-of-line marker is read from the input stream, but it is not part of the string that is returned. Different input streams use different characters as end-of-line markers, but the readLine method can deal with all the common cases. (Traditionally, Unix computers, including Linux and Mac OS X, use a line feed character, '\n', to mark an end of line; classic Macintosh used a carriage return character, '\r'; and Windows uses the two-character sequence "\r\n". In general, modern computers can deal correctly with all of these possibilities.)

Line-by-line processing is very common. Any Reader can be wrapped in a BufferedReader to make it easy to read full lines of text. If reader is of type Reader, then a BufferedReader wrapper can be created for reader with

BufferedReader in = new BufferedReader( reader );

This can be combined with the InputStreamReader class that was mentioned above to read lines of text from an InputStream. For example, we can apply this to System.in:

BufferedReader in;  // BufferedReader for reading from standard input.
in = new BufferedReader( new InputStreamReader( System.in ) );
try {
String line = in.readLine();
while ( line != null ) {
processOneLineOfInput( line );
line = in.readLine();
}
}
catch (IOException e) {
}

This code segment reads and processes lines from standard input until an end-of-stream is encountered. (An end-of-stream is possible even for interactive input. For example, on at least some computers, typing a Control-D generates an end-of-stream on the standard input stream.) The try..catch statement is necessary because the readLine method can throw an exception of type IOException, which requires mandatory exception handling; an alternative to try..catch would be to declare that the method that contains the code "throws IOException". Also, remember that BufferedReader, InputStreamReader, and IOException must be imported from the package java.io.

Note that the main purpose of BufferedReader is not simply to make it easier to read lines of text. Some I/O devices work most efficiently if data is read or written in large chunks, instead of as individual bytes or characters. A BufferedReader reads a chunk of data, and stores it in internal memory. The internal memory is known as a buffer. When you read from the BufferedReader, it will take data from the buffer if possible, and it will only go back to its input source for more data when the buffer is emptied. There is also a BufferedWriter class, and there are buffered stream classes for byte streams as well.

Previously in this book, we have used the non-standard class TextIO for input both from users and from files. The advantage of TextIO is that it makes it fairly easy to read data values of any of the primitive types. Disadvantages include the fact that TextIO can only read from one input source at a time and that it does not follow the same pattern as Java's built-in input/output classes.

I have written a class named TextReader to fix some of these disadvantages, while providing input capabilities similar to those of TextIO. Like TextIO, TextReader is a non-standard class, so you have to be careful to make it available to any program that uses it. The source code for the class can be found in the file TextReader.java.

Just as for many of Java's stream classes, an object of type TextReader can be used as a wrapper for an existing input stream, which becomes the source of the characters that will be read by the TextReader. (Unlike the standard classes, however, a TextReader is not itself a stream and cannot be wrapped inside other stream classes.) The constructors

public TextReader(Reader characterSource)

and

public TextReader(InputStream byteSource)

create objects that can be used to read character data from the given Reader or InputStream using the convenient input methods of the TextReader class. In TextIO, the input methods were static members of the class. The input methods in the TextReader class are instance methods. The instance methods in a TextReader object read from the data source that was specified in the object's constructor. This makes it possible for several TextReader objects to exist at the same time, reading from different streams; those objects can then be used to read data from several files or other input sources at the same time.

A TextReader object has essentially the same set of input methods as the TextIO class. One big difference is how errors are handled. When a TextReader encounters an error in the input, it throws an exception of type IOException. This follows the standard pattern that is used by Java's standard input streams. IOExceptions require mandatory exception handling, so TextReader methods are generally called inside try..catch statements. If an IOException is thrown by the input stream that is wrapped inside a TextReader, that IOException is simply passed along. However, other types of errors can also occur. One such possible error is an attempt to read data from the input stream when there is no more data left in the stream. A TextReader throws an exception of type TextReader.EndOfStreamException when this happens. The exception class in this case is a nested class in the TextReader class; it is a subclass of IOException, so a try..catch statement that handles IOExceptions will also handle end-of-stream exceptions. However, having a class to represent end-of-stream errors makes it possible to detect such errors and provide special handling for them. Another type of error occurs when a TextReader tries to read a data value of a certain type, and the next item in the input stream is not of the correct type. In this case, the TextReader throws an exception of type TextReader.BadDataException, which is another subclass of IOException.

For reference, here is a list of some of the more useful instance methods in the TextReader class. All of these methods can throw exceptions of type IOException:

- public char peek() -- looks ahead at the next character in the input stream, and returns that character. The character is not removed from the stream. If the next character is an end-of-line, the return value is '\n'. It is legal to call this method even if there is no more data left in the stream; in that case, the return value is the constant TextReader.EOF. ("EOF" stands for "End-Of-File," a term that is more commonly used than "End-Of-Stream", even though not all streams are files.)

- public boolean eoln() and public boolean eof() -- convenience methods for testing whether the next thing in the file is an end-of-line or an end-of-file. Note that these methods do not skip whitespace. If eof() is false, you know that there is still at least one character to be read, but there might not be any more non-blank characters in the stream.

- public void skipBlanks() and public void skipWhiteSpace() -- skip past whitespace characters in the input stream; skipWhiteSpace() skips all whitespace characters, including end-of-line while skipBlanks() only skips spaces and tabs.

- public String getln() -- reads characters up to the next end-of-line (or end-of-stream), and returns those characters in a string. The end-of-line marker is read but is not part of the returned string. This will throw an exception if there are no more characters in the stream.

- public char getAnyChar() -- reads and returns the next character from the stream. The character can be a whitespace character such as a blank or end-of-line. If this method is called after all the characters in the stream have been read, an exception is thrown.

- public int getInt(), public double getDouble(), public char getChar(), etc. -- skip any whitespace characters in the stream, including end-of-lines, then read and return a value of the specified type. Extra characters on the line are not discarded and are still available to be read by subsequent input methods. There is a method for each primitive type. An exception occurs if it's not possible to read a data value of the requested type.

- public int getlnInt(), public double getlnDouble(), public char getlnChar(), etc. -- skip any whitespace characters in the stream, including end-of-lines, then read a value of the specified type, which will be the return value of the method. Any remaining characters on the line are then discarded, including the end-of-line marker. There is a method for each primitive type. An exception occurs if it's not possible to read a data value of the requested type.

- public void close() -- Closes the input stream. This should be done when finished reading from the stream. (TextReader implements the AutoCloseable interface and so can be used as a "resource" in a try..catch statement, as discussed at the end of Subsection 8.3.2.)

11.1.5  The Scanner Class

Since its introduction, Java has been notable for its lack of built-in support for basic input, and for its reliance on fairly advanced techniques for the support that it does offer. (This is my opinion, at least.) The Scanner class was introduced to make it easier to read basic data types from a character input source. It does not (again, in my opinion) solve the problem completely, but it is a big improvement. (My TextIO and TextReader classes are not complete solutions either.) The Scanner class is in the package java.util.

Input routines are defined as instance methods in the Scanner class, so to use the class, you need to create a Scanner object. The constructor specifies the source of the characters that the Scanner will read. The scanner acts as a wrapper for the input source. The source can be a Reader, an InputStream, a String, or a File. (If a String is used as the input source, the Scanner will simply read the characters in the string from beginning to end, in the same way that it would process the same sequence of characters from a stream. The File class will be covered in the next section.) For example, you can use a Scanner to read from standard input by saying:

Scanner standardInputScanner = new Scanner( System.in );

and if charSource is of type Reader, you can create a Scanner for reading from charSource with:

Scanner scanner = new Scanner( charSource );

When processing input, a scanner usually works with tokens. A token is a meaningful string of characters that cannot, for the purposes at hand, be further broken down into smaller meaningful pieces. A token can, for example, be an individual word or a string of characters that represents a value of type double. In the case of a scanner, tokens must be separated by "delimiters." By default, the delimiters are whitespace characters such as spaces, tabs, and end-of-line markers, but you can change a Scanner's delimiters if you need to. In normal processing, whitespace characters serve simply to separate tokens and are discarded by the scanner. A scanner has instance methods for reading tokens of various types. Suppose that scanner is an object of type Scanner. Then we have:

- scanner.next() -- reads the next token from the input source and returns it as a String.

- scanner.nextInt(), scanner.nextDouble(), and so on -- read the next token from the input source and tries to convert it to a value of type int, double, and so on. There are methods for reading values of any of the primitive types.

- scanner.nextLine() -- reads an entire line from the input source, up to the next end-of-line and returns the line as a value of type String. The end-of-line marker is read but is not part of the return value. Note that this method is not based on tokens. An entire line is read and returned, including any whitespace characters in the line.

All of these methods can generate exceptions. If an attempt is made to read past the end of input, an exception of type NoSuchElementException is thrown. Methods such as scanner.getInt() will throw an exception of type InputMismatchException if the next token in the input does not represent a value of the requested type. The exceptions that can be generated do not require mandatory exception handling.

The Scanner class has very nice look-ahead capabilities. You can query a scanner to determine whether more tokens are available and whether the next token is of a given type. If scanner is of type Scanner:

- scanner.hasNext() -- returns a boolean value that is true if there is at least one more token in the input source.

- scanner.hasNextInt(), scanner.hasNextDouble(), and so on -- returns a boolean value that is true if there is at least one more token in the input source and that token represents a value of the requested type.

- scanner.hasNextLine() -- returns a boolean value that is true if there is at least one more line in the input source.

Although the insistence on defining tokens only in terms of delimiters limits the usability of scanners to some extent, they are easy to use and are suitable for many applications. With so many input classes available -- BufferedReader, TextReader, Scanner -- you might have trouble deciding which one to use! In general, I would recommend using a Scanner unless you have some particular reason for preferring the TextIO-style input routines of TextReader. BufferedReader can be used as a lightweight alternative when all that you want to do is read entire lines of text from the input source.

11.1.6  Serialized Object I/O

The classes PrintWriter, TextReader, Scanner, DataInputStream, and DataOutputStream allow you to easily input and output all of Java's primitive data types. But what happens when you want to read and write objects? Traditionally, you would have to come up with some way of encoding your object as a sequence of data values belonging to the primitive types, which can then be output as bytes or characters. This is called serializing the object. On input, you have to read the serialized data and somehow reconstitute a copy of the original object. For complex objects, this can all be a major chore. However, you can get Java to do all the work for you by using the classes ObjectInputStream and ObjectOutputStream. These are subclasses of InputStream and OutputStream that can be used for writing and reading serialized objects.

ObjectInputStream and ObjectOutputStream are wrapper classes that can be wrapped around arbitrary InputStreams and OutputStreams. This makes it possible to do object input and output on any byte stream. The methods for object I/O are readObject(), in ObjectInputStream, and writeObject(Object obj), in ObjectOutputStream. Both of these methods can throw IOExceptions. Note that readObject() returns a value of type Object, which generally has to be type-cast to the actual type of the object that was read.

ObjectOutputStream also has methods writeInt(), writeDouble(), and so on, for outputting primitive type values to the stream, and ObjectInputStream has corresponding methods for reading primitive type values. These primitive type values can be interspersed with objects in the data.

Object streams are byte streams. The objects are represented in binary, machine-readable form. This is good for efficiency, but it does suffer from the fragility that is often seen in binary data. They suffer from the additional problem that the binary format of Java objects is very specific to Java, so the data in object streams is not easily available to programs written in other programming languages. For these reasons, object streams are appropriate mostly for short-term storage of objects and for transmitting objects over a network connection from one Java program to another. For long-term storage and for communication with non-Java programs, other approaches to object serialization are usually better. (See Section 11.5 for a character-based approach.)

ObjectInputStream and ObjectOutputStream only work with objects that implement an interface named Serializable. Furthermore, all of the instance variables in the object must be serializable. However, there is little work involved in making an object serializable, since the Serializable interface does not declare any methods. It exists only as a marker for the compiler, to tell it that the object is meant to be writable and readable. You only need to add the words "implements Serializable" to your class definitions. Many of Java's standard classes are already declared to be serializable, including all the GUI component classes and many other classes in Swing and in the AWT. One of the programming examples in Section 11.3 uses object IO.

One warning about using ObjectOutputStreams: These streams are optimized to avoid writing the same object more than once. When an object is encountered for a second time, only a reference to the first occurrence is written. Unfortunately, if the object has been modified in the meantime, the new data will not be written. Because of this, ObjectOutputStreams are meant mainly for use with "immutable" objects that can't be changed after they are created. (Strings are an example of this.) However, if you do need to write mutable objects to an ObjectOutputStream, and if it is possible that you will write the same object more than once, you can ensure that the full, correct version of the object can be written by calling the stream's reset() method before writing the object to the stream.

Section 11.2

Files

The data and programs in a computer's main memory survive only as long as the power is on. For more permanent storage, computers use files, which are collections of data stored on a hard disk, on a USB memory stick, on a CD-ROM, or on some other type of storage device. Files are organized into directories (sometimes called folders). A directory can hold other directories, as well as files. Both directories and files have names that are used to identify them.

Programs can read data from existing files. They can create new files and can write data to files. In Java, such input and output can be done using streams. Human-readable character data can be read from a file using an object belonging to the class FileReader, which is a subclass of Reader. Similarly, data can be written to a file in human-readable format through an object of type FileWriter, a subclass of Writer. For files that store data in machine format, the appropriate I/O classes are FileInputStream and FileOutputStream. In this section, I will only discuss character-oriented file I/O using the FileReader and FileWriter classes. However, FileInputStream and FileOutputStream are used in an exactly parallel fashion. All these classes are defined in the java.io package.

11.2.1  Reading and Writing Files

The FileReader class has a constructor which takes the name of a file as a parameter and creates an input stream that can be used for reading from that file. This constructor will throw an exception of type FileNotFoundException if the file doesn't exist. For example, suppose you have a file named "data.txt", and you want your program to read data from that file. You could do the following to create an input stream for the file:

FileReader data;   // (Declare the variable before the
//   try statement, or else the variable
//   is local to the try block and you won't
//   be able to use it later in the program.)

try {
data = new FileReader("data.txt");  // create the stream
}
catch (FileNotFoundException e) {
... // do something to handle the error -- maybe, end the program
}

The FileNotFoundException class is a subclass of IOException, so it would be acceptable to catch IOExceptions in the above try...catch statement. More generally, just about any error that can occur during input/output operations can be caught by a catch clause that handles IOException.

Once you have successfully created a FileReader, you can start reading data from it. But since FileReaders have only the primitive input methods inherited from the basic Reader class, you will probably want to wrap your FileReader in a Scanner, in a TextReader, or in some other wrapper class. (The TextReader class is not a standard part of Java; it is described in Subsection 11.1.4. Scanner is discussed in Subsection 11.1.5.) To create a TextReader for reading from a file named data.dat, you could say:

TextReader data;

try {
data = new TextReader( new FileReader("data.dat") );
}
catch (FileNotFoundException e) {
... // handle the exception
}

To use a Scanner to read from the file, you can construct the scanner in a similar way. However, it is more common to construct it from an object of type File (to be covered below):

Scanner in;

try {
in = new Scanner( new File("data.dat") );
}
catch (FileNotFoundException e) {
... // handle the exception
}

Once you have a Scanner or TextReader for reading from a file, you can get data from the file using exactly the same methods that work with any Scanner or TextReader. When you read from a file using either of these, exceptions can occur. Since the exceptions in this case are not checked exceptions, you are not forced to enclose your input commands in a try..catch statement, but it is usually a good idea to do it anyway.

Working with output files is no more difficult than this. You simply create an object belonging to the class FileWriter. You will probably want to wrap this output stream in an object of type PrintWriter. For example, suppose you want to write data to a file named "result.dat". Since the constructor for FileWriter can throw an exception of type IOException, you should use a try..catch statement:

PrintWriter result;

try {
result = new PrintWriter(new FileWriter("result.dat"));
}
catch (IOException e) {
... // handle the exception
}

However, as with Scanner, it is more common to use a constructor that takes a File as parameter; this will automatically wrap the File in a FileWriter before creating the PrintWriter:

PrintWriter result;

try {
result = new PrintWriter(new File("result.dat"));
}
catch (IOException e) {
... // handle the exception
}

You can even use just a String as the parameter to the constructor, and it will be interpreted as a file name (but you should remember that a String in the Scanner constructor does not name a file; instead the file will read characters from the string itself).

If no file named result.dat exists, a new file will be created. If the file already exists, then the current contents of the file will be erased and replaced with the data that your program writes to the file. This will be done without any warning. To avoid overwriting a file that already exists, you can check whether a file of the same name already exists before trying to create the stream, as discussed later in this section. An IOException might occur in the PrintWriter constructor if, for example, you are trying to create a file on a disk that is "write-protected," meaning that it cannot be modified.

When you are finished with a PrintWriter, you should call its flush() method, such as "result.flush()", to make sure that all the output has been set to its destination. If you forget to do this, you might find that some of the data that you have written to a file has not actually shown up in the file.

After you are finished using a file, it's a good idea to close the file, to tell the operating system that you are finished using it. You can close a file by calling the close() method of the associated PrintWriter, TextReader, or Scanner. Once a file has been closed, it is no longer possible to read data from it or write data to it, unless you open it again as a new stream. (Note that for most stream classes, the close() method can throw an IOException, which must be handled; however, PrintWriter, TextReader, and Scanner override this method so that it cannot throw such exceptions.) If you forget to close a file, the file will ordinarily be closed automatically when the program terminates or when the file object is garbage collected, but it is better not to depend on this.

As a complete example, here is a program that will read numbers from a file named data.dat, and will then write out the same numbers in reverse order to another file named result.dat. It is assumed that data.dat contains only real numbers. The input file is read using a Scanner. Exception-handling is used to check for problems along the way. Although the application is not a particularly useful one, this program demonstrates the basics of working with files. (By the way, at the end of this program, you'll find our first useful example of a finally clause in a try statement. When the computer executes a try statement, the commands in its finally clause are guaranteed to be executed, no matter what. See Subsection 8.3.2.)

import java.io.*;
import java.util.ArrayList;

/**
* Reads numbers from a file named data.dat and writes them to a file
* named result.dat in reverse order.  The input file should contain
* exactly one real number per line.
*/
public class ReverseFile {

public static void main(String[] args) {

TextReader data;     // Character input stream for reading data.
PrintWriter result;  // Character output stream for writing data.

ArrayList<Double> numbers;  // An ArrayList for holding the data.

numbers = new ArrayList<Double>();

try {  // Create the input stream.
data = new TextReader(new FileReader("data.dat"));
}
catch (FileNotFoundException e) {
System.out.println("Can't find file data.dat!");
return;  // End the program by returning from main().
}

try {  // Create the output stream.
result = new PrintWriter(new FileWriter("result.dat"));
}
catch (IOException e) {
System.out.println("Can't open file result.dat!");
System.out.println("Error: " + e);
data.close();  // Close the input file.
return;        // End the program.
}

try {

// Read numbers from the input file, adding them to the ArrayList.

while ( data.eof() == false ) {  // Read until end-of-file.
double inputNumber = data.getlnDouble();
numbers.add( inputNumber );
}

// Output the numbers in reverse order.

for (int i = numbers.size()-1; i >= 0; i--)
result.println(numbers.get(i));

result.flush();  // Make sure data is actually sent to the file.

if (result.checkError())
System.out.println("Some error occurred while writing the file.");
else
System.out.println("Done!");

}
catch (IOException e) {
// Some problem reading the data from the input file.
// (Note that PrintWriter doesn't throw exceptions on output errors.)
System.out.println("Input Error: " + e.getMessage());
}
finally {
// Finish by closing the files, whatever else may have happened.
data.close();
result.close();
}

}  // end of main()

} // end class ReverseFileWithTextReader

A version of this program that uses a Scanner instead of a TextReader can be found in ReverseFileWithScanner.java. Note that the Scanner version does not need the final try..catch from the TextReader version, since the Scanner method for reading data doesn't throw an IOException. Instead, the program will simply stop reading data from the file if it encounters anything other than a number in the input.

As mentioned at the end of Subsection 8.3.2, the pattern of creating or opening a "resource," using it, and then closing the resource is a very common one, and the pattern is supported by the syntax of the try..catch statement. Files are resources in this sense, as are Scanner, TextReader, and all of Java's I/O streams. All of these things define close() methods, and it is good form to close them when you are finished using them. Since they all implement the AutoCloseable interface, they are all resources in the sense required by try..catch. A try..catch statement can be used to automatically close a resource when the try statement ends, which eliminates the need to close it by hand in a finally clause. This assumes that you will open the resource and use it in the same try..catch.

As an example, the sample program ReverseFileWithResources.java is another version of the example we have been looking at. In this case, try..catch statements using the resource pattern are used to read the data from a file and to write the data to a file. My original program opened a file in one try statement and used it in another try statement. The resource pattern requires that it all be done in one try, which requires some reorganization of the code (and can sometimes make it harder to determine the exact cause of an exception). Here is the try..catch statement from the sample program that opens the input file, reads from it, and closes it automatically.

try( TextReader data = new TextReader(new FileReader("data.dat")) ) {
// Read numbers, adding them to the ArrayList.
while ( data.eof() == false ) {  // Read until end-of-file.
double inputNumber = data.getlnDouble();
numbers.add( inputNumber );
}
}
catch (FileNotFoundException e) {
// Can only be caused by the TextReader constructor
System.out.println("Can't open input file data.dat!");
System.out.println("Error: " + e);
return;  // Return from main(), since an error has occurred.
//  (Otherwise, the program would try to do the output!)
}
catch (IOException e) {
// Can occur when the TextReader tries to read a number.
System.out.println("Error while reading from file: " + e);
return;  // Return from main(), since an error has occurred.
}

The resource, data, is constructed on the first line. The syntax requires a declaration of the resource, with an initial value, in parentheses after the word "try." It's possible to have several resource declarations, separated by semicolons. They will be closed in the order opposite to the order in which they are declared.

11.2.2  Files and Directories

The subject of file names is actually more complicated than I've let on so far. To fully specify a file, you have to give both the name of the file and the name of the directory where that file is located. A simple file name like "data.dat" or "result.dat" is taken to refer to a file in a directory that is called the current directory (also known as the "default directory" or "working directory"). The current directory is not a permanent thing. It can be changed by the user or by a program. Files not in the current directory must be referred to by a path name, which includes both the name of the file and information about the directory where it can be found.

To complicate matters even further, there are two types of path names, absolute path names and relative path names. An absolute path name uniquely identifies one file among all the files available to the computer. It contains full information about which directory the file is in and what the file's name is. A relative path name tells the computer how to locate the file starting from the current directory.

Unfortunately, the syntax for file names and path names varies somewhat from one type of computer to another. Here are some examples:

- data.dat -- on any computer, this would be a file named "data.dat" in the current directory.

- /home/eck/java/examples/data.dat -- This is an absolute path name in a UNIX operating system, including Linux and Mac OS X. It refers to a file named data.dat in a directory named examples, which is in turn in a directory named java, ....

- C:\eck\java\examples\data.dat -- An absolute path name on a Windows computer.

- examples/data.dat -- a relative path name under UNIX. "examples" is the name of a directory that is contained within the current directory, and data.dat is a file in that directory. The corresponding relative path name for Windows would be examples\data.dat.

- ../examples/data.dat -- a relative path name in UNIX that means "go to the directory that contains the current directory, then go into a directory named examples inside that directory, and look there for a file named data.dat." In general, ".." means "go up one directory." The corresponding path on Windows is ..\examples\data.dat.

When working on the command line, it's safe to say that if you stick to using simple file names only, and if the files are stored in the same directory with the program that will use them, then you will be OK. Later in this section, we'll look at a convenient way of letting the user specify a file in a GUI program, which allows you to avoid the issue of path names altogether.

It is possible for a Java program to find out the absolute path names for two important directories, the current directory and the user's home directory. You can then use the path name, for example, in a constructor for a File or a PrintWriter. The names of these directories are system properties, and they can be read using the function calls:

- System.getProperty("user.dir") -- returns the absolute path name of the current directory as a String.

- System.getProperty("user.home") -- returns the absolute path name of the user's home directory as a String.

To avoid some of the problems caused by differences in path names between platforms, Java has the class java.io.File. An object belonging to this class represents a file. More precisely, an object of type File represents a file name rather than a file as such. The file to which the name refers might or might not exist. Directories are treated in the same way as files, so a File object can represent a directory just as easily as it can represent a file.

A File object has a constructor, "new  File(String)", that creates a File object from a path name. The name can be a simple name, a relative path, or an absolute path. For example, new File("data.dat") creates a File object that refers to a file named data.dat, in the current directory. Another constructor, "new File(File,String)", has two parameters. The first is a File object that refers to a directory. The second can be the name of the file in that directory or a relative path from that directory to the file.

File objects contain several useful instance methods. Assuming that file is a variable of type File, here are some of the methods that are available:

- file.exists() -- This boolean-valued function returns true if the file named by the File object already exists. You can use this method if you want to avoid overwriting the contents of an existing file when you create a new output stream.

- file.isDirectory() -- This boolean-valued function returns true if the File object refers to a directory. It returns false if it refers to a regular file or if no file with the given name exists.

- file.delete() -- Deletes the file, if it exists. Returns a boolean value to indicate whether the file was successfully deleted.

- file.list() -- If the File object refers to a directory, this function returns an array of type String[] containing the names of the files in that directory. Otherwise, it returns null. The method file.listFiles() is similar, except that it returns an array of File instead of an array of String.

Here, for example, is a program that will list the names of all the files in a directory specified by the user. In this example, I have used a Scanner to read the user's input:

import java.io.File;
import java.util.Scanner;

/**
* This program lists the files in a directory specified by
* the user.  The user is asked to type in a directory name.
* If the name entered by the user is not a directory, a
* message is printed and the program ends.
*/
public class DirectoryList {

public static void main(String[] args) {

String directoryName;  // Directory name entered by the user.
File directory;        // File object referring to the directory.
String[] files;        // Array of file names in the directory.
Scanner scanner;       // For reading a line of input from the user.

scanner = new Scanner(System.in);  // scanner reads from standard input.

System.out.print("Enter a directory name: ");
directoryName = scanner.nextLine().trim();
directory = new File(directoryName);

if (directory.isDirectory() == false) {
if (directory.exists() == false)
System.out.println("There is no such directory!");
else
System.out.println("That file is not a directory.");
}
else {
files = directory.list();
System.out.println("Files in directory \"" + directory + "\":");
for (int i = 0; i < files.length; i++)
System.out.println("   " + files[i]);
}

} // end main()

} // end class DirectoryList

All the classes that are used for reading data from files and writing data to files have constructors that take a File object as a parameter. For example, if file is a variable of type File, and you want to read character data from that file, you can create a FileReader to do so by saying new FileReader(file).

11.2.3  File Dialog Boxes

In many programs, you want the user to be able to select the file that is going to be used for input or output. If your program lets the user type in the file name, you will just have to assume that the user understands how to work with files and directories. But in a graphical user interface, the user expects to be able to select files using a file dialog box, which is a window that a program can open when it wants the user to select a file for input or output. Swing includes a platform-independent technique for using file dialog boxes in the form of a class called JFileChooser. This class is part of the package javax.swing. We looked at using some basic dialog boxes in Subsection 6.7.2. File dialog boxes are similar to those, but are just a little more complicated to use.

A file dialog box shows the user a list of files and sub-directories in some directory, and makes it easy for the user to specify a file in that directory. The user can also navigate easily from one directory to another. The most common constructor for JFileChooser has no parameter and sets the starting directory in the dialog box to be the user's home directory. There are also constructors that specify the starting directory explicitly:

new JFileChooser( File startDirectory )

new JFileChooser( String pathToStartDirectory )

Constructing a JFileChooser object does not make the dialog box appear on the screen. You have to call a method in the object to do that. There are two different methods that can be used because there are two types of file dialog: An open file dialog allows the user to specify an existing file to be opened for reading data into the program; a save file dialog lets the user specify a file, which might or might not already exist, to be opened for writing data from the program. File dialogs of these two types are opened using the showOpenDialog and showSaveDialog methods. These methods make the dialog box appear on the screen; the methods do not return until the user selects a file or cancels the dialog.

A file dialog box always has a parent, another component which is associated with the dialog box. The parent is specified as a parameter to the showOpenDialog or showSaveDialog methods. The parent is a GUI component, and can often be specified as "this" in practice, since file dialogs are often used in instance methods of GUI component classes. (The parameter can also be null, in which case an invisible component is created to be used as the parent.) Both showOpenDialog and showSaveDialog have a return value, which will be one of the constants JFileChooser.CANCEL_OPTION, JFileChooser.ERROR_OPTION, or JFileChooser.APPROVE_OPTION. If the return value is JFileChooser.APPROVE_OPTION, then the user has selected a file. If the return value is something else, then the user did not select a file. The user might have clicked a "Cancel" button, for example. You should always check the return value, to make sure that the user has, in fact, selected a file. If that is the case, then you can find out which file was selected by calling the JFileChooser's getSelectedFile() method, which returns an object of type File that represents the selected file.

Putting all this together, we can look at a typical subroutine that reads data from a file that is selected using a JFileChooser:

public void readFile() {
if (fileDialog == null)   // (fileDialog is an instance variable)
fileDialog = new JFileChooser();
fileDialog.setDialogTitle("Select File for Reading");
fileDialog.setSelectedFile(null);  // No file is initially selected.
int option = fileDialog.showOpenDialog(this);
// (Using "this" as a parameter to showOpenDialog() assumes that the
//  readFile() method is an instance method in a GUI component class.)
if (option != JFileChooser.APPROVE_OPTION)
return;  // User canceled or clicked the dialog's close box.
File selectedFile = fileDialog.getSelectedFile();
TextReader in;  // (or use some other wrapper class)
try {
FileReader stream = new FileReader(selectedFile); // (or a FileInputStream)
in = new TextReader( stream );
}
catch (Exception e) {
JOptionPane.showMessageDialog(this,
"Sorry, but an error occurred while trying to open the file:\n" + e);
return;
}
try {
.
.  // Read and process the data from the input stream, in.
.
}
catch (Exception e) {
JOptionPane.showMessageDialog(this,
"Sorry, but an error occurred while trying to read the data:\n" + e);
}
finally {
in.close();
}
}

One fine point here is that the variable fileDialog is an instance variable of type JFileChooser, not a local variable. This allows the file dialog to continue to exist between calls to readFile(). The main effect of this is that the dialog box will keep the same selected directory from one call of readFile() to the next. When the dialog reappears, it will show the same directory that the user selected the previous time it appeared. This is probably what the user expects.

Note that it's common to do some configuration of a JFileChooser before calling showOpenDialog or showSaveDialog. For example, the instance method setDialogTitle(String) is used to specify a title to appear in the title bar of the window. And setSelectedFile(File) is used to set the file that is selected in the dialog box when it appears. This can be used to provide a default file choice for the user. In the readFile() method, above, fileDialog.setSelectedFile(null) specifies that no file is pre-selected when the dialog box appears. Otherwise, the selected file could be carried over from the previous time the file dialog was used.

Writing data to a file is similar, but it's a good idea to add a check to determine whether the output file that is selected by the user already exists. In that case, ask the user whether to replace the file. Here is a typical subroutine for writing to a user-selected file:

public void writeFile() {
if (fileDialog == null)
fileDialog = new JFileChooser();  // (fileDialog is an instance variable)
File selectedFile = new File("output.txt"); // (default output file name)
fileDialog.setSelectedFile(selectedFile);  // Specify a default file name.
fileDialog.setDialogTitle("Select File for Writing");
int option = fileDialog.showSaveDialog(this);
if (option != JFileChooser.APPROVE_OPTION)
return;  // User canceled or clicked the dialog's close box.
selectedFile = fileDialog.getSelectedFile();
if (selectedFile.exists()) {  // Ask the user whether to replace the file.
int response = JOptionPane.showConfirmDialog( this,
"The file \"" + selectedFile.getName()
+ "\" already exists.\nDo you want to replace it?",
"Confirm Save",
JOptionPane.YES_NO_OPTION,
JOptionPane.WARNING_MESSAGE );
if (response != JOptionPane.YES_OPTION)
return;  // User does not want to replace the file.
}
PrintWriter out;  // (or use some other wrapper class)
try {
out = new PrintWriter( selectedFile );
}
catch (Exception e) {
JOptionPane.showMessageDialog(this,
"Sorry, but an error occurred while trying to open the file:\n" + e);
return;
}
try {
.
.  // Write data to the output stream, out. (Does not throw exceptions.)
.
out.flush();
out.close();
if (out.checkError())   // (need to check for errors in PrintWriter)
throw new IOException("Error occurred while trying to write file.");
}
catch (Exception e) {
JOptionPane.showMessageDialog(this,
"Sorry, but an error occurred while trying to write the data:\n" + e);
}
}

The readFile() and writeFile() routines presented here can be used, with just a few changes, when you need to read or write a file in a GUI program. We'll look at some more complete examples of using files and file dialogs in the next section.

Section 11.3

Programming With Files

In this section, we look at several programming examples that work with files, using the techniques that were introduced in Section 11.1 and Section 11.2.

11.3.1  Copying a File

As a first example, we look at a simple command-line program that can make a copy of a file. Copying a file is a pretty common operation, and every operating system already has a command for doing it. However, it is still instructive to look at a Java program that does the same thing. Many file operations are similar to copying a file, except that the data from the input file is processed in some way before it is written to the output file. All such operations can be done by programs with the same general form. Subsection 4.3.6 included a program for copying text files using TextIO. The example in this section will work for any file.

Since the program should be able to copy any file, we can't assume that the data in the file is in human-readable form. So, we have to use byte streams InputStream and OutputStream to operate on the file. The program simply copies all the data from the InputStream to the OutputStream, one byte at a time. If source is the variable that refers to the InputStream, then the function source.read() can be used to read one byte. This function returns the value -1 when all the bytes in the input file have been read. Similarly, if copy refers to the OutputStream, then copy.write(b) writes one byte to the output file. So, the heart of the program is a simple while loop. As usual, the I/O operations can throw exceptions, so this must be done in a try..catch statement:

while(true) {
int data = source.read();
if (data < 0)
break;
copy.write(data);
}

The file-copy command in an operating system such as UNIX uses command line arguments to specify the names of the files. For example, the user might say "copy original.dat backup.dat" to copy an existing file, original.dat, to a file named backup.dat. Command-line arguments can also be used in Java programs. The command line arguments are stored in the array of strings, args, which is a parameter to the main() routine. The program can retrieve the command-line arguments from this array. (See Subsection 4.3.6.) For example, if the program is named CopyFile and if the user runs the program with the command

java CopyFile work.dat oldwork.dat

then in the program, args[0] will be the string "work.dat" and args[1] will be the string "oldwork.dat". The value of args.length tells the program how many command-line arguments were specified by the user.

The program CopyFile.java gets the names of the files from the command-line arguments. It prints an error message and exits if the file names are not specified. To add a little interest, there are two ways to use the program. The command line can simply specify the two file names. In that case, if the output file already exists, the program will print an error message and end. This is to make sure that the user won't accidently overwrite an important file. However, if the command line has three arguments, then the first argument must be "-f" while the second and third arguments are file names. The -f is a command-line option, which is meant to modify the behavior of the program. The program interprets the -f to mean that it's OK to overwrite an existing program. (The "f" stands for "force," since it forces the file to be copied in spite of what would otherwise have been considered an error.) You can see in the source code how the command line arguments are interpreted by the program:

import java.io.*;

/**
*  Makes a copy of a file.  The original file and the name of the
*  copy must be given as command-line arguments.  In addition, the
*  first command-line argument can be "-f"; if present, the program
*  will overwrite an existing file; if not, the program will report
*  an error and end if the output file already exists.  The number
*  of bytes that are copied is reported.
*/
public class CopyFile {

public static void main(String[] args) {

String sourceName;   // Name of the source file,
//    as specified on the command line.
String copyName;     // Name of the copy,
//    as specified on the command line.
InputStream source;  // Stream for reading from the source file.
OutputStream copy;   // Stream for writing the copy.
boolean force;  // This is set to true if the "-f" option
//    is specified on the command line.
int byteCount;  // Number of bytes copied from the source file.

/* Get file names from the command line and check for the
presence of the -f option.  If the command line is not one
of the two possible legal forms, print an error message and
end this program. */

if (args.length == 3 && args[0].equalsIgnoreCase("-f")) {
sourceName = args[1];
copyName = args[2];
force = true;
}
else if (args.length == 2) {
sourceName = args[0];
copyName = args[1];
force = false;
}
else {
System.out.println(
"Usage:  java CopyFile <source-file> <copy-name>");
System.out.println(
"    or  java CopyFile -f <source-file> <copy-name>");
return;
}

/* Create the input stream.  If an error occurs, end the program. */

try {
source = new FileInputStream(sourceName);
}
catch (FileNotFoundException e) {
System.out.println("Can't find file \"" + sourceName + "\".");
return;
}

/* If the output file already exists and the -f option was not
specified, print an error message and end the program. */

File file = new File(copyName);
if (file.exists() && force == false) {
System.out.println(
"Output file exists.  Use the -f option to replace it.");
return;
}

/* Create the output stream.  If an error occurs, end the program. */

try {
copy = new FileOutputStream(copyName);
}
catch (IOException e) {
System.out.println("Can't open output file \"" + copyName + "\".");
return;
}

/* Copy one byte at a time from the input stream to the output
stream, ending when the read() method returns -1 (which is
the signal that the end of the stream has been reached).  If any
error occurs, print an error message.  Also print a message if
the file has been copied successfully.  */

byteCount = 0;

try {
while (true) {
int data = source.read();
if (data < 0)
break;
copy.write(data);
byteCount++;
}
source.close();
copy.close();
System.out.println("Successfully copied " + byteCount + " bytes.");
}
catch (Exception e) {
System.out.println("Error occurred while copying.  "
+ byteCount + " bytes copied.");
System.out.println("Error: " + e);
}

}  // end main()

} // end class CopyFile

It is not terribly efficient to copy one byte at a time. Efficiency could be improved by using alternative versions of the read() and write() methods that read and write multiple bytes (see the API for details). Alternatively, the input and output streams could be wrapped in objects of type BufferedInputStream and BufferedOutputStream which automatically read from and write data to files in larger blocks, which is more efficient than reading and writing individual bytes.

(There is also a sample program CopyFileAsResources.java that does the same thing as CopyFile but uses the resource pattern in a try..catch statement to make sure that the streams are closed in all cases.)

11.3.2  Persistent Data

Once a program ends, any data that was stored in variables and objects in the program is gone. In many cases, it would be useful to have some of that data stick around so that it will be available when the program is run again. The problem is, how to make the data persistent between runs of the program? The answer, of course, is to store the data in a file (or, for some applications, in a database -- but the data in a database is itself stored in files).

Consider a "phone book" program that allows the user to keep track of a list of names and associated phone numbers. The program would make no sense at all if the user had to create the whole list from scratch each time the program is run. It would make more sense to think of the phone book as a persistent collection of data, and to think of the program as an interface to that collection of data. The program would allow the user to look up names in the phone book and to add new entries. Any changes that are made should be preserved after the program ends.

The sample program PhoneDirectoryFileDemo.java is a very simple implementation of this idea. It is meant only as an example of file use; the phone book that it implements is a "toy" version that is not meant to be taken seriously. This program stores the phone book data in a file named ".phone_book_demo" in the user's home directory. To find the user's home directory, it uses the System.getProperty() method that was mentioned in Subsection 11.2.2. When the program starts, it checks whether the file already exists. If it does, it should contain the user's phone book, which was saved in a previous run of the program, so the data from the file is read and entered into a TreeMap named phoneBook that represents the phone book while the program is running. (See Subsection 10.3.1.) In order to store the phone book in a file, some decision must be made about how the data in the phone book will be represented. For this example, I chose a simple representation in which each line of the file contains one entry consisting of a name and the associated phone number. A percent sign ('%') separates the name from the number. The following code at the beginning of the program will read the phone book data file, if it exists and has the correct format:

File userHomeDirectory = new File( System.getProperty("user.home") );
File dataFile = new File( userHomeDirectory, ".phone_book_data" );
// A file named .phone_book_data in the user's home directory.

if ( ! dataFile.exists() ) {
System.out.println("No phone book data file found.  A new one");
System.out.println("will be created, if you add any entries.");
System.out.println("File name:  " + dataFile.getAbsolutePath());
}
else {
System.out.println("Reading phone book data...");
try( Scanner scanner = new Scanner(dataFile) ) {
while (scanner.hasNextLine()) {
// Read one line from the file, containing one name/number pair.
String phoneEntry = scanner.nextLine();
int separatorPosition = phoneEntry.indexOf('%');
if (separatorPosition == -1)
throw new IOException("File is not a phonebook data file.");
name = phoneEntry.substring(0, separatorPosition);
number = phoneEntry.substring(separatorPosition+1);
phoneBook.put(name,number);
}
}
catch (IOException e) {
System.out.println("Error in phone book data file.");
System.out.println("File name:  " + dataFile.getAbsolutePath());
System.out.println("This program cannot continue.");
System.exit(1);
}
}

The program then lets the user do various things with the phone book, including making modifications. Any changes that are made are made only to the TreeMap that holds the data. When the program ends, the phone book data is written to the file (if any changes have been made while the program was running), using the following code:

if (changed) {
System.out.println("Saving phone directory changes to file " +
dataFile.getAbsolutePath() + " ...");
PrintWriter out;
try {
out = new PrintWriter( new FileWriter(dataFile) );
}
catch (IOException e) {
System.out.println("ERROR: Can't open data file for output.");
return;
}
for ( Map.Entry<String,String> entry : phoneBook.entrySet() )
out.println(entry.getKey() + "%" + entry.getValue() );
out.flush();
out.close();
if (out.checkError())
System.out.println("ERROR: Some error occurred while writing data file.");
else
System.out.println("Done.");
}

The net effect of this is that all the data, including the changes, will be there the next time the program is run. I've shown you all the file-handling code from the program. If you would like to see the rest of the program, see the source code file, PhoneDirectoryFileDemo.java.

11.3.3  Files in GUI Programs

The previous examples in this section use a command-line interface, but graphical user interface programs can also manipulate files. Programs typically have an "Open" command that reads the data from a file and displays it in a window and a "Save" command that writes the data from the window into a file. We can illustrate this in Java with a simple text editor program, TrivialEdit.java. The window for this program uses a JTextArea component to display some text that the user can edit. It also has a menu bar, with a "File" menu that includes "Open" and "Save" commands. These commands are implemented using the techniques for reading and writing files that were covered in Section 11.2.

When the user selects the Open command from the File menu in the TrivialEdit program, the program pops up a file dialog box where the user specifies the file. It is assumed that the file is a text file. A limit of 10000 characters is put on the size of the file, since a JTextArea is not meant for editing large amounts of text. The program reads the text contained in the specified file, and sets that text to be the content of the JTextArea. The program also sets the title bar of the window to show the name of the file that was opened. All this is done in the following method, which is just a variation of the readFile() method presented in Section 11.2:

/**
* Carry out the Open command by letting the user specify a file to be opened
* and reading up to 10000 characters from that file.  If the file is read
* successfully and is not too long, then the text from the file replaces the
* text in the JTextArea.
*/
public void doOpen() {
if (fileDialog == null)
fileDialog = new JFileChooser();
fileDialog.setDialogTitle("Select File to be Opened");
fileDialog.setSelectedFile(null);  // No file is initially selected.
int option = fileDialog.showOpenDialog(this);
if (option != JFileChooser.APPROVE_OPTION)
return;  // User canceled or clicked the dialog's close box.
File selectedFile = fileDialog.getSelectedFile();
Scanner in;
try {
in = new Scanner( selectedFile );
}
catch (FileNotFoundException e) {
JOptionPane.showMessageDialog(this,
"Sorry, but an error occurred while trying to open the file:\n" + e);
return;
}
try {
StringBuilder input = new StringBuilder();
while (in.hasNextLine()) {
String lineFromFile = in.nextLine();
if (lineFromFile == null)
break;  // End-of-file has been reached.
input.append(lineFromFile);
input.append('\n');
if (input.length() > 10000)
throw new IOException("Input file is too large for this program.");
}
text.setText(input.toString());
editFile = selectedFile;
setTitle("TrivialEdit: " + editFile.getName());
}
catch (Exception e) {
JOptionPane.showMessageDialog(this,
"Sorry, but an error occurred while trying to read the data:\n" + e);
}
finally {
in.close();
}
}

In this program, the instance variable editFile is used to keep track of the file that is currently being edited, if any, and the setTitle() method from class JFrame is used to set the title of the window to show the name of the file. (TrivialEdit is defined as a subclass of JFrame.)

Similarly, the response to the Save command is a minor variation on the writeFile() method from Section 11.2. I will not repeat it here. If you would like to see the entire program, you will find the source code in the file TrivialEdit.java.

11.3.4  Storing Objects in Files

Whenever data is stored in files, some definite format must be adopted for representing the data. As long as the output routine that writes the data and the input routine that reads the data use the same format, the files will be usable. However, as usual, correctness is not the end of the story. The representation that is used for data in files should also be robust. (See Section 8.1.) To see what this means, we will look at several different ways of representing the same data. This example builds on the example SimplePaint2.java from Subsection 7.3.3. (You might want to run it now to remind yourself of what it can do.) In that program, the user could use the mouse to draw simple sketches. Now, we will add file input/output capabilities to that program. This will allow the user to save a sketch to a file and later read the sketch back from the file into the program so that the user can continue to work on the sketch. The basic requirement is that all relevant data about the sketch must be saved in the file, so that the sketch can be exactly restored when the file is read by the program.

The new version of the program can be found in the source code file SimplePaintWithFiles.java. A "File" menu has been added to the new version. It contains two sets of Save/Open commands, one for saving and reloading sketch data in text form and one for data in binary form. We will consider both possibilities here, in some detail.

The data for a sketch consists of the background color of the picture and a list of the curves that were drawn by the user. A curve consists of a list of Points. Point is a standard class in package java.awt; a Point pt has instance variables pt.x and pt.y of type int that represent the pixel coordinates of a point on the xy-plane. Each curve can be a different color. Furthermore, a curve can be "symmetric," which means that in addition to the curve itself, the horizontal and vertical reflections of the curve are also drawn. The data for each curve is stored in an object of type CurveData, which is defined in the program as:

/**
* An object of type CurveData represents the data required to redraw one
* of the curves that have been sketched by the user.
*/
private static class CurveData implements Serializable {
Color color;  // The color of the curve.
boolean symmetric;  // Are horizontal and vertical reflections also drawn?
ArrayList<Point> points;  // The points on the curve.
}

Then, a list of type ArrayList<CurveData> is used to hold data for all of the curves that the user has drawn. Note that in the new version of the program, the CurveData class has been declared to "implement Serializable". This allows objects of type CurveData to be written in binary form to an ObjectOutputStream. See Subsection 11.1.6.

Let's think about how the data for a sketch could be saved to an ObjectOuputStream. The sketch is displayed on the screen in an object of type SimplePaintPanel, which is a subclass of JPanel. All the data needed for the sketch is stored in instance variables of that object. One possibility would be to simply write the entire SimplePaintPanel component as a single object to the stream. This could be done in a method in the SimplePaintPanel class with the statement

outputStream.writeObject(this);

where outputStream is the ObjectOutputStream and "this" refers to the SimplePaintPanel itself. This statement saves the entire current state of the panel. To read the data back into the program, you would create an ObjectInputStream for reading the object from the file, and you would retrieve the object from the file with the statement

SimplePaintPanel newPanel = (SimplePaintPanel)in.readObject();

where in is the ObjectInputStream. Note that the type-cast is necessary because the method in.readObject() returns a value of type Object. (To get the saved sketch to appear on the screen, the newPanel must replace the current content pane in the program's window; furthermore, the menu bar of the window must be replaced, because the menus are associated with a particular SimplePaintPanel object.)

It might look tempting to be able to save data and restore it with a single command, but in this case, it's not a good idea. The main problem with doing things this way is that the serialized form of objects that represent Swing components can change from one version of Java to the next. This means that data files that contain serialized components such as a SimplePaintPanel might become unusable in the future, and the data that they contain will be effectively lost. This is an important consideration for any serious application.

Taking this into consideration, my program uses a different format when it creates a binary file. The data written to the file consists of (1) the background color of the sketch, (2) the number of curves in the sketch, and (3) all the CurveData objects that describe the individual curves. The method that saves the data is similar to the writeFile() method from Subsection 11.2.3. Here is the complete doSaveAsBinary() method from SimplePaintWithFiles, with the changes from the generic writeFile() method shown in red italic:

/**
* Save the user's sketch to a file in binary form as serialized
* objects, using an ObjectOutputStream.  Files created by this method
* can be read back into the program using the doOpenAsBinary() method.
*/
private void doSaveAsBinary() {
if (fileDialog == null)
fileDialog = new JFileChooser();
File selectedFile;  //Initially selected file name in the dialog.
if (editFile == null)
selectedFile = new File("sketchData.binary");
else
selectedFile = new File(editFile.getName());
fileDialog.setSelectedFile(selectedFile);
fileDialog.setDialogTitle("Select File to be Saved");
int option = fileDialog.showSaveDialog(this);
if (option != JFileChooser.APPROVE_OPTION)
return;  // User canceled or clicked the dialog's close box.
selectedFile = fileDialog.getSelectedFile();
if (selectedFile.exists()) {  // Ask the user whether to replace the file.
int response = JOptionPane.showConfirmDialog( this,
"The file \"" + selectedFile.getName()
+ "\" already exists.\nDo you want to replace it?",
"Confirm Save",
JOptionPane.YES_NO_OPTION,
JOptionPane.WARNING_MESSAGE );
if (response != JOptionPane.YES_OPTION)
return;  // User does not want to replace the file.
}
ObjectOutputStream out;
try {
FileOutputStream stream = new FileOutputStream(selectedFile);
out = new ObjectOutputStream( stream );
}
catch (Exception e) {
JOptionPane.showMessageDialog(this,
"Sorry, but an error occurred while trying to open the file:\n" + e);
return;
}
try {
out.writeObject(getBackground());
out.writeInt(curves.size());
for ( CurveData curve : curves )
out.writeObject(curve);
out.flush();
out.close();
editFile = selectedFile;
setTitle("SimplePaint: " + editFile.getName());
}
catch (Exception e) {
JOptionPane.showMessageDialog(this,
"Sorry, but an error occurred while trying to write the file:\n" + e);
}
}

The heart of this method consists of the following lines, which do the actual writing of the data to the file:

out.writeObject(getBackground()); // Writes the panel's background color.
out.writeInt(curves.size());      // Writes the number of curves.
for ( CurveData curve : curves )  // For each curve...
out.writeObject(curve);        //   write the corresponding CurveData object.

The last line depends on the fact that the CurveData class implements the Serializable interface. (So does the first; the Color class, like many of Java's standard classes, implements Serializable.)

The doOpenAsBinary() method, which is responsible for reading sketch data back into the program from an ObjectInputStream, has to read exactly the same data that was written, in the same order, and use that data to build the data structures that will represent the sketch while the program is running. Once the data structures have been successfully built, they replace the data structures that describe the previous contents of the panel. This is done as follows:

/* Read data from the file into local variables */

Color newBackgroundColor = (Color)in.readObject();
int curveCount = in.readInt();
ArrayList<CurveData> newCurves = new ArrayList<CurveData>();
for (int i = 0; i < curveCount; i++)
newCurves.add( (CurveData)in.readObject() );
in.close();

/* Copy the data that was read into the instance variables that
describe the sketch that is displayed by the program.*/

curves = newCurves;
setBackground(newBackgroundColor);
repaint();

This is only a little harder than saving the entire SimplePaintPanel component to the file in one step, and it is more robust since the serialized form of the objects that are saved to file is unlikely to change in the future. But it still suffers from the general fragility of binary data.

An alternative to using object streams is to save the data in human-readable, character form. The basic idea is the same: All the data necessary to reconstitute a sketch must be saved to the output file in some definite format. The method that reads the file must follow exactly the same format as it reads the data, and it must use the data to rebuild the data structures that represent the sketch while the program is running.

When writing character data, we can't write out entire objects in one step. All the data has to be expressed, ultimately, in terms of simple data values such as strings and primitive type values. A color, for example, can be expressed in terms of three integers giving the red, green, and blue components of the color. The first (not very good) idea that comes to mind might be to just dump all the necessary data, in some definite order, into the file. Suppose that out is a PrintWriter that is used to write to the file. We could then say:

Color bgColor = getBackground();    // Write the background color to the file.
out.println( bgColor.getRed() );
out.println( bgColor.getGreen() );
out.println( bgColor.getBlue() );

out.println( curves.size() );       // Write the number of curves.

for ( CurveData curve : curves ) {  // For each curve, write...
out.println( curve.color.getRed() );      // the color of the curve
out.println( curve.color.getGreen() );
out.println( curve.color.getBlue() );
out.println( curve.symmetric ? 0 : 1 );   // the curve's symmetry property
out.println( curve.points.size() );       // the number of points on curve
for ( Point pt : curve.points ) {         // the coordinates of each point
out.println( pt.x );
out.println( pt.y );
}
}

This works in the sense that the file-reading method can read the data and rebuild the data structures. Suppose that the input method uses a Scanner named scanner to read the data file. Then it could say:

Color newBackgroundColor;                // Read the background Color.
int red = scanner.nextInt();
int green = scanner.nextInt();
int blue = scanner.nextInt();
newBackgroundColor = new Color(red,green,blue);

ArrayList<CurveData> newCurves = new ArrayList<CurveData>();

int curveCount = scanner.nextInt();      // The number of curves to be read.
for (int i = 0; i < curveCount; i++) {
CurveData curve = new CurveData();
int r = scanner.nextInt();            // Read the curve's color.
int g = scanner.nextInt();
int b = scanner.nextInt();
curve.color = new Color(r,g,b);
int symmetryCode = scanner.nextInt(); // Read the curve's symmetry property.
curve.symmetric = (symmetryCode == 1);
curveData.points = new ArrayList<Point>();
int pointCount = scanner.nextInt();  // The number of points on this curve.
for (int j = 0; j < pointCount; j++) {
int x = scanner.nextInt();        // Read the coordinates of the point.
int y = scanner.nextInt();
curveData.points.add(new Point(x,y));
}
newCurves.add(curve);
}

curves = newCurves;                     // Install the new data structures.
setBackground(newBackgroundColor);

Note how every piece of data that was written by the output method is read, in the same order, by the input method. While this does work, the data file is just a long string of numbers. It doesn't make much more sense to a human reader than a binary-format file would. Furthermore, it is still fragile in the sense that any small change made to the data representation in the program, such as adding a new property to curves, will render the data file useless (unless you happen to remember exactly which version of the program created the file).

So, I decided to use a more complex, more meaningful data format for the text files created by my program. Instead of just writing numbers, I add words to say what the numbers mean. Here is a short but complete data file for the program; just by looking at it, you can probably tell what is going on:

SimplePaintWithFiles 1.0
background 110 110 180

startcurve
color 255 255 255
symmetry true
coords 10 10
coords 200 250
coords 300 10
endcurve

startcurve
color 0 255 255
symmetry false
coords 10 400
coords 590 400
endcurve

The first line of the file identifies the program that created the data file; when the user selects a file to be opened, the program can check the first word in the file as a simple test to make sure the file is of the correct type. The first line also contains a version number, 1.0. If the file format changes in a later version of the program, a higher version number would be used; if the program sees a version number of 1.2 in a file, but the program only understands version 1.0, the program can explain to the user that a newer version of the program is needed to read the data file.

The second line of the file specifies the background color of the picture. The three integers specify the red, green, and blue components of the color. The word "background" at the beginning of the line makes the meaning clear. The remainder of the file consists of data for the curves that appear in the picture. The data for each curve is clearly marked with "startcurve" and "endcurve." The data consists of the color and symmetry properties of the curve and the xy-coordinates of each point on the curve. Again, the meaning is clear. Files in this format can easily be created or edited by hand. In fact, the data file shown above was actually created in a text editor rather than by the program. Furthermore, it's easy to extend the format to allow for additional options. Future versions of the program could add a "thickness" property to the curves to make it possible to have curves that are more than one pixel wide. Shapes such as rectangles and ovals could easily be added.

Outputting data in this format is easy. Suppose that out is a PrintWriter that is being used to write the sketch data to a file. Then the output can be done with:

out.println("SimplePaintWithFiles 1.0"); // Name and version number.
Color bgColor = getBackground();
out.println( "background " + bgColor.getRed() + " " +
bgColor.getGreen() + " " + bgColor.getBlue() );
for ( CurveData curve : curves ) {
out.println();
out.println("startcurve");
out.println("  color " + curve.color.getRed() + " " +
curve.color.getGreen() + " " + curve.color.getBlue() );
out.println( "  symmetry " + curve.symmetric );
for ( Point pt : curve.points )
out.println( "  coords " + pt.x + " " + pt.y );
out.println("endcurve");
}

Reading the data is somewhat harder, since the input routine has to deal with all the extra words in the data. In my input routine, I decided to allow some variation in the order in which the data occurs in the file. For example, the background color can be specified at the end of the file, instead of at the beginning. It can even be left out altogether, in which case white will be used as the default background color. This is possible because each item of data is labeled with a word that describes its meaning; the labels can be used to drive the processing of the input. Here is the complete method from SimplePaintWithFiles.java that reads data files in text format. It uses a Scanner to read items from the file:

private void doOpenAsText() {

if (fileDialog == null)
fileDialog = new JFileChooser();
fileDialog.setDialogTitle("Select File to be Opened");
fileDialog.setSelectedFile(null);  // No file is initially selected.
int option = fileDialog.showOpenDialog(this);
if (option != JFileChooser.APPROVE_OPTION)
return;  // User canceled or clicked the dialog's close box.
File selectedFile = fileDialog.getSelectedFile();

Scanner scanner;  // For reading from the data file.
try {
Reader stream = new BufferedReader(new FileReader(selectedFile));
scanner = new Scanner( stream );
}
catch (Exception e) {
JOptionPane.showMessageDialog(this,
"Sorry, but an error occurred while trying to open the file:\n" + e);
return;
}

try {  // Read the contents of the file.
String programName = scanner.next();
if ( ! programName.equals("SimplePaintWithFiles") )
throw new IOException("File is not a SimplePaintWithFiles data file.");
double version = scanner.nextDouble();
if (version > 1.0)
throw new IOException("File requires newer version of this program.");
Color newBackgroundColor = Color.WHITE; // default value
ArrayList<CurveData> newCurves = new ArrayList<CurveData>();
while (scanner.hasNext()) {
String itemName = scanner.next();
if (itemName.equalsIgnoreCase("background")) {
int red = scanner.nextInt();
int green = scanner.nextInt();
int blue = scanner.nextInt();
newBackgroundColor = new Color(red,green,blue);
}
else if (itemName.equalsIgnoreCase("startcurve")) {
CurveData curve = new CurveData();
curve.color = Color.BLACK;  // default value
curve.symmetric = false;    // default value
curve.points = new ArrayList<Point>();
itemName = scanner.next();
while ( ! itemName.equalsIgnoreCase("endcurve") ) {
if (itemName.equalsIgnoreCase("color")) {
int r = scanner.nextInt();
int g = scanner.nextInt();
int b = scanner.nextInt();
curve.color = new Color(r,g,b);
}
else if (itemName.equalsIgnoreCase("symmetry")) {
curve.symmetric = scanner.nextBoolean();
}
else if (itemName.equalsIgnoreCase("coords")) {
int x = scanner.nextInt();
int y = scanner.nextInt();
curve.points.add( new Point(x,y) );
}
else {
throw new Exception("Unknown term in input.");
}
itemName = scanner.next();
}
newCurves.add(curve);
}
else {
throw new Exception("Unknown term in input.");
}
}

scanner.close();
setBackground(newBackgroundColor);  // Install the new picture data.
curves = newCurves;
repaint();
editFile = selectedFile;
setTitle("SimplePaint: " + editFile.getName());
}
catch (Exception e) {
JOptionPane.showMessageDialog(this,
"Sorry, but an error occurred while trying to read the data:\n" + e);
}
}

The main reason for this long discussion of file formats has been to get you to think about the problem of representing complex data in a form suitable for storing the data in a file. The same problem arises when data must be transmitted over a network. There is no one correct solution to the problem, but some solutions are certainly better than others. In Section 11.5, we will look at one solution to the data representation problem that has become increasingly common.

In addition to being able to save sketch data in both text form and binary form, SimplePaintWithFiles can also save the picture itself as an image file that could be, for example, printed or put on a web page. This is a preview of image-handling techniques that will be covered in Chapter 13, and it uses techniques that I have not yet covered.

Section 11.4

Networking

As far as a program is concerned, a network is just another possible source of input data, and another place where data can be output. That does oversimplify things, because networks are not as easy to work with as files are. But in Java, you can do network communication using input streams and output streams, just as you can use such streams to communicate with the user or to work with files. Nevertheless, opening a network connection between two computers is a bit tricky, since there are two computers involved and they have to somehow agree to open a connection. And when each computer can send data to the other, synchronizing communication can be a problem. But the fundamentals are the same as for other forms of I/O.

One of the standard Java packages is called java.net. This package includes several classes that can be used for networking. Two different styles of network I/O are supported. One of these, which is fairly high-level, is based on the World-Wide Web, and provides the sort of network communication capability that is used by a Web browser when it downloads pages for you to view. The main classes for this style of networking are java.net.URL and java.net.URLConnection. An object of type URL is an abstract representation of a Universal Resource Locator, which is an address for an HTML document or other resource on the Web. A URLConnection represents a network connection to such a resource.

The second style of I/O, which is more general and more important, views the network at a lower level. It is based on the idea of a socket. A socket is used by a program to establish a connection with another program on a network. Communication over a network involves two sockets, one on each of the computers involved in the communication. Java uses a class called java.net.Socket to represent sockets that are used for network communication. The term "socket" presumably comes from an image of physically plugging a wire into a computer to establish a connection to a network, but it is important to understand that a socket, as the term is used here, is simply an object belonging to the class Socket. In particular, a program can have several sockets at the same time, each connecting it to another program running on some other computer on the network -- or even running on the same computer. All these connections use the same physical network connection.

This section gives a brief introduction to these basic networking classes, and shows how they relate to input and output streams.

11.4.1  URLs and URLConnections

The URL class is used to represent resources on the World-Wide Web. Every resource has an address, which identifies it uniquely and contains enough information for a Web browser to find the resource on the network and retrieve it. The address is called a "url" or "universal resource locator." (URLs can actually refer to resources from other sources besides the web; after all, they are "universal".)

An object belonging to the URL class represents such an address. Once you have a URL object, you can use it to open a URLConnection to the resource at that address. A url is ordinarily specified as a string, such as "http://math.hws.edu/eck/index.html". There are also relative url's. A relative url specifies the location of a resource relative to the location of another url, which is called the base or context for the relative url. For example, if the context is given by the url http://math.hws.edu/eck/, then the incomplete, relative url "index.html" would really refer to http://math.hws.edu/eck/index.html.

An object of the class URL is not simply a string, but it can be constructed from a string representation of a url. A URL object can also be constructed from another URL object, representing a context, plus a string that specifies a url relative to that context. These constructors have prototypes

public URL(String urlName) throws MalformedURLException

and

public URL(URL context, String relativeName) throws MalformedURLException

Note that these constructors will throw an exception of type MalformedURLException if the specified strings don't represent legal url's. The MalformedURLException class is a subclass of IOException, and it requires mandatory exception handling.

Once you have a valid URL object, you can call its openConnection() method to set up a connection. This method returns a URLConnection. The URLConnection object can, in turn, be used to create an InputStream for reading data from the resource represented by the URL. This is done by calling its getInputStream() method. For example:

URL url = new URL(urlAddressString);
URLConnection connection = url.openConnection();
InputStream in = connection.getInputStream();

The openConnection() and getInputStream() methods can both throw exceptions of type IOException. Once the InputStream has been created, you can read from it in the usual way, including wrapping it in another input stream type, such as BufferedReader, or using a Scanner. Reading from the stream can, of course, generate exceptions.

One of the other useful instance methods in the URLConnection class is getContentType(), which returns a String that describes the type of information available from the URL. The return value can be null if the type of information is not yet known or if it is not possible to determine the type. The type might not be available until after the input stream has been created, so you should generally call getContentType() after getInputStream(). The string returned by getContentType() is in a format called a mime type. Mime types include "text/plain", "text/html", "image/jpeg", "image/png", and many others. All mime types contain two parts: a general type, such as "text" or "image", and a more specific type within that general category, such as "html" or "png". If you are only interested in text data, for example, you can check whether the string returned by getContentType() starts with "text". (Mime types were first introduced to describe the content of email messages. The name stands for "Multipurpose Internet Mail Extensions." They are now used almost universally to specify the type of information in a file or other resource.)

Let's look at a short example that uses all this to read the data from a URL. This subroutine opens a connection to a specified URL, checks that the type of data at the URL is text, and then copies the text onto the screen. Many of the operations in this subroutine can throw exceptions. They are handled by declaring that the subroutine "throws IOException" and leaving it up to the main program to decide what to do when an error occurs.

static void readTextFromURL( String urlString ) throws IOException {

/* Open a connection to the URL, and get an input stream
for reading data from the URL. */

URL url = new URL(urlString);
URLConnection connection = url.openConnection();
InputStream urlData = connection.getInputStream();

/* Check that the content is some type of text.  Note: If
getContentType() method were called before getting the input
stream, it is possible for contentType to be null only because
no connection can be made.  The getInputStream() method will
throw an error if no connection can be made. */

String contentType = connection.getContentType();
System.out.println("Stream opened with content type: " + contentType);
System.out.println();
if (contentType == null || contentType.startsWith("text") == false)
throw new IOException("URL does not seem to refer to a text file.");
System.out.println("Fetching context from " + urlString + " ...");
System.out.println();

/* Copy lines of text from the input stream to the screen, until
end-of-file is encountered  (or an error occurs). */

BufferedReader in;  // For reading from the connection's input stream.
in = new BufferedReader( new InputStreamReader(urlData) );

while (true) {
String line = in.readLine();
if (line == null)
break;
System.out.println(line);
}
in.close();

} // end readTextFromURL()

A complete program that uses this subroutine can be found in the file FetchURL.java. When you run the program, you can specify the URL on the command line; if not, you will be prompted to enter the URL. For this program, a URL can begin with "http://" for a URL that refers to a resource on the web, with "file://" for a URL that refers to a file on your computer, or with "ftp://" for a URL that uses the "File Transfer Protocol." If it does not start with any of these, then "http://" is added to the start of the URL. Try the program with URL math.hws.edu/javanotes to fetch the front page of this textbook on the web. Try it with some bad inputs to see the various errors that can occur.

11.4.2  TCP/IP and Client/Server

Communication over the Internet is based on a pair of protocols called the Transmission Control Protocol and the Internet Protocol, which are collectively referred to as TCP/IP. (In fact, there is a more basic communication protocol called UDP that can be used instead of TCP in certain applications. UDP is supported in Java, but for this discussion, I'll stick to TCP/IP, which provides reliable two-way communication between networked computers.)

For two programs to communicate using TCP/IP, each program must create a socket, as discussed earlier in this section, and those sockets must be connected. Once such a connection is made, communication takes place using input streams and output streams. Each program has its own input stream and its own output stream. Data written by one program to its output stream is transmitted to the other computer. There, it enters the input stream of the program at the other end of the network connection. When that program reads data from its input stream, it is receiving the data that was transmitted to it over the network.

The hard part, then, is making a network connection in the first place. Two sockets are involved. To get things started, one program must create a socket that will wait passively until a connection request comes in from another socket. The waiting socket is said to be listening for a connection. On the other side of the connection-to-be, another program creates a socket that sends out a connection request to the listening socket. When the listening socket receives the connection request, it responds, and the connection is established. Once that is done, each program can obtain an input stream and an output stream for sending data over the connection. Communication takes place through these streams until one program or the other closes the connection.

A program that creates a listening socket is sometimes said to be a server, and the socket is called a server socket. A program that connects to a server is called a client, and the socket that it uses to make a connection is called a client socket. The idea is that the server is out there somewhere on the network, waiting for a connection request from some client. The server can be thought of as offering some kind of service, and the client gets access to that service by connecting to the server. This is called the client/server model of network communication. In many actual applications, a server program can provide connections to several clients at the same time. When a client connects to a server's listening socket, that socket does not stop listening. Instead, it continues listening for additional client connections at the same time that the first client is being serviced. To do this, it is necessary to use threads. We'll look at how it works in the next chapter.

The URL class that was discussed at the beginning of this section uses a client socket behind the scenes to do any necessary network communication. On the other side of that connection is a server program that accepts a connection request from the URL object, reads a request from that object for some particular file on the server computer, and responds by transmitting the contents of that file over the network back to the URL object. After transmitting the data, the server closes the connection.

A client program has to have some way to specify which computer, among all those on the network, it wants to communicate with. Every computer on the Internet has an IP address which identifies it. Many computers can also be referred to by domain names such as math.hws.edu or www.whitehouse.gov. (See Section 1.7.) Traditional (or IPv4) IP addresses are 32-bit integers. They are usually written in the so-called "dotted decimal" form, such as 64.89.144.135, where each of the four numbers in the address represents an 8-bit integer in the range 0 through 255. A new version of the Internet Protocol, IPv6, is currently being introduced. IPv6 addresses are 128-bit integers and are usually written in hexadecimal form (with some colons and maybe some extra information thrown in). In actual use, IPv6 addresses are still fairly rare.

A computer can have several IP addresses, and can have both IPv4 and IPv6 addresses. Usually, one of these is the loopback address, which can be used when a program wants to communicate with another program on the same computer. The loopback address has IPv4 address 127.0.0.1 and can also, in general, be referred to using the domain name localhost. In addition, there can be one or more IP addresses associated with physical network connections. Your computer probably has some utility for displaying your computer's IP addresses. I have written a small Java program, ShowMyNetwork.java, that does the same thing. When I run ShowMyNetwork on my computer, the output is:

en1 :  /192.168.1.47  /fe80:0:0:0:211:24ff:fe9c:5271%5
lo0 :  /127.0.0.1  /fe80:0:0:0:0:0:0:1%1  /0:0:0:0:0:0:0:1%0

The first thing on each line is a network interface name, which is really meaningful only to the computer's operating system. The same line also contains the IP addresses for that interface. In this example, lo0 refers to the loopback address, which has IPv4 address 127.0.0.1 as usual. The most important number here is 192.168.1.47, which is the IPv4 address that can be used for communication over the network. (The slashes at the start of each address are not part of the actual address.) The other numbers in the output are IPv6 addresses.

Now, a single computer might have several programs doing network communication at the same time, or one program communicating with several other computers. To allow for this possibility, a network connection actually has a port number in combination with an IP address. A port number is just a 16-bit positive integer. A server does not simply listen for connections -- it listens for connections on a particular port. A potential client must know both the Internet address (or domain name) of the computer on which the server is running and the port number on which the server is listening. A Web server, for example, generally listens for connections on port 80; other standard Internet services also have standard port numbers. (The standard port numbers are all less than 1024, and are reserved for particular services. If you create your own server programs, you should use port numbers greater than 1024.)

11.4.3  Sockets in Java

To implement TCP/IP connections, the java.net package provides two classes, ServerSocket and Socket. A ServerSocket represents a listening socket that waits for connection requests from clients. A Socket represents one endpoint of an actual network connection. A Socket can be a client socket that sends a connection request to a server. But a Socket can also be created by a server to handle a connection request from a client. This allows the server to create multiple sockets and handle multiple connections. A ServerSocket does not itself participate in connections; it just listens for connection requests and creates Sockets to handle the actual connections.

When you construct a ServerSocket object, you have to specify the port number on which the server will listen. The specification for the constructor is

public ServerSocket(int port) throws IOException

The port number must be in the range 0 through 65535, and should generally be greater than 1024. The constructor might throw a SecurityException if a smaller port number is specified. An IOException can occur if, for example, the specified port number is already in use. (A parameter value of 0 in this method tells the server socket to listen on any available port.)

As soon as a ServerSocket is created, it starts listening for connection requests. The accept() method in the ServerSocket class accepts such a request, establishes a connection with the client, and returns a Socket that can be used for communication with the client. The accept() method has the form

public Socket accept() throws IOException

When you call the accept() method, it will not return until a connection request is received (or until some error occurs). The method is said to block while waiting for the connection. (While the method is blocked, the program -- or more exactly, the thread -- that called the method can't do anything else. If there are other threads in the same program, they can proceed.) You can call accept() repeatedly to accept multiple connection requests. The ServerSocket will continue listening for connections until it is closed, using its close() method, or until some error occurs, or until the program is terminated in some way.

Suppose that you want a server to listen on port 1728, and that you want it to continue to accept connections as long as the program is running. Suppose that you've written a method provideService(Socket) to handle the communication with one client. Then the basic form of the server program would be:

try {
ServerSocket server = new ServerSocket(1728);
while (true) {
Socket connection = server.accept();
provideService(connection);
}
}
catch (IOException e) {
System.out.println("Server shut down with error: " + e);
}

On the client side, a client socket is created using a constructor in the Socket class. To connect to a server on a known computer and port, you would use the constructor

public Socket(String computer, int port) throws IOException

The first parameter can be either an IP number or a domain name. This constructor will block until the connection is established or until an error occurs.

Once you have a connected socket, no matter how it was created, you can use the Socket methods getInputStream() and getOutputStream() to obtain streams that can be used for communication over the connection. These methods return objects of type InputStream and OutputStream, respectively. Keeping all this in mind, here is the outline of a method for working with a client connection:

/**
* Open a client connection to a specified server computer and
* port number on the server, and then do communication through
* the connection.
*/
void doClientConnection(String computerName, int serverPort) {
Socket connection;
InputStream in;
OutputStream out;
try {
connection = new Socket(computerName,serverPort);
in = connection.getInputStream();
out = connection.getOutputStream();
}
catch (IOException e) {
System.out.println(
"Attempt to create connection failed with error: " + e);
return;
}
.
.  // Use the streams, in and out, to communicate with the server.
.
try {
connection.close();
// (Alternatively, you might depend on the server
//  to close the connection.)
}
catch (IOException e) {
}
}  // end doClientConnection()

All this makes network communication sound easier than it really is. (And if you think it sounded hard, then it's even harder.) If networks were completely reliable, things would be almost as easy as I've described. The problem, though, is to write robust programs that can deal with network and human error. I won't go into detail here. However, what I've covered here should give you the basic ideas of network programming, and it is enough to write some simple network applications. Let's look at a few working examples of client/server programming.

11.4.4  A Trivial Client/Server

The first example consists of two programs. The source code files for the programs are DateClient.java and DateServer.java. One is a simple network client and the other is a matching server. The client makes a connection to the server, reads one line of text from the server, and displays that text on the screen. The text sent by the server consists of the current date and time on the computer where the server is running. In order to open a connection, the client must know the computer on which the server is running and the port on which it is listening. The server listens on port number 32007. The port number could be anything between 1025 and 65535, as long as the server and the client use the same port. Port numbers between 1 and 1024 are reserved for standard services and should not be used for other servers. The name or IP number of the computer on which the server is running can be specified as a command-line argument. For example, if the server is running on a computer named math.hws.edu, then you could run the client with the command "java DateClient math.hws.edu". If a computer is not specified on the command line, then the user is prompted to enter one. Here is the complete client program:

import java.net.*;
import java.util.Scanner;
import java.io.*;

/**
* This program opens a connection to a computer specified
* as the first command-line argument.  If no command-line
* argument is given, it prompts the user for a computer
* to connect to.  The connection is made to
* the port specified by LISTENING_PORT.  The program reads one
* line of text from the connection and then closes the
* connection.  It displays the text that it read on
* standard output.  This program is meant to be used with
* the server program, DataServer, which sends the current
* date and time on the computer where the server is running.
*/
public class DateClient {

public static final int LISTENING_PORT = 32007;

public static void main(String[] args) {

String hostName;         // Name of the server computer to connect to.
Socket connection;       // A socket for communicating with server.
BufferedReader incoming; // For reading data from the connection.

/* Get computer name from command line. */

if (args.length > 0)
hostName = args[0];
else {
Scanner stdin = new Scanner(System.in);
System.out.print("Enter computer name or IP address: ");
hostName = stdin.nextLine();
}

/* Make the connection, then read and display a line of text. */

try {
connection = new Socket( hostName, LISTENING_PORT );
incoming = new BufferedReader(
new InputStreamReader(connection.getInputStream()) );
String lineFromServer = incoming.readLine();
if (lineFromServer == null) {
// A null from incoming.readLine() indicates that
// end-of-stream was encountered.
throw new IOException("Connection was opened, " +
"but server did not send any data.");
}
System.out.println();
System.out.println(lineFromServer);
System.out.println();
incoming.close();
}
catch (Exception e) {
System.out.println("Error:  " + e);
}

}  // end main()

} //end class DateClient

Note that all the communication with the server is done in a try..catch statement. This will catch the IOExceptions that can be generated when the connection is opened or closed and when data is read from the input stream. The connection's input stream is wrapped in a BufferedReader, which has a readLine() method that makes it easy to read one line of text. (See Subsection 11.1.4.)

In order for this program to run without error, the server program must be running on the computer to which the client tries to connect. By the way, it's possible to run the client and the server program on the same computer. For example, you can open two command windows, start the server in one window and then run the client in the other window. To make things like this easier, most computers will recognize the domain name localhost and the IP number 127.0.0.1 as referring to "this computer." This means that the command "java DateClient localhost" will tell the DateClient program to connect to a server running on the same computer. If that command doesn't work, try "java DateClient 127.0.0.1".

The server program that corresponds to the DateClient client program is called DateServer. The DateServer program creates a ServerSocket to listen for connection requests on port 32007. After the listening socket is created, the server will enter an infinite loop in which it accepts and processes connections. This will continue until the program is killed in some way -- for example by typing a CONTROL-C in the command window where the server is running. When a connection request is received from a client, the server calls a subroutine to handle the connection. In the subroutine, any Exception that occurs is caught, so that it will not crash the server. Just because a connection to one client has failed for some reason, it does not mean that the server should be shut down; the error might have been the fault of the client. The connection-handling subroutine creates a PrintWriter for sending data over the connection. It writes the current date and time to this stream and then closes the connection. (The standard class java.util.Date is used to obtain the current time. An object of type Date represents a particular date and time. The default constructor, "new Date()", creates an object that represents the time when the object is created.) The complete server program is as follows:

import java.net.*;
import java.io.*;
import java.util.Date;

/**
* This program is a server that takes connection requests on
* the port specified by the constant LISTENING_PORT.  When a
* connection is opened, the program sends the current time to
* the connected socket.  The program will continue to receive
* and process connections until it is killed (by a CONTROL-C,
* for example).  Note that this server processes each connection
* as it is received, rather than creating a separate thread
* to process the connection.
*/
public class DateServer {

public static final int LISTENING_PORT = 32007;

public static void main(String[] args) {

ServerSocket listener;  // Listens for incoming connections.
Socket connection;      // For communication with the connecting program.

/* Accept and process connections forever, or until some error occurs.
(Note that errors that occur while communicating with a connected
program are caught and handled in the sendDate() routine, so
they will not crash the server.) */

try {
listener = new ServerSocket(LISTENING_PORT);
System.out.println("Listening on port " + LISTENING_PORT);
while (true) {
// Accept next connection request and handle it.
connection = listener.accept();
sendDate(connection);
}
}
catch (Exception e) {
System.out.println("Sorry, the server has shut down.");
System.out.println("Error:  " + e);
return;
}

}  // end main()

/**
* The parameter, client, is a socket that is already connected to another
* program.  Get an output stream for the connection, send the current time,
* and close the connection.
*/
private static void sendDate(Socket client) {
try {
System.out.println("Connection from " +
client.getInetAddress().toString() );
Date now = new Date();  // The current date and time.
PrintWriter outgoing;   // Stream for sending data.
outgoing = new PrintWriter( client.getOutputStream() );
outgoing.println( now.toString() );
outgoing.flush();  // Make sure the data is actually sent!
client.close();
}
catch (Exception e){
System.out.println("Error: " + e);
}
} // end sendDate()

} //end class DateServer

When you run DateServer in a command-line interface, it will sit and wait for connection requests and report them as they are received. To make the DateServer service permanently available on a computer, the program would be run as a daemon. A daemon is a program that runs continually on a computer, independently of any user. The computer can be configured to start the daemon automatically as soon as the computer boots up. It then runs in the background, even while the computer is being used for other purposes. For example, a computer that makes pages available on the World Wide Web runs a daemon that listens for requests for web pages and responds by transmitting the pages. It's just a souped-up analog of the DateServer program! However, the question of how to set up a program as a daemon is not one I want to go into here. For testing purposes, it's easy enough to start the program by hand, and, in any case, my examples are not really robust enough or full-featured enough to be run as serious servers. (By the way, the word "daemon" is just an alternative spelling of "demon" and is usually pronounced the same way.)

Note that after calling outgoing.println() to send a line of data to the client, the server program calls outgoing.flush(). The flush() method is available in every output stream class. Calling it ensures that data that has been written to the stream is actually sent to its destination. You should generally call this function every time you use an output stream to send data over a network connection. If you don't do so, it's possible that the stream will collect data until it has a large batch of data to send. This is done for efficiency, but it can impose unacceptable delays when the client is waiting for the transmission. It is even possible that some of the data might remain untransmitted when the socket is closed, so it is especially important to call flush() before closing the connection. This is one of those unfortunate cases where different implementations of Java can behave differently. If you fail to flush your output streams, it is possible that your network application will work on some types of computers but not on others.

11.4.5  A Simple Network Chat

In the DateServer example, the server transmits information and the client reads it. It's also possible to have two-way communication between client and server. As a first example, we'll look at a client and server that allow a user on each end of the connection to send messages to the other user. The program works in a command-line interface where the users type in their messages. In this example, the server waits for a connection from a single client and then closes down its listener so that no other clients can connect. After the client and server are connected, both ends of the connection work in much the same way. The user on the client end types a message, and it is transmitted to the server, which displays it to the user on that end. Then the user of the server types a message that is transmitted to the client. Then the client user types another message, and so on. This continues until one user or the other enters "quit" when prompted for a message. When that happens, the connection is closed and both programs terminate. The client program and the server program are very similar. The techniques for opening the connections differ, and the client is programmed to send the first message while the server is programmed to receive the first message. The client and server programs can be found in the files CLChatClient.java and CLChatServer.java. (The name "CLChat" stands for "command-line chat.") Here is the source code for the server; the client is similar:

import java.net.*;
import java.util.Scanner;
import java.io.*;

/**
* This program is one end of a simple command-line interface chat program.
* It acts as a server which waits for a connection from the CLChatClient
* program.  The port on which the server listens can be specified as a
* command-line argument.  If it is not, then the port specified by the
* constant DEFAULT_PORT is used.  Note that if a port number of zero is
* specified, then the server will listen on any available port.
* This program only supports one connection.  As soon as a connection is
* opened, the listening socket is closed down.  The two ends of the connection
* each send a HANDSHAKE string to the other, so that both ends can verify
* that the program on the other end is of the right type.  Then the connected
* programs alternate sending messages to each other.  The client always sends
* the first message.  The user on either end can close the connection by
* entering the string "quit" when prompted for a message.  Note that the first
* character of any string sent over the connection must be 0 or 1; this
* character is interpreted as a command.
*/
public class CLChatServer {

/**
* Port to listen on, if none is specified on the command line.
*/
static final int DEFAULT_PORT = 1728;

/**
* Handshake string. Each end of the connection sends this  string to the
* other just after the connection is opened.  This is done to confirm that
* the program on the other side of the connection is a CLChat program.
*/
static final String HANDSHAKE = "CLChat";

/**
* This character is prepended to every message that is sent.
*/
static final char MESSAGE = '0';

/**
* This character is sent to the connected program when the user quits.
*/
static final char CLOSE = '1';

public static void main(String[] args) {

int port;   // The port on which the server listens.

ServerSocket listener;  // Listens for a connection request.
Socket connection;      // For communication with the client.

BufferedReader incoming;  // Stream for receiving data from client.
PrintWriter outgoing;     // Stream for sending data to client.
String messageOut;        // A message to be sent to the client.
String messageIn;         // A message received from the client.

Scanner userInput;        // A wrapper for System.in, for reading
// lines of input from the user.

/* First, get the port number from the command line,
or use the default port if none is specified. */

if (args.length == 0)
port = DEFAULT_PORT;
else {
try {
port= Integer.parseInt(args[0]);
if (port < 0 || port > 65535)
throw new NumberFormatException();
}
catch (NumberFormatException e) {
System.out.println("Illegal port number, " + args[0]);
return;
}
}

/* Wait for a connection request.  When it arrives, close
down the listener.  Create streams for communication
and exchange the handshake. */

try {
listener = new ServerSocket(port);
System.out.println("Listening on port " + listener.getLocalPort());
connection = listener.accept();
listener.close();
incoming = new BufferedReader(
new InputStreamReader(connection.getInputStream()) );
outgoing = new PrintWriter(connection.getOutputStream());
outgoing.println(HANDSHAKE);  // Send handshake to client.
outgoing.flush();
messageIn = incoming.readLine();  // Receive handshake from client.
if (! HANDSHAKE.equals(messageIn) ) {
throw new Exception("Connected program is not a CLChat!");
}
System.out.println("Connected.  Waiting for the first message.");
}
catch (Exception e) {
System.out.println("An error occurred while opening connection.");
System.out.println(e.toString());
return;
}

/* Exchange messages with the other end of the connection until one side
or the other closes the connection.  This server program waits for
the first message from the client.  After that, messages alternate
strictly back and forth. */

try {
userInput = new Scanner(System.in);
System.out.println("NOTE: Enter 'quit' to end the program.\n");
while (true) {
System.out.println("WAITING...");
messageIn = incoming.readLine();
if (messageIn.length() > 0) {
// The first character of the message is a command. If
// the command is CLOSE, then the connection is closed.
// Otherwise, remove the command character from the
// message and proceed.
if (messageIn.charAt(0) == CLOSE) {
System.out.println("Connection closed at other end.");
connection.close();
break;
}
messageIn = messageIn.substring(1);
}
System.out.println("RECEIVED:  " + messageIn);
System.out.print("SEND:      ");
messageOut = userInput.nextLine();
if (messageOut.equalsIgnoreCase("quit"))  {
// User wants to quit.  Inform the other side
// of the connection, then close the connection.
outgoing.println(CLOSE);
outgoing.flush();  // Make sure the data is sent!
connection.close();
System.out.println("Connection closed.");
break;
}
outgoing.println(MESSAGE + messageOut);
outgoing.flush(); // Make sure the data is sent!
if (outgoing.checkError()) {
throw new IOException("Error occurred while transmitting message.");
}
}
}
catch (Exception e) {
System.out.println("Sorry, an error has occurred.  Connection lost.");
System.out.println("Error:  " + e);
System.exit(1);
}

}  // end main()

} //end class CLChatServer

This program is a little more robust than DateServer. For one thing, it uses a handshake to make sure that a client who is trying to connect is really a CLChatClient program. A handshake is simply information sent between a client and a server as part of setting up a connection, before any actual data is sent. In this case, each side of the connection sends a string to the other side to identify itself. The handshake is part of the protocol that I made up for communication between CLChatClient and CLChatServer. A protocol is a detailed specification of what data and messages can be exchanged over a connection, how they must be represented, and what order they can be sent in. When you design a client/server application, the design of the protocol is an important consideration. Another aspect of the CLChat protocol is that after the handshake, every line of text that is sent over the connection begins with a character that acts as a command. If the character is 0, the rest of the line is a message from one user to the other. If the character is 1, the line indicates that a user has entered the "quit" command, and the connection is to be shut down.

Remember that if you want to try out this program on a single computer, you can use two command-line windows. In one, give the command "java CLChatServer" to start the server. Then, in the other, use the command "java CLChatClient localhost" to connect to the server that is running on the same machine. Note that if you run CLChatClient without specifying a computer on the command line, then you will be asked to type in the name or IP address of the server computer.

Section 11.5

A Brief Introduction to XML

When data is saved to a file or transmitted over a network, it must be represented in some way that will allow the same data to be rebuilt later, when the file is read or the transmission is received. We have seen that there are good reasons to prefer textual, character-based representations in many cases, but there are many ways to represent a given collection of data as text. In this section, we'll take a brief look at one type of character-based data representation that has become increasingly common.

XML (eXtensible Markup Language) is a syntax for creating data representation languages. There are two aspects or levels of XML. On the first level, XML specifies a strict but relatively simple syntax. Any sequence of characters that follows that syntax is a well-formed XML document. On the second level, XML provides a way of placing further restrictions on what can appear in a document. This is done by associating a DTD (Document Type Definition) with an XML document. A DTD is essentially a list of things that are allowed to appear in the XML document. A well-formed XML document that has an associated DTD and that follows the rules of the DTD is said to be a valid XML document. The idea is that XML is a general format for data representation, and a DTD specifies how to use XML to represent a particular kind of data. (There are also alternatives to DTDs, such as XML schemas, for defining valid XLM documents, but let's ignore them here.)

There is nothing magical about XML. It's certainly not perfect. It's a very verbose language, and some people think it's ugly. On the other hand it's very flexible. It can be used to represent almost any type of data. It was built from the start to support all languages and alphabets. Most important, it has become an accepted standard. There is support in just about any programming language for processing XML documents. There are standard DTDs for describing many different kinds of data. There are many ways to design a data representation language, but XML is one that has happened to come into widespread use. In fact, it has found its way into almost every corner of information technology. For example: There are XML languages for representing mathematical expressions (MathML), musical notation (MusicXML), molecules and chemical reactions (CML), vector graphics (SVG), and many other kinds of information. XML is used by OpenOffice and recent versions of Microsoft Office in the document format for office applications such as word processing, spreadsheets, and presentations. XML site syndication languages (RSS, ATOM) make it possible for web sites, newspapers, and blogs to make a list of recent headlines available in a standard format that can be used by other web sites and by web browsers; the same format is used to publish podcasts. And XML is a common format for the electronic exchange of business information.

My purpose here is not to tell you everything there is to know about XML. I will just explain a few ways in which it can be used in your own programs. In particular, I will not say anything further about DTDs and valid XML. For many purposes, it is sufficient to use well-formed XML documents with no associated DTDs.

11.5.1  Basic XML Syntax

If you know HTML, the language for writing web pages, then XML will look familiar. An XML document looks a lot like an HTML document. HTML is not itself an XML language, since it does not follow all the strict XML syntax rules, but the basic ideas are similar. Here is a short, well-formed XML document:

<?xml version="1.0"?>
<simplepaint version="1.0">
<background red='255' green='153' blue='51'/>
<curve>
<color red='0' green='0' blue='255'/>
<symmetric>false</symmetric>
<point x='83' y='96'/>
<point x='116' y='149'/>
<point x='159' y='215'/>
<point x='216' y='294'/>
<point x='264' y='359'/>
<point x='309' y='418'/>
<point x='371' y='499'/>
<point x='400' y='543'/>
</curve>
<curve>
<color red='255' green='255' blue='255'/>
<symmetric>true</symmetric>
<point x='54' y='305'/>
<point x='79' y='289'/>
<point x='128' y='262'/>
<point x='190' y='236'/>
<point x='253' y='209'/>
<point x='341' y='158'/>
</curve>
</simplepaint>

The first line, which is optional, merely identifies this as an XML document. This line can also specify other information, such as the character encoding that was used to encode the characters in the document into binary form. If this document had an associated DTD, it would be specified in a "DOCTYPE" directive on the next line of the file.

Aside from the first line, the document is made up of elements, attributes, and textual content. An element starts with a tag, such as <curve> and ends with a matching end-tag such as </curve>. Between the tag and end-tag is the content of the element, which can consist of text and nested elements. (In the example, the only textual content is the true or false in the <symmetric> elements.) If an element has no content, then the opening tag and end-tag can be combined into a single empty tag, such as <point x='83' y='96'/>, with a "/" before the final ">". This is an abbreviation for <point x='83' y='96'></point>. A tag can include attributes such as the x and y in <point x='83' y='96'/> or the version in <simplepaint version="1.0">. A document can also include a few other things, such as comments, that I will not discuss here.

The author of a well-formed XML document gets to choose the tag names and attribute names, and meaningful names can be chosen to descibe the data to a human reader. (For a valid XML document that uses a DTD, it's the author of the DTD who gets to choose the tag names.)

Every well-formed XML document follows a strict syntax. Here are some of the most important syntax rules: Tag names and attribute names in XML are case sensitive. A name must begin with a letter and can contain letters, digits and certain other characters. Spaces and ends-of-line are significant only in textual content. Every tag must either be an empty tag or have a matching end-tag. By "matching" here, I mean that elements must be properly nested; if a tag is inside some element, then the matching end-tag must also be inside that element. A document must have a root element, which contains all the other elements. The root element in the above example has tag name simplepaint. Every attribute must have a value, and that value must be enclosed in quotation marks; either single quotes or double quotes can be used for this. The special characters < and &, if they appear in attribute values or textual content, must be written as &lt; and &amp;. "&lt;" and "&amp;" are examples of entities. The entities &gt;, &quot;, and &apos; are also defined, representing >, double quote, and single quote. (Additional entities can be defined in a DTD.)

While this description will not enable you to understand everything that you might encounter in XML documents, it should allow you to design well-formed XML documents to represent data structures used in Java programs.

11.5.2  Working With the DOM

The sample XML file shown above was designed to store information about simple drawings made by the user. The drawings are ones that could be made using the sample program SimplePaint2.java from Subsection 7.3.3. We'll look at another version of that program that can save the user's drawing using an XML format for the data file. The new version is SimplePaintWithXML.java The sample XML document shown earlier in this section was produced by this program. I designed the format of that document to represent all the data needed to reconstruct a picture in SimplePaint. The document encodes the background color of the picture and a list of curves. Each <curve> element contains the data from one object of type CurveData.

It is easy enough to write data in a customized XML format, although we have to be very careful to follow all the syntax rules. Here is how I write the data for a SimplePaint picture to a PrintWriter, out. This produces an XML file with the same structure as the example shown above:

out.println("<?xml version=\"1.0\"?>");
out.println("<simplepaint version=\"1.0\">");
Color bgColor = getBackground();
out.println("   <background red='" + bgColor.getRed() + "' green='" +
bgColor.getGreen() + "' blue='" + bgColor.getBlue() + "'/>");
for (CurveData c : curves) {
out.println("   <curve>");
out.println("      <color red='" + c.color.getRed() + "' green='" +
c.color.getGreen() + "' blue='" + c.color.getBlue() + "'/>");
out.println("      <symmetric>" + c.symmetric + "</symmetric>");
for (Point pt : c.points)
out.println("      <point x='" + pt.x + "' y='" + pt.y + "'/>");
out.println("   </curve>");
}
out.println("</simplepaint>");

Reading the data back into the program is another matter. To reconstruct the data structure represented by the XML Document, it is necessary to parse the document and extract the data from it. This could be difficult to do by hand. Fortunately, Java has a standard API for parsing and processing XML Documents. (Actually, it has two, but we will only look at one of them.)

A well-formed XML document has a certain structure, consisting of elements containing attributes, nested elements, and textual content. It's possible to build a data structure in the computer's memory that corresponds to the structure and content of the document. Of course, there are many ways to do this, but there is one common standard representation known as the Document Object Model, or DOM. The DOM specifies how to build data structures to represent XML documents, and it specifies some standard methods for accessing the data in that structure. The data structure is a kind of tree whose structure mirrors the structure of the document. The tree is constructed from nodes of various types. There are nodes to represent elements, attributes, and text. (The tree can also contain several other types of node, representing aspects of XML that we can ignore here.) Attributes and text can be processed without directly manipulating the corresponding nodes, so we will be concerned almost entirely with element nodes.

(The sample program XMLDemo.java lets you experiment with XML and the DOM. It has a text area where you can enter an XML document. Initially, the input area contains the sample XML document from this section. When you click a button named "Parse XML Input", the program will attempt to read the XML from the input box and build a DOM representation of that document. If the input is not well-formed XML, an error message is displayed. If it is legal, the program will traverse the DOM representation and display a list of elements, attributes, and textual content that it encounters. The program uses a few techniques that I won't discuss here.)

In Java, the DOM representation of an XML document file can be created with just two statements. If selectedFile is a variable of type File that represents the XML file, then

DocumentBuilder docReader
= DocumentBuilderFactory.newInstance().newDocumentBuilder();
xmldoc = docReader.parse(selectedFile);

will open the file, read its contents, and build the DOM representation. The classes DocumentBuilder and DocumentBuilderFactory are both defined in the package javax.xml.parsers. The method docReader.parse() does the actual work. It will throw an exception if it can't read the file or if the file does not contain a legal XML document. If it succeeds, then the value returned by docReader.parse() is an object that represents the entire XML document. (This is a very complex task! It has been coded once and for all into a method that can be used very easily in any Java program. We see the benefit of using a standardized syntax.)

The structure of the DOM data structure is defined in the package org.w3c.dom, which contains several data types that represent an XML document as a whole and the individual nodes in a document. The "org.w3c" in the name refers to the World Wide Web Consortium, W3C, which is the standards organization for the Web. DOM, like XML, is a general standard, not just a Java standard. The data types that we need here are Document, Node, Element, and NodeList. (They are defined as interfaces rather than classes, but that fact is not relevant here.) We can use methods that are defined in these data types to access the data in the DOM representation of an XML document.

An object of type Document represents an entire XML document. The return value of docReader.parse() -- xmldoc in the above example -- is of type Document. We will only need one method from this class: If xmldoc is of type Document, then

xmldoc.getDocumentElement()

returns a value of type Element that represents the root element of the document. (Recall that this is the top-level element that contains all the other elements.) In the sample XML document from earlier in this section, the root element consists of the tag <simplepaint version="1.0">, the end-tag </simplepaint>, and everything in between. The elements that are nested inside the root element are represented by their own nodes, which are said to be children of the root node. An object of type Element contains several useful methods. If element is of type Element, then we have:

- element.getTagName() -- returns a String containing the name that is used in the element's tag. For example, the name of a <curve> element is the string "curve".

- element.getAttribute(attrName) -- if attrName is the name of an attribute in the element, then this method returns the value of that attribute. For the element, <point x="83" y="42"/>, element.getAttribute("x") would return the string "83". Note that the return value is always a String, even if the attribute is supposed to represent a numerical value. If the element has no attribute with the specified name, then the return value is an empty string.

- element.getTextContent() -- returns a String containing all the textual content that is contained in the element. Note that this includes text that is contained inside other elements that are nested inside the element.

- element.getChildNodes() -- returns a value of type NodeList that contains all the Nodes that are children of the element. The list includes nodes representing other elements and textual content that are directly nested in the element (as well as some other types of node that I don't care about here). The getChildNodes() method makes it possible to traverse the entire DOM data structure by starting with the root element, looking at children of the root element, children of the children, and so on. (There is a similar method that returns the attributes of the element, but I won't be using it here.)

- element.getElementsByTagName(tagName) -- returns a NodeList that contains all the nodes representing all elements that are nested inside element and which have the given tag name. Note that this includes elements that are nested to any level, not just elements that are directly contained inside element. The getElementsByTagName() method allows you to reach into the document and pull out specific data that you are interested in.

An object of type NodeList represents a list of Nodes. Unfortunately, it does not use the API defined for lists in the Java Collection Framework. Instead, a value, nodeList, of type NodeList has two methods: nodeList.getLength() returns the number of nodes in the list, and nodeList.item(i) returns the node at position i, where the positions are numbered 0, 1, ..., nodeList.getLength() - 1. Note that the return value of nodeList.get() is of type Node, and it might have to be type-cast to a more specific node type before it is used.

Knowing just this much, you can do the most common types of processing of DOM representations. Let's look at a few code fragments. Suppose that in the course of processing a document you come across an Element node that represents the element

<background red='255' green='153' blue='51'/>

This element might be encountered either while traversing the document with getChildNodes() or in the result of a call to getElementsByTagName("background"). Our goal is to reconstruct the data structure represented by the document, and this element represents part of that data. In this case, the element represents a color, and the red, green, and blue components are given by the attributes of the element. If element is a variable that refers to the node, the color can be obtained by saying:

int r = Integer.parseInt( element.getAttribute("red") );
int g = Integer.parseInt( element.getAttribute("green") );
int b = Integer.parseInt( element.getAttribute("blue") );
Color bgColor = new Color(r,g,b);

Suppose now that element refers to the node that represents the element

<symmetric>true</symmetric>

In this case, the element represents the value of a boolean variable, and the value is encoded in the textual content of the element. We can recover the value from the element with:

String bool = element.getTextContent();
boolean symmetric;
if (bool.equals("true"))
symmetric = true;
else
symmetric = false;

Next, consider an example that uses a NodeList. Suppose we encounter an element that represents a list of Points:

<pointlist>
<point x='17' y='42'/>
<point x='23' y='8'/>
<point x='109' y='342'/>
<point x='18' y='270'/>
</pointlist>

Suppose that element refers to the node that represents the <pointlist> element. Our goal is to build the list of type ArrayList<Point> that is represented by the element. We can do this by traversing the NodeList that contains the child nodes of element:

ArrayList<Point> points = new ArrayList<Point>();
NodeList children = element.getChildNodes();
for (int i = 0; i < children.getLength(); i++) {
Node child = children.item(i);   // One of the child nodes of element.
if ( child instanceof Element ) {
Element pointElement = (Element)child;  // One of the <point> elements.
int x = Integer.parseInt( pointElement.getAttribute("x") );
int y = Integer.parseInt( pointElement.getAttribute("y") );
Point pt = new Point(x,y); // Create the Point represented by pointElement.
points.add(pt);            // Add the point to the list of points.
}
}

All the nested <point> elements are children of the <pointlist> element. The if statement in this code fragment is necessary because an element can have other children in addition to its nested elements. In this example, we only want to process the children that are elements.

All these techniques can be employed to write the file input method for the sample program SimplePaintWithXML.java. When building the data structure represented by an XML file, my approach is to start with a default data structure and then to modify and add to it as I traverse the DOM representation of the file. It's not a trivial process, but I hope that you can follow it:

Color newBackground = Color.WHITE;
ArrayList<CurveData> newCurves = new ArrayList<CurveData>();

Element rootElement = xmldoc.getDocumentElement();

if ( ! rootElement.getNodeName().equals("simplepaint") )
throw new Exception("File is not a SimplePaint file.");
String version = rootElement.getAttribute("version");
try {
double versionNumber = Double.parseDouble(version);
if (versionNumber > 1.0)
throw new Exception("File requires a newer version of SimplePaint.");
}
catch (NumberFormatException e) {
}

NodeList nodes = rootElement.getChildNodes();

for (int i = 0; i < nodes.getLength(); i++) {
if (nodes.item(i) instanceof Element) {
Element element = (Element)nodes.item(i);
if (element.getTagName().equals("background")) { // Read background color.
int r = Integer.parseInt(element.getAttribute("red"));
int g = Integer.parseInt(element.getAttribute("green"));
int b = Integer.parseInt(element.getAttribute("blue"));
newBackground = new Color(r,g,b);
}
else if (element.getTagName().equals("curve")) { // Read data for a curve.
CurveData curve = new CurveData();
curve.color = Color.BLACK;
curve.points = new ArrayList<Point>();
newCurves.add(curve);  // Add this curve to the new list of curves.
NodeList curveNodes = element.getChildNodes();
for (int j = 0; j < curveNodes.getLength(); j++) {
if (curveNodes.item(j) instanceof Element) {
Element curveElement = (Element)curveNodes.item(j);
if (curveElement.getTagName().equals("color")) {
int r = Integer.parseInt(curveElement.getAttribute("red"));
int g = Integer.parseInt(curveElement.getAttribute("green"));
int b = Integer.parseInt(curveElement.getAttribute("blue"));
curve.color = new Color(r,g,b);
}
else if (curveElement.getTagName().equals("point")) {
int x = Integer.parseInt(curveElement.getAttribute("x"));
int y = Integer.parseInt(curveElement.getAttribute("y"));
curve.points.add(new Point(x,y));
}
else if (curveElement.getTagName().equals("symmetric")) {
String content = curveElement.getTextContent();
if (content.equals("true"))
curve.symmetric = true;
}
}
}
}
}
}
curves = newCurves;  // Change picture in window to show the data from file.
setBackground(newBackground);
repaint();

You can find the complete source code in SimplePaintWithXML.java.

XML has developed into an extremely important technology, and some applications of it are very complex. But there is a core of simple ideas that can be easily applied in Java. Knowing just the basics, you can make good use of XML in your own Java programs.

Programming Exercises for Chapter 11

Exercise 11.1:

The sample program DirectoryList.java, given as an example in Subsection 11.2.2, will print a list of files in a directory specified by the user. But some of the files in that directory might themselves be directories. And the subdirectories can themselves contain directories. And so on. Write a modified version of DirectoryList that will list all the files in a directory and all its subdirectories, to any level of nesting. You will need a recursive subroutine to do the listing. The subroutine should have a parameter of type File. You will need the constructor from the File class that has the form

public File( File dir, String fileName )
// Constructs the File object representing a file
// named fileName in the directory specified by dir.

See the Solution

Exercise 11.2:

Write a program that will count the number of lines in each file that is specified on the command line. Assume that the files are text files. Note that multiple files can be specified, as in:

java  LineCounts  file1.txt  file2.txt  file3.txt

Write each file name, along with the number of lines in that file, to standard output. If an error occurs while trying to read from one of the files, you should print an error message for that file, but you should still process all the remaining files. Do not use TextIO to process the files; use a Scanner, a BufferedReader, or a TextReader to process each file.

See the Solution

Exercise 11.3:

For this exercise, you will write a network server program. The program is a simple file server that makes a collection of files available for transmission to clients. When the server starts up, it needs to know the name of the directory that contains the collection of files. This information can be provided as a command-line argument. You can assume that the directory contains only regular files (that is, it does not contain any sub-directories). You can also assume that all the files are text files.

When a client connects to the server, the server first reads a one-line command from the client. The command can be the string "INDEX". In this case, the server responds by sending a list of names of all the files that are available on the server. Or the command can be of the form "GET <filename>", where <filename> is a file name. The server checks whether the requested file actually exists. If so, it first sends the word "OK" as a message to the client. Then it sends the contents of the file and closes the connection. Otherwise, it sends a line beginning with the word "ERROR" to the client and closes the connection. (The error response can include an error message on the rest of the line.)

Write a subroutine to handle each request. See the DirectoryList example in Subsection 11.2.2 for help with the problem of getting the list of files in the directory.

See the Solution

Exercise 11.4:

Write a client program for the server from Exercise 11.3. Design a user interface that will let the user do at least two things: (1) Get a list of files that are available on the server and display the list on standard output; and (2) Get a copy of a specified file from the server and save it to a local file (on the computer where the client is running).

See the Solution

Exercise 11.5:

The sample program PhoneDirectoryFileDemo.java, from Subsection 11.3.2, stores name/number pairs for a simple phone book in a text file in the user's home directory. Modify that program so that it uses an XML format for the data. The only significant changes that you will have to make are to the parts of the program that read and write the data file. Use the DOM to read the data, as discussed in Subsection 11.5.2. You can use the XML format illustrated in the following sample phone directory file:

<?xml version="1.0"?>
<phone_directory>
<entry name='barney' number='890-1203'/>
<entry name='fred' number='555-9923'/>
</phone_directory>

(This is just a short exercise in basic XML processing; as before, the program in this exercise is not meant to be a useful phone directory program.)

See the Solution

Exercise 11.6:

The sample program Checkers.java from Subsection 7.5.3 lets two players play checkers. It would be nice if, in the middle of a game, the state of the game could be saved to a file. Later, the file could be read back into the file to restore the game and allow the players to continue. Add the ability to save and load files to the checkers program. Design a simple text-based format for the files. Here is a picture of my solution to this exercise, just after a file has been loaded into the program:

It's a little tricky to completely restore the state of a game. The program has a variable board of type CheckersData that stores the current contents of the board, and it has a variable currentPlayer of type int that indicates whether Red or Black is currently moving. This data must be stored in the file when a file is saved. When a file is read into the program, you should read the data into two local variables newBoard of type CheckersData and newCurrentPlayer of type int. Once you have successfully read all the data from the file, you can use the following code to set up the program state correctly. This code assumes that you have introduced two new variables saveButton and loadButton of type JButton to represent the "Save Game" and "Load Game" buttons:

board = newBoard;  // Set up game with data read from file.
currentPlayer = newCurrentPlayer;
legalMoves = board.getLegalMoves(currentPlayer);
selectedRow = -1;
gameInProgress = true;
newGameButton.setEnabled(false);
loadButton.setEnabled(false);
saveButton.setEnabled(true);
resignButton.setEnabled(true);
if (currentPlayer == CheckersData.RED)
message.setText("Game loaded -- it's RED's move.");
else
message.setText("Game loaded -- it's BLACK's move.");
repaint();

(Note, by the way, that in my solution, I use a TextReader to read the data from the file. TextReader is a non-standard class introduced in Subsection 11.1.4 and defined in the file TextReader.java. How to read the data in a file depends, of course, on the format that you have chosen for the data.)

See the Solution

Quiz on Chapter 11

Question 1:

In Java, input/output is done using streams. Streams are an abstraction. Explain what this means and why it is important.

Question 2:

Java has two types of streams: character streams and byte streams. Why? What is the difference between the two types of streams?

Question 3:

What is a file? Why are files necessary?

Question 4:

What is the point of the following statement?

out = new PrintWriter( new FileWriter("data.dat") );

Why would you need a statement that involves two different stream classes, PrintWriter and FileWriter?

Question 5:

The package java.io includes a class named URL. What does an object of type URL represent, and how is it used?

Question 6:

What is the purpose of the JFileChooser class?

Question 7:

Explain what is meant by the client / server model of network communication.

Question 8:

What is a socket?

Question 9:

What is a ServerSocket and how is it used?

Question 10:

What is meant by an element in an XML document?

Question 11:

What is it about XML that makes it suitable for representing almost any type of data?

Question 12:

Write a complete program that will display the first ten lines from a text file. The lines should be written to standard output, System.out. The file name is given as the command-line argument args[0]. You can assume that the file contains at least ten lines. Don't bother to make the program robust. Do not use TextIO to process the file; read from the file using methods covered in this chapter.

See the Answers

Chapter 12

Threads and Multiprocessing

In the classic programming model, there is a single central processing unit that reads instructions from memory and carries them out, one after the other. The purpose of a program is to provide the list of instructions for the processor to execute. This is the only type of programming that we have considered so far.

However, this model of programming has limitations. Modern computers have multiple processors, making it possible for them to perform several tasks at the same time. To use the full potential of all those processors, you will need to write programs that can do parallel processing. For Java programmers, that means learning about threads. A single thread is similar to the programs that you have been writing up until now, but more than one thread can be running at the same time, "in parallel." What makes things more interesting -- and more difficult -- than single-threaded programming is the fact that the threads in a parallel program are rarely completely independent of one another. They usually need to cooperate and communicate. Learning to manage and control cooperation among threads is the main hurdle that you will face in this chapter.

There are several reasons to use parallel programming. One is simply to do computations more quickly by setting several processors to work on them simultaneously. Just as important, however, is to use threads to deal with "blocking" operations, where a process can't proceed until some event occurs. In the previous chapter, for example, we saw how programs can block while waiting for data to arrive over a network connection. Threads make it possible for one part of a program to continue to do useful work even while another part is blocked, waiting for some event to occur. In this context, threads are a vital programming tool even for a computer that has only a single processing unit.

Contents of Chapter 12:

- Section 1: Introduction to Threads

- Section 2: Programming with Threads

- Section 3: Threads and Parallel Processing

- Section 4: Threads and Networking

- Section 5: Network Programming Example: A Networked Game Framework

- Programming Exercises

- Quiz on This Chapter

Section 12.1

Introduction to Threads

Like people, computers can multitask. That is, they can be working on several different tasks at the same time. A computer that has just a single central processing unit can't literally do two things at the same time, any more than a person can, but it can still switch its attention back and forth among several tasks. Furthermore, it is increasingly common for computers to have more than one processing unit, and such computers can literally work on several tasks simultaneously. It is likely that from now on, most of the increase in computing power will come from adding additional processors to computers rather than from increasing the speed of individual processors. To use the full power of these multiprocessing computers, a programmer must do parallel programming, which means writing a program as a set of several tasks that can be executed simultaneously. Even on a single-processor computer, parallel programming techniques can be useful, since some problems can be tackled most naturally by breaking the solution into a set of simultaneous tasks that cooperate to solve the problem.

In Java, a single task is called a thread. The term "thread" refers to a "thread of control" or "thread of execution," meaning a sequence of instructions that are executed one after another -- the thread extends through time, connecting each instruction to the next. In a multithreaded program, there can be many threads of control, weaving through time in parallel and forming the complete fabric of the program. (Ok, enough with the metaphor, already!) Every Java program has at least one thread; when the Java virtual machine runs your program, it creates a thread that is responsible for executing the main routine of the program. This main thread can in turn create other threads that can continue even after the main thread has terminated. In a GUI program, there is at least one additional thread, which is responsible for handling events and drawing components on the screen. This GUI thread is created when the first window is opened. So in fact, you have already done parallel programming! When a main routine opens a window, both the main thread and the GUI thread can continue to run in parallel. Of course, parallel programming can be used in much more interesting ways.

Unfortunately, parallel programming is even more difficult than ordinary, single-threaded programming. When several threads are working together on a problem, a whole new category of errors is possible. This just means that techniques for writing correct and robust programs are even more important for parallel programming than they are for normal programming. On the other hand, fortunately, Java has a nice thread API that makes basic uses of threads reasonably easy. It also has a variety of standard classes to help with some of the more tricky parts or to hide them entirely. It won't be until midway through Section 12.3 that you'll learn about the low-level techniques that are necessary to handle the trickiest parts of parallel programming. In fact, a programmer can do a lot with threads without ever learning about the low-level stuff.

12.1.1  Creating and Running Threads

In Java, a thread is represented by an object belonging to the class java.lang.Thread (or to a subclass of this class). The purpose of a Thread object is to execute a single method and to execute it just once. This method represents the task to be carried out by the thread. The method is executed in its own thread of control, which can run in parallel with other threads. When the execution of the thread's method is finished, either because the method terminates normally or because of an uncaught exception, the thread stops running. Once this happens, there is no way to restart the thread or to use the same Thread object to start another thread.

There are two ways to program a thread. One is to create a subclass of Thread and to define the method public void run() in the subclass. This run() method defines the task that will be performed by the thread; that is, when the thread is started, it is the run() method that will be executed in the thread. For example, here is a simple, and rather useless, class that defines a thread that does nothing but print a message on standard output:

public class NamedThread extends Thread {
private String name;  // The name of this thread.
public NamedThread(String name) {  // Constructor gives name to thread.
this.name = name;
}
public void run() {  // The run method prints a message to standard output.
System.out.println("Greetings from thread '" + name + "'!");
}
}

To use a NamedThread, you must of course create an object belonging to this class. For example,

NamedThread greetings = new NamedThread("Fred");

However, creating the object does not automatically start the thread running or cause its run() method to be executed. To do that, you must call the start() method in the thread object. For the example, this would be done with the statement

greetings.start();

The purpose of the start() method is to create the new thread of control that will execute the Thread object's run() method. The new thread runs in parallel with the thread in which the start() method was called, along with any other threads that already existed. The start() method returns immediately after starting the new thread of control, without waiting for the thread to terminate. This means that the code in the thread's run() method executes at the same time as the statements that follow the call to the start() method. Consider this code segment:

NamedThread greetings = new NamedThread("Fred");
greetings.start();
System.out.println("Thread has been started");

After greetings.start() is executed, there are two threads. One of them will print "Thread has been started" while the other one wants to print "Greetings from thread 'Fred'!". It is important to note that these messages can be printed in either order. The two threads run simultaneously and will compete for access to standard output, so that they can print their messages. Whichever thread happens to be the first to get access will be the first to print its message. In a normal, single-threaded program, things happen in a definite, predictable order from beginning to end. In a multi-threaded program, there is a fundamental indeterminacy. You can't be sure what order things will happen in. This indeterminacy is what makes parallel programming so difficult!

Note that calling greetings.start() is very different from calling greetings.run(). Calling greetings.run() would execute the run() method in the same thread, rather than creating a new thread. This means that all the work of the run() method will be done before the computer moves on to the statements that follow the call to greetings.run(). There is no parallelism and no indeterminacy.

This discussion has assumed that the computer on which the program is running has more than one processing unit, so that it is possible for the original thread and the newly created thread to literally be executed at the same time. However, it's possible to create multiple threads even on a computer that has only one processor (and, more generally, it is possible to create many more threads than there are processors, on any computer). In that case, the two threads will compete for time on the processor. However, there is still indeterminacy because the processor can switch from one thread to another at unpredictable times. In fact, from the point of view of the programmer, there is no difference between programming for a single-processor computer and programming for a multi-processor computer, and we will mostly ignore the distinction from now on.

I mentioned that there are two ways to program a thread. The first way was to define a subclass of Thread. The second is to define a class that implements the interface java.lang.Runnable. The Runnable interface defines a single method, public void run(). Given a Runnable, it is possible to create a Thread whose task is to execute the Runnable's run() method.

The Thread class has a constructor that takes a Runnable as its parameter. When an object that implements the Runnable interface is passed to that constructor, the run() method of the thread will simply call the run() method from the Runnable, and calling the thread's start() method will create a new thread of control in which the Runnable's run() method is executed. For example, as an alternative to the NamedThread class, we could define the class:

public class NamedRunnable implements Runnable {
private String name;  // The name of this Runnable.
public NamedRunnable(String name) {  // Constructor gives name to object.
this.name = name;
}
public void run() {  // The run method prints a message to standard output.
System.out.println("Greetings from runnable '" + name +"'!");
}
}

To use this version of the class, we would create a NamedRunnable object and use that object to create an object of type Thread:

NamedRunnable greetings = new NamedRunnable("Fred");
Thread greetingsThread = new Thread(greetings);
greetingsThread.start();

The advantage of doing things this way is that any object can implement the Runnable interface and can contain a run() method, which can then be executed in a separate thread. That run() method has access to everything in the class, including private variables and methods. The disadvantage is that this way of doing things is not very object-oriented: It violates the principle that each object should have a single, clearly-defined responsibility. Instead of making some random object Runnable just so that you can use it to make a thread, you can consider defining the thread using a nested class that is a subclass of the Thread class. (See Section 5.8.)

Finally, I'll note that it is sometimes convenient to define a thread using an anonymous inner class (Subsection 5.8.3). For example:

Thread greetingsFromFred = new Thread() {
public void run() {
System.out.println("Greetings from Fred!");
}
};
greetingsFromFred.start();

To help you understand how multiple threads are executed in parallel, we consider the sample program ThreadTest1.java. This program creates several threads. Each thread performs exactly the same task. The task is to count the number of integers less than 5000000 that are prime. (The particular task that is done is not important for our purposes here. This is a demo program; it would be silly in a real program to have multiple threads that do the same thing.) This computation should take no more than a few seconds on a modern computer. The threads that perform this task are defined by the following static nested class:

/**
* When a thread belonging to this class is run it will count the
* number of primes between 2 and 5000000.  It will print the result
* to standard output, along with its ID number and the elapsed
* time between the start and the end of the computation.
*/
private static class CountPrimesThread extends Thread {
int id;  // An id number for this thread; specified in the constructor.
public CountPrimesThread(int id) {
this.id = id;
}
public void run() {
long startTime = System.currentTimeMillis();
int count = countPrimes(2,5000000); // Counts the primes.
long elapsedTime = System.currentTimeMillis() - startTime;
System.out.println("Thread " + id + " counted " +
count + " primes in " + (elapsedTime/1000.0) + " seconds.");
}
}

The main program asks the user how many threads to run, and then creates and starts the specified number of threads:

public static void main(String[] args) {
int numberOfThreads = 0;
while (numberOfThreads < 1 || numberOfThreads > 25) {
System.out.print("How many threads do you want to use  (1 to 25) ?  ");
numberOfThreads = TextIO.getlnInt();
if (numberOfThreads < 1 || numberOfThreads > 25)
System.out.println("Please enter a number between 1 and 25 !");
}
System.out.println("\nCreating " + numberOfThreads
+ " prime-counting threads...");
CountPrimesThread[] worker = new CountPrimesThread[numberOfThreads];
for (int i = 0; i < numberOfThreads; i++)
worker[i] = new CountPrimesThread( i );
for (int i = 0; i < numberOfThreads; i++)
worker[i].start();
System.out.println("Threads have been created and started.");
}

It would be a good idea for you to compile and run the program.

When I ran the program with one thread on my computer, it took 6.251 seconds for the computer to do the computation. When I ran it using eight threads, the output was:

Creating 8 prime-counting threads...
Threads have been created and started.
Thread 4 counted 348513 primes in 12.264 seconds.
Thread 2 counted 348513 primes in 12.569 seconds.
Thread 3 counted 348513 primes in 12.567 seconds.
Thread 0 counted 348513 primes in 12.569 seconds.
Thread 7 counted 348513 primes in 12.562 seconds.
Thread 5 counted 348513 primes in 12.565 seconds.
Thread 1 counted 348513 primes in 12.569 seconds.
Thread 6 counted 348513 primes in 12.563 seconds.

The second line was printed immediately after the first. At this point, the main program has ended but the eight threads continue to run. After a pause of about 12.5 seconds, all eight threads completed at about the same time. The order in which the threads complete is not the same as the order in which they were started, and the order is indeterminate. That is, if the program is run again, the order in which the threads complete will probably be different.

On this computer, eight threads took about twice as long as one thread. This is because the computer has four processors. With eight threads running on four processors -- one-half processor per thread -- each thread was only actively being executed for about half of the time, so it took twice as long to finish its task. On a single-processor computer, eight threads would take about eight times as long as one thread. On a computer with eight or more processors, eight threads might take no more time than a single thread. Because of overhead and other reasons, the actual speedup will probably be a little smaller than this analysis indicates, but on a multiprocessor machine, you should see a definite speedup. What happens when you run the program on your own computer? How many processors do you have?

Whenever there are more threads to be run than there are processors to run them, the computer divides its attention among all the runnable threads by switching rapidly from one thread to another. That is, each processor runs one thread for a while then switches to another thread and runs that one for a while, and so on. Typically, these "context switches" occur about 100 times or more per second. The result is that the computer makes progress on all the tasks, and it looks to the user as if all the tasks are being executed simultaneously. This is why in the sample program, in which each thread has the same amount of work to do, all the threads complete at about the same time: Over any time period longer than a fraction of a second, the computer's time is divided approximately equally among all the threads.

12.1.2  Operations on Threads

Much of Java's thread API can be found in the Thread class. However, we'll start with a thread-related method in Runtime, a class that allows a Java program to get information about the environment in which it is running. When you do parallel programming in order to spread the work among several processors, you might want to take into account the number of available processors. You might, for example, want to create one thread for each processor. In Java, you can find out the number of processors by calling the function

Runtime.getRuntime().availableProcessors()

which returns an int giving the number of processors that are available to the Java Virtual Machine. In some cases, this might be less than the actual number of processors in the computer.

A Thread object contains several useful methods for working with threads. Most important is the start() method, which was discussed above.

Once a thread has been started, it will continue to run until its run() method ends for some reason. Sometimes, it's useful for one thread to be able to tell whether another thread has terminated. If thrd is an object of type Thread, then the boolean-valued function thrd.isAlive() can be used to test whether or not thrd has terminated. A thread is "alive" between the time it is started and the time when it terminates. After the thread has terminated it is said to be "dead." (The rather gruesome metaphor is also used when we refer to "killing" or "aborting" a thread.) Remember that a thread that has terminated cannot be restarted.

The static method Thread.sleep(milliseconds) causes the thread that executes this method to "sleep" for the specified number of milliseconds. A sleeping thread is still alive, but it is not running. While a thread is sleeping, the computer can work on any other runnable threads (or on other programs). Thread.sleep() can be used to insert a pause in the execution of a thread. The sleep() method can throw an exception of type InterruptedException, which is a checked exception that requires mandatory exception handling. In practice, this means that the sleep() method is usually called inside a try..catch statement that catches the potential InterruptedException:

try {
Thread.sleep(lengthOfPause);
}
catch (InterruptedException e) {
}

One thread can interrupt another thread to wake it up when it is sleeping or paused for certain other reasons. A Thread, thrd, can be interrupted by calling the method thrd.interrupt(). Doing so can be a convenient way to send a signal from one thread to another. A thread knows it has been interrupted when it catches an InterruptedException. Outside any catch handler for the exception, the thread can check whether it has been interrupted by calling the static method Thread.interrupted(). This method tells whether the current thread -- the thread that executes the method -- has been interrupted. It also has the unusual property of clearing the interrupted status of the thread, so you only get one chance to check for an interruption. In your own programs, your threads are not going to be interrupted unless you interrupt them. So most often, you are not likely to need to do anything in response to an InterruptedException (except to catch it).

Sometimes, it's necessary for one thread to wait for another thread to die. This is done with the join() method from the Thread class. Suppose that thrd is a Thread. Then, if another thread calls thrd.join(), that other thread will go to sleep until thrd terminates. If thrd is already dead when thrd.join() is called, then it simply has no effect. The join() method can throw an InterruptedException, which must be handled as usual. As an example, the following code starts several threads, waits for them all to terminate, and then outputs the elapsed time:

CountPrimesThread[] worker = new CountPrimesThread[numberOfThreads];
long startTime = System.currentTimeMillis();
for (int i = 0; i < numberOfThreads; i++) {
worker[i] = new CountPrimesThread();
worker[i].start();
}
for (int i = 0; i < numberOfThreads; i++) {
try {
worker[i].join();  // Wait until worker[i] finishes, if it hasn't already.
}
catch (InterruptedException e) {
}
}
// At this point, all the worker threads have terminated.
long elapsedTime = System.currentTimeMillis() - startTime;
System.out.println("Total elapsed time: " + (elapsedTime/1000.0) + " seconds");

An observant reader will note that this code assumes that no InterruptedException will occur. To be absolutely sure that the thread worker[i] has terminated in an environment where InterruptedExceptions are possible, you would have to do something like:

while (worker[i].isAlive()) {
try {
worker[i].join();
}
catch (InterruptedException e) {
}
}

Another version of the join() method takes an integer parameter that specifies the maximum number of milliseconds to wait. A call to thrd.join(m) will wait until either thrd has terminated or until m milliseconds have elapsed. This can be used to allow a thread to wake up occasionally to perform some task while it is waiting. Here, for example, is a code segment that will start a thread, thrd, and then will output a period every two seconds as long as thrd continues to run:

System.out.print("Running the thread ");
thrd.start();
while (thrd.isAlive()) {
try {
thrd.join(2000);
System.out.print(".");
}
catch (InterruptedException e) {
}
}
System.out.println(" Done!");

Threads have two properties that are occasionally useful: a daemon status and a priority. A Thread thrd can be designated as a daemon thread by calling thrd.setDaemon(true). This must be done before the thread is started, and it can throw an exception of type SecurityException if the calling thread is not allowed to modify thrd's properties. This has only one effect: The Java Virtual Machine will exit as soon as there are no non-daemon threads that are still alive. That is, the fact that a daemon thread is still alive is not enough to keep the Java Virtual Machine running. A daemon thread might exist, for example, only to provide some service to other, non-daemon threads. When there are no more non-daemon threads, there will be no further call for the daemon thread's services, so the program might as well shut down.

The priority of a thread is a more important property. Every thread has a priority, specified as an integer. A thread with a greater priority value will be run in preference to a thread with a smaller priority. For example, computations that can be done in the background, when no more important thread has work to do, can be run with a low priority. In the next section, we will see how this can be useful in GUI programs. If thrd is of type Thread, then thrd.getPriority() returns the integer that specifies thrd's priority, and thrd.setPriority(p) can be used to set its priority to a given integer, p.

Priorities cannot be arbitrary integers, and thrd.setPriority() will throw an IllegalArgumentException if the specified priority is not in the legal range for the thread. The range of legal priority values can differ from one computer to another. The range of legal values is specified by the constants Thread.MIN_PRIORITY and Thread.MAX_PRIORITY, but a given thread might be further restricted to values less than Thread.MAX_PRIORITY. The default priority is given by Thread.NORM_PRIORITY. To set thrd to run with a priority value just below the normal priority, you can call

thrd.setPriority( Thread.NORM_PRIORITY - 1 );

Note that thrd.setPriority() can also throw an exception of type SecurityException, if the thread that calls the method is not allowed to set the priority of thrd.

Finally, I'll note that the static method Thread.currentThread() returns the current thread. That is, the return value of this method is the thread that executed the method. This allows a thread to get a reference to itself, so that it can modify its own properties. For example, you can determine the priority of the currently running thread by calling Thread.currentThread().getPriority().

12.1.3  Mutual Exclusion with "synchronized"

It's pretty easy to program several threads to carry out completely independent tasks. The real difficulty arises when threads have to interact in some way. One way that threads interact is by sharing resources. When two threads need access to the same resource, such as a variable or a window on the screen, some care must be taken that they don't try to use the same resource at the same time. Otherwise, the situation could be something like this: Imagine several cooks sharing the use of just one measuring cup, and imagine that Cook A fills the measuring cup with milk, only to have Cook B grab the cup before Cook A has a chance to empty the milk into his bowl. There has to be some way for Cook A to claim exclusive rights to the cup while he performs the two operations: Add-Milk-To-Cup and Empty-Cup-Into-Bowl.

Something similar happens with threads, even with something as simple as adding one to a counter. The statement

count = count + 1;

is actually a sequence of three operations:

Step 1.  Get the value of count
Step 2.  Add 1 to the value.
Step 3.  Store the new value in count

Suppose that each of several threads performs these three steps. Remember that it's possible for two threads to run at the same time, and even if there is only one processor, it's possible for that processor to switch from one thread to another at any point. Suppose that while one thread is between Step 2 and Step 3, another thread starts executing the same sequence of steps. Since the first thread has not yet stored the new value in count, the second thread reads the old value of count and adds one to that old value. Both threads have computed the same new value for count, and both threads then go on to store that value back into count by executing Step 3. After both threads have done so, the value of count has gone up only by 1 instead of by 2! This type of problem is called a race condition. This occurs when one thread is in the middle of a multi-step operation, and another thread can change some value or condition that the first thread is depending upon. (The first thread is "in a race" to complete all the steps before it is interrupted by another thread.)

Another example of a race condition can occur in an if statement. Consider the following statement, which is meant to avoid a division-by-zero error:

if ( A != 0 ) {
B = C / A;
}

Suppose that this statement is executed by some thread. If the variable A is shared by one or more other threads, and if nothing is done to guard against the race condition, then it is possible that one of those other threads will change the value of A to zero between the time that the first thread checks the condition A != 0 and the time that it does the division. This means that the thread can end up dividing by zero, even though it just checked that A was not zero!

To fix the problem of race conditions, there has to be some way for a thread to get exclusive access to a shared resource. This is not a trivial thing to implement, but Java provides a high-level and relatively easy-to-use approach to exclusive access. It's done with synchronized methods and with the synchronized statement. These are used to protect shared resources by making sure that only one thread at a time will try to access the resource. Synchronization in Java actually provides only mutual exclusion, which means that exclusive access to a resource is only guaranteed if every thread that needs access to that resource uses synchronization. Synchronization is like a cook leaving a note that says, "I'm using the measuring cup." This will get the cook exclusive access to the cup -- but only if all the cooks agree to check the note before trying to grab the cup.

Because this is a difficult topic, I will start with a simple example. Suppose that we want to avoid the race condition that occurs when several threads all want to add 1 to a counter. We can do this by defining a class to represent the counter and by using synchronized methods in that class. A method is declared to be synchronized by adding the reserved word synchronized as a modifier to the definition of the method:

public class ThreadSafeCounter {

private int count = 0;  // The value of the counter.

synchronized public void increment() {
count = count + 1;
}

synchronized public int getValue() {
return count;
}

}

If tsc is of type ThreadSafeCounter, then any thread can call tsc.increment() to add 1 to the counter in a completely safe way. The fact that tsc.increment() is synchronized means that only one thread can be in this method at a time; once a thread starts executing this method, it is guaranteed that it will finish executing it without having another thread change the value of tsc.count in the meantime. There is no possibility of a race condition. Note that the guarantee depends on the fact that count is a private variable. This forces all access to tsc.count to occur in the synchronized methods that are provided by the class. If count were public, it would be possible for a thread to bypass the synchronization by, for example, saying tsc.count++. This could change the value of count while another thread is in the middle of tsc.increment(). Remember that synchronization by itself does not guarantee exclusive access; it only guarantees mutual exclusion among all the threads that are synchronized.

However, the ThreadSafeCounter class does not prevent all possible race conditions that might arise when using a counter. Consider the if statement:

if ( tsc.getValue() == 0 ) {
doSomething();
}

where doSomething() is some method that requires the value of the counter to be zero. There is still a race condition here, which occurs if a second thread increments the counter between the time the first thread tests tsc.getValue() == 0 and the time it executes doSomething(). The first thread needs exclusive access to the counter during the execution of the whole if statement. (The synchronization in the ThreadSafeCounter class only gives it exclusive access during the time it is evaluating tsc.getValue().) We can solve the race condition by putting the if statement in a synchronized statement:

synchronized(tsc) {
if ( tsc.getValue() == 0 )
doSomething();
}

Note that the synchronized statement takes an object -- tsc in this case -- as a kind of parameter. The syntax of the synchronized statement is:

synchronized( object ) {
statements
}

In Java, mutual exclusion is always associated with an object; we say that the synchronization is "on" that object. For example, the if statement above is "synchronized on tsc." A synchronized instance method, such as those in the class ThreadSafeCounter, is synchronized on the object that contains the instance method. In fact, adding the synchronized modifier to the definition of an instance method is pretty much equivalent to putting the body of the method in a synchronized statement of the form synchronized(this) {...}. It is also possible to have synchronized static methods; a synchronized static method is synchronized on the special class object that represents the class containing the static method.

The real rule of synchronization in Java is this: Two threads cannot be synchronized on the same object at the same time; that is, they cannot simultaneously be executing code segments that are synchronized on that object. If one thread is synchronized on an object, and a second thread tries to synchronize on the same object, the second thread is forced to wait until the first thread has finished with the object. This is implemented using something called a synchronization lock. Every object has a synchronization lock, and that lock can be "held" by only one thread at a time. To enter a synchronized statement or synchronized method, a thread must obtain the associated object's lock. If the lock is available, then the thread obtains the lock and immediately begins executing the synchronized code. It releases the lock after it finishes executing the synchronized code. If Thread A tries to obtain a lock that is already held by Thread B, then Thread A has to wait until Thread B releases the lock. In fact, Thread A will go to sleep, and will not be awoken until the lock becomes available.

As a simple example of shared resources, we return to the prime-counting problem. In this case, instead of having every thread perform exactly the same task, we'll do some real parallel processing. The program will count the prime numbers in a given range of integers, and it will do so by dividing the work up among several threads. Each thread will be assigned a part of the full range of integers, and it will count the primes in its assigned part. At the end of its computation, the thread has to add its count to the overall total of primes in the entire range. The variable that represents the total is shared by all the threads, since each thread has to add a number to the total. If each thread just says

total = total + count;

then there is a (small) chance that two threads will try to do this at the same time and that the final total will be wrong. To prevent this race condition, access to total has to be synchronized. My program uses a synchronized method to add the counts to the total. This method is called once by each thread:

synchronized private static void addToTotal(int x) {
total = total + x;
System.out.println(total + " primes found so far.");
}

The source code for the program can be found in ThreadTest2.java. This program counts the primes in the range 3000001 to 6000000. (The numbers are rather arbitrary.) The main() routine in this program creates between 1 and 5 threads and assigns part of the job to each thread. It waits for all the threads to finish, using the join() method as described above. It then reports the total number of primes found, along with the elapsed time. Note that join() is required here, since it doesn't make sense to report the number of primes until all of the threads have finished. If you run the program on a multiprocessor computer, it should take less time for the program to run when you use more than one thread.

Synchronization can help to prevent race conditions, but it introduces the possibility of another type of error, deadlock. A deadlock occurs when a thread waits forever for a resource that it will never get. In the kitchen, a deadlock might occur if two very simple-minded cooks both want to measure a cup of milk at the same time. The first cook grabs the measuring cup, while the second cook grabs the milk. The first cook needs the milk, but can't find it because the second cook has it. The second cook needs the measuring cup, but can't find it because the first cook has it. Neither cook can continue and nothing more gets done. This is deadlock. Exactly the same thing can happen in a program, for example if there are two threads (like the two cooks) both of which need to obtain locks on the same two objects (like the milk and the measuring cup) before they can proceed. Deadlocks can easily occur, unless great care is taken to avoid them.

12.1.4  Volatile Variables

Synchronization is only one way of controlling communication among threads. We will cover several other techniques later in the chapter. For now, we finish this section with one more communication technique: volatile variables.

In general, threads communicate by sharing variables and accessing those variables in synchronized methods or synchronized statements. However, synchronization is fairly expensive computationally, and excessive use of it should be avoided. So in some cases, it can make sense for threads to refer to shared variables without synchronizing their access to those variables.

However, a subtle problem arises when the value of a shared variable is set in one thread and used in another. Because of the way that threads are implemented in Java, the second thread might not see the changed value of the variable immediately. That is, it is possible that a thread will continue to see the old value of the shared variable for some time after the value of the variable has been changed by another thread. This is because threads are allowed to cache shared data. That is, each thread can keep its own local copy of the shared data. When one thread changes the value of a shared variable, the local copies in the caches of other threads are not immediately changed, so the other threads can continue to see the old value, at least briefly.

When a synchronized method or statement is entered, threads are forced to update their caches to the most current values of the variables in the cache. So, using shared variables in synchronized code is always safe.

It is possible to use a shared variable safely outside of synchronized code, but in that case, the variable must be declared to be volatile. The volatile keyword is a modifier that can be added to a variable declaration, as in

private volatile int count;

If a variable is declared to be volatile, no thread will keep a local copy of that variable in its cache. Instead, the thread will always use the official, main copy of the variable. This means that any change that is made to the variable will immediately be visible to all threads. This makes it safe for threads to refer to volatile shared variables even outside of synchronized code. Access to volatile variables is less efficient than access to non-volatile variables, but more efficient than using synchronization. (Remember, though, that synchronization is still the only way to prevent race conditions.)

When the volatile modifier is applied to an object variable, only the variable itself is declared to be volatile, not the contents of the object that the variable points to. For this reason, volatile is used mostly for variables of simple types such as primitive types or immutable types like String.

A typical example of using volatile variables is to send a signal from one thread to another that tells the second thread to terminate. The two threads would share a variable

volatile boolean terminate = false;

The run method of the second thread would check the value of terminate frequently, and it would end when the value of terminate becomes true:

public void run() {
while ( terminate == false ) {
.
.  // Do some work.
.
}
}

This thread will run until some other thread sets the value of terminate to true. Something like this is really the only clean way for one thread to cause another thread to die.

(By the way, you might be wondering why threads should use local data caches in the first place, since it seems to complicate things unnecessarily. Caching is allowed because of the structure of multiprocessing computers. In many multiprocessing computers, each processor has some local memory that is directly connected to the processor. A thread's cache can be stored in the local memory of the processor on which the thread is running. Access to this local memory is much faster than access to other memory, so it is more efficient for a thread to use a local copy of a shared variable rather than some "master copy" that is stored in non-local memory.)

Section 12.2

Programming with Threads

Threads introduce new complexity into programming, but they are an important tool and will only become more essential in the future. So, every programmer should know some of the fundamental design patterns that are used with threads. In this section, we will look at some basic techniques, with more to come as the chapter progresses.

12.2.1  Threads Versus Timers

One of the most basic uses of threads is to perform some periodic task at set intervals. In fact, this is so basic that there is a specialized class for performing this task -- and you've already worked with it. The Timer class, in package javax.swing, can generate a sequence of ActionEvents separated by a specified time interval. Timers were introduced in Section 6.4, where they were used to implement animations. Behind the scenes, a Timer uses a thread. The thread sleeps most of the time, but it wakes up periodically to generate the events associated with the timer. Before timers were introduced, threads had to be used directly to implement a similar functionality.

In a typical use of a timer for animation, each event from the timer causes a new frame of the animation to be computed and displayed. In the response to the event, it is only necessary to update some state variables and to repaint the display to reflect the changes. A Timer to do that every thirty milliseconds might be created like this:

Timer timer = new Timer( 30, new ActionListener() {
public void actionPerformed(ActionEvent evt) {
updateForNextFrame();
display.repaint();
}
} );
timer.start();

Suppose that we wanted to do the same thing with a thread. The run() method of the thread would have to execute a loop in which the thread sleeps for 30 milliseconds, then wakes up to do the updating and repainting. This could be implemented in a nested class as follows using the method Thread.sleep() that was discussed in Subsection 12.1.2:

private class Animator extends Thread {
public void run() {
while (true) {
try {
Thread.sleep(30);
}
catch (InterruptedException e) {
}
updateForNextFrame();
display.repaint();
}
}
}

To run the animation, you would create an object belonging to this class and call its start() method. As it stands, there would be no way to stop the animation once it is started. One way to make it possible to stop the animation would be to end the loop when a volatile boolean variable, terminate, becomes true, as discussed in Subsection 12.1.4. Since thread objects can only be executed once, in order to restart the animation after it has been stopped in this way, it would be necessary to create a new thread. In the next section, we'll see some more versatile techniques for controlling threads.

There is a subtle difference between using threads and using timers for animation. The thread that is used by a Swing Timer does nothing but generate events. The event-handling code that is executed in response to those events is actually executed in the Swing event-handling GUI thread, which also handles repainting of components and responses to user actions. This is important because the Swing GUI is not thread-safe. That is, it does not use synchronization to avoid race conditions among threads trying to access GUI components and their state variables. As long as everything is done in the Swing event thread, there is no problem. A problem can arise when another thread manipulates components or the variables that are also used in the GUI thread. In the Animator example given above, this could happen when the thread calls the updateForNextFrame() method. The variables that are modified in updateForNextFrame() would also be used by the paintComponent() method that draws the frame. There is a race condition here: If these two methods are being executed simultaneously, there is a possibility that paintComponent() will use inconsistent variable values -- some appropriate for the new frame, some for the previous frame.

One solution to this problem would be to declare both paintComponent() and updateForNextFrame() to be synchronized methods. A better solution in this case is to use a timer rather than a thread. In practice, race conditions might not be a problem for a simple animation, even if they are implemented using threads. But it can become a real issue when threads are used for more complex tasks.

I should note that the repaint() method of a component can be safely called from any thread, without worrying about synchronization. Recall that repaint() does not actually do any painting itself. It just tells the system to schedule a paint event. The actual painting will be done later, in the Swing event-handling thread. I will also note that Java has another timer class, java.util.Timer, that is appropriate for use in non-GUI programs.

The sample program RandomArtWithThreads.java uses a thread to drive a very simple animation. You can compare it to RandomArt.java, from Section 6.4, which implemented the same animation with a timer. In this example, the thread does nothing except to call repaint(), so there is no need for synchronization.

12.2.2  Recursion in a Thread

Although timers should be used in preference to threads when possible, there are times when it is reasonable to use a thread even for a straightforward animation. One reason to do so is when the thread is running a recursive algorithm, and you want to repaint the display many times over the course of the recursion. (Recursion is covered in Section 9.1.) It's difficult to drive a recursive algorithm with a series of events from a timer; it's much more natural to use a single recursive method call to do the recursion, and it's easy to do that in a thread.

As an example, the program QuicksortThreadDemo.java uses an animation to illustrate the recursive QuickSort algorithm for sorting an array. In this case, the array contains colors, and the goal is to sort the colors into a standard spectrum from red to violet. In the program, the user randomizes the array and starts the sorting process by clicking the "Start" button below the display. The "Start" button changes to a "Finish" button that can be used to abort the sort before it finishes on its own. (It's an interesting program to watch, and it might even help you to understand QuickSort better, so you should try running it.)

In this program, the display's repaint() method is called every time the algorithm makes a change to the array. Whenever this is done, the thread sleeps for 100 milliseconds to allow time for the display to be repainted and for the user to see the change. There is also a longer delay, one full second, just after the array is randomized, before the sorting starts. Since these delays occur at several points in the code, QuicksortThreadDemo defines a delay() method that makes the thread that calls it sleep for a specified period. The delay() method calls display.repaint() just before sleeping. While the animation thread sleeps, the event-handling thread will have a chance to run and will have plenty of time to repaint the display. (I didn't use synchronization in this example, but it would have been a good idea to do so.)

An interesting question is how to implement the "Finish" button, which should abort the sort and terminate the thread. Pressing this button causes the value of a volatile boolean variable, running, to be set to false, as a signal to the thread that it should terminate. The problem is that this button can be clicked at any time, even when the algorithm is many levels down in the recursion. Before the thread can terminate, all of those recursive method calls must return. A nice way to cause that is to throw an exception. QuickSortThreadDemo defines a new exception class, ThreadTerminationException, for this purpose. The delay() method checks the value of the signal variable, running. If running is false, the delay() method throws the exception that will cause the recursive algorithm, and eventually the animation thread itself, to terminate. Here, then, is the delay() method:

private void delay(int millis) {
if (! running)
throw new ThreadTerminationException();
display.repaint();
try {
Thread.sleep(millis);
}
catch (InterruptedException e) {
}
if (! running) // Check again, in case it changed during the sleep period.
throw new ThreadTerminationException();
}

The ThreadTerminationException is caught in the thread's run() method:

/**
* This class defines the threads that run the recursive
* QuickSort algorithm.  The thread begins by randomizing the
* array, hue.  It then calls quickSort() to sort the entire array.
* If quickSort() is aborted by a ThreadTerminationException,
* which would be caused by the user clicking the Finish button,
* then the thread will restore the array to sorted order before
* terminating, so that whether or not the quickSort is aborted,
* the array ends up sorted.
*/
private class Runner extends Thread {
public void run() {
try {
for (int i = hue.length-1; i > 0; i--) { // Randomize array.
int r = (int)((i+1)*Math.random());
int temp = hue[r];
hue[r] = hue[i];
hue[i] = temp;
}
delay(1000);  // Wait one second before starting the sort.
quickSort(0,hue.length-1);  // Sort the whole array, recursively.
}
catch (ThreadTerminationException e) { // User clicked "Finish".
for (int i = 0; i < hue.length; i++)
hue[i] = i;  // restore original array values.
}
finally {// Make sure running is false and button label is correct.
running = false;
startButton.setText("Start");
display.repaint();
}
}
}

The program uses a variable, runner, of type Runner to represent the thread that does the sorting. When the user clicks the "Start" button, the following code is executed to create and start the thread:

startButton.setText("Finish");
runner = new Runner();
running = true;  // Set the signal before starting the thread!
runner.start();

Note that the value of the signal variable running is set to true before starting the thread. If running were false when the thread was started, the thread might see that value as soon as it starts and interpret it as a signal to stop before doing anything. Remember that when runner.start() is called, runner starts running in parallel with the thread that called it.

Stopping the thread is a little more interesting, because the thread might be sleeping when the "Finish" button is pressed. The thread has to wake up before it can act on the signal that it is to terminate. To make the thread a little more responsive, we can call runner.interrupt(), which will wake the thread if it is sleeping. (See Subsection 12.1.2.) This doesn't have much practical effect in this program, but it does make the program respond noticeably more quickly if the user presses "Finish" immediately after pressing "Start," while the thread is sleeping for a full second.

12.2.3  Threads for Background Computation

In order for a GUI program to be responsive -- that is, to respond to events very soon after they are generated -- it's important that event-handling methods in the program finish their work very quickly. Remember that events go into a queue as they are generated, and the computer cannot respond to an event until after the event-handler methods for previous events have done their work. This means that while one event handler is being executed, other events will have to wait. If an event handler takes a while to run, the user interface will effectively freeze up during that time. This can be very annoying if the delay is more than a fraction of a second. Fortunately, modern computers can do an awful lot of computation in a fraction of a second.

However, some computations are too big to be done in event handlers. The solution, in that case, is to do the computation in another thread that runs in parallel with the event-handling thread. This makes it possible for the computer to respond to user events even while the computation is ongoing. We say that the computation is done "in the background."

Note that this application of threads is very different from the previous example. When a thread is used to drive a simple animation, it actually does very little work. The thread only has to wake up several times each second, do a few computations to update state variables for the next frame of the animation, and call repaint() to cause the next frame to be displayed. There is plenty of time while the thread is sleeping for the computer to redraw the display and handle any other events generated by the user.

When a thread is used for background computation, however, we want to keep the computer as busy as possible working on the computation. The thread will compete for processor time with the event-handling thread; if you are not careful, event-handling -- repainting in particular -- can still be delayed. Fortunately, you can use thread priorities to avoid the problem. By setting the computation thread to run at a lower priority than the event-handling thread, you make sure that events will be processed as quickly as possible, while the computation thread will get all the extra processing time. Since event handling generally uses very little processing time, this means that most of the processing time goes to the background computation, but the interface is still very responsive. (Thread priorities were discussed in Subsection 12.1.2.)

The sample program BackgroundComputationDemo.java is an example of background processing. This program creates an image that takes a while to compute. The program uses some techniques for working with images that will not be covered until Section 13.1, for now all that you need to know is that it takes some computation to compute the color of each pixel in the image. The image itself is a piece of a mathematical object known as the Mandelbrot set. We will use the same image in several examples in this chapter, and will return to the Mandelbrot set in Section 13.5.

In outline, BackgroundComputationDemo is similar to the QuicksortThreadDemo discussed above. The computation is done in a thread defined by a nested class, Runner. A volatile boolean variable, running, is used to control the thread. If the value of running is set to false, the thread should terminate. The sample program has a button that the user clicks to start and to abort the computation. The difference is that the thread in this case is meant to run continuously, without sleeping. To allow the user to see that progress is being made in the computation (always a good idea), every time the thread computes a row of pixels, it copies those pixels to the image that is shown on the screen. The user sees the image being built up line-by-line.

When the computation thread is created in response to the "Start" button, we need to set it to run at a priority lower than the event-handling thread. The code that creates the thread is itself running in the event-handling thread, so we can use a priority that is one less than the priority of the thread that is executing the code. Note that the priority is set inside a try..catch statement. If an error occurs while trying to set the thread priority, the program will still work, though perhaps not as smoothly as it would if the priority was correctly set. Here is how the thread is created and started:

runner = new Runner();
try {
runner.setPriority( Thread.currentThread().getPriority() - 1 );
}
catch (Exception e) {
System.out.println("Error: Can't set thread priority: " + e);
}
running = true;  // Set the signal before starting the thread!
runner.start();

The other major point of interest in this program is that we have two threads that are both using the object that represents the image. The computation thread accesses the image in order to set the color of its pixels. The event-handling thread accesses the same image when it copies the image to the screen. Since the image is a resource that is shared by several threads, access to the image object should be synchronized. When the paintComponent() method copies the image to the screen (using a method that we have not yet covered), it does so in a synchronized statement:

synchronized(image) {
g.drawImage(image,0,0,null);
}

When the computation thread sets the colors of a row of pixels (using another unfamiliar method), it also uses synchronized:

synchronized(image) {
image.setRGB(0,row, width, 1, rgb, 0, width);
}

Note that both of these statements are synchronized on the same object, image. This is essential. In order to prevent the two code segments from being executed simultaneously, the synchronization must be on the same object. I use the image object here because it is convenient, but just about any object would do; it is not required that you synchronize on the object to which you are trying to control access.

Although BackgroundComputationDemo works OK, there is one problem: The goal is to get the computation done as quickly as possible, using all available processing time. The program accomplishes that goal on a computer that has only one processor. But on a computer that has several processors, we are still using only one of those processors for the computation. It would be nice to get all the processors working on the problem. To do that, we need real parallel processing, with several computation threads. We turn to that problem next.

12.2.4  Threads for Multiprocessing

Our next example, MultiprocessingDemo1.java, is a variation on BackgroundComputationDemo. Instead of doing the computation in a single thread, MultiprocessingDemo1 can divide the problem among several threads. The user can select the number of threads to be used. Each thread is assigned one section of the image to compute. The threads perform their tasks in parallel. For example, if there are two threads, the first thread computes the top half of the image while the second thread computes the bottom half. Here is picture of the program nearing the end of a computation using three threads. The gray areas represent parts of the image that have not yet been computed:

You should try out the program. On a multi-processor computer, the computation will complete more quickly when using several threads than when using just one. Note that when using one thread, this program has the same behavior as the previous example program.

The approach used in this example for dividing up the problem among threads is not optimal. We will see in the next section how it can be improved. However, MultiprocessingDemo1 makes a good first example of multiprocessing.

When the user clicks the "Start" button, the program has to create and start the specified number of threads, and it has to assign a segment of the image to each thread. Here is how this is done:

workers = new Runner[threadCount];  // Holds the computation threads.
int rowsPerThread;  // How many rows of pixels should each thread compute?
rowsPerThread  = height / threadCount;  // (height = vertical size of image)
running = true;  // Set the signal before starting the threads!
threadsCompleted = 0;  // Records how many of the threads have terminated.
for (int i = 0; i < threadCount; i++) {
int startRow;  // first row computed by thread number i
int endRow;    // last row computed by thread number i
// Create and start a thread to compute the rows of the image from
// startRow to endRow.  Note that we have to make sure that
// the endRow for the last thread is the bottom row of the image.
startRow = rowsPerThread*i;
if (i == threadCount-1)
endRow = height-1;
else
endRow = rowsPerThread*(i+1) - 1;
workers[i] = new Runner(startRow, endRow);
try {
workers[i].setPriority( Thread.currentThread().getPriority() - 1 );
}
catch (Exception e) {
}
workers[i].start();
}

Beyond creating more than one thread, very few changes are needed to get the benefits of multiprocessing. Just as in the previous example, each time a thread has computed the colors for a row of pixels, it copies that row into the image, and synchronization is used in exactly the same way to control access to the image.

One thing is new, however. When all the threads have finished running, the name of the button in the program changes from "Abort" to "Start Again", and the pop-up menu, which has been disabled while the threads were running, is re-enabled. The problem is, how to tell when all the threads have terminated? (You might think about why we can't use join() to wait for the threads to end, as was done in the example in Subsection 12.1.2; at least, we certainly can't do that in the event-handling thread!) In this example, I use an instance variable, threadsCompleted, to keep track of how many threads have terminated so far. As each thread finishes, it calls a method that adds one to the value of this variable. (The method is called in the finally clause of a try statement to make absolutely sure that it is called.) When the number of threads that have finished is equal to the number of threads that were created, the method updates the state of the program appropriately. Here is the method:

synchronized private void threadFinished() {
threadsCompleted++;
if (threadsCompleted == workers.length) { // All threads have finished.
startButton.setText("Start Again");
startButton.setEnabled(true);
running = false; // Make sure running is false after the threads end.
workers = null;  // Discard the array that holds the threads.
threadCountSelect.setEnabled(true); // Re-enable pop-up menu.
}
}

Note that this method is synchronized. This is to avoid the race condition when threadsCompleted is incremented. Without the synchronization, it is possible that two threads might call the method at the same time. If the timing is just right, both threads could read the same value for threadsCompleted and get the same answer when they increment it. The net result will be that threadsCompleted goes up by one instead of by two. One thread is not properly counted, and threadsCompleted will never become equal to the number of threads created. The program would hang in a kind of deadlock. The problem would occur only very rarely, since it depends on exact timing. But in a large program, problems of this sort can be both very serious and very hard to debug. Proper synchronization makes the error impossible.

12.2.5  The SwingUtilities Approach

Using synchronized is important for controlling access to shared resources, but it does have some performance penalty. In Java GUI programming, there is an alternative. The class SwingUtilities in package javax.swing contains a variety of static methods that are useful in GUI programming. One of those is

public static invokeLater( Runnable task )

In the statement SwingUtilities.invokeLater(task), task is an object that implements the Runnable interface and so has a run() method. The statement causes task to be added to the GUI event queue. The task's run() method will be executed some time later by the GUI thread, after any other events in the queue have been processed. Since the task is executed by the same thread that handles events and repaints the screen, there is no problem with the task accessing the same resources as the painting or event handling code. This eliminates the need for synchronizing access to those resources.

As an example, the sample program BackgroundCompWithInvoke.java is a slight modification of BackgroundComputationDemo.java, which was discussed earlier in this section. In the revised version, SwingUtilities.invokeLater replaces synchronized. You can check the source code if you are interested.

After a thread calls SwingUtilities.invokeLater(task), the thread continues immediately without waiting for the task to be completed. This is a problem if the thread depends in some way on the results of the task. The method SwingUtilities.invokeAndWait(task) schedules the task to be executed in the GUI thread, then blocks until the task has actually been completed. That is, the thread that called SwingUtilities.invokeAndWait will go to sleep until it is notified that the task has been executed. This method can throw several types of checked exceptions, so it is usually called in a try..catch statement. You can consult Java documentation for more information.

Section 12.3

Threads and Parallel Processing

The example in Subsection 12.2.4 in the previous section used parallel processing to execute pieces of a large task. On a computer that has several processors, this allows the computation to be completed more quickly. However, the way that the program divided up the computation into subtasks was not optimal. Nor was the way that the threads were managed. In this section, we will look at two more versions of that program. The first improves the way the problem is decomposed into subtasks. The second improves the way threads are used. Along the way, I'll introduce a couple of built-in classes that Java provides to support parallel processing. Later in the section, I will cover wait() and notify(), lower-level methods that can be used to control parallel processes more directly.

12.3.1  Problem Decomposition

The sample program MultiprocessingDemo1.java divides the task of computing an image into several subtasks and assigns each subtask to a thread. While this works OK, there is a problem: Some of the subtasks might take substantially longer than others. The program divides the image up into equal parts, but the fact is that some parts of the image require more computation than others. In fact, if you run the program with three threads, you'll notice that the middle piece takes longer to compute than the top or bottom piece. In general, when dividing a problem into subproblems, it is very hard to predict just how much time it will take to solve each subproblem. Let's say that one particular subproblem happens to take a lot longer than all the others. The thread that computes that subproblem will continue to run for a relatively long time after all the other threads have completed. During that time, only one of the computer's processors will be working; the rest will be idle.

As a simple example, suppose that your computer has two processors. You divide the problem into two subproblems and create a thread to run each subproblem Your hope is that by using both processors, you can get your answer in half the time that it would take when using one processor. But if one subproblem takes four times longer than the other to solve, then for most of the time, only one processor will be working. In this case, you will only have cut the time needed to get your answer by 20%.

Even if you manage to divide your problem into subproblems that require equal amounts of computation, you still can't depend on all the subproblems requiring equal amounts of time to solve. For example, some of the processors on your computer might be busy running other programs. Or perhaps some of the processors are simply slower than others. (This is not so likely when running your computation on a single computer, but when distributing computation across several networked computers, as we will do later in this chapter, differences in processor speed can be a major issue.)

The common technique for dealing with all this is to divide the problem into a fairly large number of subproblems -- many more subproblems than there are processors. This means that each processor will have to solve several subproblems. Each time a processor completes one subtask, it is assigned another subtask to work on, until all the subtasks have been assigned. Of course, there will still be variation in the time that the various subtasks require. One processor might complete several subproblems while another works on one particularly difficult case. And a slow or busy processor might complete only one or two subproblems while another processor finishes five or six. Each processor can work at its own pace. As long as the subproblems are fairly small, most of the processors can be kept busy until near the end of the computation. This is known as load balancing: the computational load is balanced among the available processors in order to keep them all as busy as possible. Of course, some processors will still finish before others, but not by longer than the time it takes to complete the longest subtask.

While the subproblems should be small, they should not be too small. There is some computational overhead involved in creating the subproblems and assigning them to processors. If the subproblems are very small, this overhead can add significantly to the total amount of work that has to be done. In my example program, the task is to compute a color for each pixel in an image. For dividing that task up into subtasks, one possibility would be to have each subtask compute just one pixel. But the subtasks produced in that way are probably too small. So, instead, each subtask in my program will compute the colors for one row of pixels. Since there are several hundred rows of pixels in the image, the number of subtasks will be fairly large, while each subtask will also be fairly large. The result is fairly good load balancing, with a reasonable amount of overhead.

Note, by the way, that the problem that we are working on is a very easy one for parallel programming. When we divide the problem of calculating an image into subproblems, all the subproblems are completely independent. It is possible to work on any number of them simultaneously, and they can be done in any order. Things get a lot more complicated when some subtasks produce results that are required by other subtasks. In that case, the subtasks are not independent, and the order in which the subtasks are performed is important. Furthermore, there has to be some way for results from one subtask to be shared with other tasks. When the subtasks are executed by different threads, this raises all the issues involved in controlling access of threads to shared resources. So, in general, decomposing a problem for parallel processing is much more difficult than it might appear from our relatively simple example. But for the most part, that's a topic for a course in distributed computing, not an introductory programming course.

12.3.2  Thread Pools and Task Queues

Once we have decided how to decompose a task into subtasks, there is the question of how to assign those subtasks to threads. Typically, in an object-oriented approach, each subtask will be represented by an object. Since a task represents some computation, it's natural for the object that represents it to have an instance method that does the computation. To execute the task, it is only necessary to call its computation method. In my program, the computation method is called run() and the task object implements the standard Runnable interface that was discussed in Subsection 12.1.1. This interface is a natural way to represent computational tasks. It's possible to create a new thread for each Runnable. However, that doesn't really make sense when there are many tasks, since there is a significant amount of overhead involved in creating each new thread. A better alternative is to create just a few threads and let each thread execute a number of tasks.

The optimal number of threads to use is not entirely clear, and it can depend on exactly what problem you are trying to solve. The goal is to keep all of the computer's processors busy. In the image-computing example, it works well to create one thread for each available processor, but that won't be true for all problems. In particular, if a thread can block for a non-trivial amount of time while waiting for some event or for access to some resource, you want to have extra threads around for the processor to run while other threads are blocked. We'll encounter exactly that situation when we turn to using threads with networking in Section 12.4.

When several threads are available for performing tasks, those threads are called a thread pool. Thread pools are used to avoid creating a new thread to perform each task. Instead, when a task needs to be performed, it can be assigned to any idle thread in the "pool."

Once all the threads in the thread pool are busy, any additional tasks will have to wait until one of the threads becomes idle. This is a natural application for a queue: Associated with the thread pool is a queue of waiting tasks. As tasks become available, they are added to the queue. Every time that a thread finishes a task, it goes to the queue to get another task to work on.

Note that there is only one task queue for the thread pool. All the threads in the pool use the same queue, so the queue is a shared resource. As always with shared resources, race conditions are possible and synchronization is essential. Without synchronization, for example, it is possible that two threads trying to get items from the queue at the same time will end up retrieving the same item. (See if you can spot the race conditions in the dequeue() method in Subsection 9.3.2.)

Java has a built-in class to solve this problem: ConcurrentLinkedQueue. This class and others that can be useful in parallel programming are defined in the package java.util.concurrent. It is a parameterized class so that to create a queue that can hold objects of type Runnable, you can say

ConcurrentLinkedQueue<Runnable> queue = new ConcurrentLinkedQueue<Runnable>();

This class represents a queue, implemented as a linked list, in which operations on the queue are properly synchronized. The operations on a ConcurrentLinkedQueue are not exactly the queue operations that we are used to. The method for adding a new item, x, to the end of queue is queue.add(x). The method for removing an item from the front of queue is queue.poll(). The queue.poll() method returns null if the queue is empty; thus, poll() can be used to test whether the queue is empty and to retrieve an item if it is not. It makes sense to do things in this way because testing whether the queue is non-empty before taking an item from the queue involves a race condition: Without synchronization, it is possible for another thread to remove the last item from the queue between the time when you check that the queue is non-empty and the time when you try to take the item from the queue. By the time you try to get the item, there's nothing there!

To use ConcurrentLinkedQueue in our image-computing example, we can use the queue along with a thread pool. To begin the computation of the image, we create all the tasks that make up the image and add them to the queue. Then, we can create and start the worker threads that will execute the tasks. Each thread will run in a loop in which it gets one task from the queue, by calling the queue's poll() method, and carries out that task. Since the task is an object of type Runnable, it is only necessary for the thread to call the task's run() method. When the poll() method returns null, the queue is empty and the thread can terminate because all the tasks have been assigned to threads.

The sample program MultiprocessingDemo2.java implements this idea. It uses a queue taskQueue of type ConcurrentLinkedQueue<Runnable> to hold the tasks. In addition, in order to allow the user to abort the computation before it finishes, it uses the volatile boolean variable running to signal the thread when the user aborts the computation. The thread should terminate when this variable is set to false. The threads are defined by a nested class named WorkerThread. It is quite short and simple to write at this point:

private class WorkerThread extends Thread {
public void run() {
try {
while (running) {
Runnable task = taskQueue.poll(); // Get a task from the queue.
if (task == null)
break; // (because the queue is empty)
task.run();  // Execute the task;
}
}
finally {
threadFinished(); // Records fact that this thread has terminated.
}
}
}

The program uses a nested class named MandelbrotTask to represent the task of computing one row of pixels in the image. This class implements the Runnable interface. Its run() method does the actual work: Compute the color of each pixel, and apply the colors to the image. Here is what the program does to start the computation (with a few details omitted):

taskQueue = new ConcurrentLinkedQueue<Runnable>(); // Create the queue.
int height = ... ; // Number of rows in the image.
for (int row = 0; row < height; row++) {
MandelbrotTask task;
task = ... ;  // Create a task to compute one row of the image.
taskQueue.add(task); // Add the task to the queue.
}

int threadCount = ... ; // Number of threads in the pool
workers = new WorkerThread[threadCount];
running = true;  // Set the signal before starting the threads!
threadsCompleted = 0;  // Records how many of the threads have terminated.
for (int i = 0; i < threadCount; i++) {
workers[i] = new WorkerThread();
try {
workers[i].setPriority( Thread.currentThread().getPriority() - 1 );
}
catch (Exception e) {
}
workers[i].start();
}

Note that it is important that the tasks be added to the queue before the threads are started. The threads see an empty queue as a signal to terminate. If the queue is empty when the threads are started, they might see an empty queue and terminate immediately after being started, without performing any tasks!

You should try out MultiprocessingDemo2. It computes the same image as MultiprocessingDemo1, but the rows of pixels are not computed in the same order as in that program (if there is more than one thread). If you look carefully, you might see that the rows of pixels are not added to the image in strict order from top to bottom. This is because it is possible for one thread to finish row number i+1 while another thread is still working on row i, or even earlier rows. (The effect might be more apparent if you use more threads than you have processors. Try it with 20 threads.)

12.3.3  Producer/Consumer and Blocking Queues

MultiprocessingDemo2 creates an entirely new thread pool every time it draws an image. This seems wasteful. Shouldn't it be possible to create one set of threads at the beginning of the program and use them whenever an image needs to be computed? After all, the idea of a thread pool is that the threads should sit around and wait for tasks to come along and should execute them when they do. The problem is that, so far, we have no way to make a thread wait for a task to come along. To do that, we will use something called a blocking queue.

A blocking queue is an implementation of one of the classic patterns in parallel processing: the producer/consumer pattern. This pattern arises when there are one or more "producers" who produce things and one or more "consumers" who consume those things. All the producers and consumers should be able to work simultaneously (hence, parallel processing). If there are no things ready to be processed, a consumer will have to wait until one is produced. In many applications, producers also have to wait sometimes: If things can only be consumed at a rate of, say, one per minute, it doesn't make sense for the producers to produce them indefinitely at a rate of two per minute. That would just lead to an unlimited build-up of things waiting to be processed. Therefore, it's often useful to put a limit on the number of things that can be waiting for processing. When that limit is reached, producers should wait before producing more things.

We need a way to get the things from the producers to the consumers. A queue is an obvious answer: Producers place items into the queue as they are produced. Consumers remove items from the other end of the queue.

We are talking parallel processing, so we need a synchronized queue, but we need more than that. When the queue is empty, we need a way to have consumers wait until an item appears in the queue. If the queue becomes full, we need a way to have producers wait until a space opens up in the queue. In our application, the producers and consumers are threads. A thread that is suspended, waiting for something to happen, is said to be blocked, and the type of queue that we need is called a blocking queue. In a blocking queue, the operation of dequeueing an item from the queue can block if the queue is empty. That is, if a thread tries to dequeue an item from an empty queue, the thread will be suspended until an item becomes available; at that time, it will wake up, retrieve the item, and proceed. Similarly, if the queue has a limited capacity, a producer that tries to enqueue an item can block if there is no space in the queue.

Java has two classes that implement blocking queues: LinkedBlockingQueue and ArrayBlockingQueue. These are parameterized types to allow you to specify the type of item that the queue can hold. Both classes are defined in the package java.util.concurrent and both implement an interface called BlockingQueue. If bqueue is a blocking queue belonging to one of these classes, then the following operations are defined:

- bqueue.take() -- Removes an item from the queue and returns it. If the queue is empty when this method is called, the thread that called it will block until an item becomes available. This method throws an InterruptedException if the thread is interrupted while it is blocked.

- bqueue.put(item) -- Adds the item to the queue. If the queue has a limited capacity and is full, the thread that called it will block until a space opens up in the queue. This method throws an InterruptedException if the thread is interrupted while it is blocked.

- bqueue.add(item) -- Adds the item to the queue, if space is available. If the queue has a limited capacity and is full, an IllegalStateException is thrown. This method does not block.

- bqueue.clear() -- Removes all items from the queue and discards them.

Java's blocking queues define many additional methods (for example, bqueue.poll(500) is similar to bqueue.take(), except that it will not block for longer than 500 milliseconds), but the four listed here are sufficient for our purposes. Note that I have listed two methods for adding items to the queue: bqueue.put(item) blocks if there is not space available in the queue and is most appropriate for use with blocking queues that have a limited capacity; bqueue.add(item) does not block and is appropriate for use with blocking queues that have an unlimited capacity.

An ArrayBlockingQueue has a maximum capacity that is specified when it is constructed. For example, to create a blocking queue that can hold up to 25 objects of type ItemType, you could say:

ArrayBlockingQueue<ItemType> bqueue = new ArrayBlockingQueue<ItemType>(25);

With this declaration, bqueue.put(item) will block if bqueue already contains 25 items, while bqueue.add(item) will throw an exception in that case. Recall that this ensures that items are not produced indefinitely at a rate faster than they can be consumed. A LinkedBlockingQueue is meant for creating blocking queues with unlimited capacity. For example,

LinkedBlockingQueue<ItemType> bqueue = new LinkedBlockingQueue<ItemType>();

creates a queue with no upper limit on the number of items that it can contain. In this case, bqueue.put(item) will never block and bqueue.add(item) will never throw an IllegalStateException. You would use a LinkedBlockingQueue when you want to avoid blocking, and you have some other way of ensuring that the queue will not grow to arbitrary size. For both types of blocking queue, bqueue.take() will block if the queue is empty.

The sample program MultiprocessingDemo3.java uses a LinkedBlockingQueue in place of the ConcurrentLinkedQueue in the previous version, MultiprocessingDemo2.java. In this example, the queue holds tasks, that is, items of type Runnable, and the queue is declared as an instance variable named taskQueue:

LinkedBlockingQueue<Runnable> taskQueue;

When the user clicks the "Start" button and it's time to compute an image, all of the tasks that make up the computation are put into this queue. This is done by calling taskQueue.add(task) for each task. It's important that this can be done without blocking, since the tasks are created in the event-handling thread, and we don't want to block that. The queue cannot grow indefinitely because the program only works on one image at a time, and there are only a few hundred tasks per image.

Just as in the previous version of the program, worker threads belonging to a thread pool will remove tasks from the queue and carry them out. However, in this case, the threads are created once at the beginning of the program -- actually, the first time the "Start" button is pressed -- and the same threads are reused for any number of images. When there are no tasks to execute, the task queue is empty and the worker threads will block until tasks become available. Each worker thread runs in an infinite loop, processing tasks forever, but it will spend a lot of its time blocked, waiting for a task to be added to the queue. Here is the inner class that defines the worker threads:

/**
* This class defines the worker threads that make up the thread pool.
* A WorkerThread runs in a loop in which it retrieves a task from the
* taskQueue and calls the run() method in that task.  Note that if
* the queue is empty, the thread blocks until a task becomes available
* in the queue.  The constructor starts the thread, so there is no
* need for the main program to do so.  The thread will run at a priority
* that is one less than the priority of the thread that calls the
* constructor.
*
* A WorkerThread is designed to run in an infinite loop.  It will
* end only when the Java virtual machine exits. (This assumes that
* the tasks that are executed don't throw exceptions, which is true
* in this program.)  The constructor sets the thread to run as
* a daemon thread; the Java virtual machine will exit automatically when
* the only threads are daemon threads.  (In this program, this is not
* necessary since the virtual machine is set to exit when the
* window is closed.  In a multi-window program, however, we can't
* simply end the program when a window is closed.)
*/
private class WorkerThread extends Thread {
WorkerThread() {
try {
setPriority( Thread.currentThread().getPriority() - 1);
}
catch (Exception e) {
}
try {
setDaemon(true);
}
catch (Exception e) {
}
start(); // Thread starts as soon as it is constructed.
}
public void run() {
while (true) {
try {
Runnable task = taskQueue.take(); // wait for task if necessary
task.run();
}
catch (InterruptedException e) {
}
}
}
}

We should look more closely at how the thread pool works. The worker threads are created and started before there is any task to perform. Each thread immediately calls taskQueue.take(). Since the task queue is empty, all the worker threads will block as soon as they are started. To start the computation of an image, the event-handling thread will create tasks and add them to the queue. As soon as this happens, worker threads will wake up and start processing tasks, and they will continue doing so until the queue is emptied. (Note that on a multi-processor computer, some worker threads can start processing even while the event thread is still adding tasks to the queue.) When the queue is empty, the worker threads will go back to sleep until processing starts on the next image.

An interesting point in this program is that we want to be able to abort the computation before it finishes, but we don't want the worker threads to terminate when that happens. When the user clicks the "Abort" button, the program calls taskQueue.clear(), which prevents any more tasks from being assigned to worker threads. However, some tasks are most likely already being executed when the task queue is cleared. Those tasks will complete after the computation in which they are subtasks has supposedly been aborted. When those subtasks complete, we don't want their output to be applied to the image. It's not a big deal in this program, but in more general applications, we don't want output meant for a previous computation job to be applied to later jobs.

My solution is to assign a job number to each computation job. The job number of the current job is stored in an instance variable named jobNum, and each task object has an instance variable that tells which task that job is part of. When a job ends -- either because the job finishes on its own or because the user aborts it -- the value of jobNum is incremented. When a task completes, the job number stored in the task object is compared to jobNum. If they are equal, then the task is part of the current job, and its output is applied to the image. If they are not equal, then the task was part of a previous job, and its output is discarded.

It's important that access to jobNum be properly synchronized. Otherwise, one thread might check the job number just as another thread is incrementing it, and output meant for an old job might sneak through after that job has been aborted. In the program, all the methods that access or change jobNum are synchronized. You can read the source code to see how it works.

One more point about MultiprocessingDemo3.... I have not provided any way to terminate the worker threads in this program. They will continue to run until the Java Virtual Machine exits. To allow thread termination before that, we could use a volatile signaling variable, running, and set its value to false when we want the worker threads to terminate. The run() methods for the threads would be replaced by

public void run() {
while ( running ) {
try {
Runnable task = taskQueue.take();
task.run();
}
catch (InterruptedException e) {
}
}
}

However, if a thread is blocked in taskQueue.take(), it will not see the new value of running until it becomes unblocked. To ensure that that happens, it is necessary to call worker.interrupt() for each worker thread worker, just after setting runner to false.

If a worker thread is executing a task when runner is set to false, the thread will not terminate until that task has completed. If the tasks are reasonably short, this is not a problem. If tasks can take longer to execute than you are willing to wait for the threads to terminate, then each task must also check the value of running periodically and exit when that value becomes false.

12.3.4  Wait and Notify

To implement a blocking queue, we must be able to make a thread block just until some event occurs. The thread is waiting for the event to occur. Somehow, it must be notified when that happens. There are two threads involved since the event that will wake one thread is caused by an action taken by another thread, such as adding an item to the queue.

Note that this is not just an issue for blocking queues. Whenever one thread produces some sort of result that is needed by another thread, that imposes some restriction on the order in which the threads can do their computations. If the second thread gets to the point where it needs the result from the first thread, it might have to stop and wait for the result to be produced. Since the second thread can't continue, it might as well go to sleep. But then there has to be some way to notify the second thread when the result is ready, so that it can wake up and continue its computation.

Java, of course, has a way to do this kind of "waiting" and "notifying": It has wait() and notify() methods that are defined as instance methods in class Object and so can be used with any object. These methods are used internally in blocking queues. They are fairly low-level, tricky, and error-prone, and you should use higher-level control strategies such as blocking queues when possible. However, it's nice to know about wait() and notify() in case you ever need to use them directly.

The reason why wait() and notify() should be associated with objects is not obvious, so don't worry about it at this point. It does, at least, make it possible to direct different notifications to different recipients, depending on which object's notify() method is called.

The general idea is that when a thread calls a wait() method in some object, that thread goes to sleep until the notify() method in the same object is called. It will have to be called, obviously, by another thread, since the thread that called wait() is sleeping. A typical pattern is that Thread A calls wait() when it needs a result from Thread B, but that result is not yet available. When Thread B has the result ready, it calls notify(), which will wake Thread A up, if it is waiting, so that it can use the result. It is not an error to call notify() when no one is waiting; it just has no effect. To implement this, Thread A will execute code similar to the following, where obj is some object:

if ( resultIsAvailable() == false )
obj.wait();  // wait for notification that the result is available
useTheResult();

while Thread B does something like:

generateTheResult();
obj.notify();  // send out a notification that the result is available

Now, there is a really nasty race condition in this code. The two threads might execute their code in the following order:

1.  Thread A checks resultIsAvailable() and finds that the result is not ready,
so it decides to execute the obj.wait() statement, but before it does,
2.  Thread B finishes generating the result and calls obj.notify()
3.  Thread A calls obj.wait() to wait for notification that the result is ready.

In Step 3, Thread A is waiting for a notification that will never come, because notify() has already been called in Step 2. This is a kind of deadlock that can leave Thread A waiting forever. Obviously, we need some kind of synchronization. The solution is to enclose both Thread A's code and Thread B's code in synchronized statements, and it is very natural to synchronize on the same object, obj, that is used for the calls to wait() and notify(). In fact, since synchronization is almost always needed when wait() and notify() are used, Java makes it an absolute requirement. In Java, a thread can legally call obj.wait() or obj.notify() only if that thread holds the synchronization lock associated with the object obj. If it does not hold that lock, then an exception is thrown. (The exception is of type IllegalMonitorStateException, which does not require mandatory handling and which is typically not caught.) One further complication is that the wait() method can throw an InterruptedException and so should be called in a try statement that handles the exception.

To make things more definite, let's consider how we can get a result that is computed by one thread to another thread that needs the result. This is a simplified producer/consumer problem in which only one item is produced and consumed. Assume that there is a shared variable named sharedResult that is used to transfer the result from the producer to the consumer. When the result is ready, the producer sets the variable to a non-null value. The consumer can check whether the result is ready by testing whether the value of sharedResult is null. We will use a variable named lock for synchronization. The code for the producer thread could have the form:

makeResult = generateTheResult();  // Not synchronized!
synchronized(lock) {
sharedResult = makeResult;
lock.notify();
}

while the consumer would execute code such as:

synchronized(lock) {
while ( sharedResult == null ) {
try {
lock.wait();
}
catch (InterruptedException e) {
}
}
useResult = sharedResult;
}
useTheResult(useResult);  // Not synchronized!

The calls to generateTheResult() and useTheResult() are not synchronized, which allows them to run in parallel with other threads that might also synchronize on lock. Since sharedResult is a shared variable, all references to sharedResult should be synchronized, so the references to sharedResult must be inside the synchronized statements. The goal is to do as little as possible (but not less) in synchronized code segments.

If you are uncommonly alert, you might notice something funny: lock.wait() does not finish until lock.notify() is executed, but since both of these methods are called in synchronized statements that synchronize on the same object, shouldn't it be impossible for both methods to be running at the same time? In fact, lock.wait() is a special case: When a thread calls lock.wait(), it gives up the lock that it holds on the synchronization object, lock. This gives another thread a chance to execute the synchronized(lock) block that contains the lock.notify() statement. After the second thread exits from this block, the lock is returned to the consumer thread so that it can continue.

In the full producer/consumer pattern, multiple results are produced by one or more producer threads and are consumed by one or more consumer threads. Instead of having just one sharedResult object, we keep a list of objects that have been produced but not yet consumed. Let's see how this might work in a very simple class that implements the three operations on a LinkedBlockingQueue<Runnable> that are used in MultiprocessingDemo3:

import java.util.LinkedList;

public class MyLinkedBlockingQueue {

private LinkedList<Runnable> taskList = new LinkedList<Runnable>();

public void clear() {
synchronized(taskList) {
taskList.clear();
}
}

public void add(Runnable task) {
synchronized(taskList) {
taskList.addLast(task);
taskList.notify();
}
}

public Runnable take() throws InterruptedException {
synchronized(taskList) {
while (taskList.isEmpty())
taskList.wait();
return taskList.removeFirst();
}
}

}

An object of this class could be used as a direct replacement for the taskQueue in MultiprocessingDemo3.

In this class, I have chosen to synchronize on the taskList object, but any object could be used. In fact, I could simply use synchronized methods, which is equivalent to synchronizing on this. (Note that you might see a call to wait() or notify() in a synchronized instance method, with no reference to the object that is being used. Remember that wait() and notify() in that context really mean this.wait() and this.notify().)

By the way, it is essential that the call to taskList.clear() be synchronized on the same object, even though it doesn't call wait() or notify(). Otherwise, there is a race condition that can occur: The list might be cleared just after the take() method checks that taskList is non-empty and before it removes an item from the list. In that case, the list is empty again by the time taskList.removeFirst() is called, resulting in an error.

It is possible for several threads to be waiting for notification. A call to obj.notify() will wake only one of the threads that is waiting on obj. If you want to wake all threads that are waiting on obj, you can call obj.notifyAll(). obj.notify() works OK in the above example because only consumer threads can be blocked. We only need to wake one consumer thread when a task is added to the queue because it doesn't matter which consumer gets the task. But consider a blocking queue with limited capacity, where producers and consumers can both block. When an item is added to the queue, we want to make sure that a consumer thread is notified, not just another producer. One solution is to call notifyAll() instead of notify(), which will notify all threads including any waiting consumer.

I should also mention a possible confusion about the name of the method obj.notify(). This method does not notify obj of anything! It notifies a thread that has called obj.wait() (if there is such a thread). Similarly, in obj.wait(), it's not obj that is waiting for something; it's the thread that calls the method.

And a final note on wait: There is another version of wait() that takes a number of milliseconds as a parameter. A thread that calls obj.wait(milliseconds) will wait only up to the specified number of milliseconds for a notification. If a notification doesn't occur during that period, the thread will wake up and continue without the notification. In practice, this feature is most often used to let a waiting thread wake periodically while it is waiting in order to perform some periodic task, such as causing a message "Waiting for computation to finish" to blink.

Let's look at an example that uses wait() and notify() to allow one thread to control another. The sample program TowersOfHanoiGUI.java solves the Towers Of Hanoi puzzle (Subsection 9.1.2), with control buttons that allow the user to control the execution of the algorithm. The user can click a "Next Step" button to execute just one step in the solution, which moves a single disk from one pile to another. Clicking "Run" lets the algorithm run automatically on its own; the text on the button changes from "Run" to "Pause", and clicking "Pause" stops the automatic execution. There is also a "Start Over" button that aborts the current solution and puts the puzzle back into its initial configuration. Here is a picture of the program in the middle of a solution, including the buttons:

In this program, there are two threads: a thread that runs a recursive algorithm to solve the puzzle, and the event-handling thread that reacts to user actions. When the user clicks one of the buttons, a method is called in the event-handling thread. But it's actually the thread that is running the recursion that has to respond by, for example, doing one step of the solution or starting over. The event-handling thread has to send some sort of signal to the solution thread. This is done by setting the value of a variable that is shared by both threads. The variable is named status, and its possible values are the constants GO, PAUSE, STEP, and RESTART.

When the event-handling thread changes the value of this variable, the solution thread should see the new value and respond. When status equals PAUSE, the solution thread is paused, waiting for the user to click "Run" or "Next Step". This is the initial state, when the program starts. If the user clicks "Next Step", the event-handling thread sets the value of status to "STEP"; the solution thread should see the new value and respond by executing one step of the solution and then resetting status to PAUSE. If the user clicks "Run", status is set to GO, which should cause the solution thread to run automatically. When the user clicks "Pause" while the solution is running, status is reset to PAUSE, and the solution thread should return to its paused state. If the user clicks "Start Over", the event-handling thread sets status to RESTART, and the solution thread should respond by ending the current recursive solution and restoring the puzzle to its initial state.

The main point for us is that when the solution thread is paused, it is sleeping. It won't see a new value for status unless it wakes up! To make that possible, the program uses wait() in the solution thread to put that thread to sleep, and it uses notify() in the event-handling thread to wake up the solution thread whenever it changes the value of status. Here is the actionPerformed() method that responds to clicks on the buttons. When the user clicks a button, this method changes the value of status and calls notify() to wake up the solution thread:

synchronized public void actionPerformed(ActionEvent evt) {
Object source = evt.getSource();
if (source == runPauseButton) {  // Toggle between running and paused.
if (status == GO) {  // Animation is running.  Pause it.
status = PAUSE;
nextStepButton.setEnabled(true);  // Enable while paused.
runPauseButton.setText("Run");
}
else {  // Animation is paused.  Start it running.
status = GO;
nextStepButton.setEnabled(false);  // Disable while running.
runPauseButton.setText("Pause");
}
}
else if (source == nextStepButton) {  // Makes animation run one step.
status = STEP;
}
else if (source == startOverButton) { // Restore to initial state.
status = RESTART;
}
notify();  // Wake up the thread so it can see the new status value!
}

This method is synchronized to allow the call to notify(). Remember that the notify() method in an object can only be called by a thread that holds that object's synchronization lock. In this case, the synchronization object is this. Synchronization is also necessary because of race conditions that arise because the value of status can also be changed by the solution thread.

The solution thread calls a method named checkStatus() to check the value of status. This method calls wait() if the status is PAUSE, which puts the solution thread to sleep until the event-handling thread calls notify(). Note that if the status is RESTART, checkStatus() throws an IllegalStateException:

synchronized private void checkStatus() {
while (status == PAUSE) {
try {
wait();
}
catch (InterruptedException e) {
}
}
// At this point, status is RUN, STEP, or RESTART.
if (status == RESTART)
throw new IllegalStateException("Restart");
// At this point, status is RUN or STEP.
}

The run() method for the solution thread runs in an infinite loop in which it sets up the initial state of the puzzle and then calls a solve() method to solve the puzzle. To implement the wait/notify control strategy, run() calls checkStatus() before starting the solution, and solve() calls checkStatus() after each move. If checkStatus() throws an IllegalStateException, the call to solve() is terminated early. (We used the method of throwing an exception to terminate a recursive algorithm before, in Subsection 12.2.2.) After the solve() method finishes or is terminated by an exception, the run() method returns to the beginning of the while loop, where the initial state of the puzzle, and of the user interface, is restored:

public void run() {
while (true) {
runPauseButton.setText("Run");   // Set user interface to initial state.
nextStepButton.setEnabled(true);
startOverButton.setEnabled(false);
setUpProblem();  // Set up the initial state of the puzzle
status = PAUSE;  // Initially, the solution thread is paused.
checkStatus(); // Returns only when user has clicked "Run" or "Next Step"
startOverButton.setEnabled(true);
try {
solve(10,0,1,2);  // Move 10 disks from pile 0 to pile 1.
}
catch (IllegalStateException e) {
// Exception was thrown because user clicked "Start Over".
}
}
}

You can check the full source code to see how this all fits into the complete program. If you want to learn how to use wait() and notify() directly, understanding this example is a good place to start!

Section 12.4

Threads and Networking

In the previous chapter, we looked at several examples of network programming. Those examples showed how to create network connections and communicate through them, but they didn't deal with one of the fundamental characteristics of network programming, the fact that network communication is asynchronous. From the point of view of a program on one end of a network connection, messages can arrive from the other side of the connection at any time; the arrival of a message is an event that is not under the control of the program that is receiving the message. Perhaps an event-oriented networking API would be a good approach to dealing with the asynchronous nature of network communication, but that is not the approach that is taken in Java (or, typically, in other languages). Instead, network programming in Java typically uses threads.

12.4.1  The Blocking I/O Problem

As covered in Section 11.4, network programming uses sockets. A socket, in the sense that we are using the term here, represents one end of a network connection. Every socket has an associated input stream and output stream. Data written to the output stream on one end of the connection is transmitted over the network and appears in the input stream at the other end.

A program that wants to read data from a socket's input stream calls one of that input stream's input methods. It is possible that the data has already arrived before the input method is called; in that case, the input method retrieves the data and returns immediately. More likely, however, the input method will have to wait for data to arrive from the other side of the connection. Until the data arrives, the input method and the thread that called it will be blocked.

It is also possible for an output method in a socket's output stream to block. This can happen if the program tries to output data to the socket faster than the data can be transmitted over the network. (It's a little complicated: a socket uses a "buffer" to hold data that is supposed to be transmitted over the network. A buffer is just a block of memory that is used like a queue. The output method drops its data into the buffer; lower-level software removes data from the buffer and transmits it over the network. The output method will block if the buffer fills up. Note that when the output method returns, it doesn't mean that the data has gone out over the network -- it just means that the data has gone into the buffer and is scheduled for later transmission.)

We say that network communication uses blocking I/O, because input and output operations on the network can block for indefinite periods of time. Programs that use the network must be prepared to deal with this blocking. In some cases, it's acceptable for a program to simply shut down all other processing and wait for input. (This is what happens when a command line program reads input typed by the user. User input is another type of blocking I/O.) However, threads make it possible for some parts of a program to continue doing useful work while other parts are blocked. A network client program that sends requests to a server might get by with a single thread, if it has nothing else to do while waiting for the server's responses. A network server program, on the other hand, can typically be connected to several clients at the same time. While waiting for data to arrive from a client, the server certainly has other things that it can do, namely communicate with other clients. When a server uses different threads to handle the communication with different clients, the fact that I/O with one client is blocked won't stop the server from communicating with other clients.

It's important to understand that using threads to deal with blocking I/O differs in a fundamental way from using threads to speed up computation. When using threads for speed-up in Subsection 12.3.2, it made sense to use one thread for each available processor. If only one processor is available, using more than one thread will yield no speed-up at all; in fact, it would slow things down because of the extra overhead involved in creating and managing the threads.

In the case of blocking I/O, on the other hand, it can make sense to have many more threads than there are processors, since at any given time many of the threads can be blocked. Only the active, unblocked threads are competing for processing time. In the ideal case, to keep all the processors busy, you would want to have one active thread per processor (actually somewhat less than that, on average, to allow for variations over time in the number of active threads). On a network server program, for example, threads generally spend most of their time blocked waiting for I/O operations to complete. If threads are blocked, say, about 90% of the time, you'd like to have about ten times as many threads as there are processors. So even on a computer that has just a single processor, server programs can make good use of large numbers of threads.

12.4.2  An Asynchronous Network Chat Program

As a first example of using threads for network communication, we consider a GUI chat program.

The command-line chat programs, CLChatClient.java and CLChatServer.java, from Subsection 11.4.5 use a straight-through, step-by-step protocol for communication. After a user on one side of a connection enters a message, the user must wait for a reply from the other side of the connection. An asynchronous chat program would be much nicer. In such a program, a user could just keep typing lines and sending messages without waiting for any response. Messages that arrive -- asynchronously -- from the other side would be displayed as soon as they arrive. It's not easy to do this in a command-line interface, but it's a natural application for a graphical user interface. The basic idea for a GUI chat program is to create a thread whose job is to read messages that arrive from the other side of the connection. As soon as the message arrives, it is displayed to the user; then, the message-reading thread blocks until the next incoming message arrives. While it is blocked, however, other threads can continue to run. In particular, the event-handling thread that responds to user actions keeps running; that thread can send outgoing messages as soon as the user generates them.

The GUIChat program can act as either the client end or the server end of a connection. (Refer back to Subsection 11.4.3 for information about how clients and servers work.) The program has a "Listen" button that the user can click to create a server socket that will listen for an incoming connection request; this makes the program act as a server. It also has a "Connect" button that the user can click to send a connection request; this makes the program act as a client. As usual, the server listens on a specified port number. The client needs to know the computer on which the server is running and the port on which the server is listening. There are input boxes in the GUIChat window where the user can enter this information.

Once a connection has been established between two GUIChat windows, each user can send messages to the other. The window has an input box where the user types a message. Pressing return while typing in this box sends the message. This means that the sending of the message is handled by the usual event-handling thread, in response to an event generated by a user action. Messages are received by a separate thread that just sits around waiting for incoming messages. This thread blocks while waiting for a message to arrive; when a message does arrive, it displays that message to the user. The window contains a large transcript area that displays both incoming and outgoing messages, along with other information about the network connection.

I urge you to compile the source code, GUIChat.java, and try the program. To make it easy to try it on a single computer, you can make a connection between one window and another window on the same computer, using "localhost" or "127.0.0.1" as the name of the computer. (Once you have one GUIChat window open, you can open a second one by clicking the "New" button.) I also urge you to read the source code. I will discuss only parts of it here.

The program uses a nested class, ConnectionHandler, to handle most network-related tasks. ConnectionHandler is a subclass of Thread. The ConnectionHandler thread is responsible for opening the network connection and then for reading incoming messages once the connection has been opened. By putting the connection-opening code in a separate thread, we make sure that the GUI is not blocked while the connection is being opened. (Like reading incoming messages, opening a connection is a blocking operation that can take some time to complete.) The ConnectionHandler handles opening the connection both when the program acts as a server and when it acts as a client. The thread is created when the user clicks either the "Listen" button or the "Connect" button. The "Listen" button should make the thread act as a server, while "Connect" should make it act as a client. To distinguish these two cases, the ConnectionHandler class has two constructors. Note that the postMessage() method posts a message to the transcript area of the window, where it will be visible to the user:

/**
* Listen for a connection on a specified port.  The constructor
* does not perform any network operations; it just sets some
* instance variables and starts the thread.  Note that the
* thread will only listen for one connection, and then will
* close its server socket.
*/
ConnectionHandler(int port) {  // For acting as the "server."
state = ConnectionState.LISTENING;
this.port = port;
postMessage("\nLISTENING ON PORT " + port + "\n");
start();
}

/**
* Open a connection to a specified computer and port.  The constructor
* does not perform any network operations; it just sets some
* instance variables and starts the thread.
*/
ConnectionHandler(String remoteHost, int port) {  // For acting as "client."
state = ConnectionState.CONNECTING;
this.remoteHost = remoteHost;
this.port = port;
postMessage("\nCONNECTING TO " + remoteHost + " ON PORT " + port + "\n");
start();
}

Here, state is an instance variable whose type is defined by an enumerated type

enum ConnectionState { LISTENING, CONNECTING, CONNECTED, CLOSED };

The values of this enum represent different possible states of the network connection. It is often useful to treat a network connection as a state machine (see Subsection 6.4.4), since the response to various events can depend on the state of the connection when the event occurs. Setting the state variable to LISTENING or CONNECTING tells the thread whether to act as server or as a client when setting up the connection.

Once the thread has been started, it executes the following run() method:

/**
* The run() method that is executed by the thread.  It opens a
* connection as a client or as a server (depending on which
* constructor was used).
*/
public void run() {
try {
if (state == ConnectionState.LISTENING) {
// Open a connection as a server.
listener = new ServerSocket(port);
socket = listener.accept();
listener.close();
}
else if (state == ConnectionState.CONNECTING) {
// Open a connection as a client.
socket = new Socket(remoteHost,port);
}
connectionOpened();  // Sets up to use the connection (including
// creating a BufferedReader, in, for reading
// incoming messages).
while (state == ConnectionState.CONNECTED) {
// Read one line of text from the other side of
// the connection, and report it to the user.
String input = in.readLine();
if (input == null)
connectionClosedFromOtherSide(); // Close socket and report to user.
else
received(input);  // Report message to user.
}
}
catch (Exception e) {
// An error occurred.  Report it to the user, but not
// if the connection has been closed (since the error
// might be the expected error that is generated when
// a socket is closed).
if (state != ConnectionState.CLOSED)
postMessage("\n\n ERROR:  " + e);
}
finally {  // Clean up before terminating the thread.
cleanUp();
}
}

This method calls several other methods to do some of its work, but you can see the general outline of how it works. After opening the connection as either a server or client, the run() method enters a while loop in which it receives and processes messages from the other side of the connection until the connection is closed. It is important to understand how the connection can be closed. The GUIChat window has a "Disconnect" button that the user can click to close the connection. The program responds to this event by closing the socket that represents the connection and by setting the connection state to CLOSED. It is likely that when this happens, the connection-handling thread is blocked in the in.readLine() method, waiting for an incoming message. When the socket is closed by the GUI thread, this method will fail and will throw an exception; this exception causes the thread to terminate. (If the connection-handling thread happens to be between calls to in.readLine() when the socket is closed, the while loop will terminate because the connection state changes from CONNECTED to CLOSED.) Note that closing the window will also close the connection in the same way.

It is also possible for the user on the other side of the connection to close the connection. When that happens, the stream of incoming messages ends, and the in.readLine() on this side of the connection returns the value null, which indicates end-of-stream and acts as a signal that the connection has been closed by the remote user.

For a final look into the GUIChat code, consider the methods that send and receive messages. These methods are called from different threads. The send() method is called by the event-handling thread in response to a user action. Its purpose is to transmit a message to the remote user. (It is conceivable, though not likely, that the data output operation could block, if the socket's output buffer fills up. A more sophisticated program might take this possibility into account.) This method uses a PrintWriter, out, that writes to the socket's output stream. Synchronization of this method prevents the connection state from changing in the middle of the send operation:

/**
* Send a message to the other side of the connection, and post the
* message to the transcript.  This should only be called when the
* connection state is ConnectionState.CONNECTED; if it is called at
* other times, it is ignored.
*/
synchronized void send(String message) {
if (state == ConnectionState.CONNECTED) {
postMessage("SEND:  " + message);
out.println(message);
out.flush();
if (out.checkError()) {
postMessage("\nERROR OCCURRED WHILE TRYING TO SEND DATA.");
close();  // Closes the connection.
}
}
}

The received() method is called by the connection-handling thread after a message has been read from the remote user. Its only job is to display the message to the user, but again it is synchronized to avoid the race condition that could occur if the connection state were changed by another thread while this method is being executed:

/**
* This is called by the run() method when a message is received from
* the other side of the connection.  The message is posted to the
* transcript, but only if the connection state is CONNECTED.  (This
* is because a message might be received after the user has clicked
* the "Disconnect" button; that message should not be seen by the
* user.)
*/
synchronized private void received(String message) {
if (state == ConnectionState.CONNECTED)
postMessage("RECEIVE:  " + message);
}

12.4.3  A Threaded Network Server

Threads are often used in network server programs. They allow the server to deal with several clients at the same time. When a client can stay connected for an extended period of time, other clients shouldn't have to wait for service. Even if the interaction with each client is expected to be very brief, you can't always assume that that will be the case. You have to allow for the possibility of a misbehaving client -- one that stays connected without sending data that the server expects. This can hang up a thread indefinitely, but in a threaded server there will be other threads that can carry on with other clients.

The DateServer.java sample program, from Subsection 11.4.4, is an extremely simple network server program. It does not use threads, so the server must finish with one client before it can accept a connection from another client. Let's see how we can turn DateServer into a threaded server. (This server is so simple that doing so doesn't make a great deal of sense. However, the same techniques will work for more complicated servers. See, for example, Exercise 12.4.)

As a first attempt, consider DateServerWithThreads.java. This sample program creates a new thread every time a connection request is received, instead of handling the connection itself by calling a subroutine. The main program simply creates the thread and hands the connection to the thread. This takes very little time, and in particular will not block. The run() method of the thread handles the connection in exactly the same way that it would be handled by the original program. This is not at all difficult to program. Here's the new version of the program, with significant changes shown in red italic. Note again that the constructor for the connection thread does very little and in particular cannot block; this is very important since the constructor runs in the main thread:

import java.net.*;
import java.io.*;
import java.util.Date;

/**
* This program is a server that takes connection requests on
* the port specified by the constant LISTENING_PORT.  When a
* connection is opened, the program sends the current time to
* the connected socket.  The program will continue to receive
* and process connections until it is killed (by a CONTROL-C,
* for example).
*
* This version of the program creates a new thread for
* every connection request.
*/
public class DateServerWithThreads {

public static final int LISTENING_PORT = 32007;

public static void main(String[] args) {

ServerSocket listener;  // Listens for incoming connections.
Socket connection;      // For communication with the connecting program.

/* Accept and process connections forever, or until some error occurs. */

try {
listener = new ServerSocket(LISTENING_PORT);
System.out.println("Listening on port " + LISTENING_PORT);
while (true) {
// Accept next connection request and create thread to handle it.
connection = listener.accept();
ConnectionHandler handler = new ConnectionHandler(connection);
handler.start();
}
}
catch (Exception e) {
System.out.println("Sorry, the server has shut down.");
System.out.println("Error:  " + e);
return;
}

}  // end main()

/**
*  Defines a thread that handles the connection with one
*  client.
*/
private static class ConnectionHandler extends Thread {
Socket client; // The connection to the client.
ConnectionHandler(Socket socket) {
client = socket;
}
public void run() {
// (code copied from the original DateServer program)
String clientAddress = client.getInetAddress().toString();
try {
System.out.println("Connection from " + clientAddress );
Date now = new Date();  // The current date and time.
PrintWriter outgoing;   // Stream for sending data.
outgoing = new PrintWriter( client.getOutputStream() );
outgoing.println( now.toString() );
outgoing.flush();  // Make sure the data is actually sent!
client.close();
}
catch (Exception e){
System.out.println("Error on connection with: "
+ clientAddress + ": " + e);
}
}
}

} //end class DateServerWithThreads

One interesting change is at the end of the run() method, where I've added the clientAddress to the output of the error message. I did this to identify which connection the error message refers to. Since threads run in parallel, it's possible for outputs from different threads to be intermingled in various orders. Messages from the same thread don't necessarily come together in the output; they might be separated by messages from other threads. This is just one of the complications that you have to keep in mind when working with threads!

12.4.4  Using a Thread Pool

It's not very efficient to create a new thread for every connection, especially when the connections are typically very short-lived. Fortunately, we have an alternative: thread pools (Subsection 12.3.2).

DateServerWithThreadPool.java is an improved version of our server that uses a thread pool. Each thread in the pool runs in an infinite loop. Each time through the loop, it handles one connection. We need a way for the main program to send connections to the threads. It's natural to use a blocking queue named connectionQueue for that purpose. A connection-handling thread takes connections from this queue. Since it is a blocking queue, the thread blocks when the queue is empty and wakes up when a connection becomes available in the queue. No other synchronization or communication technique is needed; it's all built into the blocking queue. Here is the run() method for the connection-handling threads:

public void run() {
while (true) {
Socket client;
try {
client = connectionQueue.take();  // Blocks until item is available.
}
catch (InterruptedException e) {
continue; // (If interrupted, just go back to start of while loop.)
}
String clientAddress = client.getInetAddress().toString();
try {
System.out.println("Connection from " + clientAddress );
System.out.println("Handled by thread " + this);
Date now = new Date();  // The current date and time.
PrintWriter outgoing;   // Stream for sending data.
outgoing = new PrintWriter( client.getOutputStream() );
outgoing.println( now.toString() );
outgoing.flush();  // Make sure the data is actually sent!
client.close();
}
catch (Exception e){
System.out.println("Error on connection with: "
+ clientAddress + ": " + e);
}
}
}

The main program, in the meantime, runs in an infinite loop in which connections are accepted and added to the queue:

while (true) {
// Accept next connection request and put it in the queue.
connection = listener.accept();
try {
connectionQueue.put(connection); // Blocks if queue is full.
}
catch (InterruptedException e) {
}
}

The queue in this program is of type ArrayBlockingQueue<Socket>. As such, it has a limited capacity, and the put() operation on the queue will block if the queue is full. But wait -- didn't we want to avoid blocking the main program? When the main program is blocked, the server is no longer accepting connections, and clients who are trying to connect are kept waiting. Would it be better to use a LinkedBlockingQueue, with an unlimited capacity?

In fact, connections in the blocking queue are waiting anyway; they are not being serviced. If the queue grows unreasonably long, connections in the queue will have to wait for an unreasonable amount of time. If the queue keeps growing indefinitely, that just means that the server is receiving connection requests faster than it can process them. That could happen for several reasons: Your server might simply not be powerful enough to handle the volume of traffic that you are getting; you need to buy a new server. Or perhaps the thread pool doesn't have enough threads to fully utilize your server; you should increase the size of the thread pool to match the server's capabilities. Or maybe your server is under a "Denial Of Service" attack, in which some bad guy is deliberately sending your server more requests than it can handle in an attempt to keep other, legitimate clients from getting service.

In any case, ArrayBlockingQueue with limited capacity is the correct choice. The queue should be short enough so that connections in the queue will not have to wait too long for service. In a real server, the size of the queue and the number of threads in the thread pool should be adjusted to "tune" the server to account for the particular hardware and network on which the server is running and for the nature of the client requests that it typically processes. Optimal tuning is, in general, a difficult problem.

There is, by the way, another way that things can go wrong: Suppose that the server needs to read some data from the client, but the client doesn't send the expected data. The thread that is trying to read the data can then block indefinitely, waiting for the input. If a thread pool is being used, this could happen to every thread in the pool. In that case, no further processing can ever take place! The solution to this problem is to have connections "time out" if they are inactive for an excessive period of time. Typically, each connection thread will keep track of the time when it last received data from the client. The server runs another thread (sometimes called a "reaper thread", after the Grim Reaper) that wakes up periodically and checks each connection thread to see how long it has been inactive. A connection thread that has been waiting too long for input is terminated, and a new thread is started in its place. The question of how long the timeout period should be is another difficult tuning issue.

12.4.5  Distributed Computing

We have seen how threads can be used to do parallel processing, where a number of processors work together to complete some task. So far, we have assumed that all the processors were inside one multi-processor computer. But parallel processing can also be done using processors that are in different computers, as long as those computers are connected to a network over which they can communicate. This type of parallel processing -- in which a number of computers work together on a task and communicate over a network -- is called distributed computing.

In some sense, the whole Internet is an immense distributed computation, but here I am interested in how computers on a network can cooperate to solve some computational problem. There are several approaches to distributed computing that are supported in Java. RMI and CORBA are standards that enable a program running on one computer to call methods in objects that exist on other computers. This makes it possible to design an object-oriented program in which different parts of the program are executed on different computers. RMI (Remote Method Invocation) only supports communication between Java objects. CORBA (Common Object Request Broker Architecture) is a more general standard that allows objects written in various programming languages, including Java, to communicate with each other. As is commonly the case in networking, there is the problem of locating services (where in this case, a "service" means an object that is available to be called over the network). That is, how can one computer know which computer a service is located on and what port it is listening on? RMI and CORBA solve this problem using a "request broker" -- a server program running at a known location keeps a list of services that are available on other computers. Computers that offer services register those services with the request broker; computers that need services must know the location of the broker, and they contact it to find out what services are available and where they are located.

RMI and CORBA are complex systems that are not very easy to use. I mention them here because they are part of Java's standard network API, but I will not discuss them further. Instead, we will look at a relatively simple demonstration of distributed computing that uses only basic networking.

The problem that we will consider is the same one that we used in MultiprocessingDemo1.java, and its variations, in Section 12.2 and Section 12.3, namely the computation of a complex image. This is an application that uses the simplest type of parallel programming, in which the problem can be broken down into tasks that can be performed independently, with no communication between the tasks. To apply distributed computing to this type of problem, we can use one "master" program that divides the problem into tasks and sends those tasks over the network to "worker" programs that do the actual work. The worker programs send their results back to the master program, which combines the results from all the tasks into a solution of the overall problem. In this context, the worker programs are often called "slaves," and the program uses the so-called master/slave approach to distributed computing.

The demonstration program is defined by three source code files: CLMandelbrotMaster.java defines the master program; CLMandelbrotWorker.java defines the worker programs; and CLMandelbrotTask.java defines the class, CLMandelbrotTask, that represents an individual task that is performed by the workers. The master divides the overall problem into a collection of tasks; it distributes those tasks to the workers that will execute the tasks and send the results back to the master; and the master applies the results from all the individual tasks to the overall problem.

To run the demonstration, you must first start the CLMandelbrotWorker program on several computers (probably by running it on the command line). This program uses CLMandelbrotTask, so both class files, CLMandelbrotWorker.class and CLMandelbrotTask.class, must be present on the worker computers. You can then run CLMandelbrotMaster on the master computer. Note that this program also requires the class CLMandelbrotTask. You must specify the host name or IP address of each of the worker computers as command line arguments for CLMandelbrotMaster. The worker programs listen for connection requests from the master program, and the master program must be told where to send those requests. For example, if the worker program is running on three computers with IP addresses 172.21.7.101, 172.21.7.102, and 172.21.7.103, then you can run CLMandelbrotMaster with the command

java  CLMandelbrotMaster  172.21.7.101  172.21.7.102  172.21.7.103

The master will make a network connection to the worker at each IP address; these connections will be used for communication between the master program and the workers.

It is possible to run several copies of CLMandelbrotWorker on the same computer, but they must listen for network connections on different ports. It is also possible to run CLMandelbrotWorker on the same computer as CLMandelbrotMaster. You might even see some speed-up when you do this, if your computer has several processors. See the comments in the program source code files for more information, but here are some commands that you can use to run the master program and two copies of the worker program on the same computer. Give these commands in separate command windows:

java  CLMandelbrotWorker                             (Listens on default port)

java  CLMandelbrotWorker  2501                       (Listens on port 2501)

java  CLMandelbrotMaster  localhost  localhost:2501

Every time CLMandelbrotMaster is run, it solves exactly the same problem. (For this demonstration, the nature of the problem is not important, but the problem is to compute the data needed for a picture of a small piece of the famous "Mandelbrot Set." If you are interested in seeing the picture that is produced, uncomment the call to the saveImage() method at the end of the main() routine in CLMandelbrotMaster.java.)

You can run CLMandelbrotMaster with different numbers of worker programs to see how the time required to solve the problem depends on the number of workers. (Note that the worker programs continue to run after the master program exits, so you can run the master program several times without having to restart the workers.) In addition, if you run CLMandelbrotMaster with no command line arguments, it will solve the entire problem on its own, so you can see how long it takes to do so without using distributed computing. In a trial that I ran on some very old, slow computers, it took 40 seconds for CLMandelbrotMaster to solve the problem on its own. Using just one worker, it took 43 seconds. The extra time represents extra work involved in using the network; it takes time to set up a network connection and to send messages over the network. Using two workers (on different computers), the problem was solved in 22 seconds. In this case, each worker did about half of the work, and their computations were performed in parallel, so that the job was done in about half the time. With larger numbers of workers, the time continued to decrease, but only up to a point. The master program itself has a certain amount of work to do, no matter how many workers there are, and the total time to solve the problem can never be less than the time it takes for the master program to do its part. In this case, the minimum time seemed to be about five seconds.

Let's take a look at how this distributed application is programmed. The master program divides the overall problem into a set of tasks. Each task is represented by an object of type CLMandelbrotTask. These tasks have to be communicated to the worker programs, and the worker programs must send back their results. Some protocol is needed for this communication. I decided to use character streams. The master encodes a task as a line of text, which is sent to a worker. The worker decodes the text (into an object of type CLMandelbrotTask) to find out what task it is supposed to perform. It performs the assigned task. It encodes the results as another line of text, which it sends back to the master program. Finally, the master decodes the results and combines them with the results from other tasks. After all the tasks have been completed and their results have been combined, the problem has been solved.

A worker receives not just one task, but a sequence of tasks. Each time it finishes a task and sends back the result, it is assigned a new task. After all tasks are complete, the worker receives a "close" command that tells it to close the connection. In CLMandelbrotWorker.java, all this is done in a method named handleConnection() that is called to handle a connection that has already been opened to the master program. It uses a method readTask() to decode a task that it receives from the master and a method writeResults() to encode the results of the task for transmission back to the master. It must also handle any errors that occur:

private static void handleConnection(Socket connection) {
try {
BufferedReader in = new BufferedReader(
new InputStreamReader( connection.getInputStream()) );
PrintWriter out = new PrintWriter(connection.getOutputStream());
while (true) {
String line = in.readLine();  // Message from the master.
if (line == null) {
// End-of-stream encountered -- should not happen.
throw new Exception("Connection closed unexpectedly.");
}
if (line.startsWith(CLOSE_CONNECTION_COMMAND)) {
// Represents the normal termination of the connection.
System.out.println("Received close command.");
break;
}
else if (line.startsWith(TASK_COMMAND)) {
// Represents a CLMandelbrotTask that this worker is
// supposed to perform.
CLMandelbrotTask task = readTask(line);  // Decode the message.
task.compute();  // Perform the task.
out.println(writeResults(task));  //  Send back the results.
out.flush();  // Make sure data is sent promptly!
}
else {
// No other messages are part of the protocol.
throw new Exception("Illegal command received.");
}
}
}
catch (Exception e) {
System.out.println("Client connection closed with error " + e);
}
finally {
try {
connection.close();  // Make sure the socket is closed.
}
catch (Exception e) {
}
}
}

Note that this method is not executed in a separate thread. The worker has only one thing to do at a time and does not need to be multithreaded.

Turning to the master program, CLMandelbrotMaster.java, we encounter a more complex situation. The master program must communicate with several workers over several network connections. To accomplish this, the master program is multi-threaded, with one thread to manage communication with each worker. A pseudocode outline of the main() routine is quite simple:

create a list of all tasks that must be performed
if there are no command line arguments {
// The master program does all the tasks itself.
Perform each task.
}
else {
// The tasks will be performed by worker programs.
for each command line argument:
Get information about a worker from command line argument.
Create and start a thread to communicate with the worker.
// The threads do all the work...
Wait for all threads to terminate.
}
// All tasks are now complete (assuming no error occurred).

The tasks are put into a variable of type ConcurrentBlockingQueue<CLMandelbrotTask> named tasks (see Subsection 12.3.2.) The communication threads take tasks from this list and send them to worker programs. The method tasks.poll() is used to remove a task from the queue. If the queue is empty, it returns null, which acts as a signal that all tasks have been assigned and the communication thread can terminate.

The job of a thread is to send a sequence of tasks to a worker thread and to receive the results that the worker sends back. The thread is also responsible for opening the connection in the first place. A pseudocode outline for the process executed by the thread might look like:

Create a socket connected to the worker program.
Create input and output streams for communicating with the worker.
while (true) {
Let task = tasks.poll().
If task == null
break;  // All tasks have been assigned.
Encode the task into a message and transmit it to the worker.
Read the response from the worker.
Decode and process the response.
}
Send a "close" command to the worker.
Close the socket.

This would work OK. However, there are a few subtle points. First of all, the thread must be ready to deal with a network error. For example, a worker might shut down unexpectedly. But if that happens, the master program can continue, provided other workers are still available. (You can try this when you run the program: Stop one of the worker programs, with CONTROL-C, and observe that the master program still completes successfully.) A difficulty arises if an error occurs while the thread is working on a task: If the problem as a whole is going to be completed, that task will have to be reassigned to another worker. I take care of this by putting the uncompleted task back into the task list. (Unfortunately, my program does not handle all possible errors. If the last worker thread fails, there will be no one left to take over the uncompleted task. Also, if a network connection "hangs" indefinitely without actually generating an error, my program will also hang, waiting for a response from a worker that will never arrive. A more robust program would have some way of detecting the problem reassigning the task.)

Another defect in the procedure outlined above is that it leaves the worker program idle while the thread is processing the worker's response. It would be nice to get a new task to the worker before processing the response from the previous task. This would keep the worker busy and allow two operations to proceed simultaneously instead of sequentially. (In this example, the time it takes to process a response is so short that keeping the worker waiting while it is done probably makes no significant difference. But as a general principle, it's desirable to have as much parallelism as possible in the algorithm.) We can modify the procedure to take this into account:

try {
Create a socket connected to the worker program.
Create input and output streams for communicating with the worker.
Let currentTask = tasks.poll().
Encode currentTask into a message and send it to the worker.
while (true) {
Read the response from the worker.
Let nextTask = tasks.poll().
If nextTask != null {
// Send nextTask to the worker before processing the
// response to currentTask.
Encode nextTask into a message and send it to the worker.
}
Decode and process the response to currentTask.
currentTask = nextTask.
if (currentTask == null)
break; // All tasks have been assigned.
}
Send a "close" command to the worker.
Close the socket.
}
catch (Exception e) {
Put uncompleted task, if any, back into the task queue.
}
finally {
Close the connection.
}

Finally, here is how this translates into Java. The pseudocode presented above becomes the run() method in the class that defines the communication threads used by the master program:

/**
* This class represents one worker thread.  The job of a worker thread
* is to send out tasks to a CLMandelbrotWorker program over a network
* connection, and to get back the results computed by that program.
*/
private static class WorkerConnection extends Thread {

int id;        // Identifies this thread in output statements.
String host;   // The host to which this thread will connect.
int port;      // The port number to which this thread will connect.

/**
* The constructor just sets the values of the instance
* variables id, host, and port and starts the thread.
*/
WorkerConnection(int id, String host, int port) {
this.id = id;
this.host = host;
this.port = port;
start();
}

/**
* The run() method of the thread opens a connection to the host and
* port specified in the constructor, then sends tasks to the
* CLMandelbrotWorker program on the other side of that connection.
* If the thread terminates normally, it outputs the number of tasks
* that it processed.  If it terminates with an error, it outputs
* an error message.
*/
public void run() {

int tasksCompleted = 0; // How many tasks has this thread handled.
Socket socket;  // The socket for the connection.

try {
socket = new Socket(host,port);  // open the connection.
}
catch (Exception e) {
System.out.println("Thread " + id +
" could not open connection to " + host + ":" + port);
System.out.println("   Error: " + e);
return;
}

CLMandelbrotTask currentTask = null;
CLMandelbrotTask nextTask = null;

try {
PrintWriter out = new PrintWriter(socket.getOutputStream());
BufferedReader in = new BufferedReader(
new InputStreamReader(socket.getInputStream()) );
currentTask = tasks.poll();
if (currentTask != null) {
// Send first task to the worker program.
String taskString = writeTask(currentTask);
out.println(taskString);
out.flush();
}
while (currentTask != null) {
String resultString = in.readLine(); // Get results for currentTask.
if (resultString == null)
throw new IOException("Connection closed unexpectedly.");
if (! resultString.startsWith(RESULT_COMMAND))
throw new IOException("Illegal string received from worker.");
nextTask = tasks.poll();  // Get next task and send it to worker.
if (nextTask != null) {
// Send nextTask to worker before processing results for
// currentTask, so that the worker can work on nextTask
// while the currentTask results are processed.
String taskString = writeTask(nextTask);
out.println(taskString);
out.flush();
}
readResults(resultString, currentTask);
finishTask(currentTask);  // Process results from currentTask.
tasksCompleted++;
currentTask = nextTask;   // We are finished with old currentTask.
nextTask = null;
}
out.println(CLOSE_CONNECTION_COMMAND);  // Send close command to worker.
out.flush();
}
catch (Exception e) {
System.out.println("Thread " + id + " terminated because of an error");
System.out.println("   Error: " + e);
e.printStackTrace();
// Put uncompleted tasks, if any, back into the task list.
if (currentTask != null)
reassignTask(currentTask);
if (nextTask != null)
reassignTask(nextTask);
}
finally {
System.out.println("Thread " + id + " ending after completing " +
tasksCompleted + " tasks");
try {
socket.close();
}
catch (Exception e) {
}
}

} //end run()

} // end nested class WorkerConnection

Section 12.5

Network Programming Example: A Networked Game Framework

This section presents several programs that use networking and threads. The common problem in each application is to support network communication between several programs running on different computers. A typical example of such an application is a networked game with two or more players, but the same problem can come up in less frivolous applications as well. The first part of this section describes a common framework that can be used for a variety of such applications, and the rest of the section discusses three specific applications that use that framework. This is a fairly complex example, probably the most complex that you have seen so far in this book. Understanding it is not essential for a basic understanding of networking.

This section was inspired by a pair of students, Alexander Kittelberger and Kieran Koehnlein, who wanted to write a networked poker game as a final project in a class that I was teaching. I helped them with the network part of the project by writing a basic framework to support communication between the players. Since the application illustrates a variety of important ideas, I decided to include a somewhat more advanced and general version of that framework in the current edition of this book. The final example is a networked poker game.

12.5.1  The Netgame Framework

One can imagine playing many different games over the network. As far as the network goes, all of those games have at least one thing in common: There has to be some way for actions taken by one player to be communicated over the network to other players. It makes good programming sense to make that capability available in a reusable common core that can be used in many different games. I have written such a core; it is defined by several classes in the package netgame.common.

We have not done much with packages in this book, aside from using built-in classes. Packages were introduced in Subsection 2.6.6, but we have stuck to the "default package" in our programming examples. In practice, however, packages are used in all but the simplest programming projects to divide the code into groups of related classes. It makes particularly good sense to define a reusable framework in a package that can be included as a unit in a variety of projects.

Integrated development environments such as Eclipse or Netbeans make it very easy to use packages: To use the netgame package in a project in an IDE, simply copy-and-paste the entire netgame directory into the project.

If you work on the command line, you should be in a working directory that includes the netgame directory as a subdirectory. Then, to compile all the java files in the package netgame.common, for example, you can use the following command in Mac OS or Linux:

javac netgame/common/*.java

For windows, you should use backslashes instead of forward slashes:

javac netgame\common\*.java

You will need similar commands to compile the source code for the examples in this section, which are defined in other subpackages of netgame.

To run a main program that is defined in a package, you should again be in a directory that contains the package as a subdirectory, and you should use the full name of the class that you want to run. For example, the ChatRoomServer class, discussed later in this section, is defined in the package netgame.chat, so you would run it with the command

java netgame.chat.ChatRoomServer

I will have more to say about packages in the final example of the book, in Section 13.5.

The applications discussed in this section are examples of distributed computing, since they involve several computers communicating over a network. Like the example in Subsection 12.4.5, they use a central "server," or "master," to which a number of "clients" will connect. All communication goes through the server; a client cannot send messages directly to another client. In this section, I will refer to the server as a hub, in the sense of "communications hub":

The main things that you need to understand are that: The hub must be running before any clients are started. Clients connect the hub and can send messages to the hub. The hub processes all messages from clients sequentially, in the order in which they are received. The processing can result in the hub sending messages out to one or more clients. Each client is identified by a unique ID number. This is a framework that can be used in a variety of applications, and the messages and processing will be defined by the particular application. Here are some of the details...

In Subsection 12.4.5, messages were sent back and forth between the server and the client in a definite, predetermined sequence. Communication between the server and a client was actually communication between one thread running on the server and another thread running on the client. For the netgame framework, however, I want to allow for asynchronous communication, in which it is not possible to wait for messages to arrive in a predictable sequence. To make this possible a netgame client will use two threads for communication, one for sending messages to the hub and one for receiving messages from the hub. Similarly, the netgame hub will use two threads for communicating with each client.

The hub is generally connected to many clients and can receive messages from any of those clients at any time. The hub will have to process each message in some way. To organize this processing, the hub uses a single thread to process all incoming messages. When a communication thread receives a message from a client, it simply drops that message into a queue of incoming messages. There is only one such queue, which is used for messages from all clients. The message processing thread runs in a loop in which it removes a message from the queue, processes it, removes another message from the queue, processes it, and so on. The queue itself is implemented as an object of type LinkedBlockingQueue (see Subsection 12.3.3).

There is one more thread in the hub, not shown in the illustration. This final thread creates a ServerSocket and uses it to listen for connection requests from clients. Each time it accepts a connection request, it hands off the client socket to another object, defined by the nested class ConnectionToClient, which will handle communication with that client. Each connected client is identified by an ID number. ID numbers 1, 2, 3, ... are assigned to clients as they connect. Since clients can also disconnect, the clients connected at any give time might not have consecutive IDs. A variable of type TreeMap<Integer,ConnectionToClient> associates the ID numbers of connected clients with the objects that handle their connections.

The messages that are sent and received are objects. The I/O streams that are used for reading and writing objects are of type ObjectInputStream and ObjectOutputStream. (See Subsection 11.1.6.) The output stream of a socket is wrapped in an ObjectOutputStream to make it possible to transmit objects through that socket. The socket's input stream is wrapped in an ObjectInputStream to make it possible to receive objects. Remember that the objects that are used with such streams must implement the interface java.io.Serializable.

The netgame Hub class is defined in the file Hub.java, in the package netgame.common. The port on which the server socket will listen must be specified as a parameter to the Hub constructor. The Hub class defines a method

protected void messageReceived(int playerID, Object message)

which is called to process the messages that are received from clients. The first parameter, playerID, is the ID number of the client from whom the message was received, and the second parameter is the message itself. In the Hub class, this method will simply forward the message to all connected clients. This defines the default processing for incoming messages to the hub. To forward the message, it wraps both the playerID and the message in an object of type ForwardedMessage (defined in the file ForwardedMessage.java, in the package netgame.common). In a simple application such as the chat room discussed in the next subsection, this default processing might be exactly what is needed by the application. For most applications, however, it will be necessary to define a subclass of Hub and redefine the messageReceived() method to do more complicated message processing. There are several other methods in the Hub class that you might want to redefine in a subclass, including

- protected void playerConnected(int playerID) -- This method is called each time a player connects to the hub. The parameter playerID is the ID number of the newly connected player. In the Hub class, this method does nothing. Note that the complete list of ID numbers for currently connected players can be obtained by calling getPlayerList().

- protected void playerDisconnected(int playerID) -- This is called each time a player disconnects from the hub. The parameter tells which player has just disconnected. In the Hub class, this method does nothing.

The Hub class also defines a number of useful public methods, notably

- sendToAll(message) -- sends the specified message to every client that is currently connected to the hub. The message must be a non-null object that implements the Serializable interface.

- sendToOne(recipientID,message) -- sends a specified message to just one user. The first parameter, recipientID is the ID number of the client who will receive the message. This method returns a boolean value, which is false if there is no connected client with the specified recipientID.

- shutDownServerSocket() -- shuts down the hub's server socket, so that no additional clients will be able to connect. This could be used, for example, in a two-person game, after the second client has connected.

- setAutoreset(autoreset) -- sets the boolean value of the autoreset property. If this property is true, then the ObjectOutputStreams that are used to transmit messages to clients will automatically be reset before each message is transmitted. The default value is false. (Resetting an ObjectOutputStream is something that has to be done if an object is written to the stream, modified, and then written to the stream again. If the stream is not reset before writing the modified object, then the old, unmodified value is sent to the stream instead of the new value. See Subsection 11.1.6 for a discussion of this technicality.)

For more information -- and to see how all this is implemented -- you should read the source code file Hub.java. With some effort and study, you should be able to understand everything in that file. (However, you only need to understand the public and protected interface of Hub and other classes in the the netgame framework to write applications based on it.)

Turning to the client side, the basic netgame client class is defined in the file Client.java, in the package netgame.common. The Client class has a constructor that specifies the host name (or IP address) and port number of the hub to which the client will connect. This constructor blocks until the connection has been established.

Client is an abstract class. Every netgame application must define a subclass of Client and provide a definition for the abstract method:

abstract protected void messageReceived(Object message);

This method is called each time a message is received from the netgame hub to which the client is connected. A subclass of client might also override the protected methods playerConnected, playerDisconnected, serverShutdown, and connectionClosedByError. See the source code for more information. I should also note that Client contains the protected instance variable connectedPlayerIDs, of type int[], an array containing the ID numbers of all the clients that are currently connected to the hub. The most important public methods that are provided by the Client class are

- send(message) -- transmits a message to the hub. The message can be any non-null object that implements the Serializable interface.

- getID() -- gets the ID number that was assigned to this client by the hub.

- disconnect() -- closes the client's connection to the hub. It is not possible to send messages after disconnecting. The send() method will throw an IllegalStateException if an attempt is made to do so.

The Hub and Client classes are meant to define a general framework that can be used as the basis for a variety of networked games -- and, indeed, of other distributed programs. The low level details of network communication and multithreading are hidden in the private sections of these classes. Applications that build on these classes can work in terms of higher-level concepts such as players and messages. The design of these classes was developed though several iterations, based on experience with several actual applications. I urge you to look at the source code to see how Hub and Client use threads, sockets, and streams. In the remainder of this section, I will discuss three applications built on the netgame framework. I will not discuss these applications in great detail. You can find the complete source code for all three in the netgame package.

12.5.2  A Simple Chat Room

Our first example is a "chat room," a network application where users can connect to a server and can then post messages that will be seen by all current users of the room. It is similar to the GUIChat program from Subsection 12.4.2, except that any number of users can participate in a chat. While this application is not a game, it does show the basic functionality of the netgame framework.

The chat room application consists of two programs. The first, ChatRoomServer.java, is a completely trivial program that simply creates a netgame Hub to listen for connection requests from netgame clients:

public static void main(String[] args) {
try {
new Hub(PORT);
}
catch (IOException e) {
System.out.println("Can't create listening socket.  Shutting down.");
}
}

The port number, PORT, is defined as a constant in the program and is arbitrary, as long as both the server and the clients use the same port. Note that ChatRoom uses the Hub class itself, not a subclass.

The second part of the chat room application is the program ChatRoomWindow.java, which is meant to be run by users who want to participate in the chat room. A potential user must know the name (or IP address) of the computer where the hub is running. (For testing, it is possible to run the client program on the same computer as the hub, using localhost as the name of the computer where the hub is running.) When ChatRoomWindow is run, it uses a dialog box to ask the user for this information. It then opens a window that will serve as the user's interface to the chat room. The window has a large transcript area that displays messages that users post to the chat room. It also has a text input box where the user can enter messages. When the user enters a message, that message will be posted to the transcript of every user who is connected to the hub, so all users see every message sent by every user. Let's look at some of the programming.

Any netgame application must define a subclass of the abstract Client class. For the chat room application, clients are defined by a nested class ChatClient inside ChatRoomWindow. The program has an instance variable, connection, of type ChatClient, which represents the program's connection to the hub. When the user enters a message, that message is sent to the hub by calling

connection.send(message);

When the hub receives the message, it packages it into an object of type ForwardedMessage, along with the ID number of the client who sent the message. The hub sends a copy of that ForwardedMessage to every connected client, including the client who sent the message. On the client side in each client, when the message is received from the hub, the messageReceived() method of the ChatClient object in that client is called. ChatClient overrides this method to make it add the message to the transcript of the ChatClientWindow. To summarize: Every message entered by any user is sent to the hub, which just sends out copies of each message that it receives to every client. Each client will see exactly the same stream of messages from the hub.

A client is also notified when a player connects to or disconnects from the hub and when the connection with the hub is lost. ChatClient overrides the methods that are called when these events happen so that they post appropriate messages to the transcript. Here's the complete definition of the client class for the chat room application:

/**
* A ChatClient connects to a Hub and is used to send messages to
* and receive messages from the Hub.  Messages received from the
* Hub will be of type ForwardedMessage and will contain the
* ID number of the sender and the string that was sent by that user.
*/
private class ChatClient extends Client {

/**
* Opens a connection to the chat room server on a specified computer.
*/
ChatClient(String host) throws IOException {
super(host, PORT);
}

/**
* Responds when a message is received from the server.  It should be
* a ForwardedMessage representing something that one of the participants
* in the chat room is saying.  The message is simply added to the
* transcript, along with the ID number of the sender.
*/
protected void messageReceived(Object message) {
if (message instanceof ForwardedMessage) {
// (no other message types are expected)
ForwardedMessage fm = (ForwardedMessage)message;
addToTranscript("#" + fm.senderID + " SAYS:  " + fm.message);
}
}

/**
* Called when the connection to the client is shut down because of some
* error message.  (This will happen if the server program is terminated.)
*/
protected void connectionClosedByError(String message) {
addToTranscript("Sorry, communication has shut down due to an error:\n     "
+ message);
sendButton.setEnabled(false);
messageInput.setEnabled(false);
messageInput.setEditable(false);
messageInput.setText("");
connected = false;
connection = null;
}

/**
* Posts a message to the transcript when someone joins the chat room.
*/
protected void playerConnected(int newPlayerID) {
addToTranscript("Someone new has joined the chat room, with ID number "
+ newPlayerID);
}

/**
* Posts a message to the transcript when someone leaves the chat room.
*/
protected void playerDisconnected(int departingPlayerID) {
addToTranscript("The person with ID number " + departingPlayerID
+ " has left the chat room");
}

} // end nested class ChatClient

For the full source code of the chat room application, see the source code files, which can be found in the package netgame.chat.

Note: A user of my chat room application is identified only by an ID number that is assigned by the hub when the client connects. Essentially, users are anonymous, which is not very satisfying. See Exercise 12.6 at the end of this chapter for a way of addressing this issue.

12.5.3  A Networked TicTacToe Game

My second example is a very simple game: the familiar children's game TicTacToe. In TicTacToe, two players alternate placing marks on a three-by-three board. One player plays X's; the other plays O's. The object is to get three X's or three O's in a row.

At a given time, the state of a TicTacToe game consists of various pieces of information such as the current contents of the board, whose turn it is, and -- when the game is over -- who won or lost. In a typical non-networked version of the game, this state would be represented by instance variables. The program would consult those instance variables to determine how to draw the board and how to respond to user actions such as mouse clicks. In the networked netgame version, however, there are three objects involved: Two objects belonging to a client class, which provide the interface to the two players of the game, and the hub object that manages the connections to the clients. These objects are not even on the same computer, so they certainly can't use the same state variables! Nevertheless, the game has to have a single, well-defined state at any time, and both players have to be aware of that state.

My solution is to store the "official" game state in the hub, and to send a copy of that state to each player every time the state changes. The players can't change the state directly. When a player takes some action, such as placing a piece on the board, that action is sent as a message to the hub. The hub changes the state to reflect the result of the action, and it sends the new state to both players. The window used by each player will then be updated to reflect the new state. In this way, we can be sure that the game always looks the same to both players. (Instead of sending a complete copy of the state each time the state changes, I might have sent just the change. But that would require some way to encode the changes into messages that can be sent over the network. Since the state is so simple, it seemed easier just to send the entire state as the message in this case.)

Networked TicTacToe is defined in several classes in the package netgame.tictactoe. TicTacToeGameState represents the state of a game. It includes a method

public void applyMessage(int senderID, Object message)

that modifies the state to reflect the effect of a message received from one of the players of the game. The message will represent some action taken by the player, such as clicking on the board.

The basic Hub class knows nothing about TicTacToe. Since the hub for the TicTacToe game has to keep track of the state of the game, it has to be defined by a subclass of Hub. The TicTacToeGameHub class is quite simple. It overrides the messageReceived() method so that it responds to a message from a player by applying that message to the game state and sending a copy of the new state to both players. It also overrides the playerConnected() and playerDisconnected() methods to take appropriate actions, since the game can only be played when there are exactly two connected players. Here is the complete source code:

package netgame.tictactoe;

import java.io.IOException;

import netgame.common.Hub;

/**
* A "Hub" for the network TicTacToe game.  There is only one Hub
* for a game, and both network players connect to the same Hub.
* Official information about the state of the game is maintained
* on the Hub.  When the state changes, the Hub sends the new
* state to both players, ensuring that both players see the
* same state.
*/
public class TicTacToeGameHub extends Hub {

private TicTacToeGameState state;  // Records the state of the game.

/**
* Create a hub, listening on the specified port.  Note that this
* method calls setAutoreset(true), which will cause the output stream
* to each client to be reset before sending each message.  This is
* essential since the same state object will be transmitted over and
* over, with changes between each transmission.
* @param port the port number on which the hub will listen.
* @throws IOException if a listener cannot be opened on the specified port.
*/
public TicTacToeGameHub(int port) throws IOException {
super(port);
state = new TicTacToeGameState();
setAutoreset(true);
}

/**
* Responds when a message is received from a client.  In this case,
* the message is applied to the game state, by calling state.applyMessage().
* Then the possibly changed state is transmitted to all connected players.
*/
protected void messageReceived(int playerID, Object message) {
state.applyMessage(playerID, message);
sendToAll(state);
}

/**
* This method is called when a player connects.  If that player
* is the second player, then the server's listening socket is
* shut down (because only two players are allowed), the
* first game is started, and the new state -- with the game
* now in progress -- is transmitted to both players.
*/
protected void playerConnected(int playerID) {
if (getPlayerList().length == 2) {
shutdownServerSocket();
state.startFirstGame();
sendToAll(state);
}
}

/**
* This method is called when a player disconnects.  This will
* end the game and cause the other player to shut down as
* well.  This is accomplished by setting state.playerDisconnected
* to true and sending the new state to the remaining player, if
* there is one, to notify that player that the game is over.
*/
protected void playerDisconnected(int playerID) {
state.playerDisconnected = true;
sendToAll(state);
}
}

A player's interface to the game is represented by the class TicTacToeWindow. As in the chat room application, this class defines a nested subclass of Client to represent the client's connection to the hub. One interesting point is how the client responds to a message from the hub. Such a message represents a new game state. When the message is received, the window must be updated to show the new state. The message is received and processed by one thread; the updating is done in another thread. This has the potential of introducing race conditions that require synchronization. (In particular, as I was developing the program, I found that it was possible for a message to be received before the window's constructor had finished executing. This led to a very hard-to-diagnose bug because my response to the message was trying to use objects that had not yet been created.)

When working with the Swing API, it is recommended that all modifications to the GUI be made in the GUI event thread. An alternative would be to make paintComponent() and other methods synchronized, but that would negatively impact the performance of the GUI. Swing includes a method SwingUtilitites.invokeLater(runnable) to make it possible to run arbitrary code in the GUI event thread. See Subsection 12.2.5. In the TicTacToe client class, this technique is used in the method that processes events received from the hub:

protected void messageReceived(final Object message) {
if (message instanceof TicTacToeGameState) {
SwingUtilities.invokeLater(new Runnable(){
public void run() {
// The newState() method updates the GUI for the new state.
newState( (TicTacToeGameState)message );
}
});
}
}

To run the TicTacToe netgame, the two players should each run the program Main.java in the package netgame.tictactoe. This program presents the user with a dialog box where the user can choose to start a new game or to join an existing game. If the user starts a new game, then a TicTacToeHub is created to manage the game, and a TicTacToeWindow is created that immediately connects to the hub. The game will start as soon as a second player connects to the hub. On the other hand, if the user running TicTacToeWindow chooses to connect to an existing game, then no hub is created. A window is created, and that window connects to the hub that was created by the first player. The second player has to know the name of the computer where the first player's program is running. As usual, for testing, you can run everything on one computer and use "localhost" as the computer name.

12.5.4  A Networked Poker Game

And finally, we turn very briefly to the application that inspired the netgame framework: Poker. In particular, I have implemented a two-player version of the traditional "five card draw" version of that game. This is a rather complex application, and I do not intend to say much about it here other than to describe the general design. The full source code can be found in the package netgame.fivecarddraw. To fully understand it, you will need to be familiar with the game of five card draw poker. And it uses some techniques from Section 13.1 for drawing the cards.

In general outline, the Poker game is similar to the TicTacToe game. There is a Main class that is one by both players. The first player starts a new game; the second must join that existing game. There is a class PokerGameState to represent the state of a game. And there is a subclass, PokerHub, of Hub to manage the game.

But Poker is a much more complicated game than TicTacToe, and the game state is correspondingly more complicated. It's not clear that we want to broadcast a new copy of the complete game state to the players every time some minor change is made in the state. Furthermore, it doesn't really make sense for both players to know the full game state -- that would include the opponent's hand and full knowledge of the deck from which the cards are dealt. (Of course, our client programs wouldn't have to show the full state to the players, but it would be easy enough for a player to substitute their own client program to enable cheating.) So in the Poker application, the full game state is known only to the PokerHub. A PokerGameState object represents a view of the game from the point of view of one player only. When the state of the game changes, the PokerHub creates two different PokerGameState objects, representing the state of the game from each player's point of view, and it sends the appropriate game state objects to each player. You can see the source code for details.

(One of the hard parts in poker is to implement some way to compare two hands, to see which is higher. In my game, this is handled by the class PokerRank. You might find this class useful in other poker games.)

Programming Exercises for Chapter 12

Exercise 12.1:

Subsection 12.1.3 discusses the need for synchronization in multithreaded programs, and it defines a ThreadSafeCounter class with the necessary synchronization. Is this really important? Can you really get errors by using an unsynchronized counter with multiple threads? Write a program to find out. Use the following unsynchronized counter class, which you can include as a nested class in your program:

static class Counter {
int count;
void inc() {
count = count+1;
}
int getCount() {
return count;
}
}

Write a thread class that will call the inc() method in this class a specified number of times. Create several threads, start them all, and wait for all the threads to terminate. Print the final value of the counter, and see whether it is correct.

Let the user enter the number of threads and the number of times that each thread will increment the counter. You might need a fairly large number of increments to see an error. And of course there can never be any error if you use just one thread. Your program can use join() to wait for a thread to terminate (see Subsection 12.1.2).

See the Solution

Exercise 12.2:

Exercise 3.2 asked you to find the integer in the range 1 to 10000 that has the largest number of divisors. Now write a program that uses multiple threads to solve the same problem, but for the range 1 to 100000 (or less, if you don't have a fast computer). By using threads, your program will take less time to do the computation when it is run on a multiprocessor computer. At the end of the program, output the elapsed time, the integer that has the largest number of divisors, and the number of divisors that it has. The program can be modeled on the sample prime-counting program ThreadTest2.java from Subsection 12.1.3. For this exercise, you should simply divide up the problem into parts and create one thread to do each part.

See the Solution

Exercise 12.3:

In the previous exercise, you divided up a large task into a small number of large pieces and created a thread to execute each task. Because of the nature of the problem, this meant that some threads had much more work to do than others -- it is much easier to find the number of divisors of a small number than it is of a big number. As discussed in Subsection 12.3.1, a better approach is to break up the problem into a fairly large number of smaller problems. Subsection 12.3.2 shows how to use a thread pool to execute the tasks: Each thread in the pool runs in a loop in which it repeatedly takes a task from a queue and carries out that task. Implement a thread pool strategy for solving the same maximum-number-of-divisors problem as in the previous exercise.

To make things even more interesting, you should try a new technique for combining the results from all the tasks: Use two queues in your program. Use a queue of tasks, as usual, to hold the tasks that will be executed by the thread pool (Subsection 12.3.2). But also use a queue of results produced by the threads. When a task completes, the result from that task should be placed into the result queue. The main program can read results from the second queue as they become available, and combine all the results to get the final answer. The result queue will have to be a blocking queue (Subsection 12.3.3), since the main program will have to wait for results to become available. Note that the main program knows the exact number of results that it expects to read from the queue, so it can do so in a for loop; when the for loop completes, the main program knows that all the tasks have been executed.

See the Solution

Exercise 12.4:

In Exercise 11.3, you wrote a network server program that can send text files from a specified directory to clients. That program used a single thread, which handled all the communication with each client. Modify the program to turn it into a multithreaded server. Use a thread pool of connection-handling threads and use an ArrayBlockingQueue to get connected sockets from the main() routine to the threads. The sample program DateServerWithThreads.java from Subsection 12.4.3 is an example of a multithreaded server that works in this way. Your server program will work with the same client program as the original server. You wrote the client program as the solution to Exercise 11.4.

See the Solution

Exercise 12.5:

It is possible to get an estimate of the mathematical constant π by using a random process. The idea is based on the fact that the area of a circle of radius 1 is equal to π, and the area of a quarter of that circle is π/4. Here is a picture of a quarter of a circle of radius 1, inside a 1-by-1 square:

The area of the whole square is one, while the area of the part inside the circle is π/4. If we choose a point in the square at random, the probability that it is inside the circle is π/4. If we choose N points in the square at random, and if C of them are inside the circle, we expect the fraction C/N of points that fall inside the circle to be about π/4. That is, we expect 4*C/N to be close to π. If N is large, we can expect 4*C/N to be a good estimate for π, and as N gets larger and larger, the estimate is likely to improve.

We can pick a random point in the square by choosing numbers x and y in the range 0 to 1 (using Math.random()). Since the equation of the circle is x*x+y*y=1, the point lies inside the circle if x*x+y*y is less than 1. One trial consists of picking x and y and testing whether x*x+y*y is less than 1. To get an estimate for π, you have to do many trials, count the trials, and count the number of trials in which x*x+y*y is less than 1,

For this exercise, you should write a GUI program that does this computation and displays the result. The computation should be done in a separate thread, and the results should be displayed periodically. The program can use JLabels to the display the results. It should set the text on the labels after running each batch of, say, one million trials. (Setting the text after each trial doesn't make sense, since millions of trials can be done in one second, and trying to change the display millions of times per second would be silly.

Your program should have a "Run"/"Pause" button that controls the computation. When the program starts, clicking "Run" will start the computation and change the text on the button to "Pause". Clicking "Pause" will cause the computation to pause. The thread that does the computation should be started at the beginning of the program, but should immediately go into the paused state until the "Run" button is pressed. Use the wait() method in the thread to make it wait until "Run" is pressed. Use the notify() method when the "Run" button is pressed to wake up the thread. Use a boolean signal variable running to control whether the computation thread is paused. (The wait() and notify() methods are covered in Subsection 12.3.4.)

Here is a picture of the program after it has run many trials:

You might want to start with a version of the program with no control button. In that version, the computation thread can run continually from the time it is started. Once that is working, you can add the button and the control feature.

To get you started, here is the code from the thread in my solution that runs one batch of trials and updates the display labels:

for (int i = 0; i < BATCH_SIZE; i++) {
double x = Math.random();
double y = Math.random();
trialCount++;
if (x*x + y*y < 1)
inCircleCount++;
}
double estimateForPi = 4 * ((double)inCircleCount / trialCount);
countLabel.setText(      " Number of Trials:   " + trialCount);
piEstimateLabel.setText( " Current Estimate:   " + estimateForPi);

The variables trialCount and inCircleCount are of type long in order to allow the number of trials to be more than the two billion or so that would be possible with a variable of type int.

(I was going to ask you to use multiple computation threads, one for each available processor, but I ran into an issue when using the Math.random() method in several threads. This method requires synchronization, which causes serious performance problems when several threads are using it to generate large amounts of random numbers. A solution to this problem is to have each thread use its own object of type java.util.Random to generate its random numbers (see Subsection 5.3.1). My on-line solution to this exercise discusses this problem further.)

See the Solution

Exercise 12.6:

The chat room example from Subsection 12.5.2 can be improved in several ways. First, it would be nice if the participants in the chat room could be identified by name instead of by number. Second, it would be nice if one person could send a private message to another person that would be seen just by that person rather than by everyone. Make these two changes. You can start with a copy of the package netgame.chat. You will also need the package netgame.common, which defines the netgame framework.

To make the first change, you will have to implement a subclass of Hub that can keep track of client names as well as numbers. To get the name of a client to the hub, you can override the extraHandshake() method both in the Hub subclass and in the Client subclass. The extraHandshake() method is called as part of setting up the connection between the client and the hub. It is called after the client has been assigned an ID number but before the connection is considered to be fully connected. It should throw an IOException if some error occurs during the setup process. Note that any messages that are sent by the hub as part of the handshake must be read by the client and vice versa. The extraHandshake() method in the Client is defined as:

protected void extraHandshake(ObjectInputStream in, ObjectOutputStream out)
throws IOException

while in the Hub, there is an extra parameter that tells the ID number of the client whose connection is being set up:

protected void extraHandshake(in playerID, ObjectInputStream in,
ObjectOutputStream out) throws IOException

In the ChatRoomWindow class, the main() routine asks the user for the name of the computer where the server is running. You can add some code there to ask the user their name. You will have to decide what to do if two users want to use the same name. You might consider having a list of users who are allowed to join the chat room. You might even assign them passwords.

For the second improvement, personal messages, I suggest defining a PrivateMessage class. A PrivateMessage object would include both the string that represents the message and the ID numbers of the player to whom the message is being sent and the player who sent the message. The hub will have to be programmed to know how to deal with such messages. A PrivateMessage should only be sent by the hub to the client who is listed as the recipient of the message. You need to decide how the user will input a private message and how the user will select the recipient of the message.

If you attempt this exercise, you are likely to find it quite challenging.

See the Solution

Quiz on Chapter 12

Question 1:

Write a complete subclass of Thread to represent a thread that writes out the numbers from 1 to 10. Then write some code that would create and start a thread belonging to that class.

Question 2:

Suppose that thrd is an object of type Thread. Explain the difference between calling thrd.start() and calling thrd.run().

Question 3:

What is a race condition?

Question 4:

How does synchronization prevent race conditions, and what does it mean to say that synchronization only provides mutual exclusion?

Question 5:

Suppose that a program uses a single thread that takes 4 seconds to run. Now suppose that the program creates two threads and divides the same work between the two threads. What can be said about the expected execution time of the program that uses two threads?

Question 6:

What is an ArrayBlockingQueue and how does it solve the producer/consumer problem?

Question 7:

What is a thread pool?

Question 8:

Network server programs are often multithreaded. Explain what this means and why it is true.

Question 9:

Why does a multithreaded network server program often use many times more threads than the number of available processors?

Question 10:

Consider the ThreadSafeCounter example from Subsection 12.1.3:

public class ThreadSafeCounter {

private int count = 0;  // The value of the counter.

synchronized public void increment() {
count = count + 1;
}

synchronized public int getValue() {
return count;
}

}

The increment() method is synchronized so that the caller of the method can complete the three steps of the operation "Get value of count," "Add 1 to value," "Store new value in count" without being interrupted by another thread. But getValue() consists of a single, simple step. Why is getValue() synchronized? (This is a deep and tricky question.)

See the Answers

Chapter 13

Advanced GUI Programming

It's possible to program a wide variety of GUI applications using only the techniques covered in Chapter 6. In many cases, the basic events, components, layouts, and graphics routines covered in that chapter suffice. But the Swing graphical user interface library is far richer than what we have seen so far, and it can be used to build highly sophisticated applications. This chapter is a further introduction to Swing and other aspects of GUI programming. Although the title of the chapter is "Advanced GUI Programming," it is still just an introduction. Full coverage of this topic would require at least another complete book.

Contents of Chapter 13:

- Section 1: Images and Resources

- Section 2: Fancier Graphics

- Section 3: Actions and Buttons

- Section 4: Complex Components and MVC

- Section 5: Finishing Touches

- Programming Exercises

- Quiz on This Chapter

Section 13.1

Images and Resources

We have seen how to use the Graphics class to draw on a GUI component that is visible on the computer's screen. Often, however, it is useful to be able to create a drawing off-screen, in the computer's memory. It is also important to be able to work with images that are stored in files.

To a computer, an image is just a set of numbers. The numbers specify the color of each pixel in the image. The numbers that represent the image on the computer's screen are stored in a part of memory called a frame buffer. Many times each second, the computer's video card reads the data in the frame buffer and colors each pixel on the screen according to that data. Whenever the computer needs to make some change to the screen, it writes some new numbers to the frame buffer, and the change appears on the screen a fraction of a second later, the next time the screen is redrawn by the video card.

Since it's just a set of numbers, the data for an image doesn't have to be stored in a frame buffer. It can be stored elsewhere in the computer's memory. It can be stored in a file on the computer's hard disk. Just like any other data file, an image file can be downloaded over the Internet. Java includes standard classes and subroutines that can be used to copy image data from one part of memory to another and to get data from an image file and use it to display the image on the screen.

13.1.1  Images and BufferedImages

The class java.awt.Image represents an image stored in the computer's memory. There are two fundamentally different types of Image. One kind represents an image read from a source outside the program, such as from a file on the computer's hard disk or over a network connection. The second type is an image created by the program, by drawing to it using a graphics context. I refer to this second type as an off-screen canvas. An off-screen canvas is a region of the computer's memory that can be used as a drawing surface. It is possible to draw to an off-screen image using the same Graphics class that is used for drawing on the screen.

An Image of either type can be copied onto the screen (or onto an off-screen canvas) using methods that are defined in the Graphics class. This is most commonly done in the paintComponent() method of a JComponent. Suppose that g is the Graphics object that is a parameter to the paintComponent() method, and that img is of type Image. Then the statement

g.drawImage(img, x, y, this);

will draw the image img in a rectangular area in the component. The integer-valued parameters x and y give the position of the upper-left corner of the rectangle in which the image is displayed, and the rectangle is just large enough to hold the image. The fourth parameter, this is there for technical reasons. In all cases in this book it will be either this or null. The parameter is of type ImageObserver and a non-null value is needed only when the complete image might not be available when the drawImage() method is called. This can happen, for example, if the image is being read from a file or downloaded over the network. You don't need it for an image that the program has created itself in the computer's memory or for an image that you are sure has already been completely loaded. In those cases, the image observer parameter can be null. However, even in those cases, using a non-null value does not cause any problems.

(In cases where you do need a non-null image observer, the special variable this, from Subsection 5.6.1, is usually appropriate. The image observer parameter is there for technical reasons having to do with the funny way Java treats image files. For most applications, you don't need to understand this, but here is how it works: g.drawImage() does not actually draw the image in all cases. In some circumstances, it is possible that the complete image is not available when this method is called. In that case, g.drawImage() merely initiates the drawing of the image and returns immediately. Pieces of the image are drawn later, asynchronously, as they become available. The question is, how do they get drawn? That's where the image observer comes in. When a piece of the image becomes available to be drawn, the system will inform the ImageObserver, and it is the responsibility of the observer to make sure that the new piece of the image will appear on the screen. Any JComponent object can act as an ImageObserver; it will call its repaint() method when notified that more of the image is available. The drawImage method returns a boolean value to indicate whether the image has actually been drawn or not when the method returns.)

There are a few useful variations of the drawImage() method. For example, it is possible to scale the image as it is drawn to a specified width and height. This is done with the command

g.drawImage(img, x, y, width, height, imageObserver);

The parameters width and height give the size of the rectangle in which the image is displayed. Another version makes it possible to draw just part of the image. In the command:

g.drawImage(img, dest_x1, dest_y1, dest_x2, dest_y2,
source_x1, source_y1, source_x2, source_y2, imageObserver);

the integers source_x1, source_y1, source_x2, and source_y2 specify the top-left and bottom-right corners of a rectangular region in the source image. The integers dest_x1, dest_y1, dest_x2, and dest_y2 specify opposite corners of a region in the destination graphics context. The specified rectangle in the image is drawn, with scaling if necessary, to the specified rectangle in the graphics context. For an example in which this is useful, consider a card game that needs to display 52 different cards. Dealing with 52 image files can be cumbersome and inefficient, especially for downloading over the Internet. So, all the cards might be put into a single image:

(This image is from the Gnome desktop project, http://www.gnome.org, and is shown here much smaller than its actual size.) Now just one Image object is needed. Drawing one card means drawing a rectangular region from the image. This technique is used in a variation of the sample program HighLowGUI.java from Subsection 6.6.6. In the original version, the cards are represented by textual descriptions such as "King of Hearts." In the new version, HighLowWithImages.java, the cards are shown as images. You should try the program, but here's how it looks with the card images:

In the program, the pictures of the cards are drawn using the following method. The instance variable cardImages is a variable of type Image that represents the image that is shown above, containing 52 cards, plus two Jokers and a face-down card. Each card is 79 by 123 pixels. These numbers are used, together with the suit and value of the card, to compute the corners of the source rectangle for the drawImage() command:

/**
* Draws a card in a 79x123 pixel picture of a card with its
* upper left corner at a specified point (x,y).  Drawing the card
* requires the image file "cards.png".
* @param g The graphics context used for drawing the card.
* @param card The card that is to be drawn.  If the value is null, then a
* face-down card is drawn.
* @param x the x-coord of the upper left corner of the card
* @param y the y-coord of the upper left corner of the card
*/
public void drawCard(Graphics g, Card card, int x, int y) {
int cx;    // x-coord of upper left corner of the card inside cardsImage
int cy;    // y-coord of upper left corner of the card inside cardsImage
if (card == null) {
cy = 4*123;   // coords for a face-down card.
cx = 2*79;
}
else {
cx = (card.getValue()-1)*79;
switch (card.getSuit()) {
case Card.CLUBS:
cy = 0;
break;
case Card.DIAMONDS:
cy = 123;
break;
case Card.HEARTS:
cy = 2*123;
break;
default:  // spades
cy = 3*123;
break;
}
}
g.drawImage(cardImages,x,y,x+79,y+123,cx,cy,cx+79,cy+123,this);
}

I will tell you later in this section how the image file, cards.png, can be loaded into the program.

In addition to images loaded from files, it is possible to create images by drawing to an off-screen canvas. An off-screen canvas can be represented by an object belonging to the class BufferedImage, which is defined in the package java.awt.image. BufferedImage is a subclass of Image, so that once you have a BufferedImage, you can copy it into a graphics context g using one of the g.drawImage() methods, just as you would do with any other image. A BufferedImage can be created using the constructor

public BufferedImage(int width, int height, int imageType)

where width and height specify the width and height of the image in pixels, and imageType can be one of several constants that are defined in the BufferedImage. The image type specifies how the color of each pixel is represented. One likely value for imageType is BufferedImage.TYPE_INT_RGB, which specifies that the color of each pixel is a usual RGB color, with red, green and blue components in the range 0 to 255. The image type BufferedImage.TYPE_INT_ARGB represents an RGB image with "transparency." The image type BufferedImage.TYPE_BYTE_GRAY can be used to create a grayscale image in which the only possible colors are shades of gray.

To draw to a BufferedImage, you need a graphics context that is set up to do its drawing on the image. If OSC is of type BufferedImage, then the method

OSC.createGraphics()

returns an object of type Graphics2D that can be used for drawing on the image. (Graphics2D is a subclass of Graphics. See Subsection 6.2.5 and the next section. This means you can also use the object as a regular Graphics drawing context.)

There are several reasons why a programmer might want to draw to an off-screen canvas. One is to simply keep a copy of an image that is shown on the screen. Remember that a picture that is drawn on a component can be lost, for example when the component is resized. This means that you have to be able to redraw the picture on demand, and that in turn means keeping enough information around to enable you to redraw the picture. One way to do this is to keep a copy of the picture in an off-screen canvas. Whenever the on-screen picture needs to be redrawn, you just have to copy the contents of the off-screen canvas onto the screen. Essentially, the off-screen canvas allows you to save a copy of the color of every individual pixel in the picture. The sample program PaintWithOffScreenCanvas.java is a little painting program that uses an off-screen canvas in this way. In this program, the user can draw curves, lines, and various shapes; a "Tool" menu allows the user to select the thing to be drawn. There is also an "Erase" tool and a "Smudge" tool that I will get to later. A BufferedImage is used to store the user's picture. When the user changes the picture, the changes are made to the image, and the changed image is then copied to the screen. No record is kept of the shapes that the user draws; the only record is the color of the individual pixels in the off-screen image. (You should contrast this with the program SimplePaint2.java in Subsection 7.3.3, where the user's drawing is recorded as a list of objects that represent the shapes that the user drew.)

You should try the program. Try drawing a Filled Rectangle on top of some other shapes. As you drag the mouse, the rectangle stretches from the starting point of the mouse drag to the current mouse location. As the mouse moves, the underlying picture seems to be unaffected -- parts of the picture can be covered up by the rectangle and later uncovered as the mouse moves, and they are still there. What this means is that the rectangle that is shown as you drag the mouse can't actually be part of the off-screen canvas, since drawing something into an image means changing the color of some pixels in the image. The previous colors of those pixels are not stored anywhere else and so are permanently lost. In fact, as you draw a line, rectangle, or oval in PaintWithOffScreenCanvas, the shape that is shown as you drag the mouse is not drawn to the off-screen canvas at all. Instead, the paintComponent() method draws the shape on top of the contents of the canvas. Only when you release the mouse does the shape become a permanent part of the off-screen canvas. This illustrates the point that when an off-screen canvas is used, not everything that is visible on the screen has to be drawn on the canvas. Some extra stuff can be drawn on top of the contents of the canvas by the paintComponent() method. The other tools are handled differently from the shape tools. For the curve, erase, and smudge tools, the changes are made to the canvas immediately, as the mouse is being dragged.

Let's look at how an off-screen canvas is used in this program. The canvas is represented by an instance variable, OSC, of type BufferedImage. The size of the canvas must be the same size as the panel on which the canvas is displayed. The size can be determined by calling the getWidth() and getHeight() instance methods of the panel. Furthermore, when the canvas is first created, it should be filled with the background color, which is represented in the program by an instance variable named fillColor. All this is done by the method:

/**
* This method creates the off-screen canvas and fills it with the current
* fill color.
*/
private void createOSC() {
OSC = new BufferedImage(getWidth(),getHeight(),BufferedImage.TYPE_INT_RGB);
Graphics osg = OSC.createGraphics();
osg.setColor(fillColor);
osg.fillRect(0,0,getWidth(),getHeight());
osg.dispose();
}

Note how it uses OSC.createGraphics() to obtain a graphics context for drawing to the image. Also note that the graphics context is disposed at the end of the method. When you create a graphics context, it is good practice to dispose of it when you are finished with it. There still remains the problem of where to call this method. The problem is that the width and height of the panel object are not set until some time after the panel object is constructed. If createOSC() is called in the constructor, getWidth() and getHeight() will return the value zero and we won't get an off-screen image of the correct size. The approach that I take in PaintWithOffScreenCanvas is to call createOSC() in the paintComponent() method, the first time the paintComponent() method is called. At that time, the size of the panel has definitely been set, but the user has not yet had a chance to draw anything. With this in mind you are ready to understand the paintComponent() method:

public void paintComponent(Graphics g) {

/* First create the off-screen canvas, if it does not already exist. */

if (OSC == null)
createOSC();

/* Copy the off-screen canvas to the panel.  Since we know that the
image is already completely available, the fourth, "ImageObserver"
parameter to g.drawImage() can be null.  Since the canvas completely
fills the panel, there is no need to call super.paintComponent(g). */

g.drawImage(OSC,0,0,null);

/* If the user is currently dragging the mouse to draw a line, oval,
or rectangle, draw the shape on top of the image from the off-screen
canvas, using the current drawing color.  (This is not done if the
user is drawing a curve or using the smudge, curve, or erase tool.) */

if (dragging && SHAPE_TOOLS.contains(currentTool)) {
g.setColor(currentColor);
putCurrentShape(g);
}

}

Here, dragging is a boolean instance variable that is set to true while the user is dragging the mouse, and currentTool tells which tool is currently in use. The possible tools are defined by an enum named Tool, and SHAPE_TOOLS is a variable of type EnumSet<Tool> that contains the line, oval, rectangle, filled oval, and filled rectangle tools. (See Subsection 10.2.4.)

You might notice that there is a problem if the size of the panel is ever changed, since the size of the off-screen canvas will not be changed to match. The PaintWithOffScreenCanvas program does not allow the user to resize the program's window, so this is not an issue in that program. If we want to allow resizing, however, a new off-screen canvas must be created whenever the size of the panel changes. One simple way to do this is to check the size of the canvas in the paintComponent() method and to create a new canvas if the size of the canvas does not match the size of the panel:

if (OSC == null || getWidth() != OSC.getWidth() || getHeight() != OSC.getHeight())
createOSC();

Of course, this will discard the picture that was contained in the old canvas unless some arrangement is made to copy the picture from the old canvas to the new one before the old canvas is discarded.

The other point in the program where the off-screen canvas is used is during a mouse-drag operation, which is handled in the mousePressed(), mouseDragged(), and mouseReleased() methods. The strategy that is implemented was discussed above. Shapes are drawn to the off-screen canvas only at the end of the drag operation, in the mouseReleased() method. However, as the user drags the mouse, the part of the image over which the shape appears is re-copied from the canvas onto the screen each time the mouse is moved. Then the paintComponent() method draws the shape that the user is creating on top of the image from the canvas. For the non-shape (curve, smudge, and erase) tools, on the other hand, changes are made directly to the canvas, and the region that was changed is repainted so that the change will appear on the screen. (By the way, the program uses a version of the repaint() method that repaints just a part of a component. The command repaint(x,y,width,height) tells the system to repaint the rectangle with upper left corner (x,y) and with the specified width and height. This can be substantially faster than repainting the entire component.) See the source code, PaintWithOffScreenCanvas.java, if you want to see how it's all done.

One traditional use of off-screen canvasses is for double buffering. In double-buffering, the off-screen image is an exact copy of the image that appears on screen; whenever the on-screen picture needs to be redrawn, the new picture is drawn step-by-step to an off-screen image. This can take some time. If all this drawing were done on screen, the user might see the image flicker as it is drawn. Instead, the long drawing process takes place off-screen and the completed image is then copied very quickly onto the screen. The user doesn't see all the steps involved in redrawing. This technique can be used to implement smooth, flicker-free animation.

The term "double buffering" comes from the term "frame buffer," which refers to the region in memory that holds the image on the screen. In fact, true double buffering uses two frame buffers. The video card can display either frame buffer on the screen and can switch instantaneously from one frame buffer to the other. One frame buffer is used to draw a new image for the screen. Then the video card is told to switch from one frame buffer to the other. No copying of memory is involved. Double-buffering as it is implemented in Java does require copying, which takes some time and is not perfectly flicker-free.

In Java's older AWT graphical API, it was up to the programmer to do double buffering by hand. In the Swing graphical API, double buffering is applied automatically by the system, and the programmer doesn't have to worry about it. (It is possible to turn this automatic double buffering off in Swing, but there is seldom a good reason to do so.)

One final historical note about off-screen canvasses: There is an alternative way to create them. The Component class defines the following instance method, which can be used in any GUI component object:

public Image createImage(int width, int height)

This method creates an Image with a specified width and height. You can use this image as an off-screen canvas in the same way that you would a BufferedImage. In fact, you can expect that in a modern version of Java, the image that is returned by this method is in fact a BufferedImage. The createImage() method was part of Java from the beginning, before the BufferedImage class was introduced.

13.1.2  Working With Pixels

One good reason to use a BufferedImage is that it allows easy access to the colors of individual pixels. If image is of type BufferedImage, then we have the methods:

- image.getRGB(x,y) -- returns an int that encodes the color of the pixel at coordinates (x,y) in the image. The values of the integers x and y must lie within the image. That is, it must be true that 0 <= x < image.getWidth() and 0 <= y < image.getHeight(); if not, then an exception is thrown.

- image.setRGB(x,y,rgb) -- sets the color of the pixel at coordinates (x,y) to the color encoded by rgb. Again, x and y must be in the valid range. The third parameter, rgb, is an integer that encodes the color.

These methods use integer codes for colors. If c is of type Color, the integer code for the color can be obtained by calling c.getRGB(). Conversely, if rgb is an integer that encodes a color, the corresponding Color object can be obtained with the constructor call new Color(rgb). This means that you can use

Color c = new Color( image.getRGB(x,y) )

to get the color of a pixel as a value of type Color. And if c is of type Color, you can set a pixel to that color with

image.setRGB( x, y, c.getRGB() );

The red, green, and blue components of a color are represented as 8-bit integers, in the range 0 to 255. When a color is encoded as a single int, the blue component is contained in the eight low-order bits of the int, the green component in the next lowest eight bits, and the red component in the next eight bits. (The eight high order bits store the "alpha component" of the color, which we'll encounter in the next section.) It is easy to translate between the two representations using the shift operators << and >> and the bitwise logical operators & and |. (I have not covered these operators previously in this book. Briefly: If A and B are integers, then A << B is the integer obtained by shifting each bit of A, B bit positions to the left; A >> B is the integer obtained by shifting each bit of A, B bit positions to the right; A & B is the integer obtained by applying the logical and operation to each pair of bits in A and B; and A | B is obtained similarly, using the logical or operation. For example, using 8-bit binary numbers, we have: 01100101 & 10100001 is 00100001, while 01100101 | 10100001 is 11100101.) You don't necessarily need to understand these operators. Here are incantations that you can use to work with color codes:

/* Suppose that rgb is an int that encodes a color.
To get separate red, green, and blue color components: */

int red = (rgb >> 16) & 0xFF;
int green = (rgb >> 8) & 0xFF;
int blue = rgb & 0xFF;

/* Suppose that red, green, and blue are color components in
the range 0 to 255.  To combine them into a single int: */

int rgb = (red << 16) | (green << 8) | blue;

An example of using pixel colors in a BufferedImage is provided by the smudge tool in the sample program PaintWithOffScreenCanvas.java. The purpose of this tool is to smear the colors of an image, as if it were drawn in wet paint. For example, if you rub the middle of a black rectangle with the smudge tool, you'll get something like this:

This is an effect that can only be achieved by manipulating the colors of individual pixels! Here's how it works: when the user presses the mouse using the smudge tool, the color components of a 7-by-7 block of pixels are copied from the off-screen canvas into arrays named smudgeRed, smudgeGreen and smudgeBlue. This is done in the mousePressed() routine with the following code:

int w = OSC.getWidth();
int h = OSC.getHeight();
int x = evt.getX();
int y = evt.getY();
for (int i = 0; i < 7; i++)
for (int j = 0; j < 7; j++) {
int r = y + j - 3;
int c = x + i - 3;
if (r < 0 || r >= h || c < 0 || c >= w) {
// A -1 in the smudgeRed array indicates that the
// corresponding pixel was outside the canvas.
smudgeRed[i][j] = -1;
}
else {
int color = OSC.getRGB(c,r);
smudgeRed[i][j] = (color >> 16) & 0xFF;
smudgeGreen[i][j] = (color >> 8) & 0xFF;
smudgeBlue[i][j] = color & 0xFF;
}
}

The arrays are of type double[][] because I am going to do some computations with them that require real numbers. As the user moves the mouse, the colors in the array are blended with the colors in the image, just as if you were mixing wet paint by smudging it with your finger. That is, the colors at the new mouse position in the image are replaced with a weighted average of the current colors in the image and the colors in the arrays. This has the effect of moving some of the color from the previous mouse position to the new mouse position. At the same time, the colors in the arrays are replaced by a weighted average of the old colors in the arrays and the colors from the image. This has the effect of moving some color from the image into the arrays. This is done using the following code for each pixel position, (c,r), in a 7-by-7 block around the new mouse location:

int curCol = OSC.getRGB(c,r);
int curRed = (curCol >> 16) & 0xFF;
int curGreen = (curCol >> 8) & 0xFF;
int curBlue = curCol & 0xFF;
int newRed = (int)(curRed*0.7 + smudgeRed[i][j]*0.3);
int newGreen = (int)(curGreen*0.7 + smudgeGreen[i][j]*0.3);
int newBlue = (int)(curBlue*0.7 + smudgeBlue[i][j]*0.3);
int newCol = newRed << 16 | newGreen << 8 | newBlue;
OSC.setRGB(c,r,newCol);
smudgeRed[i][j] = curRed*0.3 + smudgeRed[i][j]*0.7;
smudgeGreen[i][j] = curGreen*0.3 + smudgeGreen[i][j]*0.7;
smudgeBlue[i][j] = curBlue*0.3 + smudgeBlue[i][j]*0.7;

13.1.3  Resources

Throughout this textbook, up until now, we have been thinking of a program as made up entirely of Java code. However, programs often use other types of data, including images, sounds, and text, as part of their basic structure. These data are referred to as resources. An example is the image file, cards.png, that was used in the HighLowWithImages.java program earlier in this section. This file is part of the program. The program needs it in order to run. The user of the program doesn't need to know that this file exists or where it is located; as far as the user is concerned, it is just part of the program. The program of course, does need some way of locating the resource file and loading its data.

Resources are ordinarily stored in files that are in the same locations as the compiled class files for the program. Class files are located and loaded by something called a class loader, which is represented in Java by an object of type ClassLoader. A class loader has a list of locations where it will look for class files. This list is called the class path. It includes the location where Java's standard classes are stored. It generally includes the current directory. If the program is stored in a jar file, the jar file is included on the class path. In addition to class files, a ClassLoader is capable of finding resource files that are located on the class path or in subdirectories of locations that are on the class path.

The first step in using a resource is to obtain a ClassLoader and to use it to locate the resource file. In the HighLowWithImages program, this is done with:

ClassLoader cl = getClass().getClassLoader();
URL imageURL = cl.getResource("cards.png");

The idea of the first line is that in order to get a class loader, you have to ask a class that was loaded by the class loader. Here, getClass() is a reference to the object that represents the actual class, HighLowWithImages. The getClass() method is an instance method in class Object and so can be used with any object. Another way to get a reference to a class loader is to use ClassName.class, where ClassName is the name of any class. For example, I could have used HighLoadWithImages.class.getClassLoader() to get the class loader in this case.

The second line in the above code uses the class loader to locate the resource file named cards.png. The return value of cl.getResource() is of type java.net.URL, and it represents the location of the resource rather than the resource itself. If the resource file cannot be found, then the return value is null. The class URL was discussed in Subsection 11.4.1.

Often, resources are stored not directly on the class path but in a subdirectory. In that case, the parameter to getResource() must be a path name that includes the directory path to the resource. For example, suppose that the image file "cards.png" were stored in a directory named images inside a directory named resources, where resources is directly on the class path. Then the path to the file is "resources/images/cards.png" and the command for locating the resource would be

URL imageURL = cl.getResource("resources/images/cards.png");

Once you have a URL that represents the location of a resource file, you could use a URLConnection, as discussed in Subsection 11.4.1, to read the contents of that file. However, Java provides more convenient methods for loading several types of resources. For loading image resources, a convenient method is available in the class java.awt.Toolkit. It can be used as in the following line from HighLowWithImages, where cardImages is an instance variable of type Image and imageURL is the URL that represents the location of the image file:

cardImages = Toolkit.getDefaultToolkit().createImage(imageURL);

This still does not load the image completely -- that will only be done later, for example when cardImages is used in a drawImage command. Another technique, which does read the image completely, is to use the ImageIO.read() method, which will be discussed below in Subsection 13.1.5

Sounds represent another kind of resource that a program might want to use. For some reason, the easiest way to playing a sound is to use a static method in the Applet class, in package java.awt:

public static AudioClip newAudioClip(URL soundURL)

Since this is a static method, it can be used in any program, not just in applets, simply by calling it as Applet.newAudioClip(soundURL). The parameter is the URL of a sound resource, and the return value is of type java.applet.AudioClip. Once you have an AudioClip, you can call its play() method to play the audio clip from the beginning.

Here is a method that puts all this together to load and play the sound from an audio resource file:

private void playAudioResource(String audioResourceName) {
ClassLoader cl = getClass().getClassLoader();
URL resourceURL = cl.getResource(audioResourceName);
if (resourceURL != null) {
AudioClip sound = Applet.newAudioClip(resourceURL);
sound.play();
}
}

This method is from a sample program SoundAndCursorDemo.java that will be discussed in the next subsection. Of course, if you plan to reuse the sound often, it would be better to load the sound once into an instance variable of type AudioClip, which could then be used to play the sound any number of times, without the need to reload it each time.

The AudioClip class supports audio files in the common WAV, AIFF, and AU formats.

13.1.4  Cursors and Icons

The position of the mouse is represented on the computer's screen by a small image called a cursor. In Java, the cursor is represented by an object of type java.awt.Cursor. A Cursor has an associated image. It also has a hot spot, which is a Point that specifies the pixel within the image that corresponds to the exact position on the screen where the mouse is pointing. For example, for a typical "arrow" cursor, the hot spot is the tip of the arrow. For a "crosshair" cursor, the hot spot is the center of the crosshairs.

The Cursor class defines several standard cursors, which are identified by constants such as Cursor.CROSSHAIR_CURSOR and Cursor.DEFAULT_CURSOR. You can get a standard cursor by calling the static method Cursor.getPredefinedCursor(code), where code is one of the constants that identify the standard cursors. It is also possible to create a custom cursor from an Image. The Image might be obtained as an image resource, as described in the previous subsection. It could even be a BufferedImage that you create in your program. It should be small, maybe 16-by-16 or 24-by-24 pixels. (Some platforms might only be able to handle certain cursor sizes; see the documentation for Toolkit.getBestCursorSize() for more information.) A custom cursor can be created by calling the static method createCustomCursor() in the Toolkit class:

Cursor c = Toolkit.getDefaultToolkit().createCustomCursor(image,hotSpot,name);

where hotSpot is of type Point and name is a String that will act as a name for the cursor (and which serves no real purpose that I know of).

Cursors are associated with GUI components. When the mouse moves over a component, the cursor changes to whatever Cursor is associated with that component. To associate a Cursor with a component, call the component's instance method setCursor(cursor). For example, to set the cursor for a JPanel, panel, to be the standard "wait" cursor:

panel.setCursor( Cursor.getPredefinedCursor(Cursor.WAIT_CURSOR) );

To reset the cursor to be the default cursor, you can use:

panel.setCursor( Cursor.getDefaultCursor() );

To set the cursor to be an image that is defined in an image resource file named imageResource, you might use:

ClassLoader cl = getClass().getClassLoader();
URL resourceURL = cl.getResource(imageResource);
if (resourceURL != null) {
Toolkit toolkit = Toolkit.getDefaultToolkit();
Image image = toolkit.createImage(resourceURL);
Point hotSpot = new Point(7,7);
Cursor cursor = toolkit.createCustomCursor(image, hotSpot, "mycursor");
panel.setCursor(cursor);
}

The sample program SoundAndCursorDemo.java shows how to use predefined and custom cursors and how to play sounds from resource files. The program has several buttons that you can click. Some of the buttons change the cursor that is associated with the main panel of the program. Some of the buttons play sounds. When you play a sound, the cursor is reset to be the default cursor.

Another standard use of images in GUI interfaces is for icons. An icon is simply a small picture. As we'll see in Section 13.3, icons can be used on Java's buttons, menu items, and labels; in fact, for our purposes, an icon is simply an image that can be used in this way.

An icon is represented by an object of type Icon, which is actually an interface rather than a class. The class ImageIcon, which implements the Icon interface, is used to create icons from Images. If image is a (rather small) Image, then the constructor call new ImageIcon(image) creates an ImageIcon whose picture is the specified image. Often, the image comes from a resource file. We will see examples of this later in this chapter.

13.1.5  Image File I/O

The class javax.imageio.ImageIO makes it easy to save images from a program into files and to read images from files into a program. This would be useful in a program such as PaintWithOffScreenCanvas, so that the users would be able to save their work and to open and edit existing images. (See Exercise 13.1.)

There are many ways that the data for an image could be stored in a file. Many standard formats have been created for doing this. Java supports at least three standard image formats: PNG, JPEG, and GIF. (Individual implementations of Java might support more.) The JPEG format is "lossy," which means that the picture that you get when you read a JPEG file is only an approximation of the picture that was saved. Some information in the picture has been lost. Allowing some information to be lost makes it possible to compress the image into a lot fewer bits than would otherwise be necessary. Usually, the approximation is quite good. It works best for photographic images and worst for simple line drawings. The PNG format, on the other hand is "lossless," meaning that the picture in the file is an exact duplicate of the picture that was saved. A PNG file is compressed, but not in a way that loses information. The compression works best for images made up mostly of large blocks of uniform color; it works worst for photographic images. GIF is an older format that is limited to just 256 colors in an image; it has mostly been superseded by PNG.

Suppose that image is a BufferedImage. The image can be saved to a file simply by calling

ImageIO.write( image, format, file )

where format is a String that specifies the image format of the file and file is a File that specifies the file that is to be written. (See Subsection 11.2.2 for information about the File class.) The format string should ordinarily be either "PNG" or "JPEG", although other formats might be supported.

ImageIO.write() is a static method in the ImageIO class. It returns a boolean value that is false if the image format is not supported. That is, if the specified image format is not supported, then the image is not saved, but no exception is thrown. This means that you should always check the return value! For example:

boolean hasFormat = ImageIO.write(OSC,format,selectedFile);
if ( ! hasFormat )
throw new Exception(format + " format is not available.");

If the image format is recognized, it is still possible that an IOException might be thrown when the attempt is made to send the data to the file.

Usually, the file to be used in ImageIO.write() will be selected by the user using a JFileChooser, as discussed in Subsection 11.2.3. For example, here is a typical method for saving an image. (The use of "this" as a parameter in several places assumes that this method is defined in a subclass of JComponent.)

/**
* Attempts to save an image to a file selected by the user.
* @param image the BufferedImage to be saved to the file
* @param format the format of the image, probably either "PNG" or "JPEG"
*/
private void doSaveFile(BufferedImage image, String format) {
if (fileDialog == null)
fileDialog = new JFileChooser();
fileDialog.setSelectedFile(new File("image." + format.toLowerCase()));
fileDialog.setDialogTitle("Select File For Saved Image");
int option = fileDialog.showSaveDialog(this);
if (option != JFileChooser.APPROVE_OPTION)
return;  // User canceled or clicked the dialog's close box.
File selectedFile = fileDialog.getSelectedFile();
if (selectedFile.exists()) {  // Ask the user whether to replace the file.
int response = JOptionPane.showConfirmDialog( null,
"The file \"" + selectedFile.getName()
+ "\" already exists.\nDo you want to replace it?",
"Confirm Save",
JOptionPane.YES_NO_OPTION,
JOptionPane.WARNING_MESSAGE );
if (response != JOptionPane.YES_OPTION)
return;  // User does not want to replace the file.
}
try {
boolean hasFormat = ImageIO.write(image,format,selectedFile);
if ( ! hasFormat )
throw new Exception(format + " format is not available.");
}
catch (Exception e) {
JOptionPane.showMessageDialog(this,
"Sorry, an error occurred while trying to save image.");
e.printStackTrace();
}
}

The ImageIO class also has a static read() method for reading an image from a file into a program. The method

ImageIO.read( inputFile )

takes a variable of type File as a parameter and returns a BufferedImage. The return value is null if the file does not contain an image that is stored in a supported format. Again, no exception is thrown in this case, so you should always be careful to check the return value. It is also possible for an IOException to occur when the attempt is made to read the file. There is another version of the read() method that takes an InputStream instead of a file as its parameter, and a third version that takes a URL.

Earlier in this section, we encountered another method for reading an image from a URL, the createImage() method from the Toolkit class. The difference is that ImageIO.read() reads the image data completely and stores the result in a BufferedImage. On the other hand, createImage() does not actually read the data; it really just stores the image location and the data won't be read until later, when the image is used. This has the advantage that the createImage() method itself can complete very quickly. ImageIO.read(), on the other hand, can take some time to execute.

Section 13.2

Fancier Graphics

The graphics commands provided by the Graphics class are sufficient for many purposes. However, recent versions of Java provide a much larger and richer graphical toolbox in the form of the class java.awt.Graphics2D. I mentioned Graphics2D in Subsection 6.2.5 and promised to discuss it further in this chapter. You have already seen a few of the ideas that are covered in this section, at least briefly, but I cover them in more detail here.

Graphics2D is a subclass of Graphics, so all of the graphics commands that you already know can be used with a Graphics2D object. In fact, when you obtain a Graphics context for drawing on a Swing component, the graphics object is actually of type Graphics2D and can be type-cast to gain access to the advanced Graphics2D graphics commands. Furthermore, BufferedImage has the instance method, createGraphics(), that returns a graphics context of type Graphics2D. As mentioned in Subsection 6.2.5, to use Graphics2D commands in the paintComponent() method of a Swing component, you can use code of the form:

public void paintComponent(Graphics g) {
super.paintComponent(g);
Graphics g2 = (Graphics2D)g;
.
.   // Draw to the component using g2 (and g).
.
}

Note that when you do this, g and g2 are just two variables that refer to the same object, so they both draw to the same drawing surface and have the same state. When properties of g2, such as drawing color, are changed, the changes also apply to g. By saying

Graphics2D g2 = (Graphics2D)g.create();

you can obtain a newly created graphics context. The object created by g.create() is a graphics context that draws to the same drawing surface as g and that initially has all the same properties as g. However, it is a separate object, so that changing properties in g2 has no effect on g. This can be useful if you want to keep an unmodified copy of the original graphics context around for some drawing operations. (In this case, it is good practice to call g2.dispose() to dispose of the new graphics context when you are finished using it.)

13.2.1  Measuring Text

Although this section is mostly about Graphics2D, we start with a topic that has nothing to do with it.

Often, when drawing a string, it's important to know how big the image of the string will be. For example, you need this information if you want to center a string in a component. Or if you want to know how much space to leave between two lines of text, when you draw them one above the other. Or if the user is typing the string and you want to position a cursor at the end of the string. In Java, questions about the size of a string can be answered by an object belonging to the standard class java.awt.FontMetrics.

There are several lengths associated with any given font. Some of them are shown in this illustration:

The dashed lines in the illustration are the baselines of the two lines of text. The baseline of a string is the line on which the bases of the characters rest. The suggested distance between two baselines, for single-spaced text, is known as the lineheight of the font. The ascent is the distance that tall characters can rise above the baseline, and the descent is the distance that tails like the one on the letter "g" can descend below the baseline. The ascent and descent do not add up to the lineheight, because there should be some extra space between the tops of characters in one line and the tails of characters on the line above. The extra space is called leading. (The term comes from the time when lead blocks were used for printing. Characters were formed on blocks of lead that were lined up to make up the text of a page, covered with ink, and pressed onto paper to print the page. Extra, blank "leading" was used to separate the lines of characters.) All these quantities can be determined by calling instance methods in a FontMetrics object. There are also methods for determining the width of a character and the total width of a string of characters.

Recall that a font in Java is represented by the class Font. A FontMetrics object is associated with a given font and is used to measure characters and strings in that font. If font is of type Font and g is a graphics context, you can get a FontMetrics object for the font by calling g.getFontMetrics(font). If fm is the variable that refers to the FontMetrics object, then the ascent, descent, leading, and lineheight of the font can be obtained by calling fm.getAscent(), fm.getDescent(), fm.getLeading(), and fm.getHeight(). If ch is a character, then fm.charWidth(ch) is the width of the character when it is drawn in that font. If str is a string, then fm.stringWidth(str) is the width of the string when drawn in that font. For example, here is a paintComponent() method that shows the message "Hello World" in the exact center of the component:

public void paintComponent(Graphics g) {
super.paintComponent(g);

int strWidth, strHeight; // Width and height of the string.
int centerX, centerY;    // Coordinates of the center of the component.
int baseX, baseY;        // Coordinates of the baseline of the string.
int topOfString;         // y-coordinate of the top of the string.

centerX = getWidth() / 2;
centerY = getHeight() / 2;

Font font = g.getFont();  // What font will g draw in?
FontMetrics fm = g.getFontMetrics(font);
strWidth = fm.stringWidth("Hello World");
strHeight = fm.getAscent();  // Note: There are no tails on and
//   of the chars in the string!  So we
//   don't need to account for descent.

baseX = centerX - (strWidth/2);  // Move back from center by half the
//    width of the string.

topOfString = centerY - (strHeight/2);  // Move up from center by half
//   the height of the string.

baseY = topOfString + fm.getAscent();  // Baseline is fm.getAscent() pixels
//   below the top of the string.

g.drawString("Hello World", baseX, baseY); // Draw the string.
}

For the height of the string in this method, I use fm.getAscent(). If I were drawing "Goodbye World" instead of "Hello World," I would have used fm.getAscent() + fm.getDescent(), where the descent is added to the height in order to take into account the tail on the "y" in "Goodbye". The value of baseX is computed to be the amount of space between the left edge of the component and the start of the string. It is obtained by subtracting half the width of the string from the horizontal center of the component. This will center the string horizontally in the component. The next line computes the position of the top of the string in the same way. However, to draw the string, we need the y-coordinate of the baseline, not the y-coordinate of the top of the string. The baseline of the string is below the top of the string by an amount equal to the ascent of the font.

There is an example of centering a two-line block of text in the sample program TransparencyDemo.java, which is discussed in the next subsection.

13.2.2  Transparency

A color is represented by red, blue, and green components. In Java's usual representation, each component is an eight-bit number in the range 0 to 255. The three color components can be packed into a 32-bit integer, but that only accounts for 24 bits in the integer. What about the other eight bits? They don't have to be wasted. They can be used as a fourth component of the color, the alpha component. The alpha component can be used in several ways, but it is most commonly associated with transparency. When you draw with a transparent color, it's like laying down a sheet of colored glass. It doesn't completely obscure the part of the image that is colored over. Instead, the background image is blended with the transparent color that is used for drawing -- as if you were looking at the background through colored glass. This type of drawing is properly referred to as alpha blending, and it is not equivalent to true transparency; nevertheless, most people refer to it as transparency.

The value of the alpha component determines how transparent that color is. Actually, the alpha component gives the opaqueness of the color. Opaqueness is the opposite of transparency. If something is fully opaque, you can't see through it at all; if something is almost fully opaque, then it is just a little transparent; and so on. When the alpha component of a color has the maximum possible value, the color is fully opaque. When you draw with a fully opaque color, that color simply replaces the color of the background over which you draw. Except for a little example in Subsection 5.3.3, this is the only type of color that we have used up until now. If the alpha component of a color is zero, then the color is perfectly transparent, and drawing with that color has no effect at all. Intermediate values of the alpha component give partially opaque colors that will blend with the background when they are used for drawing.

The sample program TransparencyDemo.java can help you to understand transparency. When you run the program you will see a display area containing a triangle, an oval, a rectangle, and some text. Sliders at the bottom of the window allow you to control the degree of transparency of each shape. When a slider is moved all the way to the right, the corresponding shape is fully opaque; all the way to the left, and the shape is fully transparent.

Colors with alpha components were introduced in Java along with Graphics2D, but they can be used with ordinary Graphics objects as well. To specify the alpha component of a color, you can create the Color object using one of the following constructors from the Color class:

public Color(int red, int green, int blue, int alpha);

public Color(float red, float green, float blue, float alpha);

In the first constructor, all the parameters must be integers in the range 0 to 255. In the second, the parameters must be in the range 0.0 to 1.0. For example,

Color transparentRed = new Color( 255, 0, 0, 200 );

makes a slightly transparent red, while

Color tranparentCyan = new Color( 0.0F, 1.0F, 1.0F, 0.5F);

makes a blue-green color that is 50% opaque. (The advantage of the constructor that takes parameters of type float is that it lets you think in terms of percentages.) When you create an ordinary RGB color, as in new Color(255,0,0), you just get a fully opaque color.

Once you have a transparent color, you can use it in the same way as any other color. That is, if you want to use a Color c to draw in a graphics context g, you just say g.setColor(c), and subsequent drawing operations will use that color. As you can see, transparent colors are very easy to use.

A BufferedImage with image type BufferedImage.TYPE_INT_ARGB can use transparency. The color of each pixel in the image can have its own alpha component, which tells how transparent that pixel will be when the image is drawn over some background. A pixel whose alpha component is zero is perfectly transparent, and has no effect at all when the image is drawn; in effect, it's not part of the image at all. It is also possible for pixels to be partly transparent. When an image is saved to a file, information about transparency might be lost, depending on the file format. The PNG image format supports transparency; JPEG does not. (If you look at the images of playing cards that are used in the program HighLowWithImages in Subsection 13.1.1, you might notice that the tips of the corners of the cards are fully transparent. The card images are from a PNG file, cards.png.)

An ARGB BufferedImage should be fully transparent when it is first created, but if you want to make sure, here is one way of doing so: The Graphics2D class has a method setBackground() that can be used to set a background color for the graphics context, and it has a clearRect() method that fills a rectangle with the current background color. To create a fully transparent image with width w and height h, you can use:

BufferedImage image = new BufferedImage(w, h, BufferedImage.TYPE_INT_ARGB);
Graphics2D g2 = image.createGraphics();
g2.setBackground(new Color(0,0,0,0));  // (The R, G, and B values don't matter.)
g2.clearRect(0, 0, w, h);

(Note that simply drawing with a transparent color will not make pixels in the image transparent. The alpha component of a Color makes the color transparent when it is used for drawing; it does not change the transparency of the pixels that are modified by the drawing operation.)

As an example, just for fun, here is a method that will set the cursor of a component to be a red square with a transparent interior:

private void useRedSquareCursor() {
BufferedImage image = new BufferedImage(24,24,BufferedImage.TYPE_INT_ARGB);
Graphics2D g2 = image.createGraphics();
g2.setBackground(new Color(0,0,0,0));
g2.clearRect(0, 0, 24, 24);  // (should not be necessary in a new image)
g2.setColor(Color.RED);
g2.drawRect(0,0,23,23); // draw a red border of width 3 around the square
g2.drawRect(1,1,21,21);
g2.drawRect(2,2,19,19);
g2.dispose();
Point hotSpot = new Point(12,12);
Toolkit tk = Toolkit.getDefaultToolkit();
Cursor cursor = tk.createCustomCursor(image,hotSpot,"square");
setCursor(cursor);
}

13.2.3  Antialiasing

To draw a geometric figure such as a line or circle, you just have to color the pixels that are part of the figure, right? Actually, there is a problem with this. Pixels are little squares. Geometric figures, on the other hand, are made of geometric points that have no size at all. Think about drawing a circle, and think about a pixel on the boundary of that circle. The infinitely thin geometric boundary of the circle cuts through the pixel. Part of the pixel lies inside the circle, part lies outside. So, when we are filling the circle with color, do we color that pixel or not? A possible solution is to color the pixel if the geometric circle covers 50% or more of the pixel. Following this procedure, however, leads to a visual defect known as aliasing. It is visible in images as a jaggedness or "staircasing" effect along the borders of curved shapes. Lines that are not horizontal or vertical also have a jagged, aliased appearance. (The term "aliasing" seems to refer to the fact that many different geometric points map to the same pixel. If you think of the real-number coordinates of a geometric point as a "name" for the pixel that contains that point, then each pixel has many different names or "aliases.")

It's not possible to build a circle out of squares, but there is a technique that can eliminate some of the jaggedness of aliased images. The technique is called antialiasing. Antialiasing is based on transparency. The idea is simple: If 50% of a pixel is covered by the geometric figure that you are trying to draw, then color that pixel with a color that is 50% transparent. If 25% of the pixel is covered, use a color that is 75% transparent (25% opaque). If the entire pixel is covered by the figure, of course, use a color that is 100% opaque -- antialiasing only affects pixels that are only partly covered by the geometric shape.

In antialiasing, the color that you are drawing with is blended with the original color of the pixel, and the amount of blending depends on the fraction of the pixel that is covered by the geometric shape. (The fraction is difficult to compute exactly, so in practice, various methods are used to approximate it.) Of course, you still don't get a picture of the exact geometric shape, but antialiased images do tend to look better than jagged, aliased images.

For an example, look at the picture in the next subsection. Antialiasing is used to draw the panels in the second and third row of the picture, but it is not used in the top row. You should note the jagged appearance of the lines in the top row. (By the way, when antialiasing is applied to a line, the line is treated as a geometric rectangle whose width is equal to the line width.)

Antialiasing is supported in Graphics2D. By default, antialiasing is turned off. If g2 is a graphics context of type Graphics2D, you can turn on antialiasing in g2 by saying:

g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
RenderingHints.VALUE_ANTIALIAS_ON);

As you can see, this is only a "hint" that you would like to use antialiasing, and it is even possible that the hint will be ignored. However, it is likely that subsequent drawing operations in g2 will be antialiased. If you want to turn antialiasing off in g2, you should say:

g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
RenderingHints.VALUE_ANTIALIAS_OFF);

13.2.4  Strokes and Paints

When using the Graphics class, any line that you draw will be a solid line that is one pixel thick. The Graphics2D class makes it possible to draw a much greater variety of lines. You can draw lines of any thickness, and you can draw lines that are dotted or dashed instead of solid.

An object of type Stroke contains information about how lines should be drawn, including how thick the line should be and what pattern of dashes and dots, if any, should be used. Every Graphics2D has an associated Stroke object. The default Stroke draws a solid line of thickness one. To get lines with different properties, you just have to install a different stroke into the graphics context.

Stroke is an interface, not a class. The class BasicStroke, which implements the Stroke interface, is the one that is actually used to create stroke objects. For example, to create a stroke that draws solid lines with thickness equal to 3, use:

BasicStroke line3 = new BasicStroke(3);

If g2 is of type Graphics2D, the stroke can be installed in g2 by calling its setStroke() command:

g2.setStroke(line3)

After calling this method, subsequent drawing operations will use lines that are three times as wide as the usual thickness. The thickness of a line can be given by a value of type float, not just by an int. For example, to use lines of thickness 2.5 in the graphics context g2, you can say:

g2.setStroke( new BasicStroke(2.5F) );

(Fractional widths make more sense if antialiasing is turned on.)

When you have a thick line, the question comes up, what to do at the ends of the line. If you draw a physical line with a large, round piece of chalk, the ends of the line will be rounded. When you draw a line on the computer screen, should the ends be rounded, or should the line simply be cut off flat? With the BasicStroke class, the choice is up to you. Maybe it's time to look at examples. This illustration shows fifteen lines, drawn using different BasicStrokes. Lines in the middle row have rounded ends; lines in the other two rows are simply cut off at their endpoints. Lines of various thicknesses are shown, and the bottom row shows dashed lines. (And, as mentioned above, only the bottom two rows are antialiased.)

This illustration is a screenshot from the sample program StrokeDemo.java. In that program, you can click and drag in any of the small panels, and the lines in all the panels will be redrawn as you move the mouse. In addition, if you right-click and drag, then rectangles will be drawn instead of lines; this shows that strokes are used for drawing the outlines of shapes and not just for straight lines. If you look at the corners of the rectangles that are drawn by the program, you'll see that there are several ways of drawing a corner where two wide line segments meet.

All the options that you want for a BasicStroke have to be specified in the constructor. Once the stroke object is created, there is no way to change the options. There is one constructor that lets you specify all possible options:

public BasicStroke( float width, int capType, int joinType, float miterlimit,
float[] dashPattern, float dashPhase )

I don't want to cover all the options in detail, but here's some basic info:

- width specifies the thickness of the line

- capType specifies how the ends of a line are "capped." The possible values are BasicStroke.CAP_SQUARE, BasicStroke.CAP_ROUND and BasicStroke.CAP_BUTT. These values are used, respectively, in the first, second, and third rows of the above picture. The default is BasicStroke.CAP_SQUARE.

- joinType specifies how two line segments are joined together at corners. Possible values are BasicStroke.JOIN_MITER, BasicStroke.JOIN_ROUND, and BasicStroke.JOIN_BEVEL. Again, these are used in the three rows of panels in the sample program; you will only see the effect if you run the program and draw some rectangles. The default is BasicStroke.JOIN_MITER.

- miterLimit is used only if the value of joinType is JOIN_MITER; just use the default value, 10.0F.

- dashPattern is used to specify dotted and dashed lines. The values in the array specify lengths in the dot/dash pattern. The numbers in the array represent the length of a solid piece, followed by the length of a transparent piece, followed by the length of a solid piece, and so on. At the end of the array, the pattern wraps back to the beginning of the array. If you want a solid line, use a different constructor that has fewer parameters.

- dashPhase tells the computer where to start in the dashPattern array, for the first segment of the line. Use 0 for this parameter in most cases.

For the third row in the above picture, the dashPattern is set to new float[] {5,5}. This means that the lines are drawn starting with a solid segment of length 5, followed by a transparent section of length 5, and then repeating the same pattern. A simple dotted line would have thickness 1 and dashPattern new float[] {1,1}. A pattern of short and long dashes could be made by using new float[] {10,4,4,4}. For more information, see the Java documentation, or try experimenting with the source code for the sample program.

So now we can draw fancier lines. But any drawing operation is still restricted to drawing with a single color. We can get around that restriction by using Paint. An object of type Paint is used to assign color to each pixel that is "hit" by a drawing operation. Paint is an interface, and the Color class implements the Paint interface. When a color is used for painting, it applies the same color to every pixel that is hit. However, there are other types of paint where the color that is applied to a pixel depends on the coordinates of that pixel. Standard Java includes several classes that define paint with this property: TexturePaint and several types of gradient paint. In a texture, the pixel colors come from an image, which is repeated, if necessary, like a wallpaper pattern to cover the entire xy-plane. In a gradient, the color that is applied to pixels changes gradually from one color to another color as you move from point to point. Java has three types of gradient paints: GradientPaint, LinearGradientPaint, and RadialGradientPaint.

It will be helpful to look at some examples. This illustration shows a polygon filled with two different paints. The polygon on the left uses a GradientPaint while the one on the right uses a TexturePaint. Note that in this picture, the paint is used only for filling the polygon. The outline of the polygon is drawn in a plain black color. However, Paint objects can be used for drawing lines as well as for filling shapes. These pictures were made by the sample program PaintDemo.java. In that program, you can select among several different paints, and you can control certain properties of the paints.

Basic gradient paints are created using the constructor

public GradientPaint(float x1, float y1, Color c1,
float x2, float y2, Color c2, boolean cyclic)

This constructs a gradient that has color c1 at the point with coordinates (x1,y1) and color c2 at the point (x2,y2). As you move along the line between the two points, the color of the gradient changes from c1 to c2; along lines perpendicular to this line, the color is constant. The last parameter, cyclic, tells what happens if you move past the point (x2,y2) on the line from (x1,y1) to (x2,y2). If cyclic is false, the color stops changing and any point beyond (x2,y2) has color c2. If cyclic is true, then the colors continue to change in a cyclic pattern after you move past (x2,y2). (It works the same way if you move past the other endpoint, (x1,y1).) In most cases, you will set cyclic to true. Note that you can vary the points (x1,y1) and (x2,y2) to change the width and direction of the gradient. For example, to create a cyclic gradient that varies from black to light gray along the line from (0,0) to (100,100), use:

new GradientPaint( 0, 0, Color.BLACK, 100, 100, Color.LIGHT_GRAY, true)

The other two gradient paint classes, LinearGradientPaint and RadialGradientPaint, are more sophisticated. Linear gradient paints are similar to GradientPaint but can be based on more than two colors. Radial gradients color pixels based on their distance from a central point, which produces rings of constant color instead of lines of constant color. See the API documentation for details.

To construct a TexturePaint, you need a BufferedImage that contains the image that will be used for the texture. You also specify a rectangle in which the image will be drawn. The image will be scaled, if necessary, to exactly fill the rectangle. Outside the specified rectangle, the image will be repeated horizontally and vertically to fill the plane. You can vary the size and position of the rectangle to change the scale of the texture and its positioning on the plane. Ordinarily, however the upper left corner of the rectangle is placed at (0,0), and the size of the rectangle is the same as the actual size of the image. The constructor for TexturePaint is defined as

public TexturePaint( BufferedImage textureImage, Rectangle2D anchorRect)

The Rectangle2D class is part of the Graphics2D framework and will be discussed at the end of this section. Often, a call to the constructor takes the following form, which will show the image at its actual size:

new TexturePaint( image,
new Rectangle2D.Double(0,0,image.getWidth(),image.getHeight() )

Once you have a Paint object, you can use the setPaint() method of a Graphics2D object to install the paint in a graphics context. For example, if g2 is of type Graphics2D, then the command

g2.setPaint( new GradientPaint(0,0,Color.BLUE,100,100,Color.GREEN,true) );

sets up g2 to use a gradient paint. Subsequent drawing operations with g2 will draw using a blue/green gradient.

13.2.5  Transforms and Shapes

In the standard drawing coordinates on a component, the upper left corner of the component has coordinates (0,0). Coordinates are integers, and the coordinates (x,y) refer to the point that is x pixels over from the left edge of the component and y pixels down from the top. With Graphics2D, however, you are not restricted to using these coordinates. In fact, you can can set up a Graphics2D graphics context to use any system of coordinates that you like. You can use this capability to select the coordinate system that is most appropriate for the things that you want to draw. For example, if you are drawing architectural blueprints, you might use coordinates in which one unit represents an actual distance of one foot.

Changes to a coordinate system are referred to as transforms. There are three basic types of transform. A translate transform changes the position of the origin, (0,0). A scale transform changes the scale, that is, the unit of distance. And a rotation transform applies a rotation about some point. You can make more complex transforms by combining transforms of the three basic types. For example, you can apply a rotation, followed by a scale, followed by a translation, followed by another rotation. When you apply several transforms in a row, their effects are cumulative. It takes a fair amount of study to fully understand complex transforms, and transforms are a major topic in a course in computer graphics. I will limit myself here to discussing a few of the most simple cases, just to give you an idea of what transforms can do.

Suppose that g2 is of type Graphics2D. Then g2.translate(x,y) moves the origin, (0,0), to the point (x,y). This means that if you use coordinates (0,0) after saying g2.translate(x,y), then you are referring to the point that used to be (x,y), before the translation was applied. All other coordinate pairs are moved by the same amount. For example saying

g.translate(x,y);
g.drawLine( 0, 0, 100, 200 );

draws the same line as

g.drawLine( x, y, 100+x, 200+y );

In the second case, you are just doing the same translation "by hand." A translation (like all transforms) affects all subsequent drawing operations. Instead of thinking in terms of coordinate systems, you might find it clearer to think of what happens to the objects that are drawn. After you say g2.translate(x,y), any objects that you draw are displaced x units horizontally and y units vertically. Note that the parameters x and y can be real numbers.

As an example, perhaps you would prefer to have (0,0) at the center of a component, instead of at its upper left corner. To do this, just use the following command in the paintComponent() method of the component:

g2.translate( getWidth()/2, getHeight()/2 );

To apply a scale transform to a Graphics2D g2, use g2.scale(s,s), where s is the real number that specifies the scaling factor. If s is greater than 1, everything is magnified by a factor of s, while if s is between 0 and 1, everything is shrunk by a factor of s. The center of scaling is (0,0). That is, the point (0,0) is unaffected by the scaling, and other points move toward or away from (0,0) by a factor of s. Again, it can be clearer to think of the effect on objects that are drawn after a scale transform is applied. Those objects will be magnified or shrunk by a factor of s. Note that scaling affects everything, including thickness of lines and size of fonts. It is possible to use different scale factors in the horizontal and vertical direction with a command of the form g2.scale(sx,sy), although that will distort the shapes of objects. By the way, it is even possible to use scale factors that are less than 0, which results in reflections. For example, after calling g2.scale(-1,1), objects will be reflected horizontally through the line x=0.

The third type of basic transform is rotation. The command g2.rotate(r) rotates all subsequently drawn objects through an angle of r about the point (0,0). You can rotate instead about the point (x,y) with the command g2.rotate(r,x,y). All the parameters can be real numbers. Angles are measured in radians, where π radians are equal to 180 degrees. To rotate through an angle of d degrees, use

g2.rotate( d * Math.PI / 180 );

Positive angles are clockwise rotations, while negative angles are counterclockwise (unless you have applied a negative scale factor, which reverses the orientation).

Rotation is not as common as translation or scaling, but there are a few things that you can do with it that can't be done any other way. For example, you can use it to draw an image "on the slant." Rotation also makes it possible to draw text that is rotated so that its baseline is slanted or even vertical. To draw the string "Hello World" with its basepoint at (x,y) and rising at an angle of 30 degrees, use:

g2.rotate( -30 * Math.PI / 180, x, y );
g2.drawString( "Hello World", x, y );

To draw the message vertically, with the center of its baseline at the point (x,y), we can use FontMetrics to measure the string, and say:

FontMetrics fm = g2.getFontMetrics( g2.getFont() );
int baselineLength = fm.stringWidth("Hello World");
g2.rotate( -90 * Math.PI / 180, x, y);
g2.drawString( "Hello World", x - baselineLength/2, y );

The drawing operations in the Graphics class use integer coordinates only. Graphics2D makes it possible to use real numbers as coordinates. This becomes particularly important once you start using transforms, since after you apply a scaling transform, a square of size one might cover many pixels instead of just a single pixel. Unfortunately, the designers of Java couldn't decide whether to use numbers of type float or double as coordinates, and their indecision makes things a little more complicated than they need to be. (My guess is that they really wanted to use float, since values of type float have enough accuracy for graphics and are probably used in the underlying graphical computations of the computer. However, in Java programming, it's easier to use double than float, so they wanted to make it possible to use double values too.)

To use real number coordinates, you have to use classes defined in the package java.awt.geom. Among the classes in this package are classes that represent geometric shapes such as lines and rectangles. For example, the class Line2D represents a line whose endpoints are given as real number coordinates. The unfortunate thing is that Line2D is an abstract class, which means that you can't create objects of type Line2D directly. However, Line2D has two concrete subclasses that can be used to create objects. One subclass uses coordinates of type float, and one uses coordinates of type double. The most peculiar part is that these subclasses are defined as static nested classes inside Line2D. Their names are Line2D.Float and Line2D.Double. This means that Line2D objects can be created, for example, with:

Line2D line1 = new Line2D.Float( 0.17F, 1.3F, -2.7F, 5.21F );
Line2D line2 = new Line2D.Double( 0, 0, 1, 0);
Line2D line3 = new Line2D.Double( x1, y1, x2, y2 );

where x1, y1, x2, y2 are any numeric variables. In my own code, I generally use Line2D.Double rather than Line2D.Float.

Other shape classes in java.awt.geom are similar. The class that represents rectangles is Rectangle2D. To create a rectangle object, you have to use either Rectangle2D.Float or Rectangle2D.Double. For example,

Rectangle2D rect = new Rectangle2D.Double( -0.5, -0.5, 1.0, 1.0 );

creates a rectangle with a corner at (-0.5,-0.5) and with width and height both equal to 1. Other classes include Point2D, which represents a single point; Ellipse2D, which represents an oval; and Arc2D, which represents an arc of a circle.

If g2 is of type Graphics2D and shape is an object belonging to one of the 2D shape classes, then the command

g2.draw(shape);

draws the shape. For a shape such as a rectangle or ellipse that has an interior, only the outline is drawn. To fill in the interior of such a shape, use

g2.fill(shape)

For example, to draw a line from (x1,y1) to (x2,y2), use

g2.draw( new Line2D.Double(x1,y1,x2,y2) );

and to draw a filled rectangle with a corner at (3.5,7), with width 5 and height 3, use

g2.fill( new Rectangle2D.Double(3.5, 7, 5, 3) );

The package java.awt.geom also has a very nice class Path2D that can be used to draw polygons and curves defined by any number of points. See the Java documentation if you want to find out how to use it.

This section has introduced you to many of the interesting features of Graphics2D, but there is still a large part of the Graphics2D framework for you to explore.

Section 13.3

Actions and Buttons

For the past two sections, we have been looking at some of the more advanced aspects of the Java graphics API. But the heart of most graphical user interface programming is using GUI components. In this section and the next, we'll be looking at JComponents. We'll cover several component classes that were not covered in Chapter 6, as well as some additional features of classes that were covered there.

This section is mostly about buttons. Buttons are among the simplest of GUI components, and it seems like there shouldn't be all that much to say about them. However, buttons are not as simple as they seem. For one thing, there are many different types of buttons. The basic functionality of buttons in Java is defined by the class javax.swing.AbstractButton. Subclasses of this class represent push buttons, check boxes, and radio buttons. Menu items are also considered to be buttons. The AbstractButton class defines a surprisingly large API for controlling the appearance of buttons. This section will cover part of that API, but you should see the class documentation for full details.

In this section, we'll also encounter a few classes that do not themselves define buttons but that are related to the button API, starting with "actions."

13.3.1  Action and AbstractAction

The JButton class represents push buttons. Up until now, we have created push buttons using the constructor

public JButton(String text);

which specifies text that will appear on the button. We then added an ActionListener to the button, to respond when the user presses it. Another way to create a JButton is using an Action. The Action interface represents the general idea of some action that can be performed, together with properties associated with that action, such as a name for the action, an icon that represents the action, and whether the action is currently enabled or disabled. Actions are usually defined using the class AbstractAction, an abstract class which includes a method

public void actionPerformed(ActionEvent evt)

that must be defined in any concrete subclass. Often, this is done in an anonymous inner class. For example, suppose that display is an object that has a clear() method. Then an Action object that represents the action "clear the display" can be defined as:

Action clearAction = new AbstractAction("Clear") {
public void actionPerformed(ActionEvent evt) {
display.clear();
}
};

The parameter, "Clear", in the constructor of the AbstractAction is the name of the action. Other properties can be set by calling the method putValue(key,value), which is part of the Action interface. For example,

clearAction.putValue(Action.SHORT_DESCRIPTION, "Clear the Display");

sets the SHORT_DESCRIPTION property of the action to have the value "Clear the Display". The key parameter in the putValue() method is usually given as one of several constants defined in the Action interface. As another example, you can change the name of an action by using Action.NAME as the key in the putValue() method.

Once you have an Action, you can use it in the constructor of a button. For example, using the action clearAction defined above, we can create the JButton

JButton clearButton = new JButton( clearAction );

The name of the action will be used as the text of the button, and some other properties of the button will be taken from properties of the action. For example, if the SHORT_DESCRIPTION property of the action has a value, then that value is used as the tooltip text for the button. (The tooltip text appears when the user hovers the mouse over the button.) Furthermore, when you change a property of the action, the corresponding property of the button will also be changed. For example, changing the Action's property associated with the key Action.NAME will also change the text on the button.

The Action interface defines a setEnabled() method that is used to enable and disable the action. The clearAction action can be enabled and disabled by calling clearAction.setEnabled(true) and clearAction.setEnabled(false). When you do this, any button that has been created from the action is also enabled or disabled at the same time.

Now of course, the question is, why should you want to use Actions at all? One advantage is that using actions can help you to organize your code better. You can create separate objects that represent each of the actions that can be performed in your program. This represents a nice division of responsibility. Of course, you could do the same thing with individual ActionListener objects, but then you couldn't associate descriptions and other properties with the actions.

More important is the fact that Actions can also be used in other places in the Java API. You can use an Action to create a JMenuItem in the same way as for a JButton:

JMenuItem clearCommand = new JMenuItem( clearAction );

A JMenuItem, in fact, is a kind of button and shares many of the same properties that a JButton can have. You can use the same Action to create both a button and a menu item (or even several of each if you want). Whenever you enable or disable the action or change its name, the button and the menu item will both be changed to match. If you change the NAME property of the action, the text of both the menu item and the button will be set to the new name of the action. If you disable the action, both menu item and button will be disabled. You can think of the button and the menu items as being two presentations of the Action, and you don't have to keep track of the button or menu item after you create them. You can do everything that you need to do by manipulating the Action object.

By the way, if you want to add a menu item that is defined by an Action to a menu, you don't even need to create the JMenuItem yourself. You can add the action object directly to the menu, and the menu item will be created from the properties of the action. For example, if menu is a JMenu and clearAction is an Action, you can simply say menu.add(clearAction).

Actions have some other useful properties that we will encounter later in this section.

13.3.2  Icons on Buttons

In addition to -- or instead of -- text, buttons can also show icons. Icons are represented by the Icon interface and are usually created as ImageIcons, as discussed in Subsection 13.1.4. For example, here is a picture of a button that displays an image of a large "X" as its icon:

The icon for a button can be set by calling the button's setIcon() method, or by passing the icon object as a parameter to the constructor when the button is created. To create the button shown above, I created an ImageIcon from a BufferedImage on which I drew the picture that I wanted, and I constructed the JButton using a constructor that takes both the text and the icon for the button as parameters. Here's the code segment that does it:

BufferedImage image = new BufferedImage(24,24,BufferedImage.TYPE_INT_RGB);

Graphics2D g2 = (Graphics2D)image.getGraphics();
g2.setColor(Color.LIGHT_GRAY);             // Draw the image for the icon.
g2.fillRect(0,0,24,24);
g2.setStroke( new BasicStroke(3) );        //    Use thick lines.
g2.setColor(Color.BLACK);
g2.drawLine(4,4,20,20);                    //    Draw the "X".
g2.drawLine(4,20,20,4);
g2.dispose();

Icon clearIcon = new ImageIcon(image);     // Create the icon.

JButton clearButton = new JButton("Clear the Display", clearIcon);

You can create a button with an icon but no text by using a constructor that takes just the icon as parameter. Another alternative is for the button to get its icon from an Action. When a button is constructed from an action, it takes its icon from the value of the action property Action.SMALL_ICON. For example, suppose that we want to use an action named clearAction to create the button shown above. This could be done with:

clearAction.putValue( Action.SMALL_ICON, clearIcon );
JButton clearButton = new JButton( clearAction );

The icon could also be associated with the action by passing it as a parameter to the constructor of an AbstractAction:

Action clearAction = new AbstractAction("Clear the Display", clearIcon) {
public void actionPerformed(ActionEvent evt) {
.
.  // Carry out the action.
.
}
}
JButton clearButton = new JButton( clearAction );

The SMALL_ICON for an Action will also be used by a JMenuItem created from the action. (An action can also have an icon associated with the key Action.LARGE_ICON_KEY. If it does, then a JButton will use the "large" icon in preference to the "small" icon. However, a JMenuItem will only use a small icon.)

The appearance of buttons can be tweaked in many ways. For example, you can change the size of the gap between the button's text and its icon. You can associate additional icons with a button that are used when the button is in certain states, such as when it is pressed or when it is disabled. It is even possible to change the positioning of the text with respect to the icon. For example, to place the text centered below the icon on a button, you can say:

button.setHorizontalTextPosition(JButton.CENTER);
button.setVerticalTextPosition(JButton.BOTTOM);

These methods and many others are defined in the class AbstractButton. This class is a superclass for JMenuItem, as well as for JButton and for the classes that define check boxes and radio buttons.

Finally, I will mention that it is possible to use icons on JLabels in much the same way that they can be used on JButtons. Placing an ImageIcon on a JLabel can be a convenient way to add a static image to your GUI.

13.3.3  Making Choices

The JCheckBox class was covered in Subsection 6.5.3, and the equivalent for use in menus, JCheckBoxMenuItem, in Subsection 6.7.1. A checkbox lets the user make a choice between two alternatives. A checkbox has two states, selected and not selected, and the user can change the state by clicking on the check box. The state of a checkbox can also be set programmatically by calling its setSelected() method, and the current value of the state can be checked using the isSelected() method.

Closely related to checkboxes are radio buttons. Like a checkbox, a radio button can be either selected or not. However, radio buttons are expected to occur in groups, and at most one radio button in a group can be selected at any given time. Radio button groups let the user make a choice among several alternatives. In Java, a radio button is represented by an object of type JRadioButton. When used in isolation, a JRadioButton acts just like a JCheckBox, and it has the same methods and events. Ordinarily, however, a JRadioButton is used in a group. A group of radio buttons is represented by an object belonging to the class ButtonGroup. A ButtonGroup is not a component and does not itself have a visible representation on the screen. A ButtonGroup works behind the scenes to organize a group of radio buttons, to ensure that at most one button in the group can be selected at any given time.

To use a group of radio buttons, you must create a JRadioButton object for each button in the group, and you must create one object of type ButtonGroup to organize the individual buttons into a group. Each JRadioButton must be added individually to some container, so that it will appear on the screen. (A ButtonGroup plays no role in the placement of the buttons on the screen.) Each JRadioButton must also be added to the ButtonGroup, which has an add() method for this purpose. If you want one of the buttons to be selected initially, you can call setSelected(true) for that button. If you don't do this, then none of the buttons will be selected until the user clicks on one of them.

As an example, here is how you could set up a set of radio buttons that can be used to select a color:

JRadioButton redRadio, blueRadio, greenRadio, yellowRadio;
// Variables to represent the radio buttons.
// These should probably be instance variables, so
// that they can be used throughout the program.

ButtonGroup colorGroup = new ButtonGroup();

redRadio = new JRadioButton("Red");  // Create a button.
colorGroup.add(redRadio);            // Add it to the group.

blueRadio = new JRadioButton("Blue");
colorGroup.add(blueRadio);

greenRadio = new JRadioButton("Green");
colorGroup.add(greenRadio);

yellowRadio = new JRadioButton("Yellow");
colorGroup.add(yellowRadio);

redRadio.setSelected(true);  // Make an initial selection.

The individual buttons must still be added to a container if they are to appear on the screen. If you want to respond immediately when the user clicks on one of the radio buttons, you can register an ActionListener for each button. Just as for checkboxes, it is not always necessary to register listeners for radio buttons. In some cases, you can simply check the state of each button when you need to know it, using the button's isSelected() method.

You can add the equivalent of a group of radio buttons to a menu by using the class JRadioButtonMenuItem. To use this class, create several objects of this type, and create a ButtonGroup to manage them. Add each JRadioButtonMenuItem to the ButtonGroup, and also add them to a JMenu. If you want one of the items to be selected initially, call its setSelected() method to set its selection state to true. You can add ActionListeners to each JRadioButtonMenuItem if you need to take some action when the user selects the menu item; if not, you can simply check the selected states of the buttons whenever you need to know them. As an example, suppose that menu is a JMenu. Then you can add a group of buttons to menu as follows:

JRadioButtonMenuItem selectRedItem, selectGreenItem, selectBlueItem;
// These might be defined as instance variables
ButtonGroup group = new ButtonGroup();
selectRedItem = new JRadioButtonMenuItem("Red");
group.add(selectRedItem);
menu.add(selectRedItem);
selectGreenItem = new JRadioButtonMenuItem("Green");
group.add(selectGreenItem);
menu.add(selectGreenItem);
selectBlueItem = new JRadioButtonMenuItem("Blue");
group.add(selectBlueItem);
menu.add(selectBlueItem);

When it's drawn on the screen, a JCheckBox includes a little box that is either checked or unchecked to show the state of the box. That box is actually a pair of Icons. One icon is shown when the check box is unselected; the other is shown when it is selected. You can change the appearance of the check box by substituting different icons for the standard ones.

The icon that is shown when the check box is unselected is just the main icon for the JCheckBox. You can provide a different unselected icon in the constructor or you can change the icon using the setIcon() method of the JCheckBox object. To change the icon that is shown when the check box is selected, use the setSelectedIcon() method of the JCheckBox. All this applies equally to JRadioButton, JCheckBoxMenuItem, and JRadioButtonMenuItem.

An example of this can be found in the sample program ToolBarDemo.java, which is discussed in the next subsection. That program creates a set of radio buttons that use custom icons. The buttons are created by the following method:

/**
* Create a JRadioButton and add it to a specified button group.  The button
* is meant for selecting a drawing color in the display.  The color is used to
* create two custom icons, one for the unselected state of the button and one
* for the selected state.  These icons are used instead of the usual
* radio button icons.
* @param c the color of the button, and the color to be used for drawing.
*    (Note that c has to be "final" since it is used in the anonymous inner
*    class that defines the response to ActionEvents on the button.)
* @param grp the ButtonGroup to which the radio button will be added.
* @param selected if true, then the state of the button is set to selected.
* @return the radio button that was just created; sorry, but the button
*    is not as pretty as I would like!
*/
private JRadioButton makeColorRadioButton(final Color c,
ButtonGroup grp, boolean selected) {

/* Create an ImageIcon for the normal, unselected state of the button,
using a BufferedImage that is drawn here from scratch. */

BufferedImage image = new BufferedImage(30,30,BufferedImage.TYPE_INT_RGB);
Graphics g = image.getGraphics();
g.setColor(Color.LIGHT_GRAY);
g.fillRect(0,0,30,30);
g.setColor(c);
g.fill3DRect(1, 1, 24, 24, true);
g.dispose();
Icon unselectedIcon = new ImageIcon(image);

/* Create an ImageIcon for the selected state of the button. */

image = new BufferedImage(30,30,BufferedImage.TYPE_INT_RGB);
g = image.getGraphics();
g.setColor(Color.DARK_GRAY);
g.fillRect(0,0,30,30);
g.setColor(c);
g.fill3DRect(3, 3, 24, 24, false);
g.dispose();
Icon selectedIcon = new ImageIcon(image);

/* Create and configure the button. */

JRadioButton button = new JRadioButton(unselectedIcon);
button.setSelectedIcon(selectedIcon);
button.addActionListener( new ActionListener() {
public void actionPerformed(ActionEvent e) {
// The action for this button sets the current drawing color
// in the display to c.
display.setCurrentColor(c);
}
});
grp.add(button);
if (selected)
button.setSelected(true);

return button;
} // end makeColorRadioButton

It is possible to create radio buttons and check boxes -- both the regular sort and the corresponding menu items -- from Actions. The button or checkbox takes its name, main icon, tooltip text, and enabled/disabled state from the action. And the Action's actionPerformed() method is called when the user changes the state. Furthermore, an action has a property named Action.SELECTED_KEY, which means that the action itself can have a selected state. By default, the value of the SELECTED_KEY property is null and is not used for anything. However, you can set the value to be true or false by calling action.putValue(Action.SELECTED_KEY,true) or action.putValue(Action.SELECTED_KEY,false). Once you have done so, the action's SELECTED_KEY property and the selected state of any checkbox or radio button created from the action will automatically be kept in sync. You can find out the current value of the property by calling action.getValue(Action.SELECTED_KEY). Note that the value of a property is actually an object. When you set the value to true or false, the value is wrapped in an object of type Boolean. The return type of action.getValue() is Object, so you will probably need to type-cast the return value to Boolean.

Java has another component that lets the user choose from a set of options: the JComboBox. A combo box contains a list of items, but only displays the currently selected items. The user clicks the combo box to see a pop-up list of options and can then select from the list. The functionality is similar to a group of radio buttons.

JComboBox is a parameterized class, where the type parameter specifies the type of items that that combo box can hold. Most commonly, the items are strings, and the type is JComboBox<String>. This is the only case I will discuss. When a JComboBox<String> object is first constructed, it initially contains no items. An item is added to the bottom of the list of options by calling the combo box's instance method, addItem(str), where str is the string that will be displayed in the menu.

For example, the following code will create an object of type JComboBox that contains the options Red, Blue, Green, and Black:

JComboBox<String> colorChoice = new JComboBox<String>();
colorChoice.addItem("Red");
colorChoice.addItem("Blue");
colorChoice.addItem("Green");
colorChoice.addItem("Black");

You can call the getSelectedIndex() method of a JComboBox to find out which item is currently selected. This method returns an integer that gives the position of the selected item in the list, where the items are numbered starting from zero. Alternatively, you can call getSelectedItem() to get the selected item itself. (This method returns a value of type Object -- even when the items are limited to being strings.) You can change the selection by calling the method setSelectedIndex(n), where n is an integer giving the position of the item that you want to select.

The most common way to use a JComboBox is to call its getSelectedIndex() method when you have a need to know which item is currently selected. However, JComboBox components generate ActionEvents when the user selects an item, and you can register an ActionListener with the JComboBox if you want to respond to such events as they occur.

JComboBoxes have a nifty feature, which is probably not all that useful in practice. You can make a JComboBox "editable" by calling its method setEditable(true). If you do this, the user can edit the selection by clicking on the JComboBox and typing. This allows the user to make a selection that is not in the pre-configured list that you provide. (The "Combo" in the name "JComboBox" refers to the fact that it's a kind of combination of menu and text-input box.) If the user has edited the selection in this way, then the getSelectedIndex() method will return the value -1, and getSelectedItem() will return the string that the user typed. An ActionEvent is triggered if the user presses return while typing in the JComboBox.

There is a lot of information in this section. The sample program ChoiceDemo.java demonstrates the use of combo boxes, check boxes, and radio buttons -- including the use of Actions with check boxes and radio buttons. I encourage you to run the program to see how these things work and to read the source code. The source code has a lot of comments to explain what is going on.

13.3.4  Toolbars

It has become increasingly common for programs to have a row of small buttons along the top or side of the program window that offer access to some of the commonly used features of the program. The row of buttons is known as a tool bar. Typically, the buttons in a tool bar are presented as small icons, with no text. Tool bars can also contain other components, such as JTextFields and JLabels.

In Swing, tool bars are represented by the class JToolBar. A JToolBar is a container that can hold other components. It is also itself a component, and so can be added to other containers. In general, the parent component of the tool bar should use a BorderLayout. The tool bar should occupy one of the edge positions -- NORTH, SOUTH, EAST, or WEST -- in the BorderLayout. Furthermore, the other three edge positions should be empty. The reason for this is that it might be possible (depending on the platform and configuration) for the user to drag the tool bar from one edge position in the parent container to another. It might even be possible for the user to drag the tool bar off its parent entirely, so that it becomes a separate window.

Here is a picture of a toolbar. This is from the sample program ToolBarDemo.java, which I discuss below:

In the demo program, the user can draw colored curves in a large drawing area. The first three buttons in the tool bar are a set of radio buttons that control the drawing color. The fourth button is a push button that the user can click to clear the drawing.

Tool bars are easy to use. You just have to create the JToolBar object, add it to a container, and add some buttons and possibly other components to the tool bar. One fine point is adding space to a tool bar, such as the gap between the radio buttons and the push button in the above picture. You can leave a gap by adding a separator to the tool bar. For example:

toolbar.addSeparator(new Dimension(20,20));

This adds an invisible 20-by-20 pixel block to the tool bar. This will appear as a 20 pixel gap between components.

Here is the constructor from the ToolBarDemo program. It shows how to create the tool bar and place it in a container. Note that class ToolBarDemo is a subclass of JPanel, and the tool bar and display are added to the panel object that is being constructed:

public ToolBarDemo() {

setLayout(new BorderLayout(2,2));
setBackground(Color.GRAY);
setBorder(BorderFactory.createLineBorder(Color.GRAY,2));

display = new Display();  // the area where the user draws.
add(display, BorderLayout.CENTER);

JToolBar toolbar = new JToolBar();
add(toolbar, BorderLayout.NORTH);

ButtonGroup group = new ButtonGroup();
toolbar.add( makeColorRadioButton(Color.RED,group,true) );
toolbar.add( makeColorRadioButton(Color.GREEN,group,false) );
toolbar.add( makeColorRadioButton(Color.BLUE,group,false) );

toolbar.addSeparator(new Dimension(20,20));

toolbar.add( makeClearButton() );

}

If you want a vertical tool bar that can be placed in the EAST or WEST position of a BorderLayout, you should specify the orientation in the tool bar's constructor:

JToolBar toolbar = new JToolBar( JToolBar.VERTICAL );

The default orientation is JToolBar.HORIZONTAL. The orientation is adjusted automatically when the user drags the tool bar into a new position. If you want to prevent the user from dragging the tool bar, just say toolbar.setFloatable(false).

13.3.5  Keyboard Accelerators

In most programs, commonly used menu commands have keyboard equivalents. The user can type the keyboard equivalent instead of selecting the command from the menu, and the result will be exactly the same. Typically, for example, the "Save" command has keyboard equivalent CONTROL-S, and the "Undo" command corresponds to CONTROL-Z. (Under Mac OS, the keyboard equivalents for these commands would probably be META-S and META-Z, where META refers to holding down the "apple" key.) The keyboard equivalents for menu commands are referred to as accelerators.

The class javax.swing.KeyStroke is used to represent key strokes that the user can type on the keyboard. Keystrokes can refer to pressing a key, releasing a key, or typing a character, possibly while holding down one or more of the modifier keys control, shift, alt, and meta. The KeyStroke class has a convenient static method, getKeyStroke(String), that makes it easy to create key stroke objects representing key pressed events. For example,

KeyStroke.getKeyStroke( "ctrl S" )

returns a KeyStroke that represents the action of pressing the "S" key while holding down the control key. In addition to "ctrl", you can use the modifiers "shift", "alt", and "meta" in the string that describes the key stroke. You can even combine several modifiers, so that

KeyStroke.getKeyStroke( "ctrl shift Z" )

represents the action of pressing the "Z" key while holding down both the control and the shift keys. When the key stroke involves pressing a character key, the character must appear in the string in upper case form. You can also have key strokes that correspond to non-character keys. The number keys can be referred to as "1", "2", etc., while certain special keys have names such as "F1", "ENTER", and "LEFT" (for the left arrow key). The class KeyEvent defines many constants such as VK_ENTER, VK_LEFT, and VK_S. The names that are used for keys in the keystroke description are just these constants with the leading "VK_" removed.

There are at least two ways to associate a keyboard accelerator with a menu item. One is to use the setAccelerator() method of the menu item object:

JMenuItem saveCommand = new JMenuItem( "Save..." );
saveCommand.setAccelerator( KeyStroke.getKeyStroke("ctrl S") );

The other technique can be used if the menu item is created from an Action. The action property Action.ACCELERATOR_KEY can be used to associate a KeyStroke with an Action. When a menu item is created from the action, the keyboard accelerator for the menu item is taken from the value of this property. For example, if redoAction is an Action representing a "Redo" action, then you might say:

redoAction.putValue( Action.ACCELERATOR_KEY,
KeyStroke.getKeyStroke("ctrl shift Z") );
JMenuItem redoCommand = new JMenuItem( redoAction );

or, alternatively, you could simply add the action to a JMenu, editMenu, with editMenu.add(redoAction). (Note, by the way, that accelerators apply only to menu items, not to push buttons. When you create a JButton from an action, the ACCELERATOR_KEY property of the action is ignored.)

Note that you can use accelerators for JCheckBoxMenuItems and JRadioButtonMenuItems, as well as for simple JMenuItems.

For examples of using keyboard accelerators, see ChoiceDemo.java and the solution to Exercise 13.2.

By the way, as noted above, in the Mac OS operating system, the meta (or apple) key is usually used for keyboard accelerators instead of the control key. Java has a way of determining the correct modifier for the computer on which it is running. However, it requires the use of a different method for constructing the KeyStroke. The function call

int shortcutMask = Toolkit.getDefaultToolkit().getMenuShortcutKeyMask()

returns a "mask" that can be used to represent the correct modifier key when the keystroke is created by the function

KeyStroke stoke = KeyStroke( keyCode, shortcutMask );

The first parameter, keyCode, is one of the constants such as KeyEvent.VK_Z that represent keys on the keyboard. For example, KeyStroke.getKeyStroke(KeyEvent.VK_Z,shortcutMask) represents the action of pressing the Z key while holding down the meta key on a Mac or the control key on Windows or Linux. It is possible to add other modifiers to the shortcutMask to account for holding down the shift or alt keys; see the documentation.

As an example, here is a code segment that is used in ChoiceDemo.java to make a "Quit" menu item that will have the appropriate modifier key for the computer on which the program is running

int shortcutMask = Toolkit.getDefaultToolkit().getMenuShortcutKeyMask();
KeyStroke quitKeyStroke = KeyStroke.getKeyStroke(KeyEvent.VK_Q, shortcutMask);
JMenuItem quit = new JMenuItem("Quit");
quit.setAccelerator(quitKeyStroke);
quit.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent evt) {
System.exit(0);
}
});
controlMenu.add(quit);

13.3.6  HTML on Buttons

As a final stop in this brief tour of ways to spiff up your buttons, I'll mention the fact that the text that is displayed on a button can be specified in HTML format. HTML is the markup language that is used to write web pages. I don't cover it in this book, but I will explain a few things that you can do with it. HTML allows you to apply color or italics or other styles to just part of the text on your buttons. It also makes it possible to have buttons that display multiple lines of text. (You can also use HTML on JLabels, which can be even more useful.) Here's a picture of a button with HTML text (along with a "Java" icon):

If the string of text that is applied to a button starts with "<html>", then the string is interpreted as HTML. The string does not have to use strict HTML format; for example, you don't need a closing </html> at the end of the string. To get multi-line text, use <br> in the string to represent line breaks. If you would like the lines of text to be center justified, include the entire text (except for the <html>) between <center> and </center>. For example,

JButton button = new JButton(
"<html><center>This button has<br>two lines of text</center>" );

creates a button that displays two centered lines of text. You can apply italics to part of the string by enclosing that part between <i> and </i>. Similarly, use <b>...</b> for bold text and <u>...</u> for underlined text. For green text, enclose the text between <font color=green> and </font>. You can, of course, use other colors in place of "green." The "Java" button that is shown above was created using:

JButton javaButton = new JButton( "<html><u>Now</u> is the time for<br>" +
"a nice cup of <font color=red>coffee</font>." );

and then adding the Java image as an icon for the button.

Section 13.4

Complex Components and MVC

Since even buttons turn out to be pretty complex, as seen in the previous section, you might guess that there is a lot more complexity lurking in the Swing API. While this is true, a lot of that complexity works to your benefit as a programmer, since a lot of it is hidden in normal uses of Swing components. For example, you don't have to know about all the complex details of buttons in order to use them effectively in most programs.

Swing defines several component classes that are much more complex than those we have looked at so far, but even the most complex components are not very difficult to use for many purposes. In this section, we'll look at components that support display and manipulation of lists, tables, and text documents. To use these complex components effectively, you'll need to know something about the Model-View-Controller pattern that is used as a basis for the design of many Swing components. This pattern is discussed in the first part of this section.

This section is our last look at Swing components, but there are a number of component classes that have not even been touched on in this book. Some useful ones that you might want to look into include: JTabbedPane, JSplitPane, JTree, JSpinner, JPopupMenu, JProgressBar, and JScrollBar.

At the end of the section, we'll look briefly at the idea of writing custom component classes -- something that you might consider when even the large variety of components that are already defined in Swing don't do quite what you want (or when they do too much, and you want something simpler).

13.4.1  Model-View-Controller

One of the principles of object-oriented design is division of responsibilities. Ideally, an object should have a single, clearly defined role, with a limited realm of responsibility. One application of this principle to the design of graphical user interfaces is the MVC pattern. "MVC" stands for "Model-View-Controller" and refers to three different realms of responsibility in the design of a graphical user interface.

When the MVC pattern is applied to a component, the model consists of the data that represents the current state of the component. The view is simply the visual presentation of the component on the screen. And the controller is the aspect of the component that carries out actions in response to events generated by the user (or by other sources such as timers). The idea is to assign responsibility for the model, the view, and the controller to different objects.

The view is the easiest part of the MVC pattern to understand. It is often represented by the component object itself, and its responsibility is to draw the component on the screen. In doing this, of course, it has to consult the model, since the model represents the state of the component, and that state can determine what appears on the screen. To get at the model data -- which is stored in a separate object according to the MVC pattern -- the component object needs to keep a reference to the model object. Furthermore, when the model changes, the view often needs to be redrawn to reflect the changed state. The component needs some way of knowing when changes in the model occur. Typically, in Java, this is done with events and listeners. The model object is set up to generate events when its data changes. The view object registers itself as a listener for those events. When the model changes, an event is generated, the view is notified of that event, and the view responds by updating its appearance on the screen.

When MVC is used for Swing components, the controller is generally not so well defined as the model and view, and its responsibilities are often split among several objects. The controller might include mouse and keyboard listeners that respond to user events on the view; Actions that respond to menu commands or buttons; and listeners for other high-level events, such as those from a slider, that affect the state of the component. Usually, the controller responds to events by making modifications to the model, and the view is changed only indirectly, in response to the changes in the model.

The MVC pattern is used in many places in the design of Swing. It is even used for buttons. The state of a Swing button is stored in an object of type ButtonModel. The model stores such information as whether the button is enabled, whether it is selected, and what ButtonGroup it is part of, if any. If button is of type JButton (or one of the other subclasses of AbstractButton), then its ButtonModel can be obtained by calling button.getModel(). In the case of buttons, you might never need to use the model or even know that it exists. But for the list and table components that we will look at next, knowledge of the model is essential.

13.4.2  Lists and ListModels

A JList is a component that represents a list of items that can be selected by the user. The sample program SillyStamper.java allows the user to select one icon from a JList of Icons. The user selects an icon from the list by clicking on it. The selected icon can be "stamped" onto a drawing area by clicking on the drawing area. (The icons in this program are from the KDE desktop project.) Here is a picture of the program with several icons already stamped onto the drawing area and with the "star" icon selected:

Note that the scrollbar in this program is not part of the JList. To add a scrollbar to a list, the list must be placed into a JScrollPane. See Subsection 6.5.4, where the use of JScrollPane to hold a JTextArea was discussed. Scroll panes are used in the same way with lists and with other components. In this case, the JList, iconList, was added to a scroll pane and the scroll pane was added to a panel with the single command:

add( new JScrollPane(iconList), BorderLayout.EAST );

JList is a parameterized type, where the type parameter indicates what type of object can be displayed in the list. The most common types are JList<String> and JList<Icon>. Other types can be used, but they will be converted to strings for display. (It's possible to "teach" a JList how to display other types of items; see the setCellRenderer() method in the JList class.)

One way to construct a JList is from an array that contains the objects that will appear in the list. In the SillyStamper program, the images for the icons are read from resource files, the icons are placed into an array, and the array is used to construct a JList<Icon>. This is done by the following method:

private JList<Icon> createIconList() {

String[] iconNames = new String[] {
"icon5.png", "icon7.png", "icon8.png", "icon9.png", "icon10.png",
"icon11.png", "icon24.png", "icon25.png", "icon26.png", "icon31.png",
"icon33.png", "icon34.png"
};              // Array containing resource file names for the icon images.

iconImages = new Image[iconNames.length];

ClassLoader classLoader = getClass().getClassLoader();
Toolkit toolkit = Toolkit.getDefaultToolkit();
try {                        // Get the icon images from the resource files.
for (int i = 0; i < iconNames.length; i++) {
URL imageURL = classLoader.getResource("stamper_icons/" + iconNames[i]);
if (imageURL == null)
throw new Exception();
iconImages[i] = toolkit.createImage(imageURL);
}
}
catch (Exception e) {
iconImages = null;
return null;
}

ImageIcon[] icons = new ImageIcon[iconImages.length];
for (int i = 0; i < iconImages.length; i++)  // Create the icons.
icons[i] = new ImageIcon(iconImages[i]);

JList<Icon> list = new JList<Icon>(icons);  // A list containing image icons.
list.setSelectionMode(ListSelectionModel.SINGLE_SELECTION);
list.setSelectedIndex(0);   // First item in the list is currently selected.

return list;
}

By default, the user can select any number of items in a list. A single item is selected by clicking on it. Multiple items can be selected by shift-clicking and by either control-clicking or meta-clicking (depending on the platform). In the SillyStamper program, I wanted to restrict the selection so that only one item can be selected at a time. This restriction is imposed by calling

list.setSelectionMode(ListSelectionModel.SINGLE_SELECTION);

With this selection mode, when the user selects an item, the previously selected item, if any, is deselected. Note that the selection can be changed by the program by calling list.setSelectedIndex(itemNum). Items are numbered starting from zero. To find out the currently selected item in single selection mode, call list.getSelectedIndex(). This returns the item number of the selected item, or -1 if no item is currently selected. If multiple selections are allowed, you can call list.getSelectedIndices(), which returns an array of ints that contains the item numbers of all selected items.

Now, the list that you see on the screen is only the view aspect of the list. The controller consists of the listener objects that respond when the user clicks an item in the list. For its model, a JList uses an object of type ListModel. This is the object that knows the actual list of items. A model is defined not only by the data that it contains but by the set of operations that can be performed on the data. When a JList is constructed from an array of objects, the model that is used is very simple. The model can tell you how many items it contains and what those items are, but it can't do much else. In particular, there is no way to add items to the list or to delete items from the list! If you need that capability, you will have to use a different list model.

The class DefaultListModel defines list models that support adding items to and removing items from the list. Like JList, it is a parameterized class. (Note that the list model that you get when you create a JList from an array is not of this type.) If dlmodel is of type DefaultListModel, the following methods, among others, are defined:

- dlmodel.getSize() -- returns the number of items.

- dlmodel.getElementAt(index) -- returns the item at position index in the list.

- dlmodel.addElement(item) -- Adds item to the end of the list; item can be any Object.

- dlmodel.insertElementAt(item, index) -- inserts the specified item into the list at the specified index; items that come after that position in the list are moved down to make room for the new item.

- dlmodel.setElementAt(item, index) -- Replaces the item that is currently at position index in the list with item.

- dlmodel.remove(index) -- removes the item at position index in the list.

- dlmodel.removeAllElements() -- removes everything from the list, leaving it empty.

To use a modifiable JList<T>, you should create a DefaultListModel<T>, add any items to it that should be in the list initially, and pass it to the JList constructor. For example:

DefaultListModel<String> listModel;
JList<String> flavorList;

listModel = new DefaultListModel<String>();    // Create the model object.

listModel.addElement("Chocolate");     // Add items to the model.
listModel.addElement("Vanilla");
listModel.addElement("Strawberry");
listModel.addElement("Rum Raisin");

flavorList = new JList<String>(listModel);     // Create the list component.

By keeping a reference to the model around in an instance variable, you will be able to add and delete flavors as the program is running by calling the appropriate methods in listModel. Keep in mind that changes that are made to the model will automatically be reflected in the view. Behind the scenes, when a list model is modified, it generates an event of type ListDataEvent. The JList registers itself with its model as a listener for these events, and it responds to an event by redrawing itself to reflect the changes in the model. The programmer doesn't have to take any extra action, beyond changing the model.

By the way, the model for a JList actually has another part in addition to the ListModel: An object of type ListSelectionModel stores information about which items in the list are currently selected. When the model is complex, it's not uncommon to use several model objects to store different aspects of the state.

13.4.3  Tables and TableModels

Like a JList, a JTable displays a collection of items to the user. However, tables are much more complicated than lists. Perhaps the most important difference is that it is possible for the user to edit items in the table. Table items are arranged in a grid of rows and columns. Each grid position is called a cell of the table. Each column can have a header, which appears at the top of the column and contains a name for the column.

It is easy to create a JTable from an array that contains the names of the columns and a two-dimensional array that contains the items that go into the cells of the table. As an example, the sample program StatesAndCapitalsTableDemo.java creates a table with two columns named "State" and "Capital City." The first column contains a list of the states of the United States and the second column contains the name of the capital city of each state. The table can be created as follows:

String[][] statesAndCapitals = new String[][] {
{ "Alabama", "Montgomery" },
{ "Alaska", "Juneau" },
{ "Arizona", "Phoenix" },
.
.
.
{ "Wisconsin", "Madison" },
{ "Wyoming", "Cheyenne" }
};

String[] columnHeads = new String[] { "State", "Capital City" };

JTable table = new JTable(statesAndCapitals, columnHeads);

Since a table does not come with its own scroll bars, it is almost always placed in a JScrollPane to make it possible to scroll the table. For example:

add( new JScrollPane(table), BorderLayout.CENTER );

The column headers of a JTable are not actually part of the table; they are in a separate component. But when you add the table to a JScrollPane, the column headers are automatically placed at the top of the pane.

Using the default settings, the user can edit any cell in the table by clicking that cell. When editing a cell, the arrow keys can be used to move from one cell to another. The user can change the order of the columns by dragging a column header to a new position. The user can also change the width of the columns by dragging the line that separates neighboring column headers. You can try all this in StatesAndCapitalsTableDemo.java.

Allowing the user to edit all entries in the table is not always appropriate; certainly it's not appropriate in the "states and capitals" example. A JTable uses an object of type TableModel to store information about the contents of the table. The model object is also responsible for deciding whether or not the user should be able to edit any given cell in the table. TableModel includes the method

public boolean isCellEditable(int rowNum, int columnNum)

where rowNum and columnNum are the position of a cell in the grid of rows and columns that make up the table. When the controller wants to know whether a certain cell is editable, it calls this method in the table model. If the return value is true, the user is allowed to edit the cell.

The default model that is used when the table is created from an array of objects allows editing of all cells. For this model, the return value of isCellEditable() is true in all cases. To make some cells non-editable, you have to provide a different model for the table. One way to do this is to create a subclass of DefaultTableModel and override the isCellEditable() method. (DefaultTableModel and some other classes that are discussed in this section are defined in the package javax.swing.table.) Here is how this might have been done in the "states and capitals" program to make all cells non-editable:

TableModel model = new DefaultTableModel(statesAndCapitals,columnHeads) {
public boolean isCellEditable(int row, int col) {
return false;
}
};
JTable table = new JTable(model);

Here, an anonymous subclass of DefaultTableModel is created in which the isCellEditable() method returns false in all cases, and the model object that is created from that class is passed as a parameter to the JTable constructor.

The DefaultTableModel class defines many methods that can be used to modify the table, including for example: setValueAt(item,rowNum,colNum) to change the item in a given cell; removeRow(rowNum) to delete a row; and addRow(itemArray) to add a new row at the end of the table that contains items from the array itemArray. Note that if the item in a given cell is set to null, then that cell will be empty. Remember, again, that when you modify the model, the view is automatically updated to reflect the changes.

In addition to the isCellEditable() method, the table model method that you are most likely to want to override is getColumnClass(), which is defined as

public Class<?> getColumnClass(columnNum)

The purpose of this method is to specify what kind of values are allowed in the specified column. The return value from this method is of type Class. (The "<?>" is there for technical reasons having to do with generic programming. See Section 10.5, but don't worry about understanding it here.) Although class objects have crept into this book in a few places -- in the discussion of ClassLoaders in Subsection 13.1.3 for example -- this is the first time we have directly encountered the class named Class. An object of type Class represents a class. A Class object is usually obtained from the name of the class using expressions of the form "Double.class" or "JTable.class". If you want a three-column table in which the column types are String, Double, and Boolean, you can use a table model in which getColumnClass is defined as:

public Class<?> getColumnClass(columnNum) {
if (columnNum == 0)
return String.class;
else if (columnNum == 1)
return Double.class;
else
return Boolean.class;
}

The table will call this method and use the return value to decide how to display and edit items in the table. For example, if a column is specified to hold Boolean values, the cells in that column will be displayed and edited as check boxes. For numeric types, the table will not accept illegal input when the user types in the value. (It is possible to change the way that a table edits or displays items. See the methods setDefaultEditor() and setDefaultRenderer() in the JTable class.)

As an alternative to using a subclass of DefaultTableModel, a custom table model can also be defined using a subclass of AbstractTableModel. Whereas DefaultTableModel provides a lot of predefined functionality, AbstractTableModel provides very little. However, using AbstractTableModel gives you the freedom to represent the table data any way you want. The sample program ScatterPlotTableDemo.java uses a subclass of AbstractTableModel to define the model for a JTable. In this program, the table has three columns. The first column holds a row number and is not editable. The other columns hold values of type Double; these two columns represent the x- and y-coordinates of points in the plane. The points themselves are graphed in a "scatter plot" next to the table. Initially, the program fills in the first six points with random values. Here is a picture of the program, with the x-coordinate in row 5 selected for editing:

Note, by the way, that in this program, the scatter plot can be considered to be a view of the table model, in the same way that the table itself is. The scatter plot registers itself as a listener with the model, so that it will receive notification whenever the model changes. When that happens, the scatter plot redraws itself to reflect the new state of the model. It is an important property of the MVC pattern that several views can share the same model, offering alternative presentations of the same data. The views don't have to know about each other or communicate with each other except by sharing the model. Although I didn't do it in this program, it would even be possible to add a controller to the scatter plot view. This could let the user add a point by clicking the mouse on the scatter plot or drag a point to change its coordinates. The controller would update the table model, based on the user actions. Since the scatter plot and table share the same model, the values displayed in the table would automatically change to match.

Here is the definition of the class that defines the model in the scatter plot program. All the methods in this class must be defined in any subclass of AbstractTableModel except for setValueAt(), which only has to be defined if the table is modifiable.

/**
* This class defines the TableModel that is used for the JTable in this
* program.  The table has three columns.  Column 0 simply holds the
* row number of each row.  Column 1 holds the x-coordinates of the
* points for the scatter plot, and Column 2 holds the y-coordinates.
* The table has 25 rows.  No support is provided for adding more rows.
*/
private class CoordInputTableModel extends AbstractTableModel {

private Double[] xCoord = new Double[25];  // Data for Column 1.
private Double[] yCoord = new Double[25];  // Data for Column 2.
// Initially, all the values in the array are null, which means
// that all the cells are empty.

public int getColumnCount() {  // Tells caller how many columns there are.
return 3;
}

public int getRowCount() {  // Tells caller how many rows there are.
return xCoord.length;
}

public Object getValueAt(int row, int col) {  // Get value from cell.
if (col == 0)
return (row+1);        // Column 0 holds the row number.
else if (col == 1)
return xCoord[row];    // Column 1 holds the x-coordinates.
else
return yCoord[row];    // Column 2 holds the y-coordinates.
}

public Class<?> getColumnClass(int col) {  // Get data type of column.
if (col == 0)
return Integer.class;
else
return Double.class;
}

public String getColumnName(int col) {  // Returns a name for column header.
if (col == 0)
return "Num";
else if (col == 1)
return "X";
else
return "Y";
}

public boolean isCellEditable(int row, int col) { // Can user edit cell?
return col > 0;
}

public void setValueAt(Object obj, int row, int col) {
// (This method is called by the system if the value of the cell
// needs to be changed because the user has edited the cell.
// It can also be called to change the value programmatically.
// In this case, only columns 1 and 2 can be modified, and the data
// type for obj must be Double.  The method fireTableCellUpdated()
// has to be called to send an event to registered listeners to
// notify them of the modification to the table model.)
if (col == 1)
xCoord[row] = (Double)obj;
else if (col == 2)
yCoord[row] = (Double)obj;
fireTableCellUpdated(row, col);
}

}  // end nested class CoordInputTableModel

In addition to defining a custom table model, I customized the appearance of the table in several ways. Because this involves changes to the view, most of the changes are made by calling methods in the JTable object. For example, since the default height of the cells was too small for my taste, I called table.setRowHeight(25) to increase the height. To make lines appear between the rows and columns, I found that I had to call both table.setShowGrid(true) and table.setGridColor(Color.BLACK). Some of the customization has to be done to other objects. For example, to prevent the user from changing the order of the columns by dragging the column headers, I had to use

table.getTableHeader().setReorderingAllowed(false);

Tables are quite complex, and I have only discussed a part of the table API here. Nevertheless, I hope that you have learned enough to start using them and to learn more about them on your own.

13.4.4  Documents and Editors

As a final example of complex components, we look briefly at JTextComponent and its subclasses. A JTextComponent displays text that can, optionally, be edited by the user. Two subclasses, JTextField and JTextArea, were introduced in Subsection 6.5.4. But the real complexity comes in another subclass, JEditorPane, that supports display and editing of styled text. This allows features such as boldface and italic. A JEditorPane can even work with basic HTML documents.

It is almost absurdly easy to write a simple web browser program using a JEditorPane (although the program that you get can only handle basic web pages and does a pretty bad job on many modern web pages). This is done in the sample program SimpleWebBrowser.java. In this program, the user enters the URL of a web page, and the program tries to load and display the web page at that location. A JEditorPane can handle pages with content type "text/plain", "text/html", and "text/rtf". (The content type "text/rtf" represents styled or "rich text format" text. URLs and content types were covered in Subsection 11.4.1.) If editPane is of type JEditorPane and url is of type URL, then the statement "editPane.setPage(url);" is sufficient to load the page and display it. Since this can generate an exception, the following method is used in SimpleWebBrowser.java to display a page:

private void loadURL(URL url) {
try {
editPane.setPage(url);
}
catch (Exception e) {
editPane.setContentType("text/plain"); // Set pane to display plain text.
editPane.setText( "Sorry, the requested document was not found\n"
+"or cannot be displayed.\n\nError:" + e);
}
}

An HTML document can include links to other pages. When the user clicks on a link, the web browser should go to the linked page. A JEditorPane does not do this automatically, but it does generate an event of type HyperLinkEvent when the user clicks a link (provided that the edit pane has been set to be non-editable by the user). A program can register a listener for such events and respond by loading the new page.

There are a lot of web pages that a JEditorPane won't be able to display correctly, but it can be very useful in cases where you have control over the pages that will be displayed. A nice application is to distribute HTML-format help and information files with a program. The files can be stored as resource files in the jar file of the program, and a URL for a resource file can be obtained in the usual way, using the getResource() method of a ClassLoader. (See Subsection 13.1.3.)

It turns out, by the way, that SimpleWebBrowser.java is a little too simple. A modified version, SimpleWebBrowserWithThread.java, improves on the original by using a thread to load a page and by checking the content type of a page before trying to load it. It actually does work as a simple web browser. Try it!

The model for a JTextComponent is an object of type Document. If you want to be notified of changes in the model, you can add a listener to the model using

textComponent.getDocument().addDocumentListener(listener)

where textComponent is of type JTextComponent and listener is of type DocumentListener. The Document class also has methods that make it easy to read a document from a file and write a document to a file. I won't discuss all the things you can do with text components here. For one more peek at their capabilities, see the sample program SimpleRTFEdit.java, a very minimal editor for files that contain styled text of type "text/rtf."

13.4.5  Custom Components

Java's standard component classes are usually all you need to construct a user interface. At some point, however, you might need a component that Java doesn't provide. In that case, you can write your own component class, building on one of the components that Java does provide. We've already done this, actually, every time we've written a subclass of the JPanel class to use as a drawing surface. A JPanel is a blank slate. By defining a subclass, you can make it show any picture you like, and you can program it to respond in any way to mouse and keyboard events. Sometimes, if you are lucky, you don't need such freedom, and you can build on one of Java's more sophisticated component classes.

For example, suppose I have a need for a "stopwatch" component. When the user clicks on the stopwatch, I want it to start timing. When the user clicks again, I want it to display the elapsed time since the first click. The textual display can be done with a JLabel, but we want a JLabel that can respond to mouse clicks. We can get this behavior by defining a StopWatchLabel component as a subclass of the JLabel class. A StopWatchLabel object will listen for mouse clicks on itself. The first time the user clicks, it will change its display to "Timing..." and remember the time when the click occurred. When the user clicks again, it will check the time again, and it will compute and display the elapsed time. (Of course, I don't necessarily have to define a subclass. I could use a regular label in my program, set up a listener to respond to mouse events on the label, and let the program do the work of keeping track of the time and changing the text displayed on the label. However, by writing a new class, I have something that can be reused in other projects. I also have all the code involved in the stopwatch function collected together neatly in one place. For more complicated components, both of these considerations are very important.)

The StopWatchLabel class is not very hard to write. I need an instance variable to record the time when the user starts the stopwatch. Times in Java are measured in milliseconds and are stored in variables of type long (to allow for very large values). In the mousePressed() method, I need to know whether the timer is being started or stopped, so I need a boolean instance variable, running, to keep track of this aspect of the component's state. There is one more item of interest: How do I know what time the mouse was clicked? The method System.currentTimeMillis() returns the current time. But there can be some delay between the time the user clicks the mouse and the time when the mousePressed() routine is called. To make my stopwatch as accurate as possible, I don't want to know the current time. I want to know the exact time when the mouse was pressed. When I wrote the StopWatchLabel class, this need sent me on a search in the Java documentation. I found that if evt is an object of type MouseEvent, then the function evt.getWhen() returns the time when the event occurred. I call this function in the mousePressed() routine to determine the exact time when the user clicked on the label. The complete StopWatch class is rather short:

import java.awt.event.*;
import javax.swing.*;

/**
* A custom component that acts as a simple stop-watch.  When the user clicks
* on it, this component starts timing.  When the user clicks again,
* it displays the time between the two clicks.  Clicking a third time
* starts another timer, etc.  While it is timing, the label just
* displays the message "Timing....".
*/
public class StopWatchLabel extends JLabel implements MouseListener {

private long startTime;   // Start time of timer.
//   (Time is measured in milliseconds.)

private boolean running;  // True when the timer is running.

/**
* Constructor sets initial text on the label to
* "Click to start timer." and sets up a mouse listener
* so the label can respond to clicks.
*/
public StopWatchLabel() {
super("  Click to start timer.  ", JLabel.CENTER);
addMouseListener(this);
}

/**
* Tells whether the timer is currently running.
*/
public boolean isRunning() {
return running;
}

/**
* React when the user presses the mouse by starting or stopping
* the timer and changing the text that is shown on the label.
*/
public void mousePressed(MouseEvent evt) {
if (running == false) {
// Record the time and start the timer.
running = true;
startTime = evt.getWhen();  // Time when mouse was clicked.
setText("Timing....");
}
else {
// Stop the timer.  Compute the elapsed time since the
// timer was started and display it.
running = false;
long endTime = evt.getWhen();
double seconds = (endTime - startTime) / 1000.0;
setText("Time: " + seconds + " sec.");
}
}

public void mouseReleased(MouseEvent evt) { }
public void mouseClicked(MouseEvent evt) { }
public void mouseEntered(MouseEvent evt) { }
public void mouseExited(MouseEvent evt) { }

}

Don't forget that since StopWatchLabel is a subclass of JLabel, you can do anything with a StopWatchLabel that you can do with a JLabel. You can add it to a container. You can set its font, foreground color, and background color. You can set the text that it displays (although this would interfere with its stopwatch function). You can even add a Border if you want.

Let's look at one more example of defining a custom component. Suppose that -- for no good reason whatsoever -- I want a component that acts like a JLabel except that it displays its text in mirror-reversed form. Since no standard component does anything like this, the MirrorText class is defined as a subclass of JPanel. It has a constructor that specifies the text to be displayed and a setText() method that changes the displayed text. The paintComponent() method draws the text mirror-reversed, in the center of the component. This uses techniques discussed in Subsection 13.1.1 and Subsection 13.2.1. Information from a FontMetrics object is used to center the text in the component. The reversal is achieved by using an off-screen canvas. The text is drawn to the off-screen canvas, in the usual way. Then the image is copied to the screen with the following command, where OSC is the variable that refers to the off-screen canvas, and width and height give the size of both the component and the off-screen canvas:

g.drawImage(OSC, width, 0, 0, height, 0, 0, width, height, null);

This is the version of drawImage() that specifies corners of destination and source rectangles. The corner (0,0) in OSC is matched to the corner (width,0) on the screen, while (width,height) is matched to (0,height). This reverses the image left-to-right. Here is the complete class:

import java.awt.*;
import javax.swing.*;
import java.awt.image.BufferedImage;

/**
* A component for displaying a mirror-reversed line of text.
* The text will be centered in the available space.  This component
* is defined as a subclass of JPanel.  It respects any background
* color, foreground color, and font that are set for the JPanel.
* The setText(String) method can be used to change the displayed
* text.  Changing the text will also call revalidate() on this
* component.
*/
public class MirrorText extends JPanel {

private String text; // The text displayed by this component.

private BufferedImage OSC; // Holds an un-reversed picture of the text.

/**
* Construct a MirrorText component that will display the specified
* text in mirror-reversed form.
*/
public MirrorText(String text) {
if (text == null)
text = "";
this.text = text;
}

/**
* Change the text that is displayed on the label.
* @param text the new text to display
*/
public void setText(String text) {
if (text == null)
text = "";
if ( ! text.equals(this.text) ) {
this.text = text;  // Change the instance variable.
revalidate();      // Tell container to recompute its layout.
repaint();         // Make sure component is redrawn.
}
}

/**
* Return the text that is displayed on this component.
* The return value is non-null but can be an empty string.
*/
public String getText() {
return text;
}

/**
* The paintComponent method makes a new off-screen canvas, if necessary,
* writes the text to the off-screen canvas, then copies the canvas onto
* the screen in mirror-reversed form.
*/
public void paintComponent(Graphics g) {
int width = getWidth();
int height = getHeight();
if (OSC == null || width != OSC.getWidth()
|| height != OSC.getHeight()) {
OSC = new BufferedImage(width,height,BufferedImage.TYPE_INT_RGB);
}
Graphics OSG = OSC.getGraphics();
OSG.setColor(getBackground());
OSG.fillRect(0, 0, width, height);
OSG.setColor(getForeground());
OSG.setFont(getFont());
FontMetrics fm = OSG.getFontMetrics(getFont());
int x = (width - fm.stringWidth(text)) / 2;
int y = (height + fm.getAscent() - fm.getDescent()) / 2;
OSG.drawString(text, x, y);
OSG.dispose();
g.drawImage(OSC, width, 0, 0, height, 0, 0, width, height, null);
}

/**
* Compute a preferred size that includes the size of the text, plus
* a boundary of 5 pixels on each edge.
*/
public Dimension getPreferredSize() {
FontMetrics fm = getFontMetrics(getFont());
return new Dimension(fm.stringWidth(text) + 10,
fm.getAscent() + fm.getDescent() + 10);
}

}  // end MirrorText

This class defines the method "public Dimension getPreferredSize()". This method is called by a layout manager when it wants to know how big the component would like to be. Standard components come with a way of computing a preferred size. For a custom component based on a JPanel, it's a good idea to provide a custom preferred size. Every component has a method setPreferredSize() that can be used to set the preferred size of the component. For our MirrorText component, however, the preferred size depends on the font and the text of the component, and these can change from time to time. We need a way to compute a preferred size on demand, based on the current font and text. That's what we do by defining a getPreferredSize() method. The system calls this method when it wants to know the preferred size of the component. In response, we can compute the preferred size based on the current font and text.

The StopWatchLabel and MirrorText classes define components. Components don't stand on their own. You have to add them to a panel or other container. The sample program CustomComponentTest.java demonstrates using a MirrorText and a StopWatchLabel component, which are defined by the source code files MirrorText.java and StopWatchLabel.java.

In this program, the two custom components and a button are added to a panel that uses a FlowLayout as its layout manager, so the components are not arranged very neatly. If you click the button labeled "Change Text in this Program", the text in all the components will be changed. You can also click on the stopwatch label to start and stop the stopwatch. When you do any of these things, you will notice that the components will be rearranged to take the new sizes into account. This is known as "validating" the container. This is done automatically when a standard component changes in some way that requires a change in preferred size or location. This may or may not be the behavior that you want. (Validation doesn't always cause as much disruption as it does in this program. For example, in a GridLayout, where all the components are displayed at the same size, it will have no effect at all. I chose a FlowLayout for this example to make the effect more obvious.) When the text is changed in a MirrorText component, there is no automatic validation of its container. A custom component such as MirrorText must call the revalidate() method to indicate that the container that contains the component should be validated. In the MirrorText class, revalidate() is called in the setText() method.

Section 13.5

Finishing Touches

In this final section, I will present a program that is more complex and more polished than those we have looked at previously. Most of the examples in this book have been "toy" programs that illustrated one or two points about programming techniques. It's time to put it all together into a full-scale program that uses many of the techniques that we have covered, and a few more besides. After discussing the program and its basic design, I'll use it as an excuse to talk briefly about some of the features of Java that didn't fit into the rest of this book.

The program that we will look at is a Mandelbrot Viewer that lets the user explore the famous Mandelbrot set. I will begin by explaining what that means. Note that an even more capable Mandelbrot program can be found at http://math.hws.edu/xJava/MB.

13.5.1  The Mandelbrot Set

The Mandelbrot set is a set of points in the xy-plane that is defined by a computational procedure. To use the program, all you really need to know is that the Mandelbrot set can be used to make some pretty pictures, but here are the mathematical details: Consider the point that has real-number coordinates (a,b) and apply the following computation:

Let x = a
Let y = b
Repeat:
Let newX = x*x - y*y + a
Let newY = 2*x*y + b
Let x = newX
Let y = newY

As the loop is repeated, the point (x,y) changes. The question for the Mandelbrot set is, does (x,y) grow without bound, or is it trapped forever in a finite region of the plane? If (x,y) escapes to infinity (that is, grows without bound), then the starting point (a,b) is not in the Mandelbrot set. If (x,y) is trapped in a finite region, then (a,b) is in the Mandelbrot set. Now, it is known that if x^2 + y^2 ever becomes strictly greater than 4, then (x,y) will escape to infinity. So, if x^2 + y^2 ever becomes bigger than 4 in the above loop, we can end the loop and say that (a,b) is definitely not in the Mandelbrot set. For a point (a,b) in the Mandelbrot set, the loop will never end. When we do this on a computer, of course, we don't want to have a loop that runs forever, so we put a limit on the number of times that the loop is executed:

x = a;
y = b;
count = 0;
while ( x*x + y*y < 4.1 ) {
count++;
if (count > maxIterations)
break;
double newX = x*x - y*y + a;
double newY = 2*x*y + b;
x = newY;
y = newY;
}

After this loop ends, if count is less than or equal to maxIterations, we can say that (a,b) is definitely not in the Mandelbrot set. If count is greater than maxIterations, then (a,b) might or might not be in the Mandelbrot set, but the larger maxIterations is, the more likely that (a,b) is actually in the set.

To make a picture from this procedure, use a rectangular grid of pixels to represent some rectangle in the plane. Each pixel corresponds to some real number coordinates (a,b). (Use the coordinates of the center of the pixel.) Run the above loop for each pixel. If the count goes past maxIterations, color the pixel black; this is a point that is possibly in the Mandelbrot set. Otherwise, base the color of the pixel on the value of count after the loop ends, using different colors for different counts. In some sense, the higher the count, the closer the point is to the Mandelbrot set, so the colors give some information about points outside the set and about the shape of the set. However, it's important to understand that the colors are arbitrary and that colored points are definitely not in the set. Here is a picture that was produced by the Mandelbrot Viewer program using this computation. The black region is the Mandelbrot set (except that not all black points are known to be definitely in the set):

When you use the program, you can "zoom in" on small regions of the plane. To do so, just click-and-drag the mouse on the picture. This will draw a rectangle around part of the picture. When you release the mouse, the part of the picture inside the rectangle will be zoomed to fill the entire display. If you simply click a point in the picture, you will zoom in on the point where you click by a magnification factor of two. (Shift-click or use the right mouse button to zoom out instead of zooming in.) The interesting points are along the boundary of the Mandelbrot set. In fact, the boundary is infinitely complex. (Note that if you zoom in too far, you will exceed the capabilities of the double data type; nothing is done in the program to prevent this.)

Use the "MaxIterations" menu to increase the maximum number of iterations in the loop. Remember that black pixels might or might not be in the set; when you increase "MaxIterations," you might find that a black region becomes filled with color. The "Palette" menu determines the set of colors that are used. Different palettes give very different visualizations of the set. The "PaletteLength" menu determines how many different colors are used. In the default setting, a different color is used for each possible value of count in the algorithm. Sometimes, you can get a much better picture by using a different number of colors. If the palette length is less than maxIterations, the palette is repeated to cover all the possible values of count; if the palette length is greater than maxIterations, only part of of the palette will be used. (If the picture is of an almost uniform color, try decreasing the palette length, since that makes the color vary more quickly as count changes. If you see what look like randomly colored dots instead of bands of color, try increasing the palette length.)

The program has a "File" menu that can be used to save the picture as a PNG image file. You can also save a "param" file which simply saves the settings that produced the current picture. A param file can be read back into the program using the "Open" command.

The Mandelbrot set is named after Benoit Mandelbrot, who was the first person to note the incredible complexity of the set. It is astonishing that such complexity and beauty can arise out of such a simple algorithm.

13.5.2  Design of the Program

Most classes in Java are defined in packages. While we have used standard packages such as javax.swing and java.io extensively, almost all of my programming examples have been in the "default package," which means that they are not declared to belong to any named package. However, when doing more serious programming, it is good style to create a package to hold the classes for your program. The Oracle corporation recommends that package names should be based on an Internet domain name of the organization that produces the package. My office computer has domain name eck.hws.edu, and no other computer in the world should have the same name. According to Oracle, this allows me to use the package name edu.hws.eck, with the elements of the domain name in reverse order. I can also use sub-packages of this package, such as edu.hws.eck.mdb, which is the package name that I decided to use for my Mandelbrot Viewer application. No one else -- or at least no one else who uses the same naming convention -- will ever use the same package name, so this package name uniquely identifies my program.

I briefly discussed using packages in Subsection 2.6.6 and in the context of the programming examples in Section 12.5. Here's what you need to know for the Mandelbrot Viewer program: The program is defined in eight Java source code files. They can be found in the directory edu/hws/eck/mdb inside the source directory of the web site. (That is, they are in a directory named mdb, which is inside a directory named eck, which is inside hws, which is inside edu. The directory structure must follow the package name in this way.) The same directory also contains a file named strings.properties that is used by the program and that will be discussed below. And there is an examples folder that contains resource files used for an "Examples" menu. For an Integrated Development Environment such as Eclipse, you should just have to add the edu directory to your project. To compile the files on the command line, you must be working in the directory that contains the edu directory. Use the command

javac  edu/hws/eck/mdb/*.java

or, if you use Windows,

javac  edu\hws\eck\mdb\*.java

to compile the source code. The main routine for the program is defined by a class named Main. To run this class, use the command:

java  edu.hws.eck.mdb.Main

This command must also be given in the directory that contains the edu directory.

The work of computing and displaying images of the Mandelbrot set is done in MandelbrotDisplay.java. The MandelbrotDisplay class is a subclass of JPanel. It uses an off-screen canvas to hold a copy of the image. (See Subsection 13.1.1.) The paintComponent() method copies this image onto the panel. Then, if the user is drawing a "zoom box" with the mouse, the zoom box is drawn on top of the image. In addition to the image, the class uses a two-dimensional array to store the iteration count for each pixel in the image. If the range of xy-values changes, or if the size of the window changes, all the counts must be recomputed. Since the computation can take quite a while, it would not be acceptable to block the user interface while the computation is being performed. The solution is to do the computation in separate "worker" threads, as discussed in Chapter 12. The program uses one worker thread for each available processor. When the computation begins, the image is filled with gray. Every so often, about twice a second, the data that has been computed by the computation threads is gathered and applied to the off-screen canvas, and the part of the canvas that has been modified is copied to the screen. A Timer is used to control this process -- each time the timer fires, the image is updated with any new data that has been computed by the threads. The user can continue to use the menus and even the mouse while the image is being computed.

The file MandelbrotPanel.java defines the main panel of the Mandelbrot Viewer window. MandelbrotPanel is another subclass of JPanel. A MandelbrotPanel is mostly filled with a MandelbrotDisplay. It also adds a JLabel beneath the display. The JLabel is used as a "status bar" that shows some information that might be interesting to the user. The MandelbrotPanel also defines the program's mouse listener. In addition to handling zooming, the mouse listener puts the x and y coordinates of the current mouse location in the status bar as the user moves or drags the mouse. Also, when the mouse exits the drawing area, the text in the status bar is set to read "Idle". This is the first time that we have seen an actual use for mouseMoved and mouseExited events. (See Subsection 6.3.2 and Subsection 6.3.4.)

The menu bar for the program is defined in Menus.java. Commands in the "File" and "Control" menu are defined as Actions. (See Subsection 13.3.1.) Note that among the actions are file manipulation commands that use techniques from Subsection 11.2.3, Subsection 11.5.2, and Subsection 13.1.5. The "MaxIterations," "Palette," and "PaletteLength" menus each contain a group of JRadioButtonMenuItems. (See Subsection 13.3.3.) I have tried several approaches for handling such groups, and none of them have satisfied me completely. In this program, I have defined a nested class inside Menus to represent each group. For example, the PaletteManager class contains the menu items in the "Palette" menu as instance variables. It registers an action listener with each item, and it defines a few utility routines for operating on the menu. The classes for the three menus are very similar and should probably have been defined as subclasses of some more general class. There is an "Examples" menu that contains settings for several sample views of pieces of the Mandelbrot set.

The MandelbrotPanel that is being used in the program is a parameter to the Menus constructor. Many of the menu commands operate on this panel or on the MandelbrotDisplay that it contains. In order to carry out these commands, the Menus object needs a reference to the MandelbrotPanel. As for the MandelbrotDisplay, the panel has a method getDisplay() that returns a reference to the display that it contains. So as long as the menu bar has a reference to the panel, it can obtain a reference to the display. In previous examples, everything was written as one large class file, so all the objects were directly available to all the code. When a program is made up of multiple interacting files, getting access to the necessary objects can be more of a problem.

MandelbrotPanel, MandelbrotDisplay, and Menus are the main classes that make up the Mandelbrot Viewer program. MandelbrotFrame.java defines a simple subclass of JFrame that shows a MandelbrotPanel and its menu bar. And Main.java contains the main() routine that actually runs the program. There are a few other classes that I will discuss below.

This brief discussion of the design of the Mandelbrot Viewer has shown that it uses a wide variety of techniques that were covered earlier in this book. In the rest of this section, we'll look at a few new features of Java that were used in the program.

13.5.3  Internationalization

Internationalization refers to writing a program that is easy to adapt for running in different parts of the world. Internationalization is often referred to as I18n, where 18 is the number of letters between the "I" and the final "n" in "Internationalization." The process of adapting the program to a particular location is called localization, and the locations are called locales. Locales differ in many ways, including the type of currency used and the format used for numbers and dates, but the most obvious difference is language. Here, I will discuss how to write a program so that it can be easily translated into other languages.

The key idea is that strings that will be presented to the user should not be coded into the program source code. If they were, then a translator would have to search through the entire source code, replacing every string with its translation. Then the program would have to be recompiled. In a properly internationalized program, all the strings are stored together in one or more files that are separate from the source code, where they can easily be found and translated. And since the source code doesn't have to be modified to do the translation, no recompilation is necessary.

To implement this idea, the strings are stored in one or more properties files. A properties file is just a list of key/value pairs. For translation purposes, the values are strings that will be presented to the user; these are the strings that have to be translated. The keys are also strings, but they don't have to be translated because they will never be presented to the user. Since they won't have to be modified, the key strings can be used in the program source code. Each key uniquely identifies one of the value strings. The program can use the key string to look up the corresponding value string from the properties file. The program only needs to know the key string; the user will only see the value string. When the properties file is translated, the user of the program will see different value strings.

The format of a properties file is very simple. The key/value pairs take the form

key.string=value string

There are no spaces in the key string or before the equals sign. Periods are often used to divide words in the key string. The value string can contain spaces or any other characters. If the line ends with a backslash ("\"), the value string is continued on the next line; in this case, spaces at the beginning of that line are ignored. One unfortunate detail is that a properties file can contain only plain ASCII characters. The ASCII character set only supports the English alphabet. Nevertheless, a value string can include arbitrary UNICODE characters. Non-ASCII characters just have to be specially encoded. The JDK comes with a program, native2ascii, that can convert files that use non-ASCII characters into a form that is suitable for use as a properties file.

Suppose that the program wants to present a string to the user (as the name of a menu command, for example). The properties file would contain a key/value pair such as

menu.saveimage=Save PNG Image...

where "Save PNG Image..." is the string that will appear in the menu. The program would use the key string, "menu.saveimage", to look up the corresponding value string and would then use the value string as the text of the menu item. In Java, the look up process is supported by the ResourceBundle class, which knows how to retrieve and use properties files. Sometimes a string that is presented to the user contains substrings that are not known until the time when the program is running. A typical example is the name of a file. Suppose, for example, that the program wants to tell the user, "Sorry, the file, filename, cannot be loaded", where filename is the name of a file that was selected by the user at run time. To handle cases like this, value strings in properties files can include placeholders that will be replaced by strings to be determined by the program at run time. The placeholders take the form "{0}", "{1}", "{2}", .... For the file error example, the properties file might contain:

error.cantLoad=Sorry, the file, {0}, cannot be loaded

The program would fetch the value string for the key error.cantLoad. It would then substitute the actual file name for the placeholder, "{0}". Note that when the string is translated, the word order might be completely different. By using a placeholder for the file name, you can be sure that the file name will be put in the correct grammatical position for the language that is being used. Placeholder substitution is not handled by the ResourceBundle class, but Java has another class, MessageFormat, that makes such substitutions easy.

For the Mandelbrot Viewer program, the properties file is strings.properties. (Any properties file should have a name that ends in ".properties".) Any string that you see when you run the program comes from this file. For handling value string lookup, I wrote I18n.java. The I18n class has a static method

public static tr( String key, Object... args )

that handles the whole process. Here, key is the key string that will be looked up in strings.properties. Additional parameters, if any, will be substituted for placeholders in the value string. (Recall that the formal parameter declaration "Object..." means that there can be any number of actual parameters after key; see Subsection 7.1.2.) Typical uses would include:

String saveImageCommandText = I18n.tr( "menu.saveimage" );

String errMess = I18n.tr( "error.cantLoad" , selectedFile.getName() );

You will see function calls like this throughout the Mandelbrot Viewer source code. The I18n class is written in a general way so that it can be used in any program. As long as you provide a properties file as a resource, the only things you need to do are change the resource file name in I18n.java and put the class in your own package.

It is actually possible to provide several alternative properties files in the same program. For example, you might include French and Japanese versions of the properties file along with an English version. If the English properties file is named strings.properties, then the names for the French and Japanese versions should be strings_fr.properties and strings_ja.properties. Every language has a two-letter code, such as "fr" and "ja", that is used in constructing properties file names for that language. The program asks for the properties file using the simple name "strings". If the program is being run on a Java system in which the preferred language is French, the program will try to load "strings_fr.properties"; if that fails, it will look for "strings.properties". This means that the program will use the French properties files in a French locale; it will use the Japanese properties file in a Japanese locale; and in any other locale it will use the default properties file.

13.5.4  Events, Events, Events

We have worked extensively with mouse events, key events, and action events, but these are only a few of the event types that are used in Java. The Mandelbrot Viewer program makes use of several other types of events. It also serves as an example of the benefits of event-oriented programming.

Let's start from the following fact: The MandelbrotDisplay class knows nothing about any of the other classes that make up the program (with the single exception of one call to the internationalization method I18n.tr). Yet other classes are aware of things that are going on in the MandelbrotDisplay class. For example, when the size of the display is changed, the new size is reported in the status bar that is part of the MandelbrotPanel class. In the Menus class, certain menus are disabled when the display begins the computation of an image and are re-enabled when the computation completes. The display doesn't call methods in the MandelbrotPanel or Menus classes, so how do these classes get their information about what is going on in the display? The answer, of course, is events. The MandelbrotDisplay object emits events of various types when various things happen. The MandelbrotPanel and Menus objects set up listeners that hear those events and respond to them.

The point is that because events are used for communication, the MandelbrotDisplay class is not strongly coupled to the other classes. In fact, it can be used in other programs without any modification and without access to the other classes. The alternative to using events would be to have the display object call methods such as displaySizeChanged() or computationStarted() in the MandelbrotPanel and MandelbrotFrame objects to tell them what is going on in the display. This would be strong coupling: Any programmer who wanted to use MandelbrotDisplay would also have to use the other two classes or would have to modify the display class so that it no longer refers to the other classes. Of course, not everything can be done with events and not all strong coupling is bad: The MandelbrotPanel class refers directly to the MandelbrotDisplay class and cannot be used without it -- but since the whole purpose of a MandelbrotPanel is to hold a MandelbrotDisplay, the coupling is not a problem.

The Mandelbrot Viewer program responds to mouse events on the display. These events are generated by the display object, but the display class itself doesn't care about mouse events and doesn't do anything in response to them. Mouse events are handled by a listener in the MandelbrotPanel, which responds to them by zooming the display and by showing mouse coordinates in the status bar.

The status bar also shows the new size of the display whenever that size is changed. To handle this, events of type ComponentEvent are used. When the size of a component is changed, a ComponentEvent is generated. In the Mandelbrot Viewer program, a ComponentListener in the MandelbrotPanel class listens for size-change events in the display. When one occurs, the listener responds by showing the new size in the status bar; the display knows nothing about the status bar that shows the display's size.

Component events are also used internally in the MandelbrotDisplay class in an interesting way. When the user dynamically changes the size of the display, its size can change several times each second. Normally, a change of display size would trigger the creation of a new off-screen canvas and the start of a new asynchronous computation of the image. However, doing this is a big deal, not something I want to do several times in a second. If you try resizing the program's window, you'll notice that the image doesn't change size dynamically as the window size changes. The same image and off-screen canvas are used as long as the size is changing. Only about one-third of a second after the size has stopped changing will a new, resized image be produced. Here is how this works: The display sets up a ComponentListener to listen for resize events on itself. When a resize occurs, the listener starts a Timer that has a delay of 1/3 second. (See Subsection 6.4.1.) While this timer is running, the paintComponent() method does not resize the image; instead, it reuses the image that already exists. If the timer fires 1/3 second later, the image will be resized at that time. However, if another resize event occurs while the first timer is running, then the first timer will be stopped before it has a chance to fire, and a new timer will be started with a delay of 1/3 second. The result is that the image does not get resized until 1/3 second after the size of the window stops changing.

The Mandelbrot Viewer program also uses events of type WindowEvent, which are generated by a window when it opens or closes (among other things). Window events are used by Main.java to trigger an action that has to be taken when the program is ending; this will be discussed below.

Perhaps the most interesting use of events in the Mandelbrot Viewer program is to enable and disable menu commands based on the status of the display. For this, events of type PropertyChangeEvent are used. This event class is part of a "bean" framework that Java uses for some advanced work with objects, and class PropertyChangeEvent and related classes are defined in the package java.beans. The idea is that bean objects are defined by their "properties" (which are just aspects of the state of the bean). When a bean property changes, the bean can emit a PropertyChangeEvent to notify other objects of the change. Properties for which property change events are emitted are known technically as bound properties. A bound property has a name that identifies that particular property among all the properties of the bean. When a property change event is generated, the event object includes the name of the property that has changed, the previous value of the property, and the new value of the property.

The MandelbrotDisplay class has a bound property whose name is given by the constant MandelbrotDisplay.STATUS_PROPERTY. A display emits a property change event when its status changes. The possible values of the status property are given by other constants, such as MandelbrotDisplay.STATUS_READY. The READY status indicates that the display is not currently running a computation and is ready to do another one. There are several menu commands that should be enabled only when the status of the display is READY. To implement this, the Menus class defines a PropertyChangeListener to listen for property change events from the display. When this listener hears an event, it responds by enabling or disabling menu commands according to the new value of the status property.

All of Java's GUI components are beans and are capable of emitting property change events. In any subclass of Component, this can be done simply by calling the method

public void firePropertyChange(String propertyName,
Object oldValue, Object newValue)

For example, the MandelbrotDisplay class uses the following method for setting its current status:

private void setStatus(String status) {
if (status == this.status) {
// Note: Event should be fired only if status actually changes.
return;
}
String oldStatus = this.status;
this.status = status;
firePropertyChange(STATUS_PROPERTY, oldStatus, status);
}

When writing bean classes from scratch, you have to add support for property change events, if you need them. To make this easier, the java.beans package provides the PropertyChangeSupport class.

13.5.5  Custom Dialogs

Java has several standard dialog boxes that are defined in the classes JOptionPane, JColorChooser, and JFileChooser. These were introduced in Subsection 6.7.2 and Subsection 11.2.3. Dialogs of all these types are used in the Mandelbrot Viewer program. However, sometimes other types of dialog are needed. In such cases, you can build a custom dialog box.

Dialog boxes are defined by subclasses of the class JDialog. Like frames, dialog boxes are separate windows on the screen, and the JDialog class is very similar to the JFrame class. The big difference is that a dialog box has a parent, which is a frame or another dialog box that "owns" the dialog box. If the parent of a dialog box closes, the dialog box closes automatically. Furthermore, the dialog box might "float" on top of its parent, even when its parent is the active window.

Dialog boxes can be either modal or modeless. When a modal dialog is put up on the screen, the rest of the application is blocked until the dialog box is dismissed. This is the most common case, and all the standard dialog boxes are modal. Modeless dialog boxes are more like independent windows, since they can stay on the screen while the user interacts with other windows. There are no modeless dialogs in the Mandelbrot Viewer program.

The Mandelbrot Viewer program uses two custom dialog boxes. They are used to implement the "Set Image Size" and "Set Limits" commands and are defined by the files SetImageSizeDialog.java and SetLimitsDialog.java. The "set image size" dialog lets the user enter a new width and height for the Mandelbrot image. The "set limits" dialog lets the user input the minimum and maximum values for x and y that are shown in the image. The two dialog classes are very similar. In both classes, several JTextFields are used for user input. Two buttons named "OK" and "Cancel" are added to the window, and listeners are set up for these buttons. If the user clicks "OK", the listener checks whether the inputs in the text fields are legal; if not, an error message is displayed to the user and the dialog stays on the screen. If the input is legal when the user clicks "OK", the dialog is disposed. The dialog is also disposed if the user clicks "Cancel" or clicks the dialog box's close box. The net effect is that the dialog box stays on the screen until the user either cancels the dialog or enters legal values for the inputs and clicks "OK". The program can find out which of these occurred by calling a method named getInput() in the dialog object after showing the dialog. This method returns null if the dialog was canceled; otherwise it returns the user input.

To make my custom dialog boxes easy to use, I added a static showDialog() method to each dialog class. When this function is called, it shows the dialog, waits for it to be dismissed, and then returns the value of the getInput() method. This makes it possible to use my custom dialog boxes in much the same way as Java's standard dialog boxes are used.

Custom dialog boxes are not difficult to create and to use, if you already know about frames. I will not discuss them further here, but you can look at the source code file SetImageSizeDialog.java as a model.

13.5.6  Preferences

Most serious programs allow the user to set preferences. A preference is really just a piece of the program's state that is saved between runs of the program. In order to make preferences persistent from one run of the program to the next, the preferences could simply be saved to a file in the user's home directory. However, there would then be the problem of locating the file. There would be the problem of naming the file in a way that avoids conflicts with file names used by other programs. And there would be the problem of cluttering up the user's home directory with files that the user shouldn't even have to know about.

To deal with these problems, Java has a standard means of handling preferences. It is defined by the package java.util.prefs. In general, the only thing that you need from this package is the class named Preferences.

In the Mandelbrot Viewer program, the file Main.java has an example of using Preferences. Main.java contains the main() routine for the program.

In most programs, the user sets preferences in a custom dialog box. However, the Mandelbrot program doesn't have any preferences that are appropriate for that type of treatment. Instead, as an example, I automatically save a few aspects of the program's state as preferences. Every time the program starts up, it reads the preferences, if any are available. Every time the program terminates, it saves the preferences. (Saving the preferences poses an interesting problem because the program ends when the MandelbrotFrame window closes, not when the main() routine ends. In fact, the main() routine ends as soon as the window appears on the screen. So, it isn't possible to save the preferences at the end of the main program. The solution is to use events: A listener listens for WindowEvents from the frame. When a window-closed event is received, indicating that the program is ending, the listener saves the preferences.)

Preferences for Java programs are stored in some platform-dependent form in some platform-dependent location. As a Java programmer, you don't have to worry about it; the Java preferences system knows where to store the data. There is still the problem of identifying the preferences for one program among all the possible Java programs that might be running on a computer. Java solves this problem in the same way that it solves the package naming problem. In fact, by convention, the preferences for a program are identified by the package name of the program, with a slight change in notation. For example, the Mandelbrot Viewer program is defined in the package edu.hws.eck.mdb, and its preferences are identified by the string "/edu/hws/eck/mdb". (The periods have been changed to "/", and an extra "/" has been added at the beginning.)

The preferences for a program are stored in something called a "node." The user preferences node for a given program identifier can be accessed as follows:

Preferences root = Preferences.userRoot();
Preferences node = root.node(pathName);

where pathname is the string, such as "/edu/hws/eck/mdb", that identifies the node. The node itself consists of a simple list of key/value pairs, where both the key and the value are strings. You can store any strings you want in preferences nodes -- they are really just a way of storing some persistent data between program runs. In general, though, the key string identifies some particular preference item, and the associated value string is the value of that preference. A Preferences object, prefnode, contains methods prefnode.get(key) for retrieving the value string associated with a given key and prefnode.put(key,value) for setting the value string for a given key.

In Main.java, I use preferences to store the shape and position of the program's window. This makes the size and shape of the window persistent between runs of the program; when you run the program, the window will be right where you left it the last time you ran it. I also store the name of the directory that is currently selected in the file dialog box that is used by the program for the Save and Open commands. This is particularly satisfying, since the default behavior for a file dialog box is to start in the user's home directory, which is hardly ever the place where the user wants to keep a program's files. With the preferences feature, I can switch to the right directory the first time I use the program, and from then on I'll automatically be back in that directory when I use the program again. You can look at the source code in Main.java for the details.

And that's it.... There's a lot more that I could say about Java and about programming in general, but this book is only "An Introduction to Programming Using Java," and it's time for our journey to end. I hope that it has been a pleasant journey for you, and I hope that I have helped you establish a foundation that you can use as a basis for further exploration.

Programming Exercises for Chapter 13

Exercise 13.1:

The sample program PaintWithOffScreenCanvas.java from Section 13.1 is a simple paint program. Make two improvements to this program: First, add a "File" menu that lets the user open an image file and save the current image in either PNG or JPEG format. Second, add a basic one-level "Undo" command that lets the user undo the most recent operation that was applied to the image. (Do not try to make a multilevel Undo, which would allow the user to undo several operations.)

When you read a file into the program, you should copy the image that you read into the program's off-screen canvas. Since the canvas in the program has a fixed size, you should scale the image that you read so that it exactly fills the canvas.

See the Solution

Exercise 13.2:

For this exercise, you should continue to work on the program from the previous exercise. Add a "StrokeWidth" menu that allows the user to draw lines of varying thicknesses. Make it possible to use different colors for the interior of a filled shape and for the outline of that shape. To do this, change the "Color" menu to "StrokeColor" and add a "FillColor" menu. (My solution adds two new tools, "Stroked Filled Rectangle" and "Stroked Filled Oval", to represent filled shapes that are outlined with the current stroke.) Add support for filling shapes with transparent color. A simple approach to this is to use a JCheckboxMenuItem to select either fully opaque or 50% opaque fill. (Don't try to apply transparency to stokes -- it's very difficult to make transparency work correctly for the Curve tool, and in any case, shape outlines look better if they are opaque.) Finally, make the menus more user friendly by adding keyboard accelerators to some commands and by using JRadioButtonMenuItems where appropriate, such as in the color and tool menus. This exercise takes quite a bit of work to get it all right, so you should tackle the problem in pieces.

See the Solution

Exercise 13.3:

The StopWatchLabel component from Subsection 13.4.5 displays the text "Timing..." when the stop watch is running. It would be nice if it displayed the elapsed time since the stop watch was started. For that, you need to create a Timer. (See Subsection 6.4.1.) Add a Timer to the original source code, StopWatchLabel.java, to drive the display of the elapsed time in seconds. Create the timer in the mousePressed() routine when the stop watch is started. Stop the timer in the mousePressed() routine when the stop watch is stopped. The elapsed time won't be very accurate anyway, so just show the integral number of seconds. You only need to set the text a few times per second. For my Timer method, I use a delay of 200 milliseconds for the timer.

See the Solution

Exercise 13.4:

The custom component example MirrorText.java, from Subsection 13.4.5, uses an off-screen canvas to show mirror-reversed text in a JPanel. An alternative approach would be to draw the text after applying a transform to the graphics context that is used for drawing. (See Subsection 13.2.5.) With this approach, the custom component can be defined as a subclass of JLabel in which the paintComponent() method is overridden. Write a version of the MirrorText component that takes this approach. The solution is very short, but tricky. Note that the scale transform g2.scale(-1,1) does a left-right reflection through the left edge of the component.

See the Solution

Exercise 13.5:

The sample program PhoneDirectoryFileDemo.java from Subsection 11.3.2 keeps data for a "phone directory" in a file in the user's home directory. Exercise 11.5 asked you to revise that program to use an XML format for the data. Both programs have a simple command-line user interface. For this exercise, you should provide a GUI interface for the phone directory data. You can base your program either on the original sample program or on the modified XML version from the exercise. Use a JTable to hold the data. The user should be able to edit all the entries in the table. Also, the user should be able to add and delete rows. Include either buttons or menu commands that can be used to perform these actions. The delete command should delete the selected row, if any. New rows should be added at the end of the table. For this program, you can use a standard DefaultTableModel.

Your program should load data from the file when it starts and save data to the file when it ends, just as the two previous programs do. For a GUI program, you can't simply save the data at the end of the main() routine, since main() terminates as soon as the window shows up on the screen. You want to save the data when the user closes the window and ends the program. There are several approaches. One is to use a WindowListener to detect the event that occurs when the window closes. Another is to use a "Quit" command to end the program; when the user quits, you can save the data and close the window (by calling its dispose() method), and end the program. If you use the "Quit" command approach, you don't want the user to be able to end the program simply by closing the window. To accomplish this, you should call

frame.setDefaultCloseOperation(JFrame.DO_NOTHING_ON_CLOSE);

where frame refers to the JFrame that you have created for the program's user interface. When using a WindowListener, you want the close box on the window to close the window, not end the program. For this, you need

frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);

When the listener is notified of a window closed event, it can save the data and end the program.

Most of the JTable and DefaultTableModel methods that you need for this exercise are discussed in Subsection 13.4.3, but there are a few more that you need to know about. To determine which row is selected in a JTable, call table.getSelectedRow(). This method returns the row number of the selected row, or returns -1 if no row is selected. To specify which cell is currently being edited, you can use:

table.setRowSelectionInterval(rowNum, rowNum);  // Selects row number rowNum.
table.editCellAt( rowNum, colNum ); // Edit cell at position (rowNum,colNum).
phoneTable.getEditorComponent().requestFocus();  // Put input cursor in cell.

One particularly troublesome point is that the data that is in the cell that is currently being edited is not in the table model. The value in the edit cell is not put into the table model until after the editing is finished. This means that even though the user sees the data in the cell, it's not really part of the table data yet. If you lose that data, the user would be justified in complaining. To make sure that you get the right data when you save the data at the end of the program, you have to turn off editing before retrieving the data from the model. This can be done with the following method:

private void stopEditing() {
if (table.getCellEditor() != null)
table.getCellEditor().stopCellEditing();
}

This method must also be called before modifying the table by adding or deleting rows; if such modifications are made while editing is in progress, the effect can be very strange.

See the Solution

Quiz on Chapter 13

Question 1:

Describe the object that is created by the following statement, and give an example of how it might be used in a program:

BufferedImage OSC = new BufferedImage(32,32,BufferedImage.TYPE_INT_RGB);

Question 2:

Many programs depend on resource files. What is meant by a resource in this sense? Give an example.

Question 3:

What is the FontMetrics class used for?

Question 4:

If a Color, c, is created as c = new Color(0,0,255,125), what is the effect of drawing with this color?

Question 5:

What is antialiasing?

Question 6:

How is the ButtonGroup class used?

Question 7:

What does the acronym MVC stand for, and how does it apply to the JTable class?

Question 8:

Describe the picture that is produced by the following paintComponent() method:

public void paintComponent(Graphics g) {
super.paintComponent(g);
Graphics2D g2 = (Graphics2D)g;
g2.translate( getWidth()/2, getHeight()/2 );
g2.rotate( 30 * Math.PI / 180 );
g2.fillRect(0,0,100,100);
}

Question 9:

What is meant by Internationalization of a program?

Question 10:

Suppose that the class that you are writing has an instance method doOpen() (with no parameters) that is meant to be used to open a file selected by the user. Write a code segment that creates an Action that represents the action of opening a file. Then show how to create a button and a menu item from that action.

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Glossary

abstract class.  A class that cannot be used to create objects, and that exists only for the purpose of creating subclasses. Abstract classes in Java are defined using the modifier abstract.

abstract data type (ADT).  A data type for which the possible values of the type and the permissible operations on those values are specified, without specifying how the values and operations are to be implemented.

access specifier.  A modifier used on a method definition or variable specification that determines what classes can use that method or variable. The access specifiers in Java are public, protected, and private. A method or variable that has no access specifier is said to have "package" visibility.

activation record.  A data structure that contains all the information necessary to implement a subroutine call, including the values of parameters and local variables of the subroutine and the return address to which the computer will return when the subroutine ends. Activation records are stored on a stack, which makes it possible for several subroutine calls to be active at the same time. This is particularly important for recursion, where several calls to the same subroutine can be active at the same time.

actual parameter.  A parameter in a subroutine call statement, whose value will be passed to the subroutine when the call statement is executed. Actual parameters are also called "arguments".

address.  Each location in the computer's memory has an address, which is a number that identifies that location. Locations in memory are numbered sequentially. In modern computers, each byte of memory has its own address. Addresses are used when information is being stored into or retrieved from memory.

algorithm.  An unambiguous, step-by-step procedure for performing some task, which is guaranteed to terminate after a finite number of steps.

alpha color component.  A component of a color that says how transparent or opaque that color is. The higher the alpha component, the more opaque the color.

API.  Application Programming Interface. A specification of the interface to a software package or "toolbox." The API says what classes or subroutines are provided in the toolbox and how to use them.

applet.  A type of Java program that is meant to run on a Web page in a Web browser, as opposed to a stand-alone application.

animation.  An apparently moving picture created by rapidly showing a sequence of still images, called frames, one after the other. In Java, animations are often driven by Timer objects; a new frame of the animation is shown each time the timer fires.

antialiasing.  Adjusting the color of pixels to reduce the "jagged" effect that can occur when shapes and text are represented by pixels. For antialiased drawing, when the shape covers only part of a pixel, the color of the shape is blended with the previous color of the pixel. The degree of blending depends on how much of the pixel is covered.

array.  A list of items, sequentially numbered. Each item in the list can be identified by its index, that is, its sequence number. In Java, all the items in array must have the same type, called the base type of the array. An array is a random access data structure; that is, you can get directly at any item in the array at any time.

array type.  A data type whose possible values are arrays. If Type is the name of a type, then Type[] is the array type for arrays that have base type Type.

assignment statement.  A statement in a computer program that retrieves or computes a value and stores that value in a variable. An assignment statement in Java has the form: variable-name = expression;

asynchronous event.  An event that can occur at an unpredictable time, outside the control of a computer program. User input events, such as pressing a button on the mouse, are asynchronous.

ASCII.  American Standard Code for Information Interchange. A way of encoding characters using 7 bits for characters. ASCII code only supports 128 characters, with no accented letters, non-English alphabets, special symbols, or ideograms for non-alphabetic languages such as Chinese. Java uses the much larger and more complete Unicode code for characters.

base case.  In a recursive algorithm, a simple case that is handled directly rather than by applying the algorithm recursively.

binary number.  A number encoded as a sequence of zeros and ones. A binary number is represented in the "base 2" in the same way that ordinary numbers are represented in the "base 10."

binary tree.  A linked data structure that is either empty or consists of a root node that contains pointers to two smaller (possibly empty) binary trees. The two smaller binary trees are called the left subtree and the right subtree.

bit.  A single-digit binary number, which can be either 0 or 1.

black box.  A system or component of a system that can be used without understanding what goes on inside the box. A black box has an interface and an implementation. A black box that is meant to be used as a component in a system is called a module.

block.  In Java programming, a sequence of statements enclosed between a pair of braces, { and }. Blocks are used to group several statements into a single statement. A block can also be empty, meaning that it contains no statements at all and consists of just an empty pair of braces.

blocking operation.  An operation, such as reading data from a network connection, is said to "block" if it has to wait for some event to occur. A thread that performs a blocking operation can be "blocked" until the required event occurs. A thread cannot execute any instructions while it is blocked. Other threads in the same program, however, can continue to run.

blocking queue.  A queue in which the dequeue operation will block if the queue is empty, until an item is added to the queue. If the blocking queue has a limited capacity, the enqueue operation can also block, if the queue is full.

bottom-up design.  An approach to software design in which you start by designing basic components of the system, then combine them into more complex components, and so on.

BufferedImage.  A class representing "off-screen canvases," that is, images that are stored in the computer's memory and that can be used for drawing images off-screen.

branch.  A control structure that allows the computer to choose among two or more different courses of action. Java has two branch statements: if statements and switch statements.

byte.  A unit of memory that consists of eight bits. One byte of memory can hold an eight-bit binary number.

bytecode.  "Java bytecode" is the usual name for the machine language of the Java Virtual Machine. Java programs are compiled into Java bytecode, which can then be executed by the JVM.

charset.  A particular encoding of character data into binary form. Examples include UTF-8 and ISO-8859-1.

checked exception.  An exception in Java that must be handled, either by a try..catch statement or by a throws clause on the method that can throw he exception. Failure to handle a checked exception in one way or the other is a syntax error.

class.  The basic unit of programming in Java. A class is a collection of static and non-static methods and variables. Static members of a class are part of the class itself; non-static, or "instance," members constitute a blueprint for creating objects, which are then said to "belong" to the class.

class variables and class methods.  Alternative terms for "static variables" and "static methods", which are part of the class itself rather than of objects.

client/server.  A model of network communication in which a "server" waits at a known address on the network for connection requests that are sent to the server by "clients." This is the basic model for communication using the TCP/IP protocol.

command-line interface.  A way of interacting with the computer in which the user types in commands to the computer and the computer responds to each command.

comment.  In a computer program, text that is ignored by the computer. Comments are for human readers, to help them understand the program.

compiler.  A computer program that translates programs written in some computer language (generally a high-level language) into programs written in machine language.

component.  General term for a visual element of a GUI, such as a window, button, or menu. A component is represented in Java by an object belonging to a subclass of the class java.awt.Component.

constructor.  A special kind of subroutine in a class whose purpose is to construct objects belonging to that class. A constructor is called using the new operator, and is not considered to be a "method."

container.  A component, such as a JPanel, that can contain other GUI components. Containers have add() methods that can be used to add components.

contract of a method.  The semantic component of the method's interface. The contract specifies the responsibilities of the method and of the caller of the method. It says how to use the method correctly and specifies the task that the method will perform when it is used correctly. The contract of a method should be fully specified by its Javadoc comment.

control structure.  A program structure such as an if statement or a while loop that affects the flow of control in a program (that is, the order in which the instructions in the program are executed).

CPU.  Central Processing Unit. The CPU is the part of the program that actually performs calculations and carries out programs.

data structure.  An organized collection of data, that can be treated as a unit in a program.

deadlock.  A situation in which several threads hang indefinitely, for example because each of them is waiting for some resource that is locked by one of the other threads.

default method.  A method in a Java 8 interface that has an implementation. The default implementation is used in any class that implements the interface but does not override the method. Default methods are marked with the reserved word default. Not supported in Java 7.

default package.  The unnamed package. A class that does not declare itself to be in a named package is considered to be in the default package.

definite assignment.  Occurs at a particular point in a program if it is definitely true that a given variable must have been assigned a value before that point in the program. It is only legal to use the value of a local variable if that variable has "definitely" been assigned a value before it is used. For this to be true, the compiler must be able to verify that every path through the program from the declaration of the variable to its use must pass through a statement that assigns a value to that variable.

deprecated.  Considered to be obsolete, but still available for backwards compatibility. A deprecated Java class or method is still part of the Java language, but it is not advisable to use it in new code. Deprecated items might be removed in future versions of Java.

dialog box.  A window that is dependent on another window, called its parent. Dialog boxes are usually popped up to get information from the user or to display a message to the user. Dialog boxes in the Swing API are represented by objects of type JDialog.

distributed computing.  A kind of parallel processing in which several computers, connected by a network, work together to solve a problem.

dummy parameter.  Identifier that is used in a subroutine definition to stand for the value of an actual parameter that will be passed to the subroutine when the subroutine is called. Dummy parameters are also called "formal parameters" (or sometimes just "parameters," when the term "argument" is used instead of actual parameter).

enum.  Enumerated type. A type that is defined by listing every possible value of that type. An enum type in Java is a class, and the possible values of the type are objects.

event.  In GUI programming, something that happens outside the control of the program, such as a mouse click, and that the program must respond to when it occurs.

exception.  An error or exceptional condition that is outside the normal flow of control of a program. In Java, an exception can be represented by an object of type Throwable that can be caught and handled in a try..catch statement.

fetch-and-execute cycle.  The process by which the CPU executes machine language programs. It fetches (that is, reads) an instruction from memory and carries out (that is, executes) the instruction, and it repeats this over and over in a continuous cycle.

flag.  A boolean value that is set to true to indicate that some condition or event is true. A single bit in a binary number can also be used as a flag.

formal parameter.  Another term for "dummy parameter."

frame.  One of the images that make up an animation. Also used as another name for activation record.

function.  A subroutine that returns a value.

garbage collection.  The automatic process of reclaiming memory that is occupied by objects that can no longer be accessed.

generic programming.  Writing code that will work with various types of data, rather than with just a single type of data. The Java Collection Framework, and classes that use similar techniques, are examples of generic programming in Java.

getter.  An instance method in a class that is used to read the value of some property of that class. Usually the property is just the value of some instance variable. By convention, a getter is named getXyz() where xyz is the name of the property.

global variable.  Another name for member variable, emphasizing the fact that a member variable in a class exists outside the methods of that class.

graphics context.  The data and methods necessary for drawing to some particular destination. A graphics context in Java is an object belonging to the Graphics class.

GUI.  Graphical User Interface. The modern way of interacting with a computer, in which the computer displays interface components such as buttons and menus on a screen and the user interacts with them -- for example by clicking on them with a mouse.

hash table.  A data structure optimized for efficient search, insertion, and deletion of objects. A hash table consists of an array of locations, and the location in which an object is stored is determined by that object's "hash code," an integer that can be efficiently computed from the contents of the object.

heap.  The section of the computer's memory in which objects are stored.

high level language.  A programming language, such as Java, that is convenient for human programmers but that has to be translated into machine language before it can be executed.

HSB.  A color system in which colors are specified by three numbers (in Java, real numbers in the range 0.0 to 1.0) giving the hue, saturation, and brightness.

IDE.  Integrated Development Environment. A programming environment with a graphical user interface that integrates tools for creating, compiling, and executing programs.

identifier.  A sequence of characters that can be used as a name in a program. Identifiers are used as names of variables, methods, and classes.

index.  The position number of one item in an array.

implementation.  The inside of a black box, such as the code that defines a subroutine.

immutable object.  An immutable object cannot be modified after it is constructed, because all of its instance variables are final.

infinite loop.  A loop that never ends, because its continuation condition always evaluates to true.

inheritence.  The fact that one class can extend another. It then inherits the data and behavior of the class that it extends.

instance of a class.  An object that belongs to that class (or a subclass of that class). An object belongs to a class in this sense when the class is used as a template for the object when the object is created by a constructor defined in that class.

instance method.  A non-static method in a class and hence a method in any object that is an instance of that class.

instance variable.  A non-static variable in a class and hence a variable in any object that is an instance of that class.

interface.  As a general term, how to use a black box such as a subroutine. Knowing the interface tells you nothing about what goes on inside the box. "Interface" is also a reserved word in Java; in this sense, an interface is a type that specifies one or more abstract methods. An object that implements the interface must provide definitions for those methods.

interpreter.  A computer program that executes program written in some computer language by reading instructions from the program, one-by-one, and carrying each one out (by translating it into equivalent machine language instructions).

I/O.  Input/Output, the way a computer program communicates with the rest of the world, such as by displaying data to the user, getting information from the user, reading and writing files, and sending and receiving data over a network.

iterator.  An object associated with a collection, such a list or a set, that can be used to traverse that collection. The iterator will visit each member of the collection in turn.

Java Collection Framework (JCF).  A set of standard classed that implement generic data structures, including ArrayList and TreeSet, for example.

JavaFX.  The new toolkit for GUI applications, which becomes the preferred approach to GUI programming in Java 8. JavaFX is not covered in this book.

JDK.  Java Development Kit. Basic software that supports both compiling and running Java programs. A JDK includes a command-line programming environment as well as a JRE. You need a JDK if you want to compile Java source code, as well as executing pre-compiled programs.

JRE.  Java Runtime Environment. Basic software that supports running standard Java programs that have already been compiled. A JRE includes a Java Virtual Machine and all the standard Java classes.

just-in-time compiler.  A kind of combination interpreter/compiler that compiles parts of a program as it interprets them. This allows subsequent executions of the same parts of the program to be executed more quickly than they were the first time. This can result is greatly increased speed of execution. Modern JVMs use a just-in-time compiler.

JVM.  Java Virtual Machine. The imaginary computer whose machine language is Java bytecode. Also used to refer to computer programs that act as interpreters for programs written in bytecode; to run Java programs on your computer, you need a JVM.

layout manager.  An object whose function is to lay out the components in a container, that is, to set their sizes and locations. Different types of layout managers implement different policies for laying out components.

linked data structure.  A collection of data consisting of a number of objects that are linked together by pointers which are stored in instance variables of the objects. Examples include linked lists and binary trees.

linked list.  A linked data structure in which nodes are linked together by pointers into a linear chain.

listener.  In GUI programming, an object that can be registered to be notified when events of some given type occur. The object is said to "listen" for the events.

literal.  A sequence of characters that is typed in a program to represent a constant value. For example, 'A' is a literal that represents the constant char value, A, when it appears in a Java program.

location (in memory).  The computer's memory is made up of a sequence of locations. These locations are sequentially numbered, and the number that identifies a particular location is called the address of that location.

local variable.  A variable declared within a method, for use only inside that method. A variable declared inside a block is valid from the point where it is declared until the end of block in which the declaration occurs.

loop.  A control structure that allows a sequence of instructions to be executed repeatedly. Java has three kinds of loops: for loops, while loops, and do loops

loop control variable.  A variable in a for loop whose value is modified as the loop is executed and is checked to determine whether or not to end the loop.

machine language.  A programming language consisting of instructions that can be executed directed by a computer. Instructions in machine language are encoded as binary numbers. Each type of computer has its own machine language. Programs written in other languages must be translated into a computer's machine language before they can be executed by that computer.

main memory.  Programs and data can be stored in a computer's main memory, where they are available to the CPU. Other forms of memory, such as a disk drive, also store information, but only main memory is directly accessible to the CPU. Programs and data from a disk drive have to be copied into main memory before they can be used by the CPU.

map.  An associative array; a data structure that associates an object from some collection to each object in some set. In Java, maps are represented by the generic interface Map<T,S>

member variable.  A variable defined in a class but not inside a method, as opposed to a local variable, which is defined inside some method.

memory.  Memory in a computer is used to hold programs and data.

method.  Another term for subroutine, used in the context of object-oriented programming. A method is a subroutine that is contained in a class or in an object.

module.  A component of a larger system that interacts with the rest of the system in a simple, well-defined, straightforward manner.

multitasking.  Performing multiple tasks at once, either by switching rapidly back and forth from one task to another or by literally working on multiple tasks at the same time.

multiprocessing.  Multitasking in which more than one processor is used, so that multiple tasks can literally be worked on at the same time.

mutual exclusion.  Prevents two threads from accessing the same resource at the same time. In Java, this only applies to threads that access the resource in synchronized methods or synchronized statements. Mutual exclusion can prevent race conditions but introduces the possibility of deadlock.

MVC pattern.  The Model/View/Controller pattern, a strategy for dividing responsibility in a GUI component. The model is the data for the component. The view is the visual presentation of the component on the screen. The controller is responsible for reacting to events by changing the model. According to the MVC pattern, these responsibilities should be handled by different objects.

NaN.  Not a Number. Double.NaN is a special value of type double that represents an undefined or illegal value.

node.  Common term for one of the objects in a linked data structure.

null.  A special pointer value that means "not pointing to anything."

numerical analysis.  The field that studies algorithms that use approximations, such as real numbers, and the errors that can result from such approximation.

off-by-one error.  A common type of error in which one too few or one too many items are processed, often because counting is not being handled correctly or because the processing stops too soon or continues too long for some other reason.

object.  An entity in a computer program that can have data (variables) and behaviors (methods). An object in Java must be created using some class as a template. The class of an object determines what variables and methods it contains.

object type.  A type whose values are objects, as opposed to primitive types. Classes and interfaces are object types.

OOP.  Object-Oriented Programming. An approach to the design and implementation of computer programs in which classes and objects are created to represent concepts and entities and their interactions.

operating system.  The basic software that is always running on a computer, without which it would not be able to function. Examples include Linux, Mac OS, and Windows Vista.

operator.  A symbol such as "+", "<=", or "++" that represents an operation that can be applied to one or more values in an expression.

overloading (of operators).  The fact that the same operator can be used with different types of data. For example, the "+" operator can be applied to both numbers and strings.

overloading (of method names).  The fact that several methods that are defined in the same class can have the same name, as long as they have different signatures.

overriding.  Redefining in a subclass. When a subclass provides a new definition of a method that is inherited from a superclass, the new definition is said to override the original definition.

package.  In Java, a named collection of related classes and sub-packages, such as java.awt and javax.swing.

parallel processing.  When several tasks are being performed simultaneously, either by multiple processors or by one processor that switches back and forth among the tasks.

parameter.  Used to provide information to a subroutine when that subroutine is called. Values of "actual parameters" in the subroutine call statement are assigned to the "dummy parameters" in the subroutine definition before the code in the subroutine is executed.

parameterized type.  A type such as ArrayList<String> that includes one or more type parameters (String in the example).

parsing.  Determining the syntactical structure of a string in some language. To parse a string is to determine whether the string is legal according to the grammar of the language, and if so, how it can be created using the rules of the grammar.

partially full array.  An array that is used to store varying numbers of items. A partially full array can be represented as a normal array plus a counter to keep track of how many items are actually stored.

pixel.  A "picture element" on the screen or in an image. A picture consists of rows and columns of pixels. The color of each pixel can be individually set.

polymorphism.  The fact that the meaning of a call to an instance method can depend on the actual type of the object that is used to make the call at run time. That is, if var is a variable of object type, then the method that is called by a statement such as var.action() depends on the type of the object to which var refers when the statement is executed at run time, not on the type of variable var.

pointer.  A value that represents an address in the computer's memory, and hence can be thought of as "pointing" to the location that has that address. A variable in Java can never hold an object; it can only hold a pointer to the location where the object is stored. A pointer is also called a "reference."

pragmatics.  Rules of thumb that describe what it means to write a good program. For example, style rules and guidelines about how to structure a program are part of the pragmatics of a programming language.

precedence.  The precedence of operators determines the order in which they are applied, when several operators occur in an expression, in the absence of parentheses.

precondition.  A condition that must be true at some point in the execution of a program, in order for the program to proceed correctly from that point. A precondition of a subroutine is something that must be true when the subroutine is called, in order for the subroutine to function properly. Subroutine preconditions are often restrictions on the values of the actual parameters that can be passed into the subroutine.

priority queue.  A data structure representing a collection of items where each item has a "priority." A priority queue has operations add and remove. Items can be added in any order, but the remove operation always removes an item of minimal priority. (Some version of priority queue use maximum instead of minimum priority.)

postcondition.  A condition that is known to be true at some point in the execution of a program, as a result of the computation that has come before that point. A postcondition of a subroutine is something that must be true after the subroutine finishes its execution. A postcondition of a function often describe the return value of the function.

primitive type.  One of the eight basic built-in data types in Java, double, float, long, int, short, byte, boolean, and char. A variable of primitive type holds an actual value, as opposed to a pointer to that value.

priority of a thread.  An integer associated with a thread that can affect the order in which threads are executed. A thread with greater priority is executed in preference to a thread with lower priority.

producer/consumer.  A classic pattern in parallel programming in which one or more producers produce items that are consumed by one or more consumers, and the producers and consumers are meant to run in parallel. The problem is to get items safely and efficiently from the producers to the consumers. In Java, the producer/consumer pattern is implemented by blocking queues.

program.  A set of instructions to be carried out by a computer, written in an appropriate programming language. Used as a verb, it means to create such a set of instructions.

programming language.  A language that can be used to write programs for a computer. Programming languages range in complexity from machine language to high-level languages such as Java.

protocol.  A specification of what constitutes legal communication in a give context. A protocol specifies the format of legal messages, when they can be sent, what kind of reply is expected, and so on.

pseudocode.  Informal specification of algorithms, expressed in language that is closer to English than an actual programming language, and usually without filling in every detail of the procedure.

queue.  A data structure consisting of a list of items, where items can only be added at one end and removed at the opposite end of the list.

race condition.  A source of possible errors in parallel programming, where one thread can cause an error in another thread by changing some aspect of the state of the program that the second thread is depending on (such as the value of variable).

RAM.  Random Access Memory. This term is often used as a synonym for the main memory of a computer. Technically, however, it means memory in which all locations are equally accessible at any given time. The term also implies that data can be written to the memory as well as read from it.

recursion.  Defining something in terms of itself. In particular, a recursive subroutine is one that calls itself, either directly, or indirectly through a chain of other subroutines. Recursive algorithms work by reducing a complex problem into smaller problems which can be solved either directly or by applying the same algorithm "recursively."

RGB.  A color system in which colors are specified by three numbers (in Java, integers in the range 0 to 255) giving the red, green, and blue components of the color.

reference.  Another term for "pointer."

return type of a function.  The type of value that is returned by that function.

reserved word.  A sequence of characters that looks like an identifier but can't be used as an identifier because it has a special meaning in the language. For example, class, public, and if are reserved words in Java.

resource.  An image, sound, text, or other data file that is part of a program. Resource files for Java programs are stored on the same class path where the compiled class files for the program are stored.

robust program.  A program is robust if it is not only correct, but also is capable of handling errors such as a non-existent file or a failed network connection in a reasonable way.

set.  A collection of objects which contains no duplicates. In Java, sets are represented by the generic interface Set<T>

scope.  The region in a program where the declaration of an identifier is valid.

semantics.  Meaning. The semantics rules of a language determine the meaning of strings of symbols (such as sentences or statements) in that language.

sentinel value.  A special value that marks the end of a sequence of data values, to indicate the end of the data.

setter.  An instance method in a class that is used to set the value of some property of that class. Usually the property is just the value of some instance variable. By convention, a setter is named setXyz() where xyz is the name of the property.

signature of a method.  The name of the method, the number of formal parameters in its definition, and the type of each formal parameter. Method signatures are the information needed by a compiler to tell which method is being called by a given subroutine call statement.

socket.  An abstraction representing one end of a connection between two computers on a network. A socket represents a logical connection between computer programs, not a physical connection between computers.

stack.  A data structure consisting of a list of items where items can only be added and removed at one end of the list, which is known as the "top" of the stack. Adding an item to a stack is called "pushing," and removing an item is called "popping." The term stack also refers to the stack of activation records that is used to implement subroutine calls.

standard input.  The standard source from which a program reads input data. It is represented by the object System.in. Usually, standard input comes from text typed by the user, but standard input can be "redirected" to read from another source, such as a file, instead.

standard output.  The standard destination to which a program writes output text. It is represented by the object System.out. Usually, standard output is displayed to the user, but standard output can be "redirected" to write to another destination, such as a file, instead. There is also an object System.err that is meant for writing error messages.

state machine.  A model of computation where an abstract "machine" can be in any of some finite set of different states. The behavior of the machine depends on its state, and the state can change in response to inputs or events. The basic logical structure of a GUI program can often be represented as a state machine.

step-wise refinement.  A technique for developing an algorithm by starting with a general outline of the procedure, often expressed in pseudocode, and then gradually filling in the details.

stream.  An abstraction representing a source of input data or a destination for output data. Java has four basic stream classes representing input and output of character and binary data. These classes form the foundation for Java's input/output API.

source code.  Text written in a high-level programming language, which must be translated into a machine language such as Java bytecode before it can be executed by a computer.

subclass.  A class that extends another class, directly or indirectly, and therefore inherits its data and behaviors. The first class is said to be a subclass of the second.

subroutine.  A sequence of program instructions that have been grouped together and given a name. The name can then be used to "call" the subroutine. Subroutines are also called methods in the context of object-oriented programming.

subroutine call statement.  A statement in a program that calls a subroutine. When a subroutine call statement is executed, the computer executes the code that is inside the subroutine.

super.  A special variable, automatically defined in any instance method, that refers to the object that contains the method, but considered as belonging to the superclass of the class in which the method definition occurs. super gives access to members of the superclass that are hidden by members of the same name in the subclass.

syntax.  Grammar. The syntax rules of a language determine what strings of symbols are legal -- that is, grammatical -- in that language.

TCP/IP.  Protocols that are used for network communication on the Internet.

this.  A special variable, automatically defined in any instance method, that refers to the object that contains the method.

thread.  An abstraction representing a sequence of instructions to be executed one after the other. It is possible for a computer to execute several threads in parallel.

thread pool.  A collection of "worker threads" that are available to perform tasks. As tasks become available, they are assigned to threads in the pool. A thread pool is often used with a blocking queue that holds the tasks.

top-down design.  An approach to software design in which you start with the problems, as a whole, subdivide it into smaller problems, divide those into even smaller problems, and so on, until you get to problems that can be solved directly.

type.  Specifies some specific kind of data values. For example, the type int specifies integer data values that can be represented as 32-bit binary numbers. In Java, a type can be a primitive type, a class names, or an interface name. Type names are used to specify the types of variables, of dummy parameters in subroutines, and of return values of subroutines.

type cast.  Forces the conversion of a value of one type into another type. For example, in (int)(6*Math.random()), the (int) is a type-cast operation that converts the double value (6*Math.random()) into an integer by discarding the fractional part of the real number.

Unicode.  A way of encoding characters as binary numbers. The Unicode character set includes characters used in many languages, not just English. Unicode is the character set that is used internally by Java.

URL.  Universal Resource Locator; an address for a resource on the Internet, such as a web page.

variable.  A named memory location (or sequence of locations) that can be used to store data. A variable is created in a program, and a name is assigned to the variable, in a variable declaration statement. The name can then be used in that program to refer to the memory location, or to the data stored in that memory location, depending on context. In Java, a variable has a type, which specifies what kind of data it can hold.

wrapper class.  A class such as Double or Integer that makes it possible to "wrap" a primitive type value in an object belonging to the wrapper class. This allows primitive type values to be used in contexts were objects are required, such as with the Java Collection Framework.

XML.  eXtensible Markup Language. A very common and well-supported standard syntax for creating text-based data-representation languages.

Answers to End-Of-Chapter Quizzes

This section contains sample answer to the quizzes that occur at the end of each chapter.

Remember that, in many cases, many answers are possible. This is especially true for questions that ask you to write code!

Answers for Quiz on Chapter 1

Question 1:

One of the components of a computer is its CPU. What is a CPU and what role does it play in a computer?

Answer:

The CPU, or Central Processing Unit, is the active part of the computer. Its function is to execute programs that are coded in machine language and stored in the main memory (RAM) of the computer. It does this by repeating the fetch-and-execute cycle over and over; that is, it repeatedly fetches a machine language instruction from memory and executes it.

Question 2:

Explain what is meant by an "asynchronous event." Give some examples.

Answer:

An asynchronous event is one that occurs at an unpredictable time outside the control of the program that the CPU is running. It is not "synchronized" with the program. An example would be when the user presses a key on the keyboard or clicks the mouse button. (These events generate "interrupts" that cause the CPU to interrupt what it is doing and to take some action to handle the asynchronous event. After handling the event, the CPU returns to what it was doing before it was interrupted.)

Question 3:

What is the difference between a "compiler" and an "interpreter"?

Answer:

Compilers and interpreters have similar functions: They take a program written in some programming language and translate it into machine language. A compiler does the translation all at once. It produces a complete machine language program that can then be executed. An interpreter, on the other hand, just translates one instruction at a time, and then executes that instruction immediately. (Java uses a compiler to translate java programs into Java Bytecode, which is a machine language for the imaginary Java Virtual Machine. Java Bytecode programs are then executed by an interpreter.)

Question 4:

Explain the difference between high-level languages and machine language.

Answer:

Programs written in the machine language of a given type of computer can be directly executed by the CPU of that type of computer. High-level language programs must be translated into machine language before they can be executed. (Machine language instructions are encoded as binary numbers that are meant to be used by a machine, not read or written by people. High-level languages use a syntax that is closer to human language.)

Question 5:

If you have the source code for a Java program, and you want to run that program, you will need both a compiler and an interpreter. What does the Java compiler do, and what does the Java interpreter do?

Answer:

The Java compiler translates Java programs into a language called Java bytecode. Although bytecode is similar to machine language, it is not the machine language of any actual computer. A Java interpreter is used to run the compiled Java bytecode program. (Each type of computer needs its own Java bytecode interpreter, but all these interpreters interpret the same bytecode language.)

Question 6:

What is a subroutine?

Answer:

A subroutine is a set of instructions for performing some task that have been grouped together and given a name. Later, when that task needs to be performed, it is only necessary to call the subroutine by giving its name, rather than repeating the whole sequence of instructions.

Question 7:

Java is an object-oriented programming language. What is an object?

Answer:

An object consists of some data together with a set of subroutines that manipulate that data. (An object is a kind of "module," or self-contained entity that communicates with the rest of the world through a well-defined interface. An object should represent some coherent concept or real-world object.)

Question 8:

What is a variable? (There are four different ideas associated with variables in Java. Try to mention all four aspects in your answer. Hint: One of the aspects is the variable's name.)

Answer:

A variable is a memory location that has been given a name so that it can easily be referred to in a program. The variable holds a value, which must be of some specified type. The value can be changed during the course of the execution of the program.

Question 9:

Java is a "platform-independent language." What does this mean?

Answer:

A Java program can be compiled once into a Java Bytecode program. The compiled program can then be run on any computer that has an interpreter for the Java virtual machine. Other languages have to be re-compiled for each platform on which they are going to run. The point about Java is that it can be executed on many different types of computers without being recompiled.

Question 10:

What is the "Internet"? Give some examples of how it is used. (What kind of services does it provide?)

Answer:

The Internet is a network connecting millions of computers around the world. Computers connected to the Internet can communicate with each other. The Internet can be used for Email (which lets a user of one computer send a message to a user on another computer), file sharing (which is used to copy files between computers), and the World Wide Web (which lets a user view "pages" of information published on computers around the world).

Answers for Quiz on Chapter 2

Question 1:

Briefly explain what is meant by the syntax and the semantics of a programming language. Give an example to illustrate the difference between a syntax error and a semantics error.

Answer:

The syntax of a language is its grammar, and the semantics is its meaning. A program with a syntax error cannot be compiled. A program with a semantic error can be compiled and run, but gives an incorrect result. A missing semicolon in a program is an example of a syntax error, because the compiler will find the error and report it. If N is an integer variable, then the statement "frac = 1/N;" is probably an error of semantics. The value of 1/N will be 0 for any N greater than 1. It's likely that the programmer meant to say 1.0/N.

Question 2:

What does the computer do when it executes a variable declaration statement. Give an example.

Answer:

A variable is a "box", or location, in the computer's memory that has a name. The box holds a value of some specified type. A variable declaration statement is a statement such as

int x;

which creates the variable x. When the computer executes a variable declaration, it creates the box in memory and associates a name (in this case, x) with that box. Later in the program, that variable can be referred to by name.

Question 3:

What is a type, as this term relates to programming?

Answer:

A "type" represents a set of possible values. When you specify that a variable has a certain type, you are saying what values it can hold. When you say that an expression is of a certain type, you are saying what values the expression can have. For example, to say that a variable is of type int says that integer values in a certain range can be stored in that variable.

Question 4:

One of the primitive types in Java is boolean. What is the boolean type? Where are boolean values used? What are its possible values?

Answer:

The only values of type boolean are true and false. Expressions of type boolean are used in places where true/false values are expected, such as the conditions in while loops and if statements.

Question 5:

Give the meaning of each of the following Java operators:

a)    ++

b)    &&

c)    !=

Answer:

The operator ++ is used to add 1 to the value of a variable. For example, "count++" has the same effect as "count = count + 1".

The operator && represents the word and. It can be used to combine two boolean values, as in "(x > 0 && y > 0)", which means, "x is greater than 0 and y is greater than 0."

The operation != means "is not equal to", as in "if (x != 0)", meaning "if x is not equal to zero.".

Question 6:

Explain what is meant by an assignment statement, and give an example. What are assignment statements used for?

Answer:

An assignment statement computes a value and stores that value in a variable. Examples include:

x = 17;          // Assign a constant value to the variable, x.
newRow = row;    // Copy the value from the variable, row,
//             into the variable, newRow.
ans = 17*x + 42; // Compute the value of the expression
//     17*x + 42, and store that value in ans.

An assignment statement is used to change the value of a variable as the program is running. Since the value assigned to the variable can be another variable or an expression, assignments statements can be used to copy data from one place to another in the computer, and to do complex computations.

Question 7:

What is meant by precedence of operators?

Answer:

If two or more operators are used in an expression, and if there are no parentheses to indicate the order in which the operators are to be evaluated, then the computer needs some way of deciding which operator to evaluate first. The order is determined by the precedence of the operators. For example, * has higher precedence than +, so the expression 3+5*7 is evaluated as if it were written 3+(5*7).

Question 8:

What is a literal?

Answer:

A literal is a sequence of characters used in a program to represent a constant value. For example, 'A' is a literal that represents the value A, of type char, and 17L is a literal that represents the number 17 as a value of type long. A literal is a way of writing a value, and should not be confused with the value itself.

Question 9:

In Java, classes have two fundamentally different purposes. What are they?

Answer:

A class can be used to group together variables and subroutines that are contained in the class. These are called the static members of the class. For example, the subroutine Math.sqrt is a static member of the class called Math. Also, the main routine in any program is a static member of a class. The second possible purpose of a class is to describe and create objects. The class specifies what variables and subroutines are contained in those objects. In this role, classes are used in object-oriented programming (which we haven't studied yet in any detail.)

Question 10:

What is the difference between the statement "x = TextIO.getDouble();" and the statement "x = TextIO.getlnDouble();"

Answer:

Either statement will read a real number input by the user, and store that number in the variable, x. They would both read and return exactly the same value. The difference is that in the second statement (using getlnDouble), after reading the value the computer will continue reading characters from input up to and including the next carriage return. These extra characters are discarded.

Question 11:

Explain why the value of the expression 2 + 3 + "test" is the string "5test" while the value of the expression "test" + 2 + 3 is the string "test23". What is the value of "test" + 2 * 3 ?

Answer:

The reason is somewhat technical. The difference is due to the order of evaluation. When several + operators are used in a row, with no parentheses, they are evaluated from left to right. 2 + 3 + "test" is interpreted as (2 + 3) + "test", so 2 and 3 are added together, giving 5, and then the 5 is concatenated onto the string "test". On the other hand, "test" + 2 + 3 is interpreted as ("test" + 2) + 3, so the 2 is first concatenated onto the "test", giving "test2", and then the 3 is concatenated onto that. In the case of "test" + 2 * 3, the precedence rules for + and * come into play. Since * has higher precedence, this expression is interpreted as "test" + (2 * 3), which evaluates to "test6".

Question 12:

Integrated Development Environments such as Eclipse often use syntax coloring, which assigns various colors to the characters in a program to reflect the syntax of the language. A student notices that Eclipse colors the word String differently from int, double, and boolean. The student asks why String should be a different color, since all these words are names of types. What's the answer to the student's question?

Answer:

(This was a real question from a real student.)

Although String, like int, double, and boolean, is a type name, there is a fundamental difference between String and the other types. String is the name of a class, while int, double, and boolean are primitive types. Eclipse colors all class names in the same way that it does String, and it uses a different color for the primitive types. (Although the difference between classes and primitive types might not seem very important to you now, it really is an important distinction and it's reasonable for Eclipse to use different colors for the two concepts.)

Answers for Quiz on Chapter 3

Question 1:

What is an algorithm?

Answer:

An algorithm is an unambiguous, step-by-step procedure for performing a certain task, which is guaranteed to finish after a finite number of steps. (An algorithm is not the same thing as a program, but it can be the idea behind a program.)

Question 2:

Explain briefly what is meant by "pseudocode" and how is it useful in the development of algorithms.

Answer:

Pseudocode refers to informal descriptions of algorithms, written in a language that imitates the structure of a programming language, but without the strict syntax. Pseudocode can be used in the process of developing an algorithm with stepwise refinement. You can start with a brief pseudocode description of the algorithm and then add detail to the description through a series of refinements until you have something that can be translated easily into a program written in an actual programming language.

Question 3:

What is a block statement? How are block statements used in Java programs?

Answer:

A block statement is just a sequence of Java statements enclosed between braces, { and }. The body of a subroutine is a block statement. Block statements are often used in control structures. A block statement is generally used to group together several statements so that they can be used in a situation that only calls for a single statement. For example, the syntax of a while loop calls for a single statement: "while (condition) do statement". However, the statement can be a block statement, giving the structure: "while (condition) { statement; statement; ...}".

Question 4:

What is the main difference between a while loop and a do..while loop?

Answer:

Both types of loop repeat a block of statements until some condition becomes false. The main difference is that in a while loop, the condition is tested at the beginning of the loop, and in a do..while loop, the condition is tested at the end of the loop. It is possible that the body of a while loop might not be executed at all. However, the body of a do..while loop is executed at least once since the condition for ending the loop is not tested until the body of the loop has been executed.

Question 5:

What does it mean to prime a loop?

Answer:

The condition at the beginning of a while loop has to make sense even the first time it is tested, before the body of the loop is executed. To prime the loop is to set things up before the loop starts so that the test makes sense (that is, the variables that it contains have reasonable values). For example, if the test in the loop is "while the user's response is yes," then you will have to prime the loop by getting a response from the user (or making one up) before the loop.

Question 6:

Explain what is meant by an animation and how a computer displays an animation.

Answer:

An animation consists of a series of "frames." Each frame is a still image, but there are slight differences from one frame to the next. When the images are displayed rapidly one frame after another, the eye perceives motion. A computer displays an animation by showing one image on the screen, then replacing it with the next image, then the next, and so on.

Question 7:

Write a for loop that will print out all the multiples of 3 from 3 to 36, that is: 3 6 9 12 15 18 21 24 27 30 33 36.

Answer:

Here are two possible answers. Assume that N has been declared to be a variable of type int:

for ( N = 3;  N <= 36;  N = N + 3 ) {
System.out.println( N );
}

or
for ( N = 3;  N <= 36;  N++ ) {
if ( N % 3 == 0 )
System.out.println( N );
}

Question 8:

Fill in the following main() routine so that it will ask the user to enter an integer, read the user's response, and tell the user whether the number entered is even or odd. (You can use TextIO.getInt() to read the integer. Recall that an integer n is even if n % 2 == 0.)

public static void main(String[] args) {

// Fill in the body of this subroutine!

}

Answer:

The problem already gives an outline of the program. The last step, telling the user whether the number is even or odd, requires an if statement to decide between the two possibilities.

public static void main (String[] args) {

int n;  // the number read from the user

TextIO.put("Type an integer: ");  // ask the use to enter an integer

n = TextIO.getInt();   // read the user's response

if (n % 2 == 0)        // tell the user whether the number is even or odd
System.out.println("That's an even number.");
else
System.out.println("That's an odd number.");
}

Question 9:

Write a code segment that will print out two different random integers selected from the range 1 to 10. All possible outputs should have the same probability. Hint: You can easily select two random numbers, but you have to account for the fact that the two numbers that you pick might be the same.

Answer:

The code for selecting two random integers has to be wrapped in a loop that will end only when the two selected numbers are different. This can be done easily with a do..while loop. A while loop can also be used, but it must be "primed" in some way. Note that by using a loop to choose the numbers, we can be absolutely sure that after the loop ends, the two numbers are different. Here are three possible solutions:

(1)     int x,y; // Two random integers.
do {
x = (int)(10*Math.random() + 1);
y = (int)(10*Math.random() + 1);
} while (x == y);  // continue if x and y are the same number.
System.out.println(x + " " + y);

(2)     int x,y;  // Two random integers.
x = y = 0;  // Give them junk value to "prime" the loop to make sure it runs.
while (x == y) {
x = (int)(10*Math.random() + 1);
y = (int)(10*Math.random() + 1);
}
System.out.println(x + " " + y);

(3)     int x,y; // Two random integers.
x = (int)(10*Math.random() + 1);  // Pick x, and keep its value.
do { // The loop finds a y with a different value from x.
y = (int)(10*Math.random() + 1);
} while (x == y);
System.out.println(x + " " + y);

Question 10:

Suppose that s1 and s2 are variables of type String, whose values are expected to be string representations of values of type int. Write a code segment that will compute and print the integer sum of those values, or will print an error message if the values cannot successfully be converted into integers. (Use a try..catch statement.)

Answer:

The function Integer.parseInt can be used to convert the strings into integers. This function will throw an exception of type NumberFormatException if the conversion fails. A try..catch statement can catch this exception and print an error message. So, the code segment can be written:

try {
int n1, n2;  // The values of s1 and s2 as integers.
int sum;     // The sum of n1 and n2.
n1 = Integer.parseInt(s1);
n2 = Integer.parseInt(s2);
sum = n1 + n2;   // (If an exception occurs, we don't get to this point.)
System.out.println("The sum is " + sum);
}
catch ( NumberFormatException e ) {
System.out.println("Error!  Unable to convert strings to integers.");
}

Question 11:

Show the exact output that would be produced by the following main() routine:

public static void main(String[] args) {
int N;
N = 1;
while (N <= 32) {
N = 2 * N;
System.out.println(N);
}
}

Answer:

The exact output printed by this program is:

2
4
8
16
32
64

(The hard part to get right is the 64 at the end. The value of N doubles each time through the loop. For the final execution of the loop, N starts out with the value 32, but N is doubled to 64 before it is printed.)

Question 12:

Show the exact output produced by the following main() routine:

public static void main(String[] args) {
int x,y;
x = 5;
y = 1;
while (x > 0) {
x = x - 1;
y = y * x;
System.out.println(y);
}
}

Answer:

The way to answer this question is to trace exactly what the program does, step-by-step. The output is shown below on the right. On the left is a table that shows the values of the variables x and y as the program is being executed.

value of x   |   value of y                 OUTPUT
--------------|--------------             -------------
5       |     1  [ before loop]
4       |     4  [ = 1*4 ]               4
3       |    12  [ = 4*3 ]               12
2       |    24  [ = 12*2 ]              24
1       |    24  [ = 24*1 ]              24
0       |     0  [ = 24*0 ]              0

Question 13:

What output is produced by the following program segment? Why? (Recall that name.charAt(i) is the i-th character in the string, name.)

String name;
int i;
boolean startWord;

name = "Richard M. Nixon";
startWord = true;
for (i = 0; i < name.length(); i++) {
if (startWord)
System.out.println(name.charAt(i));
if (name.charAt(i) == ' ')
startWord = true;
else
startWord = false;
}

Answer:

This is a tough one! The output from this program consists of the three lines:

R
M
N

As the for loop in this code segment is executed, name.charAt(i) represents each of the characters in the string "Richard M. Nixon" in succession. The statement System.out.println(name.charAt(i)) outputs the single character name.charAt(i) on a line by itself. However, this output statement occurs inside an if statement, so only some of the characters are output. The character is output if startWord is true. This variable is initialized to true, so when i is 0, startWord is true, and the first character in the string, 'R', is output. Then, since 'R' does not equal ' ', startWorld becomes false, so no more characters are output until startWord becomes true again. This happens when name.charAt(i) is a space, that is, just before the 'M' is processed and again just before the 'N' is processed. In fact whatever the value of name, this for statement would print the first character in name and every character in name that follows a space. (It is almost true that this for statement prints the first character of each word in the string, but something goes wrong when the first character is a space or when there are two spaces in a row. What happens in these cases?)

Question 14:

Suppose that numbers is an array of type int[]. Write a code segment that will count and output the number of times that the number 42 occurs in the array.

Answer:

Use a for loop to go through the array and test each array element. If the value is 42, add 1 to a counter:

int count42;  // The number of 42s in the array
count42 = 0;
int i;  // loop control variable
for ( i = 0; i < numbers.length; i++ ) {
if ( numbers[i] == 42 ) {
count42++;
}
}
System.out.println("There were " + count42 + " 42s in the array.")

Question 15:

Define the range of an array of numbers to be the maximum value in the array minus the minimum value. Suppose that raceTimes is an array of type double[]. Write a code segment that will find and print the range of raceTimes.

Answer:

We need both the minimum and the maximum value in the array. We can compute both using one for loop.

double max;    // maximum value from the array
double min;    // minimum value from the array
double range;  // the range of the array, max - min
int i;         // for-loop variable
max = min = raceTimes[0];  // start with both equal to the first element
for ( i = 1; i < raceTimes.length; i++ ) {
if ( raceTimes[i] > max )
max = raceTimes[i];
if ( raceTimes[i] < min )
min = raceTimes[i];
}
range = max - min;
System.out.println("The range is " + range);

Answers for Quiz on Chapter 4

Question 1:

A "black box" has an interface and an implementation. Explain what is meant by the terms interface and implementation.

Answer:

The interface of a black box is its connection with the rest of the world, such as the name and parameters of a subroutine or the dial for setting the temperature on a thermostat. The implementation refers to internal workings of the black box. To use the black box, you need to understand its interface, but you don't need to know anything about the implementation.

Question 2:

A subroutine is said to have a contract. What is meant by the contract of a subroutine? When you want to use a subroutine, why is it important to understand its contract? The contract has both "syntactic" and "semantic" aspects. What is the syntactic aspect? What is the semantic aspect?

Answer:

The contract of a subroutine says what must be done to call the subroutine correctly and what it will do when it is called. It is, in short, everything a programmer needs to know about the subroutine in order to use it correctly. (It does not include the "insides," or implementation, of the subroutine.)

The syntactic component of a subroutine's contract includes the name of the subroutine, the number of parameters, and the type of each parameter. This is the information needed to write a subroutine call statement that can be successfully compiled. The semantic component of the contract specifies the meaning of the subroutine, that is, the task that the subroutine performs. It might also specify limitations on what parameter values the subroutine can process correctly. The semantic component is not part of the program. It is generally expressed in comments.

Question 3:

Briefly explain how subroutines can be useful in the top-down design of programs.

Answer:

Top-down refers to starting from the overall problem to be solved, and breaking it up into smaller problems that can be solved separately. When designing a program to solve the problem, you can simply make up a subroutine to solve each of the smaller problems. Then you can separately design and test each subroutine.

Question 4:

Discuss the concept of parameters. What are parameters for? What is the difference between formal parameters and actual parameters?

Answer:

Parameters are used for communication between a subroutine and the part of the program that calls the subroutine. If a subroutine is thought of as a black box, then parameters are part of the interface to that black box. Formal parameters are found in the subroutine definition. Actual parameters are found in subroutine call statements. When the subroutine is called, the values of the actual parameters are assigned to the formal parameters before the body of the subroutine is executed.

Question 5:

Give two different reasons for using named constants (declared with the final modifier).

Answer:

A constant has a meaningful name, which makes the program easier to read. It's easier to understand what a name like INTEREST_RATE is for than it is to figure out how a literal number like 0.07 is being used.

A second reason for using named constants is that it's easy to modify the value of the constant if that becomes necessary. If a literal value is used throughout the program, the programmer has to track down each occurrence of the value and change it. When a constant is used correctly, it is only necessary to change the value assigned to the constant at one point in the program.

A third reason is that using the final modifier protects the value of a variable from being changed. This is especially important for member variables that are accessible from outside the class where they are declared.

Question 6:

What is an API? Give an example.

Answer:

An API is an Applications Programming Interface. It is the interface to a "toolbox" of subroutines that someone has written. It tells you what routines are available, how to call them, and what they do, but it does not tell you how the subroutines are implemented. An example is the standard Java API which describes the interfaces of all the subroutines in all the classes that are available in such packages as java.lang and java.awt.

Question 7:

Write a subroutine named "stars" that will output a line of stars to standard output. (A star is the character "*".) The number of stars should be given as a parameter to the subroutine. Use a for loop. For example, the command "stars(20)" would output

********************

Answer:

The subroutine could be written as follows:

static void stars(int numberOfStars) {
// output a line containing the specified number of stars
for (int i = 0; i < numberOfStars; i++) {
System.out.print('*');
}
System.out.println();  // output carriage return after the *'s
}

Question 8:

Write a main() routine that uses the subroutine that you wrote for Question 7 to output 10 lines of stars with 1 star in the first line, 2 stars in the second line, and so on, as shown below.

*
**
***
****
*****
******
*******
********
*********
**********

Answer:

The main() routine can use a for loop that calls the stars() subroutine ten times, once to produce each line of output. (An occasional beginner's mistake in this problem is to rewrite the body of the subroutine inside the main() routine, instead of just calling it by name.) Here is the main routine -- which would, of course, have to be put together with the subroutine in a class in order to be used.

public static void main(String[] args) {
int line;  // Line number, and also the number of stars on that line.
for ( line = 1;  line <= 10;  line++ ) {
stars( line );
}
}

Question 9:

Write a function named countChars that has a String and a char as parameters. The function should count the number of times the character occurs in the string, and it should return the result as the value of the function.

Answer:

The returned value will be of type int. The function simply uses a for loop to look at each character in the string. When the character in the string matches the parameter value, it is counted.

static int countChars( String str, char searchChar ) {
// Count the number of times searchChar occurs in
// str and return the result.
int i;     // A position in the string, str.
char ch;   // A character in the string.
int count; // Number of times searchChar has been found in str.
count = 0;
for ( i = 0;  i < str.length();  i++ ) {
ch = str.charAt(i);  // Get the i-th character in str.
if ( ch == searchChar )
count++;
}
return count;
}

Question 10:

Write a subroutine with three parameters of type int. The subroutine should determine which of its parameters is smallest. The value of the smallest parameter should be returned as the value of the subroutine.

Answer:

I'll call the subroutine smallest and the three parameters x, y, and z. The value returned by the subroutine has to be either x or y or z. The answer will be x if x is less than or equal to both y and z. The correct syntax for checking this is "if (x <= y && x <= z)". Similarly for y. The only other remaining possibility is z, so there is no necessity for making any further test before returning z. (In fact, doing so would be an error in Java, since with no "else" clause in the if statement, the compiler cannot determine that the function definitely returns a value in all possible cases.)

static int smallest(int x, int y, int z) {
if (x <= y && x <= z) {
return x;
}
else if (y <= x && y <= z) {
return y;
}
else
return z;
}

Note: Since a return statement causes the computer to terminate the execution of a subroutine anyway, this could also be written as follows, without the elses:

static int smallest(int x, int y, int z) {
if (x <= y && x <= z) {
return x;
}
if (y <= x && y <= z) {
return y;
}
return z;
}

Question 11:

Write a function that finds the average of the first N elements of an array of type double. The array and N are parameters to the subroutine.

Answer:

Note that the array must be passed as a parameter of type double[], and that the value returned by the function will be a double. For the value of N to make sense, it should be in the range 1 up to the length of the array. My answer throws an IllegalArgumentException if N is not in this range:

static double average( double[] numbers, int N ) {
if ( N < 1 ) {
throw new IllegalArgumentException("Can't find an average of " +
N + " numbers.");
}
if ( N > numbers.length ) {
throw new IllegalArgumentsExcpetion( N +
" is more than the length of the array." );
}
double sum = 0;  // the sum of the N numbers
for ( int i = 0; i < N; i++ ) {
sum = sum + numbers[i];  // add the i-th number to the sum
}
return sum/N;  // Return the average as the value of the function.
}

Question 12:

Explain the purpose of the following function, and explain how it works:

static int[] stripZeros( int[] list ) {
int count = 0;
for (int i = 0; i < list.length; i++) {
if ( list[i] != 0 )
count++;
}
int[] newList;
newList = new int[count];
int j = 0;
for (int i = 0; i < list.length; i++) {
if ( list[i] != 0 ) {
newList[j] = list[i];
j++;
}
}
return newList;
}

Answer:

This function makes a copy of its parameter, except that it leaves out all the elements of list that are equal to zero. It builds a new array that contains all the non-zero elements of list, and it returns that array as the value of the function. (Note that this is an example of using an array type as the return type of a function.)

The function creates a new array to be the return value. But to do that, it must know how long to make the array. The first five lines of the function definition count the number of non-zero elements in list. This is how many spaces we need in the new array, so count is used as the length when the new array is created. The remainder of the function goes through the original list and copies elements into newList. An element is copied only if it is non-zero. We have to keep track of how many spaces in newList have been filled so far. That's what j is for. This is the "partially filled array" pattern from Subsection 3.8.4.

Answers for Quiz on Chapter 5

Question 1:

Object-oriented programming uses classes and objects. What are classes and what are objects? What is the relationship between classes and objects?

Answer:

When used in object-oriented programming, a class is a factory for creating objects. (We are talking here about the non-static part of the class.) An object is a collection of data and behaviors that represent some entity (real or abstract). A class defines the structure and behaviors of all entities of a given type. An object is one particular "instance" of that type of entity. For example, if Dog is a class, than Lassie would be an object of type Dog.

Question 2:

What are instance variables and instance methods?

Answer:

Instance variables and instance methods are what we have been calling "non-static variables" and "non-static subroutines." That is, they are variables and subroutines that are declared in a class but are not marked as "static." When a class is loaded into the computer, instance variables and methods do not become part of the class in memory; instead, they become part of each object that is constructed from that class, and each object gets its own copy of the instance variables and methods.

Question 3:

Explain carefully what null means in Java, and why this special value is necessary.

Answer:

When a variable is of object type (that is, declared with a class or interface as its type rather than one of Java's primitive types), the value stored in the variable is not an object. Objects exist in a part of memory called the heap, and the variable holds a pointer or reference to the object. Null is a special value that can be stored in a variable to indicate that it does not actually point to any object.

Question 4:

What is a constructor? What is the purpose of a constructor in a class?

Answer:

A constructor is a special kind of subroutine in a class. It has the same name as the name of the class, and it has no return type, not even void. A constructor is called with the new operator in order to create a new object. Its main purpose is to initialize the newly created object, but in fact, it can do anything that the programmer wants it to do.

Question 5:

Suppose that Kumquat is the name of a class and that fruit is a variable of type Kumquat. What is the meaning of the statement "fruit = new Kumquat();"? That is, what does the computer do when it executes this statement? (Try to give a complete answer. The computer does several things.)

Answer:

This statement creates a new object belonging to the class Kumquat, and it stores a reference to that object in the variable fruit. More specifically, when the computer executes this statement, it allocates memory to hold a new object of type Kumquat. It calls a constructor, which can initialize the instance variables of the object as well as perform other tasks. A reference to the new object is returned as the value of the expression "new Kumquat()". Finally, the assignment statement stores the reference in the variable, fruit. So, fruit can now be used to access the new object.

Question 6:

What is meant by the terms instance variable and instance method?

Answer:

Instance variables and instance methods are non-static variables and methods in a class. This means that they do not belong to the class itself. Instead, they specify what variables and methods are in an object that belongs to that class. That is, the class contains the source code that defines instance variables and instance methods, but actual instance variables and instance methods are contained in objects. (Such objects are called "instances" of the class.) Thus, instance variables and instance methods are the data and the behaviors of objects.

Question 7:

Explain what is meant by the terms subclass and superclass.

Answer:

In object oriented programming, one class can inherit all the properties and behaviors from another class. It can then add to and modify what it inherits. The class that inherits is called a subclass, and the class that it inherits from is said to be its superclass. In Java, the fact that ClassA is a subclass of ClassB is indicated in the definition of ClassA as follows:

class ClassA extends ClassB {...}

Question 8:

Modify the following class so that the two instance variables are private and there is a getter method and a setter method for each instance variable:

public class Player {
String name;
int score;
}

Answer:

To make a variable private, just add the word "private" in front of each declaration. We need two methods for each variable. One of them returns the value of the variable. The other provides a new value for the variable. The names for these methods should follow the usual naming convention for getter and setter methods. (Note that my setter methods use the special variable this so that I can use the same name for the parameter of the method as is used for the instance variable. This is a very common pattern.)

public class Player {
private String name;
private int score;
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;  // ("this.name" refers to the instance variable)
}
public int getScore() {
return score;
}
public void setScore(int score) {
this.score = score;
}
}

Question 9:

Explain why the class Player that is defined in the previous question has an instance method named toString(), even though no definition of this method appears in the definition of the class.

Answer:

If a class is not declared to extend any class, then it automatically extends the class Object, which is one of the built-in classes of Java. So in this case, Player is a direct subclass of Object. The Object class defines a toString() method, and the Player class inherits this toString() method from Object. The methods and member variables in a class include not just those defined in the class but also those inherited from its superclass.

Question 10:

Explain the term polymorphism.

Answer:

Polymorphism refers to the fact that different objects can respond to the same method in different ways, depending on the actual type of the object. This can occur because a method can be overridden in a subclass. In that case, objects belonging to the subclass will respond to the method differently from objects belonging to the superclass.

(Note: If B is a subclass of A, then a variable of type A can refer to either an object of type A or an object of type B. Let's say that var is such a variable and that action() is a method in class A that is redefined in class B. Consider the statement "var.action()". Does this execute the method from class A or the method from class B? The answer is that there is no way to tell! The answer depends on what type of object var refers to, a class A object or a class B object. The method executed by var.action() depends on the actual type of the object that var refers to, not on the type of the variable var. This is the real meaning of polymorphism.)

Question 11:

Java uses "garbage collection" for memory management. Explain what is meant here by garbage collection. What is the alternative to garbage collection?

Answer:

The purpose of garbage collection is to identify objects that can no longer be used, and to dispose of such objects and reclaim the memory space that they occupy. If garbage collection is not used, then the programmer must be responsible for keeping track of which objects are still in use and disposing of objects when they are no longer needed. If the programmer makes a mistake, then there is a "memory leak," which might gradually fill up memory with useless objects until the program crashes for lack of memory.

Question 12:

What is an abstract class, and how can you recognize an abstract class in Java.

Answer:

An abstract class is one that cannot be used to create objects. It exists only as a basis for making subclasses, and it expresses all the properties and behaviors that those subclasses have in common. In Java, a class can be marked with the modifier abstract to make it abstract. For example,

abstract public class Vehicle { ...

It will then be a syntax error to try to call a "new Vehicle" constructor. (Note: Only a class that has been marked as abstract can contain abstract instance methods.)

Question 13:

What is this?

Answer:

"this" is a special variable in Java, which does not have to be declared. Java makes it available automatically in instance methods. It can be used in instance methods, and it holds a reference to the object that contains the method (or, in terms of messages, the object that received the message that is being processed). It provides a way to refer to "this object." If x is an instance variable, it can also be referred to as this.x within the same class. If doSomething() is an instance method, it can also be called as this.doSomething() within the same class.

Question 14:

For this problem, you should write a very simple but complete class. The class represents a counter that counts 0, 1, 2, 3, 4, .... The name of the class should be Counter. It has one private instance variable representing the value of the counter. It has two instance methods: increment() adds one to the counter value, and getValue() returns the current counter value. Write a complete definition for the class, Counter.

Answer:

Here is a possible answer. (Note that the initialization of the instance variable, value, to zero is not really necessary, since it would be initialized to zero anyway if no explicit initialization were provided.)

/**
* An object of this class represents a counter that counts up from zero.
*/
public class Counter {

private int value = 0;  // Current value of the counter.

/**
* Add one to the value of the counter.
*/
public void increment() {
value++;
}

/**
* Returns the current value of the counter.
*/
public int getValue() {
return value;
}

} // end of class Counter

Question 15:

This problem uses the Counter class from the previous question. The following program segment is meant to simulate tossing a coin 100 times. It should use two Counter objects, headCount and tailCount, to count the number of heads and the number of tails. Fill in the blanks so that it will do so:

Counter headCount, tailCount;
tailCount = new Counter();
headCount = new Counter();
for ( int flip = 0;  flip < 100;  flip++ ) {
if (Math.random() < 0.5)    // There's a 50/50 chance that this is true.

______________________ ;   // Count a "head".

else

______________________ ;   // Count a "tail".
}

System.out.println("There were " + ___________________ + " heads.");

System.out.println("There were " + ___________________ + " tails.");

Answer:

The variable headCount is a variable of type Counter, so the only thing that you can do with it is call the instance methods headCount.increment() and headCount.getValue(). Call headCount.increment() to add one to the counter. Call headCount.getValue() to discover the current value of the counter. Note that you can't get at the value of the counter directly, since the variable that holds the value is a private instance variable in the Counter object. Similarly for tailCount. Here is the program with calls to these instance methods filled in:

Counter headCount, tailCount;
tailCount = new Counter();
headCount = new Counter();
for ( int flip = 0;  flip < 100;  flip++ ) {
if (Math.random() < 0.5)    // There's a 50/50 chance that this is true.
headCount.increment() ;   // Count a "head", using headCount
else
tailCount.increment() ;   // Count a "tail", using tailCount
}
System.out.println(("There were " + headCount.getValue() + " heads.");
System.out.println(("There were " + tailCount.getValue() + " tails.");

Answers for Quiz on Chapter 6

Question 1:

Programs written for a graphical user interface have to deal with "events." Explain what is meant by the term event. Give at least two different examples of events, and discuss how a program might respond to those events.

Answer:

An event is anything that can occur asynchronously, not under the control of the program, to which the program might want to respond. GUI programs are said to be "event-driven" because for the most part, such programs simply wait for events and respond to them when they occur. In many (but not all) cases, an event is the result of a user action, such as when the user clicks the mouse button, types a character, or clicks a button. The program might respond to a mouse-click on a canvas by drawing a shape, to a typed character by adding the character to an input box, or to a click on a button by clearing a drawing. More generally, a programmer can set up any desired response to an event by writing an event-handling routine for that event.

Question 2:

Explain carefully what the repaint() method does.

Answer:

The repaint() method of a component is called to notify the system that the component needs to be redrawn. It does not itself do any drawing, and does not itself call paintComponent(), but sometime shortly after you call it, the system will call the component's paintComponent() routine. You should call repaint() for a component when you have made some change to the state of a program that requires the appearance of that component to change.

Question 3:

Java has a standard class called JPanel. Discuss two ways in which JPanels can be used.

Answer:

A JPanel can be used as a container for other components or as a drawing surface. A JPanel is a type of component. That is, it is a visible element of a GUI. By itself, a JPanel is simply a blank rectangular region on the screen. However, a JPanel is a "container", which means that other components can be added to it and will then appear on the screen inside the JPanel. A JPanel can also be used as a drawing surface. In order to do this, it is necessary to create a subclass of JPanel and define a paintComponent() method in that subclass. An object belonging to that subclass can then be added to another panel or other container. The paintComponent() method defines how that object will draw itself on the screen.

Question 4:

Draw the picture that will be produced by the following paintComponent() method:

public static void paintComponent(Graphics g) {
super.paintComponent(g);
for (int i=10; i <= 210; i = i + 50)
for (int j = 10; j <= 210; j = j + 50)
g.drawLine(i,10,j,60);
}

Answer:

The outer loop is executed for values of i equal to 10, 60, 110, 160, and 210. For each of these values, the inner loop is executed for j equal to 10, 60, 110, 160, and 210. The drawLine command is therefore executed 25 times -- and so, 25 different lines are drawn. These lines connect the five points (10,10), (60,10), (110,10), (160,10), and (210,10) to the five points (10,60), (60,60), (110,60), (160,60), and (210,60) in all possible pairings. Here is the picture:

Question 5:

Suppose you would like a panel that displays a green square inside a red circle, as illustrated. Write a paintComponent() method for the panel class that will draw the image.

Answer:

(The size of the square and circle are not specified in the problem, so any size would be acceptable, as long as the square is in the middle of the circle. Notice that the drawing commands are fillOval and fillRect. There are no special routines for drawing circles or squares. If your circle looks more jagged around the edges than the one in the picture, it's because my solution turns on antialiasing!)

public void paintComponent(Graphics g) {

super.paintComponent(g);

Graphics2D g2 = (Graphics2D)g;  // turn on antialiasing
g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
RenderingHints.VALUE_ANTIALIAS_ON);

g.setColor(Color.RED);      // draw the shapes
g.fillOval(10,10,80,80);
g.setColor(Color.GREEN);
g.fillRect(30,30,40,40);

}

Question 6:

Java has a standard class called MouseEvent. What is the purpose of this class? What does an object of type MouseEvent do?

Answer:

When an event occurs, the system packages information about the event into an object. That object is passed as a parameter to the event-handling routine. Different types of events are represented by different classes of objects. An object of type MouseEvent represents a mouse or mouse motion event. It contains information about the location of the mouse cursor and any modifier keys that the user is holding down. This information can be obtained by calling the instance methods of the object. For example, if evt is a MouseEvent object, then evt.getX() is the x-coordinate of the mouse cursor, and evt.isShiftDown() is a boolean value that tells you whether the user was holding down the Shift key.

Question 7:

One of the main classes in Swing is the JComponent class. What is meant by a component? What are some examples?

Answer:

A JComponent represents a visual component of the computer's graphical user interface. A JComponent is not completely independent. It must be added to a "container," such as a panel. Examples of JComponents are JButtons, JTextFields, and JPanels.

Question 8:

What is the function of a LayoutManager in Java?

Answer:

A LayoutManager implements some policy for laying out all the visual components that have been added to a container, such as a JPanel. That is, it sets the sizes and positions of the components. Different types of layout managers have different rules about how components are to be arranged. Some standard layout manager classes are BorderLayout and GridLayout.

Question 9:

Consider the illustration of nested panels from the beginning of Section 6.6. What type of layout manager is being used for each of the three panels in that picture?

Answer:

The outer panel, shown in pink, seems to be using a GridLayout with two rows and one column. A GridLayout is most likely since the two components in the main panel -- the other two panels, shown in light cyan -- are exactly the same size. Similarly, the bottom subpanel seems to be using a GridLayout with one row and three columns. The top subpanel could be using a BorderLayout. The components on the left and right ends of the subpanel would be in the WEST and EAST positions of the BorderLayout. Each of these components would then be shown at its own preferred width, which would explain how their widths could be different. The third component, in the center of the subpanel, would then be in the CENTER position.

Question 10:

Explain how Timers are used to do animation.

Answer:

Displaying an animation requires showing a sequence of frames. The frames are shown one after the other, with a short delay between each frame and the next. A Timer can generate a sequence of ActionEvents. When a timer is used to do animation, each event triggers the display of another frame. The ActionListener that processes events from the timer just needs to be programmed to display a new frame each time its actionPerformed() method is called. Usually, the actionPerformed() method just changes the values of some state variables and calls repaint() to make the effect of the changes visible on the screen.

Question 11:

What is a JCheckBox and how is it used?

Answer:

A JCheckBox is a component that has two possible states, "selected" and "not selected". The user can change the state by clicking on the JCheckBox. If box is a variable of type JCheckBox, then a program can set the state of the box to "selected" by calling box.setSelected(true) and can unselect the box by calling box.setSelected(false). The current state can be determined by calling box.isSelected(), which is a boolean-valued function. A JCheckBox generates an event of type ActionEvent when it changes state. A program can listen for these events if it wants to take some action at the time the state changes. Often, however, it's enough for a program simply to look at the state of the JCheckBox when it needs it.

Question 12:

How is the preferred size of a component set, and how is it used?

Answer:

Standard components such as JButton and JLabel are responsible for computing their own preferred size, and the preferred size of a container is usually computed by the layout manager for that container, but it is possible to set the preferred size of a component, comp, by calling the method comp.setPreferredSize(dim), where dim is an object of type Dimension. When a JPanel is used as a drawing surface, it is usually necessary to set its preferred size in this way, since otherwise its preferred size will be zero.

A layout uses the preferred sizes of all the components in a container when it decides how to lay out those components. It also uses the components' preferred sizes when computing the preferred size of the container. (Note that the preferred size is only preferred; it is not necessarily the size at which the component will appear on screen. The actual size can depend on which layout manager is used, the position of the component in the layout, the size of the container, and the preferred sizes of other components in the container.)

Answers for Quiz on Chapter 7

Question 1:

What is meant by the basetype of an array?

Answer:

The base type of an array refers to the type of the items that can be stored in that array. For example, the base type of the array of type Color[] is Color.

Question 2:

What is the purpose of the following variable-arity method? What are the values of same(1,2,3), same(17,17,17,17), and same(2)? Why?

static double same( int... value ) {
for (int i = 1; i < value.length; i++) {
if ( value[i-1] != value[i] )
return false;
}
return true;
}

Answer:

This function can be called with any number of integer parameters. It tests whether all of the parameter values are equal. The value of same(1,2,3) is false. The function begins by comparing 1 and 2; since they are not equal, it immediately returns false. The value of same(17,17,17,17) is true. The function compares value[0] with value[1], value[1] with value[2], and value[2] with value[3]. All of these values are 17, so the routine goes through the for loop without returning false, and the "return true" statement after the end of the loop is executed. For same(2), value.length is 1, and the body of the loop is never executed; the return value is true.

Question 3:

What does it mean to sort an array?

Answer:

To sort an array means to rearrange the items in the array so that they are in increasing or decreasing order (according to some criterion).

(Note: To be more accurate, we should say "non-decreasing" instead of "increasing" and "non-increasing" instead of "decreasing." An array can contain duplicate values, and in that case, its elements can'be be put into strictly increasing or strictly decreasing order.)

Question 4:

What is the main advantage of binary search over linear search? What is the main disadvantage?

Answer:

The advantage of binary search is that it is much faster. On a list of one million items, linear search would take an average of five hundred thousand steps to find an item, whereas binary search would take only 20 steps. The disadvantage is that binary search only works on a sorted list, so some extra work must be done to sort the list or to keep the list in sorted order as it is created.

Question 5:

What is meant by a dynamic array? What is the advantage of a dynamic array over a regular array?

Answer:

A dynamic array is like an array in that it is a data structure that stores a sequence of items, all of the same type, in numbered locations. It is different from an array in that there is no preset upper limit on the number of items that it can contain. This is an advantage in situations where a reasonable value for the size of the array is not known at the time it is created.

Question 6:

What does it mean to say that ArrayList is a parameterized type?

Answer:

A class name is a type, but as a parameterized type, ArrayList does not define just one type. Instead, for every object type T, there is a type ArrayList<T>. T is a "type parameter." An object of type ArrayList<T> is a list that can only hold objects of type T. We can have ArrayList<String> to represent lists of Strings, ArrayList<Integer> to represent lists of Integers, and so one. The fact that ArrayList is a parameterized type means that we don't have to create a new class for each type of list.

Question 7:

Suppose that a variable strlst has been declared as

ArrayList<String> strlst = new ArrayList<String>();

Assume that the list is not empty and that all the items in the list are non-null. Write a code segment that will find and print the string in the list that comes first in lexicographic order.

Answer:

Strings can be compared for lexicographic order using the compareTo instance method in the String class. We can find the smallest string as follows:

String smallest = strlst.get(0);
for (int i = 1; i < strlist.size(); i++) {
String nextString = strlst.get(i);
if ( nextString.compareTo(smallest) < 0 )
smallest = nextString;
}
System.out.println("The smallest string lexicographically is " + smallest);

Alternatively, a for-each loop could be used:

String smallest = strlst.get(0);
for (String item : strlst) {
if ( item.compareTo(smallest) < 0 )
smallest = item;
}
System.out.println("The smallest string lexicographically is " + smallest)

Not that in this version, the first item in the list is compared to itself as the first step in the loop. There is no easy way to leave that item out of the for-each loop.

Question 8:

Show the exact output produced by the following code segment.

char[][] pic = new char[6][6];
for (int i = 0; i < 6; i++)
for (int j = 0; j < 6; j++) {
if ( i == j  ||  i == 0  ||  i == 5 )
pic[i][j] = '*';
else
pic[i][j] = '.';
}
for (int i = 0; i < 6; i++) {
for (int j = 0; j < 6; j++)
System.out.print(pic[i][j]);
System.out.println();
}

Answer:

The output consists of six lines, with each line containing six characters. In the first line, i is 0, so all the characters are *'s. In the last line, i is 5, so all the characters are *'s. In each of the four lines in the middle, one of the characters is a * and the rest are periods. The output is

******
.*....
..*...
...*..
....*.
******

It might help to look at the array items that are printed on each line. Note that pic[row][col] is '*' if row is 0 or if row is 5 or if row and col are equal.

pic[0][0] pic[0][1] pic[0][2] pic[0][3] pic[0][4] pic[0][5]
pic[1][0] pic[1][1] pic[1][2] pic[1][3] pic[1][4] pic[1][5]
pic[2][0] pic[2][1] pic[2][2] pic[2][3] pic[2][4] pic[2][5]
pic[3][0] pic[3][1] pic[3][2] pic[3][3] pic[3][4] pic[3][5]
pic[4][0] pic[4][1] pic[4][2] pic[4][3] pic[4][4] pic[4][5]
pic[5][0] pic[5][1] pic[5][2] pic[5][3] pic[5][4] pic[5][5]

Question 9:

Write a complete static method that finds the largest value in an array of ints. The method should have one parameter, which is an array of type int[]. The largest number in the array should be returned as the value of the method.

Answer:

One possible answer is:

public static int getMax(int[] list) {

int max = list[0];  // This is the largest item seen so far.

for (int i = 1; i < list.length; i++) {
// Look at each item in the array.  If the item is
// bigger than max, then set max equal to the item.
if (list[i] > max)
max = list[i];
}

// At this point, max is the largest item in the whole array.

return max;

} // end getMax

(Note that this method throws an exception if the parameter list is null or if it is an array of length 0. The exception is thrown by the line "int max = list[0];". The reference to list[0] causes a NullPointerException if list is null and an ArrayIndexOutOfBoundsException if the array has length zero.)

Question 10:

Suppose that temperature measurements were made on each day of 2014 in each of 100 cities. The measurements have been stored in an array

int[][]  temps  =  new  int[100][365];

where temps[c][d] holds the measurement for city number c on the d th day of the year. Write a code segment that will print out the average temperature, over the course of the whole year, for each city. The average temperature for a city can be obtained by adding up all 365 measurements for that city and dividing the answer by 365.0.

Answer:

A pseudocode outline of the answer is

For each city {
Add up all the temperatures for that city
Divide the total by 365 and print the answer
}

Adding up all the temperatures for a given city itself requires a for loop, so the code segment looks like this:

for (int city = 0; city < 100; city++) {
int total = 0;  // total of temperatures for this city
for (int day = 0; day < 365; day++)
total = total + temps[city][day];
double avg = total / 365.0;  // average temp for this city
System.out.println("Average temp for city number "
+ city + " is " + avg);
}

Question 11:

Suppose that a class, Employee, is defined as follows:

class Employee {
String lastName;
String firstName;
double hourlyWage;
int yearsWithCompany;
}

Suppose that data about 100 employees is already stored in an array:

Employee[] employeeData = new Employee[100];

Write a code segment that will output the first name, last name, and hourly wage of each employee who has been with the company for 20 years or more. Write two versions of the code segment, one that uses a regular for loop, and one that uses a for-each loop.

Answer:

(The data for the i-th employee is stored in an object that can be referred to as employeeData[i]. The four pieces of data about that employee are members of this object and can be referred to as:

- employeeData[i].firstName

- employeeData[i].lastName

- employeeData[i].hourlyWage

- employeeData[i].yearsWithCompany

The code segment uses a for loop to consider each employee in the array.)

for (int i=0; i < 100; i++) {
if ( employeeData[i].yearsWithCompany >= 20 )
System.out.println(employeeData[i].firstName + " " +
employeeData[i].lastName + ": " +
employeeData[i].hourlyWage);
}

Using a for-each loop:

for ( Employee emp : employeeData ) {
if ( emp.yearsWithCompany >= 20 )
System.out.println(emp.firstName + " " +
emp.lastName + ": " +
emp.hourlyWage);
}

Question 12:

What is the purpose of the following subroutine? What is the meaning of the value that it returns, in terms of the value of its parameter?

static double[] sums( double[][] data ) {
double[] answers = new double[ data.length ];
for (int i = 0; i < data.length; i++) {
double sum = 0;
for (int j = 0; j < data[i].length; i++)
sum = sum + data[i][j];
answers[i] = sum;
}
return answers;
}

Answer:

The purpose of this subroutine is to find the sum of the numbers in each row of the parameter array data. The answer is returned as a one-dimensional array of double. The value of answers[i] is the sum of the numbers in row number i of data.

(In the first two lines of the method, data.length is the number of rows in data. In the nested for loop, data[i].length is the number of values in the i-th row.)

Answers for Quiz on Chapter 8

Question 1:

What does it mean to say that a program is robust?

Answer:

A robust program is one that can handle errors and other unexpected conditions in some reasonable way. This means that the program must anticipate possible errors and respond to them if they occur.

Question 2:

Why do programming languages require that variables be declared before they are used? What does this have to do with correctness and robustness?

Answer:

It's a little inconvenient to have to declare every variable before it is used, but it's much safer. If the compiler would accept undeclared variables, then it would also accept misspelled names and treat them as valid variables. This can easily lead to incorrect programs. When variables must be declared, the unintentional creation of a variable is simply impossible, and a whole class of possible bugs is avoided.

Question 3:

What is a precondition? Give an example.

Answer:

A precondition is a condition that has to hold at a given point in the execution of a program, if the execution of the program is to continue correctly. For example, the statement "x = A[i];" has two preconditions: that A is not null and that 0 <= i < A.length. If either of these preconditions is violated, then the execution of the statement will generate an error.

Also, a precondition of a subroutine is a condition that has to be true when the subroutine is called in order for the subroutine to work correctly.

Question 4:

Explain how preconditions can be used as an aid in writing correct programs.

Answer:

Suppose that a programmer recognizes a precondition at some point in a program. This is a signal to the programmer that an error might occur if the precondition is not met. In order to have a correct and robust program, the programmer must deal with the possible error. There are several approaches that the programmer can take. One approach is to use an if statement to test whether the precondition is satisfied. If not, the programmer can take some other action such as printing an error message and terminating the program. Another approach is to use a try statement to catch and respond to the error. This is really just a cleaner way of accomplishing the same thing as the first approach. The best approach, when it is possible, is to ensure that the precondition is satisfied as a result of what has already been done in the program. For example, if the precondition is that x >= 0, and the preceding statement is "x = Math.abs(y);", then we know that the precondition is satisfied, since the absolute value of any number is greater than or equal to zero.

Question 5:

Java has a predefined class called Throwable. What does this class represent? Why does it exist?

Answer:

The class Throwable represents all possible objects that can be thrown by a throw statement and caught by a catch clause in a try..catch statement. That is, the thrown object must belong to the class Throwable or to one of its (many) subclasses such as Exception and RuntimeException. The object carries information about an exception from the point where the exception occurs to the point where it is caught and handled.

Question 6:

Write a method that prints out a 3N+1 sequence starting from a given integer, N. The starting value should be a parameter to the method. If the parameter is less than or equal to zero, throw an IllegalArgumentException. If the number in the sequence becomes too large to be represented as a value of type int, throw an ArithmeticException.

Answer:

The problem of large values in a 3N+1 sequence was discussed in Section 8.1. In that section, it is pointed out that the test "if (N > 2147483646/3)" can be used to test whether the value of N has become too large. This test is used in the following method.

/** Print the 3N+1 sequence starting from N.  If N
* is not greater than 0 or if the value of N exceeds
* the maximum legal value for ints, than an
* exception will be thrown.
*/
static void printThreeNSequence(int N) {
if (N < 1) {
throw new IllegalArgumentException(
"Starting value for 3N+1 sequence must be > 0.");
}
System.out.println("3N+1 sequence starting from " + N + " is: ");
System.out.println(N);
while (N > 1) {
if (N % 2 == 0) {  // N is even.  Divide by 2.
N = N / 2;
}
else {  // N is odd.  Multiply by 3 and add 1.
if (N > 2147483646/3) {
throw new ArithmeticException("Value has exceeded the largest int.");
}
N = 3 * N + 1;
}
System.out.println(N);
}
}

(Note that it would be possible to declare that this routine can throw exceptions by adding a "throws" clause to the heading:

static void printThreeNSequence(int N)
throws IllegalArgumentException, ArithmeticException {

However, this is not required since IllegalArgumentExceptions and ArithmeticExceptions are not checked exceptions.)

Question 7:

Rewrite the method from the previous question, using assert statements instead of exceptions to check for errors. What is the difference between the two versions of the method when the program is run?

Answer:

We can replace the if statements that check for errors with assert statements that give the same error messages:

/** Print the 3N+1 sequence starting from N.
* Precondition:  N > 0 and the 3N+1 sequence for N does not contain
* any numbers that are too big to be represented as 32-bit ints.
*/
static void printThreeNSequence(int N) {

assert  N > 0 : "Starting value for 3N+1 sequence must be > 0.";

System.out.println("3N+1 sequence starting from " + N + " is: ");

System.out.println(N);
while (N > 1) {
if (N % 2 == 0) {  // N is even.  Divide by 2.
N = N / 2;
}
else {  // N is odd.  Multiply by 3 and add 1.
assert  N <= 2147483646/3 : "Value has exceeded the largest int.";
N = 3 * N + 1;
}
System.out.println(N);
}

}

The first version of the method will always check for errors when the program is run. The second version, on the other hand, does not actually do any error checking when the program is run in the ordinary way. In order for assert statements to be executed, the program must be run with assertions enabled. The assert statements are really there only to do error-checking during debugging and testing. (In this particular case, I would say that an exception should definitely be thrown when N exceeds the maximum legal value, but that it's reasonable to use an assert to check whether N > 0.)

Question 8:

Some classes of exceptions are checked exceptions that require mandatory exception handling. Explain what this means.

Answer:

Subclasses of the class Exception which are not subclasses of RuntimeException are checked exceptions. This has two consequences: First, if a method can throw such an exception, then it must declare this fact by adding a throws clause to the method heading. Second, if a routine includes any code that can generate such an exception, then the routine must deal with the exception. It can do this by including the code in a try statement that has a catch clause to handle the exception. Or it can add a throws clause to the method definition to declare that calling the method might throw the exception.

Question 9:

Consider a subroutine processData() that has the header

static void processData() throws IOException

Write a try..catch statement that calls this subroutine and prints an error message if an IOException occurs.

Answer:

try {
processData();
}
catch (IOException e) {
System.out.println("An IOException occurred while processing the data.");
}

Question 10:

Why should a subroutine throw an exception when it encounters an error? Why not just terminate the program?

Answer:

Terminating the program is too drastic, and this tactic certainly doesn't lead to robust programs! It's likely that the subroutine doesn't know what to do with the error, but that doesn't mean that it should abort the whole program. When the subroutine throws an exception, the subroutine is terminated, but the program that called the subroutine still has a chance to catch the exception and handle it. In effect, the subroutine is saying "Alright, I'm giving up. Let's hope someone else can deal with the problem."

Question 11:

Suppose that you have a choice of two algorithms that perform the same task. One has average-case run time that is Θ(n^2) while the run time of the second algorithm has an average-case run time that is Θ(n*log(n)). Suppose that you need to process an input of size n = 100. Which algorithm would you choose? Can you be certain that you are choosing the fastest algorithm for the input that you intend to process.

Answer:

In the absence of other information, the second algorithm, with run time Θ(n*log(n)), is the better choice, since n*log(n) is much smaller than n^2, for most values of n. However, it's not completely certain that the second algorithm is the better choice in a particular case. First of all, although the n*log(n) algorithm is certainly better than the n^2 algorithm for large enough values of n, that is not necessarily true for n = 100. Second, there is the issue of "average-case" run time. Even if the n*log(n) algorithm is better for most inputs of size 100, it might not be better for all such inputs.

Question 12:

Analyze the run time of the following algorithm. That is, find a function f(n) such that the run time of the algorithm is O(f(n)) or, better, Θ(f(n)). Assume that A is an array of integers, and use the length of the array as the input size, n.

int total = 0;
for (int i = 0; i < A.length; i++) {
if (A[i] > 0)
total = total + A[i];
}

Answer:

The run time of this algorithm is Θ(n). There are several things in the code that are evaluated n times: the test "i < A.length", the increment "i++", and the test in the if statement. The initialization is done once, and nothing is executed more than n times. It follows that both the worst-case and the average case run times are Θ(n).

Answers for Quiz on Chapter 9

Question 1:

Explain what is meant by a recursive subroutine.

Answer:

A recursive subroutine is simply one that calls itself either directly or through a chain of calls involving other subroutines.

Question 2:

Consider the following subroutine:

static void printStuff(int level) {
if (level == 0) {
System.out.print("*");
}
else {
System.out.print("[");
printStuff(level - 1);
System.out.print(",");
printStuff(level - 1);
System.out.print("]");
}
}

Show the output that would be produced by the subroutine calls printStuff(0), printStuff(1), printStuff(2), and printStuff(3).

Answer:

The outputs are:

printStuff(0) outputs:   *
printStuff(1) outputs:   [*,*]
printStuff(2) outputs:   [[*,*],[*,*]]
printStuff(3) outputs:   [[[*,*],[*,*]],[[*,*],[*,*]]]

(Explanation: For printStuff(0), the value of the parameter is 0, so the first clause of the if is executed, and the output is just *. For printStuff(1), the else clause is executed. This else clause contains two recursive calls to printStuff(level-1). Since level is 1, level-1 is 0, so each call to printStuff outputs a *. The overall output from printStuff(1) is [*,*]. In a similar way, printStuff(2) includes two recursive calls to printStuff(1). Each call to printStuff(1) outputs [*,*]. And printStuff(2) just takes two copies of this and puts them between [ and ] separated by a comma: [[*,*],[*,*]]. Finally, the output from printStuff(3) outputs two copies of [[*,*],[*,*]] separated by a comma and enclosed between brackets. Once you recognize the pattern, you can do printStuff(N) for any N without trying to follow the execution of the subroutine in detail.)

Question 3:

Suppose that a linked list is formed from objects that belong to the class

class ListNode {
int item;       // An item in the list.
ListNode next;  // Pointer to next item in the list.
}

Write a subroutine that will count the number of zeros that occur in a given linked list of ints. The subroutine should have a parameter of type ListNode and should return a value of type int.

Answer:

I'll give both a non-recursive solution and a recursive solution. For a linked list, the recursion is not really necessary, but it does nicely reflect the recursive definition of ListNode

static int countZeros( ListNode head ) {
int count;        // The number of zeros in the list.
ListNode runner;  // For running along the list.
count = 0;
runner = head;    // Start at the beginning of the list.
while (runner != null) {
if ( runner.item == 0)
count++;  // Count the zero found in the current node.
runner = runner.next;  // Advance to the next node.
}
return count;
}

static int countZerosRecursively( ListNode head ) {
if ( head == null) {
// An empty list does not contain any zeros.
return 0;
}
else {
int count = countZerosRecursively( head.next );  // Count zeros in tail.
if ( head.item == 0 )
count++;  // Add 1 to account for the zero in the head node.
return count;
}
}

Question 4:

What are the three operations on a stack?

Answer:

The three stack operations are push, pop, and isEmpty. The definitions of these operations are: push(item) adds the specified item to the top of the stack; pop() removes the top item of the stack and returns it; and isEmpty() is a boolean-valued function that returns true if there are no items on the stack.

Question 5:

What is the basic difference between a stack and a queue?

Answer:

In a stack, items are added to the stack and removed from the stack on the same end (called the "top" of the stack). In a queue, items are added at one end (the "back") and removed at the other end (the "front"). Because of this difference, a queue is a FIFO structure (items are removed in the same order in which they were added), and a stack is a LIFO structure (the item that is popped from a stack is the one that was added most recently).

Question 6:

What is an activation record? What role does a stack of activation records play in a computer?

Answer:

When a subroutine is called, an activation record is created to hold the information that is needed for the execution of the subroutine, such as the values of the parameters and local variables. This activation record is stored on a stack of activation records. A stack is used since one subroutine can call another, which can then call a third, and so on. Because of this, many activation records can be in use at the same time. The data structure is a stack because an activation record has to continue to exist while all the subroutines that are called by the subroutine are executed. While they are being executed, the stack of activation records can grow and shrink as subroutines are called and return.

Question 7:

Suppose that a binary tree of integers is formed from objects belonging to the class

class TreeNode {
int item;       // One item in the tree.
TreeNode left;  // Pointer to the left subtree.
TreeNode right; // Pointer to the right subtree.
}

Write a recursive subroutine that will find the sum of all the nodes in the tree. Your subroutine should have a parameter of type TreeNode, and it should return a value of type int.

Answer:

static int treeSum( TreeNode root ) {
// Find the sum of all the nodes in the tree to which root points.
if ( root == null ) {
// The sum of the nodes in an empty tree is zero.
return 0;
}
else {
// Add the item in the root to the sum of the
// items in the left subtree and the sum of the
// items in the right subtree.
int total = root.item;
total += treeSum( root.left );
total += treeSum( root.right );
return total;
}
}

Question 8:

What is a postorder traversal of a binary tree?

Answer:

In a traversal of a binary tree, all the nodes are processed in some way. (For example, they might be printed.) In a postorder traversal, the order of processing is defined by the rule: For each node, the nodes in the left subtree of that node are processed first. Then the nodes in the right subtree are processed. Finally, the node itself is processed. This rule is applied at all levels of the tree.

Question 9:

Suppose that a <multilist> is defined by the BNF rule

<multilist>  ::=  <word>  |  "(" [ <multilist> ]... ")"

where a <word> can be any sequence of letters. Give five different <multilist>'s that can be generated by this rule. (This rule, by the way, is almost the entire syntax of the programming language LISP! LISP is known for its simple syntax and its elegant and powerful semantics.)

Answer:

Here are five possibilities (out of an infinite number of possibilities), with some explanation:

fred  --  A <multilist> can just be a word, such as "fred".

( )   --  The [ ]... around <multilist> means that there can be
any number of nested <multilist>'s, including zero.  If
there are zero, then all that's left is the empty
parentheses.

( fred mary chicago ) -- A <multilist> consisting of three
<multilist>'s -- "fred", "mary", and
"chicago" -- inside parentheses

( ( able ) ( baker charlie ) ) -- A <multilist> containing two
<multilist>'s.

( ( a ( b ) ) ( c ( d e ) g ) )  -- Even more nesting.

Question 10:

Explain what is meant by parsing a computer program.

Answer:

To parse a computer program means to determine its syntactic structure, that is, to figure out how it can be constructed using the rules of a grammar (such as a BNF grammar).

Answers for Quiz on Chapter 10

Question 1:

What is meant by generic programming and what is the alternative?

Answer:

Generic programming means writing data structures and subroutines that can be used for many different types of data. The alternative would be to write a new data structure or subroutine for each different type of data, even when they would all be essentially identical except for the type name. For example, a single generic sort routine can be used for sorting lists that contain data of any type. The alternative is one routine for sorting lists of integers, one for sorting lists of strings, one for storing arrays of real numbers, and so on.

Question 2:

Why can't you make an object of type LinkedList<int>? What should you do instead?

Answer:

LinkedList<int> is an attempt to use a generic class with a type parameter of type int, which is a primitive type. Generic programming in Java works only for objects, and not for the primitive types, so a type parameter of primitive type is not allowed. However, it is possible to use the wrapper class Integer in place of int. An object of type LinkedList<Integer> can be used almost as if it were actually a list of ints.

Question 3:

What is an iterator and why are iterators necessary for generic programming?

Answer:

One of the principle features of Java's generic programming framework is Collections. There are several types of collection (including LinkedList, ArrayList, TreeSet, and HashSet). In order to deal with all the different types of collection in a generic way, we need a generic way to access all the elements in a collection. An iterator makes this possible. An iterator is an object associated with a collection that makes it possible to traverse the collection (that is, to visit each of the items in the collection in turn). Code written using iterators will work for any type of collection. (Note, however, that explicit use of an iterator can often be avoided by using a for-each loop.)

Question 4:

Suppose that integers is a variable of type Collection<Integer>. Write a code segment that uses an iterator to compute the sum of all the integer values in the collection. Write a second code segment that does the same thing using a for-each loop.

Answer:

Using an iterator:

int sum = 0;
Iterator<Integer>  iter = integers.iterator();
while ( iter.hasNext() ) {
sum += iter.next();
}

The statement "sum += iter.next()" relies on the automatic conversion from type Integer to type int. It could also be written "sum += iter.next().intValue()".

Using a for-each loop:

int sum = 0;
for ( int number : integers ) {   // ( Could also use "Integer number : integers". )
sum += number;
}

Note that either of these two solutions will fail if integers contains a null value.

Question 5:

Interfaces such as List, Set, and Map define abstract data types. Explain what this means.

Answer:

An abstract data type is defined by the operations that can be performed on it, not by the way the data is actually stored or by the way the operations are implemented. An interface such as List defines operations that can be performed, but says nothing about how they are to be implemented. In fact, there can be many different implementations. For example, both LinkedList and ArrayList implement the List interface. They are different "concrete" data types that implement the same abstract data type.

Question 6:

What is the fundamental property that distinguishes Sets from other types of Collections?

Answer:

A Set cannot contain duplicate elements. Adding an item to the set has no effect if that item is already in the set. (Note that exactly what it means to say that two items are the same depends on the type of set. For HashSet, two items are tested for equality using the equals() method. For a TreeSet, the test for equality uses the compareTo() method from the Comparable interface or the compare() method from a Comparator.)

Question 7:

What is the essential difference in functionality between a TreeMap and a HashMap?

Answer:

The key/value pairs in a TreeMap are sorted so that the keys are in ascending order. (For this reason, it must be possible to compare the keys in a TreeMap, using a compareTo() method or compare(). Either the keys must implement the Comparable interface or a Comparator must be provided to do the comparison.)

Question 8:

Explain what is meant by a hash code.

Answer:

The hash code of an object is an integer that tells where that object should be stored in a hash table. A hash table is an array of linked lists. When an object is stored in a hash table, it is added to one of these linked lists. The object's hash code is the index of the position in the array where the object is stored. All objects with the same hash code go into the same linked list. In Java, every object obj has a method obj.hashCode() that is used to compute hash codes for the object. If the object is to be stored in a hash table of size N, then the hash code that is used for the object is Math.abs(obj.hashCode())%N.

Question 9:

Modify the following Date class so that it implements the interface Comparable<Date>. The ordering on objects of type Date should be the natural, chronological ordering.

class Date {
int month;  // Month number in range 1 to 12.
int day;    // Day number in range 1 to 31.
int year;   // Year number.
Date(int m, int d, int y) {
month = m;
day = d;
year = y;
}
}

Answer:

The interface Comparable<Date> specifies the method "public int compareTo(Date d)", which will be used to compare two objects of type Date. The compareTo() method must be added to the class, and the class must be declared to implement the interface. To compare two dates, first try comparing the years. If the years are equal, try comparing the months. If the months are also equal, compare the days.

class Date implements Comparable<Date> {
int month;  // Month number in range 1 to 12.
int day;    // Day number in range 1 to 31.
int year;   // Year number.
Date(int m, int d, int y) {
month = m;
day = d;
year = y;
}
public int compareTo( Date otherDate ) {
// Returns 1, 0, or -1 if this date is greater than, equal to,
// or less than otherDate, respectively.
if (year < otherDate.year)
return -1;
else if (year > otherDate.year)
return 1;
else { // Years are equal; compare months.
if (month < otherDate.month)
return -1;
else if (month > otherDate.month)
return 1;
else { // Years and months are equal; compare days.
if (day < otherDate.day)
return -1;
else if (day > otherDate.day)
return 1;
else
return 0;
}
}
}
}

Question 10:

Suppose that syllabus is a variable of type TreeMap<Date,String>, where Date is the class from the preceding exercise. Write a code segment that will write out the value string for every key that is in the month of December, 2014.

Answer:

I will give two solutions. One of them simply looks up each date in December, 2010 in the map and prints the corresponding value, if there is one. The other iterates though a submap that contains all the entries for dates in that month.

A solution using the map's get() method:

for (int day = 1; day <= 31; day++) {
// Get the info for one day in December, 2014
Date date = new Date(12,day,2014); // The key.
String info = syllabus.get(date); // Get the value for that key.
// (Can be null if there is no
// entry in the map for this date.)
if (info != null)
System.out.println("December " + day + ": " + info);
}

A solution using a submap (harder, but more efficient):

Date startDate = new Date(12,1,2014); // Starting date for submap.
Date endDate = new Date(1,1,2015);    // Ending date for submap.
// (Remember that the end date
// is not included.)
Map<Date,String> decemberData = syllabus.subMap(startDate, endDate);
for ( Map.Entry<Date,String> entry : decemberData ) {
Date date = entry.getKey();
String info = entry.getValue();
System.out.println("December " + date.day + ": " + info);
}

Question 11:

Write a generic class Stack<T> that can be used to represent stacks of objects of type T. The class should include methods push(), pop(), and isEmpty(). Inside the class, use an ArrayList to hold the items on the stack.

Answer:

public class Stack<T> {
ArrayList<T> stack = new ArrayList<T>();
public void push( T newItem ) {
stack.add(newItem);
}
public T pop() {
int top = stack.size() - 1;  // location of top item
return stack.remove(top);    // remove and return top item
}
public boolean isEmpty() {
return stack.size() == 0;
}
}

Question 12:

Write a generic method, using a generic type parameter <T>, that replaces every occurrence in an ArrayList<T> of a specified item with a specified replacement item. The list and the two items are parameters to the method. Both items are of type T. Take into account the fact that the item that is being replaced might be null. For a non-null item, use equals() to do the comparison.

Answer:

Since the method operates on ArrayLists, it can use indexed access with the get(i) and set(i,item) methods. These operations are efficient for array lists. I also give a second version of the method that uses a list iterator and is efficient for any type of list.

public static <T> void replaceAll(ArrayList<T> list, T oldItem, T newItem) {
if (oldItem == null) {
for (int i = 0; i < list.size(); i++) {
if ( null == list.get(i) )
list.set( i, newItem );
}
}
else {
for (int i = 0; i < list.size(); i++) {
if ( oldItem.equals(list.get(i)) )
list.set( i, newItem );
}
}
}

public static <T> void replaceAll(List<T> list, T oldItem, T newItem) {
ListIterator<T> iter = list.listIterator();
while (iter.hasNext()) {
T listItem = iter.next();
if ( oldItem == null ) {
if ( listItem == null )
iter.set(newItem);
}
else {
if ( oldItem.equals(listItem) )
iter.set(newItem);
}
}
}

(Note, by the way, that a replaceAll method is already defined as a static method in class Collections.)

Answers for Quiz on Chapter 11

Question 1:

In Java, input/output is done using streams. Streams are an abstraction. Explain what this means and why it is important.

Answer:

A stream represents a source from which data can be read or a destination to which data can be written. A stream is an abstraction because it represents the abstract idea of a source or destination of data, as opposed to specific, concrete sources and destinations such as a particular file or network connection. The stream abstraction is important because it allows programmers to do input/output using the same methods for a wide variety of data sources and destinations. It hides the details of working with files, networks, and the screen and keyboard.

Question 2:

Java has two types of streams: character streams and byte streams. Why? What is the difference between the two types of streams?

Answer:

Character streams are for working with data in human-readable format, that is, data expressed as sequences of characters. Byte streams are for data expressed in the machine-readable format that is used internally in the computer to represent the data while a program is running. It is very efficient for a computer to read and write data in machine format, since no translation of the data is necessary. However, if a person must deal directly with the data, then character streams should be used so that the data is presented in human-readable form.

Question 3:

What is a file? Why are files necessary?

Answer:

A file is a collection of data that has been given a name and stored on some permanent storage device such as a hard disk or USB memory stick. Files are necessary because data stored in the computer's RAM is lost whenever the computer is turned off. Data that is to be saved permanently must be stored in a file. (Furthermore, RAM is very expensive compared to space on a disk drive, so a computer's hard disk can typically store much more data than would fit in the computer's RAM, even if the computer were left turned on all the time.)

Question 4:

What is the point of the following statement?

out = new PrintWriter( new FileWriter("data.dat") );

Why would you need a statement that involves two different stream classes, PrintWriter and FileWriter?

Answer:

The PrintWriter class is being used as a "wrapper" for the FileWriter class. A FileWriter is a stream object that knows how to send individual characters to a file. By wrapping this in a PrintWriter, you get the ability to write other data types such as ints, doubles, and Strings to the file using the PrintWriter's print() and println() methods. Wrapping the FileWriter in a PrintWriter adds capabilities to the file output stream but still sends the data to the same destination.

Question 5:

The package java.io includes a class named URL. What does an object of type URL represent, and how is it used?

Answer:

A url is an address for a web page (or other information) on the Internet. For example, "http://math.hws.edu/javanotes/index.html" is a url that refers to the main page of the current edition of this on-line textbook. A URL object represents such an address. Once you have a URL object, you can call its openConnection() method to access the information at the web address that it represents.

Question 6:

What is the purpose of the JFileChooser class?

Answer:

An object of type JFileChooser represents a dialog box that can be used in a GUI program to let the user select a file that is to be used for input or for output.

Question 7:

Explain what is meant by the client / server model of network communication.

Answer:

In the client/server model, a server program runs on a computer somewhere on the Internet and "listens" for connection requests from client programs. The server makes some service available. A client program connects to the server to access that service. For example, a Web server has a collection of Web pages. A Web browser acts as a client for the Web server. It makes a connection to the server and sends a request for one of its pages. The server responds by transmitting a copy of the requested page back to the client.

Question 8:

What is a socket?

Answer:

A socket represents one endpoint of a network connection. A program uses a socket to communicate with another program over the network. Data written by a program to the socket at one end of the connection is transmitted to the socket on the other end of the connection, where it can be read by the program at that end.

Question 9:

What is a ServerSocket and how is it used?

Answer:

A ServerSocket is used by a server program to listen for connection requests from client programs. If listener refers to an object that belongs to Java's ServerSocket class, then calling the function listener.accept() will wait for a connection request and will return a Socket object that can be used to communicate with the client that made the request.

Question 10:

What is meant by an element in an XML document?

Answer:

An element consists of a tag (possibly containing attributes), a matching end-tag, and everything in between. There are also empty elements where an empty element consists of a single self-closing tag. An element can contain textual content and nested elements. For example, in the XML fragment

<testresult score="82"><name>Joe Smith</name><subject>Math</subject></testresult>

the entire fragment is an element, while <name>Joe Smith</name> and <subject>Math</subject> are nested elements inside the <testresult> element. "Joe Smith" and "Math" are examples of textual content, and score is an attribute with value 82.

Question 11:

What is it about XML that makes it suitable for representing almost any type of data?

Answer:

XML is a syntax for building data representation languages. In XML, the names of tags and the structure of a document can be chosen to be whatever is most appropriate for the type of data that is being represented. The tag names can be chosen to give meaningful descriptions of the data. This contrasts with HTML documents, which can include only a certain fixed set of tags. (XML also has the advantage of being a widely accepted standard that is supported in just about every programming language.)

Question 12:

Write a complete program that will display the first ten lines from a text file. The lines should be written to standard output, System.out. The file name is given as the command-line argument args[0]. You can assume that the file contains at least ten lines. Don't bother to make the program robust. Do not use TextIO to process the file; read from the file using methods covered in this chapter.

Answer:

I will give three different solutions. The first uses the non-standard TextReader class from Subsection 11.1.4 to read the lines from the file. The other two solutions use the standard classes BufferedReader and Scanner. All of these classes make it easy to read a line of text from an input stream.

For each program, I do everything in one big try statement. If anything goes wrong, an error message is printed in the catch clause of the try statement. For example, if the program is run with no command-line argument, an IndexOutOfBoundsException will be generated when the program refers to args[0]. If a file is specified, but it doesn't exist, then a FileNotFoundException will occur. (The exercise says to assume that the file has at least ten lines, but if there are fewer than ten lines in the file, TextReader and Scanner will throw an exception, while BufferedReader will just print a null for each missing line.)

import java.io.*;
// (TextReader.class must be made available to this program.)
public class TenLinesWithTextReader {

public static void main(String[] args) {
try {
TextReader in = new TextReader( new FileReader(args[0]) );
for (int lineCt = 0; lineCt < 10; lineCt++)) {
String line = in.getln();
System.out.println(line);
}
}
catch (Exception e) {
System.out.println("Error: " + e);
}
}

}  // end class TenLinesWithTextReader

//-----------------------------------------------------------------

import java.io.*;
public class TenLinesWithBufferedReader {

public static void main(String[] args) {
try {
BufferedReader in = new BufferedReader( new FileReader(args[0]) );
for (int lineCt = 0; lineCt < 10; lineCt++)) {
String line = in.readLine();
System.out.println(line);
}
}
catch (Exception e) {
System.out.println("Error: " + e);
}
}

}  // end class TenLinesWithBufferedReader

//-----------------------------------------------------------------

import java.io.*;
import java.util.Scanner;
public class TenLinesWithScanner {

public static void main(String[] args) {
try {
Scanner scanner = new Scanner( new File(args[0]) );
for (int lineCt = 0; lineCt < 10; lineCt++)) {
String line = scanner.nextLine();
System.out.println(line);
}
}
catch (Exception e) {
System.out.println("Error: " + e);
}
}

}  // end class TenLinesWithScanner

Answers for Quiz on Chapter 12

Question 1:

Write a complete subclass of Thread to represent a thread that writes out the numbers from 1 to 10. Then write some code that would create and start a thread belonging to that class.

Answer:

The class that defines the thread:

public class CountingThread extends Thread {
public static run() {
for (int i = 1; i <= 10; i++)
System.out.println(i);
}
}

Code to create an object of type CountingThread and start the thread:

CountingThread counter;     // Declare a variable to represent a thread.
counter = new CountingThread();  // Create the thread object.
counter.start();                 // Start the thread running.

(Note that it's possible to create a new thread and start it with one statement:

new CountingThread().start();

Because of the precedence of the new operator, this is equivalent to (new CountingThread()).start(), and the effect is to create a new object of type CountingThread and then to call the start() method in that object.)

Question 2:

Suppose that thrd is an object of type Thread. Explain the difference between calling thrd.start() and calling thrd.run().

Answer:

Calling thrd.start() creates a new thread of control and returns immediately after doing so; then the thread's run() method is called in the new thread of control. The code in the run() method is executed at the same time, in parallel, with the code that follows the call to thrd.start().

The statement thrd.run() calls the run method in the usual way: The run() method is executed in the same thread of control as the method that called run(). Only after the run() method returns will the code that follows thrd.run() be executed.

Question 3:

What is a race condition?

Answer:

A race condition is a problem that can occur in a multithreaded program. Suppose that a thread takes a sequence of actions in which one action can depend on the result of a previous action. A race condition occurs if it's possible for another thread to change or invalidate the result of the previous action, before the first thread completes the sequence. For example, there is a race condition in the simple assignment statement count = count+1 because it is executed as a sequence of steps. The old value of count is read, one is added to that value, and the new value is stored back into count. A race condition occurs if it is possible for another thread to increment the value of count between the time when the first thread reads the old value and the time when it stores the new value. In this case, the race condition can result in an incorrect value for count -- count might be increased by one when it was supposed to be increased by two. Another example occurs in the if statement

if ( ! list.isEmpty() )
return list.removeFirst();

There is a race condition if it is possible for another thread to empty the list between the time when the first thread tests whether the list is empty and the time when the first thread tries to remove an element from the list. In this case, the race condition can result in an exception when the first thread tries to remove an item from an empty list.

Question 4:

How does synchronization prevent race conditions, and what does it mean to say that synchronization only provides mutual exclusion?

Answer:

Synchronization makes it possible for a thread to complete a sequence of actions without interference from another thread. Two threads cannot be synchronized on the same object at the same time. While one thread is executing a synchronized block of code, it's impossible for another thread to be executing the same block of code, or any other block of code that is synchronized on the same object. For example, if a thread executes

synchronized(list) {
if ( ! list.isEmpty() )
return list.removeFirst();
}

then, assuming that all code that manipulates list is properly synchronized, it can be sure that no other thread will be able to empty the list between the time when the first thread tests whether the list is empty and the time when it removes the first element from the list.

Synchronization provides only mutual exclusion because it only protects a thread from other threads that are also synchronized on the same object. In the example, the code has exclusive access to list only if all the code segments that manipulate list are synchronized. There is no protection against a thread that executes the statement list.clear() without synchronization.

Question 5:

Suppose that a program uses a single thread that takes 4 seconds to run. Now suppose that the program creates two threads and divides the same work between the two threads. What can be said about the expected execution time of the program that uses two threads?

Answer:

The execution time will depend on whether the program is being run on a computer that has more than one processor. If so, the execution time could be as little as 2 seconds, since each of two processors can do half of the 4-seconds worth of work. If the computer has only one processor, however, the two-threaded program will still take 4 seconds, since all the work will have to be done by the single processor.

Question 6:

What is an ArrayBlockingQueue and how does it solve the producer/consumer problem?

Answer:

An ArrayBlockingQueue is a queue in which the operations of adding and removing items can block. Adding an item will block if the queue is full; removing an item will block if the queue is empty. (Furthermore, operations on the queue are properly synchronized for use in a multithreaded program.)

The producer/consumer problem is the problem of safely and efficiently getting items that are produced by one group of threads to a second group of threads that consume the items. If the items are sent through a blocking queue, then the threads in the second group will block when there are no items available for them to consume, and threads in the first group will block if they are producing items faster than they can be consumed. (Furthermore, the synchronization guarantees that no item will be lost or consumed twice.)

In an application in which only the consuming threads should block, a LinkedBlockingQueue, which has unlimited capacity, can be used.

Question 7:

What is a thread pool?

Answer:

Thread pools are used when a large number of tasks are to be performed, as an alternative to creating a new thread to execute each task. A thread pool is a relatively small collection of threads that are available for performing tasks. When a task becomes available, instead of creating a new thread for the task, the task is assigned to one of the threads from the pool. When the task is complete, the thread goes back into the pool so that more tasks can be assigned to it.

Typically, tasks are placed into a queue as they become available. Each thread in the pool runs in an infinite loop in which it repeatedly takes a task from the queue and executes it. (Blocking queues work well for this application.)

Question 8:

Network server programs are often multithreaded. Explain what this means and why it is true.

Answer:

A multi-threaded server uses threads to handle the client connections that it accepts. A server program is generally designed to process connection requests from many clients. It runs in an infinite loop in which it accepts a connection request and processes it. If the processing takes a significant amount of time, it's not a good idea to make the other clients wait while the current client is processed. The solution is for the server to make a new thread to handle each client connection, or to use a thread pool of threads that can handle the client connections. The server can continue to accept more client connections even while the first client is being serviced.

Question 9:

Why does a multithreaded network server program often use many times more threads than the number of available processors?

Answer:

Network operations can block, which means that a thread that handles communication with a client will often spend most of its time sleeping. In order to keep all the processors busy, the number of active threads should be comparable to the number of processors. If at any given time, the number of active threads is only a fraction of the total number of threads, then the total number of threads should be several times the number of processors.

Question 10:

Consider the ThreadSafeCounter example from Subsection 12.1.3:

public class ThreadSafeCounter {

private int count = 0;  // The value of the counter.

synchronized public void increment() {
count = count + 1;
}

synchronized public int getValue() {
return count;
}

}

The increment() method is synchronized so that the caller of the method can complete the three steps of the operation "Get value of count," "Add 1 to value," "Store new value in count" without being interrupted by another thread. But getValue() consists of a single, simple step. Why is getValue() synchronized? (This is a deep and tricky question.)

Answer:

The getValue() method has to be synchronized because of the caching of local data that was discussed in Subsection 12.1.4. If getValue() were not synchronized, it is possible that a thread that calls getValue() would see an old, cached value of count rather than the most current value. Synchronization ensures that the most current value of count will be seen. If count were declared to be a volatile variable, then getValue() would not have to be synchronized. However, increment() would still need to be synchronized to prevent the race condition.

Answers for Quiz on Chapter 13

Question 1:

Describe the object that is created by the following statement, and give an example of how it might be used in a program:

BufferedImage OSC = new BufferedImage(32,32,BufferedImage.TYPE_INT_RGB);

Answer:

A BufferedImage is a region in memory that can be used as a drawing surface. In this statement, the image that is created is 32 pixels wide and 32 pixels high, and the color of each pixel is an RGB color that has red, green, and blue components in the range 0 to 255. The picture in a BufferedImage can easily be copied into a graphics context g by calling one of the g.drawImage methods. However, since the image is so small in this case, it seems more likely that is going to be used to define an ImageIcon or perhaps a Cursor.

Question 2:

Many programs depend on resource files. What is meant by a resource in this sense? Give an example.

Answer:

In addition to the program code itself, programs often use other types of data such as images and sounds. These are said to be resources for the program. In order to run, the program must be able to locate and load the resources. An example of a resource is a small image file that will be used by the program to define a custom cursor.

Question 3:

What is the FontMetrics class used for?

Answer:

An object that belongs to the class FontMetrics can be used to obtain information about the sizes of characters and strings that are drawn in a specific font. The font is specified when the FontMetrics object is created. If fm is a variable of type FontMetrics, then, for example, fm.stringWidth(str) gives the width of the string str and fm.getHeight() is the usual amount of vertical space allowed for one line of text. This information could be used, for example, for positioning the string in a component.

Question 4:

If a Color, c, is created as c = new Color(0,0,255,125), what is the effect of drawing with this color?

Answer:

When a color is constructed from four integers, the fourth parameter is the alpha component of the color. When drawing, the alpha component is interpreted as the degree of opaqueness of the color. If the alpha component is less than its maximum possible value, 255, then the color is partially transparent. In this case, c represents a blue color that is about 50% transparent. When a pixel is colored with c, the pixel does not become entirely blue. Instead, the new color of the pixel is obtained by blending c with the previous color of the pixel. The effect is like adding a blue tint to the pixel, or like looking at the previous color of the pixel through blue-colored glass.

Question 5:

What is antialiasing?

Answer:

Antialiasing is used to make shapes that are drawn look less "jagged." The jaggedness is called "aliasing," and it arises because shapes have to be drawn by coloring individual pixels. If antialiasing is off, then the only decision is either to color a pixel or not to color it. As a result, a boundary that should be a smooth curve will look like a jagged staircase. When antialiasing is on, the amount of color that is applied to a pixel depends on how much of that pixel is covered by the ideal geometric shape. For pixels along the boundary of the shape, the color of the shape will be blended with the previous color of the pixel. Although antialiasing does not produce a perfect result, it does tend to make pictures look better.

Question 6:

How is the ButtonGroup class used?

Answer:

A ButtonGroup object is used with a set of radio buttons (or radio button menu items), to make sure that at most one of the radio buttons in the group can be selected at any given time. To use the ButtonGroup class, you have to create a ButtonGroup object, grp. Then each radio button, rb, that is supposed to be part of the group is added to the group by calling grp.add(rb). Nothing further needs to be done with the ButtonGroup object.

Question 7:

What does the acronym MVC stand for, and how does it apply to the JTable class?

Answer:

MVC stands for "Model-View-Controller." In a JTable, the view is the actual visible component on the screen. The model is the collection of data that specifies, among other things, what appears in each cell of the table and in the column headings. The model is represented by a separate object from the object that represents the view. The controller is responsible for interaction with the user. It consists mostly of a bunch of listener objects that listen for events generated when the user interacts with the table. The listeners respond by making changes in the model, which will in turn cause a change in the view.

Question 8:

Describe the picture that is produced by the following paintComponent() method:

public void paintComponent(Graphics g) {
super.paintComponent(g);
Graphics2D g2 = (Graphics2D)g;
g2.translate( getWidth()/2, getHeight()/2 );
g2.rotate( 30 * Math.PI / 180 );
g2.fillRect(0,0,100,100);
}

Answer:

This shows a filled black square that is 100-by-100 pixels in size. The corner of the square is at the center of the component that is being painted, and the top side of the square descends at a 30 degree angle from that point. (The translate command moves the origin, (0,0) to the point (getWidth()/2,getHeight()/2), so that when the fillRect command places the corner of the square at (0,0), the corner actually appears at the center of the component. Furthermore, the rotate command rotates the picture by 30 degrees in a clockwise direction about the origin. This means that the top of the square is rotated from the horizontal position onto a line that is 30 degrees clockwise from the horizontal. That line descends at a 30 degree angle.

Question 9:

What is meant by Internationalization of a program?

Answer:

Internationalization refers to writing the program in a way that will make it easy to adapt the program for use in a variety of "locales." For example, it should be easy to translate all the strings that are used in the program into other languages. To make this possible the strings should not be hard coded into the program itself. Instead, they are placed in a separate resource file, so that the program can be translated into another language simply by writing a resource file for that language. (Internationalization also applies to the format that is used for dates and numbers.)

Question 10:

Suppose that the class that you are writing has an instance method doOpen() (with no parameters) that is meant to be used to open a file selected by the user. Write a code segment that creates an Action that represents the action of opening a file. Then show how to create a button and a menu item from that action.

Answer:

Here is the code for the three parts of the problem:

Action openAction = new AbstractAction( "Open..." ) {
public void actionPerformed( ActionEvent e ) {
doOpen();
}
};

JButton openButton = new JButton( openAction );

JMenuItem openCommand = new JMenuItem( openAction );

(Since Action is only an interface, the class AbstractAction has to be used to create the action object. The most natural way to write the code is to create an anonymous inner class that is a subclass of AbstractAction. This subclass must define the actionPerformed() method -- which in this case only has to call the doOpen() method. As an alternative to creating the JMenuItem, the action could have simply been added directly to a JMenu. By the way, the "..." in the name of the action is there, by convention, to tell the user that selecting this command will cause a dialog box to pop up.)

Answers to End-Of-Chapter Exercises

This section contains sample solutions to the exercises that occur at the end of each chapter, starting with Chapter 2. The solution includes a discussion of the problem, as well as the code for a solution. The discussion is meant to be read, and in many cases will supplement the reading from the chapter itself.

Remember that there are many ways to solve a programming problem!

Solution for Programming Exercise 2.1

Exercise 2.1:

Write a program that will print your initials to standard output in letters that are nine lines tall. Each big letter should be made up of a bunch of *'s. For example, if your initials were "DJE", then the output would look something like:

******           *************        **********
**    **                **            **
**     **               **            **
**      **              **            **
**      **              **            ********
**      **       **     **            **
**     **         **    **            **
**    **           **  **             **
*****               ****              **********

Discussion

This is a very simple program. It consists of nothing but a sequence of System.out.println statements. The hard part is designing the letters. You have to experiment in your text editor until you get letters that look right to you. Add System.out.println(" at the beginning of the first line, then copy-and-paste it to the beginning of each of the other lines. (You don't need to type it out on each line! Creative use of copy-and-paste can save you a lot of work.) Then add "); to the end of each line. In my program, I've also added a System.out.println() statement at the beginning and the end. The extra blank lines make the output look better.

Don't forget that in order to have a complete program, you have to put the program statements in a main subroutine and put that subroutine in a public class. Also, the name of the file must correspond to the name of the class; in this case, the file must be named PrintInitials.java.

The Solution

public class PrintInitials {

/*  This program prints my initials (DJE) in big letters,
where each letter is nine lines tall.
*/

public static void main(String[] args) {
System.out.println();
System.out.println("           ******           *************        **********");
System.out.println("           **    **                **            **");
System.out.println("           **     **               **            **");
System.out.println("           **      **              **            **");
System.out.println("           **      **              **            ********");
System.out.println("           **      **       **     **            **");
System.out.println("           **     **         **    **            **");
System.out.println("           **    **           **  **             **");
System.out.println("           *****               ****              **********");
System.out.println();
}  // end main()

}  // end class

Solution for Programming Exercise 2.2

Exercise 2.2:

Write a program that simulates rolling a pair of dice. You can simulate rolling one die by choosing one of the integers 1, 2, 3, 4, 5, or 6 at random. The number you pick represents the number on the die after it is rolled. As pointed out in Section 2.5, the expression

(int)(Math.random()*6) + 1

does the computation to select a random integer between 1 and 6. You can assign this value to a variable to represent one of the dice that are being rolled. Do this twice and add the results together to get the total roll. Your program should report the number showing on each die as well as the total roll. For example:

The first die comes up 3
The second die comes up 5
Your total roll is 8

Discussion

When designing a program, one of the first things you should ask yourself is, "What values do I need to represent?" The answer helps you decide what variables to declare in the program. This program will need some variables to represent the numbers showing on each die and the total of the two dice. Since these numbers are all integers, we can use three variables of type int. I'll call the variables die1, die2, and roll. The program begins by declaring the variables:

int die1;
int die2;
int roll;

In the actual program, of course, I've added a comment to explain the purpose of each variable. The values of die1 and die2 can be computed using the expression given in the exercise:

die1 = (int)(Math.random()*6) + 1;
die2 = (int)(Math.random()*6) + 1;

Note that even though the expressions on the right-hand sides of these assignment statements are the same, the values can be different because the function Math.random() can return different values when it is called twice.

We can then compute roll = die1 + die2 and use three System.out.println statements to display the three lines of output:

System.out.println("The first die comes up " + die1);
System.out.println("The second die comes up " + die2);
System.out.println("Your total roll is " + roll);

Note that I've chosen to use the concatenation operator, +, to append the value of die1 onto the string "The first die comes up". Alternatively, I could use two output statements:

System.out.print("The first die comes up ");
System.out.println(die1);

Yet another possibility is to use System.out.printf:

System.out.printf("The first die comes up %d%n", die1);

I'll also note that I could get away without the variable roll, since I could output the value of the expression die1 + die2 directly:

System.out.println("Your total roll is " + (die1 + die2));

However, it's generally better style to have a meaningful name for a quantity. By the way, the parentheses around (die1 + die2) are essential because of the precedence rules for the + operator. You might try to experiment with leaving them out and see what happens.

The Solution

public class RollTheDice {

/*  This program simulates rolling a pair of dice.
The number that comes up on each die is output,
followed by the total of the two dice.
*/

public static void main(String[] args) {

int die1;   // The number on the first die.
int die2;   // The number on the second die.
int roll;   // The total roll (sum of the two dice).

die1 = (int)(Math.random()*6) + 1;
die2 = (int)(Math.random()*6) + 1;
roll = die1 + die2;

System.out.println("The first die comes up " + die1);
System.out.println("The second die comes up " + die2);
System.out.println("Your total roll is " + roll);

}  // end main()

}  // end class

Solution for Programming Exercise 2.3

Exercise 2.3:

Write a program that asks the user's name, and then greets the user by name. Before outputting the user's name, convert it to upper case letters. For example, if the user's name is Fred, then the program should respond "Hello, FRED, nice to meet you!".

Discussion

In order to read the name typed in by the user, this program uses one of the input routines from the non-standard TextIO class. So, this program can only be run if that class is available to the program. I will use System.out for output. (Although it is possible to use TextIO for output as well as for input, I will generally avoid that option in my examples.)

A name is a sequence of characters, so it is a value of type String. The program must declare a variable of type String to hold the user's name. I declare another variable to hold the user's name with all the letters in the name converted to upper case. The conversion might be difficult, except that String objects have a function that lets you do the conversion with one line of code. If usersName is the variable that refers to the name that the user enters, then the function call usersName.toUpperCase() returns the string obtained by replacing any lower case letters in the name with upper case letters.

There are several functions in the TextIO class that can be used for reading Strings: getWord(), getlnWord(), and getln(). The first two routines only read a single word, so if the user entered "David J. Eck", they would only read the first name, "David". The getln() routine will read the entire line, and so would get the whole name. For this program, I use getln(), but you might prefer to use just the first name.

I also give a version of the program that uses the built-in class Scanner, instead of TextIO, for reading input from the user. See Subsection 2.4.6 for a discussion of the Scanner class. In the second version of the program, a Scanner named stdin is created, and the user's input is read using the function stdin.nextLine(), which has essentially the same functionality as TextIO.getln().

(For this program, by the way, TextIO.getWord() and TextIO.getlnWord() would be equivalent. They return the same value. The second version of the routine, getlnWord(), would then discard the rest of the user's line of input. However, since this program is only doing one input operation, it doesn't matter whether it's discarded. It would only matter when it came time to read a second value from input.)

The Solution

public class Greeting {

/*  This program asks the user's name and then
greets the user by name.  This program depends
on the non-standard class, TextIO.
*/

public static void main(String[] args) {

String usersName;      // The user's name, as entered by the user.
String upperCaseName;  // The user's name, converted to upper case letters.

System.out.print("Please enter your name: ");
usersName = TextIO.getln();

upperCaseName = usersName.toUpperCase();

System.out.println("Hello, " + upperCaseName + ", nice to meet you!");

}  // end main()

}  // end class

A version using Scanner for input:

import java.util.Scanner;

public class GreetingWithScanner {

public static void main(String[] args) {

Scanner stdin = new Scanner( System.in );

String usersName;      // The user's name, as entered by the user.
String upperCaseName;  // The user's name, converted to upper case letters.

System.out.print("Please enter your name: ");
usersName = stdin.nextLine();

upperCaseName = usersName.toUpperCase();

System.out.println("Hello, " + upperCaseName + ", nice to meet you!");

}  // end main()

}  // end class

Solution for Programming Exercise 2.4

Exercise 2.4:

Write a program that helps the user count his change. The program should ask how many quarters the user has, then how many dimes, then how many nickels, then how many pennies. Then the program should tell the user how much money he has, expressed in dollars.

Discussion

The program will need variables to represent the number of each type of coin. Since the number of coins has to be an integer, these variables are of type int. I'll call the variables quarters, dimes, nickels, and pennies.

The total value of the coins, when expressed in dollars, can be a non-integer number such as 1.57 or 3.02. Since the total value in dollars is a real number, I will use a variable of type double to represent it. The variable is named dollars

The outline of the program is clear enough:

Declare the variables.
Ask the user for the number of each type of coin, and read the responses.
Compute the total value of the coins, in dollars.
Display the result to the user.

The function TextIO.getlnInt() can be used to read each of the user's responses. The alternative function TextIO.getInt() could also be used, but it is less safe. Suppose, for example, that the user responds to the request to type in the number of quarters by entering "7 quarters". After TextIO.getlnInt() reads the number 7, it will discard the extra input "quarters". TextIO.getInt() will read the 7 correctly, but the extra input is not discarded. Later, when the program tries to read the number of dimes, it sees the left-over input and tries to read that, without giving the user a chance to type in another response. You might want to experiment and see what happens if you change getlnInt() to getInt(). (Of course, if the user's response is "I have 7 quarters" or "seven", then you are out of luck in any case.)

Since one quarter is worth 0.25 dollars, the number of dollars in N quarters is 0.25*N. Similarly, a dime is worth 0.10 dollars, a nickel is 0.05 dollars, and a penny is 0.01 dollars. So, to get the total value of all the user's coins, I just have to add up (0.25*quarters) + (0.10*dimes) + (0.05*nickels) + (0.01*pennies). This value is assigned to the variable, dollars, and that is the result that is displayed to the user. Formatted output, using System.out.printf, can be used to specify that the output value should be printed with two digits after the decimal point.

Alternatively, I could first have computed the total number of cents in all the coins, and then divided by 100 to convert the amount into dollars:

int totalCents;   // Total number of cents in the coins.

totalCents = 25*quarters + 10*dimes + 5*nickels + pennies;
dollars = totalCents/100.0;

Since totalCents is of type int, it is essential here that I compute dollars as totalCents/100.0 and not as totalCents/100. The value computed by totalCents/100 is an integer. For example, if totalCents is 397, then totalCents/100 is 3. Using totalCents/100.0 forces the computer to compute the answer as a real number, giving 3.97.

A second version of the program, using a Scanner for input, is also given. (See Subsection 2.4.6.) A Scanner named stdio is created, and integers are read using the function stdio.nextInt(). This method corresponds to TextIO.getInt(), not TextIO.getlnInt(). To get something equivalent to TextIO.getlnInt(), we can follow the call to stdio.nextInt() with a call to stdio.nextLine(). The call to stdio.nextLine() will read and discard any extra characters on the same line after the integer that was read by stdio.nextInt(). If you are not worried about the user typing extra stuff on a line, you can leave out the calls to stdio.nextInt().

The Solution

public class CountChange {

/*  This program will add up the value of a number of quarters,
dimes, nickels, and pennies.  The number of each type of
coin is input by the user.  The total value is reported
in dollars.  This program depends on the non-standard class,
TextIO.
*/

public static void main(String[] args) {

int quarters;   // Number of quarters, to be input by the user.
int dimes;      // Number of dimes, to be input by the user.
int nickels;    // Number of nickels, to be input by the user.
int pennies;    // Number of pennies, to be input by the user.

double dollars; // Total value of all the coins, in dollars.

/* Ask the user for the number of each type of coin. */

System.out.print("Enter the number of quarters:  ");
quarters = TextIO.getlnInt();

System.out.print("Enter the number of dimes:     ");
dimes = TextIO.getlnInt();

System.out.print("Enter the number of nickels:   ");
nickels = TextIO.getlnInt();

System.out.print("Enter the number of pennies:   ");
pennies = TextIO.getlnInt();

/* Add up the values of the coins, in dollars. */

dollars = (0.25 * quarters) + (0.10 * dimes)
+ (0.05 * nickels) + (0.01 * pennies);

/* Report the result back to the user. */

System.out.println();
System.out.print("The total in dollars is $");
System.out.printf("%1.2f", dollars);  // Formatted output!
System.out.println();

}  // end main()

}  // end class

A version using Scanner for input:

import java.util.Scanner;

public class CountChangeWithScanner {

public static void main(String[] args) {

int quarters;   // Number of quarters, to be input by the user.
int dimes;      // Number of dimes, to be input by the user.
int nickels;    // Number of nickels, to be input by the user.
int pennies;    // Number of pennies, to be input by the user.

Scanner stdio = new Scanner( System.in );

double dollars; // Total value of all the coins, in dollars.

/* Ask the user for the number of each type of coin. */

System.out.print("Enter the number of quarters:  ");
quarters = stdio.nextInt();
stdio.nextLine();

System.out.print("Enter the number of dimes:     ");
dimes = stdio.nextInt();
stdio.nextLine();

System.out.print("Enter the number of nickels:   ");
nickels = stdio.nextInt();
stdio.nextLine();

System.out.print("Enter the number of pennies:   ");
pennies = stdio.nextInt();
stdio.nextLine();

/* Add up the values of the coins, in dollars. */

dollars = (0.25 * quarters) + (0.10 * dimes)
+ (0.05 * nickels) + (0.01 * pennies);

/* Report the result back to the user. */

System.out.println();
System.out.print("The total in dollars is $");
System.out.printf("%1.2f", dollars);  // Formatted output!
System.out.println();

}  // end main()

}  // end class

Solution for Programming Exercise 2.5

Exercise 2.5:

If you have N eggs, then you have N/12 dozen eggs, with N%12 eggs left over. (This is essentially the definition of the / and % operators for integers.) Write a program that asks the user how many eggs she has and then tells the user how many dozen eggs she has and how many extra eggs are left over.

A gross of eggs is equal to 144 eggs. Extend your program so that it will tell the user how many gross, how many dozen, and how many left over eggs she has. For example, if the user says that she has 1342 eggs, then your program would respond with

Your number of eggs is 9 gross, 3 dozen, and 10

since 1342 is equal to 9*144 + 3*12 + 10.

Discussion

All the variables for the program will be of type int. The quantities that we have to represent are: the number of eggs, the number of dozens in that many eggs, and the number of left over eggs. I will declare variables named eggs, dozens, and extras to represent these quantities.

The number of eggs can be read from the user's input using eggs = TextIO.getlnInt().

The description of the problem already tells us that dozens can be computed as eggs/12 and extras can be computed as eggs%12. So, the first version of the program is easy to write. Note that the entire output from this program could have been done with one System.out.printf:

System.out.print("Your number of eggs is %d dozen and %d extras%n",
dozens, extras);

The improved version requires a little thought. Given a pile of eggs that you want to divide into gross, dozens, and extras, you could first remove as many gross as possible, leaving a pile of between 0 and 143 eggs. This is just like dividing the pile into dozens, except that the unit of measure is 144 eggs instead of 12. So, eggs/144 will give the number of gross, and eggs%144 is the number of eggs left over. Next, the eggs in the left-over pile still have to be divided into dozens, plus some extra number of eggs between 0 and 11. Of course, we already know how to do this: If there are N eggs in the pile, the number of dozens is N/12 and the number of extra eggs is N%12. This leads to the computation in the improved version of the program, shown below. (Note that this program uses the variable named aboveGross to represent a quantity that is not mentioned explicitly in the problem description. This is far from being unusual.)

The Solution

public class Dozens {

/*  This program will convert a given number of eggs into
the number of dozens plus the number of left over eggs.
For example, 57 eggs is 4 dozen eggs plus 9 eggs left over.
The number of eggs is input by the user, and the computed
results are displayed.
*/

public static void main(String[] args) {

int eggs;    // Number of eggs, input by user.
int dozens;  // How many dozens in that number of eggs?
int extras;  // How many eggs are left over, above an integral
//    number of dozens?  This value is in the
//    range 0 to 11, and it is computed as
//    the remainder when eggs is divided by 12.

System.out.print("How many eggs do you have?  ");
eggs = TextIO.getlnInt();

dozens = eggs / 12;
extras = eggs % 12;

System.out.print("Your number of eggs is ");
System.out.print(dozens);
System.out.print(" dozen and ");
System.out.print(extras);
System.out.println();

}  // end main()

}  // end class

Improved version:

public class GrossAndDozens {

/*  This program will convert a given number of eggs into
the number of gross plus the number of dozens plus the
number of left over eggs.
For example, 1342 eggs is 9 gross plus 3 dozen plus 10.
The number of eggs is input by the user, and the computed
results are displayed.
*/

public static void main(String[] args) {

int eggs;         // Number of eggs, input by user.
int gross;        // How many gross in that number of eggs?
int aboveGross;   // How many eggs are left over, above an
//    integral number of gross?  This number
//    can be computed as eggs % 144, and is
//    in the range 0 to 143.  This number will
//    be divided into dozens and extras.
int dozens;       // How many dozens in aboveGross?
int extras;       // How many eggs are left over, above integral
//    numbers of gross and dozens?

System.out.print("How many eggs do you have?  ");
eggs = TextIO.getlnInt();

gross = eggs / 144;
aboveGross = eggs % 144;

dozens = aboveGross / 12;
extras = aboveGross % 12;

System.out.print("Your number of eggs is ");
System.out.print(gross);
System.out.print(" gross, ");
System.out.print(dozens);
System.out.print(" dozen, and ");
System.out.print(extras);
System.out.println();

}  // end main()

}  // end class

Solution for Programming Exercise 2.6

Exercise 2.6:

This exercise asks you to write a program that tests some of the built-in subroutines for working with Strings. The program should ask the user to enter their first name and their last name, separated by a space. Read the user's response using TextIO.getln(). Break the input string up into two strings, one containing the first name and one containing the last name. You can do that by using the indexOf() subroutine to find the position of the space, and then using substring() to extract each of the two names. Also output the number of characters in each name, and output the user's initials. (The initials are the first letter of the first name together with the first letter of the last name.) A sample run of the program should look something like this:

Please enter your first name and last name, separated by a space.
? Mary Smith
Your first name is Mary, which has 4 characters
Your last name is Smith, which has 5 characters
Your initials are MS

Discussion

Once you have the first name and the last name in separate String variables, it's easy to produce the desired output. The length of a string, str, can be obtained by calling the function str.length(). To get the first character of str, you can call str.charAt(0). (Remember that characters are numbered starting from zero.) If the first and last names are in the variables firstName and lastName, then the output of the program can be produced using

System.out.println("Your first name is " + firstName + ", which has "
+ firstName.length() + " characters.");
System.out.println("Your last name is " + lastName + ", which has "
+ lastName.length() + " characters.");
System.out.println("Your initials are " + firstName.charAt(0) + lastName.charAt(0));

Or, using formatted output to make it easier to read:

System.out.printf( "Your first name is %s, which has %d characters.%n",
firstName, firstName.length() );
System.out.printf( "Your last name is %s, which has %d characters.%n",
lastName, lastName.length() );
System.out.print( "Your initials are %s%s%n",
firstName.charAt(0), lastName.charAt(0) );

Note that it is not necessary to put values like firstName.length() and firstName.charAt(0) into their own variables, if all that you want to do with them is print them out. (But some people might consider it better style to use variables.)

The harder part of the problem is breaking up the input string to get the first and last name. If the user's input is in a String named input, then input.indexOf(' ') is an int that gives the position of the space in the string. (Note that the space character is written as a literal that consists of a space between two left single quotation marks.)

Once you have the position of the space, you have to figure out how to use it to get the first and last name. Let's look at exactly how the characters in a string are numbered:

M  a  r  y     S  m  i  t  h
0  1  2  3  4  5  6  7  8  9

In this example, the space is at index 4. We have to extract the substring in positions 0 through 3 and the substring in positions 5 through 9. The value of input.substring(start,end) will be a string that starts in position start and ends in position end-1. Note that the value of the second parameter is the position after the end of the substring. So, the fist name in this case is input.substring(0,4) and the last name is input.substring(5,10). (Even though there is no character in position 10, that's the correct value for the position after the end of the substring that we want.) In fact, there's an easier way to get the last name, using another version of substring: The value of input.substring(start) is the substring starting at position start and extending to the end of the string, so the last name in this example is input.substring(4).

The important thing to note here is that the "4" in input.substring(0,4) is the index of the space character, and the "5" in input.substring(5) is that index plus 1. So we can get the first and last names using:

space = input.indexOf(' ');
firstName = input.substring(0,space);  // Everything before the space
lastName = input.substring(space+1);   // Everything after the space

By the way, this program will only work correctly if the user follows the instructions. If the user's input does not contain a space, then the value of input.indexOf(' ') will be -1, and the -1 will cause the program to crash when used in the substring function. And if the input doens't have exactly one space, then the program output won't be correct. For now, you don't have any way to test the user's input to see whether it is correct. To do that, you need to know about if statements, which you will learn about very soon.

One more remark: Instead of using TextIO.getln() to read a line of text and then breaking that line into two words, you could simply use TextIO.getWord() twice to read the two words from the same line of input -- except that the exercise says to use getln().

The Solution

/**
* This program reads the user's first name and last name,
* separated by a space.  It then prints the user's first and
* last names separately, along with the number of characters
* in each name.  It also prints the user's initials.  Note that
* this program will crash if the user's input does not contain
* a space.
*/
public class FirstNameLastName {

public static void main(String[] args) {

String input;     // The input line entered by the user.
int space;        // The location of the space in the input.
String firstName; // The first name, extracted from the input.
String lastName;  // The last name, extracted from the input.

System.out.println();
System.out.println("Please enter your first name and last name, separated by a space.");
System.out.print("? ");
input = TextIO.getln();

space = input.indexOf(' ');
firstName = input.substring(0, space);
lastName = input.substring(space+1);

System.out.println("Your first name is " + firstName + ", which has "
+ firstName.length() + " characters.");
System.out.println("Your last name is " + lastName + ", which has "
+ lastName.length() + " characters.");
System.out.println("Your initials are " + firstName.charAt(0) + lastName.charAt(0));

}

}

Solution for Programming Exercise 2.7

Exercise 2.7:

Suppose that a file named "testdata.txt" contains the following information: The first line of the file is the name of a student. Each of the next three lines contains an integer. The integers are the student's scores on three exams. Write a program that will read the information in the file and display (on standard output) a message that contains the name of the student and the student's average grade on the three exams. The average is obtained by adding up the individual exam grades and then dividing by the number of exams.

Discussion

TextIO can be used to read data from a file; this is discussed in Subsection 2.4.4. To read data from a file named "testdata.txt", all you need to do is say TextIO.readFile("testdata.txt"). From then on, the input functions in TextIO will read from the file instead of reading data typed in by the user. (Note that this assumes that the file is in the same directory with the program.) In this case, we can use TextIO.getln() to read the student's name from the first line of the file, and then we can read the exam grades by calling TextIO.getln() three times.

The average should be computed as a value of type double. Don't forget that if you divide an integer by an integer in Java, the result is an integer and the remainder of the division is discarded. To get the correct average in this case, the program divides the sum of the three grades by 3.0 rather than by 3.

One final technicality is that simply outputting a double value might print out something like 83.333333333333333. By default, all significant digits in the number are output. In this case, one digit after the decimal point is probably sufficient. The program uses formatted output to achieve this:

System.out.printf("The average grade for %s was %1.1f", name, average);

The format string "The average grade for %s was %1.1f" is used to format the name and the average. The name is substituted for the format specifier %s, which means that the name is printed as a string, with no extra spaces. The average is substituted for %1.1f, which means that the average is printed as a floating point number with no extra spaces and with 1 digit after the decimal point.

You might want to run this program with no data file, or with a data file that is not in the correct format, to see what happens. (The program will crash and print an error message.)

The Solution

public class FindAverage {

public static void main(String[] args) {

String name;     // The student's name, from the first line of the file.
int exam1, exam2, exam3;     // The student's grades on the three exams.
double average;  // The average of the three exam grades.

TextIO.readFile("testdata.txt");  // Read from the file.

name = TextIO.getln();  // Reads the entire first line of the file.
exam1 = TextIO.getlnInt();
exam2 = TextIO.getlnInt();
exam3 = TextIO.getlnInt();

average = ( exam1 + exam2 + exam3 ) / 3.0;

System.out.printf("The average grade for %s was %1.1f", name, average);
System.out.println();

}

}

Solution for Programming Exercise 3.1

Exercise 3.1:

How many times do you have to roll a pair of dice before they come up snake eyes? You could do the experiment by rolling the dice by hand. Write a computer program that simulates the experiment. The program should report the number of rolls that it makes before the dice come up snake eyes. (Note: "Snake eyes" means that both dice show a value of 1.) Exercise 2.2 explained how to simulate rolling a pair of dice.

Discussion

Since we want to roll the dice at least once, a do..while is appropriate. A pseudocode algorithm for the program is

Let countRolls = 0
do:
roll the dice
count this roll by adding 1 to countRolls
while the roll is not snake eyes
Output the value of countRolls

As in Exercise 2.2, we can simulate rolling one die by computing (int)(Math.random()*6) + 1.

We want to stop rolling the dice when the roll is a double 1. We want to continue rolling the dice while the roll is not a double 1. If die1 and die2 are variables representing the values of the dice, the condition for continuing to roll can be expressed as

while ( ! (die1 == 1 && die2 == 1) )

The exclamation point means "not", so the condition says that it is not the case that (both die1 is 1 and die2 is 1). That is, it is not the case that the dice came up snake eyes. Another way to express the same condition is that at least one of the dice is not 1, that is, that either die1 is not 1 or die2 is not 1. In Java code, this is written:

while ( die1 != 1  ||  die2 != 1 )

This is the test that I use in my program. Students often get the && and || operators mixed up, especially when negation is involved. (In this case, we could have avoided the problem by testing while (die1+die2 != 2).)

Filling in some details gives an algorithm that can be easily converted into a program, which is shown below:

Let countRolls = 0
do:
die1 = (int)(Math.random()*6) + 1
die2 = (int)(Math.random()*6) + 1
count this roll by adding 1 to countRolls
while die1 is not 1 or die2 is not 1
Output the value of countRolls

You could of course write the program using a regular while loop instead of do..while. But if you move the test to the start of the loop, you have to make sure that the variables are given values before you try to test those values. You need to "prime" the loop that exist only to ensure that it runs the first time. The pseudocode then becomes

Let countRolls = 0
Let die1 = 0 // Prime the loop with any value except 1
Let die2 = 0
while die1 is not 1 or die2 is not 1:
die1 = (int)(Math.random()*6) + 1
die2 = (int)(Math.random()*6) + 1
count this roll by adding 1 to countRolls
Output the value of countRolls

Another option would be to use a "while(true)" loop. In that case, the test that is needed is the condition for stopping the loop, which is often easier to come up with than the test for continuing the loop, and no priming is necessary:

Let countRolls = 0
while true:
die1 = (int)(Math.random()*6) + 1
die2 = (int)(Math.random()*6) + 1
count this roll by adding 1 to countRolls
if die1 is 1 and die2 is 1
break
Output the value of countRolls

The Solution

/**
* This program simulates rolling a pair of dice until they
* come up snake eyes.  It reports how many rolls were needed.
*/

public class SnakeEyes {

public static void main(String[] args) {

int die1, die2;   // The values rolled on the two dice.

int countRolls;   // Used to count the number of rolls.

countRolls = 0;

do {
die1 = (int)(Math.random()*6) + 1;   // roll the dice
die2 = (int)(Math.random()*6) + 1;
countRolls++;                        // and count this roll
} while ( die1 != 1 || die2 != 1 );

System.out.println("It took " + countRolls + " rolls to get snake eyes.");

}  // end main()

}  // end class

Solution for Programming Exercise 3.2

Exercise 3.2:

Which integer between 1 and 10000 has the largest number of divisors, and how many divisors does it have? Write a program to find the answers and print out the results. It is possible that several integers in this range have the same, maximum number of divisors. Your program only has to print out one of them. An example in Subsection 3.4.2 discussed divisors. The source code for that example is CountDivisors.java.

You might need some hints about how to find a maximum value. The basic idea is to go through all the integers, keeping track of the largest number of divisors that you've seen so far. Also, keep track of the integer that had that number of divisors.

Discussion

Let's use a variable named maxDivisors to keep track of the largest number of divisors we have seen so far and use numWithMax to store the number that had that many divisors. We have to compute the number of divisors of each integer from 1 to 10000. Whenever we find a larger number of divisors than maxDivisors, we have to make note of that fact by changing the values of maxDivisors and numWithMax. At the end of the process, maxDivisors will be the absolute maximum number of divisors and numWithMax will be a number that had that many divisors. These are the values we want to print out. We can express this with a pseudocode algorithm

for each integer N from 1 to 10000:
Count the number of divisors of N
If that number is greater than maxDivisors:
Let maxDivisors = the number of divisors of N
Let numWithMax = N
Output maxDivisors and numWithMax

However, there is a problem here: The very first time maxDivisors is used in the test "If that number is greater than maxDivisors," the variable maxDivisors hasn't yet been assigned a value. The computer will report this as an error. This can be fixed by assigning a value to maxDivisors before the beginning of the for loop. One way to do this is to handle N=1 as a special case before the loop and then to let N go from 2 to 10000 in the for loop. We know that N=1 has just 1 divisor:

Let maxDivisors = 1  // number of divisors of 1
Let numWithMax = 1
for each integer N from 2 to 10000:
Count the number of divisors of N
If that number is greater than maxDivisors:
Let maxDivisors = the number of divisors of N
Let numWithMax = N
Output maxDivisors and numWithMax

Here's a curious thing: If you leave out the line "numWithMax = 1" from the program, the computer will report a syntax error where you try to output the value of numWithMax. It will say that the variable numWithMax might not have been initialized. That is, it might never have been assigned a value. Now, you know that it will be assigned a value (since when N=2 is processed, numWithMax will become 2). However, when the computer compiles the program, it doesn't know whether the body of the if statement will ever be executed, so it doesn't know whether numWithMax will ever be assigned a value. The syntax rule is that every variable must be "definitely assigned" before its value is used. This means that it is assigned a value on every possible execution path through the program. Definite assignment was discussed in Subsection 3.1.4.

We still have to expand the step "Count the number of divisors of N." This was already done in Subsection 3.4.2 for the example program CountDivisors.java. This step requires another for loop, so we have here an example of one for loop nested inside another. Here is a complete algorithm, which can be translated into a program:

Let maxDivisors = 1  // number of divisors of 1
Let numWithMax = 1
for each integer N from 2 to 10000:
Let divisorCount = 0
for each D from 1 to N:
if D is a divisor of N:
add 1 to divisorCount
If divisorCount is greater than maxDivisors:
Let maxDivisors = the number of divisors of N
Let numWithMax = N
Output maxDivisors and numWithMax

This can be translated pretty much directly into a program. By the way, the maximum number of divisors is 64. There are two numbers between 1 and 10000 that have 64 divisors, 7560 and 9240. The program will output the first of these. (It would output the second if the test "if (divisorCount > maxDivisors)" were changed to "if (divisorCount >= maxDivisors)". Do you see why?)

The Solution

/**
* This program finds an integer between 1 and 10000 that has
* the largest number of divisors.  It prints out the maximum
* number of divisors and an integer that has that many divisors.
*/

public class MostDivisors {

public static void main(String[] args) {

int N;            // One of the integers whose divisors we have to count.
int maxDivisors;  // Maximum number of divisors seen so far.
int numWithMax;   // A value of N that had the given number of divisors.

maxDivisors = 1;  // Start with the fact that 1 has 1 divisor.
numWithMax = 1;

/* Process all the remaining values of N from 2 to 10000, and
update the values of maxDivisors and numWithMax whenever we
find a value of N that has more divisors than the current value
of maxDivisors.
*/

for ( N = 2;  N <= 10000;  N++ ) {

int D;  // A number to be tested to see if it's a divisor of N.
int divisorCount;  // Number of divisors of N.

divisorCount = 0;

for ( D = 1;  D <= N;  D++ ) {  // Count the divisors of N.
if ( N % D == 0 )
divisorCount++;
}

if (divisorCount > maxDivisors) {
maxDivisors = divisorCount;
numWithMax = N;
}

}

System.out.println("Among integers between 1 and 10000,");
System.out.println("The maximum number of divisors is " + maxDivisors);
System.out.println("A number with " + maxDivisors + " divisors is " + numWithMax);

} // end main()

}

Solution for Programming Exercise 3.3

Exercise 3.3:

Write a program that will evaluate simple expressions such as 17 + 3 and 3.14159 * 4.7. The expressions are to be typed in by the user. The input always consists of a number, followed by an operator, followed by another number. The operators that are allowed are +, -, *, and /. You can read the numbers with TextIO.getDouble() and the operator with TextIO.getChar(). Your program should read an expression, print its value, read another expression, print its value, and so on. The program should end when the user enters 0 as the first number on the line.

Discussion

We need a loop to read and evaluate expressions. It's easiest to use a break statement to end the loop at the appropriate time:

Repeat indefinitely:
Get the user's input.
if the first number is 0:
Break out of the loop
Find the value of the expression
Display the value.

Getting the user's input involves reading three data values. We need three variables to store these values. It's best to test whether the first number is 0 right after we read it, so the user will just have to type a 0 to end the program, not a complete expression such as 0 + 0. "Repeat indefinitely" can be written as "while (true)":

while (true):
Let firstNum = TextIO.getDouble()
if firstNum is 0:
Break out of the loop
Let operator = TextIO.getChar()
Let secondNum = TextIO.getlnDouble()
Find the value of the expression
Display the value.

To evaluate the user's expression, we have to test the operator to find out which operation to compute. We can do this with either a multi-way if statement or with a switch statement. In the program below, I use a switch. The if statement would be:

if ( operator == '+' )
value = firstNum + secondNum;
else if ( operator == '-' )
value = firstNum - secondNum;
else if ( operator == '*' )
value = firstNum * secondNum;
else if ( operator == '/' )
value = firstNum / secondNum;
else {
System.out.println("Unknown operator: " + operator);
continue;  // back to start of loop
}

The computer won't let you get away without the else part of the if statement or the default case in the switch, since that would leave a possibility that the variable, value, is not assigned a value before it is printed out, which violates the definite assignment rule. Note that char values are tested for equality using the == operator. Chars are not Strings! Strings would have to be compared using the equals() method from the String class; chars however are primitive type values, and they don't even include any methods.

This program could be improved by having it print out an error message if the user tries to divide by zero.

(This program could use a Scanner instead of TextIO for input, replacing TextIO.getDouble() with stdin.nextDouble() and TextIO.getChar() with stdin.next(), where stdin is the Scanner. (See Subsection 2.4.6) One difference between the behavior of the two versions of the program would be that the TextIO version would accept expressions that have no spaces, such as 17+42, whereas the Scanner version would give an error for such an expression. Another is that scanner.next() returns a String rather than a char, and the character that you want would be the first, and only, character in that string.)

The Solution

/**
*  This program evaluates simple expressions such as 2 + 2
*  and 34.2 * 7.81, consisting of a number, an operator,
*  and another number.  The operators +, -, *, / are allowed.
*  The program will read and evaluate expressions until
*  the user inputs a line that starts with the number 0.
*/

public class SimpleCalculator {

public static void main(String[] args) {

double firstNum;    // First number in the expression.
double secondNum;   // Second number in the expression.
char operator;      // The operator in the expression.
double value;       // The value of the expression.

System.out.println("Enter expressions such as  2 + 2  or  34.2 * 7.81");
System.out.println("using any of the operators +, -, *, /.");
System.out.println("To end, enter a 0.");
System.out.println();

while (true) {

/* Get user's input, ending program if first number is 0. */

System.out.print("? ");
firstNum = TextIO.getDouble();
if (firstNum == 0)
break;
operator = TextIO.getChar();
secondNum = TextIO.getlnDouble();

/* Compute the value of the expression. */

switch (operator) {
case '+':
value = firstNum + secondNum;
break;
case '-':
value = firstNum - secondNum;
break;
case '*':
value = firstNum * secondNum;
break;
case '/':
value = firstNum / secondNum;
break;
default:
System.out.println("Unknown operator: " + operator);
continue;  // Back to start of loop!
} // end switch

/* Display the value. */

System.out.println("Value is " + value);
System.out.println();

} // end while

System.out.println("Good bye");

}  // end main()

}  // end class SimpleCalculator

Solution for Programming Exercise 3.4

Exercise 3.4:

Write a program that reads one line of input text and breaks it up into words. The words should be output one per line. A word is defined to be a sequence of letters. Any characters in the input that are not letters should be discarded. For example, if the user inputs the line

He said, "That's not a good idea."

then the output of the program should be

He
said
That
s
not
a
good
idea

An improved version of the program would list "that's" as a single word. An apostrophe can be considered to be part of a word if there is a letter on each side of the apostrophe.

To test whether a character is a letter, you might use (ch >= 'a' && ch <= 'z') || (ch >= 'A' && ch <= 'Z'). However, this only works in English and similar languages. A better choice is to call the standard function Character.isLetter(ch), which returns a boolean value of true if ch is a letter and false if it is not. This works for any Unicode character.

Discussion

There are many ways to approach this problem, and probably all of them are sort of tricky to get right. Here's a simple idea that almost works: Go through all the characters in the string. If the character is a letter, write it out. If it's not a letter, write a carriage return instead. If line is a String variable representing the line of text, this algorithm can be coded as

for ( i = 0;  i < line.length(); i++ ) {
ch = line.charAt(i);
if ( Character.isLetter(ch) )
System.out.print(ch);
else
System.out.println();
}

This prints all the letters in a word on the same line of output. Since words in the string are separated by non-letter characters, and the computer prints a carriage return when it finds a non-letter, words in the output are separated by carriage returns. But there are is a problem with this: If two words in the string are separated by several non-letters, then there will be one or more blank lines between the words in the output. We don't want to output two carriage returns in a row. To avoid this, we can keep track of whether the previous output was a letter or a carriage return. When we find a non-letter, we will output a carriage return only if the previous output was not a carriage return. To keep track of the necessary information, I'll use a boolean variable named didCR. The value of this variable will be true if the previous output was a carriage return. I have to remember to set the value of didCR each time I output something. With this modification, the code becomes:

for ( i = 0;  i < line.length(); i++ ) {
ch = line.charAt(i);
if ( Character.isLetter(ch) ) {
System.out.print(ch);
didCR = false;  // previous output was not a CR
}
else {
if ( didCR == false ) {  // output CR, if previous output was NOT a CR
System.out.println();
didCR = true;  // previous output was a CR
}
}
}

The program requires an initial value for didCR. In the program below, I output a carriage return before the for loop and set didCR to true. You should be able to follow the rest of the program.

An entirely different approach to this problem is given by the algorithm, "while there are more words in the string, get the next word and print it." This turns out to be even harder to implement than the above.

The Solution

/**
*  This program will read one line of input typed by the user.
*  It will print the words from the input, one word to a line.
*  A word is defined to be a sequence of letters.  All non-letters
*  in the input are discarded.
*/

public class ListWordsInString {

public static void main(String[] args) {

String line;    // A line of text, typed in by the user.
int i;          // Position in line, from 0 to line.length() - 1.
char ch;        // One of the characters in line.
boolean didCR;  // Set to true if the previous output was a carriage return.

System.out.println("Enter a line of text.");
System.out.print("? ");
line = TextIO.getln();

System.out.println();
didCR = true;

for ( i = 0;  i < line.length();  i++ ) {
ch = line.charAt(i);
if ( Character.isLetter(ch) ) {
System.out.print(ch);
didCR = false;
}
else {
if ( didCR == false ) {
System.out.println();
didCR = true;
}
}
}

System.out.println();  // Make sure there's at least one carriage return at the end.

}  // end main()

}  // end class ListWordsInString

Solution for Programming Exercise 3.5

Exercise 3.5:

Suppose that a file contains information about sales figures for a company in various cities. Each line of the file contains a city name, followed by a colon (:) followed by the data for that city. The data is a number of type double. However, for some cities, no data was available. In these lines, the data is replaced by a comment explaining why the data is missing. For example, several lines from the file might look like:

San Francisco:  19887.32
Chicago:  no report received
New York: 298734.12

Write a program that will compute and print the total sales from all the cities together. The program should also report the number of cities for which data was not available. The name of the file is "sales.dat".

To complete this program, you'll need one fact about file input with TextIO that was not covered in Subsection 2.4.4. Since you don't know in advance how many lines there are in the file, you need a way to tell when you have gotten to the end of the file. When TextIO is reading from a file, the function TextIO.eof() can be used to test for end of file. This boolean-valued function returns true if the file has been entirely read and returns false if there is more data to read in the file. This means that you can read the lines of the file in a loop while (TextIO.eof() == false).... The loop will end when all the lines of the file have been read.

Suggestion: For each line, read and ignore characters up to the colon. Then read the rest of the line into a variable of type String. Try to convert the string into a number, and use try..catch to test whether the conversion succeeds.

Discussion

The statement TextIO.readFile("sales.dat") can be used at the beginning of the program to make TextIO read from the file instead of from the user's input. This statement throws an IllegalArgumentException if the file can't be found. Although it's not required by the statement of the problem, it would be nice to detect the exception and print an error message about the missing file. This can be done as follows, using System.exit to end the program if the error occurs:

try {
TextIO.readFile("sales.dat");
}
catch (IllegalArgumentException e) {
System.out.println("Can't open file \"sales.dat\" for reading!");
System.out.println("Please make sure the file is present before");
System.out.println("running the program.");
System.exit(1);  // Terminates the program.
}

We have two main quantities to keep track of in the program: The total amount of sales and the number of cities for which data is missing. These quantities can be represented by two variables, salesTotal of type double and missingCount of type int. These variables are initialized to zero before processing any data from the file, and their values are output at the end of the program, after reading the entire file.

The main processing loop of the program is the while loop mentioned in the statement of the exercise, which begins while (TextIO.eof() == false). (In my program, I've chosen to use the equivalent but more elegant test while ( ! TextIO.eof() ); you should try to understand why this is equivalent -- and why it is more elegant!) In pseudocode form, we want a loop that does the following:

while there are more lines in the file:
read the name of the city and the colon that follows it
read the rest of the line into a string
try to convert the string into a number
if the conversion succeeds, add the number to the total sales
otherwise add 1 to the count of missing data

To read past the name of the city, we can simply read characters using TextIO.getChar() until we get to the colon:

do {  // Read past characters up to the ':'.
ch = TextIO.getChar();
} while (ch != ':');

where ch is a variable of type char. We can read the rest of the line using TextIO.getln(). (As a side note, it's important here that we read the entire line, including the carriage return at the end. TextIO.getln() does this. If we neglected to read the carriage return at the end of the line, TextIO.eof() would not detect the end of the file correctly. After the data from the last line is read, the carriage return character would still be there in the file waiting to be read; since there is still something left to read in the file, TextIO.eof() would be false false and the while loop would continue, even though all the cities have already been processed. In fact, the program would crash while looking for the next ':', which is not there. Text processing can be surprisingly subtle!)

Once we have the information in the form of a String, we can try to convert the string into a number with Double.parseDouble. If the string is not a number, this will throw an exception of type NumberFormatException. We can catch the error in a try..catch statement. The "missing data" processing is done in the catch part of this statement. In the program, this takes the form:

try {
sales = Double.parseDouble(dataString);
salesTotal += sales;  // This is skipped if the conversion fails.
}
catch (NumberFormatException e) {
// The dataString is not a number, so it counts as
// missing data.  Add 1 to the missing data count.
missingCount++;
}

(Note that my solution assumes that the data file (if it exists at all) has exactly the format that is specified in the problem. The problem would be more difficult if we had to check the format of the file, to see whether it conforms to the specification.)

The Solution

/**
* This program reads from a file named "sales.dat".  Each line of the
* file contains the name of a city, followed by a colon, followed by
* either a number giving the amount of sales in that city or by a
* message saying why the sales figure is not available.  The program
* prints the total sales for all cities and the number of cites for
* which the figure was not available.
*/

public class SalesFigures {

public static void main( String[] args ) {

/* Open file for reading; if it can't be opened, end the program */

try {
TextIO.readFile("sales.dat");
}
catch (IllegalArgumentException e) {
System.out.println("Can't open file \"sales.dat\" for reading!");
System.out.println("Please make sure the file is present before");
System.out.println("running the program.");
System.exit(1);  // Terminates the program.
}

/* Read the file, keeping track of total sales and missing data. */

double salesTotal;  // Total of all sales figures seen so far.
int missingCount;   // Number of cities for which data is missing.

salesTotal = 0;
missingCount = 0;

while ( ! TextIO.eof() ) {  // process one line of data.

char ch;  // For reading past the name of the city.
String dataString;  // Contents of line, after the city name.
double sales;  // The sales figure for the city.

do {  // Read past characters up to the ':'.
ch = TextIO.getChar();
} while (ch != ':');

dataString = TextIO.getln();  // Get the rest of the line.

try {
sales = Double.parseDouble(dataString);
salesTotal += sales;  // This is skipped if the conversion fails.
}
catch (NumberFormatException e) {
// The dataString is not a number, so it counts as
// missing data.  Add 1 to the missing data count.
missingCount++;
}

} // end while

/* Report the results. */

System.out.printf("Total sales recorded from all cities: $%1.2f\n\n", salesTotal);
if (missingCount == 0)
System.out.println("Data was received from all cities.");
else if (missingCount == 1)
System.out.println("Data was missing from 1 city.");
else
System.out.printf("Data was missing from %d cities.\n", missingCount);

} // end main()

} // end class SalesFigures

Solution for Programming Exercise 3.6

Exercise 3.6:

Exercise 3.2 asked you to find the number in the range 1 to 10000 that has the largest number of divisors. You only had to print out one such number. Revise the program so that it will print out all numbers that have the maximum number of divisors. Use an array as follows: As you count the divisors for each number, store each count in an array. Then at the end of the program, you can go through the array and print out all the numbers that have the maximum count. The output from the program should look something like this:

Among integers between 1 and 10000,
The maximum number of divisors was 64
Numbers with that many divisors include:
7560
9240

Discussion

This is a fairly straightforward exercise in using arrays. We need to save 10000 numbers in an array. The numbers are the divisor counts for the numbers 1 through 10000. The numbers that we want to store in the array are of type int, so int is the base type of the array. In the program, I use an array named saveCount of type int[] to store the data. It seems natural to store the divisor count for N in array element saveCount[N], but note that in that case, we don't use array element number 0, and we need an array element number 10000. Ignoring element 0 is fine, but for saveCount[10000] to exist, the length of the array must be at least 10001. I use an array of length 10001:

saveCount = new int[10001];

(An alternative would be to store the advisor count for N in saveCount[N-1]. In that case, we wouldn't need the extra array element, and the length of the array would be 10000.)

After computing each divisor count, we store the count in the array. At the same time, we are keeping track of the maximum number of divisors. After the end of the for loop that does the counting, we know the divisor count for each number and we know the maximum number of divisors. We just have to go through the array and print out every integer N for which the divisor count is equal to the maximum:

System.out.println("Numbers with that many divisors include:");
for ( N = 1; N <= 10000; N++ ) {
if ( saveCount[N] == maxDivisors ) {
System.out.println( "   " + N );
}
}

Note that this code checks whether the number in the array, saveCount[N], is equal to the maximum, but it prints the array index, N, which is the integer that has that many divisors.

The Solution

New code that involves the array is shown in red italic.

/**
* This program counts the number of divisors for integers in the range
* 1 to 10000.  It finds the maximum divisor count.  It outputs the
* maximum divisor count and a list of all integers in the range that
* have the maximum number of divisors.
*/

public class MostDivisorsWithArray {

public static void main(String[] args) {

int N;            // One of the integers whose divisors we have to count.
int maxDivisors;  // Maximum number of divisors seen so far.

int[] saveCount;  // Store the number of divisors for each number

saveCount = new int[10001];

maxDivisors = 1;  // Start with the fact that 1 has 1 divisor.
saveCount[1] = 1;

/* Process all the remaining values of N from 2 to 10000, and store
all the divisor counts in the array.  Update the value of maxDivisor
whenever we find a value of N that has more divisors than the current
value.
*/

for ( N = 2;  N <= 10000;  N++ ) {

int D;  // A number to be tested to see if it's a divisor of N.
int divisorCount;  // Number of divisors of N.

divisorCount = 0;

for ( D = 1;  D <= N;  D++ ) {  // Count the divisors of N.
if ( N % D == 0 )
divisorCount++;
}

saveCount[N] = divisorCount;  // Record the count for N in the array

if (divisorCount > maxDivisors) {
maxDivisors = divisorCount;
}

}

System.out.println("Among integers between 1 and 10000,");
System.out.println("The maximum number of divisors was " + maxDivisors);
System.out.println("Numbers with that many divisors include:");
for ( N = 1; N <= 10000; N++ ) {
if ( saveCount[N] == maxDivisors ) {
System.out.println( "   " + N );
}
}

} // end main()

}

Solution for Programming Exercise 3.7

Exercise 3.7:

An example in Subsection 3.8.3 tried to answer the question, How many random people do you have to select before you find a duplicate birthday? The source code for that program can be found in the file BirthdayProblem.java. Here are some related questions:

- How many random people do you have to select before you find three people who share the same birthday? (That is, all three people were born on the same day in the same month, but not necessarily in the same year.)

- Suppose you choose 365 people at random. How many different birthdays will they have? (The number could theoretically be anywhere from 1 to 365).

- How many different people do you have to check before you've found at least one person with a birthday on each of the 365 days of the year?

Write three programs to answer these questions. Each of your programs should simulate choosing people at random and checking their birthdays. (In each case, ignore the possibility of leap years.)

Discussion

The original program and the three programs for this exercise have some similarities, and we will use ideas from the original program. However, each part of this exercise presents its own problem.

The first question asks how many people you have to choose at random before finding three who share the same birthday. This is similar to the original program, but instead of just checking whether or not a given birthday has already been found, we need to keep track of how many people have been found with each birthday. Where the original program used an array of booleans, here we need an array of ints. We still want to count the number of people we check and output that count at the end. An algorithm for the simulation is:

Let count = 0
Repeat:
Select a birthday at random
Add one to count
If this is the third time that birthday has occurred:
break out of the loop
Add one to the number of times that birthday has been found
Output the count

The original program used a boolean array to keep track of whether or not each day had been seen. For this proble, we need to know how many times each day has been seen. That means that instead of one boolean value for each day, we need one integer value for each day. So, to do the counting, we need an array "int[] numFound", where numFound[i] will be the number of times the i-th day of the year has occurred as a birthday. Since numFound[i] can be used in any way that any int variable can be used, we can add one to the number stored in numFound[i] by saying "numFound[i]++" or "numFound[i] = numFound[i] + 1". When we create the array with the command "numFound = new int[365]", all the elements of the array are automatically initialized to zero. This is the initial value that we want. (This is an important thing to remember! In some programming languages, arrays are not automatically filled with zeros, so it would be necessary to use a for loop to store a zero into each place in the array. Even in Java, if you reuse the same array rather than creating a new one for each use, you would have to remember to set initialize each element of the array before reusing it.)

Given all this, we can translate the algorithm into Java as follows:

int[] numFound;  // numFound[i] will be the number of people
// who have been found who have a birthday
// on the i-th day of the year

int count;       // The number of people who have been checked.

numFound = new int[365];  // Initially, all entries are 0.

count = 0;

while (true) {
// Select a birthday at random, from 0 to 364.
// If the same birthday was already seen twice
// before, then quit.  Otherwise, add one to
// the corresponding entry in the numFound array
// to record that a person with that birthday
// has been found.
int birthday;  // The selected birthday.
birthday = (int)(Math.random()*365);
count++;
if ( numFound[birthday] == 2 )
break;  // It's the third time we've found this birthday.
numFound[birthday]++;
}

System.out.println("It took " + count +
" tries to find three people with the same birthday.");

The lines shown in red italic are the ones that differ significantly from the original program. This becomes the body of the main() subroutine in the first program.

For the second program, we know exactly how many people we want to check: 365. This calls for using a for loop. The information we need for each birthday is whether or not that birthday has occurred. For that, we can use an array of booleans. After the for loop, value stored in position i of the array will true if the i-th day of the year has occurred as a birthday and is false if not. After checking 365 people, we have to go through the boolean array and count the number of entries in the array that are true. This is the number of different birthdays that have been found. The algorithm can be expressed as:

Let used = new boolean[365]
Repeat 365 times:
Select a birthday at random
Store true into the corresponding location in the array, used
Let count = 0
for day = 0 to 364:
If used[day] is true:
Add 1 to count
Output the value of count

This translates easily into Java code:

boolean used[];  // used[i] will be true if a person is found
// whose birthday is the i-th day of the year.

used = new boolean[365];  // Initially, all entries are false!

for (i = 0; i < 365; i++) {
// Select a random birthday and record it.
int birthday;  // The selected birthday.
birthday = (int)(Math.random()*365);
used[birthday] = true;
}

int count = 0;

for (day = 0; day < 365; day++) {
// If this day occurred as a birthday, count it.
if (used[day] == true)
count++;
}

System.out.println("Among 365 people, there were " + count
+ " different birthdays.");

It might be worth noting that the test "if (used[day] == true)" can be written more simply -- and elegantly -- as "if (used[day])". Also, the three lines in the first for loop could be reduced to the single command "used[(int)(Math.random()*365)] = true;". Of course, this one-line version is harder to understand.

The third program is just a little bit trickier. We have to continue selecting people at random until we have found 365 different birthdays. We can use a counter to keep track of how many different birthdays we have found. We have to continue until this counter reaches 365. We need a second counter to keep track of how many different people we have checked. It's the second counter whose value we want to output at the end. Now, we have to be able to recognize whether a birthday we've just found is new, or whether we've encountered it previously. For this, we can again use an array of booleans. An algorithm for the simulation is:

Let used = new boolean[365]
Let count = 0   // The number of people checked
Let birthdaysFound = 0   // The number of different birthdays found
while birthdaysFound < 365:
Add one to count
Select a birthday at random
if used[birthday] is false:
Add one to birthdaysFound  // since this is a new birthday
Let used[birthday] = true   // so we don't count it again
Output the value of count

In Java, this algorithm becomes:

boolean[] used;  // For recording the possible birthdays
//   that have been seen so far.
int count;       // The number of people who have been checked.
int birthdaysFound;   // The number of different birthdays that
// have been found.

used = new boolean[365];  // Initially, all entries are false.
count = 0;
birthdaysFound = 0;

while (birthdaysFound < 365) {
// Select a birthday at random, from 0 to 364.
// If the birthday has not already been used,
// add 1 to birthdaysFound.
int birthday;  // The selected birthday.
birthday = (int)(Math.random()*365);
count++;
if ( used[birthday] == false )
birthdaysFound++;
used[birthday] = true;
}

System.out.println( count + " people were checked." );

The Solution

Finding three people with the same birthday:

/**
*  How many random people do you have to select before you find
*  THREE with the same birthday (that is, three people who were born
*  on the same day of the same month, but not necessarily in the
*  same year).  This program simulates the process.  (It ignores the
*  possibility of people born on leap day.)
*/

public class BirthdayProblem2 {

/**
* Simulate choosing people at random and checking the
* day of the year they were born on.  If the person is
* the third who was born on that day of the year, stop,
* and output the number of people who were checked.
*/
public static void main(String[] args) {

int[] numFound;  // numFound[i] will be the number of people
// who have been found who have a birthday
// on the i-th day of the year

int count;       // The number of people who have been checked.

numFound = new int[365];  // Initially, all entries are 0.

count = 0;

while (true) {
// Select a birthday at random, from 0 to 364.
// If the same birthday was already seen twice
// before, then quit.  Otherwise, add one to
// the corresponding entry in the numFound array
// to record that a person with that birthday
// has been found.
int birthday;  // The selected birthday.
birthday = (int)(Math.random()*365);
count++;
if ( numFound[birthday] == 2 )
break;
numFound[birthday]++;
}

System.out.println("It took " + count +
" tries to find three people with the same birthday.");

}

} // end class BirthdayProblem2

How many different birthdays do 365 people have?

/**
* This program simulates selecting 365 people at random and finding
* how many different birthdays they have among them.
*/

public class BirthdayProblem3 {

/**
* Simulate choosing people at random and checking the
* day of the year they were born on.  The number of
* different days found among 365 people is counted
* and output.
*/
public static void main(String[] args) {

boolean used[];  // used[i] will be true if a person is found
// whose birthday is on the i-th day of the year.

used = new boolean[365];  // Initially, all entries are false.

/* Choose 365 days at random, and mark each day by
setting the corresponding entry in the array, used,
to true.  (If the value is already true, it doesn't
matter.  We are only interested in whether or not
the birthday occurs, not how many times it occurs.)
*/

int i;
for (i = 0; i < 365; i++) {
int birthday;  // The selected birthday.
birthday = (int)(Math.random()*365);
used[birthday] = true;
}

/* Now, count how many entries in the used array are true.
This is how many different birthdays were found.
*/

int count = 0;
int day;

for (day = 0; day < 365; day++) {
// If this day occurred as a birthday, count it.
if (used[day] == true)
count++;
}

System.out.println("Among 365 people, there were " + count
+ " different birthdays.");

}

} // end class BirthdayProblem3

Finding 365 different birthdays:

/**
* How many random people do you have to select before you
*  have found someone with every possible birthday (ignoring
*  leap years)?  This program simulates the process.
*/

public class BirthdayProblem4 {

/**
* Simulate choosing people at random and checking the
* day of the year they were born on.  People are chosen
* until all 365 possible birthdays (ignoring leap years)
* have been found.  Then the number of people surveyed
* is output.
*/
public static void main(String[] args) {

boolean[] used;  // For recording the possible birthdays
//   that have been seen so far.  A value
//   of true in used[i] means that a person
//   whose birthday is the i-th day of the
//   year has been found.

int count;       // The number of people who have been checked.

int birthdaysFound;   // The number of different birthdays that
// have been found.

used = new boolean[365];  // Initially, all entries are false.

count = 0;

birthdaysFound = 0;

while (birthdaysFound < 365) {
// Select a birthday at random, from 0 to 364.
// If the birthday has not already been used,
// add 1 to birthdaysFound.
int birthday;  // The selected birthday.
birthday = (int)(Math.random()*365);
count++;
if ( used[birthday] == false )
birthdaysFound++;
used[birthday] = true;
}

System.out.println( count + " people were checked." );

}

} // end class BirthdayProblem4

Solution for Programming Exercise 3.8

Exercise 3.8:

Write a GUI program that draws a checkerboard. Base your solution on the sample program SimpleAnimationStarter.java, even though you are creating only a static picture rather than an animation. You will draw the checkerboard in the drawFrame() subroutine. You should read the comments in the file to discover other changes that you might need to make.

Assume that the size of the drawing area is 400-by-400 pixels. A checkerboard contains 8 rows and 8 columns of squares. If the size of the drawing area is 400, that means that each square can be 50-by-50 pixels. The squares are red and black (or whatever other colors you choose). Here is a tricky way to determine whether a given square should be red or black: The rows and columns can be thought of as numbered from 0 to 7. If the row number of the square and the column number of the square are either both even or both odd, then the square is red. Otherwise, it is black. Note that a square is just a rectangle in which the height is equal to the width, so you can use the subroutine g.fillRect() to draw the squares. Here is a reduced-size image of the checkerboard that you want to draw:

Discussion

The basic algorithm is obvious:

for each row of the checkerboard:
Draw all the squares in that row

Since any given row contains eight squares, one in each column of the checkerboard, we can expand the body of the for loop into another for loop:

for each of the eight rows of the checkerboard:
for each of the eight columns:
Draw the square in that row and column

Each square is a rectangle with height 50 and width 50, so it can be drawn with the command g.fillRect(x,y,50,50), where x and y are the coordinates of the top-left corner of the square. Before drawing the square, we have to determine whether it should be red or black, and we have to set the correct color with g.setColor. So, the algorithm becomes

for each row on the checkerboard:
for each of the eight columns:
Compute x,y for the top-left corner of the square
if it's a red square:
g.setColor(Color.RED)
else
g.setColor(Color.BLACK)
g.fillRect(x,y,20,20)

The top of the first row of squares is at y=0. Since each square is 50 pixels high, the top of the second row is at y=50, followed by 100 for the third row, then 150, 200, 250, 300, and 350. If we assume that the rows are numbered 0, 1, 2, ..., 7, then the tops are given by y = row*50, where row is the row number. (If you number the rows 1, 2, ..., 8, the formula would be (row-1)*20. The simpler formula in this and in many similar cases is one reason why computer scientists like to start counting with 0 instead of 1.) Similarly, the left edge of the squares in column col is given by x = col*50, where again the columns are numbered 0, 1, 2, ..., 7. I'll use "for (row=0; row<8; row++)" to count off the rows, rather than the equivalent "for (row=0; row<=7; row++)". The 8 reminds me that I am counting off the eight numbers 0, 1, 2, ..., 7. Again, this is typical computer science style.

The only problem remaining is how to determine whether the square is red. As noted in the exercise, a square is red if row and col are either both even or both odd. Since an integer N is even if N%2 is 0, the test could be expressed as

if ((row%2 == 0 && col%2 == 0) || (row%2 == 1 && col%2 == 1))

However, note that this is the same as asking whether row%2 and col%2 have the same value. So the test can be written more simply as "if (row%2 == col%2)". Putting this all together into syntactically correct Java code, the algorithm becomes

for ( row = 0;  row < 8;  row++ ) {
for ( col = 0;  col < 8;  col++ ) {
x = 50*col;
y = 50*row;
if ( (row % 2) == (col % 2) )
g.setColor(Color.RED);
else
g.setColor(Color.BLACK);
g.fillRect(x,y,50,50);
}
}

Of course, the variables row, col, x, and y have to be declared to be of type int. Then, the code goes into the body of the drawFrame()subroutine.

The complete program is shown below. In the original program, the size of the drawing area was 600-by-450. As noted in a comment, to change the size to 400-by-400, I had to change one of the lines in the main routine, replacing 600 and 450 by 400 to get:

drawingArea.setPreferredSize(new Dimension(400,400));

You don't need to understand what this means or how it works to make the change. I also wanted to stop the user from changing the size of the window, to change the text in the title bar of the window, and to change the name of the class from SimpleAnimationStarter to Checkerboard. All these things were easy to do following instructions in the program. (That leaves the program drawing the same checkerboard 50 times every second. Although the comments in the original program don't tell you how to prevent that, I did it by commenting out the very last line of main(). Without that line, the animation is never started, and the checkerboard doesn't get redrawn over and over unnecessarily.)

The Solution

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class Checkerboard extends JPanel implements ActionListener {

public void drawFrame(Graphics g, int frameNumber, int width, int height) {

int row;   // Row number, from 0 to 7
int col;   // Column number, from 0 to 7
int x,y;   // Top-left corner of square

for ( row = 0;  row < 8;  row++ ) {

for ( col = 0;  col < 8;  col++) {
x = col * 50;
y = row * 50;
if ( (row % 2) == (col % 2) )
g.setColor(Color.RED);
else
g.setColor(Color.BLACK);
g.fillRect(x, y, 50, 50);
}

}

}

//------ Implementation details: DO NOT EXPECT TO UNDERSTAND THIS ------

public static void main(String[] args) {

JFrame window = new JFrame("Checkerboard");
Checkerboard drawingArea = new Checkerboard();
drawingArea.setBackground(Color.WHITE);
window.setContentPane(drawingArea);
drawingArea.setPreferredSize(new Dimension(400,400));
window.pack();
window.setLocation(100,50);
window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
window.setResizable(false); // The user can't change the size.
Timer frameTimer = new Timer(20,drawingArea);
window.setVisible(true);
//frameTimer.start(); // commented out so we don't get an animation

} // end main

private int frameNum;

public void actionPerformed(ActionEvent evt) {
frameNum++;
repaint();
}

protected void paintComponent(Graphics g) {
super.paintComponent(g);
drawFrame(g, frameNum, getWidth(), getHeight());
}

}

Solution for Programming Exercise 3.9

Exercise 3.9:

Often, some element of an animation repeats over and over, every so many frames. Sometimes, the repetition is "cyclic," meaning that at the end it jumps back to the start. Sometimes the repetition is "oscillating," like a back-and-forth motion where the second half is the same as the first half played in reverse.

Write an animation that demonstrates both cyclic and oscillating motions at various speeds. For cyclic motion, you can use a square that moves across the drawing area, then jumps back to the start, and then repeats the same motion over and over. For oscillating motion, you can do something similar, but the square should move back and forth between the two edges of the drawing area; that is, it moves left-to-right during the first half of the animation and then backwards from right-to-left during the second half. To write the program, you can start with a copy of the sample program SimpleAnimationStarter.java, as in the previous exercise.

A cyclic motion has to repeat every N frames for some value of N. What you draw in some frame of the animation depends on the frameNumber. The frameNumber just keeps increasing forever. To implement cyclic motion, what you really want is a "cyclic frame number" that takes on the values 0, 1, 2, ..., (N-1), 0, 1, 2, ..., (N-1), 0, 1, 2, .... You can derive the value that you need from frameNumber simply by saying

cyclicFrameNumber = frameNumber % N;

Then, you just have to base what you draw on cyclicFrameNumber instead of on frameNumber. Similarly, for an oscillating animation, you need an "oscillation frame number" that takes on the values 0, 1, 2, ... (N-1), N, (N-1), (N-2), ... 2, 1, 0, 1, 2, and so on, repeating the back and forth motion forever. You can compute the value that you need with

oscilationFrameNumber = frameNumber % (2*N);
if (oscillationFrameNumber > N)
oscillationFrameNumber = (2*N) - oscillationFrameNumber;

Here is a screen shot from my version of the program. I use six squares. The top three do cyclic motion at various speeds, while the bottom three do oscillating motion. I drew black lines across the drawing area to separate the squares and to give them "channels" to move in.

Discussion

This exercise exists mostly to introduce you to cyclic and oscillating motion and the formulas that are used to implement them. But it's also a good exercise in designing an animation and working with coordinates.

Some decisions have to be made before you can start writing code. I decided to use rather small squares. (Actually, I tried it with big squares and didn't like the appearance as much.) The size of my squares is 20-by-20 pixels. For my first cyclic motion, I decided to make the length of the frame 300 frames, or about 6 seconds. This means that cyclicFrameNumber can be computed as frameNumber % 300. And I decided to move the square at one pixel per frame; that lets me use cyclicFrameNumber as the x-coordinate for the square. The top square moves along the top edge of the window, so its y-coordinate is 0. The code for the first square is:

cyclicFrameNum = frameNumber % 300;  // Repeats every 300 frames
g.setColor(Color.RED);
g.fillRect( cyclicFrameNum, 0, 20, 20 );

The exercise asks for cyclic motion at different speeds. If the second square is to be faster than the first, it should cross the drawing area in a smaller number of frames. If it is twice as fast, it will only need half as many frames to make it across the drawing area. For the second square, I use a cycle length of 150 instead of 300. To cross the entire area, the x-coordinate of the square must get to 300 after just 150 frames. We can accomplish that by using 2*cyclicFrameNumber as the x-coord. (Another way to think about it: A square with x-coord 2*cyclicFrameNumber moves twice as fast as one with x-coord cyclicFrameNumber. And it will finish the journey after 150 frames instead of 300, so the length of the cycle should be 150 frames.) The code for the second square is

cyclicFrameNum = frameNumber % 150;  // Repeats every 150 frames
g.setColor(Color.GREEN);
g.fillRect( 2*cyclicFrameNum, 20, 20, 20 );

The y-coord is 20, since the top square occupies 20 pixels at top of the window. The third square moves three times as fast as the first, and its code is similar.

For the fourth square, I want an oscillating motion. Like the first square, it takes 300 frames to move across the window from left to right. But then it takes another 300 frames to move back from right to left. The total length of the motion is 600. (In the formula given in the exercise, N is 300 and 2*N is 600.) The formula for the oscilation frame number is

oscilationFrameNum = frameNumber % 600;
if (oscilationFrameNum > 300)
oscilationFrameNum = 600 - oscilationFrameNum;

You should study how this works. When oscilationFrameNumber is between 300 and 600, the formula 600-oscilationFrameNumber gives an answer between 0 and 300. And as oscilationFrameNumber gets bigger, 600-oscilationFrameNumber gets smaller (which will mean that the x-coord of the square is decreasing). From there, you should be able to figure out the rest of the program.

One more small issue is the size of the drawing area. Since there are 6 squares, and each is 20 pixels high, the height of the drawing area must be 120. You might think at first that the width should be 300, since the x-coordinate of the square goes up to 300. But that x-coordinate is actually where the left edge of the square is. When the left edge is at 300, the right edge is at 320. So for the entire square to be visible, we have to make the width 320.

The Solution

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

/**
* This program demonstrates cyclic and oscillating animations.  For cyclic animation,
* it repeats itself every N frames, for some value of N.  An oscillating animation also
* repeats, but the repetition is "back-and-forth."  That is, the second half
* of the repeated animation is the same as the first half played backwards.
*/
public class CyclicAndOscillatingMotionDemo extends JPanel implements ActionListener {

public void drawFrame(Graphics g, int frameNumber, int width, int height) {

/* Show cyclic motion at three speeds.  In each case, a square
* moves across the drawing area from left to right, then jumps
* back to the start.
*/

int cyclicFrameNum;

cyclicFrameNum = frameNumber % 300;  // Repeats every 300 frames
g.setColor(Color.RED);
g.fillRect( cyclicFrameNum, 0, 20, 20 );

cyclicFrameNum = frameNumber % 150;  // Repeats every 150 frames
g.setColor(Color.GREEN);
g.fillRect( 2*cyclicFrameNum, 20, 20, 20 );

cyclicFrameNum = frameNumber % 100;  // Repeats every 100 frames
g.setColor(Color.BLUE);
g.fillRect( 3*cyclicFrameNum, 40, 20, 20 );

/* Show oscillating motion at three speeds.  In each case, a square
* moves across the drawing area from left to right, then reverses
* direction to move from right to left back to its starting point.
*/

int oscillationFrameNum;

oscillationFrameNum = frameNumber % 600;  // repeats every 600 frames
if (oscillationFrameNum > 300)
oscillationFrameNum = 600 - oscillationFrameNum; // after 300, the values go backwards back to 0
g.setColor(Color.CYAN);
g.fillRect( oscillationFrameNum, 60, 20, 20 );

oscillationFrameNum = frameNumber % 300; // repeats every 300 frames
if (oscillationFrameNum > 150)
oscillationFrameNum = 300 - oscillationFrameNum; // after 150, the values go backwards back to 0
g.setColor(Color.MAGENTA);
g.fillRect( 2*oscillationFrameNum, 80, 20, 20 );

oscillationFrameNum = frameNumber % 200; // repeats every 200 frames
if (oscillationFrameNum > 100)
oscillationFrameNum = 200 - oscillationFrameNum; // after 100, the values go backwards back to 0
g.setColor(Color.YELLOW);
g.fillRect( 3*oscillationFrameNum, 100, 20, 20 );

/* Draw horizontal black lines across the window to separate the
* regions used by the six squares.  Also draw a box around the outside,
* mostly for the picture that I need for the web page!
*/

int y;
g.setColor(Color.BLACK);
for ( y = 20; y < 120; y = y + 20 )
g.drawLine(0,y,320,y);
g.drawRect(0,0,319,119);  // Why not (0,0,320,120)? This is a technicality.
// If you use 320 and 120, the right and bottom edges
// of the rect will actually be outside the drawing area.
}

//------ Implementation details: DO NOT EXPECT TO UNDERSTAND THIS ------

public static void main(String[] args) {

JFrame window = new JFrame("Cyclic and Oscillating Motion");
CyclicAndOscillatingMotionDemo drawingArea = new CyclicAndOscillatingMotionDemo();
drawingArea.setBackground(Color.WHITE);
window.setContentPane(drawingArea);
drawingArea.setPreferredSize(new Dimension(320,120));  // size is 320 by 120
window.pack();
window.setLocation(100,50);
window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
window.setResizable(false); // The user can't change the size.
Timer frameTimer = new Timer(20,drawingArea);
window.setVisible(true);
frameTimer.start();

} // end main

private int frameNum;

public void actionPerformed(ActionEvent evt) {
frameNum++;
repaint();
}

protected void paintComponent(Graphics g) {
super.paintComponent(g);
drawFrame(g, frameNum, getWidth(), getHeight());
}

}

Solution for Programming Exercise 4.1

Exercise 4.1:

To "capitalize" a string means to change the first letter of each word in the string to upper case (if it is not already upper case). For example, a capitalized version of "Now is the time to act!" is "Now Is The Time To Act!". Write a subroutine named printCapitalized that will print a capitalized version of a string to standard output. The string to be printed should be a parameter to the subroutine. Test your subroutine with a main() routine that gets a line of input from the user and applies the subroutine to it.

Note that a letter is the first letter of a word if it is not immediately preceded in the string by another letter. Recall from Exercise 3.4 that there is a standard boolean-valued function Character.isLetter(char) that can be used to test whether its parameter is a letter. There is another standard char-valued function, Character.toUpperCase(char), that returns a capitalized version of the single character passed to it as a parameter. That is, if the parameter is a letter, it returns the upper-case version. If the parameter is not a letter, it just returns a copy of the parameter.

Discussion

We are told the name of the subroutine and that it has one parameter of type String. The name of the parameter is not specified. I will use str. The return type is void because the subroutine does not return a value. (It displays a value to the user, but to return a value means to return it to the line in the program where the function is called. The value returned by a function is generally not displayed to the user by the function.) The first line of the subroutine definition will be:

static  void  printCapitalized( String str )

The subroutine must look at each character in str and decide whether to capitalize it or not. An algorithm for the subroutine is

for each character in str:
if the character is the first letter of a word:
Print a capitalized version of the character
else:
Print the character
Print a carriage return to end the line of output

The test as to whether a character is the first letter of a word is surprisingly complicated. A test that almost works is: "If the character is a letter and the preceding character is not a letter." The problem is that if the character is the first character is the string, then there is no preceding character! If the character is str.charAt(i), then the preceding character would be str.charAt(i-1), but str.charAt(i-1) doesn't exist if i is 0. You should notice the problem when you look at str.charAt(i-1) and remember that the charAt() method has the precondition that its parameter must be greater than or equal to zero. When i is zero in str.charAt(i-1), that precondition is not satisfied.

Let's look at Java code that suffers from this bug. Recall that the operator "!" stands for "not."

for ( i = 0;  i < str.length();  i++ ) {        // BUGGY CODE!
ch = str.charAt( i );
if ( Character.isLetter(ch) && ! Character.isLetter(str.charAt(i-1)) )
System.out.print( Character.toUpperCase(ch) );
else
System.out.print( ch );
}
System.out.println();

This will crash when i is zero, if the first character in the string is indeed a letter. There are several ways to work around the problem, and all of them are techniques that are worth knowing. The first is to use a more complicated test in the if statement: "if the character is a letter and either it's the first character in the string or the previous character is not a letter". In Java, this is:

if (Character.isLetter(ch) && (i==0 || ! Character.isLetter(str.charAt(i-1))))

This avoids testing str.charAt(i-1) in the case when i is zero. But it can be difficult to get such a complicated test right. Another possibility is a bit sneaky: Add an extra character onto the beginning of str, and then start the for loop with i=1. Any character will do, as long as it's not a letter. For example, you could say "str = "." + str;" Since the for loop starts at i=1, the "." is not copied to output, and the problem of i == 0 doesn't arise. The method that I will use is similar, but it doesn't require any modification of str. I'll use another variable to represent the preceding character in the string, except that at the beginning of the string, I'll set it to the arbitrary value, '.'. At the end of the loop, the character that we have just processed becomes the "previous character" in the next iteration of the loop. Here is the complete subroutine, using this method:

static void printCapitalized( String str ) {
char ch;       // One of the characters in str.
char prevCh;   // The character that comes before ch in the string.
int i;         // A position in str, from 0 to str.length()-1.
prevCh = '.';  // Prime the loop with any non-letter character.
for ( i = 0;  i < str.length();  i++ ) {
ch = str.charAt(i);
if ( Character.isLetter(ch)  &&  ! Character.isLetter(prevCh) )
System.out.print( Character.toUpperCase(ch) );
else
System.out.print( ch );
prevCh = ch;  // prevCh for next iteration is ch.
}
System.out.println();
}

Keeping track of a previous value in a loop is a very common programming pattern. This doesn't exhaust the possibilities. Another idea, for example, would be to use a boolean variable to keep track of whether the previous character was a letter.

Finally, we should add a Javadoc comment to document the subroutine. Writing a main() routine to test this subroutine on a line of input is easy.

The Solution

/**
* This program will get a line of input from the user and will print a copy
* of the line in which the first character of each word has been changed to
* upper case.  The program was written to test the printCapitalized
* subroutine.  It depends on the non-standard TextIO class.
*/

public class CapitalizeOneString {

public static void main(String[] args) {
String line;  // Line of text entered by user.
System.out.println("Enter a line of text.");
line = TextIO.getln();
System.out.println();
System.out.println("Capitalized version:");
printCapitalized( line );
}

/**
*  Print a copy of a string to standard output, with the first letter
*  of each word in the string changed to upper case.
*  @param str the string that is to be output in capitalized form
*/
static void printCapitalized( String str ) {
char ch;       // One of the characters in str.
char prevCh;   // The character that comes before ch in the string.
int i;         // A position in str, from 0 to str.length()-1.
prevCh = '.';  // Prime the loop with any non-letter character.
for ( i = 0;  i < str.length();  i++ ) {
ch = str.charAt(i);
if ( Character.isLetter(ch)  &&  ! Character.isLetter(prevCh) )
System.out.print( Character.toUpperCase(ch) );
else
System.out.print( ch );
prevCh = ch;  // prevCh for next iteration is ch.
}
System.out.println();
}

} // end CapitalizeOneString

Solution for Programming Exercise 4.2

Exercise 4.2:

The hexadecimal digits are the ordinary, base-10 digits '0' through '9' plus the letters 'A' through 'F'. In the hexadecimal system, these digits represent the values 0 through 15, respectively. Write a function named hexValue that uses a switch statement to find the hexadecimal value of a given character. The character is a parameter to the function, and its hexadecimal value is the return value of the function. You should count lower case letters 'a' through 'f' as having the same value as the corresponding upper case letters. If the parameter is not one of the legal hexadecimal digits, return -1 as the value of the function.

A hexadecimal integer is a sequence of hexadecimal digits, such as 34A7, ff8, 174204, or FADE. If str is a string containing a hexadecimal integer, then the corresponding base-10 integer can be computed as follows:

value = 0;
for ( i = 0; i < str.length();  i++ )
value = value*16 + hexValue( str.charAt(i) );

Of course, this is not valid if str contains any characters that are not hexadecimal digits. Write a program that reads a string from the user. If all the characters in the string are hexadecimal digits, print out the corresponding base-10 value. If not, print out an error message.

Discussion

The subroutine has a parameter of type char and a return value of type int. It's easy to write the switch statement, although it's tedious because of the number of cases. A little creative cut-and-paste can help. The switch statement has a default case that covers all the characters that are not hexadecimal digits. For such characters, a value of -1 is returned. The subroutine is shown in the program below, and I will not repeat it here. Note that in this switch statement, the cases are terminated by return statements, rather than break.

In the main program, I will use TextIO.getlnWord() to read the user's input, rather than TextIO.getln(). This has the advantage that it will return a non-empty string that is guaranteed not to contain any blanks. We still have the problem of checking whether the user's input contains only valid hexadecimal digits. One approach is to check all the characters first and use a boolean variable to record whether they are all valid. We can test whether an individual character, ch, is valid by checking whether hexValue(ch) is -1. Let hex be a String holding the user's input. Then we have:

boolean valid; // Flag variable for testing whether the string is valid.
valid = true;  // Assume that the input is valid, and change our
// mind if we find an invalid character.
for ( i = 0; i < hex.length(); i++ ) {
if ( hexValue(hex.charAt(i)) == -1 ) {  // Character number i is bad.
valid = false;
break; // Leave the for loop, since we are now sure of the answer.
}
}

// At the point, valid is true if and only if we have examined all the
// characters in the string and found that each one is a hex digit.

if ( valid ) {  // If the input is valid, compute and print base-10 value
dec = 0;
for ( i = 0; i < hex.length(); i++ )
dec = 16*dec + hexValue( hex.charAt(i) );
System.out.println("Base-10 value is:  " + dec);
}
else {  // Input is not valid, print an error message
System.out.println("Error:  Input is not a hexadecimal number.");
}

This works, but we have to process the string twice. We can avoid this by checking the input at the same time that we do the conversion. If the input is illegal, I want to end the program. I use the fact that a return statement in the main() routine will end the program, since it returns control back to the system:

dec = 0;
for ( i = 0; i < hex.length(); i++) {
int digit = hexValue( hex.charAt(i) );
if (digit == -1) {
System.out.println("Error:  Input is not a hexadecimal number.");
return;  // Ends the main() routine.
}
dec = 16*dec + digit;
}
System.out.println("Base-10 value:  " + dec);

This is the code that is used in the main() routine of the program to do the conversion. Note that I declared dec to be of type long to allow bigger values than would fit in a variable of type int. The program still has a problem if the user enters too many characters. (It gets the wrong answer.)

It would probably be better to write a function to do the conversion of a string to base-10. It could return a -1 if the string is not a legal hexadecimal number. The main() routine could then call the function and check its return value to find out whether there was an error.

In this exercise, a special value, -1, is returned by a subroutine to indicate that its parameter value is not valid. An alternative approach would be to throw an exception if the parameter value is not legal. Instead of saying "return -1", the default case of the switch statement in the subroutine could read:

default:
throw new IllegalArgumentException("Not a legal hexadecimal digit: '" + ch + "'.");

The main program would then do the conversion from string to hexadecimal in a try..catch statement. If an exception occurs, the catch part of the statement can print an error message:

try {    // IF WE WERE USING AN EXCEPTION-THROWING VERSION OF hexValue
dec = 0;
for ( i = 0; i < hex.length(); i++ ) {
int digit = hexValue( hex.charAt(i) );  // Might throw an exception.
dec = 16*dec + digit;
}
// We get here only if no exception occurred.
System.out.println("Base-10 value:  " + dec);
}
catch ( IllegalArgumentException e ) {
System.out.println("Error:  Input is not a hexadecimal number.");
}

I think that this approach is much neater!

The Solution

/**
* This program reads a hexadecimal number input by the user and prints the
* base-10 equivalent.  If the input contains characters that are not
* hexadecimal numbers, then an error message is printed.
*/

public class Hex2Dec {

public static void main(String[] args) {
String hex;  // Input from user, containing a hexadecimal number.
long dec;    // Decimal (base-10) equivalent of hexadecimal number.
int i;       // A position in hex, from 0 to hex.length()-1.
System.out.print("Enter a hexadecimal number: ");
hex = TextIO.getlnWord();
dec = 0;
for ( i = 0; i < hex.length(); i++ ) {
int digit = hexValue( hex.charAt(i) );
if (digit == -1) {
System.out.println("Error:  Input is not a hexadecimal number.");
return;  // Ends the main() routine.
}
dec = 16*dec + digit;
}
System.out.println("Base-10 value:  " + dec);
}  // end main

/**
* Returns the hexadecimal value of a given character, or -1 if it is not
* a valid hexadecimal digit.
* @param ch the character that is to be converted into a hexadecimal digit
* @return the hexadecimal value of ch, or -1 if ch is not
*     a legal hexadecimal digit
*/
public static int hexValue(char ch) {
switch (ch) {
case '0':
return 0;
case '1':
return 1;
case '2':
return 2;
case '3':
return 3;
case '4':
return 4;
case '5':
return 5;
case '6':
return 6;
case '7':
return 7;
case '8':
return 8;
case '9':
return 9;
case 'a':     // Note:  Handle both upper and lower case letters.
case 'A':
return 10;
case 'b':
case 'B':
return 11;
case 'c':
case 'C':
return 12;
case 'd':
case 'D':
return 13;
case 'e':
case 'E':
return 14;
case 'f':
case 'F':
return 15;
default:
return -1;
}
}  // end hexValue

}

Solution for Programming Exercise 4.3

Exercise 4.3:

Write a function that simulates rolling a pair of dice until the total on the dice comes up to be a given number. The number that you are rolling for is a parameter to the function. The number of times you have to roll the dice is the return value of the function. The parameter should be one of the possible totals: 2, 3, ..., 12. The function should throw an IllegalArgumentException if this is not the case. Use your function in a program that computes and prints the number of rolls it takes to get snake eyes. (Snake eyes means that the total showing on the dice is 2.)

Discussion

The subroutine we have to write is very similar to the program from Exercise 3.1. The main difference is that instead of rolling until both dice come up 1, we roll until the total showing on the dice is equal to some specified value. That value is given by the parameter to the function. Of course, the first thing that the subroutine should do is check that the value of the parameter is in the range of possible rolls of a pair of dice; if not, it should throw an exception. Note that without this check, the subroutine would go into an infinite loop when the parameter is outside the range of possible values. I named the function rollFor:

public static int rollFor( int N ) {
if ( N < 2 || N > 12 )
throw new IllegalArgumentException("Impossible total for a pair of dice.");
int die1, die2;  // Numbers between 1 and 6 representing the dice.
int roll;        // Total showing on dice.
int rollCt;      // Number of rolls made.
rollCt = 0;
do {
die1 = (int)(Math.random()*6) + 1;
die2 = (int)(Math.random()*6) + 1;
roll = die1 + die2;
rollCt++;
} while ( roll != N );
return rollCt;
}

In the actual program, I've added an appropriate Javadoc comment.

You should understand the contract of this subroutine. The parameter, N, is supposed to be one of the numbers that could possibly come up on a pair of dice. That is, N must be one of 2, 3, ..., or 12. The condition that N have a valid value is a precondition for the subroutine. If the caller of the function violates this precondition, the subroutine can't give any sort of correct answer, so it responds by throwing an exception.

The main() routine for this program is trivial. In fact, it could even be shortened to:

public static void main(String[] args) {
System.out.println("It took " + rollFor(2) + " rolls to get snake eyes.");
}  // end main()

The Solution

/**
* This program simulates rolling a pair of dice over and over until the
* total showing on the two dice is 2.  It reports the number of rolls
* it took to get a 2.  (This was written to test the subroutine, rollFor.)
*/
public class RollFor2 {

public static void main(String[] args) {
int numberOfRolls;  // Number of rolls to get a 2.
numberOfRolls = rollFor(2);
System.out.println("It took " + numberOfRolls + " rolls to get snake eyes.");
}  // end main()

/**
* Simulates rolling a pair of dice until a given total comes up.
* Precondition:  The desired total is between 2 and 12, inclusive.
* @param N the total that we want to get on the dice
* @return the number of times the dice are rolled before the
*    desired total occurs
* @throws IllegalArgumentException if the parameter, N, is not a number
*    that could possibly come up on a pair of dice
*/
public static int rollFor( int N ) {
if ( N < 2 || N > 12 )
throw new IllegalArgumentException("Impossible total for a pair of dice.");
int die1, die2;  // Numbers between 1 and 6 representing the dice.
int roll;        // Total showing on dice.
int rollCt;      // Number of rolls made.
rollCt = 0;
do {
die1 = (int)(Math.random()*6) + 1;
die2 = (int)(Math.random()*6) + 1;
roll = die1 + die2;
rollCt++;
} while ( roll != N );
return rollCt;
}

}  // end class RollFor2

Solution for Programming Exercise 4.4

Exercise 4.4:

This exercise builds on Exercise 4.3. Every time you roll the dice repeatedly, trying to get a given total, the number of rolls it takes can be different. The question naturally arises, what's the average number of rolls to get a given total? Write a function that performs the experiment of rolling to get a given total 10000 times. The desired total is a parameter to the subroutine. The average number of rolls is the return value. Each individual experiment should be done by calling the function you wrote for Exercise 4.3. Now, write a main program that will call your function once for each of the possible totals (2, 3, ..., 12). It should make a table of the results, something like:

Total On Dice     Average Number of Rolls
-------------     -----------------------
2               35.8382
3               18.0607
.                .
.                .

Discussion

The solution uses the subroutine, rollFor, from Exercise 4.3. That subroutine will throw an exception if its parameter is not valid. However, in my program, I know that the values that I pass to the rollFor subroutine are valid and that no exception will occur. So, there is no need to use a try..catch statement to handle the exception.

The main() program simply prints a heading for the output, then uses a for loop to compute and print the data for each of the possible rolls from 2 to 12. It is not difficult to write it, with a little care to get the formatting right (using formatted output).

The only thing left is to write a function to find the average number of rolls to get a given total on the dice. The average will be a real number, so the return type of the function is double. The subroutine has a parameter of type int that specifies the number we are rolling for. I'll call the parameter "roll". An algorithm for the subroutine is

Let totalRolls = 0
Repeat 10000 times:
Call rollFor(roll) to run the experiment once
Add the returned value to totalRolls
Compute the average by dividing totalRolls by 10000
Return the average

In my program, I use a named constant, NUMBER_OF_EXPERIMENTS, to specify the number of experiments to be performed. This constant replaces the value 10000, making it easier to read the program and easier to change the number of experiments if I decide I want to do more experiments or fewer. This gives the subroutine:

public static double getAverageRollCount( int roll ) {
int rollCountThisExperiment;  // Number of rolls in one experiment.
int rollTotal;  // Total number of rolls in all the experiments.
double averageRollCount;  // Average number of rolls per experiment.
rollTotal = 0;
for ( int i = 0;  i < NUMBER_OF_EXPERIMENTS;  i++ ) {
rollCountThisExperiment = rollFor( roll );
rollTotal += rollCountThisExperiment;
}
averageRollCount = ((double)rollTotal) / NUMBER_OF_EXPERIMENTS;
return averageRollCount;
}

Note that when the average is computed, a type-cast is used to convert rollTotal to type double. This is necessary since rollCount and NUMBER_OF_EXPERIMENTS are integers, and the computer would evaluate the quotient rollCount / NUMBER_OF_EXPERIMENTS as an integer.

By the way, this subroutine could be substantially abbreviated at the expense of being somewhat less easy to understand:

public static double getAverageRollCount( int roll ) {
int rollTotal = 0;  // Total number of rolls in all the experiments.
for ( int i = 0;  i < NUMBER_OF_EXPERIMENTS;  i++ )
rollTotal += rollFor( roll );
return ((double)rollTotal) / NUMBER_OF_EXPERIMENTS;
}

The Solution

/**
* This program performs the following type of experiment:  Given a desired
* total roll, such as 7, roll a pair of dice until the given total comes up,
* and count how many rolls are necessary.  Now do that over and over, and
* find the average number of rolls.  The number of times the experiment is
* repeated is given by the constant, NUMBER_OF_EXPERIMENTS.  The average is
* computed and printed out for each possible roll = 2, 3, ..., 12.
*/

public class DiceRollStats {

/**
* The number of times that the experiment "roll for a given total"
* is to be repeated.  The program performs this many experiments, and
* prints the average of the result, for each possible roll value,
*/
public static final int NUMBER_OF_EXPERIMENTS = 10000;

public static void main(String[] args) {
double average;  // The average number of rolls to get a given total.
System.out.println("Total On Dice     Average Number of Rolls");
System.out.println("-------------     -----------------------");
for ( int dice = 2;  dice <= 12;  dice++ ) {
average = getAverageRollCount( dice );
System.out.printf("%10d%22.4f\n", dice, average);
// Use 10 spaces to output dice, and use 22 spaces to output
// average, with 4 digits after the decimal.
}
}

/**
* Find the average number of times a pair of dice must be rolled to get
* a given total.  The experiment of rolling for the given total is
* repeated NUMBER_OF_EXPERIMENTS times and the average number of rolls
* over all the experiments is computed.
* Precondition:  The given total must be be between 2 and 12, inclusive.
* @param roll the total that we want to get on the dice
* @return the average number of rolls that it takes to get the specified
*    total
*/
public static double getAverageRollCount( int roll ) {
int rollCountThisExperiment;  // Number of rolls in one experiment.
int rollTotal;  // Total number of rolls in all the experiments.
double averageRollCount;  // Average number of rolls per experiment.
rollTotal = 0;
for ( int i = 0;  i < NUMBER_OF_EXPERIMENTS;  i++ ) {
rollCountThisExperiment = rollFor( roll );
rollTotal += rollCountThisExperiment;
}
averageRollCount = ((double)rollTotal) / NUMBER_OF_EXPERIMENTS;
return averageRollCount;
}

/**
* Simulates rolling a pair of dice until a given total comes up.
* Precondition:  The desired total is between 2 and 12, inclusive.
* @param N the total that we want to get on the dice
* @return the number of times the dice are rolled before the
*    desired total occurs
* @throws IllegalArgumentException if the parameter, N, is not a number
*    that could possibly come up on a pair of dice
*/
public static int rollFor( int N ) {
if ( N < 2 || N > 12 )
throw new IllegalArgumentException("Impossible total for a pair of dice.");
int die1, die2;  // Numbers between 1 and 6 representing the dice.
int roll;        // Total showing on dice.
int rollCt;      // Number of rolls made.
rollCt = 0;
do {
die1 = (int)(Math.random()*6) + 1;
die2 = (int)(Math.random()*6) + 1;
roll = die1 + die2;
rollCt++;
} while ( roll != N );
return rollCt;
}

}  // end DiceRollStats

Solution for Programming Exercise 4.5

Exercise 4.5:

The sample program RandomMosaicWalk.java from Section 4.6 shows a "disturbance" that wanders around a grid of colored squares. When the disturbance visits a square, the color of that square is changed. Here's an idea for a variation on that program. In the new version, all the squares start out with the default color, black. Every time the disturbance visits a square, a small amount is added to the green component of the color of that square. The result will be a visually interesting effect, as the path followed by the disturbance gradually turns a brighter and brighter green.

Write a subroutine that will add 25 to the green component of one of the squares in the mosaic. (But don't let the green component go over 255, since that's the largest legal value for a color component.) The row and column numbers of the square should be given as parameters to the subroutine. Recall that you can discover the current green component of the square in row r and column c with the function call Mosaic.getGreen(r,c). Use your subroutine as a substitute for the changeToRandomColor() subroutine in the program RandomMosaicWalk2.java. (This is the improved version of the program from Section 4.7 that uses named constants for the number of rows, number of columns, and square size.) Set the number of rows and the number of columns to 80. Set the square size to 5.

By default, the rectangles in the mosaic have a "3D" appearance and a gray border that makes them look nicer in the random walk program. But for this program, you want to turn off that effect. To do so, call Mosaic.setUse3DEffect(false) in the main program.

Don't forget that you will need Mosaic.java and MosaicPanel.java to compile and run your program, since they define non-standard classes that are required by the program.

Discussion

This is an exercise in making a rather small modification to a relatively complicated existing program.

The only real problem is to write a new subroutine, which I will call brightenSquare. Much of the program comes directly from RandomMosaicWalk2.java. The randomMove() routine is unchanged. The only changes in the main() routine are to substitute a call to brightenSquare for the call to changeToRandomColor and to add the line calling Mosaic.setUse3DEffect. The subroutines fillWithRandomColors and changeToRandomColor in the RandomMosaicWalk2 program are not needed in the new program and should be removed. In the three lines that define the constants, the values are changed according the instructions in the exercise:

final static int ROWS = 80;        // Number of rows in the mosaic.
final static int COLUMNS = 80;     // Number of columns in the mosaic.
final static int SQUARE_SIZE = 5;  // Size of each square in the mosaic.

With these values, the program is interesting to watch for a while. You might want to try using shades of red, blue, or gray, instead of green. Or even use three disturbances, one incrementing the red component of the color, one incrementing the green component, and one incrementing the blue.

An outline for the brightenSquare routine is clear, remembering that the green component can't be more than 255:

Let g be the current green component of the square
Add 25 to g
If g is greater than 255
set g = 255
Set the color components of the square to  0, g, 0

The red and blue components of the color will always be zero. However, they must be specified in the Mosaic.setColor() routine. Written in Java, the body of the routine is just three lines long:

static void brightenSquare(int row, int col) {
int g = Mosaic.getGreen(row,col);
g += 25;
if (g > 255)
g = 255;
Mosaic.setColor(row,col,0,g,0);
}

The Solution

/**
* This program opens a Mosaic window that is initially filled with black.
* A "disturbance" moves randomly around in the window.  Each time it visits
* a square, the green component of the color of that square is increased
* until, after about ten visits, it has reached the maximum possible level.
* The animation continues until the user closes the window.
*/

public class RandomBrighten {

final static int ROWS = 80;        // Number of rows in the mosaic.
final static int COLUMNS = 80;     // Number of columns in the mosaic.
final static int SQUARE_SIZE = 5;  // Size of each square in the mosaic.

static int currentRow;    // Row currently containing the disturbance.
static int currentColumn; // Column currently containing disturbance.

/**
* The main program creates the window, fills it with random colors,
* and then moves the disturbance in a random walk around the window
* as long as the window is open.
*/
public static void main(String[] args) {
Mosaic.open( ROWS, COLUMNS, SQUARE_SIZE, SQUARE_SIZE );
currentRow = ROWS / 2;   // start at center of window
currentColumn = COLUMNS / 2;
Mosaic.setUse3DEffect(false);
while (Mosaic.isOpen()) {
brightenSquare(currentRow, currentColumn);
randomMove();
Mosaic.delay(1);
}
}  // end main

/**
* Add a bit of green to the rectangle in a given row and column.
* Precondition:   The specified rowNum and colNum are in the valid range
*                 of row and column numbers.
* Postcondition:  The green component of the color of the square has
*                 been increased by 25, except that it does not go
*                 over its maximum possible value, 255.
*/
static void brightenSquare(int row, int col) {
int g = Mosaic.getGreen(row,col);
g += 25;
if (g > 255)
g = 255;
Mosaic.setColor(row,col,0,g,0);
}

/**
* Move the disturbance.
* Precondition:   The global variables currentRow and currentColumn
*                 are within the legal range of row and column numbers.
* Postcondition:  currentRow or currentColumn is changed to one of the
*                 neighboring positions in the grid -- up, down, left, or
*                 right from the current position.  If this moves the
*                 position outside of the grid, then it is moved to the
*                 opposite edge of the grid.
*/
static void randomMove() {
int directionNum; // Randomly set to 0, 1, 2, or 3 to choose direction.
directionNum = (int)(4*Math.random());
switch (directionNum) {
case 0:  // move up
currentRow--;
if (currentRow < 0)
currentRow = ROWS - 1;
break;
case 1:  // move right
currentColumn++;
if (currentColumn >= COLUMNS)
currentColumn = 0;
break;
case 2:  // move down
currentRow ++;
if (currentRow >= ROWS)
currentRow = 0;
break;
case 3:  // move left
currentColumn--;
if (currentColumn < 0)
currentColumn = COLUMNS - 1;
break;
}
}  // end randomMove

} // end class RandomBrighten

Solution for Programming Exercise 4.6

Exercise 4.6:

For this exercise, you will do something even more interesting with the Mosaic class that was discussed in Section 4.6. (Again, don't forget that you will need Mosaic.java and MosaicPanel.java to compile and run your program.)

The program that you write for this exercise should start by filling a mosaic with random colors. Then repeat the following until the user closes the mosaic window: Select one of the rectangles in the mosaic at random. Then select one of the neighboring rectangles -- above it, below it, to the left of it, or to the right of it. Copy the color of the originally selected rectangle to the selected neighbor, so that the two rectangles now have the same color.

As this process is repeated over and over, it becomes more and more likely that neighboring squares will have the same color. The result is to build up larger color patches. On the other hand, once the last square of a given color disappears, there is no way for that color to ever reappear. (Extinction is forever!) If you let the program run long enough, eventually the entire mosaic will be one uniform color.

Discussion

I will call the program RandomConvert, since the basic operation is to convert one square to be the same color as a neighboring square. An outline for the main program is easy:

Open a mosaic window
Fill the mosaic with random colors
while the window is open:
Select one of the rectangles at random
Convert the color of one of that rectangle's neighbors
Short delay

We have already seen a subroutine for filling the mosaic with random color, in Subsection 4.6.2. I will also write a subroutine to do the second step in the while loop. There is some question about what it means to "select one of the rectangles at random." A rectangle in the mosaic is specified by a row number and a column number. We can select a random rectangle by choosing a row number and a column number at random. Assuming that ROWS and COLUMNS are constants that give the number of rows and the number of columns, we can do that by saying

int randomRow = (int)(ROWS * Math.random());
int randomColumn = (int)(COLUMNS * Math.random());

where I have declared each variable and initialized it in one step, as discussed in Subsection 4.7.1. For the "convert" subroutine to do its work, we will have to tell it which rectangle has been selected, so randomRow and randomColumn will be parameters to that subroutine. The code for the program's main() routine becomes:

Mosaic.open(ROWS, COLUMNS, SQUARE_SIZE, SQUARE_SIZE);
fillWithRandomColors();
while (Mosaic.isOpen()) {
int randomRow = (int)(ROWS * Math.random());
int randomColumn = (int)(COLUMNS * Math.random());
convertRandomNeighbor(randomRow, randomColumn);
Mosaic.delay(1);
}

All that remains is to write the convertRandomNeighbor() subroutine. This routine should pick a random neighbor of a given rectangle and change its color. A rectangle in the mosaic has four neighbors, above, below, to the left, and to the right. We can pick one at random by selecting a random integer less than four and using that integer to decide which neighbor to select. We have a problem, though, if the rectangle is on the edge of the mosaic. For example, if the rectangle is in the top row, then there is no neighbor above that rectangle in the mosaic. One solution to this problem is to wrap around to the bottom of the mosaic and use a square from the bottom row as the neighbor. Essentially, we think of the top of the mosaic as connected to the bottom and the left edge as connected to the right. We have seen something like this in Subsection 4.6.3, in the randomMove() subroutine. The convertRandomNeighbor() code can use some basic ideas from randomMove(). Here is a version of convertRandomNeighbor() that would work:

static void convertRandomNeighbor(int row, int col) {

/* Choose a random direction, and get the row and column
* numbers of the neighbor that lies in that direction. */

int neighborRow;       // row number of selected neighbor
int neighborColumn;    // column number of selected neighbor

int directionNum = (int)(4*Math.random());  // random direction;

switch (directionNum) {
case 0:    // Choose neighbor above.
neighborColumn = col;        // Neighbor is in the same column.
neighborRow = row - 1;       // Subtract 1 to get neighbor's row number.
if (neighborRow < 0)         // Neighbor's row number is outside the mosaic.
neighborRow = ROWS - 1;  // So wrap around to bottom of the mosaic.
break;
case 1:    // Choose neighbor to the right.
neighborRow = row;               // Same row.
neighborColumn = col + 1;        // Column to the right.
if (neighborColumn >= COLUMNS)   // Outside the mosaic?
neighborColumn = 0;          // Wrap around to the left edge
break;
case 2:    // Choose neighbor below.
neighborColumn = col;
neighborRow = row + 1;
if (neighborRow >= ROWS)
neighborRow = 0;
break;
default:    // Choose neighbor to the left.
neighborRow = row;
neighborColumn = col - 1;
if (neighborColumn < 0)
neighborColumn = COLUMNS - 1;
break;
}

/* Get the color components for position (row,col) */

int red = Mosaic.getRed(row,col);
int green = Mosaic.getGreen(row,col);
int blue = Mosaic.getBlue(row,col);

/* Change the color of the neighbor to color of the original square. */

Mosaic.setColor(neighborRow,neighborColumn,red,green,blue);

}

Note the use of a default case at the end of the switch statement. Saying "case 3" will not work here, because the computer would not be able to verify that values have definitely been assigned to neighborRow and neighborColumn.

In my program, I actually used a different algorithm that requires somewhat less code. My algorithm goes like this:

Get the color components for the rectangle at position (row,col).
Modify the value of row or col to point to a neighboring rectangle.
Set the color of position (row,col).

This is a little tricky, since the variables row and col are used both for getting the color and for setting it. But by the time row and col are used for setting the color, they are referring to a different rectangle. You can see my version of convertRandomNeighbor in the full source code listing below.

In the end, I made one change in the program. I found that I wanted the process to go more quickly, but leaving out the delay altogether made it much too fast. My solution was to do several conversions each time through the while loop in the main routine, instead of just one. I added a BATCH_SIZE constant to say how many conversions to do at a time, and I added a for loop in the main program to implement the multiple conversions. A BATCH_SIZE of 10 or even 100 gives a much more satisfactory animation.

The Solution

/**
* This program fills a mosaic with random colors.  It then enters
* a loop in which it randomly selects one of the squares in the
* mosaic, then randomly selects one of the four neighbors of that
* square and converts the selected neighbor to the color of the
* originally selected square.  The effect is to gradually build
* up larger patches of uniform color.  The animation continues
* until the user closes the window.  This program depends on
* the non-standard classes Mosaic and MosaicCanvas.
*/

public class RandomConvert {

final static int ROWS = 40;        // Number of rows in the mosaic.
final static int COLUMNS = 40;     // Number of columns in the mosaic.
final static int SQUARE_SIZE = 10; // Size of each square in the mosaic.
final static int DELAY = 1;        // Millisecond delay after each convert.
final static int BATCH_SIZE = 10;  // The number of squares tested/converted at a time.

/**
* The main() routine opens the mosaic window, then enters into
* a loop in which it repeatedly converts the color of one square.
* The loop ends when the user closes the mosaic window.
*/
public static void main(String[] args) {
Mosaic.setUse3DEffect(false);
Mosaic.open(ROWS, COLUMNS, SQUARE_SIZE, SQUARE_SIZE);
fillWithRandomColors();
while (Mosaic.isOpen()) {
for (int i = 0; i < BATCH_SIZE; i++) {
int randomRow = (int)(ROWS * Math.random());
int randomColumn = (int)(COLUMNS * Math.random());
convertRandomNeighbor(randomRow, randomColumn);
}
Mosaic.delay(DELAY);
}
}

/**
* Set each square in the mosaic to be a randomly selected color.
*/
static void fillWithRandomColors() {
for (int row = 0; row < ROWS; row++) {
for (int col = 0; col < COLUMNS; col++) {
int r = (int)(256*Math.random());
int g = (int)(256*Math.random());
int b = (int)(256*Math.random());
Mosaic.setColor(row,col,r,g,b);
}
}
}

/**
* Select one of the neighbors of the square at position (row,column) in
* the mosaic.  Change the color at position (row, column) to match the
* color of the selected neighbor.   The neighbors of a square are the
* squares above, below, to the left, and to the right of the square.
* For squares on the edge of the mosaic, wrap around to the opposite
* edge.
*/
static void convertRandomNeighbor(int row, int col) {

/* Get the color components for position (row,col) */

int red = Mosaic.getRed(row,col);
int green = Mosaic.getGreen(row,col);
int blue = Mosaic.getBlue(row,col);

/* Choose a random direction, and change the value of row
* or col to refer to the neighbor that lies in that direction. */

int directionNum = (int)(4*Math.random());
switch (directionNum) {
case 0:    // Choose neighbor above.
row--;  // Move row number one row up.
if (row < 0)  // row number is outside the mosaic.
row = ROWS - 1;  // Wrap around to bottom of the mosaic.
break;
case 1:    // Choose neighbor to the right.
col++;
if (col >= COLUMNS)
col = 0;
break;
case 2:    // Choose neighbor below.
row++;
if (row >= ROWS)
row = 0;
break;
case 3:    // Choose neighbor to the left.
col--;
if (col < 0)
col = COLUMNS - 1;
break;
}

/* Change the color of the neighbor to color of the original square. */

Mosaic.setColor(row,col,red,green,blue);
}

}

Solution for Programming Exercise 4.7

Exercise 4.7:

Write a program that administers a basic addition quiz to the user. There should be ten questions. Each question is a simple addition problem such as 17 + 42, where the numbers in the problem are chosen at random (and are not too big). The program should ask the user all ten questions and get the user's answers. After asking all the questions, the user should print each question again, with the user's answer. If the user got the answer right, the program should say so; if not, the program should give the correct answer. At the end, tell the user their score on the quiz, where each correct answer counts for ten points.

The program should use three subroutines, one to create the quiz, one to administer the quiz, and one to grade the quiz. It can use three arrays, with three global variables of type int[], to refer to the arrays. The first array holds the first number from every question, the second holds the second number from every questions, and the third holds the user's answers.

Discussion

The statement of the problem has already made a lot of decisions for us. We are told to use three global variables of type int[]. The variables must be static, and I have chosen to make them private, so the declarations become:

private static int[] firstNumbers;  // The first numbers in all ten questions.
private static int[] secondNumbers; // The second numbers in all ten questions.
private static int[] userAnswers;   // The user's answers to the ten questions.

Note that these declarations are not in any subroutine. These statements only create variables, not arrays. We still need to create three arrays of length 10. The arrays can be created using the new operator. This initialization could in fact be combined with the declaration of the variables:

private static int[] firstNumbers = new int[10];
private static int[] secondNumbers = new int[10];
private static int[] userAnswers = new int[10];

Another possibility would be to create the three arrays at the beginning of the main() routine. In fact, I chose to create them in the subroutines where they are first used.

The program has three stages -- create the problems, administer the quiz, and grade the quiz -- and we are told to write a subroutine to do each stage. The main program simply has to call the subroutines:

public static void main(String[] args) {
createQuiz();
administerQuiz();
gradeQuiz();
}

(However, I added a couple of output statements at the start.) This is an example of using subroutines to break up a large task into several simpler subtasks, so that programming each subtask becomes a separate problem. The subroutines are short and not too hard to write. Looking at administerQuiz() as an example, note that the user's answers have to be stored in the array, userAnswers, so that they can be used when grading the quiz. I create the array itself before asking any questions, and then store the user's responses into the array as they are read from the user. On the other hand, the arrays that hold the numbers that occur in the questions have already been created and filled before administerQuiz() is called. Here's the subroutine:

private static void administerQuiz() {
userAnswers = new int[10];
for (int i = 0; i < 10; i++) {
int questionNum = i + 1;
System.out.printf("Question %2d:  What is %2d + %2d  ? ",
questionNum, firstNumbers[i], secondNumbers[i]);
userAnswers[i] = TextIO.getlnInt();
}
}

A couple notes about the output: I wanted to number the questions 1, 2, ..., 10, but the elements of the arrays are numbered 0, 1, ..., 9. This means that the question number that I output has to be one more than the array index. Also, all the numbers that occur in my program are either one or two digits. By allowing exactly two digits for each number in the output, I can get everything to line up neatly. Here's the I/O for a sample run of the program:

Welcome to the addition quiz!

Question  1:  What is 40 + 14  ? 54
Question  2:  What is 18 + 38  ? 46
Question  3:  What is 21 + 31  ? 52
Question  4:  What is 18 + 36  ? 54
Question  5:  What is 43 + 12  ? 55
Question  6:  What is 21 + 17  ? 38
Question  7:  What is 33 + 25  ? 58
Question  8:  What is 39 + 21  ? 70
Question  9:  What is  1 + 25  ? 26
Question 10:  What is 39 + 26  ? 65

Here are the correct answers:
Question  1:  40 + 14  =  54.  You were CORRECT.
Question  2:  18 + 38  =  56.  You said 46, which is INCORRECT.
Question  3:  21 + 31  =  52.  You were CORRECT.
Question  4:  18 + 36  =  54.  You were CORRECT.
Question  5:  43 + 12  =  55.  You were CORRECT.
Question  6:  21 + 17  =  38.  You were CORRECT.
Question  7:  33 + 25  =  58.  You were CORRECT.
Question  8:  39 + 21  =  60.  You said 70, which is INCORRECT.
Question  9:   1 + 25  =  26.  You were CORRECT.
Question 10:  39 + 26  =  65.  You were CORRECT.

You got 8 questions correct.
Your grade on the quiz is 80

I noted in Subsection 4.3.8 that it's considered bad style to use global variables where parameters could be used instead, and that some people are quite hostile to unnecessary global variables. You might ask, how bad was it to use global variables in this case? To some extent this is a matter of opinion. You could argue that the whole program revolves around the three arrays, so it makes sense for them to be global in the program. On the other hand, it is possible to avoid using globals in this case. The three array variables can be declared in main() and the three arrays can be created there and passed as parameters to the subroutines. Here is a main routine that does that:

public static void main(String[] args) {
int[] firstNums = new int[10];  // The first numbers in the ten problems
int[] secondNums = new int[10]; // The second numbers in the ten problems
int[] answers = new int[10];    // The user's answers.
System.out.println();
System.out.println("Welcome to the addition quiz!");
System.out.println();
createQuiz(firstNums,secondNums);
administerQuiz(firstNums,secondNums,answers);
gradeQuiz(firstNums,secondNums,answers);
}

A complete second version of the program, with no global variables, is given below. One reason that I asked you to use global variables is that understanding exactly how arrays work as parameters requires an understanding of objects and how they are passed as parameters. Without that understanding, it's hard to get the global-less version correct. You will learn about passing objects as parameters in the next chapter.

The Solution

Here is the original program, using global variables:

/**
* This program administers a ten-question addition quiz to the user.  The numbers
* for the problem are chosen at random.  The numbers and the answers are one or
* two digits.  After asking the user the ten questions, the computer grades the
* quiz, telling the user the correct answer for any problem they got wrong.
*/
public class AdditionQuiz {

private static int[] firstNumbers;  // The first numbers in all ten questions.
private static int[] secondNumbers; // The second numbers in all ten questions.
private static int[] userAnswers;   // The user's answers to the ten questions.

public static void main(String[] args) {
System.out.println();
System.out.println("Welcome to the addition quiz!");
System.out.println();
createQuiz();
administerQuiz();
gradeQuiz();
}

/**
* Creates the arrays that hold the numbers for the questions and fills
* them with random numbers.
*/
private static void createQuiz() {
firstNumbers = new int[10];
secondNumbers = new int[10];
for ( int i = 0; i < 10; i++ ) {
firstNumbers[i] = (int)(Math.random() * 50 + 1);  // in the range 1 to 50
secondNumbers[i] = (int)(Math.random() * 50); // in the range 0 to 49
}
}

/**
* Asks the user each of the ten quiz questions and gets the user's answers.
* The answers are stored in an array, which is created in this subroutine.
*/
private static void administerQuiz() {
userAnswers = new int[10];
for (int i = 0; i < 10; i++) {
int questionNum = i + 1;
System.out.printf("Question %2d:  What is %2d + %2d  ? ",
questionNum, firstNumbers[i], secondNumbers[i]);
userAnswers[i] = TextIO.getlnInt();
}
}

/**
* Shows all the questions, with their correct answers, and computes a grade
* for the quiz.  For each question, the user is told whether they got
* it right.
*/
private static void gradeQuiz() {
System.out.println();
System.out.println("Here are the correct answers:");
int numberCorrect = 0;
int grade;
for (int i = 0; i < 10; i++) {
int questionNum = i + 1;
int correctAnswer = firstNumbers[i] + secondNumbers[i];
System.out.printf("   Question %2d:  %2d + %2d  =  %2d.  ",
questionNum, firstNumbers[i], secondNumbers[i], correctAnswer);
if ( userAnswers[i] == correctAnswer ) {
System.out.println("You were CORRECT.");
numberCorrect++;
}
else {
System.out.println("You said " + userAnswers[i] + ", which is INCORRECT.");
}
}
grade = numberCorrect * 10;
System.out.println();
System.out.println("You got " + numberCorrect + " questions correct.");
System.out.println("Your grade on the quiz is " + grade);
System.out.println();
}

} // end class AdditionQuiz

Here is a version that uses parameters and no global variables:

/**
* This program administers a ten-question addition quiz to the user.  The numbers
* for the problem are chosen at random.  The numbers and the answers are one or
* two digits.  After asking the user the ten questions, the computer grades the
* quiz, telling the user the correct answer for any problem they got wrong.
*/
public class AdditionQuizNoGlobals {

public static void main(String[] args) {
int[] firstNums = new int[10];  // The first numbers in the ten problems
int[] secondNums = new int[10]; // The second numbers in the ten problems
int[] answers = new int[10];    // The user's answers.
System.out.println();
System.out.println("Welcome to the addition quiz!");
System.out.println();
createQuiz(firstNums,secondNums);
administerQuiz(firstNums,secondNums,answers);
gradeQuiz(firstNums,secondNums,answers);
}

/**
* Creates the arrays that hold the numbers for the questions and fills
* them with random numbers.  The parameters are arrays that will hold
* the random numbers for the first and second operands of each addition
* problem.  The arrays must have already been created when this subroutine
* is called!
*/
private static void createQuiz(int[] firstNumbers, int[] secondNumbers) {
for ( int i = 0; i < 10; i++ ) {
firstNumbers[i] = (int)(Math.random() * 50 + 1);  // in the range 1 to 50
secondNumbers[i] = (int)(Math.random() * 50); // in the range 0 to 49
}
}

/**
* Asks the user each of the ten quiz questions and gets the user's answers.
* The answers are stored in an array, which is created in this subroutine.
* The first two parameters hold the operands for the quiz questions.  The user's
* answers to the ten problems will be stored in the third array.
*/
private static void administerQuiz(int[] firstNumbers, int[] secondNumbers, int[] userAnswers) {
for (int i = 0; i < 10; i++) {
int questionNum = i + 1;
System.out.printf("Question %2d:  What is %2d + %2d  ? ",
questionNum, firstNumbers[i], secondNumbers[i]);
userAnswers[i] = TextIO.getlnInt();
}
}

/**
* Shows all the questions, with their correct answers, and computes a grade
* for the quiz.  For each question, the user is told whether they got
* it right.  The first two parameters hold the operands for the quiz questions,
* and the third parameter holds the answers that the user gave to the quiz.
*/
private static void gradeQuiz(int[] firstNumbers, int[] secondNumbers, int[] userAnswers) {
System.out.println();
System.out.println("Here are the correct answers:");
int numberCorrect = 0;
int grade;
for (int i = 0; i < 10; i++) {
int questionNum = i + 1;
int correctAnswer = firstNumbers[i] + secondNumbers[i];
System.out.printf("   Question %2d:  %2d + %2d  =  %2d.  ",
questionNum, firstNumbers[i], secondNumbers[i], correctAnswer);
if ( userAnswers[i] == correctAnswer ) {
System.out.println("You were CORRECT.");
numberCorrect++;
}
else {
System.out.println("You said " + userAnswers[i] + ", which is INCORRECT.");
}
}
grade = numberCorrect * 10;
System.out.println();
System.out.println("You got " + numberCorrect + " questions correct.");
System.out.println("Your grade on the quiz is " + grade);
System.out.println();
}

}  // end class AdditionQuizNoGlobals

Solution for Programming Exercise 5.1

Exercise 5.1:

In all versions of the PairOfDice class in Section 5.2, the instance variables die1 and die2 are declared to be public. They really should be private, so that they would be protected from being changed from outside the class. Write another version of the PairOfDice class in which the instance variables die1 and die2 are private. Your class will need "getter" methods that can be used to find out the values of die1 and die2. (The idea is to protect their values from being changed from outside the class, but still to allow the values to be read.) Include other improvements in the class, including at least a toString() method. Test your class with a short program that counts how many times a pair of dice is rolled, before the total of the two dice is equal to two.

Discussion

The versions of the PairOfDice class in Section 2 differ in how the dice are initialized. I like the idea of initializing the dice to random values, so I will work with the following version:

public class PairOfDice {

public int die1;   // Number showing on the first die.
public int die2;   // Number showing on the second die.

/**
* Constructor creates a pair of dice and rolls them so that
* they initially show some random value.
*/
public PairOfDice() {
roll();  // Call the roll() method to roll the dice.
}

/**
* Roll the dice by setting each die to be a random number between 1 and 6.
*/
public void roll() {
die1 = (int)(Math.random()*6) + 1;
die2 = (int)(Math.random()*6) + 1;
}

} // end class PairOfDice

After a PairOfDice object has just been created, the number on each die is definitely between 1 and 6, just like the number on a real die. Can we be sure that this will always be true? Not if the instance variables die1 and die2 are public, since they can be changed from outside the class. There is nothing to stop someone from changing them to 42 and -17 or anything else. It's not good enough to say that you're not supposed to do that with dice. I want an absolute guarantee that my dice objects can only have the values that real dice could have. By making die1 and die2 private, I can have that guarantee, because the code that I write in the PairOfDice class is the only code that will ever affect the values of the variables.

So, we will make die1 and die2 private, and add instance methods getDie1() and getDie2() to return the values of die1 and die2. These are "getter" methods that get the value of a member variable, and their names follow the convention that the name of a getter method consists of "get" followed by a capitalized version of the variable name. It would still be useful to be able to set the values of the dice, but we want to make it impossible to set the value to anything outside the range 1 to 6. The solution is to provide "setter" methods that throw an exception when the value is illegal.

I also added the toString() method from Subsection 5.3.3. As for other improvements, I can foresee that people who use my class will often be interested in the total on the dice, so they will tend to say things like "dice.getDie1() + dice.getDie2()" a lot. If this is going to be done over and over, why not provide a method in the class to do it? So, I will also add a method getTotal() that returns the total value showing on the two dice. The complete, modified PairOfDice class is shown below.

(Here is another improvement that I thought about. We could modify the roll() method so that in addition to rolling the dice, it would also return an int value giving the total on the dice. For example, this would allow us to replace "dice.roll(); val = dice.getTotal()" with "val = dice.roll()". Since it's legal to call a function with a subroutine call statement, we could still say "dice.roll()" if we just want to roll the dice without recording the total immediately. However, I decided that it was a little clearer to leave this feature out.)

The main program is easy, especially since we've done the same problem before without using objects (in Exercise 3.1). Note how the pair of dice object is used. To test whether or not the total on the dice is 2, I use the test "while (dice.getTotal() != 2)". Because of the toString() method, I just have to say say

System.out.println("The dice come up " + dice );

to show the numbers on the two dice.

The Solution

The Modified PairOfDice Class

/**
* An object of class PairOfDice represents a pair of dice,
* where each die shows a number between 1 and 6.  The dice
* can be rolled, which randomizes the numbers showing on the
* dice.
*/
public class PairOfDice {

private int die1;   // Number showing on the first die.
private int die2;   // Number showing on the second die.

/**
* Constructor creates a pair of dice and rolls them so that
* they initially show some random value.
*/
public PairOfDice() {
roll();  // Call the roll() method to roll the dice.
}

/**
* Roll the dice by setting each die to be a random number between 1 and 6.
*/
public void roll() {
die1 = (int)(Math.random()*6) + 1;
die2 = (int)(Math.random()*6) + 1;
}

/**
* Return the number showing on the first die.
*/
public int getDie1() {
return die1;
}

/**
* Set the value of the first die.  Throws an IllegalArgumentException
* if the value is not in the range 1 to 6.
*/
public void setDie1( int value ) {
if ( value < 1 || value > 6 )
throw new IllegalArgumentException("Illegal dice value " + value);
die1 = value;
}

/**
* Return the number showing on the second die.
*/
public int getDie2() {
return die2;
}

/**
* Set the value of the second die.  Throws an IllegalArgumentException
* if the value is not in the range 1 to 6.
*/
public void setDie2( int value ) {
if ( value < 1 || value > 6 )
throw new IllegalArgumentException("Illegal dice value " + value);
die2 = value;
}

/**
* Return the total showing on the two dice.
*/
public int getTotal() {
return die1 + die2;
}

/**
* Return a String representation of a pair of dice, where die1
* and die2 are instance variables containing the numbers that are
* showing on the two dice.
*/
public String toString() {
if (die1 == die2)
return "double " + die1;
else
return die1 + " and " + die2;
}

}  // end class PairOfDice

The Main Program

/**
* Rolls a pair of dice until the dice come up snake eyes
* (with a total value of 2).  Counts and reports the
* number of rolls.
*/
public class RollFor2 {

public static void main(String[] args) {

PairOfDice dice;  // A variable that will refer to the dice.
int rollCount;    // Number of times the dice have been rolled.

dice = new PairOfDice();  // Create the PairOfDice object.
rollCount = 0;

/* Roll the dice until they come up snake eyes. */

do {
dice.roll();
System.out.println("The dice come up " + dice );
rollCount++;
} while (dice.getTotal() != 2);

/* Report the number of rolls. */

System.out.println("\nIt took " + rollCount + " rolls to get a 2.");

/* Now, generate an exception. */

System.out.println();
System.out.println("This program will now crash with an error");
System.out.println("when it tries to set the value of a die to 42.");
System.out.println();

dice.setDie1(42);
System.out.println(dice);  // This statement will not be executed!

}

}  // end class RollFor2

Solution for Programming Exercise 5.2

Exercise 5.2:

A common programming task is computing statistics of a set of numbers. (A statistic is a number that summarizes some property of a set of data.) Common statistics include the mean (also known as the average) and the standard deviation (which tells how spread out the data are from the mean). I have written a little class called StatCalc that can be used to compute these statistics, as well as the sum of the items in the dataset and the number of items in the dataset. You can read the source code for this class in the file StatCalc.java. If calc is a variable of type StatCalc, then the following instance methods are available:

- calc.enter(item)  where item is a number, adds the item to the dataset.

- calc.getCount()  is a function that returns the number of items that have been added to the dataset.

- calc.getSum()  is a function that returns the sum of all the items that have been added to the dataset.

- calc.getMean()  is a function that returns the average of all the items.

- calc.getStandardDeviation()  is a function that returns the standard deviation of the items.

Typically, all the data are added one after the other by calling the enter() method over and over, as the data become available. After all the data have been entered, any of the other methods can be called to get statistical information about the data. The methods getMean() and getStandardDeviation() should only be called if the number of items is greater than zero.

Modify the current source code, StatCalc.java, to add instance methods getMax() and getMin(). The getMax() method should return the largest of all the items that have been added to the dataset, and getMin() should return the smallest. You will need to add two new instance variables to keep track of the largest and smallest items that have been seen so far.

Test your new class by using it in a program to compute statistics for a set of non-zero numbers entered by the user. Start by creating an object of type StatCalc:

StatCalc  calc;   // Object to be used to process the data.
calc = new StatCalc();

Read numbers from the user and add them to the dataset. Use 0 as a sentinel value (that is, stop reading numbers when the user enters 0). After all the user's non-zero numbers have been entered, print out each of the six statistics that are available from calc.

Discussion

For the StatCalc class to handle minimums and maximums, some of what must be added to the class is obvious. We need two new instance variables, min and max, and two getter methods to return the values of those instance variables. So, we can add these lines to the class definition:

private double min;   // Smallest item that has been entered.
private double max;   // Largest item that has been entered.

public double getMin() {
return min;
}

public double getMax() {
return max;
}

But then there is the problem of making sure that min and max have the right values. min records the smallest number seen so far. Every time we have a new number to add to the dataset there is a possibility that min will change, so we have to compare min with the newly added number. If the new number is smaller than the current min, then the number becomes the new value of min (since the new number is now the smallest number we have seen so far). We do something similar for max. This has to be done whenever a number is entered into the dataset, so it has to be added to the enter() method, giving:

public void enter(double num) {
// This is not completely correct!
count++;
sum += num;
squareSum += num*num;
if (num > max)          // We have a new maximum.
max = num;
if (num < min)          // We have a new minimum.
min = num;
}

Unfortunately, if this is all we do, there is a bug in our program! For example, if the dataset consists of the numbers 21, 17, and 4, the computer will insist that the minimum is 0, rather than 4. The problem is that the variables min and max are initialized to zero. (If no initial value is provided for a numerical instance variable, it gets the default initial value, zero.) Since min is 0, none of the numbers in the dataset pass the test "if (num < min)", and therefore the value of min never changes. A similar problem holds for max, but it will only show up if all the numbers in the dataset are less than zero. For the other instance variables, count, sum, and squareSum, the default initial value of zero is correct. For min and max, we have to do something different.

One possible way to fix the problem is to treat the first number entered as a special case. When only one number has been entered, it's certainly the largest number so far and also the smallest number so far, so it should be assigned to both min and max. This can be handled in the enter() method:

public void enter(double num) {
// (This is NOT the version I used in my final answer.)
count++;
sum += num;
squareSum += num*num;
if (count == 1) {  // This is the first number.
max = num;
min = num;
}
else {
if (num > max)     // We have a new maximum.
max = num;
if (num < min)     // We have a new minimum.
min = num;
}
}

This works fine. However, I decided to use an alternative approach. We would be OK if we could initialize min to have a value that is bigger than any possible number. Then, when the first number is entered, it definitely will satisfy the test "if (num < min)", and therefore the first number will become the value of min. But to be "bigger than any possible number," min would have to be infinity. The initial value for max has to be smaller than any possible number, so max has to be initialized to negative infinity. And that's what we'll do!

Recall from Section 2.5 that the standard class Double contains constants Double.POSITIVE_INFINITY and Double.NEGATIVE_INFINITY that represent positive and negative infinity. We can use these named constants to provide initial values for the instance variables min and max. So, the declarations become:

private double max = Double.NEGATIVE_INFINITY;  // Largest item seen.
private double min = Double.POSITIVE_INFINITY;  // Smallest item seen.

With this change, the StatCalc class works correctly. The complete class is shown below. (By the way, you might think about what happens if getMin() or getMax() is called before any data has been entered. What actually happens? What should happen? What is the minimum or maximum of a set of numbers that contains no numbers at all?)

The main program is fairly straightforward, so just for fun I decided to use a Scanner instead of TextIO to read the user's input (see Subsection 2.4.6. The user's data are read and entered into the StatCalc object in a loop:

do {
System.out.print("? ");
item = in.nextDouble();  // (in is the Scanner.)
if (item != 0)
calc.enter(item);
} while ( item != 0 );

The subroutine call "calc.enter(item);" enters the user's item. That is, it does all the processing necessary to include this data item in the statistics it is computing. After all the data have been entered, the statistics can be obtained by using function calls such as "calc.getMean()". The statistics are output in statements such as:

System.out.println("   Average:            " + calc.getMean());

Note that a function call represents a value, and so can be used anyplace where a variable or literal value could be used. I don't have to assign the value of the function to a variable. I can use the function call directly in the output statement. Another note: In this program, I decided not to use formatted output, since it seems appropriate to print the answers with as much accuracy as possible. For formatted output, the statement used to print the mean could be something like:

System.out.printf("    Average:            %10.3f\n", calc.getMean());

The complete main program is shown below.

Although that completes the exercise, one might wonder: Instead of modifying the source code of StatCalc, could we make a subclass of StatCalc and put the modifications in that? The answer is yes. We will need need to use the slightly obscure special variable super that was discussed in Subsection 5.6.2.

The new instance variables and instance methods can simply be put into the subclass. The problem arises with the enter() method. We have to redefine this method so that it will update the values of min and max. But it also has to do all the processing that is done by the original enter() method in the StatCalc class. This is what super is for. It lets us call a method from the superclass of the class we are writing. So, the subclass can be written:

class StatCalcWithMinMax extends StatCalc {

private double max = Double.NEGATIVE_INFINITY;  // Largest item seen.
private double min = Double.POSITIVE_INFINITY;  // Smallest item seen.

public void enter(double num) {
super.enter(num);  // Call the enter method from the StatCalc class.
if (num > max)  // Then do the extra processing for min and max.
max = num;
if (num < min)
min = num;
}

public double getMin() {
return min;
}

public double getMax() {
return max;
}

}  // end class StatCalcWithMinMax

The Solution

Revised StatCalc Class.  Changes from the original are shown in red italic.

/*
* An object of class StatCalc can be used to compute several simple statistics
* for a set of numbers.  Numbers are entered into the dataset using
* the enter(double) method.  Methods are provided to return the following
* statistics for the set of numbers that have been entered: The number
* of items, the sum of the items, the average, the standard deviation,
* the maximum, and the minimum.
*/
public class StatCalc {

private int count;   // Number of numbers that have been entered.
private double sum;  // The sum of all the items that have been entered.
private double squareSum;  // The sum of the squares of all the items.
private double max = Double.NEGATIVE_INFINITY;  // Largest item seen.
private double min = Double.POSITIVE_INFINITY;  // Smallest item seen.

/**
* Add a number to the dataset.  The statistics will be computed for all
* the numbers that have been added to the dataset using this method.
*/
public void enter(double num) {
count++;
sum += num;
squareSum += num*num;
if (num > max)
max = num;
if (num < min)
min = num;
}

/**
* Return the number of items that have been entered into the dataset.
*/
public int getCount() {
return count;
}

/**
* Return the sum of all the numbers that have been entered.
*/
public double getSum() {
return sum;
}

/**
* Return the average of all the items that have been entered.
* The return value is Double.NaN if no numbers have been entered.
*/
public double getMean() {
return sum / count;
}

/**
* Return the standard deviation of all the items that have been entered.
* The return value is Double.NaN if no numbers have been entered.
*/
public double getStandardDeviation() {
double mean = getMean();
return Math.sqrt( squareSum/count - mean*mean );
}

/**
* Return the smallest item that has been entered.
* The return value will be infinity if no numbers have been entered.
*/
public double getMin() {
return min;
}

/**
* Return the largest item that has been entered.
* The return value will be -infinity if no numbers have been entered.
*/
public double getMax() {
return max;
}

}  // end class StatCalc

Main Program

/**
* Computes and display several statistics for a set of non-zero
* numbers entered by the user.  (Input ends when user enters 0.)
* This program uses StatCalc.java.
*/

import java.util.Scanner;

public class SimpleStats {

public static void main(String[] args) {

Scanner in = new Scanner(System.in);

StatCalc calc; // Computes stats for numbers entered by user.
calc = new StatCalc();

double item; // One number entered by the user.

System.out.println("Enter your numbers.  Enter 0 to end.");
System.out.println();

do {
System.out.print("? ");
item = in.nextDouble();
if (item != 0)
calc.enter(item);
} while (item != 0);

System.out.println("\nStatistics about your calc:\n");
System.out.println("   Count:              " + calc.getCount());
System.out.println("   Sum:                " + calc.getSum());
System.out.println("   Minimum:            " + calc.getMin());
System.out.println("   Maximum:            " + calc.getMax());
System.out.println("   Average:            " + calc.getMean());
System.out.println("   Standard Deviation: "
+ calc.getStandardDeviation());

} // end main()

} // end SimpleStats

Solution for Programming Exercise 5.3

Exercise 5.3:

This problem uses the PairOfDice class from Exercise 5.1 and the StatCalc class from Exercise 5.2.

The program in Exercise 4.4 performs the experiment of counting how many times a pair of dice is rolled before a given total comes up. It repeats this experiment 10000 times and then reports the average number of rolls. It does this whole process for each possible total (2, 3, ..., 12).

Redo that exercise. But instead of just reporting the average number of rolls, you should also report the standard deviation and the maximum number of rolls. Use a PairOfDice object to represent the dice. Use a StatCalc object to compute the statistics. (You'll need a new StatCalc object for each possible total, 2, 3, ..., 12. You can use a new pair of dice if you want, but it's not required.)

Discussion

The program from Exercise 4.4 defines a function, rollFor(N), that performs the basic experiment once. It rolls the dice until the total on the dice is N, and it returns the number of rolls. Using a PairOfDice object, dice, the body of this subroutine becomes

int rollCt = 0;  // Number of rolls made.
do {
dice.roll();
rollCt++;
} while ( dice.getTotal() != N );
return rollCt;

This is significantly simpler than the original version. But where does the dice object come from? One possibility is to create a new PairOfDice object at the beginning of the function. This will work, but then a new object is created each time the function is called. In the program we are writing, the function is called 110,000 times. It seems a waste to manufacture 110,000 pairs of dice when one would do! To avoid this, I create the dice as a static member variable:

private static PairOfDice dice = new PairOfDice();

The variable must be static since it is used in the static function, rollFor. Since dice is a static member variable, it is created and initialized when the class is first loaded and it exists as long as the program is running. The rollFor() method always uses this one pair of dice. (Some people might prefer to create the dice as a local variable in the main() routine. The dice could be passed as a parameter to the rollFor() method. Then, rollFor(N,dice) would mean "roll for a total of N using this pair of dice." But in my opinion, the dice are not logically a part of the interface of this subroutine, and I prefer to use the member variable.)

The original program also had a method called getAverageRollCount() to find the average number of rolls, when the basic experiment is repeated 10000 times. We could rename this to getRollCountStats and use it to compute all the statistics, not just the average. The actual computation is to be done by a StatCalc object. Let stats be a variable that refers to that object. The results of each experiment will be fed into this object, something like this:

for ( int i = 0;  i < 10000;  i++ ) {
// Assume "total" is the number we are rolling for.
rollCountThisExperiment = rollFor( total );  // Do one experiment.
stats.enter( rollCountThisExperiment );      // Enter the data.
}

At the end of this process, stats is ready to report the statistics. All you have to do is call its functions, such as stats.getMean().

In my program, I use the named constant NUMBER_OF_EXPERIMENTS instead of the literal number, 10000. I abbreviate the for loop to

for ( int i = 0;  i < NUMBER_OF_EXPERIMENTS;  i++ )
stats.enter( rollFor(total) );

and, since it has become so short, I deleted the subroutine and moved the for loop into the main() routine. The main() routine prints the output in neat columns, using formatted output. For example, "System.out.printf("%6d",total);" prints the value of total in a column of width 6. (The hardest part was figuring out how wide to make the columns!). After printing out some headings for the columns, the main() routine says

for ( int total = 2;  total <= 12;  total++ ) {
StatCalc stats;  // An object that will compute the statistics.
stats = new StatCalc();
for ( int i = 0; i < NUMBER_OF_EXPERIMENTS; i++ ) {
// Do the experiment of counting the number of rolls
// required to roll the desired total, and enter the
// number of rolls into stats' dataset.
stats.enter( rollFor(total) );
}
System.out.printf("%6d", total);
System.out.printf("%18.3f", stats.getMean());
System.out.printf("%19.3f", stats.getStandardDeviation());
System.out.printf("%14.3f", stats.getMax());
System.out.println();
}

The body of the for loop processes one of the possible totals on a pair of dice, and it produces one line of output with the statistics for that total. A new StatCalc object is created for each execution of the for loop. This is necessary because we want separate statistics for each total. A single StatCalc object would just accumulate all the data for all the possible values of total into a single dataset. It might be worth noting that the five output statements in this method could be written as a single statement:

System.out.printf( "%6d%18.3f%19.3f%14.3f%n", total, stats.getMean(),
stats.getStandardDeveialtion(), stats.getMax() );

By the way, you might wonder what would happen if I had not eliminated the getRollCountStats() subroutine? In that case, the statistics data is generated in the subroutine and is used in the main() routine. So, the same StatCalc object has to be used in both routines. There are several ways to handle this. The variable, stats, could be a static member variable. Then it could simply be used in both routines. Alternatively, the StatCalc object could be passed as a parameter to the subroutine. Or, as a final and best alternative, the object could be created in the subroutine and sent back to the main() routine as a return value. Let's look at this last possibility. The subroutine would be:

static StatCalc getRollCountStats( int total ) {
StatCalc calc;         // An object to compute the statistics.
calc = new StatCalc();
for ( int i = 0; i < NUMBER_OF_EXPERIMENTS; i++ )
calc.enter( rollFor(total) );
return calc;  // Send back the object, with the statistics.
}

In the main program, this would be used as follows:

for ( int total = 2; total <= 12; total++ ) {
StatCalc stats;  // The stats for this total
stats = getRollCountStats( total ); // Get stats from subroutine.
System.out.printf("%6d", total);
System.out.printf("%18.3f", stats.getMean());
System.out.printf("%19.3f", stats.getStandardDeviation());
System.out.printf("%14.3f", stats.getMax());
System.out.println();
}

Note in particular that not every object variable needs to be initialized with a new object. In the case of the variable calc in the main() routine, an object is computed elsewhere. The variable, calc, is set to refer to that existing object.

The Solution

/**
* This program performs the following type of experiment:
* Given a desired total roll, such as 7, roll a pair of
* dice until the given total comes up, and count how many
* rolls are necessary.  Now do the experiment over and over,
* and find the average number of rolls.  The number of times
* the experiment is repeated is given by the constant,
* NUMBER_OF_EXPERIMENTS.  Several statistics are computed and
* printed out for each possible roll = 2, 3, ..., 12:
* the average number of rolls, the standard deviation,
* and the maximum number of rolls.
*/

public class DiceRollStats2 {

static final int NUMBER_OF_EXPERIMENTS = 10000;

private static PairOfDice dice = new PairOfDice();
// A single pair of dice, which will be used for all
// the experiments.

public static void main(String[] args) {

System.out.println("Dice Total   Avg # of Rolls   Stand. Deviation   Max # of Rolls");
System.out.println("----------   --------------   ----------------   --------------");

for ( int total = 2;  total <= 12;  total++ ) {
StatCalc stats;  // An object that will compute the statistics.
stats = new StatCalc();
for ( int i = 0; i < NUMBER_OF_EXPERIMENTS; i++ ) {
// Do the experiment of counting the number of rolls
// required to roll the desired total, and enter the
// number of rolls into stats' dataset.
stats.enter( rollFor(total) );
}
System.out.printf("%6d", total);
System.out.printf("%18.3f", stats.getMean());
System.out.printf("%19.3f", stats.getStandardDeviation());
System.out.printf("%14.3f", stats.getMax());
System.out.println();
}

} // end main

/**
* Roll the dice repeatedly until the total on the
* two dice comes up to be N.  N MUST be one of the numbers
* 2, 3, ..., 12.  (If not, this routine will go into an
* infinite loop!).  The number of rolls is returned.
*/
static int rollFor( int N ) {
int rollCt = 0;  // Number of rolls made.
do {
dice.roll();
rollCt++;
} while ( dice.getTotal() != N );
return rollCt;
}

}  // end class DiceRollStats2

Solution for Programming Exercise 5.4

Exercise 5.4:

The BlackjackHand class from Subsection 5.5.1 is an extension of the Hand class from Section 5.4. The instance methods in the Hand class are discussed in that section. In addition to those methods, BlackjackHand includes an instance method, getBlackjackValue(), which returns the value of the hand for the game of Blackjack. For this exercise, you will also need the Deck and Card classes from Section 5.4.

A Blackjack hand typically contains from two to six cards. Write a program to test the BlackjackHand class. You should create a BlackjackHand object and a Deck object. Pick a random number between 2 and 6. Deal that many cards from the deck and add them to the hand. Print out all the cards in the hand, and then print out the value computed for the hand by getBlackjackValue(). Repeat this as long as the user wants to continue.

In addition to TextIO.java, your program will depend on Card.java, Deck.java, Hand.java, and BlackjackHand.java.

Discussion

This problem is mostly a warm-up for the next one. It uses objects of three different types, Card, Deck, and BlackjackHand. The Hand class is used indirectly, as the superclass of BlackjackHand. To use these objects, you need to know what methods are available in each class, so you should review the information that you have about the classes before beginning the program.

An algorithm for the program is

Create a deck
repeat while user wants to continue:
Shuffle the deck
Create a new BlackjackHand
Decide the number of cards in the hand
Deal cards from the deck into the hand, and print them out
Display the value of the hand

Some variation is possible. You could use just one BlackjackHand object, and remove all the cards from it between hands. The Hand class includes an instance method, clear(), that could be used for this purpose. Similarly, you could create a new Deck object each time through the loop. Or, you might want to use one deck and shuffle it only when the number of cards in the deck gets too small. You could say:

if (deck.cardsLeft() < 6)
deck.shuffle();

Since we always want to do at least one hand, we can use a do..while statement for the loop. Putting in some variable names, we can refine the algorithm to

deck  =  new Deck();
do:
deck.shuffle();
hand  =  new BlackjackHand();
cardsInHand = a random number between 2 and 6
Deal cards from deck into hand, and print them out.
Display hand.getBlackjackValue()
Ask if user wants to go again
while user wants to go again

The number of cards in the hand is supposed to be a random number between 2 and 6. There are five possible values. The expression "(int)(Math.random()*5)" has one of the 5 possible values 0, 1, 2, 3, or 4. Adding 2 to the result gives one of the values 2, 3, 4, 5, or 6. So, the number of cards can be computed as "2 + (int)(Math.random()*5)".

Once we know the number of cards, we can use a for loop to deal cards into the hand, one at a time. The function call deck.dealCard() gets a card from the deck. Once we have a card, we can add it to the hand with the subroutine call hand.addCard(card). This allows us to refine the algorithm to

deck  =  new Deck();
do:
deck.shuffle();
hand  =  new BlackjackHand();
cardsInHand = 2 + (int)(Math.random()*5)
for i = 0 to cardsInHand:
card  =  deck.dealCard()
hand.addCard(card)
Display the card
Display hand.getBlackjackValue()
Ask if user wants to go again
while user wants to go again

Alternatively, dealing the cards and displaying them could be done in separate for loops.

This algorithm can be translated pretty directly into the main() routine of the program, which is shown below.

The Solution

/**
* Creates random blackjack hands, with 2 to 6 cards,
* and prints out the blackjack value of each hand.
* The user decides when to stop.
*/
public class TestBlackjackHand {

public static void main(String[] args) {

Deck deck;            // A deck of cards.
Card card;            // A card dealt from the deck.
BlackjackHand hand;   // A hand of from two to six cards.
int cardsInHand;      // Number or cards in the hand.
boolean again;        // Set to true if user wants to continue.

deck = new Deck();    // Create the deck.

do {
deck.shuffle();
hand = new BlackjackHand();
cardsInHand = 2 + (int)(Math.random()*5);
System.out.println();
System.out.println();
System.out.println("Hand contains:");
for ( int i = 1; i <= cardsInHand; i++ ) {
// Get a card from the deck, print it out,
//   and add it to the hand.
card = deck.dealCard();
hand.addCard(card);
System.out.println("    " + card);
}
System.out.println("Value of hand is " + hand.getBlackjackValue());
System.out.println();
System.out.print("Again? ");
again = TextIO.getlnBoolean();
} while (again == true);

}

}  // end class TestBlackjackHand

Solution for Programming Exercise 5.5

Exercise 5.5:

Write a program that lets the user play Blackjack. The game will be a simplified version of Blackjack as it is played in a casino. The computer will act as the dealer. As in the previous exercise, your program will need the classes defined in Card.java, Deck.java, Hand.java, and BlackjackHand.java. (This is the longest and most complex program that has come up so far in the exercises.)

You should first write a subroutine in which the user plays one game. The subroutine should return a boolean value to indicate whether the user wins the game or not. Return true if the user wins, false if the dealer wins. The program needs an object of class Deck and two objects of type BlackjackHand, one for the dealer and one for the user. The general object in Blackjack is to get a hand of cards whose value is as close to 21 as possible, without going over. The game goes like this.

- First, two cards are dealt into each player's hand. If the dealer's hand has a value of 21 at this point, then the dealer wins. Otherwise, if the user has 21, then the user wins. (This is called a "Blackjack".) Note that the dealer wins on a tie, so if both players have Blackjack, then the dealer wins.

- Now, if the game has not ended, the user gets a chance to add some cards to her hand. In this phase, the user sees her own cards and sees one of the dealer's two cards. (In a casino, the dealer deals himself one card face up and one card face down. All the user's cards are dealt face up.) The user makes a decision whether to "Hit", which means to add another card to her hand, or to "Stand", which means to stop taking cards.

- If the user Hits, there is a possibility that the user will go over 21. In that case, the game is over and the user loses. If not, then the process continues. The user gets to decide again whether to Hit or Stand.

- If the user Stands, the game will end, but first the dealer gets a chance to draw cards. The dealer only follows rules, without any choice. The rule is that as long as the value of the dealer's hand is less than or equal to 16, the dealer Hits (that is, takes another card). The user should see all the dealer's cards at this point. Now, the winner can be determined: If the dealer has gone over 21, the user wins. Otherwise, if the dealer's total is greater than or equal to the user's total, then the dealer wins. Otherwise, the user wins.

Two notes on programming: At any point in the subroutine, as soon as you know who the winner is, you can say "return true;" or "return false;" to end the subroutine and return to the main program. To avoid having an overabundance of variables in your subroutine, remember that a function call such as userHand.getBlackjackValue() can be used anywhere that a number could be used, including in an output statement or in the condition of an if statement.

Write a main program that lets the user play several games of Blackjack. To make things interesting, give the user 100 dollars, and let the user make bets on the game. If the user loses, subtract the bet from the user's money. If the user wins, add an amount equal to the bet to the user's money. End the program when the user wants to quit or when she runs out of money.

Discussion

Let's start by designing the main program. We want to give the user $100 for betting on the games. Then, the user plays Blackjack games until the user runs out of money or until the user wants to quit. We could ask the user after each game whether she wants to continue. But instead of this, I just tell the user to enter a bet amount of 0 if she wants to quit. We need variables to represent the amount of money that the user has and the amount that the user bets on a given game. Let money and bet be variables of type int to represent these quantities. Then, we can write an algorithm for the main program:

Let money = 100
while (true):
Input the user's bet
if the bet is 0:
Break out of the loop
User plays a game of Blackjack
if the user won the game
Pay off the user's bet (money = money + bet)
else
Collect the user's bet (money = money - bet)
If the user is out of money:
Break out of the loop.

Since the Blackjack game will take place in a subroutine, we need some way for the main() routine to find out whether the user won. The exercise says that the subroutine should be a function that returns a boolean value with this information. We should record the return value and test it to see whether the user won. The other point that needs some refinement is inputting the user's bet. We better make sure that the user's bet is a reasonable amount, that is, something between 0 and the amount of money the user has. So, the algorithm can be refined as:

Let money = 100
while (true):
do {
Ask the user to enter a bet
Let bet = the user's response
} while bet is < 0 or > money
if  bet is 0:
Break out of the loop
Let userWins = playBlackjack()
if userWins:
Pay off the user's bet (money = money + bet)
else
Collect the user's bet (money = money - bet)
If money == 0:
Break out of the loop.

This algorithm can be translated into the main() routine in the program given below.

Of course, the major part of the problem is to write the playBlackjack() routine. Fortunately, the exercise gives what amounts to a fairly detailed outline of the algorithm. Things are a little complicated because the game can end at various points along the way. When this happens, the subroutine ends immediately, and any remaining steps in the algorithm are skipped. In outline, the game goes like this:

Create and shuffle a deck of cards
Create two BlackjackHands, userHand and dealerHand
Deal two cards into each hand
Check if dealer has blackjack (if so, game ends)
Check if user has blackjack (if so, game ends)
User draws cards (if user goes over 21, game ends)
Dealer draws cards (if dealer goes over 21, games ends)
Check for winner

The last three steps need to be expanded, again using the information stated in the exercise. The user can draw several cards, so we need a loop. The loop ends when the user wants to "stand". In the loop, if the value of userHand goes over 21, then the whole subroutine ends. The dealer simply draws cards in a loop as long as the value of dealerHand is 16 or less. Again, if the value goes over 21, the whole subroutine ends. In the last step, we determine the winner by comparing the values of the two hands. With these refinements, the algorithm becomes

Create and shuffle a deck of cards
Create two BlackjackHands, userHand and dealerHand

Deal two cards into each hand
if dealer has blackjack
User loses and the game ends now
If user has blackjack
User wins and the game ends now

Repeat:
Ask whether user wants to hit or stand
if user stands:
break out of loop
if user hits:
Give user a card
if userHand.getBlackjackValue() > 21:
User loses and the game ends now

while  dealerHand.getBlackjackValue() <= 16 :
Give dealer a card
if dealerHand.getBlackjackValue() > 21:
User wins and game ends now

if dealerHand.getBlackjackValue() >= userHand.getBlackjackValue()
User loses
else
User wins

This is ready to be translated into Java. One point of coding is the question of how to deal a card to the user or to the dealer. If deck refers to the object of type Deck, then the function call deck.dealCard() returns the card we want. We can add the card to a hand with the addCard() instance method from the Hand class. We can do this in one step, if we want. For example, to deal two cards into each hand, we just have to say

dealerHand.addCard( deck.dealCard() );
dealerHand.addCard( deck.dealCard() );
userHand.addCard( deck.dealCard() );
userHand.addCard( deck.dealCard() );

Of course, a lot of output statements have to be added to the algorithm to keep the user informed about what is going on. For example, I expanded the step where it says "Ask whether user wants to hit or stand" to

Display all the cards in the user's hand
Display the user's total
Display the dealers face-up card, i.e. dealerHand.getCard(0)
Ask if user wants to hit or stand
Get user's response, and make sure it's legal

The last step listed here expands to a loop that ends when the user inputs a valid response, 'H' or 'S'. The first step uses a for loop to display the cards in the user's hand:

for ( int i = 0;  i < userHand.getCardCount();  i++ )
System.out.println("   " + userHand.getCard(i));

The function call userHand.getCardCount() gives the number of cards in the hand. The cards are numbered from 0 to userHand.getCardCount() - 1, and userHand.getCard(i) is the card in the i-position. Of course, to produce code like this, you have to make sure that you are familiar with the methods in the classes that you are using.

Although there are many other details to get right, it's mostly routine from here on. I encourage you to read the entire program below and make sure that you understand it.

The Solution

/**
* This program lets the user play Blackjack.  The computer
* acts as the dealer.  The user has a stake of $100, and
* makes a bet on each game.  The user can leave at any time,
* or will be kicked out when he loses all the money.
* House rules:  The dealer hits on a total of 16 or less
* and stands on a total of 17 or more.  Dealer wins ties.
* A new deck of cards is used for each game.
*/
public class Blackjack {

public static void main(String[] args) {

int money;          // Amount of money the user has.
int bet;            // Amount user bets on a game.
boolean userWins;   // Did the user win the game?

System.out.println("Welcome to the game of blackjack.");
System.out.println();

money = 100;  // User starts with $100.

while (true) {
System.out.println("You have " + money + " dollars.");
do {
System.out.println("How many dollars do you want to bet?  (Enter 0 to end.)");
System.out.print("? ");
bet = TextIO.getlnInt();
if (bet < 0 || bet > money)
System.out.println("Your answer must be between 0 and " + money + '.');
} while (bet < 0 || bet > money);
if (bet == 0)
break;
userWins = playBlackjack();
if (userWins)
money = money + bet;
else
money = money - bet;
System.out.println();
if (money == 0) {
System.out.println("Looks like you've run out of money!");
break;
}
}

System.out.println();
System.out.println("You leave with $" + money + '.');

} // end main()

/**
* Let the user play one game of Blackjack, with the computer as dealer.
* @return true if the user wins the game, false if the user loses.
*/
static boolean playBlackjack() {

Deck deck;                  // A deck of cards.  A new deck for each game.
BlackjackHand dealerHand;   // The dealer's hand.
BlackjackHand userHand;     // The user's hand.

deck = new Deck();
dealerHand = new BlackjackHand();
userHand = new BlackjackHand();

/*  Shuffle the deck, then deal two cards to each player. */

deck.shuffle();
dealerHand.addCard( deck.dealCard() );
dealerHand.addCard( deck.dealCard() );
userHand.addCard( deck.dealCard() );
userHand.addCard( deck.dealCard() );

System.out.println();
System.out.println();

/* Check if one of the players has Blackjack (two cards totaling to 21).
The player with Blackjack wins the game.  Dealer wins ties.
*/

if (dealerHand.getBlackjackValue() == 21) {
System.out.println("Dealer has the " + dealerHand.getCard(0)
+ " and the " + dealerHand.getCard(1) + ".");
System.out.println("User has the " + userHand.getCard(0)
+ " and the " + userHand.getCard(1) + ".");
System.out.println();
System.out.println("Dealer has Blackjack.  Dealer wins.");
return false;
}

if (userHand.getBlackjackValue() == 21) {
System.out.println("Dealer has the " + dealerHand.getCard(0)
+ " and the " + dealerHand.getCard(1) + ".");
System.out.println("User has the " + userHand.getCard(0)
+ " and the " + userHand.getCard(1) + ".");
System.out.println();
System.out.println("You have Blackjack.  You win.");
return true;
}

/*  If neither player has Blackjack, play the game.  First the user
gets a chance to draw cards (i.e., to "Hit").  The while loop ends
when the user chooses to "Stand".  If the user goes over 21,
the user loses immediately.
*/

while (true) {

/* Display user's cards, and let user decide to Hit or Stand. */

System.out.println();
System.out.println();
System.out.println("Your cards are:");
for ( int i = 0; i < userHand.getCardCount(); i++ )
System.out.println("    " + userHand.getCard(i));
System.out.println("Your total is " + userHand.getBlackjackValue());
System.out.println();
System.out.println("Dealer is showing the " + dealerHand.getCard(0));
System.out.println();
System.out.print("Hit (H) or Stand (S)? ");
char userAction;  // User's response, 'H' or 'S'.
do {
userAction = Character.toUpperCase( TextIO.getlnChar() );
if (userAction != 'H' && userAction != 'S')
System.out.print("Please respond H or S:  ");
} while (userAction != 'H' && userAction != 'S');

/* If the user Hits, the user gets a card.  If the user Stands,
the loop ends (and it's the dealer's turn to draw cards).
*/

if ( userAction == 'S' ) {
// Loop ends; user is done taking cards.
break;
}
else {  // userAction is 'H'.  Give the user a card.
// If the user goes over 21, the user loses.
Card newCard = deck.dealCard();
userHand.addCard(newCard);
System.out.println();
System.out.println("User hits.");
System.out.println("Your card is the " + newCard);
System.out.println("Your total is now " + userHand.getBlackjackValue());
if (userHand.getBlackjackValue() > 21) {
System.out.println();
System.out.println("You busted by going over 21.  You lose.");
System.out.println("Dealer's other card was the "
+ dealerHand.getCard(1));
return false;
}
}

} // end while loop

/* If we get to this point, the user has Stood with 21 or less.  Now, it's
the dealer's chance to draw.  Dealer draws cards until the dealer's
total is > 16.  If dealer goes over 21, the dealer loses.
*/

System.out.println();
System.out.println("User stands.");
System.out.println("Dealer's cards are");
System.out.println("    " + dealerHand.getCard(0));
System.out.println("    " + dealerHand.getCard(1));
while (dealerHand.getBlackjackValue() <= 16) {
Card newCard = deck.dealCard();
System.out.println("Dealer hits and gets the " + newCard);
dealerHand.addCard(newCard);
if (dealerHand.getBlackjackValue() > 21) {
System.out.println();
System.out.println("Dealer busted by going over 21.  You win.");
return true;
}
}
System.out.println("Dealer's total is " + dealerHand.getBlackjackValue());

/* If we get to this point, both players have 21 or less.  We
can determine the winner by comparing the values of their hands. */

System.out.println();
if (dealerHand.getBlackjackValue() == userHand.getBlackjackValue()) {
System.out.println("Dealer wins on a tie.  You lose.");
return false;
}
else if (dealerHand.getBlackjackValue() > userHand.getBlackjackValue()) {
System.out.println("Dealer wins, " + dealerHand.getBlackjackValue()
+ " points to " + userHand.getBlackjackValue() + ".");
return false;
}
else {
System.out.println("You win, " + userHand.getBlackjackValue()
+ " points to " + dealerHand.getBlackjackValue() + ".");
return true;
}

}  // end playBlackjack()

} // end class Blackjack

Solution for Programming Exercise 5.6

Exercise 5.6:

Exercise 4.7 asked you to write a program that administers a 10-question addition quiz. Rewrite that program so that it uses the following class to represent addition questions:

public class AdditionQuestion {

private int a, b;  // The numbers in the problem.

public AdditionQuestion() { // constructor
a = (int)(Math.random() * 50 + 1);
b = (int)(Math.random() * 50);
}

public String getQuestion() {
return "What is " + a + " + " + b + " ?";
}

public int getCorrectAnswer() {
return a + b;
}

}

Discussion

The original program used two array of type int[] to represent the addition questions. One array held the first number from each of the ten problems, while the other array held the second number. This representation is an example of parallel arrays: The data for a single question is split between two arrays. (The arrays are called "parallel" because to see all the data for a problem next to each other, you would have to lay the arrays down next to each other, like parallel lines, so the i-th element in one array lies next to the i-th element in the other array.)

The new version of the program can use an array of ten objects of type AdditionQuestion to represent the quiz. The data that we need for any one question can now be found in the same place -- the object that represents that question. This makes for a neater solution than using parallel arrays.

Some of the functionality that we need to create, administer, and grade the quiz has been incorporated into the AdditionQuestion class. For example, the constructor in that class already selects random numbers to be used in the problem, so creating a problem is just a matter of calling the constructor, so there is no need to use random numbers when making a problem in the createQuiz() method. Also, to get the question and correct answer for a problem, we can just call questions[i].getQuestion() and questions[i].getCorrectAnswer(). In fact, we can't get the question any other way, since the two instance variables in an ArithmeticQuestion are private. One downside of this is that I couln't format the output from gradeQuis() as neatly as in the first version.

All-in-all, t is fairly straightforward to translate the old version of the program into the new version. You can read my solution below.

By the way, I considered adding the user's answer to the AdditionQuestion class, along with the correct answer, but the user's answer doesn't seem to me to be logically part of a question. So I think it makes more sense to keep it out of the object that represents the question.

Remember that this program depends on the AdditionQuiz class that is given in the statement of the problem.

The Solution

/**
* This program administers a ten-question addition quiz to the user.  The numbers
* for the problem are chosen at random.  The numbers and the answers are one or
* two digits.  After asking the user the ten questions, the computer grades the
* quiz, telling the user the correct answer for any problem they got wrong.
*/
public class AdditionQuizWithObjects {

private static AdditionQuestion[] questions;  // The questions for the quiz

private static int[] userAnswers;   // The user's answers to the ten questions.

public static void main(String[] args) {
System.out.println();
System.out.println("Welcome to the addition quiz!");
System.out.println();
createQuiz();
administerQuiz();
gradeQuiz();
}

/**
* Creates the array of objects that holds the quiz questions
*/
private static void createQuiz() {
questions = new AdditionQuestion[10];
for ( int i = 0; i < 10; i++ ) {
questions[i] = new AdditionQuestion();
}
}

/**
* Asks the user each of the ten quiz questions and gets the user's answers.
* The answers are stored in an array, which is created in this subroutine.
*/
private static void administerQuiz() {
userAnswers = new int[10];
for (int i = 0; i < 10; i++) {
int questionNum = i + 1;
System.out.printf("Question %2d:  %s ",
questionNum, questions[i].getQuestion());
userAnswers[i] = TextIO.getlnInt();
}
}

/**
* Shows all the questions, with their correct answers, and computes a grade
* for the quiz.  For each question, the user is told whether they got
* it right.
*/
private static void gradeQuiz() {
System.out.println();
System.out.println("Here are the correct answers:");
int numberCorrect = 0;
int grade;
for (int i = 0; i < 10; i++) {
int questionNum = i + 1;
System.out.printf("   Question %2d:  %s  Correct answer is %d  ",
questionNum, questions[i].getQuestion(), questions[i].getCorrectAnswer());
if ( userAnswers[i] == questions[i].getCorrectAnswer() ) {
System.out.println("You were CORRECT.");
numberCorrect++;
}
else {
System.out.println("You said " + userAnswers[i] + ", which is INCORRECT.");
}
}
grade = numberCorrect * 10;
System.out.println();
System.out.println("You got " + numberCorrect + " questions correct.");
System.out.println("Your grade on the quiz is " + grade);
System.out.println();
}

} // end class AdditionQuizWithObjects

Solution for Programming Exercise 5.7

Exercise 5.7:

Rewrite the program from the previous exercise so that it administers a quiz with several different kinds of questions. In the previous exercise, you used a class to represent addition questions. For this exercise, you will use the following interface, or an equivalent abstract class, to represent the more general idea of a question that has an integer as its answer:

public interface IntQuestion {
public String getQuestion();
public int getCorrectAnswer();
}

You can make the AdditionQuestion class implement the interface simply by adding "implements IntQuestion" to its definition. Write a similar class to represent subtraction questions. When creating a subtraction problem, you should make sure that the answer is not negative.

For the new program, use an array of type IntQuestion[] to hold the quiz questions. Include some addition questions and some subtraction questions in the quiz. You can also add a couple non-math questions, including this one, created as an anonymous class:

IntQuestion bigQuestion = new IntQuestion() {
public String getQuestion() {
return "What is the answer to the ultimate question " +
" of life, the universe, and everything?";
}
public int getCorrectAnswer() {
return 42;
}
};

Discussion

Where the solution to the previous exercise used an array of AdditionQuestion to store addition problems, the new program has to store questions of various types. An array of type IntQuestion[] can hold any object that implements the IntQuestion interface. That includes objects of type AdditionQuestion as long as we modify that class so that it implements IntQuestion. But it can also include other types of objects. For one thing, we want to have subtraction problems. We can implement subtraction with a class very similar to the class for addition questions:

class SubtractionQuestion implements IntQuestion {
private int a, b;  // The numbers in the problem.
public SubtractionQuestion() { // constructor
a = (int)(Math.random() * 50 + 1);
b = (int)(Math.random() * 50);
if (ba - b < 0) { // swap a and b so answer won't be negative
int temp = a;
a = b;
b = temp;
}
}
public String getQuestion() {
return "What is " + a + " - " + b + " ?";
}
public int getCorrectAnswer() {
return a - b;
}
}

The exercise asks for subtraction problems for which the answer is not negative. There are a number of ways to accomplish that. For example, you could use a loop that would continue selecting random values for a and b as long as a - b < 0. In my solution, I just swap the values of a and b when a - b < 0.

Creating the quiz becomes a more difficult problem in the new version, since we need to use several different kinds of questions. In my solution, the first seven questions are math problems. For each math problem, the program decides randomly whether to make it an addition question or a subtraction question. The last three questions are created using anonymous classes, just like the example given in the exercise. You can see my solution below.

But the three anonymous classes in my solution have a lot in common: They are the same except for the particular string that is used to ask the question and the particular number that is the correct answer. A better approach would probably have been to create a class to represent such questions:

public class GeneralQuestion implements IntQuestion {
private String question;
private int answer;
public GeneralQuestion( String question, int answer ) {
this.question = question;
this.answer = answer;
}
public String getQuestion() {
return question;
}
public int getCorrectAnswer() {
return answer;
}
}

In that case, the subroutine for creating the quiz would become:

private static void createQuiz() {
questions = new IntQuestion[10];
for ( int i = 0; i < 7; i++ ) {
if (Math.random() < 0.5)
questions[i] = new AdditionQuestion();
else
questions[i] = new SubtractionQuestion();
}
questions[7] = new GeneralQuestion("How many states are there in the United States?",50);
questions[8] = new GeneralQuestion("In what year did the First World War begin?",1914);
questions[9] = new GeneralQuestion("What is the answer to the ultimate question " +
"of life, the universe, and everything?",42);
}

Aside from the declaration of the array questions and the routine for creating the quiz, everything else in the program can be exactly as it was in the previous exercise. (However, I did change the format of the output in gradeQuiz() to make it look better for longer questions.)

In my solution, just to keep everything in one file, I nested the interface and the two classes that implement it inside the main program class.

The Solution

/**
* This program creates, administers, and grades a quiz made up of ten questions,
* where each question has an integer answer.  The quiz includes some simple addition
* problems, some subtraction problems, and some non-math questions.
*/
public class GeneralQuiz {

// -------------------- Nested classes and interface -----------------------

interface IntQuestion {
public String getQuestion();
public int getCorrectAnswer();
}

static class AdditionQuestion implements IntQuestion {
private int a, b;  // The numbers in the problem.
public AdditionQuestion() { // constructor
a = (int)(Math.random() * 50 + 1);
b = (int)(Math.random() * 50);
}
public String getQuestion() {
return "What is " + a + " + " + b + " ?";
}
public int getCorrectAnswer() {
return a + b;
}
}

static class SubtractionQuestion implements IntQuestion {
private int a, b;  // The numbers in the problem.
public SubtractionQuestion() { // constructor
a = (int)(Math.random() * 50 + 1);
b = (int)(Math.random() * 50);
if (b > a) { // swap a and b so answer won't be negative
int temp = a;
a = b;
b = temp;
}
}
public String getQuestion() {
return "What is " + a + " - " + b + " ?";
}
public int getCorrectAnswer() {
return a - b;
}
}

// -------------------- The Program --------------------------------------

private static IntQuestion[] questions;  // The questions for the quiz

private static int[] userAnswers;   // The user's answers to the ten questions.

public static void main(String[] args) {
System.out.println();
System.out.println("Welcome to the quiz");
System.out.println();
System.out.println("There are some math questions and a few non-math");
System.out.println("questions, but the answer to every question is");
System.out.println("an integer.");
System.out.println();
createQuiz();
administerQuiz();
gradeQuiz();
}

/**
* Creates the array of objects that holds the quiz questions
*/
private static void createQuiz() {
questions = new IntQuestion[10];
for ( int i = 0; i < 7; i++ ) {
if (Math.random() < 0.5)
questions[i] = new AdditionQuestion();
else
questions[i] = new SubtractionQuestion();
}
questions[7] = new IntQuestion() {
public String getQuestion() {
return "How many states are there in the United States?";
}
public int getCorrectAnswer() {
return 50;
}
};
questions[8] = new IntQuestion() {
public String getQuestion() {
return "In what year did the First World War begin?";
}
public int getCorrectAnswer() {
return 1914;
}
};
questions[9] = new IntQuestion() {
public String getQuestion() {
return "What is the answer to the ultimate question " +
"of life, the universe, and everything?";
}
public int getCorrectAnswer() {
return 42;
}
};
}

/**
* Asks the user each of the ten quiz questions and gets the user's answers.
* The answers are stored in an array, which is created in this subroutine.
*/
private static void administerQuiz() {
userAnswers = new int[10];
for (int i = 0; i < 10; i++) {
int questionNum = i + 1;
System.out.printf("Question %2d:  %s ",
questionNum, questions[i].getQuestion());
userAnswers[i] = TextIO.getlnInt();
}
}

/**
* Shows all the questions, with their correct answers, and computes a grade
* for the quiz.  For each question, the user is told whether they got
* it right.
*/
private static void gradeQuiz() {
System.out.println();
System.out.println("Here are the correct answers:");
System.out.println();
int numberCorrect = 0;
int grade;
for (int i = 0; i < 10; i++) {
System.out.println("Question number " + (i+1) + ":");
System.out.println("    " + questions[i].getQuestion());
System.out.println("    Correct answer:  " + questions[i].getCorrectAnswer());
if ( userAnswers[i] == questions[i].getCorrectAnswer() ) {
System.out.println("    You were CORRECT.");
numberCorrect++;
}
else {
System.out.println("    You said " + userAnswers[i] + ", which is INCORRECT.");
}
System.out.println();
}
grade = numberCorrect * 10;
System.out.println();
System.out.println("You got " + numberCorrect + " questions correct.");
System.out.println("Your grade on the quiz is " + grade);
System.out.println();
}

} // end class GeneralQuiz

Solution for Programming Exercise 6.1

Exercise 6.1:

In the SimpleStamper example from Subsection 6.3.3, a rectangle or oval is drawn on the panel when the user clicks the mouse. Except, when the user shift-clicks, the panel is cleared instead. Modify this class so that the modified version will continue to draw figures as the user drags the mouse. That is, the mouse will leave a trail of figures as the user drags. However, if the user shift-clicks, the panel should simply be cleared and no figures should be drawn even if the user drags the mouse after shift-clicking. Here is a picture of my solution:

The source code for the original program is SimpleStamper.java. See the discussion of dragging in Subsection 6.3.4. (Note that the original version uses a black background, with a black border around each shape. That didn't work well with a lot of closely spaced shapes, so the new version uses a white background.)

If you want to make the problem a little more challenging, when drawing shapes during a drag operation, make sure that the shapes that are drawn are at least, say, 5 pixels apart. To implement this, you have to keep track of the position of the last shape that was drawn.

Discussion

In order to implement dragging in the new version, we need a MouseMotionListener in addition to the MouseListener that is already present in the original version. In the original, the panel class itself implements MouseListener, so I just added MouseMotionListener:

public class SimpleStamperWithDrag extends JPanel
implements MouseListener, MouseMotionListener { . . .

Of course, the mouse motion listener has to be registered with the panel in order for it to hear any events from the panel. This is done by adding the line

addMouseMotionListener(this);

to the constructor. Here "this" refers to the panel object itself and is used because the panel itself implements MouseMotionListener, and it will listen for mouse motion events from itself. (It's awfully easy to forget this step when implementing event-handling! Nothing will happen when the event occurs, and it can be hard to realize what the problem is.)

To finish the implementation of the mouse motion listener interface, the mouseMoved and mouseDragged methods must be added to the class. The program does not respond when the user moves the mouse without holding down any mouse button, so the mouseMoved method is empty. The mouseDragged method must draw a figure at the current mouse position; the code for this is almost identical to the existing drawing code in the mousePressed routine and can be copied from there. However, nothing should be drawn in the mouseDragged method if the user started the mouse drag gesture by shift-clicking. The discussion of dragging in Subsection 6.3.4, suggests that the program should use an instance variable named dragging to keep track of whether or not to draw anything in the mouseDragged method. In the mousePressed routine, this variable is set to false if the user shift-clicked, and to true otherwise. The mouseDragged routine checks the value of dragging; if the value is false, it means that the drag started with a shift-click and therefore nothing should be drawn. The complete source code is shown below.

The picture produced by the program would look better if there were always at least a few pixels between the shapes that are drawn as the user drags the mouse, as suggested at the end of the exercise. It is not difficult to make the change. The panel needs two new instance variables, prevX and prevY, of type int, to store the position of the shape that was drawn most recently. Their values should be set after drawing a shape in both mousePressed() and mouseDragged() with the statements

prevX = x;
prevY = y;

The values of prevX and prevY can then be tested at the beginning of the mouseDragged() method to decide whether or not to draw a shape. The shape should be drawn only if either the x-coordinate or the y-coordinate has changed by at least 5 pixels since the last time a shape was drawn. For example, mouseDragged() could make the test as follows:

public void mouseDragged(MouseEvent evt) {
if (dragging == false) {
return;
}
int x = evt.getX();  // x-coordinate where user clicked.
int y = evt.getY();  // y-coordinate where user clicked.
if ( Math.abs( prevX - x ) < 5 && Math.abs( prevY - y) < 5 )
return;
.
.
.

The Solution

Here is the code for the modified panel class, with changes from the original (SimpleStamper.java) shown in red italic:

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

/**
* A simple demonstration of MouseEvents.  Shapes are drawn
* on a black background when the user clicks the panel.  If
* the user Shift-clicks, the panel is cleared.  If the user
* right-clicks the panel, a blue oval is drawn.  Otherwise,
* when the user clicks, a red rectangle is drawn.  The contents of
* the panel are not persistent.  For example, they might disappear
* if the panel is resized or is covered and uncovered.
* Ovals and rects continue to be drawn as the user drags the mouse.
* This class has a main() routine to allow it to be run as an application.
*/
public class SimpleStamperWithDrag extends JPanel
implements MouseListener, MouseMotionListener {

public static void main(String[] args) {
JFrame window = new JFrame("Simple Stamper");
SimpleStamperWithDrag content = new SimpleStamperWithDrag();
window.setContentPane(content);
window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
window.setLocation(120,70);
window.setSize(400,300);
window.setVisible(true);
}

// ----------------------------------------------------------------------

/**
* This variable is set to true during a drag operation, unless the
* user was holding down the shift key when the mouse was first
* pressed (since in that case, the mouse gesture simply clears the
* panel and no figures should be drawn if the user drags the mouse).
*/
private boolean dragging;

/**
* This constructor simply sets the background color of the panel to be black
* and sets the panel to listen for mouse events on itself.
*/
public SimpleStamperWithDrag() {
setBackground(Color.WHITE);
addMouseListener(this);
addMouseMotionListener(this);
}

/**
*  Since this panel has been set to listen for mouse events on itself,
*  this method will be called when the user clicks the mouse on the panel.
*  This method is part of the MouseListener interface.
*/
public void mousePressed(MouseEvent evt) {

if ( evt.isShiftDown() ) {
// The user was holding down the Shift key.  Just repaint the panel.
// Since this class does not define a paintComponent() method, the
// method from the superclass, JPanel, is called.  That method simply
// fills the panel with its background color, which is black.  The
// effect is to clear the panel.
dragging = false;
repaint();
return;
}

dragging = true;

int x = evt.getX();  // x-coordinate where user clicked.
int y = evt.getY();  // y-coordinate where user clicked.

Graphics g = getGraphics();  // Graphics context for drawing directly.
// NOTE:  This is considered to be bad style!

if ( evt.isMetaDown() ) {
// User right-clicked at the point (x,y). Draw a blue oval centered
// at the point (x,y). (A black outline around the oval will make it
// more distinct when shapes overlap.)
g.setColor(Color.BLUE);  // Blue interior.
g.fillOval( x - 30, y - 15, 60, 30 );
g.setColor(Color.BLACK); // Black outline.
g.drawOval( x - 30, y - 15, 60, 30 );
}
else {
// User left-clicked (or middle-clicked) at (x,y).
// Draw a red rectangle centered at (x,y).
g.setColor(Color.RED);   // Red interior.
g.fillRect( x - 30, y - 15, 60, 30 );
g.setColor(Color.BLACK); // Black outline.
g.drawRect( x - 30, y - 15, 60, 30 );
}

g.dispose();  // We are finished with the graphics context, so dispose of it.

} // end mousePressed();

/**
*  This method is called when the user drags the mouse.  If a the value of the
*  instance variable dragging is true, it will draw a rect or oval at the
*  current mouse position.
*/
public void mouseDragged(MouseEvent evt) {
if ( dragging == false ) {
return;
}
int x = evt.getX();  // x-coordinate where user clicked.
int y = evt.getY();  // y-coordinate where user clicked.
Graphics g = getGraphics();  // Graphics context for drawing directly.
// NOTE:  This is considered to be bad style!
if ( evt.isMetaDown() ) {
// The user is using the right mouse button; draw an oval.
g.setColor(Color.BLUE);  // Blue interior.
g.fillOval( x - 30, y - 15, 60, 30 );
g.setColor(Color.BLACK); // Black outline.
g.drawOval( x - 30, y - 15, 60, 30 );
}
else {
g.setColor(Color.RED);   // Red interior.
g.fillRect( x - 30, y - 15, 60, 30 );
g.setColor(Color.BLACK); // Black outline.
g.drawRect( x - 30, y - 15, 60, 30 );
}
g.dispose();  // We are finished with the graphics context, so dispose of it.
} // end mouseDragged();

// The next four empty routines are required by the MouseListener interface.
// They don't do anything in this class, so their definitions are empty.

public void mouseEntered(MouseEvent evt) { }
public void mouseExited(MouseEvent evt) { }
public void mouseClicked(MouseEvent evt) { }
public void mouseReleased(MouseEvent evt) { }

// The next routines is required by the MouseMotionListener interface.

public void mouseMoved(MouseEvent evt) { }

} // end class SimpleStamperWithDrag

Solution for Programming Exercise 6.2

Exercise 6.2:

Write a program that shows a small red square and a small blue square. The user should be able to drag either square with the mouse. (You'll need an instance variable to remember which square the user is dragging.) The user can drag the square out of the panel if she wants; if she does this, there is no way to get it back.

Note that for this exercise, you should do all the drawing in the paintComponent() method (as indeed you should whenever possible).

Discussion

To write this program, you need to understand dragging, as discussed in Subsection 6.3.4. To support dragging, you have to implement both the MouseListener and MouseMotionListener interfaces and register some object to listen for both mouse and mouse motion events. The code for dragging a square is spread out over three methods, mousePressed, mouseReleased, and mouseDragged. Several instance variables are needed to keep track of what is going on while a dragging operation is being executed. A general framework for dragging is given in Subsection 6.3.4. This example is simplified a bit because while dragging the square, we only need to know the current position of the mouse, so that we can move the square to that position. We don't need to keep track of the previous position of the mouse. In my solution, I decided to write a nested class inside the panel class to define the listening object. Another possibility would have been to let the panel class implement MouseListener and MouseMotionListener, and then register the panel with itself to listen for events.

As usual for dragging, I use a boolean variable, dragging, to keep track of whether or not a drag operation is in progress. Not every mouse press starts a drag operation. If the user clicks the panel outside of the squares, there is nothing to drag, and dragging is set to false in the mousePressed method. If the user clicks one of the squares, dragging is set to true. Since there are two squares to be dragged, we have to keep track of which is being dragged. I use a boolean variable, dragRedSquare, which is true if the red square is being dragged and is false if the blue square is being dragged. (A boolean variable is actually not the best choice in this case. It would be a problem if we wanted to add another square. A boolean variable only has two possible values, so an integer variable would probably be a better choice.) I keep track of the locations of the squares with integer instance variables x1 and y1 for the upper left corner of the red square and x2 and y2 for the upper left corner of the blue square. These variables are in the main class, not the nested class that defines the listener, since the panel needs to use the location of the squares in the paintComponent() method.

There is one little problem. The mouse location is a single (x,y) point. A square occupies a whole bunch of points. When we move the square to follow the mouse, where exactly should we put the square? One possibility is to put the upper left corner of the square at the mouse location. If we did this, the mouseDragged routine would look like:

public void mouseDragged(MouseEvent evt) {
if (dragging == false)
return;
int x = evt.getX();  // Get mouse position.
int y = evt.getY();
if (dragRedSquare) {  // Move the red square.
x1 = x;  // Put top-left corner at mouse position.
y1 = y;
}
else {   // Move the blue square.
x2 = x;  // Put top-left corner at mouse position.
y2 = y;
}
repaint();
}

This works, but it is not very aesthetic. When the user starts dragging a square, no matter where in the square the user clicks, the square will jump so that its top-left corner is at the mouse position. This is not what a user typically expects. If I grab a square by clicking its center, then I want the center to stay under the mouse cursor as I move it. If I grab the lower right corner, I want the lower right corner to follow the mouse, not the upper left corner. There is a solution to this, and it's one that is often needed for dragging operations. We need to record the original position of the mouse relative to the upper left corner of the square. This tells us where in the square the user clicked. This is done in the mousePressed routine by assigning appropriate values to instance variables offsetX and offsetY:

int x = evt.getX();  // Location where user clicked.
int y = evt.getY();

if (x >= x2 && x < x2+30 && y >= y2 && y < y2+30) {
// It's the blue square (which should be checked first,
// since it's drawn on top of the red square.)
dragging = true;
dragRedSquare = false;
offsetX = x - x2;  // Distance from corner of square to (x,y).
offsetY = y - y2;
}
else if (x >= x1 && x < x1+30 && y >= y1 && y < y1+30) {
// It's the red square.
dragging = true;
dragRedSquare = true;
offsetX = x - x1;  // Distance from corner of square to (x,y).
offsetY = y - y1;
}

In mouseDragged, when the mouse moves to a new (x,y) point, we move the square so that the vertical and horizontal distances between the mouse location and the top left corner of the square remain the same:

if (dragRedSquare) {  // Move the red square.
x1 = x - offsetX;  // Offset corner from mouse location.
y1 = y - offsetY;
}
else {   // Move the blue square.
x2 = x - offsetX;  // Offset corner from mouse location.
y2 = y - offsetY;
}

The program is written as a subclass of JPanel. I included a main() routine in the class, to allow it to be run as an application, but main() could also be in its own class. The constructor in the DrawTwoSquares class sets up the initial positions of the two squares, creates a listening object, and registers the listening object to listen for mouse and mouse motion events. It also adds a border around the panel to make it more attractive. The class has a paintComponent() method, which just has to draw the two squares in their current positions; this is easy to write. The mouse-handling code is in a nested class. Note that the nested class is non-static, since it need access to instance variables from the main class.

By the way, if you wanted to stop the user from dragging the square outside the panel, you would just have to add code to the mouseDragged routine to "clamp" the variables x1, y1, x2, and y2 so that they lie in the acceptable range. Here is a modified routine that keeps the square entirely within the panel:

public void mouseDragged(MouseEvent evt) {
if (dragging == false)
return;
int x = evt.getX();
int y = evt.getY();
if (dragRedSquare) {  // Move the red square.
x1 = x - offsetX;
y1 = y - offsetY;
if (x1 < 0)  // Clamp (x1,y1) so the square lies in the panel.
x1 = 0;
else if (x1 >= getWidth() - 30)
x1 = getWidth() - 30;
if (y1 < 0)
y1 = 0;
else if (y1 >= getHeight() - 30)
y1 = getHeight() - 30;
}
else {   // Move the blue square.
x2 = x - offsetX;
y2 = y - offsetY;
if (x2 < 0)  // Clamp (x2,y2) so the square lies in the panel.
x2 = 0;
else if (x2 >= getWidth() - 30)
x2 = getWidth() - 30;
if (y2 < 0)
y2 = 0;
else if (y2 >= getHeight() - 30)
y2 = getHeight() - 30;
}
repaint();
}

The Solution

import java.awt.*;
import java.awt.event.*;

import javax.swing.*;

/**
* A panel showing a red square and a blue square that the user
* can drag with the mouse.   The user can drag the squares off
* the panel and drop them.  There is no way of getting them back.
*/
public class DragTwoSquares extends JPanel {

/**
* A main routine allows this class to be run as an application.
*/
public static void main(String[] args) {
JFrame window = new JFrame("Drag Either Square");
DragTwoSquares content = new DragTwoSquares();
window.setContentPane(content);
window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
window.setLocation(120,70);
window.setSize(400,300);
window.setVisible(true);
}

//---------------------------------------------------------------------

private int x1, y1;   // Coords of top-left corner of the red square.
private int x2, y2;   // Coords of top-left corner of the blue square.

/**
*  The constructor places the two squares in their initial positions and
*  sets up listening for mouse events and mouse motion events.
*/
public DragTwoSquares() {

x1 = 10;  // Set up initial positions of the squares.
y1 = 10;
x2 = 50;
y2 = 10;

setBackground(Color.LIGHT_GRAY);  // Set up appearance of the panel
setBorder(BorderFactory.createLineBorder(Color.BLACK, 1) );

Dragger listener = new Dragger();  // Listening object, belonging to a nested
//     class that is defined below.

addMouseListener(listener);        // Set up listening.
addMouseMotionListener(listener);

}

/**
* paintComponent just draws the two squares in their current positions.
*/
public void paintComponent(Graphics g) {
super.paintComponent(g);  // Fill with background color.
g.setColor(Color.RED);
g.fillRect(x1, y1, 30, 30);
g.setColor(Color.BLUE);
g.fillRect(x2, y2, 30, 30);
}

/**
*  This private class is used to define the listener that listens
*  for mouse events and mouse motion events on the panel.
*/
private class Dragger implements MouseListener, MouseMotionListener {

/* Some variables used during dragging */

boolean dragging;      // Set to true when a drag is in progress.

boolean dragRedSquare; // True if red square is being dragged, false
//    if blue square is being dragged.

int offsetX, offsetY;  // Offset of mouse-click coordinates from the
//   top-left corner of the square that was
//   clicked.

/**
* Respond when the user presses the mouse on the panel.
* Check which square the user clicked, if any, and start
* dragging that square.
*/
public void mousePressed(MouseEvent evt) {

if (dragging)  // Exit if a drag is already in progress.
return;

int x = evt.getX();  // Location where user clicked.
int y = evt.getY();

if (x >= x2 && x < x2+30 && y >= y2 && y < y2+30) {
// It's the blue square (which should be checked first,
// since it's drawn on top of the red square.)
dragging = true;
dragRedSquare = false;
offsetX = x - x2;  // Distance from corner of square to (x,y).
offsetY = y - y2;
}
else if (x >= x1 && x < x1+30 && y >= y1 && y < y1+30) {
// It's the red square.
dragging = true;
dragRedSquare = true;
offsetX = x - x1;  // Distance from corner of square to (x,y).
offsetY = y - y1;
}

}

/**
* Dragging stops when user releases the mouse button.
*/
public void mouseReleased(MouseEvent evt) {
dragging = false;
}

/**
* Respond when the user drags the mouse.  If a square is
* not being dragged, then exit. Otherwise, change the position
* of the square that is being dragged to match the position
* of the mouse.  Note that the corner of the square is placed
* in the same relative position with respect to the mouse that i
* had when the user started dragging it.
*/
public void mouseDragged(MouseEvent evt) {

if (dragging == false)
return;
int x = evt.getX();
int y = evt.getY();
if (dragRedSquare) {  // Move the red square.
x1 = x - offsetX;
y1 = y - offsetY;
}
else {   // Move the blue square.
x2 = x - offsetX;
y2 = y - offsetY;
}
repaint();  // (Calls the repaint() method in the DragTwoSquaresPanel class.)
}

public void mouseMoved(MouseEvent evt) { }  // empty methods required by interfaces.
public void mouseClicked(MouseEvent evt) { }
public void mouseEntered(MouseEvent evt) { }
public void mouseExited(MouseEvent evt) { }

} // end nested class Dragger

} // end class DragTwoSquaresPanel

Solution for Programming Exercise 6.3

Exercise 6.3:

Write a program that shows a pair of dice. When the user clicks on the panel in the program, the dice should be rolled (that is, the dice should be assigned newly computed random values). Each die should be drawn as a square showing from 1 to 6 dots. Since you have to draw two dice, its a good idea to write a subroutine, "void drawDie(Graphics g, int val, int x, int y)", to draw a die at the specified (x,y) coordinates. The second parameter, val, specifies the value that is showing on the die. Assume that the size of the panel is 100 by 100 pixels. Here is a picture of the panel that displays the dice:

Discussion

This is largely an exercise in precision drawing.

We need a subclass of JPanel to make the panel. We will also need an object to respond to mouse events. As always, there are several ways to approach this: Let the panel implement MouseListener and listen for events on itself, use a separate class to define the listener object, or use an anonymous inner class (see Subsection 6.3.5). In this case, I decided to use an anonymous class, since the mouse-handling code is very simple. The listener object is created and registered to listen for mouse events in the constructor of the main class:

addMouseListener( new MouseAdapter() {
public void mousePressed(MouseEvent evt) {
roll();
}
});

By using MouseAdapter instead of MouseListener as the superclass for the listener, I avoid the necessity of providing empty definitions for mouseReleased, mouseClicked, mouseEntered, and mouseExited. MouseAdapter is discussed in Subsection 6.3.5. In the mousePressed() method, roll() is a method that is defined in the main class; it just rolls the dice. It would be reasonable to put the code for rolling the dice in mousePressed(), but writing a subroutine to do it makes the program a little easier to modify for the next exercise. Anyway, when you can identify a self-contained, meaningful task to be performed, it's never a bad a idea to write a subroutine to do it. It will make the program more readable, if nothing else. My roll subroutine assigns random values to the dice and calls repaint() so that the new values will be shown.

The hardest part of this exercise is drawing the dice. I made each die 35 pixels wide, leaving a 10 pixel border on each side and 10 pixels between the dice, for a total of 100 pixels. The top left corner of the left die is at (10,10), the top left corner of the right die is at (55,55). The 55 includes the 10 pixel border on the left, the 35 pixel width of the other die, and the 10 pixels between the dice. The paintComponent() method calls a drawDie() routine to draw each die, using the commands:

drawDie(g, die1, 10, 10);
drawDie(g, die2, 55, 55);

where die1 is the numerical value shown on the first die and die2 is the numerical value of the second die.

As for the drawDie routine, there are two quite different algorithms that could have been used for drawing the dots. Either:

if the value shown is 1
draw 1 dot (in the center)
else if the value shown is 2
draw 2 dots (in the top-left and bottom-right corners)
.
.
.
else if the value shown is 6
draw 6 dots (along the left and right edges)

Or:

if the value has a dot in the top-left corner
draw the top-left dot
else if the value has a dot in the top-right corner
draw the top-right dot
.
.
.
else if the value has a dot in the bottom-right corner
draw the bottom-right dot

Although the first algorithm is more obvious, the second requires much less typing. (The first algorithm ends up using 21 drawOval() commands, while the second uses only 7.) Furthermore, after drawing the dice on paper, I found that the conditions for testing when a given dot needs to be drawn are simpler than I expected. For example, the values that need a dot in the top-left position are all the values greater than 1. The algorithm leads to my drawDie() routine:

/**
* Draw a die with upper left corner at (x,y).  The die is
* 35 by 35 pixels in size.  The val parameter gives the
* value showing on the die (that is, the number of dots).
*/
private void drawDie(Graphics g, int val, int x, int y) {
g.setColor(Color.white);
g.fillRect(x, y, 35, 35);
g.setColor(Color.black);
g.drawRect(x, y, 34, 34);
if (val > 1)  // upper left dot
g.fillOval(x+3, y+3, 9, 9);
if (val > 3)  // upper right dot
g.fillOval(x+23, y+3, 9, 9);
if (val == 6) // middle left dot
g.fillOval(x+3, y+13, 9, 9);
if (val % 2 == 1) // middle dot (for odd-numbered val's)
g.fillOval(x+13, y+13, 9, 9);
if (val == 6) // middle right dot
g.fillOval(x+23, y+13, 9, 9);
if (val > 3)  // bottom left dot
g.fillOval(x+3, y+23, 9, 9);
if (val > 1)  // bottom right dot
g.fillOval(x+23, y+23, 9,9);
}

It took some care to figure out the numbers to use in the fillOval commands. The individual dots have a diameter of 9 pixels. There are three rows of dots, which have a combined height of 27 pixels. That leaves 35 minus 27, or 8 pixels for spacing. I use 3 pixels between the dots and the edge of the die, and 1 pixel between rows. This puts the tops of the rows at 3, 3+9+1, and 3+9+1+9+1, that is, at 3, 13, and 23. The columns use the same numbers. (If you believe that I got all this right the first time, I won't disillusion you!)

The paintComponent method just draws the two dice and the border around the panel

public void paintComponent(Graphics g) {
super.paintComponent(g);  // fill with background color.
Graphics2D g2 = (Graphics2D)g;
g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
RenderingHints.VALUE_ANTIALIAS_ON);
g.setColor( Color.BLUE );
g.drawRect(0,0,99,99);
g.drawRect(1,1,97,97);
drawDie(g, die1, 10, 10);
drawDie(g, die2, 55, 55);
}

The paintComponent() method turns on antialiasing, since without it the dots on the dice have a jagged appearance (see Subsection 6.2.5).

One more small remark on the solution: The constructor of the DicePanel class sets the preferred size of the panel to be 100-by-100 pixels, using the command:

setPreferredSize( new Dimension(100,100) );

Since the main() routine for the program will use the pack() method to set the size of the frame, it is essential that the panel have a preferred size. If not, the size of the frame will not be set correctly.

The Solution

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

/**
* Shows a pair of dice that are rolled when the user clicks on the
* program.  It is assumed that the panel is 100-by-100 pixels.
*/
public class DicePanel extends JPanel {

/**
* A main routine allows this class to be run as an application.
*/
public static void main(String[] args) {
JFrame window = new JFrame("Dice");
DicePanel content = new DicePanel();
window.setContentPane(content);
window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
window.setLocation(120,70);
window.pack();
window.setVisible(true);
}

//---------------------------------------------------------------------

private int die1 = 4;  // The values shown on the dice.
private int die2 = 3;

/**
*  The constructor adds a mouse listener to the panel.  The listener
*  will roll the dice when the user clicks the panel.  Also, the
*  background color and the preferred size of the panel are set.
*/
public DicePanel() {
setPreferredSize( new Dimension(100,100) );
setBackground( new Color(200,200,255) );  // light blue
addMouseListener( new MouseAdapter() {
public void mousePressed(MouseEvent evt) {
roll();
}
});
}

/**
* Draw a die with upper left corner at (x,y).  The die is
* 35 by 35 pixels in size.  The val parameter gives the
* value showing on the die (that is, the number of dots).
*/
private void drawDie(Graphics g, int val, int x, int y) {
g.setColor(Color.white);
g.fillRect(x, y, 35, 35);
g.setColor(Color.black);
g.drawRect(x, y, 34, 34);
if (val > 1)  // upper left dot
g.fillOval(x+3, y+3, 9, 9);
if (val > 3)  // upper right dot
g.fillOval(x+23, y+3, 9, 9);
if (val == 6) // middle left dot
g.fillOval(x+3, y+13, 9, 9);
if (val % 2 == 1) // middle dot (for odd-numbered val's)
g.fillOval(x+13, y+13, 9, 9);
if (val == 6) // middle right dot
g.fillOval(x+23, y+13, 9, 9);
if (val > 3)  // bottom left dot
g.fillOval(x+3, y+23, 9, 9);
if (val > 1)  // bottom right dot
g.fillOval(x+23, y+23, 9,9);
}

/**
* Roll the dice by randomizing their values.  Tell the
* system to repaint the panel, to show the new values.
*/
void roll() {
die1 = (int)(Math.random()*6) + 1;
die2 = (int)(Math.random()*6) + 1;
repaint();
}

/**
* The paintComponent method just draws the two dice and draws
* a one-pixel wide blue border around the panel.  Antialiasing
* is turned on to make the ovals look nicer.
*/
public void paintComponent(Graphics g) {
super.paintComponent(g);  // fill with background color.
Graphics2D g2 = (Graphics2D)g;
g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
RenderingHints.VALUE_ANTIALIAS_ON);
g.setColor( Color.BLUE );
g.drawRect(0,0,99,99);
g.drawRect(1,1,97,97);
drawDie(g, die1, 10, 10);
drawDie(g, die2, 55, 55);
}

} // end class DicePanel

Solution for Programming Exercise 6.4

Exercise 6.4:

In Exercise 6.3, you wrote a pair-of-dice panel where the dice are rolled when the user clicks on the panel. Now make a pair-of-dice program in which the user rolls the dice by clicking a button. The button should appear under the panel that shows the dice. Also make the following change: When the dice are rolled, instead of just showing the new value, show a short animation during which the values on the dice are changed in every frame. The animation is supposed to make the dice look more like they are actually rolling.

Discussion

In Exercise 6.3, there was a single panel, which was being used as a drawing surface. In the new version, there are two panels: One is the drawing surface on which the dice are drawn; the other is a container that holds the button and the drawing surface panel. If we write a class to define the container, we need another class to define the drawing surface. One way to write that class is as a simple nested class that contains only a paintComponent method. Since it is so simple, I decided to define it as an anonymous inner class:

JPanel dicePanel = new JPanel() {
public void paintComponent(Graphics g) {
super.paintComponent(g);  // fill with background color.
Graphics2D g2 = (Graphics2D)g;
g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
RenderingHints.VALUE_ANTIALIAS_ON);
drawDie(g, die1, 10, 10); // Just draw the dice.
drawDie(g, die2, 55, 55);
}
};

This just makes dicePanel into a JPanel object that contains a paintComponent method that differs from the one defined in the JPanel class.

The constructor of the main class is responsible for setting up the user interface and the event handling. It sets the layout manager of the panel to be a BorderLayout, creates the drawing surface and button, adds the drawing surface in the CENTER position and the button in the SOUTH position. It adds a blue border to the panel, and leaves a gap in the border layout through which a blue background color will show. It also adds an action listener to the button that will call the roll() method to roll the dice when the button is pressed. Here is the complete constructor:

public DicePanelWithButton() {

setLayout(new BorderLayout(2,2));
setBackground(Color.BLUE);  // Will show through the gap in the BorderLayout.
setBorder(BorderFactory.createLineBorder(Color.BLUE,2));

JPanel dicePanel = new JPanel() { // the drawing surface, where dice are shown
public void paintComponent(Graphics g) {
super.paintComponent(g);  // fill with background color.
Graphics2D g2 = (Graphics2D)g;
g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
RenderingHints.VALUE_ANTIALIAS_ON);
drawDie(g, die1, 10, 10); // Just draw the dice.
drawDie(g, die2, 55, 55);
}
};
dicePanel.setPreferredSize( new Dimension(100,100) );
dicePanel.setBackground( new Color(200,200,255) );  // light blue
add(dicePanel, BorderLayout.CENTER);

JButton rollButton = new JButton("Roll!");  // the button that rolls the dice
rollButton.addActionListener( new ActionListener() {
public void actionPerformed(ActionEvent evt) {
roll();
}
});
add(rollButton, BorderLayout.SOUTH);

} // end constructor

The method for drawing the dice was discussed in the solution to Exercise 6.3. But we still have to think about animating the rolling of the dice. The roll() method is responsible for rolling the dice. In the original version, this method simply set the numbers showing on the dice to random values and called repaint(). In the new version, we want to repeat this action several times over a period of time. That is, we want an animation in which the action for each frame is to randomize the numbers on the dice and call repaint(). The code for doing one frame goes in the ActionListener that responds to events from the timer. The roll() method simply creates the timer and starts it running. (Timers and animation are discussed in Subsection 6.4.1.)

There is, however, one big problem: How is the timer stopped? If it's not stopped, the dice will keep rolling forever! The solution is not so hard. We have to stop the timer after a certain number of frames. The action listener can keep track of how many frames it has handled, and after a certain number of frames it can stop the timer. After some experimentation, I found that 10 frames, with a delay between frames of 100 milliseconds, looks pretty good. So in the tenth frame, the actionPerformed method stops the timer.

There is still one little problem: The user might click the "Roll" button while the animation is in progress. If the dice are already ready rolling, it doesn't make sense to start another animation. So, the program needs a way of determining whether an animation is in progress when the "Roll" button is clicked; if it is, then the click should be ignored. In my program, there is a Timer variable that is set to a non-null value when an animation is in progress. When the timer is stopped, the animation is finished, and the Timer variable is set back to null. The roll() method checks the Timer variable to determine whether or not an animation is already in progress. (Another way of handling the problem would be to disable the Roll button while the animation is in progress.) Here's the roll() method. Note that the ActionListener for the timer is defined as an anonymous inner class.

/**
* Run an animation that randomly changes the values shown on
* the dice 10 times, every 100 milliseconds.
*/
private void roll() {
if (timer != null)
return;
timer = new Timer(100, new ActionListener() {
int frames = 1;
public void actionPerformed(ActionEvent evt) {
die1 = (int)(Math.random()*6) + 1;
die2 = (int)(Math.random()*6) + 1;
repaint();
frames++;
if (frames == 10) {
timer.stop();
timer = null;
}
}
});
timer.start();
}

For this program, I decided to put the main() routine for the program in a separate class, just to remind you that it can be done that way!

The Solution

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

/**
* Shows a pair of dice that are rolled when the user clicks a button
* that appears below the dice.
*/
public class DicePanelWithButton extends JPanel {

private int die1 = 4;  // The values shown on the dice.
private int die2 = 3;

private Timer timer;   // Used to animate rolling of the dice.

/**
*  The constructor sets up the panel.  It creates the button and
*  the drawing surface panel on which the dice are drawn and puts
*  them into a BorderLayout.  It adds an ActionListener to the button
*  that rolls the dice when the user clicks the button.
*/
public DicePanelWithButton() {

setLayout(new BorderLayout(2,2));
setBackground(Color.BLUE);  // Will show through the gap in the BorderLayout.
setBorder(BorderFactory.createLineBorder(Color.BLUE,2));

JPanel dicePanel = new JPanel() {
public void paintComponent(Graphics g) {
super.paintComponent(g);  // fill with background color.
drawDie(g, die1, 10, 10); // Just draw the dice.
drawDie(g, die2, 55, 55);
}
};
dicePanel.setPreferredSize( new Dimension(100,100) );
dicePanel.setBackground( new Color(200,200,255) );  // light blue
add(dicePanel, BorderLayout.CENTER);

JButton rollButton = new JButton("Roll!");
rollButton.addActionListener( new ActionListener() {
public void actionPerformed(ActionEvent evt) {
roll();
}
});
add(rollButton, BorderLayout.SOUTH);

} // end constructor

/**
* Draw a die with upper left corner at (x,y).  The die is
* 35 by 35 pixels in size.  The val parameter gives the
* value showing on the die (that is, the number of dots).
*/
void drawDie(Graphics g, int val, int x, int y) {
g.setColor(Color.white);
g.fillRect(x, y, 35, 35);
g.setColor(Color.black);
g.drawRect(x, y, 34, 34);
if (val > 1)  // upper left dot
g.fillOval(x+3, y+3, 9, 9);
if (val > 3)  // upper right dot
g.fillOval(x+23, y+3, 9, 9);
if (val == 6) // middle left dot
g.fillOval(x+3, y+13, 9, 9);
if (val % 2 == 1) // middle dot (for odd-numbered val's)
g.fillOval(x+13, y+13, 9, 9);
if (val == 6) // middle right dot
g.fillOval(x+23, y+13, 9, 9);
if (val > 3)  // bottom left dot
g.fillOval(x+3, y+23, 9, 9);
if (val > 1)  // bottom right dot
g.fillOval(x+23, y+23, 9,9);
}

/**
* Run an animation that randomly changes the values shown on
* the dice 10 times, every 100 milliseconds.
*/
private void roll() {
if (timer != null)
return;
timer = new Timer(100, new ActionListener() {
int frames = 1;
public void actionPerformed(ActionEvent evt) {
die1 = (int)(Math.random()*6) + 1;
die2 = (int)(Math.random()*6) + 1;
repaint();
frames++;
if (frames == 10) {
timer.stop();
timer = null;
}
}
});
timer.start();
}

} // end class DicePanelWithButton

To use this in a stand-alone application, we need a class that defines the main() routine of the program. For this exercise, I used a separate class, which can be written as follows:

import javax.swing.JFrame;

/**
* A main program that just opens a window that shows a DicePanelWithButton.
*/
public class DiceWithButtonMain {

public static void main(String[] args) {
JFrame window = new JFrame();
DicePanelWithButton content = new DicePanelWithButton();
window.setContentPane(content);
window.pack();
window.setLocation(100,100);
window.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
window.setResizable(false);  // User can't change the window's size.
window.setVisible(true);
}

}

Solution for Programming Exercise 6.5

Exercise 6.5:

In Exercise 3.8, you drew a checkerboard. For this exercise, write a program where the user can select a square by clicking on it. (Use a JPanel for the checkerboard.) Highlight the selected square by drawing a colored border around it. When the program starts, no square is selected. When the user clicks on a square that is not currently selected, it becomes selected (and the previously selected square, if any, is unselected). If the user clicks the square that is selected, it becomes unselected. Assume that the size of the panel is exactly 160 by 160 pixels, so that each square on the checkerboard is 20 by 20 pixels. Here is my checkerboard, with the square in row 3, column 3 selected:

Discussion

See the solution to Exercise 3.8 for a discussion of how to draw the checkerboard. In that exercise, the code for drawing the board was in a drawFrame() method. Now, we can use a JPanel as a drawing surface for the checkerboard, with the drawing code in the panel's paintComponent() method. The code for drawing the checkerboard is the same.

As always, there are many ways to organize the program. In this case, I decided to let the main panel class implement MouseListener. The panel listens for mouse events on itself.

To keep track of which square is selected, if any, the class contains instance variables, selectedRow and selectedCol. When no square is selected, selectedRow is -1 (and I don't care what selectedCol is). When a square is selected, selectedRow is the number of the row that contains that square and selectedCol is the number of the column that contains the selected square. Remember that rows and columns are numbered from 0 to 7. This makes some of the calculations easier than numbering them from 1 to 8.

After drawing the checkerboard, the paintComponent() method has to highlight the selected square, if there is one. I do this by drawing a cyan border around the inside of the selected square. This is the new code that is added to the checkerboard-drawing code:

if (selectedRow >= 0) {
// Since there is a selected square, draw a cyan
// border around it.
g.setColor(Color.CYAN);
y = selectedRow * 20;
x = selectedCol * 20;
g.drawRect(x, y, 19, 19);
g.drawRect(x+1, y+1, 17, 17);
}

Since the squares are 20 pixels on each side, you might wonder why the first drawRect() command specifies a width and height of 19 instead of 20. In the fillRect() method that is used earlier in the paintComponent() method to fill in the square, a width and height of 20 is used. Remember that the drawRect() method actually draws a rectangle whose width and height are one more than the values specified in the parameters. (Remember the bit about the pen that hangs one pixel outside the rectangle?)

To respond to user mouse clicks, the panel must implement the MouseListener interface. The constructor calls addMouseListener(this) to register the board to listen for mouse events on itself. (Remember that calling addMouseListener(this) is the same as calling this.addMouseListener(this).) Of the five methods specified in the MouseListener interface, only mousePressed has a non-empty definition. This method must figure out which square the user clicked and adjust the values of the instance variables selectedRow and selectedCol accordingly.

Let's say that the user clicked at the point (x,y). The problem is to determine which square on the checkerboard contains that point. The column number of the square is obtained by dividing the x coordinate by the width of the squares. Since the squares are 20 pixels wide, the row number of the clicked square is x/20. For values of x between 0 and 19, this gives a column number of 0, which is correct. For the next 20 pixels, from 20 to 39, x/20 is 1, which is the correct column number. For the next strip of pixels, from 40 to 59, the answer is 2. And so on. Similarly, y/20 gives the row number of the square where the user clicked. (I often get rows and columns mixed up -- remember that the x coordinate corresponds to columns and the y coordinate corresponds to rows.)

Once we know the row and column where the user clicked, we can compare them to selectedRow and selectedCol. If the values are the same, then the user clicked in a square that was already selected. We want to remove the highlighting. That can be done by setting selectedRow = -1, the value that indicates that no square is selected. Otherwise, the values of selectedRow and selectedCol are set to the row and column that the user clicked. In the end, repaint() is called so that the change will be reflected in what is shown on the screen.

All this explains the reasoning behind the mousePressed() routine, which you can see below.

The Solution

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

/**
*  This program draws a red-and-black checkerboard.
*  It is assumed that the size of the panel is 160
*  by 160 pixels.  When the user clicks a square, that
*  square is selected, unless it is already selected.
*  When the user clicks the selected square, it is
*  unselected.  If there is a selected square, it is
*  highlighted with a cyan border.
*/
public class ClickableCheckerboard extends JPanel implements MouseListener {

/**
* A main routine lets this class be run as an application.
*/
public static void main(String[] args) {
JFrame window = new JFrame("Clickable Checkerboard");
ClickableCheckerboard content = new ClickableCheckerboard();
window.setContentPane(content);
window.pack(); // Size the window to the preferred size of its content.
window.setLocation(100,100);
window.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
window.setResizable(false);  // User can't change the window's size.
window.setVisible(true);
}

//-------------------------------------------------------------------

int selectedRow; // Row and column of selected square.  If no
int selectedCol; //      square is selected, selectedRow is -1.

/**
* Constructor.  Set selectedRow to -1 to indicate that
* no square is selected.  And set the board object
* to listen for mouse events on itself.
*/
public ClickableCheckerboard() {
selectedRow = -1;
addMouseListener(this);
setPreferredSize( new Dimension(160,160) );
}

/**
* Draw the checkerboard and highlight selected square, if any.
* (Note: super.paintComponent(g) is not necessary, since this
* method already paints the entire surface of the object.
* This assumes that the object is exactly 160-by-160 pixels.
*/
public void paintComponent(Graphics g) {

int row;   // Row number, from 0 to 7
int col;   // Column number, from 0 to 7
int x,y;   // Top-left corner of square

for ( row = 0;  row < 8;  row++ ) {

for ( col = 0;  col < 8;  col++) {
x = col * 20;
y = row * 20;
if ( (row % 2) == (col % 2) )
g.setColor(Color.red);
else
g.setColor(Color.black);
g.fillRect(x, y, 20, 20);
}

} // end for row

if (selectedRow >= 0) {
// Since there is a selected square, draw a cyan
// border around it.  (If selectedRow < 0, then
// no square is selected and no border is drawn.)
g.setColor(Color.CYAN);
y = selectedRow * 20;
x = selectedCol * 20;
g.drawRect(x, y, 19, 19);
g.drawRect(x+1, y+1, 17, 17);
}

}  // end paint()

/**
* When the user clicks on the panel, figure out which
* row and column the click was in and change the
* selected square accordingly.
*/
public void mousePressed(MouseEvent evt) {

int col = evt.getX() / 20;   // Column where user clicked.
int row = evt.getY() / 20;   // Row where user clicked.

if (selectedRow == row && selectedCol == col) {
// User clicked on the currently selected square.
// Turn off the selection by setting selectedRow to -1.
selectedRow = -1;
}
else {
// Change the selection to the square the user clicked on.
selectedRow = row;
selectedCol = col;
}
repaint();

}  // end mousePressed()

public void mouseReleased(MouseEvent evt) { }
public void mouseClicked(MouseEvent evt) { }
public void mouseEntered(MouseEvent evt) { }
public void mouseExited(MouseEvent evt) { }

} // end class ClickableCheckerboard

Solution for Programming Exercise 6.6

Exercise 6.6:

For this exercise, you should modify the SubKiller game from Subsection 6.4.4. You can start with the existing source code, from the file SubKiller.java. Modify the game so it keeps track of the number of hits and misses and displays these quantities. That is, every time the depth charge blows up the sub, the number of hits goes up by one. Every time the depth charge falls off the bottom of the screen without hitting the sub, the number of misses goes up by one. There is room at the top of the panel to display these numbers. To do this exercise, you only have to add a half-dozen lines to the source code. But you have to figure out what they are and where to add them. To do this, you'll have to read the source code closely enough to understand how it works.

Discussion

You can do this exercise by adding just seven lines to the original version, SubKiller.java (plus changing the name of the class, if you want to do that). I used two lines to declare instance variables named hits and misses. These variables have to be updated whenever the depth charge hits the sub or falls off the bottom of the panel. These events are already detected by the panel, in the updateForNextFrame() method of the Bomb class. At the point where this method detects that the depth charge has hit the sub, I add the command "hits++;" to chalk up another hit for the user. At the point in where it is determined that the y-coordinate of the depth charge has exceeded the height of the panel, I add the command "misses++;" to record the fact that the sub has escaped destruction this time.

The only other thing to do is to display the number of hits and misses at the top of the panel. This is part of drawing the picture, so it is done in the paintComponent() method. The information is output with two drawString commands. We need one more command to make sure that the strings are displayed in a color that can be seen:

g.setColor(Color.BLACK);
g.drawString("Number of hits:   " + hits, 15, 24);
g.drawString("Number of misses: " + misses, 15, 45);

(Using the coordinates in these statements, the messages about hits and misses were drawn over the "CLICK TO ACTIVATE" message, so I also move the "CLICK TO ACIVATE" message to the bottom of the screen.)

I made one further change when I decided that I wanted the output to be displayed in a larger font. I create a new font, store it in an instance variable named infoFont, and use the command "g.setFont(infoFont);" before drawing the strings.

The source code is shown below. Changes from the original version are shown in red italic.

The Solution

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

/**
* This panel implements a simple arcade game in which the user tries to blow
* up a "submarine" (a black oval) by dropping "depth charges" (a red disk) from
* a "boat" (a blue roundrect).  The user moves the boat with the left- and
* right-arrow keys and drops the depth charge with the down-arrow key.
* The sub moves left and right erratically along the bottom of the panel.
* This class contains a main() routine to allow it to be run as a program.
* The number of hits and the number of misses are shown at the top of the panel.
*/
public class SubKillerWithScore extends JPanel {

public static void main(String[] args) {
JFrame window = new JFrame("Sub Killer Game");
SubKillerWithScore content = new SubKillerWithScore();
window.setContentPane(content);
window.setSize(600, 480);
window.setLocation(100,100);
window.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
window.setResizable(false);  // User can't change the window's size.
window.setVisible(true);
}

//------------------------------------------------------------------------

private Timer timer;        // Timer that drives the animation.

private int width, height;  // The size of the panel -- the values are set
//    the first time the paintComponent() method
//    is called.  This class is not designed to
//    handle changes in size; once the width and
//    height have been set, they are not changed.
//    Note that width and height cannot be set
//    in the constructor because the width and
//    height of the panel have not been set at
//    the time that the constructor is called.

private Boat boat;          // The boat, bomb, and sub objects are defined
private Bomb bomb;          //    by nested classes Boat, Bomb, and Submarine,
private Submarine sub;      //    which are defined later in this class.
//    Note that the objects are created in the
//    paintComponent() method, after the width
//    and height of the panel are known.

private int hits;           // The number of times the user has hit the sub.
private int misses;         // The number of times the user has missed the sub.

private Font infoFont = new Font("Monospaced", Font.PLAIN, 16);
// A font for displaying the numbers of hits and misses.

/**
* The constructor sets the background color of the panel, creates the
* timer, and adds a KeyListener, FocusListener, and MouseListener to the
* panel.  These listeners, as well as the ActionListener for the timer
* are defined by anonymous inner classes.  The timer will run only
* when the panel has the input focus.
*/
public SubKillerWithScore() {

setBackground( new Color(0,200,0) );

ActionListener action = new ActionListener() {
// Defines the action taken each time the timer fires.
public void actionPerformed(ActionEvent evt) {
if (boat != null) {
boat.updateForNewFrame();
bomb.updateForNewFrame();
sub.updateForNewFrame();
}
repaint();
}
};
timer = new Timer( 30, action );  // Fires every 30 milliseconds.

addMouseListener( new MouseAdapter() {
// The mouse listener simply requests focus when the user
// clicks the panel.
public void mousePressed(MouseEvent evt) {
requestFocus();
}
} );

addFocusListener( new FocusListener() {
// The focus listener starts the timer when the panel gains
// the input focus and stops the timer when the panel loses
// the focus.  It also calls repaint() when these events occur.
public void focusGained(FocusEvent evt) {
timer.start();
repaint();
}
public void focusLost(FocusEvent evt) {
timer.stop();
repaint();
}
} );

addKeyListener( new KeyAdapter() {
// The key listener responds to keyPressed events on the panel. Only
// the left-, right-, and down-arrow keys have any effect.  The left- and
// right-arrow keys move the boat while down-arrow releases the bomb.
public void keyPressed(KeyEvent evt) {
int code = evt.getKeyCode();  // Which key was pressed?
if (code == KeyEvent.VK_LEFT) {
// Move the boat left.  (If this moves the boat out of the frame, its
// position will be adjusted in the boat.updateForNewFrame() method.)
boat.centerX -= 15;
}
else if (code == KeyEvent.VK_RIGHT) {
// Move the boat right.  (If this moves boat out of the frame, its
// position will be adjusted in the boat.updateForNewFrame() method.)
boat.centerX += 15;
}
else if (code == KeyEvent.VK_DOWN) {
// Start the bomb falling, if it is not already falling.
if ( bomb.isFalling == false )
bomb.isFalling = true;
}
}
} );

} // end constructor

/**
* The paintComponent() method draws the current state of the game.  It
* draws a gray or cyan border around the panel to indicate whether or not
* the panel has the input focus.  It draws the boat, sub, and bomb by
* calling their respective draw() methods.
*/
public void paintComponent(Graphics g) {

super.paintComponent(g);  // Fill panel with background color, green.

Graphics2D g2 = (Graphics2D)g;
g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);

if (boat == null) {
// The first time that paintComponent is called, it assigns
// values to the instance variables.
width = getWidth();
height = getHeight();
boat = new Boat();
sub = new Submarine();
bomb = new Bomb();
}

if (hasFocus())
g.setColor(Color.CYAN);
else {
g.setColor(Color.BLACK);
g.drawString("CLICK TO ACTIVATE", 20, height - 10);
g.setColor(Color.GRAY);
}
g.drawRect(0,0,width-1,height-1);  // Draw a 3-pixel border.
g.drawRect(1,1,width-3,height-3);
g.drawRect(2,2,width-5,height-5);

boat.draw(g);
sub.draw(g);
bomb.draw(g);

g.setFont(infoFont);
g.setColor(Color.BLACK);
g.drawString("Number of hits:   " + hits, 15, 24);
g.drawString("Number of misses: " + misses, 15, 45);

} // end paintComponent()

/**
* This nested class defines the boat.  Note that its constructor cannot
* be called until the width of the panel is known!
*/
private class Boat {
int centerX, centerY;  // Current position of the center of the boat.
Boat() { // Constructor centers the boat horizontally, 80 pixels from top.
centerX = width/2;
centerY = 80;
}
void updateForNewFrame() { // Makes sure boat has not moved off screen.
if (centerX < 0)
centerX = 0;
else if (centerX > width)
centerX = width;
}
void draw(Graphics g) {  // Draws the boat at its current location.
g.setColor(Color.BLUE);
g.fillRoundRect(centerX - 40, centerY - 20, 80, 40, 20, 20);
}
} // end nested class Boat

/**
* This nested class defines the bomb.
*/
private class Bomb {
int centerX, centerY; // Current position of the center of the bomb.
boolean isFalling;    // If true, the bomb is falling; if false, it
// is attached to the boat.
Bomb() { // Constructor creates a bomb that is initially attached to boat.
isFalling = false;
}
void updateForNewFrame() {  // If bomb is falling, take appropriate action.
if (isFalling) {
if (centerY > height) {
// Bomb has missed the submarine.  It is returned to its
// initial state, with isFalling equal to false.
isFalling = false;
misses++;   // USER HAS MISSED THE SUB
}
else if (Math.abs(centerX - sub.centerX) <= 36 &&
Math.abs(centerY - sub.centerY) <= 21) {
// Bomb has hit the submarine.  The submarine
// enters the "isExploding" state.
sub.isExploding = true;
sub.explosionFrameNumber = 1;
isFalling = false;  // Bomb reappears on the boat.
hits++;   // USER HAS HIT THE SUB
}
else {
// If the bomb has not fallen off the panel or hit the
// sub, then it is moved down 10 pixels.
centerY += 10;
}
}
}
void draw(Graphics g) { // Draw the bomb.
if ( ! isFalling ) {  // If not falling, set centerX and centerY
// to show the bomb on the bottom of the boat.
centerX = boat.centerX;
centerY = boat.centerY + 23;
}
g.setColor(Color.RED);
g.fillOval(centerX - 8, centerY - 8, 16, 16);
}
} // end nested class Bomb

/**
* This nested class defines the sub.  Note that its constructor cannot
* be called until the width of the panel is known!
*/
private class Submarine {
int centerX, centerY; // Current position of the center of the sub.
boolean isMovingLeft; // Tells whether the sub is moving left or right
boolean isExploding;  // Set to true when the sub is hit by the bomb.
int explosionFrameNumber;  // If the sub is exploding, this is the number
//   of frames since the explosion started.
Submarine() {  // Create the sub at a random location 40 pixels from bottom.
centerX = (int)(width*Math.random());
centerY = height - 40;
isExploding = false;
isMovingLeft = (Math.random() < 0.5);
}
void updateForNewFrame() { // Move sub or increase explosionFrameNumber.
if (isExploding) {
// If the sub is exploding, add 1 to explosionFrameNumber.
// When the number reaches 15, the explosion ends and the
// sub reappears in a random position.
explosionFrameNumber++;
if (explosionFrameNumber == 15) {
centerX = (int)(width*Math.random());
centerY = height - 40;
isExploding = false;
isMovingLeft = (Math.random() < 0.5);
}
}
else { // Move the sub.
if (Math.random() < 0.04) {
// In one frame out of every 25, on average, the sub
// reverses its direction of motion.
isMovingLeft = ! isMovingLeft;
}
if (isMovingLeft) {
// Move the sub 5 pixels to the left.  If it moves off
// the left edge of the panel, move it back to the left
// edge and start it moving to the right.
centerX -= 5;
if (centerX <= 0) {
centerX = 0;
isMovingLeft = false;
}
}
else {
// Move the sub 5 pixels to the right.  If it moves off
// the right edge of the panel, move it back to the right
// edge and start it moving to the left.
centerX += 5;
if (centerX > width) {
centerX = width;
isMovingLeft = true;
}
}
}
}
void draw(Graphics g) {  // Draw sub and, if it is exploding, the explosion.
g.setColor(Color.BLACK);
g.fillOval(centerX - 30, centerY - 15, 60, 30);
if (isExploding) {
// Draw an "explosion" that grows in size as the number of
// frames since the start of the explosion increases.
g.setColor(Color.YELLOW);
g.fillOval(centerX - 4*explosionFrameNumber,
centerY - 2*explosionFrameNumber,
8*explosionFrameNumber,
4*explosionFrameNumber);
g.setColor(Color.RED);
g.fillOval(centerX - 2*explosionFrameNumber,
centerY - explosionFrameNumber/2,
4*explosionFrameNumber,
explosionFrameNumber);
}
}
} // end nested class Submarine

} // end class SubKillerWithScore

Solution for Programming Exercise 6.7

Exercise 6.7:

Exercise 5.2 involved a class, StatCalc.java, that could compute some statistics of a set of numbers. Write a GUI program that uses the StatCalc class to compute and display statistics of numbers entered by the user. The panel will have an instance variable of type StatCalc that does the computations. The panel should include a JTextField where the user enters a number. It should have four labels that display four statistics for the numbers that have been entered: the number of numbers, the sum, the mean, and the standard deviation. Every time the user enters a new number, the statistics displayed on the labels should change. The user enters a number by typing it into the JTextField and pressing return. There should be a "Clear" button that clears out all the data. This means creating a new StatCalc object and resetting the displays on the labels. My panel also has an "Enter" button that does the same thing as pressing the return key in the JTextField. (Recall that a JTextField generates an ActionEvent when the user presses return, so your panel should register itself to listen for ActionEvents from the JTextField as well as the buttons.) Here is a picture of my solution to this problem:

Discussion

In my solution, I used four labels to display results and another label at the top of the panel to display a message to the user. Aside from these labels, one row of the panel holds three other components: a JTextField and two JButtons. The panel uses a GridLayout with six rows. Five of the rows hold JLabels. The other row contains a JPanel that holds the JTextField and JButtons. This JPanel uses a GridLayout with three columns and just one row.

The constructor creates and lays out the components. Since I want the program to look nice, I set a background color and a foreground color for most of the components. I set the labels to be opaque, to make sure that the background of each label will actually be filled in with the label's background color. After looking at my first attempt, I decided to use a Monospaced font for the display labels. In a Monospaced font, all the characters are the same size. This makes it possible to line up the output values vertically by putting the same number of characters in each label. To make it easy to play with the colors and fonts, I declared three named constants

final static Color labelBG = new Color(240,225,200);  // beige
final static Color labelFG = new Color(180,0,0);  // dark red
final static Font labelFont = new Font("Monospaced", Font.PLAIN, 12);

I could then make one of the labels, such as countLabel, with the commands:

countLabel = new JLabel("Number of Entries:   0");
countLabel.setBackground(labelBG);
countLabel.setForeground(labelFG);
countLabel.setOpaque(true);
countLabel.setFont(labelFont);

However, since there are four labels to create, I wrote a subroutine to create a display label to show a given string:

/**
* A utility routine for creating the labels that are used
* for display.  This routine is called by the constructor.
* @param text The text to show on the label.
*/
private JLabel makeLabel(String text) {
JLabel label = new JLabel(text);
label.setBackground(labelBG);
label.setForeground(labelFG);
label.setOpaque(true);
label.setFont(labelFont);
return label;
}

Then in the constructor, the labels can be created with four lines, instead of 16:

countLabel =   makeLabel(" Number of Entries:  0");
sumLabel =     makeLabel(" Sum:                0.0");
meanLabel =    makeLabel(" Average:            undefined");
standevLabel = makeLabel(" Standard Deviation: undefined");

Utility routines like makeLabel() are very commonly used when there are a lot of similar components to create. Note that when the labels are first created, the text on the labels is appropriate for a dataset that contains zero elements. In particular, if there are no data, the average and standard deviation are undefined.

The panel registers itself to listen for action events from the JTextField and from the JButtons. In the actionPerformed() method, the function evt.getSource() is called to find the Object that generated the event. This will be either the numberInput box, the enterButton, or the clearButton. The source of the event is checked to decide how to respond. (This is an alternative to checking the event's action command.)

If the user clicked the "Clear" button, the response is to create a new StatCalc object and to reset the display labels to reflect the fact that there is no data in the dataset. It's important to understand the effect of the command "stats = new StatCalc();". The panel will continue to use the same StatCalc variable, stats. However, now the variable refers to a new StatCalc object. The new object does not yet have any data in its dataset. The next time the user enters a number, the dataset will get its first value. Always keep in mind the difference between variables and objects. Also, keep in mind that you have to think in terms of changing the state of the panel in response to events. I change the panel's state by starting to use a new StatCalc object, and the display labels are changed to keep them consistent with the new state.

When the user clicks the "Enter" button or presses return in the JTextField, we have to get the user's input and add it to the StatCalc object. This will cause the values of the four statistics to change. We have to change the display labels to show the new values. The code for getting the user's number from the input box includes a check to make sure that the user's input is a legal number. If the input is not legal, then I show an error message in the JLabel named message and return from the actionPerformed() method without entering any new data:

double num;  // The user's number.
try {
num = Double.parseDouble(numberInput.getText());
}
catch (NumberFormatException e) {
// The user's entry is not a legal number.
// Put an error message in the message label
// and return without entering a number.
message.setText("\"" + numberInput.getText() +
"\" is not a legal number.");
numberInput.selectAll();
numberInput.requestFocus();
return;
}

The commands "numberInput.selectAll();" and "numberInput.requestFocus();" are there as a convenience for the user. The first command, which was not covered in this chapter, selects all the text in the number input box. The second command gives the input focus to the input box. That way, the user can just start typing the next number, without having to click on the input box or erase the content of the box. (Since the contents of the box are selected, they will disappear automatically when the user starts typing, to be replaced with the new input. A surprising number of people have never learned that text selections work this way.)

Once we have the user's number, the command "stats.enter(num);" adds the number num into the dataset. The statistics about the data set can be obtained by calling the functions stats.getCount(), stats.getSum(), stats.getMean(), and stats.getStandardDeviation(). This information can be found by reading the source code for the StatCalc class. These values are used on the labels that display the statistics. For example,

countLabel.setText(" Number of Entries:  " + stats.getCount());

Returning to the topic of variables versus objects, a common novice mistake would be to try to change the text that is displayed on the label by saying

countLabel = new JLabel(" Number of Entries:  " + stats.getCount());  // WRONG

since that type of command is what was used to set the text in the first place. But this statement has no effect on what is displayed on the screen. The reason why this doesn't work is instructive: The assignment command creates a new JLabel and sets countLabel to refer to that new object. The new label does indeed have the desired text. But the new label has nothing to do with what's on the screen! The original label was created in the constructor and added to the panel. The panel has a reference to the original label, and that original label continues to appear on the screen even if countLabel now refers to a new label. The correct command, using countLabel.setText(), modifies the original label, which is what is shown on the screen.

As a point of interest, I will mention another technique that has not been covered elsewhere in the book. You might want to limit the number of decimal places that are displayed in the statistics. You know how to do this in output to the command line, using System.out.printf or TextIO.putf, but we are not outputting the data here, just putting it into a String. The String class has a static method String.format() that solves this problem. String.format(fmtString,val,val,val,...) works similarly to System.out.prinf(fmtString,val,val,val,...), but instead of producing output, String.format just creates a string and returns it. To set the text of the label that displays the average, for example, you Might say:

meanLabel.setText( String.format(" Average:            %1.9g", stats.getMean()) );

I don't do this in my solution, but String.format can be very useful, especially in GUI programs.

The Solution

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

/**
* In this panel, the user enters numbers in a text field box.
* After entering each number, the user presses return (or clicks
* on a button).  Some statistics are displayed about all the
* numbers that the user has entered.
*/
public class StatCalcGUI extends JPanel implements ActionListener {

/**
* A main routine allows this class to be run as an application.
*/
public static void main(String[] args) {
JFrame window = new JFrame("Stat Calc");
StatCalcGUI content = new StatCalcGUI();
window.setContentPane(content);
window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
window.setLocation(120,70);
window.setSize(350,200);
window.setVisible(true);
}

//---------------------------------------------------------------------

final static Color labelBG = new Color(240,225,200);  // For creating labels
final static Color labelFG = new Color(180,0,0);
final static Font labelFont = new Font("Monospaced", Font.PLAIN, 12);

private JLabel countLabel;    // A label for displaying the number of numbers.
private JLabel sumLabel;      // A label for displaying the sum of the numbers.
private JLabel meanLabel;     // A label for displaying the average.
private JLabel standevLabel;  // A label for displaying the standard deviation.

private JLabel message;  // A message at the top of the panel.  It will
//   show an error message if the user's input is
//   not a legal number.  Otherwise, it just tells
//   the user to enter a number and press return.

private JButton enterButton;   // A button the user can press to enter a number.
//    This is an alternative to pressing return.
private JButton clearButton;   // A button that clears all the data that the
//    user has entered.

private JTextField numberInput;  // The input box where the user enters numbers.

private StatCalc stats;  // An object that keeps track of the statistics
//   for all the numbers that have been entered.

/**
* The constructor creates the objects used by the panel.  The panel
* will listen for action events from the buttons and from the text
* field.  (A JTextField generates an ActionEvent when the user presses
* return while typing in the text field.)
*/
public StatCalcGUI() {

stats = new StatCalc();

numberInput = new JTextField();
numberInput.setBackground(Color.WHITE);
numberInput.addActionListener(this);

enterButton = new JButton("Enter");
enterButton.addActionListener(this);

clearButton = new JButton("Clear");
clearButton.addActionListener(this);

JPanel inputPanel = new JPanel();  // A panel that will hold the
//   JTextField and JButtons.
inputPanel.setLayout( new GridLayout(1,3) );
inputPanel.add(numberInput);
inputPanel.add(enterButton);
inputPanel.add(clearButton);

countLabel =   makeLabel(" Number of Entries:  0");
sumLabel =     makeLabel(" Sum:                0.0");
meanLabel =    makeLabel(" Average:            undefined");
standevLabel = makeLabel(" Standard Deviation: undefined");

message = new JLabel("Enter a number, press return:",
JLabel.CENTER);
message.setBackground(labelBG);
message.setForeground(Color.BLUE);
message.setOpaque(true);
message.setFont(new Font("SansSerif", Font.BOLD, 12));

/* Use a GridLayout with 6 rows and 1 column, and add all the
components that have been created to the panel. */

setBackground(Color.BLUE);
setLayout( new GridLayout(6,1,2,2) );
add(message);
add(inputPanel);
add(countLabel);
add(sumLabel);
add(meanLabel);
add(standevLabel);

/* Add a blue border around the panel. */

setBorder( BorderFactory.createLineBorder(Color.BLUE, 2) );

} // end constructor

/**
* A utility routine for creating the labels that are used
* for display.  This routine is called by the constructor.
* @param text The text to show on the label.
*/
private JLabel makeLabel(String text) {
JLabel label = new JLabel(text);
label.setBackground(labelBG);
label.setForeground(labelFG);
label.setFont(labelFont);
label.setOpaque(true);
return label;
}

/**
* This is called when the user clicks one of the buttons or
* presses return in the input box.  The response to clicking
* on the Enter button is the same as the response to pressing
* return in the JTextField.
*/
public void actionPerformed(ActionEvent evt) {

Object source = evt.getSource();  // Object that generated
//   the action event.

if (source == clearButton) {
// Handle the clear button by starting with a new,
// empty StatCalc object and resetting the display
// labels to show no data entered.  The TextField
// is also made empty.
stats = new StatCalc();
countLabel.setText(" Number of Entries:  0");
sumLabel.setText(" Sum:                0.0");
meanLabel.setText(" Average:            undefined");
standevLabel.setText(" Standard Deviation: undefined");
numberInput.setText("");
}
else if (source == enterButton || source == numberInput) {
// Get the user's number, enter it into the StatCalc
// object, and set the display on the display labels
// to reflect the new data.
double num;  // The user's number.
try {
num = Double.parseDouble(numberInput.getText());
}
catch (NumberFormatException e) {
// The user's entry is not a legal number.
// Put an error message in the message label
// and return without entering a number.
message.setText("\"" + numberInput.getText() +
"\" is not a legal number.");
numberInput.selectAll();
numberInput.requestFocus();
return;
}
stats.enter(num);
countLabel.setText(" Number of Entries:  " + stats.getCount());
sumLabel.setText(" Sum:                " + stats.getSum());
meanLabel.setText(" Average:            " + stats.getMean());
standevLabel.setText(" Standard Deviation: "
+ stats.getStandardDeviation());
}

/* Set the message label back to its normal text, in case it has
been showing an error message.  For the user's convenience,
select the text in the TextField and give the input focus
to the text field.  That way the user can just start typing
the next number. */

message.setText("Enter a number, press return:");
numberInput.selectAll();
numberInput.requestFocus();

}  // end ActionPerformed

}  // end StatsCalcGUI

Solution for Programming Exercise 6.8

Exercise 6.8:

Write a program that has a JTextArea where the user can enter some text. Then program should have a button such that when the user clicks on the button, the panel will count the number of lines in the user's input, the number of words in the user's input, and the number of characters in the user's input. This information should be displayed on three labels. Recall that if textInput is a JTextArea, then you can get the contents of the JTextArea by calling the function textInput.getText(). This function returns a String containing all the text from the text area. The number of characters is just the length of this String. Lines in the String are separated by the new line character, '\n', so the number of lines is just the number of new line characters in the String, plus one. Words are a little harder to count. Exercise 3.4 has some advice about finding the words in a String. Essentially, you want to count the number of characters that are first characters in words. Don't forget to put your JTextArea in a JScrollPane, and add the scroll pane to the container, not the text area. Scrollbars should appear when the user types more text than will fit in the available area. Here is a picture of my solution:

Discussion

The panel contains five components. There are several ways to lay them out. A GridLayout with five rows certainly will not work, because the JTextArea should be taller than the other components. One possible layout is to use a GridLayout with two rows. The JTextArea would occupy the first row. The bottom half would contain a JPanel that holds the other four components. (A GridLayout with two columns and one row would also work, if you wanted a panel that was wider and not so tall. You could put the JTextArea in the left half and the other components in a JPanel in the right half.) However, I decided to use a BorderLayout. The JTextArea occupies the CENTER position, and the SOUTH position is occupied by a JPanel that contains the other components. The nested JPanel uses a GridLayout with four rows. My main() program sets the size of the window to 300-by-350, and the text area gets the space not occupied by the bottom panel. Once this choice has been made, writing the constructor is not hard.

I use an anonymous inner class to listen for ActionEvents from the button. The actionPerformed() method of the listener just calls a method named processInput() in the main class; this method does the real work. The processInput() method just has to get the text from the JTextArea, do the counting, and set the labels. The only interesting part is counting the words. Back in Exercise 3.4, words such as "can't", that contain an apostrophe, were counted as two words. This time around, let's handle this special case. Two letters with an apostrophe between them should be counted as part of the same word. The algorithm for counting words is still

wordCt = 0
for each character in the string:
if the character is the first character of a word:
Add 1 to wordCt

but testing whether a given character is the first character in a word has gotten a little more complicated. To make the test easier, I use a boolean variable, startOfWord. The value of this variable is set to true if the character is the start of a word and to false if not. That is, the algorithm becomes:

wordCt = 0
for each character in the string:
Let startOfWord be true if at start of word, false otherwise
if startOfWord is true:
Add 1 to wordCt

The use of a "flag variable" like startOfWord can simplify the calculation of a complicated boolean condition. The value is computed as a series of tests:

boolean startOfWord;  // Is character i the start of a word?
if ( Character.isLetter(text.charAt(i)) == false )
startOfWord = false;  // No.  It's not a letter.
else if (i == 0)
startOfWord = true;   // Yes.  It's a letter at start of text.
else if ( Character.isLetter(text.charAt(i-1)) )
startOfWord = false;  // No.  It's a letter preceded by a letter.
else if ( text.charAt(i-1) == '\'' && i > 1
&& Character.isLetter(text.charAt(i-2)) )
startOfWord = false;  // No.  It's a continuation of a word
//      after an apostrophe.
else
startOfWord = true;   // Yes.  It's a letter preceded by
//       a non-letter.

The first test checks whether the character in position i is a letter. If it is not, then we know that it can't be the start of a word, so startOfWord is false. If it is a letter, it might be the start of a word, so we go on to make additional tests. Note that if we get to the other tests at all, we already know that the character in position i is a letter. And so on. This style of "cascading tests" is very useful. In each test, we already have all the information from the previous tests. Note that the cascade effect works only with "else if". Using "if" in place of "else if" in the preceding code would not give the right answer. (You should be sure to understand why this is so.) You should also note why the test if (i == 0) has to be made before the test if ( Character.isLetter(text.charAt(i-1)) ) -- it's because text.charAt(i-1) gives an index-out-of-bounds exception if i is zero.

The Solution

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

/**
* In this panel, the user types some text in a JTextArea and presses
* a button.  The panel computes and displays the number of lines
* in the text, the number of words in the text, and the number of
* characters in the text.  A word is defined to be a sequence of
* letters, except that an apostrophe with a letter on each side
* of it is considered to be a letter.  (Thus "can't" is one word,
* not two.)
*/
public class TextCounter extends JPanel {

/**
* A main routine allows this class to be run as an application.
*/
public static void main(String[] args) {
JFrame window = new JFrame("Text Counter");
TextCounter content = new TextCounter();
window.setContentPane(content);
window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
window.setLocation(120,70);
window.setSize(300,350);
window.setVisible(true);
}

//---------------------------------------------------------------------

private JTextArea textInput;     // For the user's input text.

private JLabel lineCountLabel;   // For displaying the number of lines.
private JLabel wordCountLabel;   // For displaying the number of words.
private JLabel charCountLabel;   // For displaying the number of chars.

/**
* The constructor creates components and lays out the panel.
*/
public TextCounter() {

setBackground(Color.DARK_GRAY);

/* Create the text input area and make sure it has a
white background. */

textInput = new JTextArea();
textInput.setBackground(Color.WHITE);

/* Create a panel to hold the button and three display
labels.  These will be laid out in a GridLayout with
4 rows and 1 column. */

JPanel south = new JPanel();
south.setBackground(Color.DARK_GRAY);
south.setLayout( new GridLayout(4,1,2,2) );

/* Create the button and a listener to listen for
clicks on the button, and add it to the panel. */

JButton countButton = new JButton("Process the Text");
countButton.addActionListener( new ActionListener() {
public void actionPerformed(ActionEvent evt) {
processInput();
}
});
south.add(countButton);

/* Create each of the labels, set their colors, and
add them to the panel. */

lineCountLabel = new JLabel("  Number of lines:");
lineCountLabel.setBackground(Color.WHITE);
lineCountLabel.setForeground(Color.BLUE);
lineCountLabel.setOpaque(true);
south.add(lineCountLabel);

wordCountLabel = new JLabel("  Number of words:");
wordCountLabel.setBackground(Color.WHITE);
wordCountLabel.setForeground(Color.BLUE);
wordCountLabel.setOpaque(true);
south.add(wordCountLabel);

charCountLabel = new JLabel("  Number of chars:");
charCountLabel.setBackground(Color.WHITE);
charCountLabel.setForeground(Color.BLUE);
charCountLabel.setOpaque(true);
south.add(charCountLabel);

/* Use a BorderLayout on the panel.  Although a BorderLayout
is the default, I want one with a vertical gap of two
pixels, to let the dark gray background color show through.
Also add a gray border around the panel. */

setLayout( new BorderLayout(2,2) );
setBorder(BorderFactory.createLineBorder(Color.DARK_GRAY));

/* The text area is put into a JScrollPane to provide
scroll bars for the TextArea, and the scroll pane is put in
the Center position.  The panel that holds the button and
labels is in the South position.  Note that the text area
will be sized to fill the space that is left after the
panel is assigned its preferred height. */

JScrollPane scroller = new JScrollPane( textInput );
add(scroller, BorderLayout.CENTER);
add(south, BorderLayout.SOUTH);

} // end constructor

/**
* This will be called by the action listener for the button when the user
* clicks the button.  It gets the text from the text area, counts the number
* of chars, words, and lines that it contains, and sets the labels to
* display the results.
*/
public void processInput() {

String text;  // The user's input from the text area.

int charCt, wordCt, lineCt;  // Char, word, and line counts.

text = textInput.getText();

charCt = text.length();  // The number of characters in the
//    text is just its length.

/* Compute the wordCt by counting the number of characters
in the text that lie at the beginning of a word.  The
beginning of a word is a letter such that the preceding
character is not a letter.  This is complicated by two
things:  If the letter is the first character in the
text, then it is the beginning of a word.  If the letter
is preceded by an apostrophe, and the apostrophe is
preceded by a letter, than its not the first character
in a word.
*/

wordCt = 0;
for (int i = 0; i < charCt; i++) {
boolean startOfWord;  // Is character i the start of a word?
if ( Character.isLetter(text.charAt(i)) == false )
startOfWord = false;  // No.  It's not a letter.
else if (i == 0)
startOfWord = true;   // Yes.  It's a letter at start of text.
else if ( Character.isLetter(text.charAt(i-1)) )
startOfWord = false;  // No.  It's a letter preceded by a letter.
else if ( text.charAt(i-1) == '\'' && i > 1
&& Character.isLetter(text.charAt(i-2)) )
startOfWord = false;  // No.  It's a continuation of a word
//      after an apostrophe.
else
startOfWord = true;   // Yes.  It's a letter preceded by
//       a non-letter.
if (startOfWord)
wordCt++;
}

/* The number of lines is just one plus the number of times the
end of line character, '\n', occurs in the text. */

lineCt = 1;
for (int i = 0; i < charCt; i++) {
if (text.charAt(i) == '\n')
lineCt++;
}

/* Set the labels to display the data. */

lineCountLabel.setText("  Number of Lines:  " + lineCt);
wordCountLabel.setText("  Number of Words:  " + wordCt);
charCountLabel.setText("  Number of Chars:  " + charCt);

}  // end processInput()

} // end class TextCounter

Solution for Programming Exercise 6.9

Exercise 6.9:

A polygon is a geometric figure made up of a sequence of connected line segments. The points where the line segments meet are called the vertices of the polygon. The Graphics class includes commands for drawing and filling polygons. For these commands, the coordinates of the vertices of the polygon are stored in arrays. If g is a variable of type Graphics then

- g.drawPolygon(xCoords, yCoords, pointCt) will draw the outline of the polygon with vertices at the points (xCoords[0],yCoords[0]), (xCoords[1],yCoords[1]), ..., (xCoords[pointCt-1],yCoords[pointCt-1]). The third parameter, pointCt, is an int that specifies the number of vertices of the polygon. Its value should be 3 or greater. The first two parameters are arrays of type int[]. Note that the polygon automatically includes a line from the last point, (xCoords[pointCt-1],yCoords[pointCt-1]), back to the starting point (xCoords[0],yCoords[0]).

- g.fillPolygon(xCoords, yCoords, pointCt) fills the interior of the polygon with the current drawing color. The parameters have the same meaning as in the drawPolygon() method. Note that it is OK for the sides of the polygon to cross each other, but the interior of a polygon with self-intersections might not be exactly what you expect.

Write a program that lets the user draw polygons. As the user clicks a sequence of points, count them and store their x- and y-coordinates in two arrays. These points will be the vertices of the polygon. As the user is creating the polygon, you should just connect all the points with line segments. When the user clicks near the starting point, draw the complete polygon. Draw it with a red interior and a black border. Once the user has completed a polygon, the next click will clear the data and start a new polygon from scratch. All drawing should be done in the paintComponent() method.

Here is a picture of my solution after the user has drawn a fairly complex polygon:

Discussion

This is an exercise in using arrays, but it is also an exercise in using instance variables to record the state of the program. The paintComponent() method needs enough information to correctly draw the picture. Obviously, that includes the coordinate arrays and the number of points that have been stored in the arrays. But the picture is different depending on whether or not the user has completed the polygon. If the polygon is complete, the picture shows a polygon; if not, the picture shows line segments connecting each point to the next. We need to record that basic distinction as part of the state. We can do that using a boolean variable, complete, which is true when a complete polygon should be drawn and false while the polygon is under construction. So, here are the necessary instance variables:

int[] xCoord, yCoord;  // Arrays to hold the coordinates for up to 500 points.
int pointCt;           // Number of points in the arrays.
boolean complete;      // Set to true when a polygon is complete.

The arrays xCoord and yCoord are examples of partially full arrays, although there is only one counter variable that applies to both arrays.

Given these instance variables, the paintComponent() method can be written. To make the picture look nicer, I decided to use two Graphics2D features: antialiasing and wide lines. (See Subsection 6.2.5). There is also a question about what to draw when there is only one point. It's nice if the user can see that the point has been added to the data, but one point is not enough to draw a line or polygon. I decided to draw a small rectangle at the point, just to make it visible.

protected void paintComponent(Graphics g) {
super.paintComponent(g);
if (pointCt == 0)
return; // Nothing at all to draw
Graphics2D g2 = (Graphics2D)g;
g2.setStroke( new BasicStroke(2) );
g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
RenderingHints.VALUE_ANTIALIAS_ON);
if (pointCt == 1) {
// just one point -- draw a small dot there
g.fillRect(xCoord[0], yCoord[0], 2, 2);
}
else if (complete) {
// The polygon is complete; draw it, with a black border
g.setColor(polygonColor);
g.fillPolygon(xCoord, yCoord, pointCt);
g.setColor(Color.BLACK);
g.drawPolygon(xCoord, yCoord, pointCt);
}
else {
// The polygon is not complete, just draw a series of lines.
for (int i = 0; i < pointCt - 1; i++)
g.drawLine( xCoord[i], yCoord[i], xCoord[i+1], yCoord[i+1]);
}
}

The rest of the program logic is in the mousePressed() routine. In that method, state variables have to change in response to the user's actions. It requires some care to do things in the right order. The three things that can happen are that the user starts a new polygon, the user completes the current polygon, or the user just adds a point to the current polygon. The conditions should be tested in that order:

if the current polygon is complete
start a new polygon with the point where the user clicked
else if the user clicked near the starting point
complete the current polygon
else
add the point that the user clicked to the data
call repaint() to make the change visible

Actually, in my solution, I decided to add another case: The polygon can also be completed by right-clicking, or -- to be very safe -- if the number of points has reached 500. Also, there is a bug in the algorithm as stated, where it tests "if the user clicked near the first point". This test doesn't make sense unless there actually is a first point, that is unless pointCt is greater than zero. The test should really read "if pointCt > 0 and if the user clicked near (xCoord[0],yCoord[0])."

To complete a polygon just means setting the value of the variable complete to true. When repaint() is called, the data will be displayed as a polygon. When a new polygon is stated, the value of the variable complete has to be reset to false. Also, the first point on the polygon has to be put into the coordinate arrays, and pointCt must be set to 1 to indicate that there is only one point in the data. This much is pretty straightforward to implement.

The only thing in the algorithm that still needs implementing is to test whether the user clicks "near the starting point". The starting point has coordinates (xCoord[0],yCoord[0]) and the point where the user clicked has coordinates (evt.getX(),evt.getY()). In my program, I check whether the x-coordinates of these points are three pixels or less apart and the y-coordinates are also two pixels or less apart. This is done by checking whether "Math.abs(xCoord[0] - evt.getX()) <= 3 && Math.abs(yCoord[0] - evt.getY()) <= 3".

The Solution

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

/**
* This program lets the user draw colored polygons.
* The user inputs a polygon by clicking a series of points.
* The points are connected with lines from each point to the
* next Clicking near the starting point (within 3 pixels) or
* right-clicking (or Command-clicking) will complete the
* polygon, so the user can begin a new one.  As soon as the
* user begins drawing a new polygon, the old one is discarded.
*/
public class SimplePolygons extends JPanel implements MouseListener {

/**
* A main() routine to allow this program to be run as an application.
*/
public static void main(String[] args) {
JFrame window = new JFrame("SimplePolygons");
SimplePolygons content = new SimplePolygons();
window.setContentPane(content);
window.pack();
window.setLocation(100,100);
window.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
window.setResizable(false);
window.setVisible(true);
}

/* Variables for implementing polygon input. */

private int[] xCoord, yCoord;   // Arrays containing the points of
//   the polygon.  Up to 500 points
//   are allowed.

private int pointCt;  // The number of points that have been input.

private boolean complete;   // Set to true when the polygon is complete.
// When this is false, only a series of lines are drawn.
// When it is true, a filled polygon is drawn.

private final static Color polygonColor = Color.RED;
// Color that is used to draw the polygons.

/**
* Initialize the panel and its data; add a black border and set
* the panel to listen for mouse events on itself.  Also sets
* the preferred size of the panel to be 300-by-300.
*/
public SimplePolygons() {
setBackground(Color.WHITE);
setBorder(BorderFactory.createLineBorder(Color.BLACK,1));
setPreferredSize( new Dimension(300,300) );
addMouseListener(this);
xCoord = new int[500];
yCoord = new int[500];
pointCt = 0;
}

protected void paintComponent(Graphics g) {
super.paintComponent(g);
if (pointCt == 0)
return;
Graphics2D g2 = (Graphics2D)g;
g2.setStroke( new BasicStroke(2) );
g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
RenderingHints.VALUE_ANTIALIAS_ON);
if (pointCt == 1) {
g.fillRect(xCoord[0], yCoord[0], 2, 2);
}
else if (complete) { // draw a polygon
g.setColor(polygonColor);
g.fillPolygon(xCoord, yCoord, pointCt);
g.setColor(Color.BLACK);
g.drawPolygon(xCoord, yCoord, pointCt);
}
else { // draw a series of lines
for (int i = 0; i < pointCt - 1; i++)
g.drawLine( xCoord[i], yCoord[i], xCoord[i+1], yCoord[i+1]);
}
}

/**
* Processes a mouse click.
*/
public void mousePressed(MouseEvent evt) {

if (complete) {
// Start a new polygon at the point that was clicked.
complete = false;
xCoord[0] = evt.getX();
yCoord[0] = evt.getY();
pointCt = 1;
}
else if ( pointCt > 0 && pointCt > 0 && (Math.abs(xCoord[0] - evt.getX()) <= 3)
&& (Math.abs(yCoord[0] - evt.getY()) <= 3) ) {
// User has clicked near the starting point.
// The polygon is complete.
complete = true;
}
else if (evt.isMetaDown() || pointCt == 500) {
// The polygon is complete.
complete = true;
}
else {
// Add the point where the user clicked to the list of
// points in the polygon, and draw a line between the
// previous point and the current point.  A line can
// only be drawn if there are at least two points.
xCoord[pointCt] = evt.getX();
yCoord[pointCt] = evt.getY();
pointCt++;
}
repaint();  // in all cases, repaint.
} // end mousePressed()

public void mouseReleased(MouseEvent evt) { }
public void mouseClicked(MouseEvent evt) { }
public void mouseEntered(MouseEvent evt) { }
public void mouseExited(MouseEvent evt) { }

}  // end class SimplePolygons

Solution for Programming Exercise 6.10

Exercise 6.10:

Write a GUI Blackjack program that lets the user play a game of Blackjack, with the computer as the dealer. The program should draw the user's cards and the dealer's cards, just as was done for the graphical HighLow card game in Subsection 6.6.6. You can use the source code for that game, HighLowGUI.java, for some ideas about how to write your Blackjack game. The structures of the HighLow panel and the Blackjack panel are very similar. You will certainly want to use the drawCard() method from the HighLow program.

You can find a description of the game of Blackjack in Exercise 5.5. Add the following rule to that description: If a player takes five cards without going over 21, that player wins immediately. This rule is used in some casinos. For your program, it means that you only have to allow room for five cards. You should assume that the panel is just wide enough to show five cards, and that it is tall enough show the user's hand and the dealer's hand.

Note that the design of a GUI Blackjack game is very different from the design of the text-oriented program that you wrote for Exercise 5.5. The user should play the game by clicking on "Hit" and "Stand" buttons. There should be a "New Game" button that can be used to start another game after one game ends. You have to decide what happens when each of these buttons is pressed. You don't have much chance of getting this right unless you think in terms of the states that the game can be in and how the state can change.

Your program will need the classes defined in Card.java, Hand.java, Deck.java, and BlackjackHand.java.

The next exercise has a picture of a blackjack game that you can use a guide, except that the version for this exercise does not allow betting.

Discussion

The constructor for this exercise can be almost identical to that in the HighLow game. The text of the buttons just has to be changed from "Higher" and "Lower" to "Hit" and "Stand". However, the nested class, CardPanel has to be rewritten to implement a game of Blackjack instead of a game of HighLow. The basic structure of the revised class remains similar to the original. All the programming for the game is in this class.

In the HighLow game, there is one "hand," which holds all the cards that have been dealt. Blackjack is a two-player game, so there are two hands, one for the player and one for the dealer. These hands are of type BlackjackHand. So, we need instance variables

BlackjackHand dealerHand;   // The dealer's cards.
BlackjackHand playerHand;   // The user's cards.

We also need a deck and a boolean-valued instance variable, gameInProgress, to keep track of the two basic states of the game: Is a game in progress, or are we between games. Finally, there is a message variable, which holds the string that is shown at the bottom of the game board.

The paintComponent() method uses the information in the dealerHand, playerHand, message, and gameInProgress variables. The reason it needs to look at the gameInProgress variable is that when a game is in progress, one of the dealer's cards is drawn face down, so the user can't see it. Once the game is over, the card is drawn face up so the user can see what the dealer was holding. Note that there is no point in the program where I say, "turn the dealer's first card face up"! It happens automatically because the state of the game changes, and the paintComponent() method checks the state when it draws the panel. If the game is over, the card is face up. If the game is in progress, the card is face down. This is nice example of state-machine thinking.

Note that writing the paintComponent() method required some calculation. The cards are 80 pixels wide and 100 pixels tall. Horizontally, there is a gap of 10 pixels between cards, and there are gaps of 10 pixels between the cards and the left and right edges. The total width needed for the card panel, 460, allows for five 80-pixel cards and six 10-pixel gaps: 5*80 + 6*10 = 460. The panel is another 6 pixels wide because of a 3-pixel wide border on each side. The N-th card, counting from 0, has its left edge at 10+90*N. It might be easier to see this as 10+80*N+10*N, 10 pixels on the left plus N 80-pixel cards, plus N 10-pixel gaps between cards. Vertically, I allow 30 pixels for each string, "Dealer's Cards" and "Your Cards". This puts the top of the first row of cards at y=30. Allowing 100 pixels for that row of cards and 30 pixels for the string "Your Cards", the top of the second row of cards is at 160. Given all this, you should be able to understand the paintComponent() method. Allowing 100 pixels for the second row of cards and 30 pixels for the message at the bottom of the board, we need a height of at least 290 pixels for the drawing area. I set the preferred height of the CardPanel to 310 to for some extra space between the cards and the message at the bottom of the panel.

In this GUI version of Blackjack, things happen when the user clicks the "Hit", "Stand", and "New Game" buttons. The program handles these events by calling the routines doHit(), doStand(), and doNewGame(). Each of these routines has responsibility for one part of the game of Blackjack. Note that each routine starts by checking the state of the game to make sure that it is legal to call the routine at this time. If gameInProgress is true, the user can legally click "Hit" or "Stand". If gameInProgress is false, the user can legally click "New Game". If the user made an illegal move, an error message is stored in the message variable, and repaint() is called so the user will see the new message. This is similar to the way the three buttons in HighLowGUI are handled.

The doNewGame() routine has to set up a new game. This means creating the deck and hands, shuffling the deck and dealing two cards into each hand. At this point, the first time I wrote the game, I just set gameInProgress to true, to record the fact that the state of the game has changed. Later, I realized that the doNewGame() routine also has to check whether one of the players has Blackjack, since there is really no other place where this can be done. If one of the players has Blackjack, the game is over as soon as it starts, so gameIsProgress has to be false, and the only action that the user can take at that point is to click the "New Game" button again. (Note that the doNewGame() routine is also called by the constructor of the CardPanel class. This sets up the first game, when the panel is first created, so the user doesn't have to click on the "New Game" button to start the first game.)

When the user clicks "Hit", if the game is in progress, we deal a card into the user's hand. At this point, the state of the game might have changed. If the user has over 21, the user loses and the game is over. If the user has taken 5 cards without going over 21, the user wins and the game is over. In either of these cases, the value of the state variable gameInProgress becomes false. Otherwise, gameInProgress retains the value true, and the game will continue. Since gameInProgress is true, the user still has the choice of clicking "Hit" or "Stand". (Note that there is no loop in the program that says "while the user continues to hit." The progress of the game is driven by events.)

Finally, when the user clicks "Stand", the game is definitely over, so gameInProgress is set to false. However, before the game can end, the dealer gets to draw cards and a winner is determined. This all has to be done in the doStand() routine. Then, the panel is repainted to show the final state of the game.

The Solution

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

/**
* In this program, the user plays a game of Blackjack.  The
* computer acts as the dealer.  The user plays by clicking
* "Hit!" and "Stand!" buttons.
*
* This class defines a panel, but it also contains a main()
* routine that makes it possible to run the program as a
* stand-alone application.
*
* This program depends on the following classes:  Card, Hand,
* BlackjackHand, Deck.
*/
public class BlackjackGUI extends JPanel {

/**
* The main routine simply opens a window that shows a BlackjackGUI.
*/
public static void main(String[] args) {
JFrame window = new JFrame("Blackjack");
BlackjackGUI content = new BlackjackGUI();
window.setContentPane(content);
window.pack();  // Set size of window to preferred size of its contents.
window.setResizable(false);  // User can't change the window's size.
Dimension screensize = Toolkit.getDefaultToolkit().getScreenSize();
window.setLocation( (screensize.width - window.getWidth())/2,
(screensize.height - window.getHeight())/2 );
window.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
window.setVisible(true);
}

/**
* The constructor lays out the panel.  A CardPanel occupies the CENTER
* position of the panel (where CardPanel is a subclass of JPanel that is
* defined below).  On the bottom is a panel that holds three buttons.
* The CardPanel listens for ActionEvents from the buttons and does all
* the real work of the program.
*/
public BlackjackGUI() {

setBackground( new Color(130,50,40) );

setLayout( new BorderLayout(3,3) );

CardPanel board = new CardPanel();
add(board, BorderLayout.CENTER);

JPanel buttonPanel = new JPanel();
buttonPanel.setBackground( new Color(220,200,180) );
add(buttonPanel, BorderLayout.SOUTH);

JButton hitButton = new JButton( "Hit!" );
hitButton.addActionListener(board);
buttonPanel.add(hitButton);

JButton standButton = new JButton( "Stand!" );
standButton.addActionListener(board);
buttonPanel.add(standButton);

JButton newGame = new JButton( "New Game" );
newGame.addActionListener(board);
buttonPanel.add(newGame);

setBorder(BorderFactory.createLineBorder( new Color(130,50,40), 3) );

}  // end constructor

/**
* A nested class that displays the game and does all the work
* of keeping track of the state and responding to user events.
*/
private class CardPanel extends JPanel implements ActionListener {

Deck deck;         // A deck of cards to be used in the game.

BlackjackHand dealerHand;   // Hand containing the dealer's cards.
BlackjackHand playerHand;   // Hand containing the user's cards.

String message;  // A message drawn on the canvas, which changes
//    to reflect the state of the game.

boolean gameInProgress;  // Set to true when a game begins and to false
//   when the game ends.

Font bigFont;      // Font that will be used to display the message.
Font smallFont;    // Font that will be used to draw the cards.

/**
* The constructor creates the fonts and starts the first game.
* It also sets a preferred size of 460-by-310 for the panel.
* The paintComponent() method assumes that this is in fact the
* size of the panel (although it can be a little taller with
* no bad effect).
*/
CardPanel() {
setPreferredSize( new Dimension(460,310) );
setBackground( new Color(0,120,0) );
smallFont = new Font("SansSerif", Font.PLAIN, 12);
bigFont = new Font("Serif", Font.BOLD, 14);
doNewGame();
}

/**
* Respond when the user clicks on a button by calling the appropriate
* method.  Note that the buttons are created and listening is set
* up in the constructor of the BlackjackGUI class.
*/
public void actionPerformed(ActionEvent evt) {
String command = evt.getActionCommand();
if (command.equals("Hit!"))
doHit();
else if (command.equals("Stand!"))
doStand();
else if (command.equals("New Game"))
doNewGame();
}

/**
* This method is called when the user clicks the "Hit!" button.  First
* check that a game is actually in progress.  If not, give  an error
* message and exit.  Otherwise, give the user a card.  The game can end
* at this point if the user goes over 21 or if the user has taken 5 cards
* without going over 21.
*/
void doHit() {
if (gameInProgress == false) {
message = "Click \"New Game\" to start a new game.";
repaint();
return;
}
playerHand.addCard( deck.dealCard() );
if ( playerHand.getBlackjackValue() > 21 ) {
message = "You've busted!  Sorry, you lose.";
gameInProgress = false;
}
else if (playerHand.getCardCount() == 5) {
message = "You win by taking 5 cards without going over 21.";
gameInProgress = false;
}
else {
message = "You have " + playerHand.getBlackjackValue() + ".  Hit or Stand?";
}
repaint();
}

/**
* This method is called when the user clicks the "Stand!" button.
* Check whether a game is actually in progress.  If it is, the game
* ends.  The dealer takes cards until either the dealer has 5 cards
* or more than 16 points.  Then the  winner of the game is determined.
*/
void doStand() {
if (gameInProgress == false) {
message = "Click \"New Game\" to start a new game.";
repaint();
return;
}
gameInProgress = false;
while (dealerHand.getBlackjackValue() <= 16 && dealerHand.getCardCount() < 5)
dealerHand.addCard( deck.dealCard() );
if (dealerHand.getBlackjackValue() > 21)
message = "You win!  Dealer has busted with " + dealerHand.getBlackjackValue() + ".";
else if (dealerHand.getCardCount() == 5)
message = "Sorry, you lose.  Dealer took 5 cards without going over 21.";
else if (dealerHand.getBlackjackValue() > playerHand.getBlackjackValue())
message = "Sorry, you lose, " + dealerHand.getBlackjackValue()
+ " to " + playerHand.getBlackjackValue() + ".";
else if (dealerHand.getBlackjackValue() == playerHand.getBlackjackValue())
message = "Sorry, you lose.  Dealer wins on a tie.";
else
message = "You win, " + playerHand.getBlackjackValue()
+ " to " + dealerHand.getBlackjackValue() + "!";
repaint();
}

/**
* Called by the constructor, and called by actionPerformed() if  the
* user clicks the "New Game" button.  Start a new game.  Deal two cards
* to each player.  The game might end right then  if one of the players
* had blackjack.  Otherwise, gameInProgress is set to true and the game
* begins.
*/
void doNewGame() {
if (gameInProgress) {
// If the current game is not over, it is an error to try
// to start a new game.
message = "You still have to finish this game!";
repaint();
return;
}
deck = new Deck();   // Create the deck and hands to use for this game.
dealerHand = new BlackjackHand();
playerHand = new BlackjackHand();
deck.shuffle();
dealerHand.addCard( deck.dealCard() );  // Deal two cards to each player.
dealerHand.addCard( deck.dealCard() );
playerHand.addCard( deck.dealCard() );
playerHand.addCard( deck.dealCard() );
if (dealerHand.getBlackjackValue() == 21) {
message = "Sorry, you lose.  Dealer has Blackjack.";
gameInProgress = false;
}
else if (playerHand.getBlackjackValue() == 21) {
message = "You win!  You have Blackjack.";
gameInProgress = false;
}
else {
message = "You have " + playerHand.getBlackjackValue() + ".  Hit or stand?";
gameInProgress = true;
}
repaint();
}  // end newGame();

/**
* The paint method shows the message at the bottom of the
* canvas, and it draws all of the dealt cards spread out
* across the canvas.
*/
public void paintComponent(Graphics g) {

super.paintComponent(g); // fill with background color.

g.setFont(bigFont);
g.setColor(Color.GREEN);
g.drawString(message, 10, getHeight() - 10);

// Draw labels for the two sets of cards.

g.drawString("Dealer's Cards:", 10, 23);
g.drawString("Your Cards:", 10, 153);

// Draw dealer's cards.  Draw first card face down if
// the game is still in progress,  It will be revealed
// when the game ends.

g.setFont(smallFont);
if (gameInProgress)
drawCard(g, null, 10, 30);
else
drawCard(g, dealerHand.getCard(0), 10, 30);
for (int i = 1; i < dealerHand.getCardCount(); i++)
drawCard(g, dealerHand.getCard(i), 10 + i * 90, 30);

// Draw the user's cards.

for (int i = 0; i < playerHand.getCardCount(); i++)
drawCard(g, playerHand.getCard(i), 10 + i * 90, 160);

}  // end paint();

/**
* Draws a card as a 80 by 100 rectangle with upper left corner at (x,y).
* The card is drawn in the graphics context g.  If card is null, then
* a face-down card is drawn.  (The cards are rather primitive!)
*/
void drawCard(Graphics g, Card card, int x, int y) {
if (card == null) {
// Draw a face-down card
g.setColor(Color.blue);
g.fillRect(x,y,80,100);
g.setColor(Color.white);
g.drawRect(x+3,y+3,73,93);
g.drawRect(x+4,y+4,71,91);
}
else {
g.setColor(Color.white);
g.fillRect(x,y,80,100);
g.setColor(Color.gray);
g.drawRect(x,y,79,99);
g.drawRect(x+1,y+1,77,97);
if (card.getSuit() == Card.DIAMONDS || card.getSuit() == Card.HEARTS)
g.setColor(Color.red);
else
g.setColor(Color.black);
g.drawString(card.getValueAsString(), x + 10, y + 30);
g.drawString("of", x+ 10, y + 50);
g.drawString(card.getSuitAsString(), x + 10, y + 70);
}
}  // end drawCard()

} // end nested class CardPanel

} // end class BlackjackGUI

Solution for Programming Exercise 6.11

Exercise 6.11:

In the Blackjack game from Exercise 6.10, the user can click on the "Hit", "Stand", and "NewGame" buttons even when it doesn't make sense to do so. It would be better if the buttons were disabled at the appropriate times. The "New Game" button should be disabled when there is a game in progress. The "Hit" and "Stand" buttons should be disabled when there is not a game in progress. The instance variable gameInProgress tells whether or not a game is in progress, so you just have to make sure that the buttons are properly enabled and disabled whenever this variable changes value. I strongly advise writing a subroutine that can be called whenever it is necessary to set the value of the gameInProgress variable. Then the subroutine can take responsibility for enabling and disabling the buttons. Recall that if bttn is a variable of type JButton, then bttn.setEnabled(false) disables the button and bttn.setEnabled(true) enables the button.

As a second (and more difficult) improvement, make it possible for the user to place bets on the Blackjack game. When the program starts, give the user $100. Add a JTextField to the strip of controls along the bottom of the panel. The user can enter the bet in this JTextField. When the game begins, check the amount of the bet. You should do this when the game begins, not when it ends, because several errors can occur: The contents of the JTextField might not be a legal number, the bet that the user places might be more money than the user has, or the bet might be <= 0. You should detect these errors and show an error message instead of starting the game. The user's bet should be an integral number of dollars.

It would be a good idea to make the JTextField uneditable while the game is in progress. If betInput is the JTextField, you can make it editable and uneditable by the user with the commands betInput.setEditable(true) and betInput.setEditable(false).

In the paintComponent() method, you should include commands to display the amount of money that the user has left.

There is one other thing to think about: Ideally, the program should not start a new game when it is first created. The user should have a chance to set a bet amount before the game starts. So, in the constructor for the drawing surface class, you should not call doNewGame(). You might want to display a message such as "Welcome to Blackjack" before the first game starts.

Here is a picture of my program:

Discussion

In the original program, the button variables are declared in the constructor of the main class, where the buttons are created. There are no instance variables that refer to the buttons, so it is not possible to do anything with the buttons outside the constructor. For this exercise, references to the buttons must be stored in instance variables. The program uses instance variables hitButton, standButton, and newGameButton to refer to the buttons. These variables are required in order to call the buttons' setEnabled() methods. I also introduced a JTextField, betInput, as an instance variable and modified the constructor so that it creates the text field and adds it to the panel that occupies the SOUTH position in the main panel.

The buttons must be enabled and disabled whenever the value of the variable gameInProgress changes. At the same time, the text field should be made editable or non-editable. As recommended in the exercise, I wrote a method for changing the value of gameInProgress. This method also sets the buttons and text field to reflect the state of the program, which allows the state of the buttons and text field to be controlled in one location:

/**
* This method is called whenever the value of the gameInProgress
* property has to be changed.  In addition to setting the value
* of the gameInProgress variable, it also enables and disables
* the buttons and text input box to reflect the state of the
* game.
* @param inProgress The new value of gameInProgress.
*/
private void setGameInProgress( boolean inProgress ) {
gameInProgress = inProgress;
if (gameInProgress) {
hitButton.setEnabled(true);
standButton.setEnabled(true);
newGameButton.setEnabled(false);
betInput.setEditable(false);
}
else {
hitButton.setEnabled(false);
standButton.setEnabled(false);
newGameButton.setEnabled(true);
betInput.setEditable(true);
}
}

Once this routine is available, then any line in the old program that said "gameInProgress = false;" should be changed to "setGameInProgress(false);". And any line that said "gameInProgress = true;" should be changed to "setGameInProgress(true);". In this way, we can be sure that the buttons are always properly enabled and disabled. Note that I added lines to the BlackjackGUI2 constructor to disable hitButton and standButton when they are created. This ensures that they have the correct state at the start of the program, before the first game has been started.

You should understand why I used a subroutine to set the value of gameInProgress. Every time gameInProgress changes, each of the buttons has to be enabled or disabled and the text field has to be made editable or uneditable. That's four extra lines of code each time the program says gameInProgress = true or gameInProgress = false. We can avoid some extra typing by calling the subroutine. Furthermore, if we always call the subroutine to set the value of gameInProgress, we can be sure that the states of the buttons and text field will always be set correctly to match the value of gameInProgress.

The changes that I've discussed so far are enough to complete the first part of the exercise, enabling and disabling the buttons. We still have to implement betting.

The JTextField for the user's input is created with the command "betInput = new JTextField("10",5);". The first parameter in the constructor specifies the initial content of the text input box. This is meant as a reasonable value for the bet, but the user can change it if he wants to. The second parameter is important. It specifies the number of characters that the text field is designed to hold. The preferred size of the JTextField is computed based on this number of characters. If the text field were being used in a context where it would be stretched to fit the available size, such as in a GridLayout, the preferred size would not be so important. However, in this program, the JTextField is used with a FlowLayout, and it will appear at exactly its preferred size. If you leave out the second parameter in the constructor, the JTextField will be sized to fit its contents, "10", and it will look too small.

The CardPanel class contains two new instance variables for managing the user's bets. One variable, usersMoney, records the amount of money that the user has. The other, betAmount, records the amount of the user's bet on the current game. The value of usersMoney is initialized to 100 in the constructor. At the end of a game, if the user wins, the betAmount is added to the user's money, and if the user loses, the betAmount is subtracted from the user's money. We have to decide what happens if the user runs out of money. One possibility would be to shut the game down, but that seems drastic since it's only play money anyway. So, if the value of usersMoney drops to zero, I give the user another $100 at the start of the next game.

At the beginning of a game, the program has to look at the number in the text field to determine how much money the user wants to bet on the game. Several things can go wrong at this time. Since it's a little complicated, I wrote a method to check the contents of betInput:

/**
* This is called when the user wants to start a new game.  It tries to
* read the amount of the user's bet from the betInput text field.  If an error
* occurs, the message in the panel is changed to inform the user of the error.
* @return true if the bet is read without error, or false if an error occurs
*/
private boolean checkBet() {
int amount;
try {
amount = Integer.parseInt( betInput.getText() );
}
catch (NumberFormatException e) {
message = "The bet amount must be a legal positive integer.";
repaint();
return false;
}
if (amount <= 0) {
message = "The bet amount must be a positive integer.";
repaint();
return false;
}
if (amount > usersMoney) {
message = "You can't bet more money than you have!";
repaint();
return false;
}
betAmount = amount;
return true;
}

The doNewGame() method starts by checking if usersMoney is 0; if so, it gives the user another $100 by setting usersMoney to 100. Then it checks the user's bet by calling the checkBet() method. If the return value is false, meaning that the value in the text field was not a valid bet, then the doNewGame() method returns without starting a new game. Otherwise, it starts the game as it did in the old program.

When the game ends for any reason, the user's money has to be adjusted. There are many points in the source code where the game ends. In each of those places, I inserted a line "usersMoney = usersMoney + betAmount" if the user won or "usersMoney = usersMoney - betAmount" if the user lost. It might have been a good idea to write another subroutine to handle this task.

I also added some code to the paintComponent() method to display the user's current amount of money. To accommodate this, this version of the panel has to be about 30 pixels taller than the previous version.

One of the tricky parts of this assignment is to arrange things so that a game does not start as soon as the panel is created. It's no problem to take the "doNewGame();" statement out of the constructor in the CardPanel class. Some initialization has to be done there instead:

usersMoney = 100;
message = "Welcome to Blackjack!  You start with $100.";

However, when I did this, I ran into a NullPointerException in the paintComponent() method because the paintComponent() method assumed that two Hand objects, dealerHand and playerHand, exist. These objects are created in the doNewGame() method, so removing the call to doNewGame() from the constructor meant that the hands were null when paintComponent() was first called. This just required a simple modification in the paintComponent() method to deal with this possibility. I decided simply to return from paintComponent() after drawing the messages, if the hands don't exist. (An alternative would be to have some fancy introductory screen.)

(I should also confess that I had problems with null pointer exceptions when I tried to run the program without removing the call to doNewGame() from the constructor. In this case, the problem was more subtle: The CardPanel constructor was being called before the button and text field objects had been created, so doNewGame() was trying to enable/disable buttons that didn't exist yet.)

The Solution

Here is the new version of the blackjack source code, with changes from Exercise 6.10 shown in red italic:

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

/**
* In this program, the user plays a game of Blackjack.  The
* computer acts as the dealer.  The user plays by clicking
* "Hit!" and "Stand!" buttons.  The user can place bets.
* At the beginning of the game, the user is given $100.
*
* This class defines a panel, but it also contains a main()
* routine that makes it possible to run the program as a
* stand-alone application.
*
* This program depends on the following classes:  Card, Hand,
* BlackjackHand, Deck.
*/
public class BlackjackGUI2 extends JPanel {

/**
* The main routine simply opens a window that shows a BlackjackGUI2.
*/
public static void main(String[] args) {
JFrame window = new JFrame("Blackjack");
BlackjackGUI2 content = new BlackjackGUI2();
window.setContentPane(content);
window.pack();  // Set size of window to preferred size of its contents.
window.setResizable(false);  // User can't change the window's size.
Dimension screensize = Toolkit.getDefaultToolkit().getScreenSize();
window.setLocation( (screensize.width - window.getWidth())/2,
(screensize.height - window.getHeight())/2 );
window.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
window.setVisible(true);
}

private JButton hitButton;     // The three buttons that control the game.
private JButton standButton;
private JButton newGameButton;
private JTextField betInput;  // Where the user inputs the amount of his bet.

/**
* The constructor lays out the panel.  A CardPanel occupies the CENTER
* position of the panel (where CardPanel is a subclass of JPanel that is
* defined below).  On the bottom is a panel that holds three buttons.
* The CardPanel listens for ActionEvents from the buttons and does all
* the real work of the program.
*/
public BlackjackGUI2() {

setBackground( new Color(130,50,40) );

setLayout( new BorderLayout(3,3) );

CardPanel board = new CardPanel();
add(board, BorderLayout.CENTER);

JPanel buttonPanel = new JPanel();
buttonPanel.setBackground( new Color(220,200,180) );
add(buttonPanel, BorderLayout.SOUTH);

// NOTE: Declarations of hitButton, standButton, newGameButton were moved
// out of the constructor.  Previously, they were local variables.

hitButton = new JButton( "Hit!" );
hitButton.setEnabled(false);
hitButton.addActionListener(board);
buttonPanel.add(hitButton);

standButton = new JButton( "Stand!" );
standButton.setEnabled(false);
standButton.addActionListener(board);
buttonPanel.add(standButton);

newGameButton = new JButton( "New Game" );
newGameButton.addActionListener(board);
buttonPanel.add(newGameButton);

buttonPanel.add(new JLabel("  Bet:", JLabel.RIGHT));

betInput = new JTextField("10", 5);
betInput.setMargin( new Insets(3,3,3,3) );
buttonPanel.add(betInput);

setBorder(BorderFactory.createLineBorder( new Color(130,50,40), 3) );

}  // end constructor

/**
* A nested class that displays the game and does all the work
* of keeping track of the state and responding to user events.
*/
private class CardPanel extends JPanel implements ActionListener {

Deck deck;         // A deck of cards to be used in the game.

BlackjackHand dealerHand;   // Hand containing the dealer's cards.
BlackjackHand playerHand;   // Hand containing the user's cards.

String message;  // A message drawn on the canvas, which changes
//    to reflect the state of the game.

boolean gameInProgress;  // Set to true when a game begins and to false
//   when the game ends.

Font bigFont;      // Font that will be used to display the message.
Font smallFont;    // Font that will be used to draw the cards.

int usersMoney;    // The amount of money that the user currently has.
int betAmount;     // The bet amount, read from betInput when game starts.

/**
* The constructor creates the fonts and starts the first game.
* It also sets a preferred size of 460-by-330 for the panel.
* The paintComponent() method assumes that this is in fact the
* size of the panel (although it can be a little taller with
* no bad effect).
*/
CardPanel() {
setPreferredSize( new Dimension(460,330) );
setBackground( new Color(0,120,0) );
smallFont = new Font("SansSerif", Font.PLAIN, 12);
bigFont = new Font("Serif", Font.BOLD, 14);
usersMoney = 100;
message = "Welcome to Blackjack!  You start with $100.";
}

/**
* This method is called whenever the value of the gameInProgress
* property has to be changed.  In addition to setting the value
* of the gameInProgress variable, it also enables and disables
* the buttons and text input box to reflect the state of the
* game.
* @param inProgress The new value of gameInProgress.
*/
private void setGameInProgress( boolean inProgress ) {
gameInProgress = inProgress;
if (gameInProgress) {
hitButton.setEnabled(true);
standButton.setEnabled(true);
newGameButton.setEnabled(false);
betInput.setEditable(false);
}
else {
hitButton.setEnabled(false);
standButton.setEnabled(false);
newGameButton.setEnabled(true);
betInput.setEditable(true);
}
}

/**
* This is called when the user wants to start a new game.  It tries to
* read the amount of the user's bet from the betInput text field.  If an error
* occurs, the message in the panel is changed to inform the user of the error.
* @return true if the bet is read without error, or false if an error occurs
*/
private boolean checkBet() {
int amount;
try {
amount = Integer.parseInt( betInput.getText() );
}
catch (NumberFormatException e) {
message = "The bet amount must be a legal positive integer.";
repaint();
return false;
}
if (amount <= 0) {
message = "The bet amount must be a positive integer.";
repaint();
return false;
}
if (amount > usersMoney) {
message = "You can't bet more money than you have!";
repaint();
return false;
}
betAmount = amount;
return true;
}

/**
* Respond when the user clicks on a button by calling the appropriate
* method.  Note that the buttons are created and listening is set
* up in the constructor of the BlackjackPanel class.
*/
public void actionPerformed(ActionEvent evt) {
String command = evt.getActionCommand();
if (command.equals("Hit!"))
doHit();
else if (command.equals("Stand!"))
doStand();
else if (command.equals("New Game"))
doNewGame();
}

/**
* This method is called when the user clicks the "Hit!" button.  First
* check that a game is actually in progress.  If not, give  an error
* message and exit.  Otherwise, give the user a card.  The game can end
* at this point if the user goes over 21 or if the user has taken 5 cards
* without going over 21.
*/
void doHit() {
if (gameInProgress == false) {  // Should not be possible!
message = "Click \"New Game\" to start a new game.";
repaint();
return;
}
playerHand.addCard( deck.dealCard() );
if ( playerHand.getBlackjackValue() > 21 ) {
usersMoney = usersMoney - betAmount;
message = "You've busted!  Sorry, you lose.";
setGameInProgress(false);
}
else if (playerHand.getCardCount() == 5) {
usersMoney = usersMoney + betAmount;
message = "You win by taking 5 cards without going over 21.";
setGameInProgress(false);
}
else {
message = "You have " + playerHand.getBlackjackValue() + ".  Hit or Stand?";
}
repaint();
}

/**
* This method is called when the user clicks the "Stand!" button.
* Check whether a game is actually in progress.  If it is, the game
* ends.  The dealer takes cards until either the dealer has 5 cards
* or more than 16 points.  Then the  winner of the game is determined.
*/
void doStand() {
if (gameInProgress == false) {  // Should not be possible!
message = "Click \"New Game\" to start a new game.";
repaint();
return;
}
setGameInProgress(false);
while (dealerHand.getBlackjackValue() <= 16 && dealerHand.getCardCount() < 5)
dealerHand.addCard( deck.dealCard() );
if (dealerHand.getBlackjackValue() > 21) {
usersMoney = usersMoney + betAmount;
message = "You win!  Dealer has busted with " + dealerHand.getBlackjackValue() + ".";
}
else if (dealerHand.getCardCount() == 5) {
usersMoney = usersMoney - betAmount;
message = "Sorry, you lose.  Dealer took 5 cards without going over 21.";
}
else if (dealerHand.getBlackjackValue() > playerHand.getBlackjackValue()) {
usersMoney = usersMoney - betAmount;
message = "Sorry, you lose, " + dealerHand.getBlackjackValue()
+    " to " + playerHand.getBlackjackValue() + ".";
}
else if (dealerHand.getBlackjackValue() == playerHand.getBlackjackValue()) {
usersMoney = usersMoney - betAmount;
message = "Sorry, you lose.  Dealer wins on a tie.";
}
else {
usersMoney = usersMoney + betAmount;
message = "You win, " + playerHand.getBlackjackValue()
+ " to " + dealerHand.getBlackjackValue() + "!";
}
repaint();
}

/**
* Called by the constructor, and called by actionPerformed() if  the
* user clicks the "New Game" button.  Start a new game.  Deal two cards
* to each player.  The game might end right then  if one of the players
* had blackjack.  Otherwise, gameInProgress is set to true and the game
* begins.
*/
void doNewGame() {
if (gameInProgress) {
// If the current game is not over, it is an error to try
// to start a new game.  This shouldn't be possible because
// the new game button is disabled while a game is in progress,
// but it doesn't hurt anything to check anyway.
message = "You still have to finish this game!";
repaint();
return;
}
if (usersMoney == 0) {
// The user has run out of money; give the user another $100.
usersMoney = 100;
}
if ( ! checkBet() ) {
// The user's bet was not legal, so we can't start a game.
// The checkBet method has already given an error message.
return;
}
deck = new Deck();   // Create the deck and hands to use for this game.
dealerHand = new BlackjackHand();
playerHand = new BlackjackHand();
deck.shuffle();
dealerHand.addCard( deck.dealCard() );  // Deal two cards to each player.
dealerHand.addCard( deck.dealCard() );
playerHand.addCard( deck.dealCard() );
playerHand.addCard( deck.dealCard() );
if (dealerHand.getBlackjackValue() == 21) {
message = "Sorry, you lose.  Dealer has Blackjack.";
usersMoney = usersMoney - betAmount;
setGameInProgress(false);
}
else if (playerHand.getBlackjackValue() == 21) {
message = "You win!  You have Blackjack.";
usersMoney = usersMoney + betAmount;
setGameInProgress(false);
}
else {
message = "You have " + playerHand.getBlackjackValue() + ".  Hit or stand?";
setGameInProgress(true);
}
repaint();
}  // end newGame();

/**
* The paint method shows the message at the bottom of the
* canvas, and it draws all of the dealt cards spread out
* across the canvas.
*/
public void paintComponent(Graphics g) {

super.paintComponent(g); // fill with background color.

g.setFont(bigFont);
g.setColor(Color.GREEN);
g.drawString(message, 10, getHeight() - 10);

// Draw a message telling how much money the user has.

g.setColor(Color.YELLOW);
if (usersMoney > 0)
g.drawString("You have $" + usersMoney, 10, getHeight() - 35);
else
g.drawString("YOU ARE BROKE!  (I will give you another $100.)",
10, getHeight() - 32 );

if (dealerHand == null)
return;  // the first game has not yet started.

// Draw labels for the two sets of cards.

g.setColor(Color.GREEN);
g.drawString("Dealer's Cards:", 10, 23);
g.drawString("Your Cards:", 10, 153);

// Draw dealer's cards.  Draw first card face down if
// the game is still in progress,  It will be revealed
// when the game ends.

g.setFont(smallFont);
if (gameInProgress)
drawCard(g, null, 10, 30);
else
drawCard(g, dealerHand.getCard(0), 10, 30);
for (int i = 1; i < dealerHand.getCardCount(); i++)
drawCard(g, dealerHand.getCard(i), 10 + i * 90, 30);

// Draw the user's cards.

for (int i = 0; i < playerHand.getCardCount(); i++)
drawCard(g, playerHand.getCard(i), 10 + i * 90, 160);

}  // end paintComponent();

/**
* Draws a card as a 80 by 100 rectangle with upper left corner at (x,y).
* The card is drawn in the graphics context g.  If card is null, then
* a face-down card is drawn.  (The cards are rather primitive!)
*/
void drawCard(Graphics g, Card card, int x, int y) {
if (card == null) {
// Draw a face-down card
g.setColor(Color.blue);
g.fillRect(x,y,80,100);
g.setColor(Color.white);
g.drawRect(x+3,y+3,73,93);
g.drawRect(x+4,y+4,71,91);
}
else {
g.setColor(Color.white);
g.fillRect(x,y,80,100);
g.setColor(Color.gray);
g.drawRect(x,y,79,99);
g.drawRect(x+1,y+1,77,97);
if (card.getSuit() == Card.DIAMONDS || card.getSuit() == Card.HEARTS)
g.setColor(Color.red);
else
g.setColor(Color.black);
g.drawString(card.getValueAsString(), x + 10, y + 30);
g.drawString("of", x+ 10, y + 50);
g.drawString(card.getSuitAsString(), x + 10, y + 70);
}
}  // end drawCard()

} // end nested class CardPanel

} // end class BlackjackGUI2

Solution for Programming Exercise 7.1

Exercise 7.1:

Write a subroutine that creates an ArrayList containing several different random integers in the range from 1 up to some specified maximum. The number of integers and the maximum allowed value for the integers should be parameters to the subroutine. Write a main() routine to test your subroutine.

Discussion

The exercise asks for an ArrayList containing integers. That has to mean using ArrayList<Integer>, since an ArrayList can't contain values of type int. The problem is pretty easy: Start with an empty list, then generate random integers and add them to the list until you have as many integers as you want. Since all the integers in the list must be different, before adding an integer to the list, we need to test whether that integer is already in the list. One way to do that is with the indexOf() method from the ArrayList class. If numbers is an ArrayList<Integer> and num is an int, then numbers.indexOf(num) returns -1 if num is not in the list. (Note that this relies on autoboxing to convert num into an object of type Integer, since indexOf() requires an object as its parameter.) To create the list of numbers, we just add integers to the list until it reaches the desired size. Assuming that count is the desired number of integers and max is the maximum allowed value for the integers, this can be done with

ArrayList<Integer> numbers = new ArrayList<Integer>();
while (numbers.size() < count) {
int num = (int)(Math.random()*max) + 1;
if ( numbers.indexOf(num) == -1 )
numbers.add( num );
}

This code is the heart of the solution. There is one problem, however. If count > max, then it's impossible to get count different integers in the range from 1 to max, because there aren't that many different numbers in the range! In fact, the while loop will be an infinite loop in that case since the size of the ArrayList can never become larger than max. My solution avoids the infinite loop by throwing an IllegalArgumentException when count > max. You can see the complete subroutine, and a main() routine that tests it, below.

The Solution

import java.util.ArrayList;

public class SeveralRandomIntegers {

/**
* Creates an ArrayList that contains several integers chosen at
* random from a specified range of values.  All the integers
* in the list are different.
* @param count  the number of random integers to be created
* @param max  the integers are chosen in the range 1 to max, inclusive
* @return an ArrayList containing the integers.  The integers are
*    random and are in a random order.
* @throws IllegalArgumentException if max is greater than count.  In
*    that case there are fewer than count different integers in the
*    range 1 to max.
*/
public static ArrayList<Integer> makeRandomInts( int count, int max ) {
if (count > max) {
throw new IllegalArgumentException("Can't have " + count +
" different integers in the range  1 to " + max);
}
ArrayList<Integer> numbers = new ArrayList<Integer>();
while (numbers.size() < count) {
int num = (int)(Math.random()*max) + 1;
if ( numbers.indexOf(num) == -1 )
numbers.add( num );
}
return numbers;
}

private static void printList( ArrayList<Integer> numbers ) {
System.out.print("    [ ");
for (int i = 0; i < numbers.size(); i++) {
if (i > 0)
System.out.print(", ");
System.out.print( numbers.get(i) );
}
System.out.println(" ]");
}

public static void main(String[] args) {
System.out.println("10 integers chosen from the range 1 to 100:");
for (int i = 0; i < 8; i++)
printList( makeRandomInts(10,100) );
System.out.println();
System.out.println("10 integers chosen from the range 1 to 10:");
for (int i = 0; i < 3; i++)
printList( makeRandomInts(10,10) );
System.out.println();
System.out.println("25 integers chosen from the range 1 to 10000:");
printList( makeRandomInts(25,10000) );
System.out.println();
System.out.println("End with an exception!");
makeRandomInts(10,9);
}

}

Solution for Programming Exercise 7.2

Exercise 7.2:

Suppose that M is a two-dimensional array that has R rows and C columns. The transpose of M is defined to be an array T that has C rows and R columns such that T[i][j] = M[j][i] for each i and j. Write a function that takes an array of type int[][] as a parameter, and returns the transpose of that array. (Assume that the parameter is a typical 2D array in which all the rows have the same length.) Also write a subroutine to print a 2D array of integers in neat rows and columns, and include a main() routine to test your work.

Discussion

To create the transpose, we need to know how many rows and how many columns are in the original array. As noted in Subsection 7.5.1, the number of rows in a 2D array A is given by A.length, and the number of columns is A[0].length. (A[0].length is the length of the first row of A, which is the same as the number of columns in A. Remember that all rows have the same length.) Given that, we can create a transpose array of the correct size, and -- with a little care to get the indices right -- we can copy all the items from the original array into the transpose. If matrix is the original array, then the transpose can be created with

int[][] transpose;
int R = matrix.length;     // the number of rows in matrix
int C = matrix[0].length;  // the number of columns in matrix
transpose = new int[C][R];
for ( int i = 0; i < C; i++) { // goes through ROWS of the transpose
for ( int j = 0; j < R; j++ ) { // goes through COLUMNS of the transpose
transpose[i][j] = matrix[j][i];
}
}

One other note of interest is in the code for printing the array. See the comments on the source code below.

By the way, one purpose of this exercise was to remind you that arrays can be parameters and they can be the return value of a function. In this case, the parameter type and the return type for the function are both given by int[][].

The Solution

public class TransposeMatrix {

/**
* Creates the transpose of a given 2D array of integers.
* @param matrix  the original array
* @return the transpose of matrix
*/
public static int[][] computeTranspose( int[][] matrix ) {
int[][] transpose;
int R = matrix.length;     // the number of rows in matrix
int C = matrix[0].length;  // the number of columns in matrix
transpose = new int[C][R];
for ( int i = 0; i < C; i++) { // goes through ROWS of the transpose
for ( int j = 0; j < R; j++ ) { // goes through COLUMNS of the transpose
transpose[i][j] = matrix[j][i];
}
}
return transpose;
}

/**
* Prints out the items of a 2D array of ints in rows and columns,
* with 6 spaces in each column.
*/
public static void print( int[][] array ) {
// Note that this uses a for-each loop where the loop
// control variable is of type int[].  This works because
// a 2D array is actually a 1D array of 1D arrays, where
// each 1D array is one of the rows of the 2D array.
for ( int[] row : array ) {
// print out one row from the array
System.out.print("   ");
for ( int item : row ) {
// print with 1 blank space and 5 spaces for the integer;
// if an integer needs more than 5 spaces, the columns will
// be messed up, but all the integers will still be readable.
System.out.printf(" %5d", item);
}
System.out.println();
}
}

/**
* Test the subroutines by creating two arrays and printing them and
* their transposes.  The arrays are constructed so that it is easy
* to see that the transposes are correct.
*/
public static void main(String[] args) {
int[][] orig = {
{ 1, 2, 3, 4, 5, 6 },
{ 10, 20, 30, 40, 50, 60 },
{ 100, 200, 300, 400, 500, 600 }
};
System.out.println("Original matrix:");
System.out.println();
print(orig);
System.out.println();
System.out.println("The transpose:");
System.out.println();
print( computeTranspose(orig) );
System.out.println();
System.out.println();

orig = new int[][] {
{1, 1, 1, 1, 1, 1, 1},
{2, 2, 2, 2, 2, 2, 2},
{3, 3, 3, 3, 3, 3, 3},
{4, 4, 4, 4, 4, 4, 4},
{5, 5, 5, 5, 5, 5, 5},
{6, 6, 6, 6, 6, 6, 6},
{7, 7, 7, 7, 7, 7, 7},
};
System.out.println("Original matrix:");
System.out.println();
print(orig);
System.out.println();
System.out.println("The transpose:");
System.out.println();
print( computeTranspose(orig) );
System.out.println();

}

}

Solution for Programming Exercise 7.3

Exercise 7.3:

In Subsection 7.4.4, it is mentioned that the standard sorting method Arrays.sort() is much faster and efficient than selection sort. Write a program to test this claim. To be specific, your program should create a large array filled with random real numbers. It should use both Arrays.sort() and selectionSort() to sort the array, and it should time how long it takes to perform each sort. Furthermore, it should do the same thing for a large array of random Strings. To find the times, you can use System.currentTimeMillis() (see Subsection 2.3.1 and the example TimedComputation.java).

Discussion

This exercise is most interesting for the results of the timing experiments, but one point of interest is how to make a "random string." In my program, I make a string containing uppercase letters, with a random length:

private static String randomString() {
int length = 5 + (int)(21*Math.random());
StringBuilder str = new StringBuilder();
for (int i = 0; i < length; i++) {
char ch = (char)('A' + (int)(26*Math.random())); // a random letter
str.append(ch);
}
return str.toString();
}

This method is then used to fill an array with random strings. I wrote methods for creating random arrays of a given size. I used Arrays.copyOf() to make copies of the arrays. I need two copies of each random array, since I want to apply selectionSort() and Arrays.sort() to identical data. So the arrays are created with

numberList1 = randomNumbers(SIZE);
numberList2 = Arrays.copyOf(numberList1, SIZE);
stringList1 = randomStrings(SIZE);
stringList2 = Arrays.copyOf(stringList1, SIZE);

where SIZE is a constant. (By using a constant here, I can easily adapt the program to run with different array sizes.)

The selection sort algorithm is copied from Subsection 7.4.4. We actually need two selectionSort methods, one to sort numbers an one to sort strings. You will see that my program includes tests of those methods. Without that test, I wouldn't have been confident that my subroutines were sorting the arrays correctly.

The code for timing the sorting algorithms is straightforward. Here are sort times from my program. Note that Arrays.sort() is much faster than selectionSort. Also, it takes longer to sort strings that it takes to sort numbers, since comparing two strings takes longer than comparing two numbers. The advantage of Arrays.sort() is greater for strings than it is for numbers. I'm not sure why that happens.

Milliseconds to sort 100000 numbers with selectionSort: 14299
Milliseconds to sort 100000 numbers with Arrays.sort(): 79
Milliseconds to sort 100000 strings with selectionSort: 251792
Milliseconds to sort 100000 strings with Arrays.sort(): 216

You can try running the program with different array sizes. A large difference is noticeable even for arrays of size 1000, although all of the times are rather short in that case. The advantage of Arrays.sort() increases as the array size increases.

The Solution

import java.util.Arrays;

public class SortExperiments {

final static int SIZE = 100000; // The length of arrays that will be sorted.

/**
* Creates a random string.  The length of the string is between 5 and 25,
* and it is made up of randomly selected uppercase letters.
*/
private static String randomString() {
int length = 5 + (int)(21*Math.random());
StringBuilder str = new StringBuilder();
for (int i = 0; i < length; i++) {
char ch = (char)('A' + (int)(26*Math.random()));
str.append(ch);
}
return str.toString();
}

/**
* Creates an array of random real numbers.  The items in the array
* are random numbers in the range 0.0 to 1.0.
* @param count The length of the array that is created.
*/
private static double[] randomNumbers(int count) {
double[] numbers = new double[count];
for (int i = 0; i < count; i++)
numbers[i] = Math.random();
return numbers;
}

/**
* Creates an array of random strings. The items in the
* array are created by calling the function randomString();
* @param count the size of the array that is created
*/
private static String[] randomStrings(int count) {
String[] strings = new String[count];
for (int i = 0; i < count; i++)
strings[i] = randomString();
return strings;
}

/**
* Sort an array of real numbers using the selection sort algorithm.
*/
private static void selectionSort(double[] numbers) {
for (int top = numbers.length-1; top > 0; top-- ) {
int maxloc = 0;
for (int i = 1; i <= top; i++) {
if (numbers[i] > numbers[maxloc])
maxloc = i;
}
double temp = numbers[top];
numbers[top] = numbers[maxloc];
numbers[maxloc] = temp;
}
}

/**
* Sort an array of strings using the selection sort algorithm.
*/
private static void selectionSort(String[] numbers) {
for (int top = numbers.length-1; top > 0; top-- ) {
int maxloc = 0;
for (int i = 1; i <= top; i++) {
if (numbers[i].compareTo(numbers[maxloc]) > 0)
maxloc = i;
}
String temp = numbers[top];
numbers[top] = numbers[maxloc];
numbers[maxloc] = temp;
}
}

public static void main(String[] args) {

long startTime;  // time when a sort begin.
long endTime;    // time when a sort ends.

double[] numberList1;  // An array of random numbers.
double[] numberList2;  // A copy of numberList1.

String[] stringList1;  // An array of random strings.
String[] stringList2;  // A copy of stringList1.

/* Make sure the selection sort methods are correct.  The outputs
should be correctly sorted. */

System.out.println("First, test that selection sort works on doubles.");
System.out.println("The 10 output numbers should be in increasing order.");
numberList1 = randomNumbers(10);
selectionSort(numberList1);
for (double n : numberList1)
System.out.println( "   " + n );
System.out.println();

System.out.println("Next, test that selection sort works on strings.");
System.out.println("The 10 output strings should be in alphabetical order.");
System.out.println("(Also tests that random strings are made correctly.");
stringList1 = randomStrings(10);
selectionSort(stringList1);
for (String str : stringList1)
System.out.println( "   " + str );
System.out.println();

System.out.println();
System.out.println("Times for sorting arrays of size " + SIZE + ":");
System.out.println();

/* Create the arrays. */

numberList1 = randomNumbers(SIZE);
numberList2 = Arrays.copyOf(numberList1, SIZE);
stringList1 = randomStrings(SIZE);
stringList2 = Arrays.copyOf(stringList1, SIZE);

/* Do the sorts and output the times. */

startTime = System.currentTimeMillis();
selectionSort(numberList1);
endTime = System.currentTimeMillis();
System.out.printf("Milliseconds to sort %d numbers with selectionSort: %d",
SIZE, endTime-startTime);
System.out.println();

startTime = System.currentTimeMillis();
Arrays.sort(numberList2);
endTime = System.currentTimeMillis();
System.out.printf("Milliseconds to sort %d numbers with Arrays.sort(): %d",
SIZE, endTime-startTime);
System.out.println();

startTime = System.currentTimeMillis();
selectionSort(stringList1);
endTime = System.currentTimeMillis();
System.out.printf("Milliseconds to sort %d strings with selectionSort: %d",
SIZE, endTime-startTime);
System.out.println();

startTime = System.currentTimeMillis();
Arrays.sort(stringList2);
endTime = System.currentTimeMillis();
System.out.printf("Milliseconds to sort %d strings with Arrays.sort(): %d",
SIZE, endTime-startTime);
System.out.println();
System.out.println();

}

}

Solution for Programming Exercise 7.4

Exercise 7.4:

In Exercise 6.1, you wrote a program SimpleStamperWithDrag that allows the user to place red rectangles and blue ovals in a panel by clicking and dragging the mouse. However, that program does not store any information about what has been drawn, so the panel cannot repaint itself correctly. Revise the program to use an ArrayList to store data about the contents of the panel. All drawing should be done in a paintComponent() method.

Discussion

We need three pieces of information in order to draw a shape: its x-coordinate, its y-coordinate, and whether it is a red rectangle or a blue oval. We can pack the three pieces of information into a class. I've added a constructor to the class to make it easier to create an object and set the values of its instance variables at the same time:

/**
* A class to hold information about one shape.
*/
private static class ShapeInfo {
int x,y;  // The location of a shape
boolean rect;  // true if it's a red rectangle, false for a blue oval
ShapeInfo(int x, int y, boolean isRect) {
this.x = x;
this.y = y;
rect = isRect;
}
}

The data for the whole collection of shapes is then stored using a variable named shapes of type ArrayList<ShapeInfo>. By using an ArrayList, we allow for an unlimited number of shapes.

Given this data structure, it's easy to write the paintComponent() method. It uses a for-each loop to go through the ArrayList. The loop control variable, shape, is of type ShapeInfo, and the data needed to draw the shape is given by its three instance variables shape.x, shape.y, and shapes.rect. You can look at the method in the program below.

In the original program, the mousePressed() and mouseDragged() methods actually draw the shapes. In the new version, these methods simply add an item to the shapes ArrayList, and they call repaint() to make the change show up on the screen. (I will admit that in my first attempt, I forgot the repaint(), with the result that the mouse actions had no visible effect even though the shape was correctly added to the list.)

If the user is holding down the shift key in mousePressed(), the panel has to be cleared. I do that by calling shapes.clear(), which removes all the items from the ArrayList and sets its size to zero.

(By the way, one of the reasons to use an ArrayList in this program is to avoid avoid using the getGraphics() method to get a graphics context for drawing on the panel. The use of this method is frowned upon. The new program follows the recommended rule, to do all drawing in the paintComponent() method.)

The Solution

Changes from SimpleStamperWithDrag are shown in red italic.

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import java.util.ArrayList;

/**
* A simple demonstration of MouseEvents.  Shapes are drawn
* on a black background when the user clicks the panel.  If
* the user Shift-clicks, the panel is cleared.  If the user
* right-clicks the panel, a blue oval is drawn.  Otherwise,
* when the user clicks, a red rectangle is drawn.  Information
* about the shapes that have been drawn is stored in an ArrayList,
* which is used in paintComponent() to draw the contents of
* the panel.
*/
public class SimpleStamperWithArrayList extends JPanel
implements MouseListener, MouseMotionListener {

public static void main(String[] args) {
JFrame window = new JFrame("Simple Stamper");
SimpleStamperWithArrayList content = new SimpleStamperWithArrayList();
window.setContentPane(content);
window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
window.setLocation(120,70);
window.setSize(400,300);
window.setVisible(true);
}

// ----------------------------------------------------------------------

/**
* A class to hold information about one shape.
*/
private static class ShapeInfo {
int x,y;  // The location of a shape
boolean rect;  // true if it's a red rectangle, false for a blue oval
ShapeInfo(int x, int y, boolean isRect) {
this.x = x;
this.y = y;
rect = isRect;
}
}

private ArrayList<ShapeInfo> shapes;  // Holds shapes that have been added.

/**
* This variable is set to true during a drag operation, unless the
* user was holding down the shift key when the mouse was first
* pressed (since in that case, the mouse gesture simply clears the
* panel and no figures should be drawn if the user drags the mouse).
*/
private boolean dragging;

/**
* This constructor simply sets the background color of the panel to be black
* and sets the panel to listen for mouse events on itself.
*/
public SimpleStamperWithArrayList() {
setBackground(Color.WHITE);
addMouseListener(this);
addMouseMotionListener(this);
shapes = new ArrayList<ShapeInfo>();
}

protected void paintComponent(Graphics g) {
super.paintComponent(g);
for ( ShapeInfo shape : shapes ) {
if ( shape.rect ) {
// Draw a red rectangle centered at (x,y).
g.setColor(Color.RED);   // Red interior.
g.fillRect( shape.x - 30, shape.y - 15, 60, 30 );
g.setColor(Color.BLACK); // Black outline.
g.drawRect( shape.x - 30, shape.y - 15, 60, 30 );
}
else {
// draw a blue oval centered at (x,y)
g.setColor(Color.BLUE);  // Blue interior.
g.fillOval( shape.x - 30, shape.y - 15, 60, 30 );
g.setColor(Color.BLACK); // Black outline.
g.drawOval( shape.x - 30, shape.y - 15, 60, 30 );
}
}
}

/**
*  This method will be called when the user clicks the mouse on the panel.
*  If the shift key is down, the panel is cleared.  Otherwise, it adds
*  a shape to the panel and starts a drag operation.
*/
public void mousePressed(MouseEvent evt) {

if ( evt.isShiftDown() ) {
// The user was holding down the Shift key.
// Set shapeCount to 0 to record that there are no shapes.
dragging = false;
shapes.clear();
repaint();
return;
}

dragging = true;

int x = evt.getX();  // x-coordinate where user clicked.
int y = evt.getY();  // y-coordinate where user clicked.

ShapeInfo newShape;
if ( evt.isMetaDown() ) {
newShape = new ShapeInfo(x,y,false);
}
else {
newShape = new ShapeInfo(x,y,true);
}
shapes.add(newShape);
repaint();  // repaint so that user can see the new shape!

} // end mousePressed();

/**
*  This method is called when the user drags the mouse.  If a the value of the
*  instance variable dragging is true, it will add a shape to the panel.
*/
public void mouseDragged(MouseEvent evt) {
if ( dragging == false ) {
return;
}
int x = evt.getX();  // x-coordinate where user clicked.
int y = evt.getY();  // y-coordinate where user clicked.

ShapeInfo newShape;
if ( evt.isMetaDown() ) {
newShape = new ShapeInfo(x,y,false);
}
else {
newShape = new ShapeInfo(x,y,true);
}
shapes.add(newShape);

repaint();  // repaint so that user can see the new shape!
} // end mouseDragged();

// The next four empty routines are required by the MouseListener interface.
// They don't do anything in this class, so their definitions are empty.

public void mouseEntered(MouseEvent evt) { }
public void mouseExited(MouseEvent evt) { }
public void mouseClicked(MouseEvent evt) { }
public void mouseReleased(MouseEvent evt) { }

// The next routines is required by the MouseMotionListener interface.

public void mouseMoved(MouseEvent evt) { }

} // end class SimpleStamperWithArrayList

Solution for Programming Exercise 7.5

Exercise 7.5:

Write a program that will read a sequence of positive real numbers entered by the user and will print the same numbers in sorted order from smallest to largest. The user will input a zero to mark the end of the input. Assume that at most 100 positive numbers will be entered. Do not use any built-in function such as Arrays.sort(). Do the sorting yourself.

Discussion

The sample program ReverseWithDynamicArray.java from Subsection 7.2.4 reads in up to 100 positive integers from the user and outputs them in the reverse of the order in which the user entered them. This is similar to what we have to do for this exercise, except that the numbers we have to read are real numbers (of type double) and they have to be output in sorted order.

There are two basic approaches to this problem. The first is to store all the numbers in an array in the order in which they are input. After all the numbers have been input, the array can be sorted, and then the contents of the array can be output. The second approach is to always keep the array in sorted order as numbers are added to it. When a new number is input, that number must be inserted into its correct location in the array, in order to keep the array sorted. After all the numbers have been input, the contents of the array are ready to be printed.

Two solutions to the exercise, based on these two approaches, are shown below. They use techniques for sorting and inserting that were covered in Section 7.4. In my first program, I've chosen to use Selection Sort to sort the array. Insertion Sort would work just as well. The Selection Sort subroutine is taken from Section 7.4 with two changes: It sorts an array of double values instead of an array of ints, and it has been modified to work with a "partially full" array. In order to make the subroutine work with a partially full array, it is necessary to add a parameter that tells the subroutine how many entries in the array are in use. The modified Selection Sort routine is as follows, with changes from the original version shown in red italic

static void selectionSort(double[] A, int count) {
// Sort the numbers in A[0], A[1], ..., A[count-1] into
// increasing order using Selection Sort.
for ( int lastPlace = count - 1; lastPlace > 0; lastPlace-- ) {
int maxLoc = 0;
for (int j = 1; j <= lastPlace; j++) {
if (A[j] > A[maxLoc]) {
maxLoc = j;
}
}
double temp = A[maxLoc];
A[maxLoc] = A[lastPlace];
A[lastPlace] = temp;
}
} // end selectionSort

In the first version of the program, this subroutine is called just after all the numbers have been input from the user.

The second version of the program is straightforward. It uses the insert() subroutine from Section 7.4, modified to work with an array of doubles instead of an array of ints.

The Solution

First solution, with Selection Sort:

/**
* This program reads up to 100 positive integers from the user and
* prints them in sorted order.  Input ends when the user enters a
* non-positive integer.  The numbers are read and stored in an array.
* That array is sorted using selection sort, and then the array is
* printed.
*/

public class SortInputNumbers {

public static void main(String[] args) {

double[] numbers;  // An array for storing the input values.
int numCt;         // The number of numbers saved in the array.
double num;        // One of the numbers input by the user.

numbers = new double[100];   // Space for 100 numbers.
numCt = 0;                   // No numbers have been saved yet.

System.out.println("Enter up to 100 positive numbers; Enter 0 to end");

while (true) {   // Get the numbers and put them in the array.
System.out.print("? ");
num = TextIO.getlnDouble();
if (num <= 0)
break;
numbers[numCt] = num;
numCt++;
}

selectionSort(numbers, numCt);  // Sort the numbers.

System.out.println("\nYour numbers in sorted order are:\n");

for (int i = 0; i < numCt; i++) {
System.out.println( numbers[i] );
}

} // end main();

/**
* Sort the numbers in A[0], A[1], ..., A[count-1] into
* increasing order using Selection Sort.
*/
static void selectionSort(double[] A, int count) {
for ( int lastPlace = count - 1; lastPlace > 0; lastPlace-- ) {
int maxLoc = 0;
for (int j = 1; j <= lastPlace; j++) {
if (A[j] > A[maxLoc]) {
maxLoc = j;
}
}
double temp = A[maxLoc];
A[maxLoc] = A[lastPlace];
A[lastPlace] = temp;
}
} // end selectionSort

}  // end class SortInputNumbers

Second solution, with Insert:

/**
* This program reads up to 100 positive integers from the user and
* prints them in sorted order.  Input ends when the user enters a
* non-positive integer.  The numbers are read and inserted into
* an array.  The array is maintained at all times in sorted order.
*/

public class SortInputNumbers2 {

public static void main(String[] args) {

double[] numbers;  // An array for storing the input values.
int numCt;         // The number of numbers saved in the array.
double num;        // One of the numbers input by the user.

numbers = new double[100];   // Space for 100 numbers.
numCt = 0;                   // No numbers have been saved yet.

System.out.println("Enter up to 100 positive numbers; Enter 0 to end");

while (true) {   // Get the numbers and insert them into the array.
System.out.print("? ");
num = TextIO.getlnDouble();
if (num <= 0)
break;
insert(numbers, numCt, num);
numCt++;
}

System.out.println("\nYour numbers in sorted order are:\n");

for (int i = 0; i < numCt; i++) {
System.out.println( numbers[i] );
}

} // end main();

/**
* Assume that A contains itemsInArray in increasing order.
* Insert newItem into its correct position in the sorted array.
*/
static void insert(double[] A, int itemsInArray, double newItem) {
int loc = itemsInArray - 1;
while (loc >= 0 && A[loc] > newItem) {
// Move the item from A[loc] up one space.
A[loc + 1] = A[loc];
loc = loc - 1;
}
A[loc + 1] = newItem;  // Put newItem in the last vacated space.
} // end insert

}  // end class SortInputNumbers2

Solution for Programming Exercise 7.6

Exercise 7.6:

The sample program RandomArt.java from Subsection 6.4.1 shows a different random "artwork" every four seconds. There are three types of "art", one made from lines, one from circles, and one from filled squares. However, the program does not save the data for the picture that is shown on the screen. As a result, the picture cannot be redrawn when necessary. In fact, every time paintComponent() is called, a new picture is drawn.

Write a new version of RandomArt.java that saves the data needed to redraw its pictures. The paintComponent() method should simply use the data to draw the picture. New data should be recomputed only every four seconds, in response to an event from the timer that drives the program.

To make this interesting, write a separate class for each of the three different types of art. Also write an abstract class to serve as the common base class for the three classes. Since all three types of art use a random gray background, the background color can be defined in their superclass. The superclass also contains a draw() method that draws the picture; this is an abstract method because its implementation depends on the particular type of art that is being drawn. The abstract class can be defined as:

private abstract class ArtData {
Color backgroundColor;  // The background color for the art.
ArtData() {  // Constructor sets background color to be a random gray.
int x = (int)(256*Math.random());
backgroundColor = new Color( x, x, x );
}
abstract void draw(Graphics g);  // Draws this artwork.
}

Each of the three subclasses of ArtData must define its own draw() method. It must also define instance variables to hold the data necessary to draw the picture. I suggest that you should create random data for the picture in the constructor of the class, so that constructing the object will automatically create the data for the random artwork. (One problem with this is that you can't create the data until you know the size of the panel, so you can't create an ArtData object in the constructor of the panel. One solution is to create an ArtData object at the beginning of the paintComponent() method, if the object has not already been created.) In each of the three subclasses, you will need to use one or more arrays or ArrayLists to store the data.

Discussion

In my solution, I defined subclasses LineArtData, CircleArtData, and SquareArtData of the basic abstract ArtData class. An instance variable named artData, of type ArtData, points to the object that holds the data for the current picture. When it's time to create a new artwork, I call the following method, which changes the value of artData. Note that this method has an equal chance of producing each of the three types of art:

/**
* Creates an object belonging to one of the three subclasses of
* ArtData, and assigns that object to the instance variable, artData.
* The subclass to use (that is, the type of art) is chosen at random.
*/
private void createArtData() {
switch ( (int)(3*Math.random()) ) {
case 0:
artData = new LineArtData();
break;
case 1:
artData = new CircleArtData();
break;
case 2:
artData = new SquareArtData();
break;
}
}

This method is called by the action listener that responds to events from the timer, so a new artwork is created every time the timer generates an event, that is, every four seconds.

The paintComponent() method uses the background color from artData to fill the drawing area. It then tells artData to draw itself by calling its draw() method. As suggested in the exercise, it first makes sure that an art data object has been created by calling createArtData() if artData is still null; this can only be true the first time paintComponent() is called. So, the paintComponent() method is quite simple:

public void paintComponent(Graphics g) {

if (artData == null)  // If no artdata has yet been created, create it.
createArtData();

// Note:  Since the next two lines fill the entire panel, there is
// no need to call super.paintComponent(g), since any drawing
// that it does will only be covered up anyway.

g.setColor(artData.backgroundColor); // Fill with the art's background color.
g.fillRect( 0, 0, getWidth(), getHeight() );

artData.draw(g);  // Draw the art.

} // end paintComponent()

Note in particular the last line, artData.draw(g). This is a polymorphic method, since what gets drawn will depend on what class the object artData belongs to, and that changes as the program runs.

The only thing that remains for discussion is the three classes that define the three types of art. Note that we have been able to get this far without thinking about creating or drawing the actual art. This is because we have been thinking "abstractly," in terms of the abstract class.

Let's look in detail at LineArtData, one of the three concrete subclasses of ArtData. In the original program, line art was drawn in the paintComponent() method as follows:

for (int i = 0; i < 500; i++) {
int x1 = (int)(getWidth() * Math.random());
int y1 = (int)(getHeight() * Math.random());
int x2 = (int)(getWidth() * Math.random());
int y2 = (int)(getHeight() * Math.random());
Color randomHue = Color.getHSBColor( (float)Math.random(), 1.0F, 1.0F);
g.setColor(randomHue);
g.drawLine(x1,y1,x2,y2);
}

Here, the coordinates and colors for each line are chosen at random and the line is immediately drawn using that data. For the new version, we want to save the data in instance variables so that the picture can be drawn and redrawn on demand. The creation of the data will be split from the drawing; the data will be created in the constructor and will be used to draw the picture in the draw() method. To save the data for all 500 lines, we have to store the coordinates and colors in arrays. There are several ways to do this, but I used one array for each piece of data, x1, y1, x2, y2, and the color. This is an example of parallel arrays. To store data for 500 lines, each array should be of length 500. For example, x1 is declared as an instance variable of type int[], and the array is created with the command "x1 = new int[500];". The arrays are created in the LineArtData constructor and are filled with random data. In the draw() method, the i-th line can be drawn with the commands:

g.setColor( color[i] );    // Use the i-th color in the array
g.drawLine( x1[i], y1[i], x2[i], y2[i] );

We just need a for loop to draw all the lines. The complete definition of LineArtData class is as follows:

/**
* Stores data for a picture that contains 500 random lines drawn in
* different random colors.
*/
private class LineArtData extends ArtData {
Color[] color;         // color[i] is the color of line number i
int[] x1, y1, x2, y2;  // line i goes from (x1[i],y1[i]) to (x2[i],y2[i]).
LineArtData() {  // Constructor creates arrays and fills then randomly.
color = new Color[500];
x1 = new int[500];
y1 = new int[500];
x2 = new int[500];
y2 = new int[500];
for (int i = 0; i < 500; i++) {
x1[i] = (int)(getWidth() * Math.random());
y1[i] = (int)(getHeight() * Math.random());
x2[i] = (int)(getWidth() * Math.random());
y2[i] = (int)(getHeight() * Math.random());
color[i] = Color.getHSBColor( (float)Math.random(), 1.0F, 1.0F);
}
}
void draw(Graphics g) {  // Draw the picture.
for (int i = 0; i < 500; i++) {
g.setColor(color[i]);
g.drawLine( x1[i], y1[i], x2[i], y2[i] );
}
}
}

This is, by the way, a non-static nested class in the RandomArtPanel class. It can't be static because it uses the instance methods getWidth() and getHeight() from the containing class.

The other two classes can be approached in a similar way, but to mix things up a bit, I decided to use an ArrayList of objects, instead of parallel arrays, for the other two classes. For the circles, I created a class OneCircle to hold the data for one circle, and I used an ArrayList<OneCircle> to hold data for 100 circles. (An array of OneCircle would have worked just as well.) You can see the result in the complete solution below. Note, by the way, that OneCircle is a nested class that is inside another nested class. Java allows multiple levels of nesting.

The Solution

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import java.util.ArrayList;

/**
* A RandomArtPanel draws random pictures which might be taken to have
* some vague resemblance to abstract art.  A new picture is produced every
* four seconds.  There are three types of pictures:  random lines,
* random circles, and random 3D rects.  The figures are drawn in
* random colors on a background that is a random shade of gray.  The
* data for a given piece of art is stored in a data structure so
* that the picture can be redrawn if necessary.  The data is created
* in response to the action event from a timer.
*/
public class RandomArt2 extends JPanel {

/**
* A main routine to make it possible to run this program as an application.
*/
public static void main(String[] args) {
JFrame window = new JFrame("Random Art ??");
RandomArt2 content = new RandomArt2();
window.setContentPane(content);
window.setSize(400,400);
window.setLocation(100,100);
window.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
window.setVisible(true);
}

//---------------------------------------------------------------------
/**
* The data for the currently displayed picture (unless it is null).
*/
private ArtData artData;

/**
* The constructor creates a timer with a delay time of four seconds
* (4000 milliseconds), and with a RepaintAction object as its
* ActionListener.  It also starts the timer running.  The
* RepaintAction class is a nested class, defined below.
*/
public RandomArt2() {
RepaintAction action = new RepaintAction();
Timer timer = new Timer(4000, action);
timer.start();
}

/**
* The paintComponent() method fills the panel with a random shade of
* gray and then draws one of three types of random "art".  The data for
* the picture is in the variable artData (if this variable is null,
* then an artData object is created here).
*/
public void paintComponent(Graphics g) {

if (artData == null)  // If no artdata has yet been created, create it.
createArtData();

// Note:  Since the next two lines fill the entire panel, there is
// no need to call super.paintComponent(g), since any drawing
// that it does will only be covered up anyway.

g.setColor(artData.backgroundColor); // Fill with the art's background color.
g.fillRect( 0, 0, getWidth(), getHeight() );

artData.draw(g);  // Draw the art.

} // end paintComponent()

/**
* Creates an object belonging to one of the three subclasses of
* ArtData, and assigns that object to the instance variable, artData.
* The subclass to use (that is, the type of art) is chosen at random.
*/
private void createArtData() {
switch ( (int)(3*Math.random()) ) {
case 0:
artData = new LineArtData();
break;
case 1:
artData = new CircleArtData();
break;
case 2:
artData = new SquareArtData();
break;
}
}

/**
* An abstract class that represents the data for a random work
* of "art".  Different concrete subclasses of this class represent
* different types of art.  This class contains a background
* color which is a random shade of gray, selected when the object
* is constructed.
*/
private abstract class ArtData {
Color backgroundColor;  // The background color for the art.
ArtData() {  // Constructor sets background color to be a random shade of gray.
int x = (int)(256*Math.random());
backgroundColor = new Color( x, x, x );
}
abstract void draw(Graphics g);  // Draw the picture.
}

/**
* Stores data for a picture that contains 500 random lines drawn in
* different random colors.
*/
private class LineArtData extends ArtData {

Color[] color;         // color[i] is the color of line number i
int[] x1, y1, x2, y2;  // line i goes from (x1[i],y1[i]) to (x2[i],y2[i]).
LineArtData() {  // Constructor creates arrays and fills then randomly.
color = new Color[500];
x1 = new int[500];
y1 = new int[500];
x2 = new int[500];
y2 = new int[500];
for (int i = 0; i < 500; i++) {
x1[i] = (int)(getWidth() * Math.random());
y1[i] = (int)(getHeight() * Math.random());
x2[i] = (int)(getWidth() * Math.random());
y2[i] = (int)(getHeight() * Math.random());
color[i] = Color.getHSBColor( (float)Math.random(), 1.0F, 1.0F);
}
}
void draw(Graphics g) {  // Draw the picture.
for (int i = 0; i < 500; i++) {
g.setColor(color[i]);
g.drawLine( x1[i], y1[i], x2[i], y2[i] );
}
}
}

/**
* Stores data for a picture that contains 200 circles with
* radius 50, with random centers, and drawn in random colors.
*/
private class CircleArtData extends ArtData {
class OneCircle {
Color color;  // the color of the th circle
int centerX;  // center of circle is at (centerX, centerY)
int centerY;
}
ArrayList<OneCircle> circles;
CircleArtData() {  // Constructor creates arrays and fills then randomly.
circles = new ArrayList<OneCircle>();
for (int i = 0; i < 200; i++) {
OneCircle c = new OneCircle();
c.centerX =  (int)(getWidth() * Math.random());
c.centerY = (int)(getHeight() * Math.random());
c.color = Color.getHSBColor( (float)Math.random(), 1.0F, 1.0F);
circles.add(c);
}
}
void draw(Graphics g) {  // Draw the picture.
for (OneCircle circle : circles) {
g.setColor(circle.color);
g.drawOval(circle.centerX - 50, circle.centerY - 50, 100, 100);
}
}
}

/**
* Stores data for a picture that contains 25 filled squares with
* random sizes and  with random centers, and drawn in random colors.
*/
private class SquareArtData extends ArtData {
class OneSquare {
Color color;  // the color of the square
int centerX;  // the center of square is (centerX, centerY)
int centerY;
int size;     // the length of a side of the square
}
ArrayList<OneSquare> squares = new ArrayList<OneSquare>();
SquareArtData() {  // Constructor creates arrays and fills then randomly.
for (int i = 0; i < 25; i++) {
OneSquare s = new OneSquare();
s.centerX =  (int)(getWidth() * Math.random());
s.centerY = (int)(getHeight() * Math.random());
s.size = 30 + (int)(170*Math.random());
s.color = new Color( (int)(256*Math.random()),
(int)(256*Math.random()), (int)(256*Math.random()) );
squares.add(s);
}
}
void draw(Graphics g) {  // Draw the picture.
for ( OneSquare square : squares ) {
g.setColor(square.color);
g.fill3DRect(square.centerX - square.size/2, square.centerY - square.size/2,
square.size, square.size, true);
}
}
}

/**
* A RepaintAction object creates a new artData object and calls the repaint
* method of this panel each time its actionPerformed() method is called.
* An object of this type is used as an action listener for a Timer that
* generates an ActionEvent every four seconds.  The result is a new work of
* art every four seconds.
*/
private class RepaintAction implements ActionListener {
public void actionPerformed(ActionEvent evt) {
createArtData();
repaint();
}
}

} // end class RandomArt2

Solution for Programming Exercise 7.7

Exercise 7.7:

Write a program that will read a text file selected by the user, and will make an alphabetical list of all the different words in that file. All words should be converted to lower case, and duplicates should be eliminated from the list. The list should be written to an output file selected by the user. As discussed in Subsection 2.4.4, you can use TextIO to read and write files. Use a variable of type ArrayList<String> to store the words. It is not easy to separate a file into words as you are reading it. You can use the following method:

/**
* Read the next word from TextIO, if there is one.  First, skip past
* any non-letters in the input.  If an end-of-file is encountered before
* a word is found, return null.  Otherwise, read and return the word.
* A word is defined as a sequence of letters.  Also, a word can include
* an apostrophe if the apostrophe is surrounded by letters on each side.
* @return the next word from TextIO, or null if an end-of-file is
*     encountered
*/
private static String readNextWord() {
char ch = TextIO.peek(); // Look at next character in input.
while (ch != TextIO.EOF && ! Character.isLetter(ch)) {
// Skip past non-letters.
TextIO.getAnyChar();  // Read the character.
ch = TextIO.peek();   // Look at the next character.
}
if (ch == TextIO.EOF) // Encountered end-of-file
return null;
// At this point, we know the next character is a letter, so read a word.
String word = "";  // This will be the word that is read.
while (true) {
word += TextIO.getAnyChar();  // Append the letter onto word.
ch = TextIO.peek();  // Look at next character.
if ( ch == '\'' ) {
// The next character is an apostrophe.  Read it, and
// if the following character is a letter, add both the
// apostrophe and the letter onto the word and continue
// reading the word.  If the character after the apostrophe
// is not a letter, the word is done, so break out of the loop.
TextIO.getAnyChar();   // Read the apostrophe.
ch = TextIO.peek();    // Look at char that follows apostrophe.
if (Character.isLetter(ch)) {
word += "\'" + TextIO.getAnyChar();
ch = TextIO.peek();  // Look at next char.
}
else
break;
}
if ( ! Character.isLetter(ch) ) {
// If the next character is not a letter, the word is
// finished, so break out of the loop.
break;
}
// If we haven't broken out of the loop, next char is a letter.
}
return word;  // Return the word that has been read.
}

Note that this method will return null when the file has been entirely read. You can use this as a signal to stop processing the input file.

Discussion

This is actually not a very difficult program to write. The main point of the exercise is to get you to use a list of strings and to do something with files.

There are several possible approaches to this problem. One approach is to simply dump all the words from the file into a list, without worrying about eliminating duplicates or keeping the list in order. After the file has been read, the list can be sorted and printed. Although the list can contain duplicates, the output file should list each word only once. However, it's easy to leave out the duplicates as the file is being written. If wordList is the variable of type ArrayList<String> that holds the already sorted list of words, then the following code will output the list without duplicates. The idea is that a word is written only if it is different from the previous word in the list. Word number 0 is a special case, because there is no previous location in the list in that case:

for (int i = 0; i < wordlist.size(); i++) {
if (i == 0 || ! wordlist.get(i).equals(wordList.get(i-1) )
System.out.println(wordlist.get(i));
}

A second approach to the problem would be to keep the list of words in sorted order as it is being constructed. This can be done by applying the insert() routine from Subsection 7.4.3 to insert each word into the list.

A third approach, and the one that I use in my solution, is to eliminate duplicates from the list as it being constructed. Each time a word is read from the input file, I first check whether the word is already in the list. If so, I discard it; if not, I add it to the end of the list. The function wordList.indexOf(word) can be used to test whether a given word is already in the list; this function returns the value -1 if word is not in the list. After the input file has been read, the list contains one copy of each word that was found in the file. At this point the list still has to be sorted. I use a selection sort algorithm (Subsection 7.4.4), adapted to work for an ArrayList, to do the sorting. (Another option would be to copy the strings into an array, and use Arrays.sort() to sort the array. A third would be to discover that there is a standard method Collections.sort(arraylist), which can be used to sort the ArrayList directly.) Then, all the elements of the list are output using a for-each loop:

for (String w : wordList)
TextIO.putln("   " + w);

To let the user select the input and the output files, I use the methods TextIO.readUserSelectedFile() and TextIO.writeUserSelectedFile(), which are discussed in Subsection 2.4.4. These methods put up a file dialog where the user can select a file. After the user selects an input file, TextIO reads from that file instead of from the user's input. After the user selects an output file, TextIO writes to that file instead of to standard output. If the user cancels the input file dialog, then there is no input file to process, so I exit the program. If the user cancels the output file dialog, I write the list of words anyway -- it will go to standard output so the user will see it on the screen.

When TextIO is working with files and an error occurs, it will generate an error of type IllegalArgumentException. My program catches the error if one occurs, and it prints an error message. An error is not very likely in this case but one could occur if, for example, the user selects an input file that the user does not have permission to read.

The complete code for my solution is shown below.

The Solution

import java.util.ArrayList;

/**
* Makes an alphabetical list of all the words in a file selected
* by the user.  The list can be written to a file.
*/
public class ListAllWordsFromFile {

public static void main(String[] args) {

System.out.println("\n\nThis program will ask you to select an input file");
System.out.println("It will read that file and make an alphabetical");
System.out.println("list of all the words in the file.  After reading");
System.out.println("the file, the program asks you to select an output");
System.out.println("file.  If you select a file, the list of words will");
System.out.println("be written to that file; if you cancel, the list");
System.out.println("be written to standard output.  All words are converted");
System.out.println("lower case, and duplicates are eliminated from the list.\n\n");
System.out.print("Press return to begin.");
TextIO.getln();  // Wait for user to press return.

try {
if (TextIO.readUserSelectedFile() == false) {
System.out.println("No input file selected.  Exiting.");
System.exit(1);
}
ArrayList<String> wordList = new ArrayList<String>();
String word = readNextWord();
while (word != null) {
word = word.toLowerCase();  // convert word to lower case
if ( wordList.indexOf(word) == -1 ) {
// This is a new word, so add it to the list
wordList.add(word);
}
word = readNextWord();
}
System.out.println("Number of different words found in file:  "
+ wordList.size());
System.out.println();
if (wordList.size() == 0) {
System.out.println("No words found in file.");
System.out.println("Exiting without saving data.");
System.exit(0);
}
selectionSort(wordList);
TextIO.writeUserSelectedFile(); // If user cancels, output automatically
// goes to standard output.
TextIO.putln(wordList.size() + " words found in file:\n");
for (String w : wordList)
TextIO.putln("   " + w);
System.out.println("\n\nDone.\n\n");
}
catch (Exception e) {
System.out.println("Sorry, an error has occurred.");
System.out.println("Error Message:  " + e.getMessage());
}
System.exit(0);  // Might be necessary, because of use of file dialogs.
}

/**
* Sorts a list of strings into lexicographical order, using
* selection sort and treating the list much like an array.  In this
* program, the list only contains words made up of lower case
* letters, so lexicographic order is the same as alphabetical order.
*/
private static void selectionSort(ArrayList<String> list) {
for (int top = list.size() - 1; top > 0; top--) {
int indexOfBiggest = 0;
for (int j = 0; j < top; j++) {
String str = list.get(j);
if (str.compareTo( list.get(indexOfBiggest) ) > 0) {
indexOfBiggest = j;
}
}
String temp = list.get(top);
list.set( top, list.get(indexOfBiggest) );
list.set( indexOfBiggest, temp );
}
}

/**
* Read the next word from TextIO, if there is one.  First, skip past
* any non-letters in the input.  If an end-of-file is encountered before
* a word is found, return null.  Otherwise, read and return the word.
* A word is defined as a sequence of letters.  Also, a word can include
* an apostrophe if the apostrophe is surrounded by letters on each side.
* @return the next word from TextIO, or null if an end-of-file is encountered
*/
private static String readNextWord() {
char ch = TextIO.peek(); // Look at next character in input.
while (ch != TextIO.EOF && ! Character.isLetter(ch)) {
// Skip past non-letters.
TextIO.getAnyChar();  // Read the character.
ch = TextIO.peek();   // Look at the next character.
}
if (ch == TextIO.EOF) // Encountered end-of-file
return null;
// At this point, we know that the next character, so read a word.
String word = "";  // This will be the word that is read.
while (true) {
word += TextIO.getAnyChar();  // Append the letter onto word.
ch = TextIO.peek();  // Look at next character.
if ( ch == '\'' ) {
// The next character is an apostrophe.  Read it, and
// if the following character is a letter, add both the
// apostrophe and the letter onto the word and continue
// reading the word.  If the character after the apostrophe
// is not a letter, the word is done, so break out of the loop.
TextIO.getAnyChar();   // Read the apostrophe.
ch = TextIO.peek();    // Look at char that follows apostrophe.
if (Character.isLetter(ch)) {
word += "\'" + TextIO.getAnyChar();
ch = TextIO.peek();  // Look at next char.
}
else
break;
}
if ( ! Character.isLetter(ch) ) {
// If the next character is not a letter, the word is
// finished, so break out of the loop.
break;
}
// If we haven't broken out of the loop, next char is a letter.
}
return word;  // Return the word that has been read.
}

} // end ListAllWordsFromFile

Solution for Programming Exercise 7.8

Exercise 7.8:

The game of Go Moku (also known as Pente or Five Stones) is similar to Tic-Tac-Toe, except that it is played on a much larger board and the object is to get five squares in a row rather than three. Players take turns placing pieces on a board. A piece can be placed in any empty square. The first player to get five pieces in a row -- horizontally, vertically, or diagonally -- wins. If all squares are filled before either player wins, then the game is a draw. Write a program that lets two players play Go Moku against each other.

Your program will be simpler than the Checkers program from Subsection 7.5.3. Play alternates strictly between the two players, and there is no need to highlight the legal moves. You will only need two classes, a short panel class to set up the interface and a Board class to draw the board and do all the work of the game. Nevertheless, you will probably want to look at the source code for the checkers program, Checkers.java, for ideas about the general outline of the program.

The hardest part of the program is checking whether the move that a player makes is a winning move. To do this, you have to look in each of the four possible directions from the square where the user has placed a piece. You have to count how many pieces that player has in a row in that direction. If the number is five or more in any direction, then that player wins. As a hint, here is part of the code from my program. This code counts the number of pieces that the user has in a row in a specified direction. The direction is specified by two integers, dirX and dirY. The values of these variables are 0, 1, or -1, and at least one of them is non-zero. For example, to look in the horizontal direction, dirX is 1 and dirY is 0.

int ct = 1;  // Number of pieces in a row belonging to the player.

int r, c;    // A row and column to be examined

r = row + dirX;  // Look at square in specified direction.
c = col + dirY;
while ( r >= 0 && r < 13 && c >= 0 && c < 13
&& board[r][c] == player ) {
// Square is on the board, and it
// contains one of the players' pieces.
ct++;
r += dirX;  // Go on to next square in this direction.
c += dirY;
}

r = row - dirX;  // Now, look in the opposite direction.
c = col - dirY;
while ( r >= 0 && r < 13 && c >= 0 && c < 13
&& board[r][c] == player ) {
ct++;
r -= dirX;   // Go on to next square in this direction.
c -= dirY;
}

Here is a picture of my program, just after black has won the game.

Discussion

This is a fairly complicated program, but it's possible to design and build it in stages, testing each stage separately. The first stage, the general layout of the panel, is already done in the Checkers.java program. With just a few changes, the main panel class, the layout of the panel, and the button and message handling come directly from that program. Let's take the rest of the Go Moku game one stage at a time.

A two-dimensional array is used to store the contents of the board. This array is of type int[][] and is named board. It is defined as an instance variable in the Board class, and it is initialized in the constructor of that class to be a 13-by-13 array. The value in each position of the array is one of three constants: EMPTY, WHITE, or BLACK. When a game begins, each of the entries in the array is set to empty. When a player clicks on an empty square, the corresponding entry in the array is changed from EMPTY to the player's color, BLACK or WHITE. In the paintComponent() method, the contents of the board array are used to decide what pieces to draw on the board.

Drawing the Board

We need a paintComponent() method for the Board class that can draw the board. The board has 13 rows and 13 columns of spaces. How wide should the board be? If each square in the board is x pixels wide, we need a total of 13*x pixels just for the spaces. But there are also lines between the spaces. These require another 12 pixels. And there is a 2-pixel border on each side, for another 4 pixels added to the width. So, with squares of side x, we need a board that is 13*x+16 pixels wide. Since I wanted something about the same size as the original checkerboard, I choose x to be 12, giving a board width of 172 pixels. The height is also 172. The dimension of the board is actually set in the constructor for the main class, which uses a null layout and so has to set the sizes of all the components that it contains by hand.

The left edge of the col-th column of squares in the board is 2+13*col. This allows for the two-pixel border on the left and for 13 pixels for each of the preceding columns of squares. (That's 12 pixels for the square plus one pixel for the line between that column and the next.) The lines between the columns are drawn one pixel to the left of each column, at x values 1 + 13*i, for i from 1 to 12. Rows work the same way. To draw a piece in row number row and column number col, the command

g.fillOval(3 + 13*col, 3 + 13*row, 10, 10);

can be used. This allows a one-pixel border between the oval that represents that piece and the side of the square. In my program, I defined a method to draw a piece:

private void drawPiece(Graphics g, int piece, int row, int col) {
if (piece == WHITE)
g.setColor(Color.WHITE);
else
g.setColor(Color.BLACK);
g.fillOval(3 + 13*col, 3 + 13*row, 10, 10);
}

The background of the canvas is gray, so the paintComponent() method only has to draw the black border around the board, the lines between the squares, and all the pieces on the board. Remember that the piece in a given row and column is recorded in the board array as board[row][col]. The board can be drawn by the following paintComponent() method:

public void paintComponent(Graphics g) {

super.paintComponent(g); // Fill with background color, lightGray.

/* Draw grid lines in dark gray.  */

g.setColor(Color.DARK_GRAY);
for (int i = 1; i < 13; i++) {
g.drawLine(1 + 13*i, 0, 1 + 13*i, getSize().height);
g.drawLine(0, 1 + 13*i, getSize().width, 1 + 13*i);
}

/* Draw a two-pixel black border around the edges of the board. */

g.setColor(Color.BLACK;
g.drawRect(0,0,getSize().width-1,getSize().height-1);
g.drawRect(1,1,getSize().width-3,getSize().height-3);

/* Draw the pieces that are on the board. */

for (int row = 0; row < 13; row++)
for (int col = 0; col < 13; col++)
if (board[row][col] != EMPTY)
drawPiece(g, board[row][col], row, col);

}  // end paintComponent()

Playing the Game

The logic of the GoMoku game itself is mostly in the method "void doClickSquare(int row, int col)", which is called by the mousePressed() method when the user clicks on the square in row number row and column number col. This method must check whether the move is legal. If so, the move is made. The method then checks whether the move wins the game. If so, the game ends. The game will also end if the board has become completely full. Otherwise, play passes to the other player.

The current player is recorded in an instance variable named currentPlayer. The value of this variable is one of the two constants WHITE or BLACK. The game can be ended by calling a method named gameOver(). I wrote a boolean-valued method called winner() to check whether a move wins the game. (When I first wrote this method, it did nothing but "return false". This let me try out the program at this stage of development, before I started working on the difficult problem of testing for a winner.) The doClickSquare() method can be written:

void doClickSquare(int row, int col) {
// This is called by mousePressed() when a player clicks
// on the square in the specified row and col.  It has already
// been checked that a game is, in fact, in progress.

/* Check that the user clicked an empty square.  If not, show an
error message and exit. */

if ( board[row][col] != EMPTY ) {
if (currentPlayer == BLACK)
message.setText("BLACK:  Please click an empty square.");
else
message.setText("WHITE:  Please click an empty square.");
return;
}

/* Make the move.  Check if the board is full or if the move
is a winning move.  If so, the game ends.  If not, then it's
the other user's turn. */

board[row][col] = currentPlayer;  // Make the move.
repaint();

if (winner(row,col)) {  // First, check for a winner.
if (currentPlayer == WHITE)
gameOver("WHITE wins the game!");
else
gameOver("BLACK wins the game!");
return;
}

boolean emptySpace = false;     // Check if the board is full.
for (int i = 0; i < 13; i++)
for (int j = 0; j < 13; j++)
if (board[i][j] == EMPTY)
emptySpace = true;  // The board contains an empty space.
if (emptySpace == false) {
gameOver("The game ends in a draw.");
return;
}

/* Continue the game.  It's the other player's turn. */

if (currentPlayer == BLACK) {
currentPlayer = WHITE;
message.setText("WHITE:  Make your move.");
}
else {
currentPlayer = BLACK;
message.setText("BLACK:  Make your move.");
}

}  // end doClickSquare()

Determining the Winner

The winner() method is certainly the hardest part of the program. The method must look in each of the four possible directions from the square where the user has placed a piece. If the player has five or more pieces in a row in that direction, then the player has won. As indicated in the exercise, a direction can be indicated by two variables, dirX and dirY. The values of these variables for each of the four directions are:

dirX    dirY    Why?
----    ----    --------------------------------
horizontal direction    1       0       Only x changes.
vertical direction      0       1       Only y changes.
falling diagonal        1       1       Both x and y change.
rising diagonal         1      -1       Change in opposing directions.

I wrote a method "int count(int player, int row, int col, int dirX, int dirY) that counts the number of pieces the specified player has in a row, starting from the square in row number row and column number col and looking in the direction indicated by dirX and dirY. This method contains the code given in the exercise. It returns the number of pieces found. My winner method just calls this method for each of the four directions:

/**
* This is called just after a piece has been played on the
* square in the specified row and column.  It determines
* whether that was a winning move by counting the number
* of squares in a line in each of the four possible
* directions from (row,col).  If there are 5 squares (or more)
* in a row in any direction, then the game is won.
*/
private boolean winner(int row, int col) {

if (count( board[row][col], row, col, 1, 0 ) >= 5)
return true;
if (count( board[row][col], row, col, 0, 1 ) >= 5)
return true;
if (count( board[row][col], row, col, 1, -1 ) >= 5)
return true;
if (count( board[row][col], row, col, 1, 1 ) >= 5)
return true;

/* When we get to this point, we know that the game is not won. */

return false;

}  // end winner()

When I first wrote this method, I checked whether the number of pieces was "== 5" instead of ">= 5". This was a bug. It's possible for a player to get more than 5 pieces in a row, if the player plays a piece in an empty square that joins two shorter rows of pieces together.

Marking the Winning Pieces

In my program, when a player wins, the row of pieces that wins the game is marked with a red line. To do this, I added four instance variables of type int to the Board class. The instance variables are named win_r1, win_c1, win_r2, and win_c2. If the game has not yet been won, then the value of win_r1 is -1. The paintComponent() method uses this value as a signal that it should not draw any red line. After a player has won the game, the values of these variables are set to mark the squares at the two ends of the winning row of pieces. The positions of these squares are given by (win_r1,win_c1) and (win_r2,win_c2). If win_r1 is greater than -1, then the paintComponent() method draws a red line between these two squares.

I added some code to the winner() and count() methods to set the values of these variables properly. As the count() method counts pieces in a row, it sets win_r1, win_c1, win_r2, and win_c2 to mark the location of the last piece it finds in the two directions it checks. If the game is won, this will set the values correctly. To handle the case where the game is not won, I added the line "win_r1 = -1;" to the winner() method, just before the "return false;" statement. This ensures that the value of this variable will be -1 whenever the game is not yet won. (Perhaps this is all too tricky, but I really wanted to mark the winning pieces...)

The Solution

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

/**
* This panel lets two users play Go Moku (a.k.a Pente) against each
* other.  Black always starts the game.  When a player gets five-in-a-row,
* that player wins.  The game ends in a draw if the board is filled
* before either player wins.
*
* The class has a main() routine that lets it be run as a stand-alone
* application.  The application just opens a window that uses an object
* of type GoMoku as its content pane.
*
*/

public class GoMoku extends JPanel {

/**
* Main routine makes it possible to run GoMoku as a stand-alone
* application.  Opens a window showing a GoMoku panel; the program
* ends when the user closes the window.
*/
public static void main(String[] args) {
JFrame window = new JFrame("GoMoku");
GoMoku content = new GoMoku();
window.setContentPane(content);
window.pack();
Dimension screensize = Toolkit.getDefaultToolkit().getScreenSize();
window.setLocation( (screensize.width - window.getWidth())/2,
(screensize.height - window.getHeight())/2 );
window.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
window.setResizable(false);
window.setVisible(true);
}

private JButton newGameButton;  // Button for starting a new game.

private JButton resignButton;   // Button that a player can use to end the
//   game by resigning.

private JLabel message;  // Label for displaying messages to the user.

/**
*  The constructor lays out the panel.  The work of
*  the game is all done in the Board object.  A null layout
*  is used, and all setup of sizes and positions is done here.
*/
public GoMoku() {

setLayout(null);  // I will do the layout myself.

setPreferredSize( new Dimension(350,250) );

setBackground(new Color(0,150,0));  // Dark green background.

/* Create the components and add them to the panel. */

Board board = new Board();  // Note: The constructor for the
//   board also creates the buttons
//   and label.
add(board);
add(newGameButton);
add(resignButton);
add(message);

/* Set the position and size of each component by calling
its setBounds() method. */

board.setBounds(16,16,172,172); // Note:  size MUST be 172-by-172 !
newGameButton.setBounds(210, 60, 120, 30);
resignButton.setBounds(210, 120, 120, 30);
message.setBounds(0, 200, 350, 30);
}

// ----------------------- Nested class -----------------------------------

/**
* This panel displays a 168-by-168 pixel checkerboard pattern with
* a 2-pixel black border.  It is assumed that the size of the
* canvas is set to exactly 172-by-172 pixels.  This class does
* the work of letting the users play Go Moku, and it displays
* the checkerboard.  In this program, the board has 13 rows and
* columns of squares.
*/
class Board extends JPanel implements ActionListener, MouseListener {

int[][] board;   // The data for the board is kept here.  The values
//   in this array are chosen from the following constants.

static final int EMPTY = 0,       // Represents an empty square.
WHITE = 1,       // A white piece.
BLACK = 2;       // A black piece.

boolean gameInProgress; // Is a game currently in progress?

int currentPlayer;      // Whose turn is it now?  The possible values
//    are WHITE and BLACK.  (This is valid only while
//    a game is in progress.)

int win_r1, win_c1, win_r2, win_c2;  // When a player wins by getting five or more
// pieces in a row, the squares at the
// ends of the row are (win_r1,win_c1)
// and (win_r2,win_c2).  A red line is
// drawn between these squares.  When there
// are not five pieces in a row, the value of
// win_r1 is -1.  The values are set in the
// count() method.  The value of win_r1 is
// tested in the paintComponent() method.

/**
* Constructor.  Create the buttons and label.  Listen for mouse
* clicks and for clicks on the buttons.  Create the board and
* start the first game.
*/
public Board() {
setBackground(Color.LIGHT_GRAY);
addMouseListener(this);
resignButton = new JButton("Resign");
resignButton.addActionListener(this);
newGameButton = new JButton("New Game");
newGameButton.addActionListener(this);
message = new JLabel("",JLabel.CENTER);
message.setFont(new  Font("Serif", Font.BOLD, 14));
message.setForeground(Color.GREEN);
board = new int[13][13];
doNewGame();
}

/**
* Respond to user's click on one of the two buttons.
*/
public void actionPerformed(ActionEvent evt) {
Object src = evt.getSource();
if (src == newGameButton)
doNewGame();
else if (src == resignButton)
doResign();
}

/**
* Begin a new game; this is called by the actionPerformed()
* method when a user clicks the New Game button.
*/
void doNewGame() {
if (gameInProgress == true) {
// This should not be possible because New Game button
// is enabled only when it is legal to use it, but it doesn't
// hurt to check.
message.setText("Finish the current game first!");
return;
}
for (int row = 0; row < 13; row++)       // Fill the board with EMPTYs
for (int col = 0; col < 13; col++)
board[row][col] = EMPTY;
currentPlayer = BLACK;   // BLACK moves first.
message.setText("BLACK:  Make your move.");
gameInProgress = true;
newGameButton.setEnabled(false);
resignButton.setEnabled(true);
win_r1 = -1;  // This value indicates that no red line is to be drawn.
repaint();
}

/**
* Current player resigns; this is called by the actionPerformed()
* method when a user clicks the Resign button.  Game ends, and
* opponent wins.
*/
void doResign() {
if (gameInProgress == false) {
// This should not be possible.
message.setText("There is no game in progress!");
return;
}
if (currentPlayer == WHITE)
message.setText("WHITE resigns.  BLACK wins.");
else
message.setText("BLACK resigns.  WHITE wins.");
newGameButton.setEnabled(true);
resignButton.setEnabled(false);
gameInProgress = false;
}

/**
* This method is called whenever the game ends.  The parameter, str,
* is displayed as a message, and the buttons are enabled/disabled
* to reflect the fact that a game is not currently in progress.
*/
void gameOver(String str) {
message.setText(str);
newGameButton.setEnabled(true);
resignButton.setEnabled(false);
gameInProgress = false;
}

/**
* This is called by mousePressed() when a player clicks on the
* square in the specified row and col.  It has already been checked
* that a game is, in fact, in progress.
*/
void doClickSquare(int row, int col) {

/* Check that the user clicked an empty square.  If not, show an
error message and exit. */

if ( board[row][col] != EMPTY ) {
if (currentPlayer == BLACK)
message.setText("BLACK:  Please click an empty square.");
else
message.setText("WHITE:  Please click an empty square.");
return;
}

/* Make the move.  Check if the board is full or if the move
is a winning move.  If so, the game ends.  If not, then it's
the other user's turn. */

board[row][col] = currentPlayer;  // Make the move.
repaint();

if (winner(row,col)) {  // First, check for a winner.
if (currentPlayer == WHITE)
gameOver("WHITE wins the game!");
else
gameOver("BLACK wins the game!");
return;
}

boolean emptySpace = false;     // Check if the board is full.
for (int i = 0; i < 13; i++)
for (int j = 0; j < 13; j++)
if (board[i][j] == EMPTY)
emptySpace = true;
if (emptySpace == false) {
gameOver("The game ends in a draw.");
return;
}

/* Continue the game.  It's the other player's turn. */

if (currentPlayer == BLACK) {
currentPlayer = WHITE;
message.setText("WHITE:  Make your move.");
}
else {
currentPlayer = BLACK;
message.setText("BLACK:  Make your move.");
}

}  // end doClickSquare()

/**
* This is called just after a piece has been played on the
* square in the specified row and column.  It determines
* whether that was a winning move by counting the number
* of squares in a line in each of the four possible
* directions from (row,col).  If there are 5 squares (or more)
* in a row in any direction, then the game is won.
*/
private boolean winner(int row, int col) {

if (count( board[row][col], row, col, 1, 0 ) >= 5)
return true;
if (count( board[row][col], row, col, 0, 1 ) >= 5)
return true;
if (count( board[row][col], row, col, 1, -1 ) >= 5)
return true;
if (count( board[row][col], row, col, 1, 1 ) >= 5)
return true;

/* When we get to this point, we know that the game is not
won.  The value of win_r1, which was changed in the count()
method, has to be reset to -1, to avoid drawing a red line
on the board. */

win_r1 = -1;
return false;

}  // end winner()

/**
* Counts the number of the specified player's pieces starting at
* square (row,col) and extending along the direction specified by
* (dirX,dirY).  It is assumed that the player has a piece at
* (row,col).  This method looks at the squares (row + dirX, col + dirY),
* (row + 2*dirX, col + 2*dirY), ... until it hits a square that is
* off the board or is not occupied by one of the player's pieces.
* It counts the squares that are occupied by the player's pieces.
* Furthermore, it sets (win_r1,win_c1) to mark last position where
* it saw one of the player's pieces.  Then, it looks in the
* opposite direction, at squares (row - dirX, col-dirY),
* (row - 2*dirX, col - 2*dirY), ... and does the same thing.
* Except, this time it sets (win_r2,win_c2) to mark the last piece.
* Note:  The values of dirX and dirY must be 0, 1, or -1.  At least
* one of them must be non-zero.
*/
private int count(int player, int row, int col, int dirX, int dirY) {

int ct = 1;  // Number of pieces in a row belonging to the player.

int r, c;    // A row and column to be examined

r = row + dirX;  // Look at square in specified direction.
c = col + dirY;
while ( r >= 0 && r < 13 && c >= 0 && c < 13 && board[r][c] == player ) {
// Square is on the board and contains one of the players's pieces.
ct++;
r += dirX;  // Go on to next square in this direction.
c += dirY;
}

win_r1 = r - dirX;  // The next-to-last square looked at.
win_c1 = c - dirY;  //    (The LAST one looked at was off the board or
//    did not contain one of the player's pieces.

r = row - dirX;  // Look in the opposite direction.
c = col - dirY;
while ( r >= 0 && r < 13 && c >= 0 && c < 13 && board[r][c] == player ) {
// Square is on the board and contains one of the players's pieces.
ct++;
r -= dirX;   // Go on to next square in this direction.
c -= dirY;
}

win_r2 = r + dirX;
win_c2 = c + dirY;

// At this point, (win_r1,win_c1) and (win_r2,win_c2) mark the endpoints
// of the line of pieces belonging to the player.

return ct;

}  // end count()

/**
* Draws the board and the pieces on the board.  If the game has
* been won by getting five or more pieces in a row, draws a red line
* through the pieces.
*/
public void paintComponent(Graphics g) {

super.paintComponent(g); // Fill with background color, lightGray

/* Draw a two-pixel black border around the edges of the canvas,
and draw grid lines in darkGray.  */

g.setColor(Color.DARK_GRAY);
for (int i = 1; i < 13; i++) {
g.drawLine(1 + 13*i, 0, 1 + 13*i, getSize().height);
g.drawLine(0, 1 + 13*i, getSize().width, 1 + 13*i);
}
g.setColor(Color.BLACK);
g.drawRect(0,0,getSize().width-1,getSize().height-1);
g.drawRect(1,1,getSize().width-3,getSize().height-3);

/* Draw the pieces that are on the board. */

for (int row = 0; row < 13; row++)
for (int col = 0; col < 13; col++)
if (board[row][col] != EMPTY)
drawPiece(g, board[row][col], row, col);

/* If the game has been won, then win_r1 >= 0.  Draw a line to mark
the five (or more) winning pieces. */

if (win_r1 >= 0)
drawWinLine(g);

}  // end paintComponent()

/**
* Draw a piece in the square at (row,col).  The color is specified
* by the piece parameter, which should be either BLACK or WHITE.
*/
private void drawPiece(Graphics g, int piece, int row, int col) {
if (piece == WHITE)
g.setColor(Color.WHITE);
else
g.setColor(Color.BLACK);
g.fillOval(3 + 13*col, 3 + 13*row, 10, 10);
}

/**
* Draw a 2-pixel wide red line from the middle of the square at
* (win_r1,win_c1) to the middle of the square at (win_r2,win_c2).
* This routine is called to mark the pieces that won the game.
* The values of the variables are set in the count() method.
*/
private void drawWinLine(Graphics g) {
g.setColor(Color.RED);
g.drawLine( 8 + 13*win_c1, 8 + 13*win_r1, 8 + 13*win_c2, 8 + 13*win_r2 );
if (win_r1 == win_r2)
g.drawLine( 8 + 13*win_c1, 7 + 13*win_r1, 8 + 13*win_c2, 7 + 13*win_r2 );
else
g.drawLine( 7 + 13*win_c1, 8 + 13*win_r1, 7 + 13*win_c2, 8 + 13*win_r2 );
}

/**
* Respond to a user click on the board.  If no game is
* in progress, show an error message.  Otherwise, find
* the row and column that the user clicked and call
* doClickSquare() to handle it.
*/
public void mousePressed(MouseEvent evt) {
if (gameInProgress == false)
message.setText("Click \"New Game\" to start a new game.");
else {
int col = (evt.getX() - 2) / 13;
int row = (evt.getY() - 2) / 13;
if (col >= 0 && col < 13 && row >= 0 && row < 13)
doClickSquare(row,col);
}
}

public void mouseReleased(MouseEvent evt) { }
public void mouseClicked(MouseEvent evt) { }
public void mouseEntered(MouseEvent evt) { }
public void mouseExited(MouseEvent evt) { }

}  // end nested class Board

} // end class GoMoku

Solution for Programming Exercise 8.1

Exercise 8.1:

Write a program that uses the following subroutine, from Subsection 8.3.3, to solve equations specified by the user.

/**
* Returns the larger of the two roots of the quadratic equation
* A*x*x + B*x + C = 0, provided it has any roots.  If A == 0 or
* if the discriminant, B*B - 4*A*C, is negative, then an exception
* of type IllegalArgumentException is thrown.
*/
static public double root( double A, double B, double C )
throws IllegalArgumentException {
if (A == 0) {
throw new IllegalArgumentException("A can't be zero.");
}
else {
double disc = B*B - 4*A*C;
if (disc < 0)
throw new IllegalArgumentException("Discriminant < zero.");
return  (-B + Math.sqrt(disc)) / (2*A);
}
}

Your program should allow the user to specify values for A, B, and C. It should call the subroutine to compute a solution of the equation. If no error occurs, it should print the root. However, if an error occurs, your program should catch that error and print an error message. After processing one equation, the program should ask whether the user wants to enter another equation. The program should continue until the user answers no.

Discussion

This is really just a fairly easy exercise in using exceptions. The root() subroutine must be called in a try..catch statement. There must be a catch clause to handle the IllegalArgumentException that might be thrown by the routine. The catch clause can simply print an error message:

try {
solution = root(A,B,C);
System.out.println("A solution of the equation is " + solution);
}
catch (IllegalArgumentException e) {
System.out.println("Sorry, I can't find a solution.");
System.out.println(e.getMessage());
}

Note that I've put the output statement that prints the solution inside the try statement. If an IllegalArgumentException is thrown by the subroutine, then this output statement will not be executed since the computer will jump immediately to the catch clause. You have to be careful about things like this. It wouldn't do to have the output statement after the try..catch statement, since then the computer would still try to execute the output statement after handling an IllegalArgumentException. There are other ways to deal with this problem. For example, since the try..catch statement occurs in a while loop, we could put a continue statement in the catch clause to abort further processing when an exception occurs:

try {
solution = root(A,B,C);
}
catch (IllegalArgumentException e) {
System.out.println("Sorry, I can't find a solution.");
System.out.println(e.getMessage());
continue;  // jump back to start of the while loop.
}

/* We only get to this point if NO exception was thrown.
So we know that the solution was computed successfully. */

System.out.println("A solution of the equation is " + solution);

The rest of the main program is pretty standard. A complete solution is shown below.

The Solution

/**
* This program will compute and print one of the solutions
* to an equation of the form A*X*X + B*X + C = 0, where
* A, B, and C are numbers entered by the user.  It depends
* on the non-standard class TextIO for doing input.
*/
public class Quadratic {

public static void main(String[] args) {

double A, B, C;   // Coefficients in the equation.
double solution;  // The solution computed for the equation.
boolean goAgain;  // Set to true if the user wants to
//   solve another equation.

System.out.println("This program will print a solution of an equation");
System.out.println("of the form A*X*X + B*X + C = 0, where A, B, and");
System.out.println("C are values that you enter.");

do {

/* Get the coefficients from the user. */

System.out.println();
System.out.println("Enter values for A, B, and C:");
System.out.print("A = ");
A = TextIO.getlnDouble();
System.out.print("B = ");
B = TextIO.getlnDouble();
System.out.print("C = ");
C = TextIO.getlnDouble();
System.out.println();

/* Print the solution, or an error message, if
there is no solution. */

try {
solution = root(A,B,C);
System.out.println("A solution of the equation is " + solution);
}
catch (IllegalArgumentException e) {
System.out.println("Sorry, I can't find a solution.");
System.out.println(e.getMessage());
}

/* Find out whether the user wants to go again. */

System.out.println();
System.out.print("Do you want to solve another equation? ");
goAgain = TextIO.getlnBoolean();

} while (goAgain);

} // end main

/**
* Returns the larger of the two roots of the quadratic equation
* A*x*x + B*x + C = 0, provided it has any roots.  If A == 0 or
* if the discriminant, B*B - 4*A*C, is negative, then an exception
* of type IllegalArgumentException is thrown.
*/
static public double root( double A, double B, double C )
throws IllegalArgumentException {
if (A == 0) {
throw new IllegalArgumentException("A can't be zero.");
}
else {
double disc = B*B - 4*A*C;
if (disc < 0)
throw new IllegalArgumentException("Discriminant < zero.");
return  (-B + Math.sqrt(disc)) / (2*A);
}
}

}  // end class Quadratic

Solution for Programming Exercise 8.2

Exercise 8.2:

As discussed in Section 8.1, values of type int are limited to 32 bits. Integers that are too large to be represented in 32 bits cannot be stored in an int variable. Java has a standard class, java.math.BigInteger, that addresses this problem. An object of type BigInteger is an integer that can be arbitrarily large. (The maximum size is limited only by the amount of memory available to the Java Virtual Machine.) Since BigIntegers are objects, they must be manipulated using instance methods from the BigInteger class. For example, you can't add two BigIntegers with the + operator. Instead, if N and M are variables that refer to BigIntegers, you can compute the sum of N and M with the function call N.add(M). The value returned by this function is a new BigInteger object that is equal to the sum of N and M.

The BigInteger class has a constructor new BigInteger(str), where str is a string. The string must represent an integer, such as "3" or "39849823783783283733". If the string does not represent a legal integer, then the constructor throws a NumberFormatException.

There are many instance methods in the BigInteger class. Here are a few that you will find useful for this exercise. Assume that N and M are variables of type BigInteger.

- N.add(M) -- a function that returns a BigInteger representing the sum of N and M.

- N.multiply(M) -- a function that returns a BigInteger representing the result of multiplying N times M.

- N.divide(M) -- a function that returns a BigInteger representing the result of dividing N by M, discarding the remainder.

- N.signum() -- a function that returns an ordinary int. The returned value represents the sign of the integer N. The returned value is 1 if N is greater than zero. It is -1 if N is less than zero. And it is 0 if N is zero.

- N.equals(M) -- a function that returns a boolean value that is true if N and M have the same integer value.

- N.toString() -- a function that returns a String representing the value of N.

- N.testBit(k) -- a function that returns a boolean value. The parameter k is an integer. The return value is true if the k-th bit in N is 1, and it is false if the k-th bit is 0. Bits are numbered from right to left, starting with 0. Testing "if (N.testBit(0))" is an easy way to check whether N is even or odd. N.testBit(0) is true if and only if N is an odd number.

For this exercise, you should write a program that prints 3N+1 sequences with starting values specified by the user. In this version of the program, you should use BigIntegers to represent the terms in the sequence. You can read the user's input into a String with the TextIO.getln() function. Use the input value to create the BigInteger object that represents the starting point of the 3N+1 sequence. Don't forget to catch and handle the NumberFormatException that will occur if the user's input is not a legal integer! You should also check that the input number is greater than zero.

If the user's input is legal, print out the 3N+1 sequence. Count the number of terms in the sequence, and print the count at the end of the sequence. Exit the program when the user inputs an empty line.

Discussion

My solution uses a subroutine, printThreeNSequence(N), to print out the 3N+1 sequence starting from the BigInteger, N. The subroutine assumes that N is not null and that it represents a value that is greater than one. Given these assumptions, the subroutine cannot generate any errors. (These conditions are ensured by the main program which calls the subroutine, but I use assert statements to test the assumptions during debugging.) The only interesting aspect of the subroutine is that all operations on N must be performed using instance methods from the BigInteger class. For example, to multiply N by 2, I use a statement "N = N.multiply(TWO);", where TWO is a BigInteger that represents the integer 2. My program defines TWO as a constant, along with several other BigIntegers that represent values that I need:

static final BigInteger THREE = new BigInteger("3");
static final BigInteger ONE = new BigInteger("1");
static final BigInteger TWO = new BigInteger("2");

With these constants, the code for computing the next term in a 3N+1 sequence becomes:

if (N.testBit(0) == false) {
// N is even.  Divide N by 2.
N = N.divide(TWO);
}
else {
// N is odd.  Multiply N by 3, then add 1.
N = N.multiply(THREE);
N = N.add(ONE);
}

You can find the complete subroutine in the solution that is given below.

In the main() routine, the user's input is read into a variable, line, of type String. The input is used to construct a BigInteger with the statement "N = new BigInteger(line);". Since this statement can produce a NumberFormatException, it is placed in a try statement that can catch and handle the error. The test "if (N.signum() == 1)" is used to make sure that N >= 1. The value of N.signum() is 1 if and only if N is a positive integer. Here is the loop form the main() routine that processes the user's input:

while (true) {
System.out.println();
System.out.println("Enter the starting value, or press return to end.");
System.out.print("? ");
line = TextIO.getln().trim();
if (line.length() == 0)
break;
try {
N = new BigInteger(line);
if (N.signum() == 1)
printThreeNSequence(N);
else
System.out.println("Error:  The starting value cannot be less than or equal to zero.");
}
catch (NumberFormatException e) {
System.out.println("Error:  \"" + line + "\" is not a legal integer.");
}
}

The Solution

import java.math.BigInteger;

/**
* This program prints out 3N+1 sequences for starting values of N that
* are entered by the user.  Since integers are represented as objects of
* type BigInteger, it will work for arbitrarily large integers.  The
* starting value specified by the user must be greater than zero.  The
* program continues to read input from the user and print 3N+1 sequences
* until the user inputs an empty line.  If the user's input is illegal,
* the program will print an error message and continue.
*/
public class BigThreeN {

private static final BigInteger THREE = new BigInteger("3");
private static final BigInteger ONE = new BigInteger("1");
private static final BigInteger TWO = new BigInteger("2");

public static void main(String[] args) {

String line;   // A line of input from the user.

BigInteger N;  // The starting point for the 3N+1 sequence,
//   as specified by the user.

System.out.println("This program will print 3N+1 sequences for positive starting values");
System.out.println("that you enter.  There is no pre-set limit on the number of");
System.out.println("digits in the numbers that you enter.  The program will end when");
System.out.println("you enter an empty line.");

while (true) {
System.out.println();
System.out.println("Enter the starting value, or press return to end.");
System.out.print("? ");
line = TextIO.getln().trim();
if (line.length() == 0)
break;
try {
N = new BigInteger(line);
if (N.signum() == 1)
printThreeNSequence(N);
else
System.out.println("Error:  The starting value cannot be less than or equal to zero.");
}
catch (NumberFormatException e) {
System.out.println("Error:  \"" + line + "\" is not a legal integer.");
}
}

System.out.println();
System.out.println("OK.  Bye for now!");

}  // end main()

/**
* Print the 3N+1 sequence starting from N, and count the number
* of terms in the sequence.  It is assumed that N is not null and
* that it is greater than zero.
*/
private static void printThreeNSequence(BigInteger N) {

assert N != null && N.signum() == 1 : "Illegal parameter value.";

int count;  // The number of terms in the sequence.

System.out.println();
System.out.println("The 3N+1 sequence starting with " + N + " is:");
System.out.println();

System.out.println(N.toString());   // Print N as the first term of the sequence
count = 1;

while ( ! N.equals(ONE) ){   // Compute and print the next term
if (N.testBit(0) == false) {
// N is even.  Divide N by 2.
N = N.divide(TWO);
}
else {
// N is odd.  Multiply N by 3, then add 1.
N = N.multiply(THREE);
N = N.add(ONE);
}
System.out.println(N.toString());
count++;
}

System.out.println();
System.out.println("There were " + count + " terms in the sequence.");

}  // end printThreeNSequence

} // end BigThreeN

Solution for Programming Exercise 8.3

Exercise 8.3:

A Roman numeral represents an integer using letters. Examples are XVII to represent 17, MCMLIII for 1953, and MMMCCCIII for 3303. By contrast, ordinary numbers such as 17 or 1953 are called Arabic numerals. The following table shows the Arabic equivalent of all the single-letter Roman numerals:

M    1000            X   10
D     500            V    5
C     100            I    1
L      50

When letters are strung together, the values of the letters are just added up, with the following exception. When a letter of smaller value is followed by a letter of larger value, the smaller value is subtracted from the larger value. For example, IV represents 5 - 1, or 4. And MCMXCV is interpreted as M + CM + XC + V, or 1000 + (1000 - 100) + (100 - 10) + 5, which is 1995. In standard Roman numerals, no more than three consecutive copies of the same letter are used. Following these rules, every number between 1 and 3999 can be represented as a Roman numeral made up of the following one- and two-letter combinations:

M    1000            X   10
CM    900            IX   9
D     500            V    5
CD    400            IV   4
C     100            I    1
XC     90
L      50
XL     40

Write a class to represent Roman numerals. The class should have two constructors. One constructs a Roman numeral from a string such as "XVII" or "MCMXCV". It should throw a NumberFormatException if the string is not a legal Roman numeral. The other constructor constructs a Roman numeral from an int. It should throw a NumberFormatException if the int is outside the range 1 to 3999.

In addition, the class should have two instance methods. The method toString() returns the string that represents the Roman numeral. The method toInt() returns the value of the Roman numeral as an int.

At some point in your class, you will have to convert an int into the string that represents the corresponding Roman numeral. One way to approach this is to gradually "move" value from the Arabic numeral to the Roman numeral. Here is the beginning of a routine that will do this, where number is the int that is to be converted:

String roman = "";
int N = number;
while (N >= 1000) {
// Move 1000 from N to roman.
roman += "M";
N -= 1000;
}
while (N >= 900) {
// Move 900 from N to roman.
roman += "CM";
N -= 900;
}
.
.  // Continue with other values from the above table.
.

(You can save yourself a lot of typing in this routine if you use arrays in a clever way to represent the data in the above table.)

Once you've written your class, use it in a main program that will read both Arabic numerals and Roman numerals entered by the user. If the user enters an Arabic numeral, print the corresponding Roman numeral. If the user enters a Roman numeral, print the corresponding Arabic numeral. (You can tell the difference by using TextIO.peek() to peek at the first character in the user's input (see Subsection 8.2.2). If the first character is a digit, then the user's input is an Arabic numeral. Otherwise, it's a Roman numeral.) The program should end when the user inputs an empty line.

Discussion

My class is called RomanNumeral. An object of type RomanNumeral has a private instance variable of type int that stores the integer value of the Roman numeral. When the toString() method is called, it computes the string that represents the Roman number based on the value of this int. By contrast, the toInt() method simply returns the value of the instance variable. This is not the only way that things could be done. I might have stored the string representation of the Roman numeral in an instance variable. In that case, the toString() method would simply return the stored value, while the toInt() method would have to compute the int value from the stored String. It would even be possible, and more efficient, to store both the int and String representations in the object. (The point, though, is that it's not necessary to do so. The two representations hold exactly the same information.)

In my version of the class, the constructor which takes a parameter of type int simply has to store the parameter value in the instance variable, after checking that it is in the legal range of values.

The constructor that takes a parameter of type String must interpret the string as a Roman numeral and convert it to the corresponding int value. This is done by adding up the integer value associated with each character or pair of characters in the string. The fact that characters sometimes need to be considered in pairs complicates things a bit. An algorithm for converting a String, roman, to an int, arabic, is:

Let arabic = 0
Let i = 0  // representing a position in the string

while i is a legal position in the string:
Let ch be the character in position i
Let N be the numeric equivalent of ch
i++   // to account for the character, ch
if there are no additional characters in the string:
// (We need to make this test first, to avoid an error
// when we try to look at the next character.)
Add N to arabic
else:   // Try pairing the ch with the next character
Let N2 be the numeric equivalent of the NEXT character
If N < N2:  // Evaluate the characters as a pair
Add (N2 - N) to arabic
i++    // to account for the extra character
else:
Add N to arabic

This algorithm does not take into account that the string might not be a legal Roman numeral. If a character in the string is not one of the characters M, D, C, L, X, V, or I, then a NumberFormatException must be thrown. And the algorithm allows some unusual two-letter combinations such as IM to represent 999; it is not clear whether these should be considered to be errors.

The job of converting an int into an equivalent Roman numeral is handled in my toString() method. The exercise includes code that shows how to write this method as a long sequence of while loops. Consider the loop

while (N >= 1000) {
roman += "M";
N -= 1000;
}

After this loop, all the 1000's in N have been converted to M's in roman, and we can be sure that N is 999 or less. So what's left of N can be expressed in terms of the smaller numbers in the table: 900, 500, 400, and so on. Each of these numbers can be processed by a while loop (although an if statement would also work in some cases, where the number that is being tested can only occur once.). Note that the numbers in these loops must be in decreasing order for this to work.

However, all the loops in this algorithm have the same form. They just use different numbers and letters. In my program, I use two arrays to store the numbers and letters from the table:

private static int[]    numbers = { 1000,  900,  500,  400,  100,   90,
50,   40,   10,    9,    5,    4,    1 };

private static String[] letters = { "M",  "CM",  "D",  "CD", "C",  "XC",
"L",  "XL",  "X",  "IX", "V",  "IV", "I" };

For each index i, numbers[i] is the int equivalent of the Roman numeral letters[i]. All the processing can then be done with a for loop that does all the required while loops one after the other:

public String toString() {
// Return the standard representation of this Roman numeral.
String roman = "";  // The Roman numeral.
int N = num;        // N represents the part of num that still has
//   to be converted to Roman numeral representation.
for (int i = 0; i < numbers.length; i++) {
while (N >= numbers[i]) {
roman += letters[i];
N -= numbers[i];
}
}
return roman;
}

An algorithm for the main program is given by:

while (true):
Prompt the user for input
If the first thing on the line is the end-of-line:
break
else if the first character on the line is a digit:
Let arabic = TextIO.getlnInt()
Let roman = new RomanNumeral(arabic)
Print out roman.toString()
else:
Let str = TextIO.getln();
Let roman = new RomanNumeral(str);
Print out roman.toInt();

This algorithm ignores the possibility that the user's input might be illegal. If it is, then the RomanNumeral constructor will throw a NumberFormatException. This exception must be caught and handled. With this in mind, the algorithm becomes:

while (true):
Prompt the user for input
If the first thing on the line is the end-of-line:
break
else if the first character on the line is a digit:
Let arabic = TextIO.getlnInt()
try {
Let roman = new RomanNumeral(arabic)
Print out roman.toString()
}
catch (NumberFormatException e) {
Print an error message
}
else:
Let str = TextIO.getln();
try {
Let roman = new RomanNumeral(str);
Print out roman.toInt();
}
catch (NumberFormatException e) {
Print an error message
}

This can be easily coded into Java. By the way, the test as to whether the first character on the input line is a digit can be performed using the standard boolean-valued function Character.isDigit(ch), which returns true if the character ch is a digit.

The Solution

The Roman numerals class:

/**
* An object of type RomanNumeral is an integer between 1 and 3999.  It can
* be constructed either from an integer or from a string that represents
* a Roman numeral in this range.  The function toString() will return a
* standardized Roman numeral representation of the number.  The function
* toInt() will return the number as a value of type int.
*/
public class RomanNumeral {

private final int num;   // The number represented by this Roman numeral.

/* The following arrays are used by the toString() function to construct
* the standard Roman numeral representation of the number.  For each i,
* the number numbers[i] is represented by the corresponding string, letters[i].
*/

private static int[]    numbers = { 1000,  900,  500,  400,  100,   90,
50,   40,   10,    9,    5,    4,    1 };

private static String[] letters = { "M",  "CM",  "D",  "CD", "C",  "XC",
"L",  "XL",  "X",  "IX", "V",  "IV", "I" };

/**
* Constructor.  Creates the Roman number with the int value specified
* by the parameter.
* @throws NumberFormatException if the parameter is not in the range 1 to 3999 inclusive.
*/
public RomanNumeral(int arabic) {
if (arabic < 1)
throw new NumberFormatException("Value of RomanNumeral must be positive.");
if (arabic > 3999)
throw new NumberFormatException("Value of RomanNumeral must be 3999 or less.");
num = arabic;
}

/*
* Constructor.  Creates the Roman number with the given representation.
* For example, RomanNumeral("xvii") is 17.  If the parameter is not a
* legal Roman numeral, a NumberFormatException is thrown.  Both upper and
* lower case letters are allowed.
* @throws NumberFormatException if the parameter is not a legal Roman numeral
*/
public RomanNumeral(String roman) {

if (roman.length() == 0)
throw new NumberFormatException("An empty string does not define a Roman numeral.");

roman = roman.toUpperCase();  // Convert to upper case letters.

int i = 0;       // A position in the string, roman;
int arabic = 0;  // Arabic numeral equivalent of the part of the string that has
//    been converted so far.

while (i < roman.length()) {

char letter = roman.charAt(i);        // Letter at current position in string.
int number = letterToNumber(letter);  // Numerical equivalent of letter.

i++;  // Move on to next position in the string

if (i == roman.length()) {
// There is no letter in the string following the one we have just processed.
// So just add the number corresponding to the single letter to arabic.
arabic += number;
}
else {
// Look at the next letter in the string.  If it has a larger Roman numeral
// equivalent than number, then the two letters are counted together as
// a Roman numeral with value (nextNumber - number).
int nextNumber = letterToNumber(roman.charAt(i));
if (nextNumber > number) {
// Combine the two letters to get one value, and move on to next position in string.
arabic += (nextNumber - number);
i++;
}
else {
// Don't combine the letters.  Just add the value of the one letter onto the number.
arabic += number;
}
}

}  // end while

if (arabic > 3999)
throw new NumberFormatException("Roman numeral must have value 3999 or less.");

num = arabic;

} // end constructor

/**
* Find the integer value of letter considered as a Roman numeral.  Throws
* NumberFormatException if letter is not a legal Roman numeral.  The letter
* must be upper case.
*/
private int letterToNumber(char letter) {
switch (letter) {
case 'I':  return 1;
case 'V':  return 5;
case 'X':  return 10;
case 'L':  return 50;
case 'C':  return 100;
case 'D':  return 500;
case 'M':  return 1000;
default:   throw new NumberFormatException(
"Illegal character \"" + letter + "\" in Roman numeral");
}
}

/**
* Return the standard representation of this Roman numeral.
*/
public String toString() {
String roman = "";  // The roman numeral.
int N = num;        // N represents the part of num that still has
//   to be converted to Roman numeral representation.
for (int i = 0; i < numbers.length; i++) {
while (N >= numbers[i]) {
roman += letters[i];
N -= numbers[i];
}
}
return roman;
}

/**
* Return the value of this Roman numeral as an int.
*/
public int toInt() {
return num;
}

} // end class RomanNumeral

The main program class:

/**
* This program will convert Roman numerals to ordinary arabic numerals
* and vice versa.  The user can enter a numerals of either type.  Arabic
* numerals must be in the range from 1 to 3999 inclusive.  The user ends
* the program by entering an empty line.
*/
public class RomanConverter {

public static void main(String[] args) {

System.out.println("Enter a Roman numeral and I will convert it to an ordinary");
System.out.println("arabic integer.  Enter an integer in the range 1 to 3999");
System.out.println("and I will convert it to a Roman numeral.  Press return when");
System.out.println("you want to quit.");

while (true) {

System.out.println();
System.out.print("? ");

/* Skip past any blanks at the beginning of the input line.
Break out of the loop if there is nothing else on the line. */

while (TextIO.peek() == ' ' || TextIO.peek() == '\t')
TextIO.getAnyChar();
if ( TextIO.peek() == '\n' )
break;

/* If the first non-blank character is a digit, read an arabic
numeral and convert it to a Roman numeral.  Otherwise, read
a Roman numeral and convert it to an arabic numeral. */

if ( Character.isDigit(TextIO.peek()) ) {
int arabic = TextIO.getlnInt();
try {
RomanNumeral N = new RomanNumeral(arabic);
System.out.println(N.toInt() + " = " + N.toString());
}
catch (NumberFormatException e) {
System.out.println("Invalid input.");
System.out.println(e.getMessage());
}
}
else {
String roman = TextIO.getln();
try {
RomanNumeral N = new RomanNumeral(roman);
System.out.println(N.toString() + " = " + N.toInt());
}
catch (NumberFormatException e) {
System.out.println("Invalid input.");
System.out.println(e.getMessage());
}
}

}  // end while

System.out.println("OK.  Bye for now.");

}  // end main()

} // end class RomanConverter

Solution for Programming Exercise 8.4

Exercise 8.4:

The source code file Expr.java defines a class, Expr, that can be used to represent mathematical expressions involving the variable x. The expression can use the operators +, -, *, /, and ^ (where ^ represents the operation of raising a number to a power). It can use mathematical functions such as sin, cos, abs, and ln. See the source code file for full details. The Expr class uses some advanced techniques which have not yet been covered in this textbook. However, the interface is easy to understand. It contains only a constructor and two public methods.

The constructor new Expr(def) creates an Expr object defined by a given expression. The parameter, def, is a string that contains the definition. For example, new Expr("x^2") or new Expr("sin(x)+3*x"). If the parameter in the constructor call does not represent a legal expression, then the constructor throws an IllegalArgumentException. The message in the exception describes the error.

If func is a variable of type Expr and num is of type double, then func.value(num) is a function that returns the value of the expression when the number num is substituted for the variable x in the expression. For example, if Expr represents the expression 3*x+1, then func.value(5) is 3*5+1, or 16. If the expression is undefined for the specified value of x, then the special value Double.NaN is returned; no exception is thrown.

Finally, func.toString() returns the definition of the expression. This is just the string that was used in the constructor that created the expression object.

For this exercise, you should write a program that lets the user enter an expression. If the expression contains an error, print an error message. Otherwise, let the user enter some numerical values for the variable x. Print the value of the expression for each number that the user enters. However, if the expression is undefined for the specified value of x, print a message to that effect. You can use the boolean-valued function Double.isNaN(val) to check whether a number, val, is Double.NaN.

The user should be able to enter as many values of x as desired. After that, the user should be able to enter a new expression.

Discussion

A pseudocode algorithm for the program is given by:

while (true):
Get a line of input from the user
if the line is empty:
break
Convert the input line to an Expr
Read and process the user's numbers

Converting the input line into an object of type Expr involves calling the constructor from the Expr class. This call might generate an IllegalArgumentException. The algorithm must be expanded to handle this exception and print an error message if it occurs. When an error occurs, I use a continue statement to jump back to the start of the loop without reading any numbers from the user:

while (true):
Get a line of input from the user
if the line is empty:
break
try {
Let expression = new Expr(line)
}
catch (IllegalArgumentException e) {
Print an error message
continue   // jumps back to start of loop
}
Read and process the user's numbers

The last step, reading and processing the user's numbers, expands into a loop, which is nested inside the main while loop. In this loop, I could use TextIO.getDouble() to read one of the user's numbers, but instead I chose to read the user's input into a string and convert that string into a value of type double. This has two advantages: I can end the loop when the user presses return. And I can do nicer error handling than the default error-handling that is provided by TextIO. The conversion from a string, line, to a double is done using a method Double.parseDouble(string). The conversion will generate a NumberFormatException if the user's input is not a legal number. The algorithm for reading and processing the user's numbers becomes:

while (true):
Get a line of input from the user
if the line is empty:
break
try {
Let x = Double.parseDouble(line)
}
catch (NumberFormatException e) {
Print an error message
continue
}
Let val = expression.value(x)
if val is Double.NaN:
Print an error message
else:
Output val

All this can be easily translated into the complete solution, which follows.

The Solution

public class FunctionEvaluator {

public static void main(String[] args) {

String line;      // A line of input read from the user.
Expr expression;  // The definition of the function f(x).
double x;         // A value of x for which f(x) is to be calculated.
double val;       // The value of f(x) for the specified value of x.

System.out.println("This program will evaluate a specified function, f(x), at");
System.out.println("specified values of the variable x.  The definition of f(x)");
System.out.println("can use the operators +, -, *, /, and ^ as well as mathematical");
System.out.println("functions such as sin, abs, and ln.");

while (true) {

/* Get the function from the user.  A line of input is read and
used to construct an object of type Expr.  If the input line is
empty, then the loop will end, and the program will terminate. */

System.out.println("\n\n\nEnter definition of f(x), or press return to quit.");
System.out.print("\nf(x) = ");
line = TextIO.getln().trim();
if (line.length() == 0)
break;

try {
expression = new Expr(line);
}
catch (IllegalArgumentException e) {
// An error was found in the input.  Print an error
//    message and go back to the beginning of the loop.
System.out.println("Error!  The definition of f(x) is not valid.");
System.out.println(e.getMessage());
continue;
}

/* Read values of x from the user, until the user presses return.
If the user's input is not a legal number, print an error message.
Otherwise, compute f(x) and print the result. */

System.out.println("\nEnter values of x where f(x) is to be evaluated.");
System.out.println("Press return to end.");

while (true) {
System.out.print("\nx = ");
line = TextIO.getln().trim();
if (line.length() == 0)
break;
try {
x = Double.parseDouble(line);
}
catch (NumberFormatException e) {
System.out.println("\"" + line + "\" is not a legal number.");
continue;
}
val = expression.value(x);
if (Double.isNaN(val))
System.out.println("f(" + x + ") is undefined.");
else
System.out.println("f(" + x + ") = " + val);
}  // end while

} // end while

System.out.println("\n\n\nOK.  Bye for now.");

}  // end main();

} // end class FunctionEvaluator

Solution for Programming Exercise 8.5

Exercise 8.5:

This exercise uses the class Expr, which was described in Exercise 8.4 and which is defined in the source code file Expr.java. For this exercise, you should write a GUI program that can graph a function, f(x), whose definition is entered by the user. The program should have a text-input box where the user can enter an expression involving the variable x, such as x^2 or sin(x-3)/x. This expression is the definition of the function. When the user presses return in the text input box, the program should use the contents of the text input box to construct an object of type Expr. If an error is found in the definition, then the program should display an error message. Otherwise, it should display a graph of the function. (Recall: A JTextField generates an ActionEvent when the user presses return.)

The program will need a JPanel for displaying the graph. To keep things simple, this panel should represent a fixed region in the xy-plane, defined by -5 <= x <= 5 and -5 <= y <= 5. To draw the graph, compute a large number of points and connect them with line segments. (This method does not handle discontinuous functions properly; doing so is very hard, so you shouldn't try to do it for this exercise.) My program divides the interval -5 <= x <= 5 into 300 subintervals and uses the 301 endpoints of these subintervals for drawing the graph. Note that the function might be undefined at one of these x-values. In that case, you have to skip that point.

A point on the graph has the form (x,y) where y is obtained by evaluating the user's expression at the given value of x. You will have to convert these real numbers to the integer coordinates of the corresponding pixel on the canvas. The formulas for the conversion are:

a  =  (int)( (x + 5)/10 * width );
b  =  (int)( (5 - y)/10 * height );

where a and b are the horizontal and vertical coordinates of the pixel, and width and height are the width and height of the panel.

Discussion

I wrote my solution as a subclass, SimpleGrapher, of JPanel that represents the main panel of the program. The class includes a main() routine that makes it possible to run the class as a stand-alone application; main() just opens a window and sets its content pane to be a SimpleGrapher.

My solution uses a nested class, GraphPanel, for displaying the graph. This class is defined as a subclass of JPanel. It has an instance variable, func, of type Expr that represents the function to be drawn. If there is no function, then func is null. This is true when the program is first started and when the user's input has been found to be illegal. The paintComponent() method checks the value of func to decide what to draw. If func is null, then the paint method simply draws a message on the panel stating that no function is available. Otherwise, it draws a pair of axes and the graph of the function.

The interesting work in class GraphPanel is done in the drawFunction() method, which is called by the paintComponent() method. This function draws the graph of the function for -5 <= x <= 5. This interval on the x axis is divided into 300 subintervals. Since the length of the interval is 10, the length of each subinterval is given by dx, where dx is 10.0/300. The x values for the points that I want to plot are given by -5, -5+dx, -5+2*dx, and so on. Each x-value is obtained by adding dx to the previous value. For each x value, the y-value of the point on the graph is computed as func.value(x). As the points on the graph are computed, line segments are drawn to connect pairs of points (unless the y-value of either point is undefined). An algorithm for the drawFunction() method is:

Let dx = 10.0 / 300;
Let x = -5                // Get the first point
Let y = func.value(x)
for i = 1 to 300:
Let prevx = x         // Save the previous point
Let prevy = y
Let x = x + dx        // Get the next point
Let y = func.value(x)
if neither y nor prevy is Double.NaN:
draw a line segment from (prevx,prevy) to (x,y)

The method for drawing the line segment uses the conversion from real coordinates to integer pixel coordinates that is given in the exercise. By the way, more general conversion formulas can be given in the case where x extends from xmin to xmax and y extends from ymin to ymax. The general formulas are:

a  =  (int)( (x - xmin) / (xmax - xmin) * width );
b  =  (int)( (ymax - y) / (ymax - ymin) * height );

The formulas for a and b are of slightly different form to reflect the fact that a increases from 0 to width as x increases from xmin to xmax, while b decreases from height to 0 as y increases from ymin to ymax. You could improve the program by adding text input boxes where the user can enter values for xmin, xmax, ymin, and ymax.

In my main panel class, the constructor lays out the components in the panel with a BorderLayout that has a vertical gap, to allow some space between the graph and the components that are above it and below it. The "North" component is a JLabel that is used to display messages to the user. The "South" component holds a JTextField where the user enters the definition of the function. Since I wanted to add a label, "f(x) = ", next to the text field, I created a sub-panel to hold both the label and the text field, and I put the sub-panel in the "South" position of the main panel. Finally, the "Center" component of the main panel is the GraphPanel where the graph is drawn. A listener for ActionEvents is registered with the JTextField. When the user presses return in the JTextField, the listener's actionPerformed() method will be called.

The actionPerformed() method gets the user's input string from the JTextField. It tries to use this string to construct an object of type Expr. The constructor throws an IllegalArgumentException if the string contains an error, so the constructor is called in a try statement that can catch and handle the error. If an error occurs, then the error message in the exception object is displayed in the JLabel at the top of the window, and the graph is cleared. If no error occurs, the graph is set to display the user's function, and the JLabel is set to display the generic message, "Enter a function and press return." The code for all this is:

Expr function;  // The user's function.

try {
String def = functionInput.getText();
function = new Expr(def);
graph.setFunction(function);
message.setText(" Enter a function and press return.");
}
catch (IllegalArgumentException e) {
graph.clearFunction();
message.setText(e.getMessage());
}

(After viewing my program for the first time, I was dissatisfied with the appearance of the label at the top. There was no space between the text of the label and the gray background of the component. I decided to fix this by adding an EmptyBorder to the label to allow more space around the text where the white background of the label shows through. I also added borders around the main panel and around the subpanel that contains the text field. Borders were covered in Subsection 6.6.2)

The Solution

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

/**
The SimpleGrapher program can draw graphs of functions input by the
user.  The user enters the definition of the function in a text
input box.  When the user presses return, the function is graphed.
(Unless the definition contains an error.  In that case, an error
message is displayed.)

The graph is drawn on a canvas which represents the region of the
(x,y)-plane given by  -5 <= x <= 5  and  -5 <= y <= 5.  Any part of
the graph that lies outside this region is not shown.  The graph
is drawn by plotting 301 points and joining them with lines.  This
does not handle discontinuous functions properly.

This program requires the class Expr,
which is defined in by a separate file, Expr.java.
That file contains a full description of the syntax
of legal function definitions.

This class has a main() routine so that it can be run as an application.
*/

public class SimpleGrapher extends JPanel {

//-- Support for running this class as a stand-alone application --

public static void main(String[] args) {
// Open a window that shows a SimpleGrapher panel.
JFrame window = new JFrame("SimpleGrapher");
window.setContentPane( new SimpleGrapher() );
window.setLocation(50,50);
window.setSize(500,540);
window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
window.setVisible(true);
}

//---------------------------------------------------------------------------------

private GraphPanel graph;  // The JPanel that will display the graph.
// The nested class GraphPanel is defined below.

private JTextField functionInput;  // A text input box where the user enters
// the definition of the function.

private JLabel message;  // A label for displaying messages to the user,
// including error messages when the function
// definition is illegal.

public SimpleGrapher() {
// Initialize the panel by creating and laying out the components
// and setting up an action listener for the text field.

setBackground(Color.GRAY);
setLayout(new BorderLayout(3,3));
setBorder(BorderFactory.createLineBorder(Color.GRAY,3));

graph = new GraphPanel();
add(graph, BorderLayout.CENTER);

message = new JLabel(" Enter a function and press return");
message.setBackground(Color.WHITE);
message.setForeground(Color.RED);
message.setOpaque(true);
message.setBorder( BorderFactory.createEmptyBorder(5,0,5,0) );
add(message, BorderLayout.NORTH);

functionInput = new JTextField();
JPanel subpanel = new JPanel();
subpanel.setLayout(new BorderLayout());
subpanel.setBorder(BorderFactory.createEmptyBorder(3,3,3,3));
subpanel.add(new JLabel("f(x) = "), BorderLayout.WEST);
subpanel.add(functionInput, BorderLayout.CENTER);
add(subpanel, BorderLayout.SOUTH);

functionInput.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent evt) {
// Get the user's function definition from the box and use it
// to create a new object of type Expr.  Tell the GraphPanel to
// graph this function.  If the  definition is illegal, an
// IllegalArgumentException is  thrown by the Expr constructor.
// If this happens, the graph is cleared and an error message
// is displayed in the message label.
Expr function;  // The user's function.
try {
String def = functionInput.getText();
function = new Expr(def);
graph.setFunction(function);
message.setText(" Enter a function and press return.");
}
catch (IllegalArgumentException e) {
graph.clearFunction();
message.setText(e.getMessage());
}
functionInput.selectAll();
functionInput.requestFocus();  // Let's user start typing in input box.
}
});

}  // end constructor

//   -------------------------- Nested class ----------------------------

private static class GraphPanel extends JPanel {

// A object of this class can display the graph of a function
// on the region of the (x,y)-plane given by -5 &lt;= x &lt;= 5 and
// -5 &lt;= y &lt;= 5.  The graph is drawn very simply, by plotting
// 301 points and connecting them with line segments.

Expr func;  // The definition of the function that is to be graphed.
// If the value is null, no graph is drawn.

GraphPanel() {
// Constructor.
setBackground(Color.WHITE);
func = null;
}

public void setFunction(Expr exp) {
// Set the canvas to graph the function whose definition is
// given by the function exp.
func = exp;
repaint();
}

public void clearFunction() {
// Set the canvas to draw no graph at all.
func = null;
repaint();
}

public void paintComponent(Graphics g) {
// Draw the graph of the function or, if func is null,
// display a message that there is no function to be graphed.
super.paintComponent(g); // Fill with background color, white.
if (func == null) {
g.drawString("No function is available.", 20, 30);
}
else {
g.drawString("y = " + func, 5, 15);
drawAxes(g);
drawFunction(g);
}
}

void drawAxes(Graphics g) {
// Draw horizontal and vertical axes in the middle of the
// canvas.  A 5-pixel border is left at the ends of the axes.
int width = getWidth();
int height = getHeight();
g.setColor(Color.BLUE);
g.drawLine(5, height/2, width-5, height/2);
g.drawLine(width/2, 5, width/2, height-5);
}

void drawFunction(Graphics g) {
// Draw the graph of the function defined by the instance
// variable func.  Just plot 301 points with lines
// between them.

double x, y;          // A point on the graph.  y is f(x).
double prevx, prevy;  // The previous point on the graph.

double dx;  // Difference between the x-values of consecutive
// points on the graph.

dx  = 10.0 / 300;

g.setColor(Color.RED);

/* Compute the first point. */

x = -5;
y = func.value(x);

/* Compute each of the other 300 points, and draw a line segment
between each consecutive pair of points.  Note that if the
function is undefined at one of the points in a pair, then
the line segment is not drawn.  */

for (int i = 1; i <= 300; i++) {

prevx = x;           // Save the coords of the previous point.
prevy = y;

x += dx;            // Get the coords of the next point.

y = func.value(x);

if ( (! Double.isNaN(y)) && (! Double.isNaN(prevy)) ) {
// Draw a line segment between the two points.
putLine(g, prevx, prevy, x, y);
}

}  // end for

}  // end drawFunction()

void putLine(Graphics g, double x1, double y1,
double x2, double y2) {
// Draw a line segment from the point (x1,y1) to (x2,y2).
// These real values must be scaled to get the integer
// coordinates of the corresponding pixels.

int a1, b1;   // Pixel coordinates corresponding to (x1,y1).
int a2, b2;   // Pixel coordinates corresponding to (x2,y2).

int width = getWidth();     // Width of the canvas.
int height = getHeight();   // Height of the canvas.

a1 = (int)( (x1 + 5) / 10 * width );
b1 = (int)( (5 - y1) / 10 * height );
a2 = (int)( (x2 + 5) / 10 * width );
b2 = (int)( (5 - y2) / 10 * height );

if (Math.abs(y1) < 30000 && Math.abs(y2) < 30000) {
// Only draw lines for reasonable y-values.
// This should not be necessary, I think,
// but I got a problem when y was very large.)
g.drawLine(a1,b1,a2,b2);
}

}  // end putLine()

}  // end nested class GraphPanel

} // end class SimpleGrapher

Solution for Programming Exercise 9.1

Exercise 9.1:

In many textbooks, the first examples of recursion are the mathematical functions factorial and fibonacci. These functions are defined for non-negative integers using the following recursive formulas:

factorial(0)  =  1
factorial(N)  =  N*factorial(N-1)   for N > 0

fibonacci(0)  =  1
fibonacci(1)  =  1
fibonacci(N)  =  fibonacci(N-1) + fibonacci(N-2)   for N > 1

Write recursive functions to compute factorial(N) and fibonacci(N) for a given non-negative integer N, and write a main() routine to test your functions. Consider using the BigInteger class (see Exercise 8.2)

(In fact, factorial and fibonacci are really not very good examples of recursion, since the most natural way to compute them is to use simple for loops. Furthermore, fibonacci is a particularly bad example, since the natural recursive approach to computing this function is extremely inefficient.)

Discussion

The recursive definitions of the two functions can be translated rather easily into Java code. Note that for factorial(N), the base case is N=0, while fibonacci(N) has two base cases, N=0 and N=1. The obvious recursive function for computing fibonacci(N) is:

static int fibonacci( int N ) {
if ( N == 0 || N == 1 ) {
// Base cases; the answer is 1.
return 1;
}
else {
// Recursive case; the answer is obtained by applying the function
// recursively to N-1 and to N-2, and adding the two answers.
return fibonacci(N-1) + fibonacci(N-2);
}
}

and for factorial(N) is

static int factorial( int N ) {
if ( N == 0) {
// Base case; the answer is 1.
return 1;
}
else {
// Recursive case; the answer is obtained by applying the function
// recursively to N-1 and multiplying the answer by N.
return factorial(N-1) * N;
}
}

Adding a main() routine that calls these methods for several values of N would be an acceptable solution to the exercise. However, when testing the program, you will quickly notice some problems. First of all, the recursive algorithm for fibonacci(N) is so inefficient that it takes an unreasonably long time for it to run even for values of N as small as 40. (In fact, the recursive evaluation of fibonacci(N) calls both fibonacci(N-1) and fibonacci(N-2) and so takes almost twice as long as the evaluation of fibonacci(N-1); with the computation time for fibonacci(N) almost doubling every time N goes up by 1, the computation time quickly becomes unreasonable.) Although I include the recursive version of fibonacci in my solution, my program refuses to use this function if N is greater than 40.

Another problem occurs because of the limited size of values of type int. The value of factorial(N) grows very quickly as N increases -- so quickly that by the time N reaches 13, the value of factorial(N) is already too large to be expressed as a 32-bit integer! So, the factorial function as defined above only gives the correct answer for numbers 0 through 12. Although fibonacci(N) does not grow nearly so fast as factorial(N), it is still true that by the time N reaches 46, fibonacci(N) is outside the range of 32-bit integers.

The problem of the limited size of values of type int was discussed in Subsection 8.1.3. One approach to dealing with the problem was presented in Exercise 8.2: The class java.math.BigInteger represents integer values that can be arbitrarily large (within the limits of the computer's memory). In my solution to the exercise, I decided to use BigInteger values to compute factorial(N) and to compute fibonacci(N) non-recursively. This allows my program to work even for fairly large values of N, say up to a few thousand.

You can read my solution below to see how I used the BigInteger class and how I computed fibonacci(N) non-recursively.

The Solution

import java.math.BigInteger;

/**
* Computes factorial(N) and fibonacci(N) for integers N entered by
* the user, as a demonstration of recursion.
*/
public class FibonacciAndFactorial {

/**
* Main routine reads integers N from the user and prints the
* values of factorial(N) and fibonacci(N), stopping when the
* user inputs a zero.
*/
public static void main( String[] args ) {
while (true) {
System.out.print("\n\nEnter a postive integer, or 0 to end:  ");
int N = TextIO.getlnInt();
if (N == 0)
break;
else if (N < 0) {
System.out.println("Negative numbers are not allowed.");
continue;
}
BigInteger NasBigInteger = BigInteger.valueOf(N);
System.out.println("\n  factorial(" + N + ") is " + factorial(NasBigInteger));
if (N > 40) {
System.out.println("\n  N is too big to compute fibonacci(N) recursively");
}
else {
System.out.println("\n  fibonacci(" + N + ") is " +
fibonacci(N) + "   (recursively)");
}
System.out.println("\n  fibonacci(" + N + ") is " +
fibonacci_nonrecursive(N) + "   (non-recursively)");
}
}

/**
* Compute fibonacci(N) using recursion.  Because this is so inefficient,
* even for fairly small values of N, N should be less than or equal to 40.
* Also, N must be greater than or equal to zero, or an infinite recursion
* will occur.
*/
static int fibonacci( int N ) {
assert N >= 0 : "fibonacci(n) is only defined for non-negative n";
assert N <= 40 : "n is to large to compute fibonacci(N) recursively";
if ( N == 0 || N == 1 ) {
// Base cases; the answer is 1.
return 1;
}
else {
// Recursive case; the answer is obtained by applying the function
// recursively to N-1 and to N-2, and adding the two answers.
return fibonacci(N-1) + fibonacci(N-2);
}
}

/**
* Compute fibonacci(N) using a for loop.  The answer is returned as
* a BigInteger and can be very large even for fairly small values
* of N.  N must be greater than or equal to zero.
*/
static BigInteger fibonacci_nonrecursive( int N ) {
assert N >= 0 : "fibonacci(n) is only defined for non-negative n";
if (N == 0 || N == 1) {
// fibonacci(0) = fibonacci(1) = 1;
return BigInteger.ONE;
}
else {
BigInteger f0 = BigInteger.ONE;  // In the loop, this is fibonacci(i-2)
BigInteger f1 = BigInteger.ONE;  // In the loop, this is fibonacci(i-1)
for (int i = 2; i <= N; i++) {
BigInteger fi = f0.add(f1);  // Computes fibonacci(i)
f0 = f1;  // Update to account for i++
f1 = fi;  // Update to account for i++
}
return f1;  // Final value of f1 is fibonacci(N)
}
}

/**
* Compute factorial(N) using recursion.  The computation is done using
* BigIntegers and can be very large even for fairly small values of N.
* N must be greater than or equal to zero.
*/
static BigInteger factorial( BigInteger N ) {
assert N.signum() >= 0 : "factorial(n) is only defined for non-negative n";
if ( N.equals(BigInteger.ZERO) ) {
// Base case; the answer is 1.
return BigInteger.ONE;
}
else {
// Recursive case; the answer is obtained by applying the function
// recursively to N-1 and multiplying the answer by N.
BigInteger factorialOfNMinus1 = factorial(N.subtract(BigInteger.ONE));
return N.multiply(factorialOfNMinus1);
}
}

}

Solution for Programming Exercise 9.2

Exercise 9.2:

Exercise 7.7 asked you to read a file, make an alphabetical list of all the words that occur in the file, and write the list to another file. In that exercise, you were asked to use an ArrayList<String> to store the words. Write a new version of the same program that stores the words in a binary sort tree instead of in an arraylist. You can use the binary sort tree routines from SortTreeDemo.java, which was discussed in Subsection 9.4.2.

Discussion

In my solution to Exercise 7.7, words are added to an arraylist in the order in which they are encountered. After the file has been completely read, the arraylist is sorted into alphabetical order before the list of words is printed. Since a binary sort tree is designed to store words in alphabetical order at all times, there is no need for sorting. At the end of the program, an inorder traversal of the tree can be used to output the words to the file. Using an inorder traversal guarantees that the words will be output in increasing order.

For my solution to this exercise, I copied the routines treeInsert, treeContains, and countNodes from SortTreeDemo.java. I also copied the declaration of root as a static member variable, since that's the variable that represents the tree itself. (It's unfortunate that root has to be a global variable rather than a local variable in main(), but it's used as a global variable in the treeInsert routine. A better solution to the exercise would define a BinarySortTree class to encapsulate the data and routines needed to represent the tree and to use a variable of type BinarySortTree in the program.)

Only a few changes are needed in the main() routine of the original program. They are shown in red italic in the solution shown below. All-in-all, the substitution of the binary tree for the arraylist is very straightforward.

The Solution

/**
* Makes an alphabetical list of all the words in a file selected
* by the user.  The list can be written to a file.
* The words are stored in a binary sort tree.
*/
public class ListAllWordsFromFileWithTree {

private static TreeNode root;  // Pointer to the root node in a binary tree.
// This tree is used in this program as a
// binary sort tree.  When the tree is empty,
// root is null (as it is initially).

public static void main(String[] args) {

System.out.println("\n\nThis program will ask you to select an input file");
System.out.println("It will read that file and make an alphabetical");
System.out.println("list of all the words in the file.  After reading");
System.out.println("the file, the program asks you to select an output");
System.out.println("file.  If you select a file, the list of words will");
System.out.println("be written to that file; if you cancel, the list");
System.out.println("be written to standard output.  All words are converted");
System.out.println("lower case, and duplicates are eliminated from the list.\n\n");
System.out.print("Press return to begin.");
TextIO.getln();  // Wait for user to press return.

try {
if (TextIO.readUserSelectedFile() == false) {
System.out.println("No input file selected.  Exiting.");
System.exit(1);
}
// ArrayList<String> wordList = new ArrayList<String>(); DELETED LINE
String word = readNextWord();
while (word != null) {
word = word.toLowerCase();  // convert word to lower case
if ( treeContains(root,word) == false ) {
// This is a new word, so add it to the list
treeInsert(word);
}
word = readNextWord();
}
int wordsInTree = countNodes(root);
System.out.println("Number of different words found in file:  "
+ wordsInTree);
System.out.println();
if (wordsInTree == 0) {
System.out.println("No words found in file.");
System.out.println("Exiting without saving data.");
System.exit(0);
}
// selectionSort(wordList);  DELETED LINE
TextIO.writeUserSelectedFile(); // If user cancels, output automatically
// goes to standard output.
System.out.println(wordsInTree + " words found in file:\n");
treeList(root);
System.out.println("\n\nDone.\n\n");
}
catch (Exception e) {
System.out.println("Sorry, an error has occurred.");
System.out.println("Error Message:  " + e.getMessage());
}
System.exit(0);  // Might be necessary, because of use of file dialogs.
}

/**
* Read the next word from TextIO, if there is one.  First, skip past
* and non-letters in the input.  If an end-of-file is encountered before
* a word is found, return null.  Otherwise, read and return the word.
* A word is defined as a sequence of letters.  Also, a word can include
* an apostrophe if the apostrophe is surrounded by letters on each side.
* @return the next word from TextIO, or null if an end-of-file is encountered
*/
private static String readNextWord() {
char ch = TextIO.peek(); // Look at next character in input.
while (ch != TextIO.EOF && !Character.isLetter(ch)) {
TextIO.getAnyChar();  // Read the character.
ch = TextIO.peek();   // Look at the next character.
}
if (ch == TextIO.EOF) // Encountered end-of-file
return null;
// At this point, we know that the next character, so read a word.
String word = "";  // This will be the word that is read.
while (true) {
word += TextIO.getAnyChar();  // Append the letter onto word.
ch = TextIO.peek();  // Look at next character.
if ( ch == '\'' ) {
// The next character is an apostrophe.  Read it, and
// if the following character is a letter, add both the
// apostrophe and the letter onto the word and continue
// reading the word.  If the character after the apostrophe
// is not a letter, the word is done, so break out of the loop.
TextIO.getAnyChar();   // Read the apostrophe.
ch = TextIO.peek();    // Look at char that follows apostrophe.
if (Character.isLetter(ch)) {
word += "\'" + TextIO.getAnyChar();
ch = TextIO.peek();  // Look at next char.
}
else
break;
}
if ( ! Character.isLetter(ch) ) {
// If the next character is not a letter, the word is
// finished, so bread out of the loop.
break;
}
// If we haven't broken out of the loop, next char is a letter.
}
return word;  // Return the word that has been read.
}

//------------- Binary Sort Tree data structures and methods ------------------
//------------- (Copied from SortTreeDemo.java) -------------------------------

/**
* An object of type TreeNode represents one node in a binary tree of strings.
*/
private static class TreeNode {
String item;      // The data in this node.
TreeNode left;    // Pointer to left subtree.
TreeNode right;   // Pointer to right subtree.
TreeNode(String str) {
// Constructor.  Make a node containing the specified string.
// Note that left and right pointers are initially null.
item = str;
}
}  // end nested class TreeNode

/**
* Add the item to the binary sort tree to which the global variable
* "root" refers.  (Note that root can't be passed as a parameter to
* this routine because the value of root might change, and a change
* in the value of a formal parameter does not change the actual parameter.)
*/
private static void treeInsert(String newItem) {
if ( root == null ) {
// The tree is empty.  Set root to point to a new node containing
// the new item.  This becomes the only node in the tree.
root = new TreeNode( newItem );
return;
}
TreeNode runner;  // Runs down the tree to find a place for newItem.
runner = root;   // Start at the root.
while (true) {
if ( newItem.compareTo(runner.item) < 0 ) {
// Since the new item is less than the item in runner,
// it belongs in the left subtree of runner.  If there
// is an open space at runner.left, add a new node there.
// Otherwise, advance runner down one level to the left.
if ( runner.left == null ) {
runner.left = new TreeNode( newItem );
return;  // New item has been added to the tree.
}
else
runner = runner.left;
}
else {
// Since the new item is greater than or equal to the item in
// runner it belongs in the right subtree of runner.  If there
// is an open space at runner.right, add a new node there.
// Otherwise, advance runner down one level to the right.
if ( runner.right == null ) {
runner.right = new TreeNode( newItem );
return;  // New item has been added to the tree.
}
else
runner = runner.right;
}
} // end while
}  // end treeInsert()

/**
* Return true if item is one of the items in the binary
* sort tree to which root points.   Return false if not.
*/
static boolean treeContains( TreeNode root, String item ) {
if ( root == null ) {
// Tree is empty, so it certainly doesn't contain item.
return false;
}
else if ( item.equals(root.item) ) {
// Yes, the item has been found in the root node.
return true;
}
else if ( item.compareTo(root.item) < 0 ) {
// If the item occurs, it must be in the left subtree.
return treeContains( root.left, item );
}
else {
// If the item occurs, it must be in the right subtree.
return treeContains( root.right, item );
}
}  // end treeContains()

/**
* Print the items in the tree in inorder, one item to a line.
* Since the tree is a sort tree, the output  will be in increasing order.
*/
private static void treeList(TreeNode node) {
if ( node != null ) {
treeList(node.left);             // Print items in left subtree.
System.out.println("  " + node.item);  // Print item in the node.
treeList(node.right);            // Print items in the right subtree.
}
} // end treeList()

/**
* Count the nodes in the binary tree.
* @param node A pointer to the root of the tree.  A null value indicates
* an empty tree
* @return the number of nodes in the tree to which node points.  For an
* empty tree, the value is zero.
*/
private static int countNodes(TreeNode node) {
if ( node == null ) {
// Tree is empty, so it contains no nodes.
return 0;
}
else {
// Add up the root node and the nodes in its two subtrees.
int leftCount = countNodes( node.left );
int rightCount = countNodes( node.right );
return  1 + leftCount + rightCount;
}
} // end countNodes()

}

Solution for Programming Exercise 9.3

Exercise 9.3:

Suppose that linked lists of integers are made from objects belonging to the class

class ListNode {
int item;       // An item in the list.
ListNode next;  // Pointer to the next node in the list.
}

Write a subroutine that will make a copy of a list, with the order of the items of the list reversed. The subroutine should have a parameter of type ListNode, and it should return a value of type ListNode. The original list should not be modified.

You should also write a main() routine to test your subroutine.

Discussion

To make any linked list from scratch, you have to create nodes one-by-one and link them together. In this case, we want to make nodes that contain copies of the items from the original list. We can run through the original list, look at each item, create a new node that contains a copy of that item, and link that new node into the reversed list that we are constructing. We just have to make sure that the nodes in the new list are in the desired order.

In fact this is pretty easy: As we run down the original list from start to finish, we need to build the reversed list from back to front. The first item in the original list should be at the back of the reversed list, the second item from the original goes in front of that item, and so on. This is equivalent to "pushing" the items onto the reversed list, using the same push operation that is used for a stack. An algorithm for this is:

Let rev be an empty list
for each item in the original list:
Push the item onto rev
Move on to the next item

This can be coded into the subroutine we need as follows:

/**
* Return a new list containing the same items as the list,
* but in the reverse order.
*/
static ListNode reverse( ListNode list ) {
ListNode rev = null;     // rev will be the reversed list.
ListNode runner = list;  // For running through the nodes of list.
while (runner != null) {
// "Push" the next node of list onto the front of rev.
ListNode newNode = new ListNode();
newNode.item = runner.item;
newNode.next = rev;
rev = newNode;
// Move on to the next node in the list.
runner = runner.next;
}
return rev;
} // end reverse()

The exercise lets you design your own routine for testing the subroutine. It should be tested on several lists, including an empty list. It's important to test it on the empty list since a null pointer often represents a special case in an algorithm, and is therefore a common source of bugs. It's also a good idea to test a list of length one, for similar reasons. In my main() routine, I build up a random list one node at a time and test the reverse() subroutine on the list at each step. The main() routine was probably harder to write than the subroutine!

The Solution

/**
* This program includes a subroutine that makes a reversed copy of a
* list of ints.  The main program simply tests that routine on a few lists.
*/
public class ReverseListDemo {

/**
* Objects of type ListNode are linked together into linked lists.
*/
static class ListNode {
int item;        // An item in the list.
ListNode next;   // Pointer to the next node in the list.
}

/**
* Return a new list containing the same items as the list,
* but in the reverse order.
*/
static ListNode reverse( ListNode list ) {
ListNode rev = null;     // rev will be the reversed list.
ListNode runner = list;  // For running through the nodes of list.
while (runner != null) {
// "Push" the next node of list onto the front of rev.
ListNode newNode = new ListNode();
newNode.item = runner.item;
newNode.next = rev;
rev = newNode;
// Move on to the next node in the list.
runner = runner.next;
}
return rev;
} // end reverse()

/**
* Prints the items in the list to which the parameter, start, points.
* They are printed on one line, separated by spaces and enclosed in
* parentheses.
*/
static void printList(ListNode start) {
ListNode runner;  // For running along the list.
runner = start;
System.out.print("(");
while (runner != null) {
System.out.print(" " + runner.item);
runner = runner.next;
}
System.out.print(" )");
} // end printList()

public static void main(String[] args) {

System.out.println("I will print out a list and its reverse, for");
System.out.println("several different lists.  The first list is empty.\n");

ListNode list = null;   // A list, initially empty.
ListNode reversedList;  // The reversed list.

int ct = 0;  // How many lists have we processed?

while (true) {
// Print the current list and its reverse.  Then
// add a new node onto the head of the list before
// repeating.
System.out.print("The list:          ");
printList(list);
System.out.println();
reversedList = reverse(list);
System.out.print("The reversed list: ");
printList(reversedList);
System.out.println();
System.out.println();
ct++;
if (ct == 6)
break;
ListNode head = new ListNode();  // A new node to add to the list.
head.item = (int)(Math.random()*100);  // A random item.
head.next = list;
list = head;
}

} // end main()

} // end class ReverseListDemo

Solution for Programming Exercise 9.4

Exercise 9.4:

Subsection 9.4.1 explains how to use recursion to print out the items in a binary tree in various orders. That section also notes that a non-recursive subroutine can be used to print the items, provided that a stack or queue is used as an auxiliary data structure. Assuming that a queue is used, here is an algorithm for such a subroutine:

Add the root node to an empty queue
while the queue is not empty:
Get a node from the queue
Print the item in the node
if node.left is not null:
add it to the queue
if node.right is not null:
add it to the queue

Write a subroutine that implements this algorithm, and write a program to test the subroutine. Note that you will need a queue of TreeNodes, so you will need to write a class to represent such queues.

(Note that the order in which items are printed by this algorithm is different from all three of the orders considered in Subsection 9.4.1.)

Discussion

There's really not a lot to think about here, since such a complete algorithm is given. However, we do have to assemble the pieces. I use the standard binary tree node from Section 9.4 (except that I changed the name of the tree node class to StrTreeNode). The algorithm needs a queue of tree nodes. To implement this, I copied the QueueOfInts class from Subsection 9.3.2 and changed the type of the items in the queue to StrTreeNode. I also changed the name to TreeQueue. I did this literally by copying the class from a Web browser window and pasting it into my source code file. With these classes in hand, the algorithm given in the exercise can be coded as:

/**
* Use a queue to print all the strings in the tree to which
* root points.  (The nodes will be listed in "level order",
* that is:  first the root, then children of the root, then
* grandchildren of the root, and so on.)
*/
static void levelOrderPrint(StrTreeNode root) {
if (root == null)
return;  // There is nothing to print in an empty tree.
TreeQueue queue;   // The queue.
queue = new TreeQueue();
queue.enqueue(root);
while ( queue.isEmpty() == false ) {
StrTreeNode node = queue.dequeue();
System.out.println( node.item );
if ( node.left != null )
queue.enqueue( node.left );
if ( node.right != null )
queue.enqueue( node.right );
}
} // end levelOrderPrint()

The name of this routine comes from the order in which it prints out the nodes of the tree. Think of the root of the tree as being on the top "level" of the tree, the children of the root on the second level, the children of the children of the root on the third level, and so on. Then the subroutine prints the items in level order. That is, all the nodes on one level are printed before any of the nodes on the next level. This is a consequence of the way the algorithm processes the items. As items from one level are removed from the queue and printed, their children (which are the nodes on the next level) are added to the back of the queue. Just after all the items from one level have been processed, the queue contains all the children of those items, ready to be processed, and those children are exactly the nodes on the next level of the tree. Level-order tree traversals can't be done by recursion, and they are a standard application of queues.

To test my subroutine, I wanted a reasonably large tree whose structure I knew, so I could check whether the nodes are printed in the correct order. (Since you didn't know about level-order traversals until now, on the other hand, you should have been mainly concerned with checking that all the nodes in the tree are printed, period.) I decided to create the binary sort tree shown in Subsection 9.4.2. To do this, I copied the treeInsert()subroutine from that section and used it to add names to the tree in an order that would produce the tree I wanted. Finally, I called levelOrderPrint() to output the names from the tree. (It worked!)

By the way, you might notice that the levelOrderPrint() routine is very similar to the technique used in the grid-marking algorithm in the sample program DepthBreadth.java from Subsection 9.3.2. In fact they are just variations on the same idea. One difference is that in DepthBreadth.java, the squares of the grid had to be marked as "visited" as they were processed to avoid going into an infinite loop. The levelOrderPrint() subroutine doesn't have to do the same type of marking because it is working on a tree. One of the defining properties of a tree is that it cannot contain a loop of nodes. That is, it is not possible for a node to be its own descendant. This restriction guarantees that levelOrderPrint() will not go into an infinite loop. The same property guarantees that all of our recursive tree-processing methods will not suffer from infinite recursion when they are applied to a tree. You should note, however, that it is possible to connect tree nodes into data structures that contain loops and are therefore not trees at all. While these data structures are not trees, they might have other uses. Many of the subroutines we've looked will fail if applied to these loopy structures.

The Solution

/**
* This program includes a non-recursive subroutine that prints the
* nodes of a binary tree, using a queue.  The main program simply
* tests that routine.  (The nodes are printed in what is called
* "level order".)
*
* This file defines the queue and tree classes as nested classes.
* Since they are general-purpose classes, it would really be better
* to put them in separate files.
*/

public class TreePrintNonRecursive {

//--------------------------------- NESTED CLASSES -----------------------

/**
* An object in this class is a node in a binary tree
* in which the nodes contain items of type String.
*/
static class StrTreeNode {
String item;  // The item
StrTreeNode left;  // Pointer to left subtree.
StrTreeNode right; // Pointer to right subtree.
StrTreeNode( String str ) {
// Constructor.  Make a node to contain str.
item = str;
}
} // end class StrTreeNode

/**
* An object of this type represents a queue of StrTreeNodes,
* with the usual operations: dequeue, enqueue, isEmpty.
*/
static class TreeQueue {

/**
* An object of type Node holds one of the items
* in the linked list that represents the queue.
*/
private static class Node {
StrTreeNode item;
Node next;
}

private Node head = null;  // Points to first Node in the queue.
// The queue is empty when head is null.

private Node tail = null;  // Points to last Node in the queue.

/**
* Add N to the back of the queue.
*/
void enqueue( StrTreeNode tree ) {
Node newTail = new Node();  // A Node to hold the new item.
newTail.item = tree;
if (head == null) {
// The queue was empty.  The new Node becomes
// the only node in the list.  Since it is both
// the first and last node, both head and tail
// point to it.
head = newTail;
tail = newTail;
}
else {
// The new node becomes the new tail of the list.
// (The head of the list is unaffected.)
tail.next = newTail;
tail = newTail;
}
}

/**
* Remove and return the front item in the queue.
* Throws an IllegalStateException if the queue is empty.
*/
StrTreeNode dequeue() {
if ( head == null)
throw new IllegalStateException("Can't dequeue from an empty queue.");
StrTreeNode firstItem = head.item;
head = head.next;  // The previous second item is now first.
if (head == null) {
// The queue has become empty.  The Node that was
// deleted was the tail as well as the head of the
// list, so now there is no tail.  (Actually, the
// class would work fine without this step.)
tail = null;
}
return firstItem;
}

/**
* Return true if the queue is empty, false if contains one
* or more items
*/
boolean isEmpty() {
return (head == null);
}

} // end class TreeQueue

//-------------------- END OF NESTED CLASSES ---------------------------

static StrTreeNode root;  // A pointer to the root of the binary tree.

/**
* Use a queue to print all the strings in the tree to which
* root points.  (The nodes will be listed in "level order",
* that is:  first the root, then children of the root, then
* grandchildren of the root, and so on.)
*/
static void levelOrderPrint(StrTreeNode root) {
if (root == null)
return;  // There is nothing to print in an empty tree.
TreeQueue queue;   // The queue, which will only hold non-null nodes.
queue = new TreeQueue();
queue.enqueue(root);
while ( queue.isEmpty() == false ) {
StrTreeNode node = queue.dequeue();
System.out.println( node.item );
if ( node.left != null )
queue.enqueue( node.left );
if ( node.right != null )
queue.enqueue( node.right );
}
} // end levelOrderPrint()

/**
* Add the word to the binary sort tree to which the
* global variable "root" refers.  I will use this
* routine only to create the sample tree on which
* I will test levelOrderPrint().
*/
static void treeInsert(String newItem) {
if ( root == null ) {
// The tree is empty.  Set root to point to a new node
// containing the new item.
root = new StrTreeNode( newItem );
return;
}
StrTreeNode runner; // Runs down the tree to find a place for newItem.
runner = root;   // Start at the root.
while (true) {
if ( newItem.compareTo(runner.item) < 0 ) {
// Since the new item is less than the item in runner,
// it belongs in the left subtree of runner.  If there
// is an open space at runner.left, add a node there.
// Otherwise, advance runner down one level to the left.
if ( runner.left == null ) {
runner.left = new StrTreeNode( newItem );
return;  // New item has been added to the tree.
}
else
runner = runner.left;
}
else {
// Since the new item is greater than or equal to the
// item in runner, it belongs in the right subtree of
// runner.  If there is an open space at runner.right,
// add a new node there.  Otherwise, advance runner
// down one level to the right.
if ( runner.right == null ) {
runner.right = new StrTreeNode( newItem );
return;  // New item has been added to the tree.
}
else
runner = runner.right;
}
} // end while
}  // end treeInsert()

/**
* Make a tree with a known form, then call levelOrderPrint()
* for that tree.  (I want to check that all the items from
* the tree are printed, and I want to see the order in which
* they are printed.  The expected order of output is
* judy bill mary alice fred tom dave jane joe.  The
* tree that is built here is from an illustration in
* Section 9.4.)
*/
public static void main(String[] args) {
treeInsert("judy");
treeInsert("bill");
treeInsert("fred");
treeInsert("mary");
treeInsert("dave");
treeInsert("jane");
treeInsert("alice");
treeInsert("joe");
treeInsert("tom");
levelOrderPrint(root);
} // end main()

} // end class TreePrintNonRecursive

Solution for Programming Exercise 9.5

Exercise 9.5:

In Subsection 9.4.2, I say that "if the [binary sort] tree is created by inserting items in a random order, there is a high probability that the tree is approximately balanced." For this exercise, you will do an experiment to test whether that is true.

The depth of a node in a binary tree is the length of the path from the root of the tree to that node. That is, the root has depth 0, its children have depth 1, its grandchildren have depth 2, and so on. In a balanced tree, all the leaves in the tree are about the same depth. For example, in a perfectly balanced tree with 1023 nodes, all the leaves are at depth 9. In an approximately balanced tree with 1023 nodes, the average depth of all the leaves should be not too much bigger than 9.

On the other hand, even if the tree is approximately balanced, there might be a few leaves that have much larger depth than the average, so we might also want to look at the maximum depth among all the leaves in a tree.

For this exercise, you should create a random binary sort tree with 1023 nodes. The items in the tree can be real numbers, and you can create the tree by generating 1023 random real numbers and inserting them into the tree, using the usual treeInsert() method for binary sort trees. Once you have the tree, you should compute and output the average depth of all the leaves in the tree and the maximum depth of all the leaves. To do this, you will need three recursive subroutines: one to count the leaves, one to find the sum of the depths of all the leaves, and one to find the maximum depth. The latter two subroutines should have an int-valued parameter, depth, that tells how deep in the tree you've gone. When you call this routine from the main program, the depth parameter is 0; when you call the routine recursively, the parameter increases by 1.

Discussion

To create the tree, I copied the TreeNode class and the insertTree() subroutine from Subsection 9.4.2, and I changed the type of the items in the tree from String to double. The main program uses a for loop to add 1023 random real numbers to the tree:

for (int i = 0; i < 1023; i++)
treeInsert(Math.random());

After that, it's just a matter of writing the routines described in the exercise and calling them to get the desired statistics.

A routine for counting the leaves in the tree is similar to the countNodes() routine from Subsection 9.4.2. That routine, however, counts every node in the tree and now we only want to count the leaves. A leaf is defined to be a node in which both the left and right pointers are null. In the recursion, one of the base cases is when we come to a tree that consists of nothing but a leaf. In that case, the number of leaves is 1. If the node is not a leaf, then we have to count the number of leaves in each of its subtrees and add the results:

/**
* Return the number of leaves in the tree to which node points.
*/
static int countLeaves(TreeNode node) {
if (node == null)
return 0;  // An empty tree has no leaves.
else if (node.left == null && node.right == null)
return 1;  // Node is a leaf.
else
return countLeaves(node.left) + countLeaves(node.right);
} // end countNodes()

In general structure, the other two routines are similar. That is, there are two base cases: an empty tree and a tree consisting just of a leaf. In the remaining case -- a node that has one or both subtrees non-empty -- the routine has to be applied recursively to the subtrees of the node. Look, for example, at the routine for finding the sum of the depths of all the leaves in the tree:

/**
* When called as sumOfLeafDepths(root,0), this will compute the
* sum of the depths of all the leaves in the tree to which root
* points.  When called recursively, the depth parameter gives
* the depth of the node, and the routine returns the sum of the
* depths of the leaves in the subtree to which node points.
* In each recursive call to this routine, depth goes up by one.
*/
static int sumOfLeafDepths( TreeNode node, int depth ) {
if ( node == null ) {
// Since the tree is empty and there are no leaves,
// the sum is zero.
return 0;
}
else if ( node.left == null && node.right == null) {
// The node is a leaf, and there are no subtrees of node, so
// the sum of the leaf depths is just the depth of this node.
return depth;
}
else {
// The node is not a leaf.  Return the sum of the
// the depths of the leaves in the subtrees.
return sumOfLeafDepths(node.left, depth + 1)
+ sumOfLeafDepths(node.right, depth + 1);
}
} // end sumOfLeafDepth()

The most interesting aspect of this routine is the way it uses its depth parameter, which is used to keep track of the depth of the node in the complete tree (not just the subtree to which node points). For the root, the depth is 0. Each time the subroutine is called recursively, the node is one level deeper in the tree, and the depth parameter is correspondingly increased by 1. When we get down to a leaf node, where node.left and node.right are null, the value of depth is the depth of that node in the original tree, and the sum of the depths of the leaves in the subtree, which consists of just this one leaf node, is depth. When node is not a leaf, the sums for the two subtrees of node are computed recursively and are added together to give the sum for all the leaves in the whole subtree to which node refers. (If you have trouble believing that this works, remember that recursion works if it works for the base cases and if it correctly breaks down big problems into smaller problems. You don't have to follow the details.)

The routine for computing the maximum depth is similar.

When I ran my program several times, I found that the average depth of the leaves in the tree tended to be about 12 -- higher than I expected but still only 1/3 more than the average depth in a perfectly balanced tree. The height of the tree tended to be about 20. (The height of a tree is defined to be the maximum depth of any node in the tree.)

The Solution

/**
* This program makes a random binary sort tree containing 1023 random
* real numbers.  It then computes the height of the tree and the
* average depth of the leaves of the tree.  Hopefully, the average
* depth will tend to be close to 9, which is what it would be
* if the tree were perfectly balanced.  The height of the tree,
* which is the same as the maximum depth of any leaf, can be
* significantly larger.
*/
public class RandomSortTree {

static TreeNode root;   // Pointer to the binary sort tree.

/**
* An object of type TreeNode represents one node in a binary tree of real numbers.
*/
static class TreeNode {
double item;      // The data in this node.
TreeNode left;    // Pointer to left subtree.
TreeNode right;   // Pointer to right subtree.
TreeNode(double x) {
// Constructor.  Make a node containing x.
item = x;
}
} // end class TreeNode

/**
* Add x to the binary sort tree to which the global variable "root" refers.
*/
static void treeInsert(double x) {
if ( root == null ) {
// The tree is empty.  Set root to point to a new node
// containing the new item.
root = new TreeNode( x );
return;
}
TreeNode runner; // Runs down the tree to find a place for newItem.
runner = root;   // Start at the root.
while (true) {
if ( x < runner.item ) {
// Since the new item is less than the item in runner,
// it belongs in the left subtree of runner.  If there
// is an open space at runner.left, add a node there.
// Otherwise, advance runner down one level to the left.
if ( runner.left == null ) {
runner.left = new TreeNode( x );
return;  // New item has been added to the tree.
}
else
runner = runner.left;
}
else {
// Since the new item is greater than or equal to the
// item in runner, it belongs in the right subtree of
// runner.  If there is an open space at runner.right,
// add a new node there.  Otherwise, advance runner
// down one level to the right.
if ( runner.right == null ) {
runner.right = new TreeNode( x );
return;  // New item has been added to the tree.
}
else
runner = runner.right;
}
} // end while
}  // end treeInsert()

/**
* Return the number of leaves in the tree to which node points.
*/
static int countLeaves(TreeNode node) {
if (node == null)
return 0;
else if (node.left == null && node.right == null)
return 1;  // Node is a leaf.
else
return countLeaves(node.left) + countLeaves(node.right);
} // end countNodes()

/**
* When called as sumOfLeafDepths(root,0), this will compute the
* sum of the depths of all the leaves in the tree to which root
* points.  When called recursively, the depth parameter gives
* the depth of the node, and the routine returns the sum of the
* depths of the leaves in the subtree to which node points.
* In each recursive call to this routine, depth goes up by one.
*/
static int sumOfLeafDepths( TreeNode node, int depth ) {
if ( node == null ) {
// Since the tree is empty and there are no leaves,
// the sum is zero.
return 0;
}
else if ( node.left == null && node.right == null) {
// The node is a leaf, and there are no subtrees of node, so
// the sum of the leaf depth is just the depths of this node.
return depth;
}
else {
// The node is not a leaf.  Return the sum of the
// the depths of the leaves in the subtrees.
return sumOfLeafDepths(node.left, depth + 1)
+ sumOfLeafDepths(node.right, depth + 1);
}
} // end sumOfLeafDepths()

/**
* When called as maximumLeafDepth(root,0), this will compute the
* max of the depths of all the leaves in the tree to which root
* points.  When called recursively, the depth parameter gives
* the depth of the node, and the routine returns the max of the
* depths of the leaves in the subtree to which node points.
* In each recursive call to this routine, depth goes up by one.
*/
static int maximumLeafDepth( TreeNode node, int depth ) {
if ( node == null ) {
// The tree is empty.  Return 0.
return 0;
}
else if ( node.left == null && node.right == null) {
// The node is a leaf, so the maximum depth in this
// subtree is the depth of this node (the only leaf
// that it contains).
return depth;
}
else {
// Get the maximum depths for the two subtrees of this
// node.  Return the larger of the two values, which
// represents the maximum in the tree overall.
int leftMax = maximumLeafDepth(node.left, depth + 1);
int rightMax =  maximumLeafDepth(node.right, depth + 1);
if (leftMax > rightMax)
return leftMax;
else
return rightMax;
}
} // end maximumLeafDepth()

/**
* The main routine makes the random tree and prints the statistics.
*/
public static void main(String[] args) {

root = null;  // Start with an empty tree.  Root is a global
// variable, defined at the top of the class.

// Insert 1023 random items.

for (int i = 0; i < 1023; i++)
treeInsert(Math.random());

// Get the statistics.

int leafCount = countLeaves(root);
int depthSum = sumOfLeafDepths(root,0);
int depthMax = maximumLeafDepth(root,0);
double averageDepth = ((double)depthSum) / leafCount;

// Display the results.

System.out.println("Number of leaves:         " + leafCount);
System.out.println("Average depth of leaves:  " + averageDepth);
System.out.println("Maximum depth of leaves:  " + depthMax);

}  // end main()

} // end class RandomSortTree

Solution for Programming Exercise 9.6

Exercise 9.6:

The parsing programs in Section 9.5 work with expressions made up of numbers and operators. We can make things a little more interesting by allowing the variable "x" to occur. This would allow expression such as "3*(x-1)*(x+1)", for example. Make a new version of the sample program SimpleParser3.java that can work with such expressions. In your program, the main() routine can't simply print the value of the expression, since the value of the expression now depends on the value of x. Instead, it should print the value of the expression for x=0, x=1, x=2, and x=3.

The original program will have to be modified in several other ways. Currently, the program uses classes ConstNode, BinOpNode, and UnaryMinusNode to represent nodes in an expression tree. Since expressions can now include x, you will need a new class, VariableNode, to represent an occurrence of x in the expression.

In the original program, each of the node classes has an instance method, "double value()", which returns the value of the node. But in your program, the value can depend on x, so you should replace this method with one of the form "double value(double xValue)", where the parameter xValue is the value of x.

Finally, the parsing subroutines in your program will have to take into account the fact that expressions can contain x. There is just one small change in the BNF rules for the expressions: A <factor> is allowed to be the variable x:

<factor>  ::=  <number>  |  <x-variable>  |  "(" <expression> ")"

where <x-variable> can be either a lower case or an upper case "X". This change in the BNF requires a change in the factorTree() subroutine.

Discussion

Like the other expression node classes, the VariableNode class is a subclass of ExpNode, and it must implement the value(x) and printStackCommands() methods that it inherits from that class. The value(x) method has been modified to have a parameter of type double, which gives the value of the variable x. Since a VariableNode represents an occurrence of the variable x, the value of the node is simply the value of x. As for the stack commands to evaluate the node: When we encounter an x in an expression, we need to push the value of x onto the stack, just as we would push a constant value onto the stack. I represent this with a stack operation "Push X". Note that x can have different values at different times, so we can't say what value will be pushed. We are generating instructions for a stack machine. At the time when the stack machine evaluates the expression, it has to know the value of x. The "Push X" command tells it to push a copy of that value onto the stack. The VariableNode class is defined as:

/**
* An expression node that represents a reference to the variable, x.
*/
private static class VariableNode extends ExpNode {
VariableNode() {
// Construct a VariableNode. (There is nothing to do!)
}
double value(double xValue) {
// The value of the node is the value of x.
return xValue;
}
void printStackCommands() {
// On a stack machine, just push the value of X onto the stack.
System.out.println("  Push X");
}
}

One curious thing about this class is that it doesn't have any instance variables. A VariableNode represents an occurrence of x. There is no other information to record. It's not like a ConstNode where we need an instance variable to tell us which numerical constant has been found. There is only one "x". Of course, if we expanded our definition of expression to allow other variables such as y and z, we might add an instance variable to VariableNode to say which of the possible variables is represented.

The factorTree() subroutine from SimpleParser3.java has to be modified to check for an x. If it finds one, it has to return a new VariableNode. This change and the others you have to make are fairly straightforward. They are shown in red italic in the solution that follows.

The Solution

/*
This program reads standard expressions typed in by the user.
The program constructs an expression tree to represent the
expression.  It then prints the value of the tree.  It also uses
the tree to print out a list of commands that could be used
on a stack machine to evaluate the expression.
The expressions can use the variable "x", positive real numbers, and
the binary operators +, -, *, and /.  The unary minus operation
is supported.  The expressions are defined by the BNF rules:

<expression>  ::=  [ "-" ] <term> [ [ "+" | "-" ] <term> ]...

<term>  ::=  <factor> [ [ "*" | "/" ] <factor> ]...

<factor>  ::=  <number>  |  <x-variable> | "(" <expression> ")"

A number must begin with a digit (i.e., not a decimal point).
A line of input must contain exactly one such expression.  If extra
data is found on a line after an expression has been read, it is
considered an error.

In addition to the main program class, SimpleParser4, this program
defines a set of five nested classes for implementing expression trees.

*/

public class SimpleParser4 {

//   -------------------- Nested classes for Expression Trees ------------------------------

/**
*  An abstract class representing any node in an expression tree.
*  The four concrete node classes are concrete subclasses.
*  Two instance methods are specified, so that they can be used with
*  any ExpNode.  The value() method returns the value of the
*  expression for a specified value of the variable, x.
*  The printStackCommands() method prints a list
*  of commands that could be used to evaluate the expression on
*  a stack machine (assuming that the value of the expression is
*  to be left on the stack).
*/
abstract private static class ExpNode {
abstract double value(double xValue);
abstract void printStackCommands();
}

/**
* Represents an expression node that holds a number.
*/
private static class ConstNode extends ExpNode {
double number;  // The number.
ConstNode(double val) {
// Construct a ConstNode containing the specified number.
number = val;
}
double value(double xValue) {
// The value of the node is the number that it contains.
return number;
}
void printStackCommands() {
// On a stack machine, just push the number onto the stack.
System.out.println("  Push " + number);
}
}

/**
* An expression node representing a binary operator,
*/
private static class BinOpNode extends ExpNode {
char op;        // The operator.
ExpNode left;   // The expression for its left operand.
ExpNode right;  // The expression for its right operand.
BinOpNode(char op, ExpNode left, ExpNode right) {
// Construct a BinOpNode containing the specified data.
assert op == '+' || op == '-' || op == '*' || op == '/';
assert left != null && right != null;
this.op = op;
this.left = left;
this.right = right;
}
double value(double xValue) {
// The value is obtained by evaluating the left and right
// operands and combining the values with the operator.
double x = left.value(xValue);
double y = right.value(xValue);
switch (op) {
case '+':  return x + y;
case '-':  return x - y;
case '*':  return x * y;
case '/':  return x / y;
default:   return Double.NaN;  // Bad operator!
}
}
void  printStackCommands() {
// To evaluate the expression on a stack machine, first do
// whatever is necessary to evaluate the left operand, leaving
// the answer on the stack.  Then do the same thing for the
// second operand.  Then apply the operator (which means popping
// the operands, applying the operator, and pushing the result).
left.printStackCommands();
right.printStackCommands();
System.out.println("  Operator " + op);
}
}

/**
* An expression node to represent a unary minus operator.
*/
private static class UnaryMinusNode extends ExpNode {
ExpNode operand;  // The operand to which the unary minus applies.
UnaryMinusNode(ExpNode operand) {
// Construct a UnaryMinusNode with the specified operand.
assert operand != null;
this.operand = operand;
}
double value(double xValue) {
// The value is the negative of the value of the operand.
double neg = operand.value(xValue);
return -neg;
}
void printStackCommands() {
// To evaluate this expression on a stack machine, first do
// whatever is necessary to evaluate the operand, leaving the
// operand on the stack.  Then apply the unary minus (which means
// popping the operand, negating it, and pushing the result).
operand.printStackCommands();
System.out.println("  Unary minus");
}
}

/**
* An expression node that represents a reference to the variable, x.
*/
private static class VariableNode extends ExpNode {
VariableNode() {
// Construct a VariableNode. (There is nothing to do!)
}
double value(double xValue) {
// The value of the node is the value of x.
return xValue;
}
void printStackCommands() {
// On a stack machine, just push the value of X onto the stack.
System.out.println("  Push X");
}
}

//   -------------------------------------------------------------------------------

/**
* An object of type ParseError represents a syntax error found in
* the user's input.
*/
private static class ParseError extends Exception {
ParseError(String message) {
super(message);
}
} // end nested class ParseError

public static void main(String[] args) {

while (true) {
System.out.println("\n\nEnter an expression, or press return to end.");
System.out.print("\n?  ");
TextIO.skipBlanks();
if ( TextIO.peek() == '\n' )
break;
try {
ExpNode exp = expressionTree();
TextIO.skipBlanks();
if ( TextIO.peek() != '\n' )
throw new ParseError("Extra data after end of expression.");
TextIO.getln();
System.out.println("\nValue at x = 0 is " + exp.value(0));
System.out.println("Value at x = 1 is " + exp.value(1));
System.out.println("Value at x = 2 is " + exp.value(2));
System.out.println("Value at x = 3 is " + exp.value(3));
System.out.println("\nOrder of postfix evaluation is:\n");
exp.printStackCommands();
}
catch (ParseError e) {
System.out.println("\n*** Error in input:    " + e.getMessage());
System.out.println("*** Discarding input:  " + TextIO.getln());
}
}

System.out.println("\n\nDone.");

} // end main()

/**
* Reads an expression from the current line of input and builds
* an expression tree that represents the expression.
* @return an ExpNode which is a pointer to the root node of the
*    expression tree
* @throws ParseError if a syntax error is found in the input
*/
private static ExpNode expressionTree() throws ParseError {
TextIO.skipBlanks();
boolean negative;  // True if there is a leading minus sign.
negative = false;
if (TextIO.peek() == '-') {
TextIO.getAnyChar();
negative = true;
}
ExpNode exp;       // The expression tree for the expression.
exp = termTree();  // Start with the first term.
if (negative)
exp = new UnaryMinusNode(exp);
TextIO.skipBlanks();
while ( TextIO.peek() == '+' || TextIO.peek() == '-' ) {
// Read the next term and combine it with the
// previous terms into a bigger expression tree.
char op = TextIO.getAnyChar();
ExpNode nextTerm = termTree();
exp = new BinOpNode(op, exp, nextTerm);
TextIO.skipBlanks();
}
return exp;
} // end expressionTree()

/**
* Reads a term from the current line of input and builds
* an expression tree that represents the expression.
* @return an ExpNode which is a pointer to the root node of the
*    expression tree
* @throws ParseError if a syntax error is found in the input
*/
private static ExpNode termTree() throws ParseError {
TextIO.skipBlanks();
ExpNode term;  // The expression tree representing the term.
term = factorTree();
TextIO.skipBlanks();
while ( TextIO.peek() == '*' || TextIO.peek() == '/' ) {
// Read the next factor, and combine it with the
// previous factors into a bigger expression tree.
char op = TextIO.getAnyChar();
ExpNode nextFactor = factorTree();
term = new BinOpNode(op,term,nextFactor);
TextIO.skipBlanks();
}
return term;
} // end termValue()

/**
* Reads a factor from the current line of input and builds
* an expression tree that represents the expression.
* @return an ExpNode which is a pointer to the root node of the
*    expression tree
* @throws ParseError if a syntax error is found in the input
*/

private static ExpNode factorTree() throws ParseError {
TextIO.skipBlanks();
char ch = TextIO.peek();
if ( Character.isDigit(ch) ) {
// The factor is a number.  Return a ConstNode.
double num = TextIO.getDouble();
return new ConstNode(num);
}
else if ( ch == 'x' || ch == 'X' ) {
// The factor is the variable x.
TextIO.getAnyChar();   // Read the X.
return new VariableNode();
}
else if ( ch == '(' ) {
// The factor is an expression in parentheses.
// Return a tree representing that expression.
TextIO.getAnyChar();  // Read the "("
ExpNode exp = expressionTree();
TextIO.skipBlanks();
if ( TextIO.peek() != ')' )
throw new ParseError("Missing right parenthesis.");
TextIO.getAnyChar();  // Read the ")"
return exp;
}
else if ( ch == '\n' )
throw new ParseError("End-of-line encountered in the middle of an expression.");
else if ( ch == ')' )
throw new ParseError("Extra right parenthesis.");
else if ( ch == '+' || ch == '-' || ch == '*' || ch == '/' )
throw new ParseError("Misplaced operator.");
else
throw new ParseError("Unexpected character \"" + ch + "\" encountered.");
}  // end factorTree()

} // end class SimpleParser4

Solution for Programming Exercise 9.7

Exercise 9.7:

This exercise builds on the previous exercise, Exercise 9.6. To understand it, you should have some background in Calculus. The derivative of an expression that involves the variable x can be defined by a few recursive rules:

- The derivative of a constant is 0.

- The derivative of x is 1.

- If A is an expression, let dA be the derivative of A. Then the derivative of -A is -dA.

- If A and B are expressions, let dA be the derivative of A and let dB be the derivative of B. Then the derivative of A+B is dA+dB.

- The derivative of A-B is dA-dB.

- The derivative of A*B is A*dB + B*dA.

- The derivative of A/B is (B*dA - A*dB) / (B*B).

For this exercise, you should modify your program from the previous exercise so that it can compute the derivative of an expression. You can do this by adding a derivative-computing method to each of the node classes. First, add another abstract method to the ExpNode class:

abstract ExpNode derivative();

Then implement this method in each of the four subclasses of ExpNode. All the information that you need is in the rules given above. In your main program, instead of printing the stack operations for the original expression, you should print out the stack operations that define the derivative. Note that the formula that you get for the derivative can be much more complicated than it needs to be. For example, the derivative of 3*x+1 will be computed as (3*1+0*x)+0. This is correct, even though it's kind of ugly, and it would be nice for it to be simplified. However, simplifying expressions is not easy.

As an alternative to printing out stack operations, you might want to print the derivative as a fully parenthesized expression. You can do this by adding a printInfix() routine to each node class. It would be nice to leave out unnecessary parentheses, but again, the problem of deciding which parentheses can be left out without altering the meaning of the expression is a fairly difficult one, which I don't advise you to attempt.

(There is one curious thing that happens here: If you apply the rules, as given, to an expression tree, the result is no longer a tree, since the same subexpression can occur at multiple points in the derivative. For example, if you build a node to represent B*B by saying "new BinOpNode('*',B,B)", then the left and right children of the new node are actually the same node! This is not allowed in a tree. However, the difference is harmless in this case since, like a tree, the structure that you get has no loops in it. Loops, on the other hand, would be a disaster in most of the recursive tree-processing subroutines that we have written, since it would lead to infinite recursion. The type of structure that is built by the derivative functions is technically referred to as a directed acyclic graph.)

Discussion

The solution to Exercise 9.6 already allows the variable x to occur in expressions. Since we are building on that solution, no changes are needed in the parsing routines. There are a few easy changes in the main() routine, since it must take the derivative of the expression entered by the user and then work with that derivative. The changes are shown in the solution that is given below.

Aside from that, we only need to add the "ExpNode derivative()" method to each of the node classes. Since I want to print out the derivative in fully parenthesized infix form, I also add another method, "void printInfix()". Since this is not a required part of the exercise -- and since it's fairly simple to do -- I won't discuss the printInfix() method further.

All the information that is needed for writing the derivative() methods is given in the derivative rules that are listed in the exercise. The first three rules are pretty simple:

- Since the derivative of a constant is 0, the derivative() method in the ConstNode class has to return an ExpNode that represents the expression "0". That's easy. We just need a constant node that contains the number 0. The definition of derivative() in the ConstNode class is just: "return new ConstNode(0);".

- Similarly, the derivative of x is 1, so the definition of derivative() in the VariableNode class is "return new ConstNode(1);".

- The derivative of -A is -dA, that is, it consists of a unary minus operator applied to the derivative of the operand A. So, in the UnaryMinusNode class, we have to compute the derivative of the operand and then create an ExpNode that applies a unary minus to that derivative. The derivative of operand is operand.derivative(), so we only need to "return new UnaryMinusNode(operand.derivative());".

In the BinOpNode class, the derivative rule that we need to apply depends on the value of the binary operator, +, -, *, or /. The rules for A+B and A-B are easy to implement. Let's look at the case of *, where the rule is that the derivative of A*B is A*dB+B*dA. In the BinOpNode class, A is the left operand and B is the right operand. We can compute the derivatives dA and dB as left.derivative() and right.derivative(). We then have to build an expression tree to represent A*dB+B*dA. We need one node to represent the + operation and two more nodes to represent the two * operations. We can create the tree step-by-step:

ExpNode dA = left.derivative();
ExpNode dB = right.derivative();
ExpNode firstHalf = new BinOpNode('*', left, dB);   // A*dB
ExpNode secondHalf = new BinOpNode('*', right, dA); // B*dA
ExpNode answer = new BinOpNode('+', firstHalf, secondHalf);
return answer;  // This is the derivative we want!

In my solution, however, I did the same thing in one statement:

return new BinOpNode( '+',
new BinOpNode('*', left, right.derivative()),
new BinOpNode('*', right, left.derivative()) );

This uses the fact that a constructor call is an expression and can be used as an actual parameter in a subroutine. This statement returns a node that represents the sum of two things. The first thing is a node that represents the product of left and right.derivative(), and the second is a node that represents the product of right and left.derivative(). This is exactly the same thing that is returned by the previous sequence of six statements. There are reasonable arguments for doing things either way.

The rule for A/B is even more complicated: (B*dA-A*dB)/(B*B). Nevertheless, using left for A and right for B, I can compute the value with a single statement:

return new BinOpNode( '/',
new BinOpNode('-',
new BinOpNode('*', right, left.derivative()),
new BinOpNode('*', left, right.derivative())),
new BinOpNode('*', right, right) );

As an exercise, you might try doing the same thing with a sequence of simple statements.

The Solution

/*
This program reads standard expressions typed in by the user.
The program constructs an expression tree to represent the
expression.  It computes the derivative of the expression and
prints out the derivative and the value of the derivative at
several values of x.  It also prints out a list of commands
that could be used on a stack machine to evaluate the derivative.
The expressions can use the variable "x", positive real numbers, and
the binary operators +, -, *, and /.  The unary minus operation
is supported.  The expressions are defined by the BNF rules:

<expression>  ::=  [ "-" ] <term> [ [ "+" | "-" ] <term> ]...

<term>  ::=  <factor> [ [ "*" | "/" ] <factor> ]...

<factor>  ::=  <number>  |  <x-variable> | "(" <expression> ")"

A number must begin with a digit (i.e., not a decimal point).
A line of input must contain exactly one such expression.  If extra
data is found on a line after an expression has been read, it is
considered an error.

In addition to the main program class, SimpleParser5, this program
defines a set of five nested classes for implementing expression trees.

*/

public class SimpleParser5 {

//   -------------------- Nested classes for Expression Trees ------------------------------

/**
*  An abstract class representing any node in an expression tree.
*  The four concrete node classes are concrete subclasses.
*  Two instance methods are specified, so that they can be used with
*  any ExpNode.  The value() method returns the value of the
*  expression for a specified value of the variable, x.
*  The printStackCommands() method prints a list
*  of commands that could be used to evaluate the expression on
*  a stack machine (assuming that the value of the expression is
*  to be left on the stack).
*  The derivative() method returns an expression tree for the derivative
*  of the expression (with no attempt at simplification).  Actually,
*  this might not be a tree, but it is a "directed acyclic graph",
*  with no loops, so it's OK for our purposes.  The printInfix()
*  method prints the expression in fully parenthesized form.
*/
abstract private static class ExpNode {
abstract double value(double xValue);
abstract void printStackCommands();
abstract void printInfix();
abstract ExpNode derivative();
}

/**
* Represents an expression node that holds a number.
*/
private static class ConstNode extends ExpNode {
double number;  // The number.
ConstNode(double val) {
// Construct a ConstNode containing the specified number.
number = val;
}
double value(double xValue) {
// The value of the node is the number that it contains.
return number;
}
void printStackCommands() {
// On a stack machine, just push the number onto the stack.
System.out.println("  Push " + number);
}
void printInfix() {
System.out.print(number);
}
ExpNode derivative() {
// The derivative of a constant is zero.
return new ConstNode(0);
}
}

/**
* An expression node representing a binary operator,
*/
private static class BinOpNode extends ExpNode {
char op;        // The operator.
ExpNode left;   // The expression for its left operand.
ExpNode right;  // The expression for its right operand.
BinOpNode(char op, ExpNode left, ExpNode right) {
// Construct a BinOpNode containing the specified data.
assert op == '+' || op == '-' || op == '*' || op == '/';
assert left != null && right != null;
this.op = op;
this.left = left;
this.right = right;
}
double value(double xValue) {
// The value is obtained by evaluating the left and right
// operands and combining the values with the operator.
double x = left.value(xValue);
double y = right.value(xValue);
switch (op) {
case '+':  return x + y;
case '-':  return x - y;
case '*':  return x * y;
case '/':  return x / y;
default:   return Double.NaN;  // Bad operator!
}
}
void  printStackCommands() {
// To evaluate the expression on a stack machine, first do
// whatever is necessary to evaluate the left operand, leaving
// the answer on the stack.  Then do the same thing for the
// second operand.  Then apply the operator (which means popping
// the operands, applying the operator, and pushing the result).
left.printStackCommands();
right.printStackCommands();
System.out.println("  Operator " + op);
}
void printInfix() {
// Print the expression, in parentheses.
System.out.print('(');
left.printInfix();
System.out.print(" " + op + " ");
right.printInfix();
System.out.print(')');
}
ExpNode derivative() {
// Apply the derivative formulas.
switch (op) {
case '+':
return new BinOpNode('+', left.derivative(), right.derivative());
case '-':
return new BinOpNode('-', left.derivative(), right.derivative());
case '*':
return new BinOpNode( '+',
new BinOpNode('*', left, right.derivative()),
new BinOpNode('*', right, left.derivative()) );
case '/':
return new BinOpNode( '/',
new BinOpNode('-',
new BinOpNode('*', right, left.derivative()),
new BinOpNode('*', left, right.derivative())),
new BinOpNode('*', right, right) );
default:
return null;
}
}
}

/**
* An expression node to represent a unary minus operator.
*/
private static class UnaryMinusNode extends ExpNode {
ExpNode operand;  // The operand to which the unary minus applies.
UnaryMinusNode(ExpNode operand) {
// Construct a UnaryMinusNode with the specified operand.
assert operand != null;
this.operand = operand;
}
double value(double xValue) {
// The value is the negative of the value of the operand.
double neg = operand.value(xValue);
return -neg;
}
void printStackCommands() {
// To evaluate this expression on a stack machine, first do
// whatever is necessary to evaluate the operand, leaving the
// operand on the stack.  Then apply the unary minus (which means
// popping the operand, negating it, and pushing the result).
operand.printStackCommands();
System.out.println("  Unary minus");
}
void printInfix() {
// Print the expression, in parentheses.
System.out.print("(-");
operand.printInfix();
System.out.print(')');
}
ExpNode derivative() {
// The derivative of -A is -(derivative of A).
return new UnaryMinusNode(operand.derivative());
}
}

/**
* An expression node that represents a reference to the variable, x.
*/
private static class VariableNode extends ExpNode {
VariableNode() {
// Construct a VariableNode. (There is nothing to do!)
}
double value(double xValue) {
// The value of the node is the value of x.
return xValue;
}
void printStackCommands() {
// On a stack machine, just push the value of X onto the stack.
System.out.println("  Push X");
}
void printInfix() {
System.out.print("x");
}
ExpNode derivative() {
// The derivative of x is the constant 1.
return new ConstNode(1);
}
}

//   -------------------------------------------------------------------------------

/**
* An object of type ParseError represents a syntax error found in
* the user's input.
*/
private static class ParseError extends Exception {
ParseError(String message) {
super(message);
}
} // end nested class ParseError

public static void main(String[] args) {

while (true) {
System.out.println("\n\nEnter an expression, or press return to end.");
System.out.print("\n?  ");
TextIO.skipBlanks();
if ( TextIO.peek() == '\n' )
break;
try {
ExpNode exp = expressionTree();
TextIO.skipBlanks();
if ( TextIO.peek() != '\n' )
throw new ParseError("Extra data after end of expression.");
TextIO.getln();
ExpNode deriv = exp.derivative();
System.out.println("\nA fully parenthesized expression for the derivative is:");
System.out.print("   ");
deriv.printInfix();
System.out.println();
System.out.println("\nValue of derivative at x = 0 is " + deriv.value(0));
System.out.println("Value of derivative at x = 1 is " + deriv.value(1));
System.out.println("Value of derivative at x = 2 is " + deriv.value(2));
System.out.println("Value of derivative at x = 3 is " + deriv.value(3));
System.out.println("\nOrder of postfix evaluation for derivative is:\n");
deriv.printStackCommands();
}
catch (ParseError e) {
System.out.println("\n*** Error in input:    " + e.getMessage());
System.out.println("*** Discarding input:  " + TextIO.getln());
}
}

System.out.println("\n\nDone.");

} // end main()

/**
* Reads an expression from the current line of input and builds
* an expression tree that represents the expression.
* @return an ExpNode which is a pointer to the root node of the
*    expression tree
* @throws ParseError if a syntax error is found in the input
*/
private static ExpNode expressionTree() throws ParseError {
TextIO.skipBlanks();
boolean negative;  // True if there is a leading minus sign.
negative = false;
if (TextIO.peek() == '-') {
TextIO.getAnyChar();
negative = true;
}
ExpNode exp;       // The expression tree for the expression.
exp = termTree();  // Start with the first term.
if (negative)
exp = new UnaryMinusNode(exp);
TextIO.skipBlanks();
while ( TextIO.peek() == '+' || TextIO.peek() == '-' ) {
// Read the next term and combine it with the
// previous terms into a bigger expression tree.
char op = TextIO.getAnyChar();
ExpNode nextTerm = termTree();
exp = new BinOpNode(op, exp, nextTerm);
TextIO.skipBlanks();
}
return exp;
} // end expressionTree()

/**
* Reads a term from the current line of input and builds
* an expression tree that represents the expression.
* @return an ExpNode which is a pointer to the root node of the
*    expression tree
* @throws ParseError if a syntax error is found in the input
*/
private static ExpNode termTree() throws ParseError {
TextIO.skipBlanks();
ExpNode term;  // The expression tree representing the term.
term = factorTree();
TextIO.skipBlanks();
while ( TextIO.peek() == '*' || TextIO.peek() == '/' ) {
// Read the next factor, and combine it with the
// previous factors into a bigger expression tree.
char op = TextIO.getAnyChar();
ExpNode nextFactor = factorTree();
term = new BinOpNode(op,term,nextFactor);
TextIO.skipBlanks();
}
return term;
} // end termValue()

/**
* Reads a factor from the current line of input and builds
* an expression tree that represents the expression.
* @return an ExpNode which is a pointer to the root node of the
*    expression tree
* @throws ParseError if a syntax error is found in the input
*/

private static ExpNode factorTree() throws ParseError {
TextIO.skipBlanks();
char ch = TextIO.peek();
if ( Character.isDigit(ch) ) {
// The factor is a number.  Return a ConstNode.
double num = TextIO.getDouble();
return new ConstNode(num);
}
else if ( ch == 'x' || ch == 'X' ) {
// The factor is the variable x.
TextIO.getAnyChar();   // Read the X.
return new VariableNode();
}
else if ( ch == '(' ) {
// The factor is an expression in parentheses.
// Return a tree representing that expression.
TextIO.getAnyChar();  // Read the "("
ExpNode exp = expressionTree();
TextIO.skipBlanks();
if ( TextIO.peek() != ')' )
throw new ParseError("Missing right parenthesis.");
TextIO.getAnyChar();  // Read the ")"
return exp;
}
else if ( ch == '\n' )
throw new ParseError("End-of-line encountered in the middle of an expression.");
else if ( ch == ')' )
throw new ParseError("Extra right parenthesis.");
else if ( ch == '+' || ch == '-' || ch == '*' || ch == '/' )
throw new ParseError("Misplaced operator.");
else
throw new ParseError("Unexpected character \"" + ch + "\" encountered.");
}  // end factorTree()

} // end class SimpleParser5

Solution for Programming Exercise 10.1

Exercise 10.1:

Rewrite the PhoneDirectory class from Subsection 7.4.2 so that it uses a TreeMap to store directory entries, instead of an array. (Doing this was suggested in Subsection 10.3.1.)

Discussion

The class that you are asked to write for this exercise really is very trivial. Everything that the class needs to do is already done by the TreeMap class, and TreeMap does much more besides. In fact the only reason to write the PhoneDirectory class is to hide a lot of the capabilities of TreeMaps. A PhoneDirectory object is a "thin wrapper" around a TreeMap object. It allows only the operations getNumber(name) and putNumber(name,number). Each of these can be implemented in a single line, as a call to one of the methods in the TreeMap. The PhoneDirectory does provide nice, meaningful names for the methods, and it adds a check to the implementation of putNumber(name,number) to ensure that name and number are not null.

It would be nice for phone directories to have a few more capabilities. A method for listing the contents of the directory would be especially nice, so I have added one to my class even though there was no such method in the original PhoneDirectory class. The print method uses a for-each loop to iterate through the entry set of the phone directory. It is very similar to examples in Subsection 10.3.2 Even with this addition, though, the PhoneDirectory class is still very far from being useful in real applications.

The Solution

import java.util.Map;
import java.util.TreeMap;

/**
* A phone directory holds a list of names with a phone number for
* each name.  It is possible to find the number associated with
* a given name, and to specify the phone number for a given name.
* (This version of the class uses a TreeMap to store the data.)
*/
public class PhoneDirectoryWithTreeMap {

/**
* The TreeMap that will store the data.  Both key and value are
* of type String.  The key represents a name and the value represents
* the associated phone number.
*/
private TreeMap<String,String> data;

/**
* Constructor creates an initially empty directory.
*/
public PhoneDirectoryWithTreeMap() {
data = new TreeMap<String,String>();
}

/**
* Finds the phone number, if any, for a given name.
* @return The phone number associated with the name; if the name does
*    not occur in the phone directory, then the return value is null.
*/
public String getNumber( String name ) {
return data.get(name);
}

/**
* Associates a given name with a given phone number.  If the name
* already exists in the phone directory, then the new number replaces
* the old one.  Otherwise, a new name/number pair is added.  The
* name and number should both be non-null.  An IllegalArgumentException
* is thrown if this is not the case.
*/
public void putNumber( String name, String number ) {
if (name == null || number == null)
throw new IllegalArgumentException("name and number cannot be null");
data.put(name,number);
}

/**
*  Write the contents of the phone directory to System.out.
*/
public void print() {
for ( Map.Entry<String,String> entry : data.entrySet() )
System.out.println( entry.getKey() + ":  " + entry.getValue() );
}

} // end class PhoneDirectoryWithTreeMap

Here is a very small program to test the class:

public class TestPhoneDirectoryWithTreeMap {

public static void main(String[] args) {
PhoneDirectoryWithTreeMap directory = new PhoneDirectoryWithTreeMap();
System.out.println("This program creates a PhoneDirectoryWithTreeMap and");
System.out.println("adds several entries.  It then prints the contents of");
System.out.println("directory and does a few lookups.");
System.out.println();
directory.putNumber("Fred","555-1234");
directory.putNumber("Barney","555-2345");
directory.putNumber("Wilma","555-3456");
System.out.println("Contents are:");
System.out.println();
directory.print();
System.out.println();
System.out.println("Number for Fred is " + directory.getNumber("Fred"));
System.out.println("Number for Wilma is " + directory.getNumber("Wilma"));
System.out.println("Number for Tom is " + directory.getNumber("Tom"));
// The output from the last line should be null.
}

}

Solution for Programming Exercise 10.2

Exercise 10.2:

In mathematics, several operations are defined on sets. The union of two sets A and B is a set that contains all the elements that are in A together with all the elements that are in B. The intersection of A and B is the set that contains elements that are in both A and B. The difference of A and B is the set that contains all the elements of A except for those elements that are also in B.

Suppose that A and B are variables of type set in Java. The mathematical operations on A and B can be computed using methods from the Set interface. In particular: A.addAll(B) computes the union of A and B; A.retainAll(B) computes the intersection of A and B; and A.removeAll(B) computes the difference of A and B. (These operations change the contents of the set A, while the mathematical operations create a new set without changing A, but that difference is not relevant to this exercise.)

For this exercise, you should write a program that can be used as a "set calculator" for simple operations on sets of non-negative integers. (Negative integers are not allowed.) A set of such integers will be represented as a list of integers, separated by commas and, optionally, spaces and enclosed in square brackets. For example: [1,2,3] or [17, 42, 9, 53, 108]. The characters +, *, and - will be used for the union, intersection, and difference operations. The user of the program will type in lines of input containing two sets, separated by an operator. The program should perform the operation and print the resulting set. Here are some examples:

Input                                 Output
-------------------------           -------------------
[1, 2, 3] + [3,  5,  7]             [1, 2, 3, 5, 7]
[10,9,8,7] * [2,4,6,8]              [8]
[ 5, 10, 15, 20 ] - [ 0, 10, 20 ]   [5, 15]

To represent sets of non-negative integers, use sets of type TreeSet<Integer>. Read the user's input, create two TreeSets, and use the appropriate TreeSet method to perform the requested operation on the two sets. Your program should be able to read and process any number of lines of input. If a line contains a syntax error, your program should not crash. It should report the error and move on to the next line of input. (Note: To print out a Set, A, of Integers, you can just say System.out.println(A). I've chosen the syntax for sets to be the same as that used by the system for outputting a set.)

Discussion

This program would be much easier to write if we could assume that the user's input was in the correct format. Unfortunately, the exercise says that we have to detect errors in the user's input and handle them by printing error messages. To do this we have to constantly look ahead while reading the user's input, to see whether the input agrees with what we are expecting. The techniques for doing this were covered for the example LengthConverter2.java from Subsection 8.2.2. We can use the function TextIO.peek() to look ahead at the next character in the user's input. We need to be able to skip past any blanks in the input. Instead of writing my own method to do that, I use the method TextIO.skipBlanks() in my program.

My program uses exceptions to handle the errors. Exceptions make it possible to organize the error-handling code in a straightforward way. When an error is discovered, an exception is thrown. The main program uses a try..catch statement to try to process one line of input. If an error occurs, the program does not crash. The error is caught, an error message is printed, and the program continues. I throw an error of type IllegalArgumentException when an error is found. This exception is a standard part of Java, but it might be better style to define a new error class to represent the specific type of error that occurs in this program.

My program uses a method named calc() to read and process one line of input. This method in turn uses another method, readSet() to read each of the two sets of integers from the input line. Without error handling, an algorithm for readSet() would be:

Start with an empty set.
Read the '[' that begins the set.
Repeat:
Read the next number and add it to the set.
If the next character is ']':
break.
Read the comma that separates one number from the next.
Read the ']'.
Return the set.

To turn this into a robust routine, we just have to check, before reading anything, that the next character is legal. If not, throw an exception. This adds a lot of code, but the nice thing about throwing exceptions is that it doesn't disturb the logical flow of the routine.

There is one possible bug in the algorithm. It assumes that there is at least one number in the set. But in mathematics, a set can be empty. I decided to allow for the possibility of empty sets in my program. See the solution, below.

An algorithm for the calc() method is even more straightforward. Again, the basic algorithm ignores the possibility of errors:

Read the first set, A.
Read the operator.
Read the second set, B.
if the operator is '+'
A.addAll(B)  // Sets A equal to the union of A and B.
else if the operator is '*'
A.retainAll(B)  // Sets A to the intersection.
else
A.removeAll(B)  // Sets A to the difference.
Print A.

In the program, an error check has to be added to make sure that there is a legal operator in the correct position. I also add an error check to make sure that there is no extra data on the line.

The Solution

import java.util.TreeSet;

/**
* This program is a very simple "set calculator" that can compute
* the intersection, union, and set difference of two sets of
* non-negative integers.  Each line of the user's input contains two
* such sets, separated by one of the operators +, *, or -, standing
* for union, intersection, and set difference respectively.  A set
* must be given in the form of a list of non-negative integers, separated
* by commas, and enclosed in square brackets.  For example:
* [1, 2, 3] + [4, 3, 10, 0].  Spaces can occur anywhere in the input.
* If an error is found in the input, the program will report it.
* The program ends when the user inputs an empty line.
*
* This program is mainly a demonstration of Sets.
*/

public class SetCalc {

public static void main(String[] args) {

System.out.println("This program will compute union, intersection,");
System.out.println("and set difference of sets of integers.");
System.out.println("");
System.out.println("");
System.out.println("Enter set computations (press return to end):");

while (true) {
char ch;
System.out.print("\n? ");
TextIO.skipBlanks();
if (TextIO.peek() == '\n') {
// The input line is empty.
// Exit the loop and end the program.
break;
}
try {
calc(); // Reads and processes one line of input.
}
catch (IllegalArgumentException e) {
// An error was found in the input line.
System.out.println("Error in input: " + e.getMessage());
System.out.print("Discarding input: ");
}
do {  // Copy extra characters on line to output.  If there
// was no error, then the only character that is copied
// is the end-of-line character.
ch = TextIO.getAnyChar();
System.out.print(ch);
} while (ch != '\n');
}

} // end main()

/**
* Read a line of input, consisting of two sets separated by
* an operator.  Perform the operation and output the value.
* If any syntax error is found in the input, an
* IllegalArgumentException is thrown
*/
private static void calc() {

TreeSet<Integer> A, B;  // The two sets of integers.
char op;                // The operator, +, *, or -.

A = readSet(); // Read the first set.

TextIO.skipBlanks();
if (TextIO.peek() != '*' && TextIO.peek() != '+'
&& TextIO.peek() != '-')
throw new IllegalArgumentException(
"Expected *, +, or  - after first set.");
op = TextIO.getAnyChar(); // Read the operator.

B = readSet(); // Read the second set.

TextIO.skipBlanks();
if (TextIO.peek() != '\n')
throw new IllegalArgumentException("Extra unexpected input.");

// Perform the operation.  This modifies set A to represent
// the answer.  Display the answer by printing out A. The
// output format for a set of integers is the same as the
// input format used by this program.

if (op == '+')
A.addAll(B);     // Union.
else if (op == '*')
A.retainAll(B);  // Intersection.
else
A.removeAll(B);  // Set difference.

System.out.print("Value:  " + A);

} // end calc()

/**
* Reads a set of non-negative integers from standard input,
* and stores them in a TreeSet that contains objects belonging
* to the wrapper class Integer.  The set must be enclosed
* between square brackets and must contain a list of zero or
* more non-negative integers, separated by commas.  Spaces
* are allowed anywhere.  If the input is not of the correct
* form, an IllegalArgumentException is thrown.
*/
private static TreeSet<Integer> readSet() {

TreeSet<Integer> set = new TreeSet<Integer>();  // The set that will be read.

TextIO.skipBlanks();
if (TextIO.peek() != '[')
throw new IllegalArgumentException("Expected '[' at start of set.");
TextIO.getAnyChar(); // Read the '['.

TextIO.skipBlanks();
if (TextIO.peek() == ']') {
// The set has no integers.  This is the empty set, which
// is legal.  Return the value.
TextIO.getAnyChar(); // Read the ']'.
return set;
}

while (true) {
// Read the next integer and add it to the set.
TextIO.skipBlanks();
if (! Character.isDigit(TextIO.peek()))
throw new IllegalArgumentException("Expected an integer.");
int n = TextIO.getInt(); // Read the integer.
set.add( new Integer(n) );
TextIO.skipBlanks();
if (TextIO.peek() == ']')
break;  // ']' marks the end of the set.
else if (TextIO.peek() == ',')
TextIO.getAnyChar(); // Read a comma and continue.
else
throw new IllegalArgumentException("Expected ',' or ']'.");
}

TextIO.getAnyChar(); // Read the ']' that ended the set.

return set;

} // end readSet()

} // end class SetCalc

Solution for Programming Exercise 10.3

Exercise 10.3:

The fact that Java has a HashMap class means that no Java programmer has to write an implementation of hash tables from scratch -- unless, of course, that programmer is a computer science student.

For this exercise, you should write a hash table in which both the keys and the values are of type String. (This is not an exercise in generic programming; do not try to write a generic class.) Write an implementation of hash tables from scratch. Define the following methods: get(key), put(key,value), remove(key), containsKey(key), and size(). Remember that every object, obj, has a method obj.hashCode() that can be used for computing a hash code for the object, so at least you don't have to define your own hash function. Do not use any of Java's built-in generic types; create your own linked lists using nodes as covered in Subsection 9.2.2. However, you do not have to worry about increasing the size of the table when it becomes too full.

You should also write a short program to test your solution.

Discussion

Hash tables are discussed in Subsection 10.3.3. A hash table is just an array of linked lists. Each linked list holds all the items in the table that share the same hash code. Initially, all the lists are empty (represented as null in the array). We need to be able to add and delete items in the list. Methods for inserting and deleting items can be found in Subsection 9.2.2. In a hash table, the order of items in a particular list doesn't matter, so I simply insert each new item at the beginning of the list. This makes the insert operation fairly simple. Deletion, however, is more complicated, since we need to be able to delete an item no matter where it occurs in a list.

A hash table really contains pairs of items, where each pair consists of a key and an associated value. To implement this, I use linked lists in which each node contains both a key and a value, as well as the pointer to the next node in the list. The end of a list is marked, as usual, by a null pointer. The nodes are defined by a static nested class:

private static class ListNode {
String key;
String value;
ListNode next;
}

The array of linked lists that stores all the data is of type ListNode[]. Each item in the array is either null to indicate an empty list, or it is a pointer to the first node in a linked list.

Given any key, we can find the linked list that should contain that key by looking at the hash code of the key. The code computed by calling key.hashCode() is of type int. We need a value that is in the range of legal indices for the array. As noted in Subsection 10.3.3, the value can be computed as Math.abs(key.hashCode()) % table.length, where table is the array.

The hash code is used in all the methods that deal with keys, to decide which linked list to look at. Once a list has been selected, the operations on the list are pretty straightforward (given all the information and code in Subsection 9.2.2), so I will not discuss them further here. You can look at the solution, below.

Although it is not required by the exercise, I defined a resize() method that is used to increase the size of the table when the table becomes too full. I call this method in the put() method when the table becomes more than 3/4 full.

A major part of developing a class for general use is testing. It's important to design a testing procedure that will test all aspects of the class. For this problem, I wrote a testing program that would allow me to test each of the methods in the class. I also added a method dump() to the class; this method displays the entire hash table. This method does not really belong in the class, since users of a hash table shouldn't care how the data is stored. But I needed it to make sure that my resize() method is working properly and that I could delete items correctly from all positions in the lists (beginning, middle, and end).

By the way, you might be tempted to try to write a generic version of HashTable in which the types of the keys and values are given as type parameters in the definition of the class. Unfortunately, you would run into problems as soon as you tried to use an array in your class. Generic classes don't work well with arrays, because arrays of generic type can't be created at run time, as noted in Subsection 10.1.3.

The Solution

The HashTable Class

/**
* This file defines a HashTable class.  Keys and values in the table are of
* type String.  Keys cannot be null.  The default constructor creates a table
* that initially has 64 locations, but a different initial size can be specified
* as a parameter to the constructor.  The table increases in size if it
* becomes more than 3/4 full; to help implement this, a count of the number
* of items in the hash table is kept in an instance variable.
*/
public class HashTable {

/**
* Keys that have the same hash code are placed together in a linked list.
* This private nested class is used internally to implement linked lists.
* A ListNode holds a (key,value) pair.
*/
private static class ListNode {
String key;
String value;
ListNode next;  // Pointer to next node in the list;
// A null marks the end of the list.
}

private ListNode[] table;  // The hash table, represented as
// an array of linked lists.

private int count;  // The number of (key,value) pairs in the
// hash table.

/**
* Create a hash table with an initial size of 64.
*/
public HashTable() {
table = new ListNode[64];
}

/**
* Create a hash table with a specified initial size.
* Precondition: initalSize > 0.
*/
public HashTable(int initialSize) {
if (initialSize <= 0)
throw new IllegalArgumentException("Illegal table size");
table = new ListNode[initialSize];
}

/**
* This method is NOT part of the usual interface for a hash table.
* It is here only to be used for testing purposes, and should be
* removed before the class is released for general use.  This
* lists the (key,value) pairs in each location of the table.
*/
void dump() {
System.out.println();
for (int i = 0; i < table.length; i++) {
// Print out the location number and the list of
// key/value pairs in this location.
System.out.print(i + ":");
ListNode list = table[i]; // For traversing linked list number i.
while (list != null) {
System.out.print("  (" + list.key + "," + list.value + ")");
list = list.next;
}
System.out.println();
}
} // end dump()

/**
* Associate the specified value with the specified key.
* Precondition:  The key is not null.
*/
public void put(String key, String value) {

assert key != null : "The key must be non-null";

int bucket = hash(key); // Which location should this key be in?

ListNode list = table[bucket]; // For traversing the linked list
// at the appropriate location.
while (list != null) {
// Search the nodes in the list, to see if the key already exists.
if (list.key.equals(key))
break;
list = list.next;
}

// At this point, either list is null, or list.key.equals(key).

if (list != null) {
// Since list is not null, we have found the key.
// Just change the associated value.
list.value = value;
}
else {
// Since list == null, the key is not already in the list.
// Add a new node at the head of the list to contain the
// new key and its associated value.
if (count >= 0.75*table.length) {
// The table is becoming too full.  Increase its size
// before adding the new node.
resize();
bucket = hash(key);  // Recompute hash code, since it
// depends on the table size.
}
ListNode newNode = new ListNode();
newNode.key = key;
newNode.value = value;
newNode.next = table[bucket];
table[bucket] = newNode;
count++;  // Count the newly added key.
}
}

/**
* Retrieve the value associated with the specified key in the table,
* if there is any.  If not, the value null will be returned.
* @param key The key whose associated value we want to find
* @return the associated value, or null if there is no associated value
*/
public String get(String key) {

int bucket = hash(key);  // At what location should the key be?

ListNode list = table[bucket];  // For traversing the list.
while (list != null) {
// Check if the specified key is in the node that
// list points to.  If so, return the associated value.
if (list.key.equals(key))
return list.value;
list = list.next;  // Move on to next node in the list.
}

// If we get to this point, then we have looked at every
// node in the list without finding the key.  Return
// the value null to indicate that the key is not in the table.

return null;
}

/**
* Remove the key and its associated value from the table,
* if the key occurs in the table.  If it does not occur,
* then nothing is done.
*/
public void remove(String key) {

int bucket = hash(key);  // At what location should the key be?

if (table[bucket] == null) {
// There are no keys in that location, so key does not
// occur in the table.  There is nothing to do, so return.
return;
}

if (table[bucket].key.equals(key)) {
// If the key is the first node on the list, then
// table[bucket] must be changed to eliminate the
// first node from the list.
table[bucket] = table[bucket].next;
count--; // Remove new number of items in the table.
return;
}

// We have to remove a node from somewhere in the middle
// of the list, or at the end.  Use a pointer to traverse
// the list, looking for a node that contains the
// specified key, and remove it if it is found.

ListNode prev = table[bucket];  // The node that precedes
// curr in the list.  Prev.next
// is always equal to curr.
ListNode curr = prev.next;  // For traversing the list,
// starting from the second node.
while (curr != null && ! curr.key.equals(key)) {
curr = curr.next;
prev = curr;
}

// If we get to this point, then either curr is null,
// or curr.key is equal to key.  In the latter case,
// we have to remove curr from the list.  Do this
// by making prev.next point to the node after curr,
// instead of to curr.  If curr is null, it means that
// the key was not found in the table, so there is nothing
// to do.

if (curr != null) {
prev.next = curr.next;
count--;  // Record new number of items in the table.
}
}

/**
* Test whether the specified key has an associated value in the table.
* @param key The key that we want to search for.
* @return true if the key exists in the table, false if not
*/
public boolean containsKey(String key) {

int bucket = hash(key);  // In what location should key be?

ListNode list = table[bucket];  // For traversing the list.
while (list != null) {
// If we find the key in this node, return true.
if (list.key.equals(key))
return true;
list = list.next;
}

// If we get to this point, we know that the key does
// not exist in the table.

return false;
}

/**
* Return the number of key/value pairs in the table.
*/
public int size() {
return count;
}

/**
* Compute a hash code for the key; key cannot be null.
* The hash code depends on the size of the table as
* well as on the value returned by key.hashCode().
*/
private int hash(Object key) {
return (Math.abs(key.hashCode())) % table.length;
}

/**
* Double the size of the table, and redistribute the
* key/value pairs to their proper locations in the
* new table.
*/
private void resize() {
ListNode[] newtable = new ListNode[table.length*2];
for (int i = 0; i < table.length; i++) {
// Move all the nodes in linked list number i into the new table.
// No new ListNodes are created.  The existing ListNode for each
// key/value pair is moved to the newtable.  This is done by
// changing the "next" pointer in the node and by making a pointer
// in the new table point to the node.
ListNode list = table[i]; // For traversing linked list number i.
while (list != null) {
// Move the node pointed to by list to the new table.
ListNode next = list.next;  // The is the next node in the list.
// Remember it, before changing the value of list!
int hash = (Math.abs(list.key.hashCode())) % newtable.length;
// hash is the hash code of list.key that is
// appropriate for the new table size.  The
// next two lines add the node pointed to by list
// onto the head of the linked list in the new table
// at position number hash.
list.next = newtable[hash];
newtable[hash] = list;
list = next;  // Move on to the next node in the OLD table.
}
}
table = newtable;  // Replace the table with the new table.
} // end resize()

} // end class HashTable

A Class for Testing:

/**
* A little program to test the HashTable class.  Note that I
* start with a really small table so that I can easily test
* the resize() method.
*/

public class TestHashTable {

public static void main(String[] args){
HashTable table = new HashTable(2);  // Initial size of table is 2.
String key,value;
while (true) {
System.out.println("\nMenu:");
System.out.println("   1. test put(key,value)");
System.out.println("   2. test get(key)");
System.out.println("   3. test containsKey(key)");
System.out.println("   4. test remove(key)");
System.out.println("   5. show complete contents of hash table.");
System.out.println("   6. EXIT");
System.out.print("Enter your command:  ");
switch ( TextIO.getlnInt()) {
case 1:
System.out.print("\n   Key = ");
key = TextIO.getln();
System.out.print("   Value = ");
value = TextIO.getln();
table.put(key,value);
break;
case 2:
System.out.print("\n   Key = ");
key = TextIO.getln();
System.out.println("   Value is " + table.get(key));
break;
case 3:
System.out.print("\n   Key = ");
key = TextIO.getln();
System.out.println("   containsKey(" + key + ") is "
+ table.containsKey(key));
break;
case 4:
System.out.print("\n   Key = ");
key = TextIO.getln();
table.remove(key);
break;
case 5:
table.dump();
break;
case 6:
return;  // End program by returning from main()
default:
System.out.println("   Illegal command.");
break;
}
System.out.println("\nHash table size is " + table.size());
}
}

} // end class TestHashTable

Solution for Programming Exercise 10.4

Exercise 10.4:

A predicate is a boolean-valued function with one parameter. Some languages use predicates in generic programming. Java 7 doesn't, but this exercise looks at how predicates might work.

In Java, we could implement "predicate objects" by defining a generic interface:

public interface Predicate<T> {
public boolean test( T obj );
}

The idea is that an object that implements this interface knows how to "test" objects of type T in some way. Define a class that contains the following generic static methods for working with predicate objects. The name of the class should be Predicates, in analogy with the standard class Collections that provides various static methods for working with collections.

public static <T> void remove(Collection<T> coll, Predicate<T> pred)
// Remove every object, obj, from coll for which
// pred.test(obj) is true.

public static <T> void retain(Collection<T> coll, Predicate<T> pred)
// Remove every object, obj, from coll for which
// pred.test(obj) is false.  (That is, retain the
// objects for which the predicate is true.)

public static <T> List<T> collect(Collection<T> coll, Predicate<T> pred)
// Return a List that contains all the objects, obj,
// from the collection, coll, such that pred.test(obj)
// is true.

public static <T> int find(ArrayList<T> list, Predicate<T> pred)
// Return the index of the first item in list
// for which the predicate is true, if any.
// If there is no such item, return -1.

(In C++, methods similar to these are included as a standard part of the generic programming framework. And Java 8 has a similar predicate interface, and the Collections class has a removeIf() method that uses a predicate.)

Discussion

The code for this exercise is pretty short, but it involves some advanced ideas, including methods with bounded type parameters (from Subsection 10.5.4). Aside from that, the main point of the exercise is to introduce predicates and show how they might be useful in generic programming. The operation of finding all the items in a collection that satisfy some condition is very common. The code to do it is not hard to write, but even so, it is nice to have it done once and for all. In this exercise, the condition that the items have to satisfy is represented as a Predicate. The collect() method provides a general way to find all the items that satisfy the condition. The items are dumped into a collection, which can then be processed further if necessary. If I wanted to carry this generic programming thing even further, I would define another interface:

public interface Processor<T> {
public void process( T obj );
}

and define a generic method for applying a Processor object to each item in a collection. With Predicates and Processors in hand, it would hardly ever be necessary to write a loop for iterating through all the items in a collection.

The implementation of the collect() method simply uses a for-each loop to go through the collection, coll, and puts every item that satisfies the predicate into a list. The method returns an object of type List, but List is only an interface. Any real list must belong to one of the classes that implement this interface, such as LinkedList or ArrayList. I create the list as an ArrayList, but a LinkedList would also be reasonable. (Perhaps the return type of the method should have been specified as ArrayList rather than List so that users of the method would know exactly what they are getting.)

/**
* Return a List that contains all the objects, obj, from the collection,
* coll, for which pred.test(obj) is true.
*/
public static <T> List<T> collect(Collection<T> coll, Predicate<T> pred) {
List<T> list = new ArrayList<T>();
for ( T item : coll ) {
if (pred.test(item))
list.add(item);
}
return list;
} // end collect()

The remove() and retain() methods are similar to collect(), except that instead of making a new collection, they modify the collection, coll. A for-each loop cannot modify the collection over which it is iterating, so the remove() and retain() methods will have to use Iterators. Recall that if iter is an Iterator for a collection, then the method iter.remove() will remove from the collection the item that was seen most recently by the iterator. The remove() method can be written as:

/**
* Remove every object, obj, from coll for which pred.test(obj) is true.
*/
public static <T> void remove(Collection<T> coll, Predicate<T> pred) {
Iterator<T> iter = coll.iterator();
while (iter.hasNext()) {
T item = iter.next();
if (pred.test(item))
iter.remove();
}
} // end remove()

The retain() method is almost identical to this, and the find() method is easy to write using an ordinary for loop. See my solution, below.

My test program simply creates some collections and applies the methods from the Predicates class to them. My collections contain objects of type Integer and I made two Predicates that work on objects of this type. To define such a predicate, we need a class that implements the interface Predicate<Integer>, and then we have to make an object belonging to that class. For example, I define the class:

/**
* An object of type Even tests an Object of type Integer to see whether the
* integer that it represents is an even number. Note that the test() method
* should only be applied to non-null values of type Integer.
*/
static class Even implements Predicate<Integer> {
public boolean test(Integer i) {
if ( i % 2 == 0 )
return true;
else
return false;
}
} // end class Even

The predicate object is then created with a command:

Predicate<Integer> pred = new Even():

A predicate object belonging to class Even will throw an exception if its test() method is applied to a null value. If this class were meant for more general use, it might be better to define test() so that it simply returns the answer "false" in this case:

/**
* An object of type Even tests an Object of type Integer to see whether the
* integer that it represents is an even number.  When applied to a null
* object, the test returns the value false.
*/
static class Even implements Predicate<Integer> {
public boolean test(Integer i) {
if (i == null)
return false;
else if ( i % 2 == 0 )
return true;
else
return false;
}
} // end class Even

Note that a Predicate is an object, but we are really only interested in the test() method of the object. In some languages, including C++, it is possible to pass a function as a parameter to another function. In those languages, predicates are simply functions that return boolean values, and working with predicates does not require the overhead of making classes and objects. Of course, things are not so bad in Java if you make use of an anonymous nested class to define the predicate. In fact, you can define the predicate right in the function call where you need it. For example:

result = Predicates.collect( coll,
new Predicate<Integer>() {
public boolean test(Integer i) {
return (i != null && i > 100);
}
} );

Furthermore, in Java 8, the predicate can be defined as a lamda expression (see Subsection 5.8.4), which simplifies the code quite a bit:

result = Predicates.collect(coll, i -> i != null && i > 0))

The Solution

The Predicate interface must be defined:

interface Predicate<T> {
public boolean test( T obj );
}

The Predicates class:

import java.util.*;

/**
* This class defines some static methods for working
* with Collections and Predicates.  (The Predicate
* interface is NOT a standard part of Java.)
*/
public class Predicates {

/**
* Remove every object, obj, from coll for which pred.test(obj) is true.
*/
public static <T> void remove(Collection<T> coll, Predicate<T> pred) {
Iterator<T> iter = coll.iterator();
while (iter.hasNext()) {
T item = iter.next();
if (pred.test(item))
iter.remove();
}
} // end remove()

/**
* Remove every object, obj, from coll for which pred.test(obj) is false.
* (That is, retain the objects for which the predicate is true.)
*/
public static <T> void retain(Collection<T> coll, Predicate<T> pred){
Iterator<T> iter = coll.iterator();
while (iter.hasNext()) {
T item = iter.next();
if ( ! pred.test(item) )
iter.remove();
}
} // end retain()

/**
* Return a List that contains all the objects, obj, from the collection,
* coll, for which pred.test(obj) is true.
*/
public static <T> List<T> collect(Collection<T> coll, Predicate<T> pred) {
List<T> list = new ArrayList<T>();
for ( T item : coll ) {
if (pred.test(item))
list.add(item);
}
return list;
} // end collect()

/**
* Return the index of the first item in list for which the predicate is true, if any.
* If there is no such item, return -1.
*/
public static <T> int find(ArrayList<T> list, Predicate<T> pred) {
//
for (int i = 0; i < list.size(); i++) {
T item = list.get(i);
if (pred.test(item))
return i;
}
return -1;
} // end find()

} // end class Predicates

The Test program:

import java.util.*;

/**
* Perform some simple tests on the Predicates class.
*/
public class TestPredicates {

/**
* An object of type Even tests an Object of type Integer to see whether the
* integer that it represents is an even number. Note that the test() method
* should only be applied to non-null values of type Integer.
*/
static class Even implements Predicate<Integer> {
public boolean test(Integer i) {
if ( i % 2 == 0 )
return true;
else
return false;
}
} // end class Even

/**
* An object of type Big tests an Object of type Integer to see whether the
* integer that it represents is bigger than 100.  Note that the test() method
* should only be applied to non-null values of type Integer.
*/
static class Big implements Predicate<Integer> {
public boolean test(Integer i) {
if ( i > 100 )
return true;
else
return false;
}
} // end class Big

/**
* For convenience make a Collection containing some integers.  The Collection
* is actually a TreeSet, but that is not relevant to the main program.
*/
static Collection<Integer> makeSet() {
Collection<Integer> set = new TreeSet<Integer>();
set.add(new Integer(32));
set.add(new Integer(17));
set.add(new Integer(142));
set.add(new Integer(56));
set.add(new Integer(1899));
set.add(new Integer(57));
set.add(new Integer(999));
set.add(new Integer(86));
set.add(new Integer(83));
set.add(new Integer(100));
return set;
} // end makeSet()

/**
* Main routine tests the Predicates class by making Collections of Integers
* and applying the methods from the Predicates class to them.
*/
public static void main(String[] args) {

Collection<Integer> coll;    // A collection (from makeSet() method).

List<Integer> result;        // The result of applying collect() to coll.

Predicate<Integer> pred = new Even();  // Tests if an Integer is even.

coll = makeSet();
System.out.println("Original collection: " + coll);

Predicates.remove(coll,pred);
System.out.println("Remove evens: " + coll);

coll = makeSet();
Predicates.retain(coll,pred);
System.out.println("Retain evens: " + coll);

coll = makeSet();
result = Predicates.collect(coll,pred);
System.out.println("Collect evens: " + result);

ArrayList<Integer> list = new ArrayList<Integer>();
int i = Predicates.find(list,pred);
System.out.println("Find first even: at index " + i);

pred = new Big();        // Tests if an Integer is bigger than 100.

coll = makeSet();
System.out.println("Original collection: " + coll);

Predicates.remove(coll,pred);
System.out.println("Remove big: " + coll);

coll = makeSet();
Predicates.retain(coll,pred);
System.out.println("Retain big: " + coll);

coll = makeSet();
result = Predicates.collect(coll,pred);
System.out.println("Collect big: " + result);

list = new ArrayList<Integer>(coll);
i = Predicates.find(list,pred);
System.out.println("Find first big: at index " + i);

} // end main()

} // end class TestPredicates

Solution for Programming Exercise 10.5

Exercise 10.5:

An example in Subsection 10.4.2 concerns the problem of making an index for a book. A related problem is making a concordance for a document. A concordance lists every word that occurs in the document, and for each word it gives the line number of every line in the document where the word occurs. All the subroutines for creating an index that were presented in Subsection 10.4.2 can also be used to create a concordance. The only real difference is that the integers in a concordance are line numbers rather than page numbers.

Write a program that can create a concordance. The document should be read from an input file, and the concordance data should be written to an output file. You can use the indexing subroutines from Subsection 10.4.2, modified to write the data to TextIO instead of to System.out. (You will need to make these subroutines static.) The input and output files should be selected by the user when the program is run. The sample program WordCount.java, from Subsection 10.4.4, can be used as a model of how to use files. That program also has a useful subroutine that reads one word from input.

As you read the file, you want to take each word that you encounter and add it to the concordance along with the current line number. Keeping track of the line numbers is one of the trickiest parts of the problem. In an input file, the end of each line in the file is marked by the newline character, '\n'. Every time you encounter this character, you have to add one to the line number. WordCount.java ignores ends of lines. Because you need to find and count the end-of-line characters, your program cannot process the input file in exactly the same way as does WordCount.java. Also, you will need to detect the end of the file. The function TextIO.peek(), which is used to look ahead at the next character in the input, returns the value TextIO.EOF at end-of-file, after all the characters in the file have been read.

Because it is so common, don't include the word "the" in your concordance. Also, do not include words that have length less than 3.

Discussion

Solving this exercise, for the most part, means collecting subroutines that were presented in Subsection 10.4.2 into a complete program. I copied the method addReference() directly, just making it static and changing the names of the parameters to names that are more appropriate for a concordance. I copied printIntegers(), changed its name to printConcordance, made it static, and changed "System.out.print" to "TextIO.put" so that it would send its output to the file selected by the user.

In Subsection 10.4.3, I discussed the problem that arises when comparing strings that differ only in their use of upper and lower case. In that subsection, I used a Comparator to solve the problem. In my solution to this exercise, however, I used the simpler solution of converting all words to lower case immediately after reading them from the file. For lower case words, the default ordering is alphabetical order, so I don't need a Comparator.

It remains to write the main() routine, which is similar in outline to the main() routine in WordCount.java. One difference is that we have to keep track of line numbers. We can do this by looking for new-line characters as we skip over the characters between words. While we do this, we have to be on the alert for the special character TextIO.EOF that is returned by TextIO.peek() when the end of the file is reached. A pseudocode algorithm can be given as:

Open the input files.

Let lineNum = 1.

Repeat:

// Skip over any non-letters in the input, stopping when a
// letter (marking the beginning of a word) or EOF is found.

while the next character is not end-of-file or a letter:
Read the next character.
If it is a new line character:
Count the line by adding 1 to lineNum

// After the while loop, we are looking at either the end of file
//    or at a letter that is the beginning of the next word.

If at end-of-file:
Exit from the loop.

Get the next word from the input file.
Convert the word to lower case.
if the word is not "the" and has length > 2:
Add the word and lineNum to the concordance.

Open the output file.

Print the concordance.

This can be translated directly (using some code from WordCount.java) to give the main() routine. For reading individual words from input, the method readNextWord() can be copied directly from WordCount.java. (In my program, I used a slightly simpler method named readWord() which uses the fact that when readWord() is called by the main() routine, the next character in input is already known to be a letter.)

The Solution

import java.util.Map;
import java.util.TreeMap;
import java.util.TreeSet;

/**
* This program makes a concordance for a file.  A concordance
* lists all the words that occur in the file, along with all
* the line numbers on which each word occurs.  (Words of
* length less than 3 are omitted, and "the" is omitted.)  The
* concordance is written to an output file.  The user selects
* the input and output files using file dialog boxes.  This
* program uses the non-standard class, TextIO.
*/
public class Concordance {

/**
* This TreeMap holds the concordance.  Words from the file
* are used as keys in the map.  The value associated with
* each word is a set that contains the line numbers on which
* the word occurs in the file.  The set contains values
* belonging to the wrapper class, Integer.
*/
private static TreeMap<String, TreeSet<Integer>>  concordance;

public static void main(String[] args) {

System.out.println("\n\n   This program will ask you to select an input file.");
System.out.println("It makes a list of all the words that occur in the file");
System.out.println("along with the line number of each line that contained");
System.out.println("that word.  This is called a \"concordance\" for the file.");
System.out.println("   After reading the input file, the program asks you to");
System.out.println("select an output file.  If you select a file, the list of");
System.out.println("words will be written to that file; if you cancel, the list");
System.out.println("be written to standard output.  All words are converted to");
System.out.println("lower case.\n\n");
System.out.print("Press return to begin.");

TextIO.getln();  // Wait for user to press return.

try {

// Let user select the input file.  If the user cancels,
// the program ends immediately.

if (TextIO.readUserSelectedFile() == false) {
System.out.println("No input file selected.  Exiting.");
System.exit(0);
}

// Create the data structure that will hold the concordance.

concordance = new TreeMap<String, TreeSet<Integer>>();

int lineNum = 1;  // The number of the line in the input
// file that is currently being processed.

// Read words from the file until end of file is reached,
// and add each word to the data.

while (true) {
char ch = TextIO.peek(); // Look ahead at next character
while ( ch != TextIO.EOF && ! Character.isLetter(ch) ) {
// Skip over non-letter characters, stopping when
// end-of-file (TextIO.EOF) or a letter is seen.  If the
// character is an end-of-line character, add 1
// to lineNum to reflect the fact that we are moving
// on to the next line in the file.
TextIO.getAnyChar();  // Reads the next character, which is junk.
if (ch == '\n') {
lineNum++;  // Start of a new line.
}
ch = TextIO.peek();  // Look at the next character.
}
if (ch == TextIO.EOF) {
// The end-of-file has been reached, so exit from the loop.
break;
}
String word = readWord();  // The next word from the file.
word = word.toLowerCase();
if (word.length() > 2  && !word.equalsIgnoreCase("the")) {
// Add the reference to word to the concordance, unless
// the word is "the" or the word has length <= 2.
addReference(word,lineNum);
}
}

// Write the data to a user-selected file, or to standard
// output if the user does not select an output file.

System.out.println(concordance.size() + " distinct words were found in the file.\n");
System.out.println();
if (concordance.size() == 0) {
System.out.println("No words found in file.");
System.out.println("Exiting without saving data.");
System.exit(0);
}

TextIO.writeUserSelectedFile(); // If user cancels, output automatically
// goes to standard output.

printConcordance();  // Print the data to the output file.

}
catch (IllegalArgumentException e) {
System.out.println( "Sorry, some error occurred:  " + e.getMessage() );
}

} // end main()

/**
* Writes the data in the concordance to TextIO.  (The output will go
* to the output file, if one has been selected; otherwise, it will go
* to standard output.)  Each line of output contains one word from the
* file and a list of lines on which that word occurred.  The words
* are in alphabetical order.
*/
private static void printConcordance() {

for ( Map.Entry<String, TreeSet<Integer>>  entry :  concordance.entrySet() ) {

String term = entry.getKey();
TreeSet<Integer> pageSet = entry.getValue();

TextIO.put( term + " " );
for ( int page : pageSet ) {
TextIO.put( page + " " );
}
TextIO.putln();

}
}

/**
* Add a word reference to the concordance.
*/
private static void addReference(String word, int lineNum) {
TreeSet<Integer> references; // The set of lines where we have
//    previously found the word.
references = concordance.get(word);
if (references == null){
// This is the first reference that we have
// found for the word.  Make a new set containing
// the line number and add it to the concordance, with
// the word as the key.
TreeSet<Integer> firstRef = new TreeSet<Integer>();
firstRef.add( lineNum );  // lineNum is "autoboxed" to give an Integer!
concordance.put(word,firstRef);
}
else {
// The variable references is the set of line references
// that we have found previously for the word.
// Add the new line number to that set.  This
// set is already associated to word in the concordance.
references.add( lineNum ); // pageNum is "autoboxed" to give an Integer!
}
}

/**
* Read the next word from TextIO.  It is assumed that the next character
* in input is a letter.
*    A word is defined as a sequence of letters.  Also, a word can include
* an apostrophe if the apostrophe is surrounded by letters on each side.
* @return the next word from TextIO, or null if an end-of-file is encountered
*/
private static String readWord() {
char ch = TextIO.peek(); // Look at next character in input.
assert Character.isLetter(ch);
String word = "";  // This will be the word that is read.
while (true) {
word += TextIO.getAnyChar();  // Append the letter onto word.
ch = TextIO.peek();  // Look at next character.
if ( ch == '\'' ) {
// The next character is an apostrophe.  Read it, and
// if the following character is a letter, add both the
// apostrophe and the letter onto the word and continue
// reading the word.  If the character after the apostrophe
// is not a letter, the word is done, so break out of the loop.
TextIO.getAnyChar();   // Read the apostrophe.
ch = TextIO.peek();    // Look at char that follows apostrophe.
if (Character.isLetter(ch)) {
word += "\'" + TextIO.getAnyChar();
ch = TextIO.peek();  // Look at next char.
}
else
break;
}
if ( ! Character.isLetter(ch) ) {
// If the next character is not a letter, the word is
// finished, so break out of the loop.
break;
}
// If we haven't broken out of the loop, next char is a letter.
}
return word;  // Return the word that has been read.
}

} // end class Concordance

Solution for Programming Exercise 10.6

Exercise 10.6:

The sample program SimpleInterpreter.java from Subsection 10.4.1 can carry out commands of the form "let variable = expression" or "print expression". That program can handle expressions that contain variables, numbers, operators, and parentheses. Extend the program so that it can also handle the standard mathematical functions sin, cos, tan, abs, sqrt, and log. For example, the program should be able to evaluate an expression such as sin(3*x-7)+log(sqrt(y)), assuming that the variables x and y have been given values. Note that the name of a function must be followed by an expression that is enclosed in parentheses.

In the original program, a symbol table holds a value for each variable that has been defined. In your program, you should add another type of symbol to the table to represent standard functions. You can use the following nested enumerated type and class for this purpose:

private enum Functions { SIN, COS, TAN, ABS, SQRT, LOG }

/**
* An object of this class represents one of the standard functions.
*/
private static class StandardFunction {

/**
* Tells which function this is.
*/
Functions functionCode;

/**
* Constructor creates an object to represent one of
* the standard functions
* @param code which function is represented.
*/
StandardFunction(Functions code) {
functionCode = code;
}

/**
* Finds the value of this function for the specified
* parameter value, x.
*/
double evaluate(double x) {
switch(functionCode) {
case SIN:
return Math.sin(x);
case COS:
return Math.cos(x);
case TAN:
return Math.tan(x);
case ABS:
return Math.abs(x);
case SQRT:
return Math.sqrt(x);
default:
return Math.log(x);
}
}

} // end class StandardFunction

Add a symbol to the symbol table to represent each function. The key is the name of the function and the value is an object of type StandardFunction that represents the function. For example:

symbolTable.put("sin", new StandardFunction(Function.SIN));

In SimpleInterpreter.java, the symbol table is a map of type HashMap<String,Double>. It's not legal to use a StandardFunction as the value in such a map, so you will have to change the type of the map. The map has to hold two different types of objects. The easy way to make this possible is to create a map of type HashMap<String,Object>. (A better way is to create a general type to represent objects that can be values in the symbol table, and to define two subclasses of that class, one to represent variables and one to represent standard functions, but for this exercise, you should do it the easy way.)

In your parser, when you encounter a word, you have to be able to tell whether it's a variable or a standard function. Look up the word in the symbol table. If the associated object is non-null and is of type Double, then the word is a variable. If it is of type StandardFunction, then the word is a function. Remember that you can test the type of an object using the instanceof operator. For example: if (obj instanceof Double)

Discussion

The changes that must be made to SimpleInterpreter.java are remarkably small, aside from defining the StandardFunction class. In my solution, I included this class as a static nested class in my main program class. It would also make sense to define it as a separate class.

I changed the type of the map to HashMap<String,Object>, as suggested in the exercise. I added a few lines of code at the beginning of the main() routine to add the six standard functions to the symbol table, using statements similar to the example given in the exercise:

symbolTable.put("sin", new StandardFunction(Functions.SIN));
symbolTable.put("cos", new StandardFunction(Functions.COS));
symbolTable.put("tan", new StandardFunction(Functions.TAN));
symbolTable.put("abs", new StandardFunction(Functions.ABS));
symbolTable.put("sqrt", new StandardFunction(Functions.SQRT));
symbolTable.put("log", new StandardFunction(Functions.LOG));

The only other change is in the parsing routines. The change is in the primaryValue() method. When a word is encountered, the computer must check whether the word is a variable or a standard function. This is done, as suggested in the exercise, by looking up the word in the symbol table and checking the type of the associated value. If the word is a standard function, then we have to read and evaluate the argument of the function and then find the value of the function at that argument value:

else if ( Character.isLetter(ch) ) {
// The factor is a variable or a standard function.  Read its name and
// look up its value in the symbol table.  If the name is not in the symbol table,
// an error occurs.  (Note that the values in the symbol table are objects of type
// Double or StandardFunction.)
String name = readWord();
Object obj = symbolTable.get(name);
if (obj == null)
throw new ParseError("Unknown word \"" + name + "\"");
assert (obj instanceof Double || obj instanceof StandardFunction);
if (obj instanceof Double) {
// The name is a variable; return value of that variable.
Double val = (Double)obj;
return val.doubleValue();
}
else {
// The name is a standard function.  Read the argument
// of the function and return the value of the function
// at that argument.  The argument must be an expression
// in parentheses.
StandardFunction func = (StandardFunction)obj;
TextIO.skipBlanks();
if ( TextIO.peek() != '(' )
throw new ParseError("Parenthesis missing after standard function");
TextIO.getChar(); // discard the '('
double argument = expressionValue();  // read and evaluate expression
TextIO.skipBlanks();
if ( TextIO.peek() != ')' )
throw new ParseError("Missing right parenthesis.");
TextIO.getChar(); // discard the ')'
return func.evaluate(argument);
}
}

There is one additional change that I might have made. As the program stands, it allows the user to use a "let" command to assign a value to one of the standard function names. For example: "let sin = 42". This effectively changes the name so that it is a variable, and the standard function is no longer available for use in expressions. It would probably be better to give an error message when the user tries to assign a value to a standard function name. This could be done in the doLetCommand() method. After reading the variable name in the "let" command, we have to check whether the name is already in the symbol table as the name of a standard function:

private static void doLetCommand() throws ParseError {
TextIO.skipBlanks();
if ( ! Character.isLetter(TextIO.peek()) )
throw new ParseError("Expected variable name after 'let'.");
String varName = readWord();  // The name of the variable.
Object symbol = symbolTable.get(name);
if (symbol != null && symbol instanceof StandardFunction)
throw new ParseError("Can't assign a value to a standard function name.");
TextIO.skipBlanks();
if ( TextIO.peek() != '=' )
throw new ParseError("Expected '=' operator for 'let' command.");
TextIO.getChar();
double val = expressionValue();  // The value of the variable.
TextIO.skipBlanks();
if ( TextIO.peek() != '\n' )
throw new ParseError("Extra data after end of expression.");
symbolTable.put( varName, val );  // Add to symbol table.
System.out.println("ok");
}

The Solution

Changes from the program SimpleInterpreter.java are shown in red italic.

/*
This program can evaluate expressions that can include
numbers, variables, parentheses, and the operators +,
-, *, /, and ^ (where ^ indicates raising to a power),
and standard functions sin, cos, tan, abs, sqrt, and log.
A variable name must consist of letters and digits,
beginning with a letter.  Names are case-sensitive.
This program accepts commands of two types from the user.
For a command of the form  print <expression> , the expression
is evaluated and the value is output.  For a command of
the form  let <variable> = <expression> , the expression is
evaluated and the value is assigned to the variable.
If a variable is used in an expression before it has been
assigned a value, an error occurs.  A number must begin with
a digit (i.e., not a decimal point).

Commands are formally defined by the BNF rules:

<command>  ::=  "print" <expression>
|  "let" <variable> "=" <expression>

<expression>  ::=  [ "-" ] <term> [ [ "+" | "-" ] <term> ]...

<term>  ::=  <factor> [ [ "*" | "/" ] <factor> ]...

<factor>  ::=  <primary> [ "^" <primary> ]...

<primary>  ::=  <number> | <variable> | "(" <expression> ")"
| <standard-function-name> "(" <expression> ")"

A line of input must contain exactly one such command.  If extra
data is found on a line after an expression has been read, it is
considered an error.  The variables "pi" and "e" are defined
when the program starts to represent the usual mathematical
constants.

This program demonstrates the use of a HashMap as a symbol
table.

SimpleInterpreter2.java is based on the program SimpleInterpreter.java,
which did not handle standard functions in expressions.

This program depends on the non-standard class, TextIO.
*/

import java.util.HashMap;

public class SimpleInterpreter2 {

/**
* Represents a syntax error found in the user's input.
*/
private static class ParseError extends Exception {
ParseError(String message) {
super(message);
}
} // end nested class ParseError

/**
* An enumerated type whose values represent the possible
* standard functions.
*/
private enum Functions { SIN, COS, TAN, ABS, SQRT, LOG }

/**
* An object of this class represents one of the standard functions.
* Objects of this type are stored in the symbol table, associated
* with the name of the standard functions.  Note that an object
* of this type also knows how to evaluate the corresponding function.
*/
private static class StandardFunction {

/**
* Tells which function this is.
*/
Functions functionCode;

/**
* Constructor creates an object to represent one of
* the standard functions
* @param code which function is represented.
*/
StandardFunction(Functions code) {
functionCode = code;
}

/**
* Finds the value of this function for the specified
* parameter value, x.
*/
double evaluate(double x) {
switch(functionCode) {
case SIN:
return Math.sin(x);
case COS:
return Math.cos(x);
case TAN:
return Math.tan(x);
case ABS:
return Math.abs(x);
case SQRT:
return Math.sqrt(x);
default:
return Math.log(x);
}
}

} // end class StandardFunction

/**
* The symbolTable contains information about the values of variables.  When a variable
* is assigned a value, it is recorded in the symbol table. The key is the name of the
* variable, and the  value is an object of type Double that contains the value of the
* variable.
*    The symbol table can also contain standard functions.  The key is the name of
* of the function, and the value is the corresponding StandardFunction object.
*/
private static HashMap<String,Object> symbolTable;

public static void main(String[] args) {

// Create the map that represents symbol table.

symbolTable = new HashMap<String,Object>();

// To start, add variables named "pi" and "e" to the symbol
// table.  Their values are the usual mathematical constants.

symbolTable.put("pi", Math.PI);
symbolTable.put("e", Math.E);

// Add the standard functions to the hash table.

symbolTable.put("sin", new StandardFunction(Functions.SIN));
symbolTable.put("cos", new StandardFunction(Functions.COS));
symbolTable.put("tan", new StandardFunction(Functions.TAN));
symbolTable.put("abs", new StandardFunction(Functions.ABS));
symbolTable.put("sqrt", new StandardFunction(Functions.SQRT));
symbolTable.put("log", new StandardFunction(Functions.LOG));

System.out.println("\n\nEnter commands; press return to end.");
System.out.println("Commands must have the form:\n");
System.out.println("      print <expression>");
System.out.println("  or");
System.out.println("      let <variable> = <expression>");

while (true) {
System.out.print("\n?  ");
TextIO.skipBlanks();
if ( TextIO.peek() == '\n' ) {
break;  // A blank input line ends the while loop and the program.
}
try {
String command = TextIO.getWord();
if (command.equalsIgnoreCase("print"))
doPrintCommand();
else if (command.equalsIgnoreCase("let"))
doLetCommand();
else
throw new ParseError("Command must begin with 'print' or 'let'.");
TextIO.getln();
}
catch (ParseError e) {
System.out.println("\n*** Error in input:    " + e.getMessage());
System.out.println("*** Discarding input:  " + TextIO.getln());
}
}

System.out.println("\n\nDone.");

} // end main()

/**
* Process a command of the form  let <variable> = <expression>.
* When this method is called, the word "let" has already
* been read.  Read the variable name and the expression, and
* store the value of the variable in the symbol table.
*/
private static void doLetCommand() throws ParseError {
TextIO.skipBlanks();
if ( ! Character.isLetter(TextIO.peek()) )
throw new ParseError("Expected variable name after 'let'.");
String varName = readWord();  // The name of the variable.
TextIO.skipBlanks();
if ( TextIO.peek() != '=' )
throw new ParseError("Expected '=' operator for 'let' command.");
TextIO.getChar();
double val = expressionValue();  // The value of the variable.
TextIO.skipBlanks();
if ( TextIO.peek() != '\n' )
throw new ParseError("Extra data after end of expression.");
symbolTable.put( varName, val );  // Add to symbol table.
System.out.println("ok");
}

/**
* Process a command of the form  print <expression>.
* When this method is called, the word "print" has already
* been read.  Evaluate the expression and print the value.
*/
private static void doPrintCommand() throws ParseError {
double val = expressionValue();
TextIO.skipBlanks();
if ( TextIO.peek() != '\n' )
throw new ParseError("Extra data after end of expression.");
System.out.println("Value is " + val);
}

/**
* Read an expression from the current line of input and return its value.
*/
private static double expressionValue() throws ParseError {
TextIO.skipBlanks();
boolean negative;  // True if there is a leading minus sign.
negative = false;
if (TextIO.peek() == '-') {
TextIO.getAnyChar();
negative = true;
}
double val;  // Value of the expression.
val = termValue();  // An expression must start with a term.
if (negative)
val = -val; // Apply the leading minus sign
TextIO.skipBlanks();
while ( TextIO.peek() == '+' || TextIO.peek() == '-' ) {
// Read the next term and add it to or subtract it from
// the value of previous terms in the expression.
char op = TextIO.getAnyChar();
double nextVal = termValue();
if (op == '+')
val += nextVal;
else
val -= nextVal;
TextIO.skipBlanks();
}
return val;
} // end expressionValue()

/**
* Read a term from the current line of input and return its value.
*/
private static double termValue() throws ParseError {
TextIO.skipBlanks();
double val;  // The value of the term.
val = factorValue();  // A term must start with a factor.
TextIO.skipBlanks();
while ( TextIO.peek() == '*' || TextIO.peek() == '/' ) {
// Read the next factor, and multiply or divide
// the value-so-far by the value of this factor.
char op = TextIO.getAnyChar();
double nextVal = factorValue();
if (op == '*')
val *= nextVal;
else
val /= nextVal;
TextIO.skipBlanks();
}
return val;
} // end termValue()

/**
* Read a factor from the current line of input and return its value.
*/
private static double factorValue() throws ParseError {
TextIO.skipBlanks();
double val;  // Value of the factor.
val = primaryValue();  // A factor must start with a primary.
TextIO.skipBlanks();
while ( TextIO.peek() == '^' ) {
// Read the next primary, and exponentiate
// the value-so-far by the value of this primary.
TextIO.getChar();
double nextVal = primaryValue();
val = Math.pow(val,nextVal);
if (Double.isNaN(val))
throw new ParseError("Illegal values for ^ operator.");
TextIO.skipBlanks();
}
return val;
} // end termValue()

/**
*  Read a primary from the current line of input and
*  return its value.  A primary must be a number,
*  a variable, or an expression enclosed in parentheses.
*/
private static double primaryValue() throws ParseError {
TextIO.skipBlanks();
char ch = TextIO.peek();
if ( Character.isDigit(ch) ) {
// The factor is a number.  Read it and
// return its value.
return TextIO.getDouble();
}
else if ( Character.isLetter(ch) ) {
// The factor is a variable or a standard function.  Read its name and
// look up its value in the symbol table.  If the name is not in the symbol table,
// an error occurs.  (Note that the values in the symbol table are objects of type
// Double or StandardFunction.)
String name = readWord();
Object obj = symbolTable.get(name);
if (obj == null)
throw new ParseError("Unknown word \"" + name + "\"");
assert (obj instanceof Double || obj instanceof StandardFunction);
if (obj instanceof Double) {
// The name is a variable; return value of that variable.
Double val = (Double)obj;
return val.doubleValue();
}
else {
// The name is a standard function.  Read the argument
// of the function and return the value of the function
// at that argument.  The argument must be an expression
// in parentheses.
StandardFunction func = (StandardFunction)obj;
TextIO.skipBlanks();
if ( TextIO.peek() != '(' )
throw new ParseError("Parenthesis missing after standard function");
TextIO.getChar(); // discard the '('
double argument = expressionValue();  // read and evaluate expression
TextIO.skipBlanks();
if ( TextIO.peek() != ')' )
throw new ParseError("Missing right parenthesis.");
TextIO.getChar(); // discard the ')'
return func.evaluate(argument);
}
}
else if ( ch == '(' ) {
// The factor is an expression in parentheses.
// Return the value of the expression.
TextIO.getAnyChar();  // Read the "("
double val = expressionValue();
TextIO.skipBlanks();
if ( TextIO.peek() != ')' )
throw new ParseError("Missing right parenthesis.");
TextIO.getAnyChar();  // Read the ")"
return val;
}
else if ( ch == '\n' )
throw new ParseError("End-of-line encountered in the middle of an expression.");
else if ( ch == ')' )
throw new ParseError("Extra right parenthesis.");
else if ( ch == '+' || ch == '-' || ch == '*' || ch == '/')
throw new ParseError("Misplaced operator.");
else
throw new ParseError("Unexpected character \"" + ch + "\" encountered.");
}

/**
*  Reads a word from input.  A word is any sequence of
*  letters and digits, starting with a letter.  When
*  this subroutine is called, it should already be
*  known that the next character in the input is
*  a letter.
*/
private static String readWord() {
String word = "";  // The word.
char ch = TextIO.peek();
while (Character.isLetter(ch) || Character.isDigit(ch)) {
word += TextIO.getChar(); // Add the character to the word.
ch = TextIO.peek();
}
return word;
}

} // end class SimpleInterpreter2

Solution for Programming Exercise 11.1

Exercise 11.1:

The sample program DirectoryList.java, given as an example in Subsection 11.2.2, will print a list of files in a directory specified by the user. But some of the files in that directory might themselves be directories. And the subdirectories can themselves contain directories. And so on. Write a modified version of DirectoryList that will list all the files in a directory and all its subdirectories, to any level of nesting. You will need a recursive subroutine to do the listing. The subroutine should have a parameter of type File. You will need the constructor from the File class that has the form

public File( File dir, String fileName )
// Constructs the File object representing a file
// named fileName in the directory specified by dir.

Discussion

One possible recursive algorithm for printing all the files in a directory and its subdirectories is:

Print the name of the directory
for each file in the directory:
if the file is a directory:
Print its contents recursively
else
Print the name of the file.

If the directory is given as an object of type File, then this can be coded easily. We need some instance methods from the File class. (This subroutine is not quite what I ended up using in my solution.)

static void listDirectoryContents( File dir ) {
System.out.println("Directory \"" + dir.getName() + "\"");
String[] files;  // The names of the files in the directory.
files = dir.list();
for (int i = 0; i < files.length; i++) {
File f;  // One of the files in the directory.
f = new File(dir, files[i]);
if ( f.isDirectory() ) {
// Call listDirectoryContents() recursively to
// list the contents of the directory, f.
listDirectoryContents(f);
}
else {
// For a regular file, just print the name, files[i].
System.out.println(files[i]);
}
}
} // end listDirectoryContents()

Every time this routine finds a directory, it lists not just the name of the directory but also, recursively, the names of everything that it contains. The only problem with this is that it doesn't indicate which items are in which directory. For example, consider the output:

Directory "games"
blackbox
Directory "CardGames"
cribbage
euchre
tetris

There is no way to tell where the list of items in "CardGames" ends. Possibly, for example, "euchre" is in the "CardGames" directory. But possibly, "cribbage" is the only file in "CardGames" and "euchre" is actually part of the listing for "games". It would be nice to use indentation to show the nesting, like this:

Directory "games"
blackbox
Directory "CardGames"
cribbage
euchre
tetris

In this listing, you can tell that "euchre" is in fact in "CardGames" while "tetris" is in "games". To implement this, we just have to recognize that the indentation can be different in each call to the listDirectoryContents() method. Since it can be different, it should be a parameter. When we call listDirectoryContents() recursively, we should increase the indentation. You can see how this is done in the actual solution, given below.

The Solution

import java.io.File;
import java.util.Scanner;

/**
* This program lists the files in a directory specified by
* the user.  If one of those files is itself a directory, the
* program recursively lists the contents of that subdirectory.
* The user is asked to type in the name of the directory that is
* to be listed.  If the name entered by the user is not a directory,
* a message is printed and the program ends.
*/
public class RecursiveDirectoryList {

public static void main(String[] args) {

String directoryName;  // Directory name entered by the user.
File directory;        // File object referring to the directory.
Scanner scanner;       // For reading a line of input from the user.

scanner = new Scanner(System.in);  // scanner reads from standard input.

System.out.print("Enter a directory name: ");
directoryName = scanner.nextLine().trim();
directory = new File(directoryName);

if (directory.isDirectory() == false) {
if (directory.exists() == false)
System.out.println("There is no such directory!");
else // the file exists but is not a directory
System.out.println("That file is not a directory.");
}
else {
// List the contents of the directory recursively, with
// no indentation at the top level.
listDirectoryContents(directory,"");
}

} // end main()

/**
* A recursive subroutine that lists the contents of a directory,
* including the contents of its subdirectories to any level of nesting.
* @param dir the directory whose contents are to be listed.  It is assumed
*   that dir is in fact a directory.
* @param indent a string of blanks that is prepended to each item in
*    the directory listing.  This string grows in length with each increase
*    in the level of directory nesting.
*/
private static void listDirectoryContents(File dir, String indent) {
String[] files;  // List of names of files in the directory.
System.out.println(indent + "Directory \"" + dir.getName() + "\":");
indent += "   ";  // Increase the indentation for listing the contents.
files = dir.list();
for (int i = 0; i < files.length; i++) {
// If the file is a  directory, list its contents
// recursively.  Otherwise, just print its name.
File f = new File(dir, files[i]);
if (f.isDirectory())
listDirectoryContents(f, indent);
else
System.out.println(indent + files[i]);
}
} // end listContents()

} // end class RecursiveDirectoryList

Solution for Programming Exercise 11.2

Exercise 11.2:

Write a program that will count the number of lines in each file that is specified on the command line. Assume that the files are text files. Note that multiple files can be specified, as in:

java  LineCounts  file1.txt  file2.txt  file3.txt

Write each file name, along with the number of lines in that file, to standard output. If an error occurs while trying to read from one of the files, you should print an error message for that file, but you should still process all the remaining files. Do not use TextIO to process the files; use a Scanner, a BufferedReader, or a TextReader to process each file.

Discussion

The main() routine for this program is a simple for loop that processes the command-line arguments. Each argument is supposed to be the name of a file. The loop just prints the file name and calls a subroutine that counts the number of lines in the file and outputs the result:

for (int i = 0; i < args.length; i++) {
System.out.print(args[i] + ": ");
countLines(args[i]);
}

The countLines() subroutine will catch any errors that occur when it tries to access the file. If an error occurs, it will output an error message instead of the number of lines. Since the error is handled in the subroutine, it won't crash the program or stop the main() routine from going on to process any remaining files.

The countLines() subroutine can be written using either a TextReader, a BufferedReader, or a Scanner to read lines from the file; each of these classes has a simple method for reading a complete line of text from the file. (An alternative approach would read individual characters from the file and count the number of end-of-line markers. Unfortunately, ends-of-line can be marked by any of the strings "\n", "\r", or "\r\n" and that would add to the difficulty of the task. The line reading methods in TextReader, BufferedReader, and Scanner will work no matter which end-of-line marker is used.)

In my solution, I use a BufferedReader. The countLines() method creates a BufferedReader stream to read from the file. It then calls the readLine() routine from the BufferedReader class to read lines of text from the file, until the end of the file is encountered. For a BufferedReader, the end-of-file can be recognized when readLine() returns null. The method counts each line as it reads it. In the end, the value of the counter is the number of lines in the file. The subroutine writes this value to standard output. All this is done in a try..catch statement, treating the BufferedReader as a resource, so that an error can be handled if it occurs. It's pretty straightforward. The complete solution is given below.

One note about the command line for calling this program. If you are using UNIX, including Mac OS or Linux, you can take advantage of something called "wildcards" in the command-line arguments. For example, if you say, "java LineCounts *.txt", the "*" is a wildcard. The operating system will expand "*.txt" into a list of all the files in the current directory that end in ".txt". Similarly, "*xx*" expands into a list of all file names that contain "xx", and "fil*dat" expands into the list of file names that begin with "fil" and end with "dat". The "*" matches any number of characters in the file name, including zero characters. This expansion is done before your program sees the command-line arguments, so you don't have to worry about it in your program. Typing the command "java LineCounts *.txt" would be exactly equivalent to typing something like "java LineCounts file1.txt file2.txt file3.txt". This type of expansion happens for any command in UNIX, not just for the "java" command.

The Solution

import java.io.*;

/**
* This program reports the number of lines in each of the files
* that are specified as command line arguments.  The files are
* assumed to be text files.  If a file does not exist, or if
* some error occurs when the attempt is made to read the file,
* then an error message is printed (but the other files are
* still processed).
*/
public class LineCounts {

/**
* The main() routine simply gets the file names from the
* command line and calls the countLines() routine to process
* each name.  Since any errors are handled in the countLines()
* routine, the main program will continue after an error occurs
* while trying to process one of the files.
*/
public static void main(String[] args) {

if (args.length == 0) {
// This program must have at least one command-line
// argument to work with.
System.out.println("Usage:   java LineCounts <file-names>");
return;
}

for (int i = 0; i < args.length; i++) {
System.out.print(args[i] + ":  ");
countLines(args[i]);
}

}  // end main()

/**
* Count the number of lines in the specified file, and
* print the number to standard output.  If an error occurs
* while processing the file, print an error message instead.
* Two try..catch statements are used so I can give a
* different error message in each case.
*/
private static void countLines(String fileName) {

try(BufferedReader in = new BufferedReader( new FileReader(fileName) )) {

int lineCount = 0;  // number of lines read from the file

String line = in.readLine();   // Read the first line.
while (line != null) {
lineCount++;               // Count this line.
line = in.readLine();      // Read the next line.
}
System.out.println(lineCount + " lines");

}
catch (FileNotFoundException e) {
System.out.println("Error.  Can't open file.");
}
catch (Exception e) {
System.out.println("Error.   Problem with reading from file.");
}

}  // end countLines()

} // end class LineCounts

Solution for Programming Exercise 11.3

Exercise 11.3:

For this exercise, you will write a network server program. The program is a simple file server that makes a collection of files available for transmission to clients. When the server starts up, it needs to know the name of the directory that contains the collection of files. This information can be provided as a command-line argument. You can assume that the directory contains only regular files (that is, it does not contain any sub-directories). You can also assume that all the files are text files.

When a client connects to the server, the server first reads a one-line command from the client. The command can be the string "INDEX". In this case, the server responds by sending a list of names of all the files that are available on the server. Or the command can be of the form "GET <filename>", where <filename> is a file name. The server checks whether the requested file actually exists. If so, it first sends the word "OK" as a message to the client. Then it sends the contents of the file and closes the connection. Otherwise, it sends a line beginning with the word "ERROR" to the client and closes the connection. (The error response can include an error message on the rest of the line.)

Write a subroutine to handle each request. See the DirectoryList example in Subsection 11.2.2 for help with the problem of getting the list of files in the directory.

Discussion

The sample program DateServer.java, from Subsection 11.4.4, shows the typical outline of a server program. For this exercise, the processing of each connection request is more complicated, but the basic outline of the program is the same.

My program begins by getting the directory name from the command line. An object of type File is created which can be used to check whether the specified directory exists and is in fact a directory rather than a regular file. This allows for some basic error checking before the server starts running:

if (args.length == 0) {
System.out.println("Usage:  java FileServer <directory>");
return;
}
directory = new File(args[0]);
if ( ! directory.exists() ) {
System.out.println("Specified directory does not exist.");
return;
}
if (! directory.isDirectory() ) {
System.out.println("The specified file is not a directory.");
return;
}

The File object is needed later, in any case, for reading the list of files in the directory.

The rest of the main() routine takes the typical form for a server program. A listening socket is established. Then the server accepts and processes connection requests in an infinite loop. In this case, the processing is done by calling a handleConnection() method that is defined later in the program. The actual communication with the client is done in this method. (Since it can potentially take a long time to transmit a file, it would be desirable to use threads to handle the connections in this program, instead of running them in the same thread that listens for connection requests. That would allow several requests to be handled simultaneously. Using threads for network communication will be covered in the next chapter.

The handleConnection() method implements the communication protocol of the server in a straightforward way: It reads a line from the client. If the line is "index", it sends a list of available files. If the line starts with the string "get", it sends the requested file. Finally, it closes the connection. (In fact, the connection is closed in the finally clause of a try statement to make sure that the connection is closed before the method returns.) You should be able to follow all this in the solution that follows.

(One minor point: Suppose that the last line in a file is not terminated by an end-of-line marker. That is, there is no end-of-line directly preceding the end-of-file. When my program transmits the file, it will nevertheless send an end-of-line marker after the last line of the file. So, the file that is transmitted is not identical to the file on the computer's hard disk. Actually, things are even worse than that because the end-of-line markers used when the file is transmitted are not necessarily the same as the ones that are used in the file as stored on the disk. It would probably be better to consider the file to be binary data and to send an exact, byte-by-byte copy.)

The Solution

import java.net.*;
import java.io.*;
import java.util.Scanner;

/**
* This program is a very simple network file server.  The
* server has a list of available text files that can be
* downloaded by the client.  The client can also download
* the list of files.  When the connection is opened, the
* client sends one of two possible commands to the server:
* "INDEX" or "GET <file-name>".  The server replies to
* the first command by sending the list of available files.
* It responds to the second with a one-line message,
* either "OK" or "ERROR".  If the message is "OK", it is
* followed by the contents of the file with the specified
* name.  The "ERROR" message indicates that the specified
* file does not exist on the server. (The server can also
* respond with the message "unknown command" if the command
* it reads is not one of the two possible legal commands.)
* (The commands INDEX and GET are not case-sensitive.)
*
* The server program requires a command-line parameter
* that specifies the directory that contains the files
* that the server can serve.  The files should all be
* text files, but this is not checked.  Also, the server
* must have permission to read all the files.
*/
public class FileServer {

static final int LISTENING_PORT = 3210;

public static void main(String[] args) {

File directory;        // The directory from which the server
//    gets the files that it serves.

ServerSocket listener; // Listens for connection requests.

Socket connection;     // A socket for communicating with a client.

/* Check that there is a command-line argument.
If not, print a usage message and end. */

if (args.length == 0) {
System.out.println("Usage:  java FileServer <directory>");
return;
}

/* Get the directory name from the command line, and make
it into a file object.  Check that the file exists and
is in fact a directory. */

directory = new File(args[0]);
if ( ! directory.exists() ) {
System.out.println("Specified directory does not exist.");
return;
}
if (! directory.isDirectory() ) {
System.out.println("The specified file is not a directory.");
return;
}

/* Listen for connection requests from clients.  For
each connection, call the handleConnection() method
to process it.  The server runs until the program
is terminated, for example by a CONTROL-C. */

try {
listener = new ServerSocket(LISTENING_PORT);
System.out.println("Listening on port " + LISTENING_PORT);
while (true) {
connection = listener.accept();
handleConnection(directory,connection);
}
}
catch (Exception e) {
System.out.println("Server shut down unexpectedly.");
System.out.println("Error:  " + e);
return;
}

} // end main()

/**
* This method processes the connection with one client.
* It creates streams for communicating with the client,
* reads a command from the client, and carries out that
* command.  The connection is also logged to standard output.
* An output beginning with ERROR indicates that a network
* error occurred.  A line beginning with OK means that
* there was no network error, but does not imply that the
* command from the client was a legal command.
*/
private static void handleConnection(File directory, Socket connection) {
Scanner incoming;       // For reading data from the client.
PrintWriter outgoing;   // For transmitting data to the client.
String command = "Command not read";
try {
incoming = new Scanner( connection.getInputStream() );
outgoing = new PrintWriter( connection.getOutputStream() );
command = incoming.nextLine();
if (command.equalsIgnoreCase("index")) {
sendIndex(directory, outgoing);
}
else if (command.toLowerCase().startsWith("get")){
String fileName = command.substring(3).trim();
sendFile(fileName, directory, outgoing);
}
else {
outgoing.println("ERROR unsupported command");
outgoing.flush();
}
System.out.println("OK    " + connection.getInetAddress()
+ " " + command);
}
catch (Exception e) {
System.out.println("ERROR " + connection.getInetAddress()
+ " " + command + " " + e);
}
finally {
try {
connection.close();
}
catch (IOException e) {
}
}
}

/**
* This is called by the handleConnection() method in response to an "INDEX" command
* from the client.  Send the list of files in the server's directory.
*/
private static void sendIndex(File directory, PrintWriter outgoing) throws Exception {
String[] fileList = directory.list();
for (int i = 0; i < fileList.length; i++)
outgoing.println(fileList[i]);
outgoing.flush();
outgoing.close();
if (outgoing.checkError())
throw new Exception("Error while transmitting data.");
}

/**
* This is called by the handleConnection() command in response to "GET <fileName>"
* command from the client.  If the file doesn't exist, send the message "error".
* Otherwise, send the message "ok" followed by the contents of the file.
*/
private static void sendFile(String fileName, File directory, PrintWriter outgoing) throws Exception {
File file = new File(directory,fileName);
if ( (! file.exists()) || file.isDirectory() ) {
// (Note:  Don't try to send a directory, which
// shouldn't be there anyway.)
outgoing.println("ERROR");
}
else {
outgoing.println("OK");
BufferedReader fileIn = new BufferedReader( new FileReader(file) );
while (true) {
// Read and send lines from the file until
// an end-of-file is encountered.
String line = fileIn.readLine();
if (line == null)
break;
outgoing.println(line);
}
}
outgoing.flush();
outgoing.close();
if (outgoing.checkError())
throw new Exception("Error while transmitting data.");
}

} //end class FileServer

Solution for Programming Exercise 11.4

Exercise 11.4:

Write a client program for the server from Exercise 11.3. Design a user interface that will let the user do at least two things: (1) Get a list of files that are available on the server and display the list on standard output; and (2) Get a copy of a specified file from the server and save it to a local file (on the computer where the client is running).

Discussion

One possible user interface would be to present the user with a menu of actions that the program can perform. For variety, however, I decided on an interface based entirely on the command line. The first command line argument must be the name or IP address of the computer where a copy of the FileServer program from Exercise 11.3 is running. If that is the only command-line argument, then the client will contact the server and send an "index" command to the server. The server responds with a list of file names. The client reads these names and displays them on the screen. The program then ends. (If you want to give another command, you have to run the client program again with a new command line.)

If there are two command-line arguments, then the second argument must be the name of a file on the server. The client contacts the server and sends a "GET" command. The server responds with a one-line message, either an "ERROR" or "OK". The client reads this message. If the message starts with "ERROR", indicating that the requested file can't be sent, then the client just displays an error message to the user. If the message is "OK", then the server also sends the contents of the file. The client tries to create a local file of the same name. It reads the data from the server and saves it to that file. However, for safety, the client will not create a new file if a local file of the same name already exists. This is considered to be an error. (An alternative would be to ask the user whether to replace the existing file.)

Finally, I wanted to make it possible to save a downloaded file in a local file with a different name. For this, three command line arguments are used. The first is the server, the second is the name of the file on the server, and the third is the name of the local file where the downloaded file is to be saved. In this case, the program is willing to overwrite an existing file of the same name, so the command must be used with some care.

If the server program is running on the same computer as the client (for demonstration purposes), the following command lines can be used to run the client:

java FileClient  localhost
java FileClient  localhost  datafile.txt
java FileClient  localhost  datafile.txt  mycopy.dat
java FileClient  localhost  datafile.txt  datafile.txt

The first command shows a list of files that are available on the server. The other three all try to get a file named "datafile.txt" from the server. The second command would refuse the save the file, if a file named datafile.txt already exists. The last command would replace an existing datafile.txt with the file retrieved from the server.

The actual programming of the client is fairly straightforward. The example program DateClient.java, from Subsection 11.4.4 provides a model for opening a connection to the server. You should be able to follow the solution, given below.

Note that the client program must know the protocol that is used to communicate with the server. The user of the program, however, doesn't need to know anything about the protocol. The user only has to know how to use the program, and the user interface does not reflect or depend on the details of the protocol.

By the way, once you understand how the FileClient and FileServer examples work, it's not a big conceptual leap to understanding how the World Wide Web works. A Web server program is just a greatly souped-up version of the FileServer program. It has access to a collection of files. It receives and responds to requests for those files from client programs. To get a file, the client program -- a Web browser -- needs to know the computer on which the server is running and the name of the file. This information is encoded in the url address of a Web page -- just like it's given on the command line of the FileClient program. A Web page, of course, can contain links to other Web pages. The link includes a url with the necessary information for finding the page. To get the page, the Web browser just goes through the same process of contacting the specified server and requesting the specified file. (One big complication is that not all the files on a Web server are text files, so the client needs some way of knowing what type of data is stored in the file, and it needs to know how to handle data of that type. Another complication is that Web servers support "server side programming." That is, in response to certain URLs, instead of just sending a file, the Web server will run a program and send the output to the client.)

The Solution

import java.net.*;
import java.io.*;

/**
* This program is a client for the FileServer server.  The
* server has a list of available text files that can be
* downloaded by the client.  The client can also download
* the list of files.  When the connection is opened, the
* client sends one of two possible commands to the server:
* "index" or "get <file-name>".  The server replies to
* the first command by sending the list of available files.
* It responds to the second with a one-line message,
* either "ok" or "error".  If the message is "ok", it is
* followed by the contents of the file with the specified
* name.  The "error" message indicates that the specified
* file does not exist on the server.  (The server can also
* respond with the message "unsupported command" if the command
* it reads is not one of the two possible legal commands.)
*
* The client program works with command-line arguments.
* The first argument must be the name or IP address of the
* computer where the server is running.  If that is the
* only argument on the command line, then the client
* gets the list of files from the server and displays
* it on standard output.  If there are two parameters,
* the second parameter is interpreted as the name of a
* file to be downloaded.  A copy of the file is saved
* as a local file of the same name, unless a file of
* the same name already exists.  If there are three
* arguments, the second is the name of the file to be
* downloaded and the third is the name under which the
* local copy of the file is to be saved.  This will
* work even if a file of the same name already exists.
*/
public class FileClient {

static final int LISTENING_PORT = 3210;

public static void main(String[] args) {

String computer;          // Name or IP address of server.
Socket connection;        // Socket for communicating with that computer.
PrintWriter outgoing;     // Stream for sending a command to the server.
BufferedReader incoming;  // Stream for reading data from the connection.
String command;           // Command to send to the server.

/* Check that the number of command-line arguments is legal.
If not, print a usage message and end. */

if (args.length == 0 || args.length > 3) {
System.out.println("Usage:  java FileClient <server>");
System.out.println("    or  java FileClient <server> <file>");
System.out.println(
"    or  java FileClient <server> <file> <local-file>");
return;
}

/* Get the server name and the message to send to the server. */

computer = args[0];

if (args.length == 1)
command = "INDEX";
else
command = "GET " + args[1];

/* Make the connection and open streams for communication.
Send the command to the server.  If something fails
during this process, print an error message and end. */

try {
connection = new Socket( computer, LISTENING_PORT );
incoming = new BufferedReader(
new InputStreamReader(connection.getInputStream()) );
outgoing = new PrintWriter( connection.getOutputStream() );
outgoing.println(command);
outgoing.flush(); // ESSENTIAL: Make sure command is dispatched to server!
}
catch (Exception e) {
System.out.println(
"Can't make connection to server at \"" + args[0] + "\".");
System.out.println("Error:  " + e);
return;
}

/* Read and process the server's response to the command. */

try {
if (args.length == 1) {
// The command was "index".  Read and display lines
// from the server until the end-of-stream is reached.
System.out.println("File list from server:");
while (true) {
String line = incoming.readLine();
if (line == null)
break;
System.out.println("   " + line);
}
}
else {
// The command was "get <file-name>".  Read the server's
// response message.  If the message is "OK", get the file.
String message = incoming.readLine();
if (! message.equalsIgnoreCase("OK")) {
System.out.println("File not found on server.");
System.out.println("Message from server: " + message);
return;
}
PrintWriter fileOut;  // For writing the received data to a file.
if (args.length == 3) {
// Use the third parameter as a file name.
// This will overwrite a file with the same name!
fileOut = new PrintWriter( new FileWriter(args[2]) );
}
else {
// Use the second parameter as a file name,
// but don't replace an existing file.
File file = new File(args[1]);
if (file.exists()) {
System.out.println("A file with that name already exists.");
System.out.println("To replace it, use the three-argument");
System.out.println("version of the command.");
return;
}
fileOut = new PrintWriter( new FileWriter(args[1]) );
}
while (true) {
// Copy lines from incoming to the file until
// the end of the incoming stream is encountered.
String line = incoming.readLine();
if (line == null)
break;
fileOut.println(line);
}
if (fileOut.checkError()) {
System.out.println("Some error occurred while writing the file.");
System.out.println("Output file might be empty or incomplete.");
}
}
}
catch (Exception e) {
System.out.println(
"Sorry, an error occurred while reading data from the server.");
System.out.println("Error: " + e);
}
finally {
try {
connection.close();
}
catch (IOException e) {
}
}

}  // end main()

} //end class FileClient

Solution for Programming Exercise 11.5

Exercise 11.5:

The sample program PhoneDirectoryFileDemo.java, from Subsection 11.3.2, stores name/number pairs for a simple phone book in a text file in the user's home directory. Modify that program so that it uses an XML format for the data. The only significant changes that you will have to make are to the parts of the program that read and write the data file. Use the DOM to read the data, as discussed in Subsection 11.5.2. You can use the XML format illustrated in the following sample phone directory file:

<?xml version="1.0"?>
<phone_directory>
<entry name='barney' number='890-1203'/>
<entry name='fred' number='555-9923'/>
</phone_directory>

(This is just a short exercise in basic XML processing; as before, the program in this exercise is not meant to be a useful phone directory program.)

Discussion

The data for the phone book is stored in a TreeMap in which the entries are name/number pairs. The original program uses a two-line for-each loop to output the data to a PrintWriter named out:

for ( Map.Entry<String,String> entry : phoneBook.entrySet() )
out.println(entry.getKey() + "%" + entry.getValue() );

The data file is written at the end of the program. Writing out the data in the XML format shown in the exercise is only a little harder. The same values are output, but we have to add in a lot of extra text:

out.println("<?xml version=\"1.0\"?>");
out.println("<phone_directory>");
for ( Map.Entry<String,String> entry : phoneBook.entrySet() ) {
out.print("  <entry name='");
out.print(entry.getKey());
out.print("' number='");
out.print(entry.getValue());
out.println("'/>");
}
out.println("</phone_directory>");

The data file is read at the beginning of the program. To use the Document Object Model for this, we have to create a DocumentBuilder and use it to parse the data file. The code comes directory from Subsection 11.5.2:

DocumentBuilder docReader =
DocumentBuilderFactory.newInstance().newDocumentBuilder();
Document xmldoc = docReader.parse(dataFile);

If this succeeds, we know that the file contained a well-formed XML document. We can then call xmldoc.getDocumentElement() to get the root element of the document. This element should have tag name "phone_directory." The phone directory entries are stored in elements that are directly nested inside the root element. We can get a list of the children of the root element by calling root.getChildNodes() and we can traverse that list to get the individual directory entries. The name and number for an entry are stored as values of attributes named "name" and "number", and we can retrieve them using the getAttribute() method. Once we have a name and number, we can add it to the TreeMap in the usual way. Here is the code that gets the data from the DOM representation of the XML data file:

Element root = xmldoc.getDocumentElement();
if (! root.getTagName().equals("phone_directory"))
throw new Exception();
NodeList nodes = root.getChildNodes();
for (int i = 0; i < nodes.getLength(); i++) {
if ( nodes.item(i) instanceof Element ) {
Element entry = (Element)nodes.item(i);
if (! entry.getTagName().equals("entry"))
throw new Exception();
String entryName = entry.getAttribute("name");
String entryNumber = entry.getAttribute("number");
if (entryName.length() == 0 || entryNumber.length() == 0)
throw new Exception();
phoneBook.put(entryName,entryNumber);
}
}

A complete solution is given below, with changes from PhoneDirectoryFileDemo.java shown in red italic.

The Solution

import java.io.*;

import java.util.Map;
import java.util.TreeMap;
import java.util.Scanner;

import javax.xml.parsers.*;
import org.w3c.dom.*;

/**
* This program lets the user keep a persistent "phone book" that
* contains names and phone numbers.  The data for the phone book
* is stored in a file in the user's home directory.
*
* The program is meant only as a demonstration of files and XML.
* The data file uses XML to represent the phone book data.
*/
public class PhoneDirectoryXMLDemo {

/**
* The name of the file in which the phone book data is kept.  The
* file is stored in the user's home directory.  The "." at the
* beginning of the file name means that the file will be a
* "hidden" file on Unix-based computers, including Linux and
* Mac OS X.
*/
private static String DATA_FILE_NAME = ".phone_book_xml_demo";

public static void main(String[] args) {

String name, number;  // Name and number of an entry in the directory
// (used at various places in the program).

TreeMap<String,String>  phoneBook;   // Phone directory data structure.
// Entries are name/number pairs.

phoneBook = new TreeMap<String,String>();

/* Create a dataFile variable of type File to represent the
* data file that is stored in the user's home directory.
*/

File userHomeDirectory = new File( System.getProperty("user.home") );
File dataFile = new File( userHomeDirectory, DATA_FILE_NAME );

/* If the data file already exists, then the data in the file is
* read and is used to initialize the phone directory.
*/

if ( ! dataFile.exists() ) {
System.out.println("No phone book data file found.  A new one");
System.out.println("will be created, if you add any entries.");
System.out.println("File name:  " + dataFile.getAbsolutePath());
}
else {
System.out.println("Reading phone book data...");
try {
DocumentBuilder docReader =
DocumentBuilderFactory.newInstance().newDocumentBuilder();
Document xmldoc = docReader.parse(dataFile);
Element root = xmldoc.getDocumentElement();
if (! root.getTagName().equals("phone_directory"))
throw new Exception();
NodeList nodes = root.getChildNodes();
for (int i = 0; i < nodes.getLength(); i++) {
if ( nodes.item(i) instanceof Element ) {
Element entry = (Element)nodes.item(i);
if (! entry.getTagName().equals("entry"))
throw new Exception();
String entryName = entry.getAttribute("name");
String entryNumber = entry.getAttribute("number");
if (entryName.length() == 0 || entryNumber.length() == 0)
throw new Exception();
phoneBook.put(entryName,entryNumber);
}
}
}
catch (Exception e) {
System.out.println("Error in read phone book data file.");
System.out.println("File name:  " + dataFile.getAbsolutePath());
System.out.println("This program cannot continue.");
System.exit(1);
}
}

/* Read commands from the user and carry them out, until the
* user gives the "Exit from program" command.
*/

Scanner in = new Scanner( System.in );
boolean changed = false;  // Have any changes been made to the directory?

mainLoop: while (true) {
System.out.println("\nSelect the action that you want to perform:");
System.out.println("   1.  Look up a phone number.");
System.out.println("   2.  Add or change a phone number.");
System.out.println("   3.  Remove an entry from your phone directory.");
System.out.println("   4.  List the entire phone directory.");
System.out.println("   5.  Exit from the program.");
System.out.println("Enter action number (1-5):  ");
int command;
if ( in.hasNextInt() ) {
command = in.nextInt();
in.nextLine();
}
else {
System.out.println("\nILLEGAL RESPONSE.  YOU MUST ENTER A NUMBER.");
in.nextLine();
continue;
}
switch(command) {
case 1:
System.out.print("\nEnter the name whose number you want to look up: ");
name = in.nextLine().trim().toLowerCase();
number = phoneBook.get(name);
if (number == null)
System.out.println("\nSORRY, NO NUMBER FOUND FOR " + name);
else
System.out.println("\nNUMBER FOR " + name + ":  " + number);
break;
case 2:
System.out.print("\nEnter the name: ");
name = in.nextLine().trim().toLowerCase();
if (name.length() == 0)
System.out.println("\nNAME CANNOT BE BLANK.");
else if (name.indexOf('%') >= 0)
System.out.println("\nNAME CANNOT CONTAIN THE CHARACTER \"%\".");
else {
System.out.print("Enter phone number: ");
number = in.nextLine().trim();
if (number.length() == 0)
System.out.println("\nPHONE NUMBER CANNOT BE BLANK.");
else {
phoneBook.put(name,number);
changed = true;
}
}
break;
case 3:
System.out.print("\nEnter the name whose entry you want to remove: ");
name = in.nextLine().trim().toLowerCase();
number = phoneBook.get(name);
if (number == null)
System.out.println("\nSORRY, THERE IS NO ENTRY FOR " + name);
else {
phoneBook.remove(name);
changed = true;
System.out.println("\nDIRECTORY ENTRY REMOVED FOR " + name);
}
break;
case 4:
System.out.println("\nLIST OF ENTRIES IN YOUR PHONE BOOK:\n");
for ( Map.Entry<String,String> entry : phoneBook.entrySet() )
System.out.println("   " + entry.getKey() + ": " + entry.getValue() );
break;
case 5:
System.out.println("\nExiting program.");
break mainLoop;
default:
System.out.println("\nILLEGAL ACTION NUMBER.");
}
}

/* Before ending the program, write the current contents of the
* phone directory, but only if some changes have been made to
* the directory.
*/

if (changed) {
System.out.println("Saving phone directory changes to file " +
dataFile.getAbsolutePath() + " ...");
PrintWriter out;
try {
out = new PrintWriter( new FileWriter(dataFile) );
}
catch (IOException e) {
System.out.println("ERROR: Can't open data file for output.");
return;
}
out.println("<?xml version=\"1.0\"?>");
out.println("<phone_directory>");
for ( Map.Entry<String,String> entry : phoneBook.entrySet() ) {
out.print("  <entry name='");
out.print(entry.getKey());
out.print("' number='");
out.print(entry.getValue());
out.println("'/>");
}
out.println("</phone_directory>");
out.close();
if (out.checkError())
System.out.println("ERROR: Some error occurred while writing data file.");
else
System.out.println("Done.");
}

}

}

Solution for Programming Exercise 11.6

Exercise 11.6:

The sample program Checkers.java from Subsection 7.5.3 lets two players play checkers. It would be nice if, in the middle of a game, the state of the game could be saved to a file. Later, the file could be read back into the file to restore the game and allow the players to continue. Add the ability to save and load files to the checkers program. Design a simple text-based format for the files. Here is a picture of my solution to this exercise, just after a file has been loaded into the program:

It's a little tricky to completely restore the state of a game. The program has a variable board of type CheckersData that stores the current contents of the board, and it has a variable currentPlayer of type int that indicates whether Red or Black is currently moving. This data must be stored in the file when a file is saved. When a file is read into the program, you should read the data into two local variables newBoard of type CheckersData and newCurrentPlayer of type int. Once you have successfully read all the data from the file, you can use the following code to set up the program state correctly. This code assumes that you have introduced two new variables saveButton and loadButton of type JButton to represent the "Save Game" and "Load Game" buttons:

board = newBoard;  // Set up game with data read from file.
currentPlayer = newCurrentPlayer;
legalMoves = board.getLegalMoves(currentPlayer);
selectedRow = -1;
gameInProgress = true;
newGameButton.setEnabled(false);
loadButton.setEnabled(false);
saveButton.setEnabled(true);
resignButton.setEnabled(true);
if (currentPlayer == CheckersData.RED)
message.setText("Game loaded -- it's RED's move.");
else
message.setText("Game loaded -- it's BLACK's move.");
repaint();

(Note, by the way, that in my solution, I use a TextReader to read the data from the file. TextReader is a non-standard class introduced in Subsection 11.1.4 and defined in the file TextReader.java. How to read the data in a file depends, of course, on the format that you have chosen for the data.)

Discussion

Many different formats could be used to represent the data for a checkers game in text form. In the program, the board data is an 8-by-8 two-dimensional array of integers. One way to represent the board would be to simply output 64 integers. However, the possible values in the array are given by constants CheckersData.RED, CheckersData.BLACK, CheckersData.RED_KING, CheckersData.BLACK_KING, and CheckersData.EMPTY, and it would be nice to use a representation that uses the meaning of these constants. I decided to represent the possible values with individual characters. An empty square is represented by '.'; a regular red piece by 'r'; a regular black piece by 'b'; a red king by 'R'; and a black king by 'B'. The contents of the 8-by-8 board are then represented by 8 lines with 8 characters on each line. I added a line at the beginning containing the name of the program "CheckersWithFiles"; this will make it easy to check that a file that the user selects for input is of the correct type. The last line of the file contains the string "Red" if it is Red's turn to move or "Black" if it is Black's turn. Here is a sample file created by my program:

CheckersWithFiles
b.......
.b.b.R..
b.......
........
........
.....r..
r.......
.r.r.B.r
Black

I wrote methods doSave() and doLoad() to save and load files. They are modeled directly on the sample writeFile() and readFile() methods that were presented in Subsection 11.2.3. The doSave() method uses a PrintWriter to write the file. The data is taken from the variables board and currentPlayer, as described in the exercise. This is very straightforward:

out.println("CheckersWithFiles"); // Identifies file as a Checkers game.
for (int row = 0; row < 8; row++) {
for (int col = 0; col < 8; col++) {
int piece = board.pieceAt(row, col);  // Contents of one square of board.
switch (piece) {
case CheckersData.EMPTY:      out.print('.');  break;
case CheckersData.RED:        out.print('r');  break;
case CheckersData.BLACK:      out.print('b');  break;
case CheckersData.RED_KING:   out.print('R');  break;
case CheckersData.BLACK_KING: out.print('B');  break;
}
}
out.println();
}
out.println(currentPlayer == CheckersData.RED ? "Red" : "Black");
out.flush();
out.close();
if (out.checkError())
throw new IOException("Some error occurred while saving the file.");

Reading the data back in is not much harder. As indicated in the exercises, I used a TextReader named in to read the data from the file. The method in.getln() can be used to read a line of text; in.getChar() reads a single non-blank character; and in.getWord() reads a word (that is, a string of non-blank characters). Both in.getChar() and in.getWord() will skip over any spaces and end-of-lines to get to a non-blank character; this makes it very easy to read the data format that I chose for the file. Here is the section of my doLoad() method that reads the data. It has a little more error-checking than you might have used in your own solution:

CheckersData newBoard = new CheckersData();
int newCurrentPlayer;
String programNameFromFile = in.getln();
if (! programNameFromFile.equals("CheckersWithFiles"))
throw new Exception("Selected file does not contain a checkers game.");
for (int row = 0; row < 8; row++) {
for (int col = 0; col < 8; col++) {
char pieceCode = in.getChar();
switch (pieceCode) {
case '.': newBoard.setPieceAt(row, col, CheckersData.EMPTY); break;
case 'r': newBoard.setPieceAt(row, col, CheckersData.RED); break;
case 'b': newBoard.setPieceAt(row, col, CheckersData.BLACK); break;
case 'R': newBoard.setPieceAt(row, col, CheckersData.RED_KING); break;
case 'B': newBoard.setPieceAt(row, col, CheckersData.BLACK_KING); break;
default: throw new Exception("Illegal board data found in file.");
}
if (row % 2 != col % 2 &&
newBoard.pieceAt(row, col) != CheckersData.EMPTY) {
// The logic here is that half the squares on a legal board must
// be empty.  A tricky way to recognize the squares that are
// always empty is that either the row number is odd and the
// column number is even, or the row number is even and the
// column number is odd.  The test "row % 2 != col % 2" is
// a quick way to test for this.
throw new Exception("Illegal board data found in file.");
}
}
}
String currentPlayerString = in.getWord();
if (currentPlayerString.equals("Red"))
newCurrentPlayer = CheckersData.RED;
else if (currentPlayerString.equals("Black"))
newCurrentPlayer = CheckersData.BLACK;
else
throw new Exception("Illegal current player found in file.");
board = newBoard;  // Set up game with data read from file.
currentPlayer = newCurrentPlayer;
legalMoves = board.getLegalMoves(currentPlayer);
selectedRow = -1;
gameInProgress = true;
newGameButton.setEnabled(false);
loadButton.setEnabled(false);
saveButton.setEnabled(true);
resignButton.setEnabled(true);
if (currentPlayer == CheckersData.RED)
message.setText("Game loaded -- it's RED's move.");
else
message.setText("Game loaded -- it's BLACK's move.");
repaint();

Aside from writing the doSave() and doLoad methods, you must also add "Save Game" and "Load Game" buttons to the program, and you have to decide when they should be enabled and disabled. I decided that it would be legal to save a game only when a game is in progress, not when it is over; the "Resign" button is enabled at exactly the same times, so I just searched for resignButton.setEnabled to find the places where I would have to call saveButton.setEnabled. Similarly, I made it legal to load a game from a file only when a game is not in progress, and the "New Game" button is enabled at exactly the same times. For creating the new buttons and adding them to the panel, I also imitated the existing code for the "New Game" and "Resign" buttons. You can see the changes that I made in the solution below.

The Solution

The checkers program is very long. All the changes from the original version are in the first 325 lines and are shown in red italic.

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import java.io.*;
import java.util.ArrayList;
import java.util.Scanner;

/**
* This panel lets two users play checkers against each other.
* Red always starts the game.  If a player can jump an opponent's
* piece, then the player must jump.  When a player can make no more
* moves, the game ends.
*
* The state of the game can be saved into a file, which can be
* reloaded later to continue the game.
*
* The class has a main() routine that lets it be run as a stand-alone
* application.  The application just opens a window that uses an object
* of type Checkers as its content pane.
*
*/
public class CheckersWithFiles extends JPanel {

/**
* Main routine makes it possible to run Checkers as a stand-alone
* application.  Opens a window showing a Checkers panel; the program
* ends when the user closes the window.
*/
public static void main(String[] args) {
JFrame window = new JFrame("Checkers");
CheckersWithFiles content = new CheckersWithFiles();
window.setContentPane(content);
window.pack();
Dimension screensize = Toolkit.getDefaultToolkit().getScreenSize();
window.setLocation( (screensize.width - window.getWidth())/2,
(screensize.height - window.getHeight())/2 );
window.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
window.setResizable(false);
window.setVisible(true);
}

private JButton newGameButton;  // Button for starting a new game.
private JButton resignButton;   // Button that a player can use to end
//    the game by resigning.
private JButton saveButton;     // Button for saving the state of a game.
private JButton loadButton;     // Button for loading a saved game back
//   into the program.

private JLabel message;  // Label for displaying messages to the user.

/**
* The constructor creates the Board (which in turn creates and manages
* the buttons and message label), adds all the components, and sets
* the bounds of the components.  A null layout is used.  (This is
* the only thing that is done in the main Checkers class.)
*/
public CheckersWithFiles() {

setLayout(null);  // I will do the layout myself.
setPreferredSize( new Dimension(350,250) );

setBackground(new Color(0,150,0));  // Dark green background.

/* Create the components and add them to the panel. */

Board board = new Board();  // Note: The constructor for the
//   board also creates the buttons
//   and label.
add(board);
add(newGameButton);
add(resignButton);
add(saveButton);
add(loadButton);
add(message);

/* Set the position and size of each component by calling
its setBounds() method. */

board.setBounds(20,20,164,164); // Note:  size MUST be 164-by-164 !
newGameButton.setBounds(210, 20, 120, 30);
loadButton.setBounds(210, 65, 120, 30);
saveButton.setBounds(210, 110, 120, 30);
resignButton.setBounds(210, 155, 120, 30);
message.setBounds(0, 200, 350, 30);

} // end constructor

// --------------------  Nested Classes -------------------------------

/**
* A CheckersMove object represents a move in the game of Checkers.
* It holds the row and column of the piece that is to be moved
* and the row and column of the square to which it is to be moved.
* (This class makes no guarantee that the move is legal.)
*/
private static class CheckersMove {
int fromRow, fromCol;  // Position of piece to be moved.
int toRow, toCol;      // Square it is to move to.
CheckersMove(int r1, int c1, int r2, int c2) {
// Constructor.  Just set the values of the instance variables.
fromRow = r1;
fromCol = c1;
toRow = r2;
toCol = c2;
}
boolean isJump() {
// Test whether this move is a jump.  It is assumed that
// the move is legal.  In a jump, the piece moves two
// rows.  (In a regular move, it only moves one row.)
return (fromRow - toRow == 2 || fromRow - toRow == -2);
}
}  // end class CheckersMove.

/**
* This panel displays a 160-by-160 checkerboard pattern with
* a 2-pixel black border.  It is assumed that the size of the
* canvas is set to exactly 164-by-164 pixels.  This class does
* the work of letting the users play checkers, and it displays
* the checkerboard.
*/
private class Board extends JPanel implements ActionListener, MouseListener {

CheckersData board;  // The data for the checkers board is kept here.
//    This board is also responsible for generating
//    lists of legal moves.

boolean gameInProgress; // Is a game currently in progress?

/* The next three variables are valid only when the game is in progress. */

int currentPlayer;      // Whose turn is it now?  The possible values
//    are CheckersData.RED and CheckersData.BLACK.

int selectedRow, selectedCol;  // If the current player has selected a piece to
//     move, these give the row and column
//     containing that piece.  If no piece is
//     yet selected, then selectedRow is -1.

CheckersMove[] legalMoves;  // An array containing the legal moves for the
//   current player.

JFileChooser fileDialog;   // For use in save and load commands.

/**
* Constructor.  Create the buttons and label.  Listens for mouse
* clicks and for clicks on the buttons.  Create the board and
* start the first game.
*/
Board() {
setBackground(Color.BLACK);
addMouseListener(this);
resignButton = new JButton("Resign");
resignButton.addActionListener(this);
newGameButton = new JButton("New Game");
newGameButton.addActionListener(this);
saveButton = new JButton("Save Game");
saveButton.addActionListener(this);
loadButton = new JButton("Load Game");
loadButton.addActionListener(this);
message = new JLabel("",JLabel.CENTER);
message.setFont(new  Font("Serif", Font.BOLD, 14));
message.setForeground(Color.green);
board = new CheckersData();
doNewGame();
}

/**
* Respond to user's click on one of the two buttons.
*/
public void actionPerformed(ActionEvent evt) {
Object src = evt.getSource();
if (src == newGameButton)
doNewGame();
else if (src == resignButton)
doResign();
else if (src == saveButton)
doSave();
else if (src == loadButton)
doLoad();
}

/**
* Save the current game state to a file.
*/
void doSave() {
if (fileDialog == null)
fileDialog = new JFileChooser();
File selectedFile;  // Initially selected file name in the dialog.
selectedFile = new File("CheckersGame.text");
fileDialog.setSelectedFile(selectedFile);
fileDialog.setDialogTitle("Select File for Saving the Game");
int option = fileDialog.showSaveDialog(this);
if (option != JFileChooser.APPROVE_OPTION)
return;  // User canceled or clicked the dialog's close box.
selectedFile = fileDialog.getSelectedFile();
if (selectedFile.exists()) {  // Ask the user whether to replace the file.
int response = JOptionPane.showConfirmDialog( this,
"The file \"" + selectedFile.getName()
+ "\" already exists.\nDo you want to replace it?",
"Confirm Save",
JOptionPane.YES_NO_OPTION,
JOptionPane.WARNING_MESSAGE );
if (response != JOptionPane.YES_OPTION)
return;  // User does not want to replace the file.
}
PrintWriter out;
try {
FileWriter stream = new FileWriter(selectedFile);
out = new PrintWriter( stream );
}
catch (Exception e) {
JOptionPane.showMessageDialog(this,
"Sorry, but an error occurred while trying to open the file:\n" + e);
return;
}
try {
out.println("CheckersWithFiles"); // Identifies file as a Checkers game.
for (int row = 0; row < 8; row++) {
for (int col = 0; col < 8; col++) {
int piece = board.pieceAt(row, col);
switch (piece) {
case CheckersData.EMPTY:      out.print('.');  break;
case CheckersData.RED:        out.print('r');  break;
case CheckersData.BLACK:      out.print('b');  break;
case CheckersData.RED_KING:   out.print('R');  break;
case CheckersData.BLACK_KING: out.print('B');  break;
}
}
out.println();
}
out.println(currentPlayer == CheckersData.RED ? "Red" : "Black");
out.flush();
out.close();
if (out.checkError())
throw new IOException("Some error occurred while saving the file.");
}
catch (Exception e) {
JOptionPane.showMessageDialog(this,
"Sorry, but an error occurred while trying to write the text:\n" + e);
}
}

/**
* Load a game that was previous saved to a file back into
* the program.
*/
void doLoad() {
if (fileDialog == null)
fileDialog = new JFileChooser();
fileDialog.setDialogTitle("Select Checkers Game File");
fileDialog.setSelectedFile(null);  // No file is initially selected.
int option = fileDialog.showOpenDialog(this);
if (option != JFileChooser.APPROVE_OPTION)
return;  // User canceled or clicked the dialog's close box.
File selectedFile = fileDialog.getSelectedFile();
TextReader in;
try {
Reader stream = new BufferedReader(new FileReader(selectedFile));
in = new TextReader( stream );
}
catch (Exception e) {
JOptionPane.showMessageDialog(this,
"Sorry, but an error occurred while trying to open the file:\n" + e);
return;
}
try {
CheckersData newBoard = new CheckersData();
int newCurrentPlayer;
String programNameFromFile = in.getln();
if (! programNameFromFile.equals("CheckersWithFiles"))
throw new Exception("Selected file does not contain a checkers game.");
for (int row = 0; row < 8; row++) {
for (int col = 0; col < 8; col++) {
char pieceCode = in.getChar();
switch (pieceCode) {
case '.': newBoard.setPieceAt(row, col, CheckersData.EMPTY); break;
case 'r': newBoard.setPieceAt(row, col, CheckersData.RED); break;
case 'b': newBoard.setPieceAt(row, col, CheckersData.BLACK); break;
case 'R': newBoard.setPieceAt(row, col, CheckersData.RED_KING); break;
case 'B': newBoard.setPieceAt(row, col, CheckersData.BLACK_KING); break;
default: throw new Exception("Illegal board data found in file.");
}
if (row % 2 != col % 2 &&
newBoard.pieceAt(row, col) != CheckersData.EMPTY)
throw new Exception("Illegal board data found in file.");
}
}
String currentPlayerString = in.getWord();
if (currentPlayerString.equals("Red"))
newCurrentPlayer = CheckersData.RED;
else if (currentPlayerString.equals("Black"))
newCurrentPlayer = CheckersData.BLACK;
else
throw new Exception("Illegal current player found in file.");
board = newBoard;  // Set up game with data read from file.
currentPlayer = newCurrentPlayer;
legalMoves = board.getLegalMoves(currentPlayer);
selectedRow = -1;
gameInProgress = true;
newGameButton.setEnabled(false);
loadButton.setEnabled(false);
saveButton.setEnabled(true);
resignButton.setEnabled(true);
if (currentPlayer == CheckersData.RED)
message.setText("Game loaded -- it's RED's move.");
else
message.setText("Game loaded -- it's BLACK's move.");
repaint();
}
catch (Exception e) {
JOptionPane.showMessageDialog(this,
"Sorry, but an error occurred while trying to read the data:\n" + e);
}
}

/**
* Start a new game
*/
void doNewGame() {
if (gameInProgress == true) {
// This should not be possible, but it doesn't hurt to check.
message.setText("Finish the current game first!");
return;
}
board.setUpGame();   // Set up the pieces.
currentPlayer = CheckersData.RED;   // RED moves first.
legalMoves = board.getLegalMoves(CheckersData.RED);  // Get RED's legal moves.
selectedRow = -1;   // RED has not yet selected a piece to move.
message.setText("Red:  Make your move.");
gameInProgress = true;
newGameButton.setEnabled(false);
loadButton.setEnabled(false);
saveButton.setEnabled(true);
resignButton.setEnabled(true);
repaint();
}

/**
* Current player resigns.  Game ends.  Opponent wins.
*/
void doResign() {
if (gameInProgress == false) {
message.setText("There is no game in progress!");
return;
}
if (currentPlayer == CheckersData.RED)
gameOver("RED resigns.  BLACK wins.");
else
gameOver("BLACK resigns.  RED wins.");
}

/**
* The game ends.  The parameter, str, is displayed as a message
* to the user.  The states of the buttons are adjusted so the players
* can start a new game.  This method is called when the game
* ends at any point in this class.
*/
void gameOver(String str) {
message.setText(str);
newGameButton.setEnabled(true);
loadButton.setEnabled(true);
saveButton.setEnabled(false);
resignButton.setEnabled(false);
gameInProgress = false;
}

/**
* This is called by mousePressed() when a player clicks on the
* square in the specified row and col.  It has already been checked
* that a game is, in fact, in progress.
*/
void doClickSquare(int row, int col) {

/* If the player clicked on one of the pieces that the player
can move, mark this row and col as selected and return.  (This
might change a previous selection.)  Reset the message, in
case it was previously displaying an error message. */

for (int i = 0; i < legalMoves.length; i++)
if (legalMoves[i].fromRow == row && legalMoves[i].fromCol == col) {
selectedRow = row;
selectedCol = col;
if (currentPlayer == CheckersData.RED)
message.setText("RED:  Make your move.");
else
message.setText("BLACK:  Make your move.");
repaint();
return;
}

/* If no piece has been selected to be moved, the user must first
select a piece.  Show an error message and return. */

if (selectedRow < 0) {
message.setText("Click the piece you want to move.");
return;
}

/* If the user clicked on a square where the selected piece can be
legally moved, then make the move and return. */

for (int i = 0; i < legalMoves.length; i++)
if (legalMoves[i].fromRow == selectedRow && legalMoves[i].fromCol == selectedCol
&& legalMoves[i].toRow == row && legalMoves[i].toCol == col) {
doMakeMove(legalMoves[i]);
return;
}

/* If we get to this point, there is a piece selected, and the square where
the user just clicked is not one where that piece can be legally moved.
Show an error message. */

message.setText("Click the square you want to move to.");

}  // end doClickSquare()

/**
* This is called when the current player has chosen the specified
* move.  Make the move, and then either end or continue the game
* appropriately.
*/
void doMakeMove(CheckersMove move) {

board.makeMove(move);

/* If the move was a jump, it's possible that the player has another
jump.  Check for legal jumps starting from the square that the player
just moved to.  If there are any, the player must jump.  The same
player continues moving.
*/

if (move.isJump()) {
legalMoves = board.getLegalJumpsFrom(currentPlayer,move.toRow,move.toCol);
if (legalMoves != null) {
if (currentPlayer == CheckersData.RED)
message.setText("RED:  You must continue jumping.");
else
message.setText("BLACK:  You must continue jumping.");
selectedRow = move.toRow;  // Since only one piece can be moved, select it.
selectedCol = move.toCol;
repaint();
return;
}
}

/* The current player's turn is ended, so change to the other player.
Get that player's legal moves.  If the player has no legal moves,
then the game ends. */

if (currentPlayer == CheckersData.RED) {
currentPlayer = CheckersData.BLACK;
legalMoves = board.getLegalMoves(currentPlayer);
if (legalMoves == null)
gameOver("BLACK has no moves.  RED wins.");
else if (legalMoves[0].isJump())
message.setText("BLACK:  Make your move.  You must jump.");
else
message.setText("BLACK:  Make your move.");
}
else {
currentPlayer = CheckersData.RED;
legalMoves = board.getLegalMoves(currentPlayer);
if (legalMoves == null)
gameOver("RED has no moves.  BLACK wins.");
else if (legalMoves[0].isJump())
message.setText("RED:  Make your move.  You must jump.");
else
message.setText("RED:  Make your move.");
}

/* Set selectedRow = -1 to record that the player has not yet selected
a piece to move. */

selectedRow = -1;

/* As a courtesy to the user, if all legal moves use the same piece, then
select that piece automatically so the user won't have to click on it
to select it. */

if (legalMoves != null) {
boolean sameStartSquare = true;
for (int i = 1; i < legalMoves.length; i++)
if (legalMoves[i].fromRow != legalMoves[0].fromRow
|| legalMoves[i].fromCol != legalMoves[0].fromCol) {
sameStartSquare = false;
break;
}
if (sameStartSquare) {
selectedRow = legalMoves[0].fromRow;
selectedCol = legalMoves[0].fromCol;
}
}

/* Make sure the board is redrawn in its new state. */

repaint();

}  // end doMakeMove();

/**
* Draw checkerboard pattern in gray and lightGray.  Draw the
* checkers.  If a game is in progress, highlight the legal moves.
*/
public void paintComponent(Graphics g) {

/* Draw a two-pixel black border around the edges of the canvas. */

g.setColor(Color.black);
g.drawRect(0,0,getSize().width-1,getSize().height-1);
g.drawRect(1,1,getSize().width-3,getSize().height-3);

/* Draw the squares of the checkerboard and the checkers. */

for (int row = 0; row < 8; row++) {
for (int col = 0; col < 8; col++) {
if ( row % 2 == col % 2 )
g.setColor(Color.LIGHT_GRAY);
else
g.setColor(Color.GRAY);
g.fillRect(2 + col*20, 2 + row*20, 20, 20);
switch (board.pieceAt(row,col)) {
case CheckersData.RED:
g.setColor(Color.RED);
g.fillOval(4 + col*20, 4 + row*20, 15, 15);
break;
case CheckersData.BLACK:
g.setColor(Color.BLACK);
g.fillOval(4 + col*20, 4 + row*20, 15, 15);
break;
case CheckersData.RED_KING:
g.setColor(Color.RED);
g.fillOval(4 + col*20, 4 + row*20, 15, 15);
g.setColor(Color.WHITE);
g.drawString("K", 7 + col*20, 16 + row*20);
break;
case CheckersData.BLACK_KING:
g.setColor(Color.BLACK);
g.fillOval(4 + col*20, 4 + row*20, 15, 15);
g.setColor(Color.WHITE);
g.drawString("K", 7 + col*20, 16 + row*20);
break;
}
}
}

/* If a game is in progress, highlight the legal moves.   Note that legalMoves
is never null while a game is in progress. */

if (gameInProgress) {
/* First, draw a 2-pixel cyan border around the pieces that can be moved. */
g.setColor(Color.cyan);
for (int i = 0; i < legalMoves.length; i++) {
g.drawRect(2 + legalMoves[i].fromCol*20, 2 + legalMoves[i].fromRow*20, 19, 19);
g.drawRect(3 + legalMoves[i].fromCol*20, 3 + legalMoves[i].fromRow*20, 17, 17);
}
/* If a piece is selected for moving (i.e. if selectedRow >= 0), then
draw a 2-pixel white border around that piece and draw green borders
around each square that that piece can be moved to. */
if (selectedRow >= 0) {
g.setColor(Color.white);
g.drawRect(2 + selectedCol*20, 2 + selectedRow*20, 19, 19);
g.drawRect(3 + selectedCol*20, 3 + selectedRow*20, 17, 17);
g.setColor(Color.green);
for (int i = 0; i < legalMoves.length; i++) {
if (legalMoves[i].fromCol == selectedCol && legalMoves[i].fromRow == selectedRow) {
g.drawRect(2 + legalMoves[i].toCol*20, 2 + legalMoves[i].toRow*20, 19, 19);
g.drawRect(3 + legalMoves[i].toCol*20, 3 + legalMoves[i].toRow*20, 17, 17);
}
}
}
}

}  // end paintComponent()

/**
* Respond to a user click on the board.  If no game is in progress, show
* an error message.  Otherwise, find the row and column that the user
* clicked and call doClickSquare() to handle it.
*/
public void mousePressed(MouseEvent evt) {
if (gameInProgress == false)
message.setText("Click \"New Game\" to start a new game.");
else {
int col = (evt.getX() - 2) / 20;
int row = (evt.getY() - 2) / 20;
if (col >= 0 && col < 8 && row >= 0 && row < 8)
doClickSquare(row,col);
}
}

public void mouseReleased(MouseEvent evt) { }
public void mouseClicked(MouseEvent evt) { }
public void mouseEntered(MouseEvent evt) { }
public void mouseExited(MouseEvent evt) { }

}  // end class Board

/**
* An object of this class holds data about a game of checkers.
* It knows what kind of piece is on each square of the checkerboard.
* Note that RED moves "up" the board (i.e. row number decreases)
* while BLACK moves "down" the board (i.e. row number increases).
* Methods are provided to return lists of available legal moves.
*/
private static class CheckersData {

/*  The following constants represent the possible contents of a square
on the board.  The constants RED and BLACK also represent players
in the game. */

static final int
EMPTY = 0,
RED = 1,
RED_KING = 2,
BLACK = 3,
BLACK_KING = 4;

int[][] board;  // board[r][c] is the contents of row r, column c.

/**
* Constructor.  Create the board and set it up for a new game.
*/
CheckersData() {
board = new int[8][8];
setUpGame();
}

/**
* Set up the board with checkers in position for the beginning
* of a game.  Note that checkers can only be found in squares
* that satisfy  row % 2 == col % 2.  At the start of the game,
* all such squares in the first three rows contain black squares
* and all such squares in the last three rows contain red squares.
*/
void setUpGame() {
for (int row = 0; row < 8; row++) {
for (int col = 0; col < 8; col++) {
if ( row % 2 == col % 2 ) {
if (row < 3)
board[row][col] = BLACK;
else if (row > 4)
board[row][col] = RED;
else
board[row][col] = EMPTY;
}
else {
board[row][col] = EMPTY;
}
}
}
}  // end setUpGame()

/**
* Return the contents of the square in the specified row and column.
*/
int pieceAt(int row, int col) {
return board[row][col];
}

/**
* Set the contents of the square in the specified row and column.
* piece must be one of the constants EMPTY, RED, BLACK, RED_KING,
* BLACK_KING.
*/
void setPieceAt(int row, int col, int piece) {
board[row][col] = piece;
}

/**
* Make the specified move.  It is assumed that move
* is non-null and that the move it represents is legal.
*/
void makeMove(CheckersMove move) {
makeMove(move.fromRow, move.fromCol, move.toRow, move.toCol);
}

/**
* Make the move from (fromRow,fromCol) to (toRow,toCol).  It is
* assumed that this move is legal.  If the move is a jump, the
* jumped piece is removed from the board.  If a piece moves
* the last row on the opponent's side of the board, the
* piece becomes a king.
*/
void makeMove(int fromRow, int fromCol, int toRow, int toCol) {
board[toRow][toCol] = board[fromRow][fromCol];
board[fromRow][fromCol] = EMPTY;
if (fromRow - toRow == 2 || fromRow - toRow == -2) {
// The move is a jump.  Remove the jumped piece from the board.
int jumpRow = (fromRow + toRow) / 2;  // Row of the jumped piece.
int jumpCol = (fromCol + toCol) / 2;  // Column of the jumped piece.
board[jumpRow][jumpCol] = EMPTY;
}
if (toRow == 0 && board[toRow][toCol] == RED)
board[toRow][toCol] = RED_KING;
if (toRow == 7 && board[toRow][toCol] == BLACK)
board[toRow][toCol] = BLACK_KING;
}

/**
* Return an array containing all the legal CheckersMoves
* for the specified player on the current board.  If the player
* has no legal moves, null is returned.  The value of player
* should be one of the constants RED or BLACK; if not, null
* is returned.  If the returned value is non-null, it consists
* entirely of jump moves or entirely of regular moves, since
* if the player can jump, only jumps are legal moves.
*/
CheckersMove[] getLegalMoves(int player) {

if (player != RED && player != BLACK)
return null;

int playerKing;  // The constant representing a King belonging to player.
if (player == RED)
playerKing = RED_KING;
else
playerKing = BLACK_KING;

ArrayList<CheckersMove> moves = new ArrayList<CheckersMove>();  // Moves will be stored in this list.

/*  First, check for any possible jumps.  Look at each square on the board.
If that square contains one of the player's pieces, look at a possible
jump in each of the four directions from that square.  If there is
a legal jump in that direction, put it in the moves ArrayList.
*/

for (int row = 0; row < 8; row++) {
for (int col = 0; col < 8; col++) {
if (board[row][col] == player || board[row][col] == playerKing) {
if (canJump(player, row, col, row+1, col+1, row+2, col+2))
moves.add(new CheckersMove(row, col, row+2, col+2));
if (canJump(player, row, col, row-1, col+1, row-2, col+2))
moves.add(new CheckersMove(row, col, row-2, col+2));
if (canJump(player, row, col, row+1, col-1, row+2, col-2))
moves.add(new CheckersMove(row, col, row+2, col-2));
if (canJump(player, row, col, row-1, col-1, row-2, col-2))
moves.add(new CheckersMove(row, col, row-2, col-2));
}
}
}

/*  If any jump moves were found, then the user must jump, so we don't
add any regular moves.  However, if no jumps were found, check for
any legal regular moves.  Look at each square on the board.
If that square contains one of the player's pieces, look at a possible
move in each of the four directions from that square.  If there is
a legal move in that direction, put it in the moves ArrayList.
*/

if (moves.size() == 0) {
for (int row = 0; row < 8; row++) {
for (int col = 0; col < 8; col++) {
if (board[row][col] == player || board[row][col] == playerKing) {
if (canMove(player,row,col,row+1,col+1))
moves.add(new CheckersMove(row,col,row+1,col+1));
if (canMove(player,row,col,row-1,col+1))
moves.add(new CheckersMove(row,col,row-1,col+1));
if (canMove(player,row,col,row+1,col-1))
moves.add(new CheckersMove(row,col,row+1,col-1));
if (canMove(player,row,col,row-1,col-1))
moves.add(new CheckersMove(row,col,row-1,col-1));
}
}
}
}

/* If no legal moves have been found, return null.  Otherwise, create
an array just big enough to hold all the legal moves, copy the
legal moves from the ArrayList into the array, and return the array. */

if (moves.size() == 0)
return null;
else {
CheckersMove[] moveArray = new CheckersMove[moves.size()];
for (int i = 0; i < moves.size(); i++)
moveArray[i] = moves.get(i);
return moveArray;
}

}  // end getLegalMoves

/**
* Return a list of the legal jumps that the specified player can
* make starting from the specified row and column.  If no such
* jumps are possible, null is returned.  The logic is similar
* to the logic of the getLegalMoves() method.
*/
CheckersMove[] getLegalJumpsFrom(int player, int row, int col) {
if (player != RED && player != BLACK)
return null;
int playerKing;  // The constant representing a King belonging to player.
if (player == RED)
playerKing = RED_KING;
else
playerKing = BLACK_KING;
ArrayList<CheckersMove> moves = new ArrayList<CheckersMove>();  // The legal jumps will be stored in this list.
if (board[row][col] == player || board[row][col] == playerKing) {
if (canJump(player, row, col, row+1, col+1, row+2, col+2))
moves.add(new CheckersMove(row, col, row+2, col+2));
if (canJump(player, row, col, row-1, col+1, row-2, col+2))
moves.add(new CheckersMove(row, col, row-2, col+2));
if (canJump(player, row, col, row+1, col-1, row+2, col-2))
moves.add(new CheckersMove(row, col, row+2, col-2));
if (canJump(player, row, col, row-1, col-1, row-2, col-2))
moves.add(new CheckersMove(row, col, row-2, col-2));
}
if (moves.size() == 0)
return null;
else {
CheckersMove[] moveArray = new CheckersMove[moves.size()];
for (int i = 0; i < moves.size(); i++)
moveArray[i] = moves.get(i);
return moveArray;
}
}  // end getLegalMovesFrom()

/**
* This is called by the two previous methods to check whether the
* player can legally jump from (r1,c1) to (r3,c3).  It is assumed
* that the player has a piece at (r1,c1), that (r3,c3) is a position
* that is 2 rows and 2 columns distant from (r1,c1) and that
* (r2,c2) is the square between (r1,c1) and (r3,c3).
*/
private boolean canJump(int player, int r1, int c1, int r2, int c2, int r3, int c3) {

if (r3 < 0 || r3 >= 8 || c3 < 0 || c3 >= 8)
return false;  // (r3,c3) is off the board.

if (board[r3][c3] != EMPTY)
return false;  // (r3,c3) already contains a piece.

if (player == RED) {
if (board[r1][c1] == RED && r3 > r1)
return false;  // Regular red piece can only move  up.
if (board[r2][c2] != BLACK && board[r2][c2] != BLACK_KING)
return false;  // There is no black piece to jump.
return true;  // The jump is legal.
}
else {
if (board[r1][c1] == BLACK && r3 < r1)
return false;  // Regular black piece can only move down.
if (board[r2][c2] != RED && board[r2][c2] != RED_KING)
return false;  // There is no red piece to jump.
return true;  // The jump is legal.
}

}  // end canJump()

/**
* This is called by the getLegalMoves() method to determine whether
* the player can legally move from (r1,c1) to (r2,c2).  It is
* assumed that (r1,r2) contains one of the player's pieces and
* that (r2,c2) is a neighboring square.
*/
private boolean canMove(int player, int r1, int c1, int r2, int c2) {

if (r2 < 0 || r2 >= 8 || c2 < 0 || c2 >= 8)
return false;  // (r2,c2) is off the board.

if (board[r2][c2] != EMPTY)
return false;  // (r2,c2) already contains a piece.

if (player == RED) {
if (board[r1][c1] == RED && r2 > r1)
return false;  // Regular red piece can only move down.
return true;  // The move is legal.
}
else {
if (board[r1][c1] == BLACK && r2 < r1)
return false;  // Regular black piece can only move up.
return true;  // The move is legal.
}

}  // end canMove()

} // end class CheckersData

} // end class CheckersWithFiles

Solution for Programming Exercise 12.1

Exercise 12.1:

Subsection 12.1.3 discusses the need for synchronization in multithreaded programs, and it defines a ThreadSafeCounter class with the necessary synchronization. Is this really important? Can you really get errors by using an unsynchronized counter with multiple threads? Write a program to find out. Use the following unsynchronized counter class, which you can include as a nested class in your program:

static class Counter {
int count;
void inc() {
count = count+1;
}
int getCount() {
return count;
}
}

Write a thread class that will call the inc() method in this class a specified number of times. Create several threads, start them all, and wait for all the threads to terminate. Print the final value of the counter, and see whether it is correct.

Let the user enter the number of threads and the number of times that each thread will increment the counter. You might need a fairly large number of increments to see an error. And of course there can never be any error if you use just one thread. Your program can use join() to wait for a thread to terminate (see Subsection 12.1.2).

Discussion

The necessary thread class is very simple to write. In my program, it's a static nested class that uses two global variables, which represent the counter and the number of times that the thread is to increment the counter:

static Counter counter;          // The counter that will be incremented.
static int numberOfIncrements;   // Number of times each thread will increment it.

static class IncrementerThread extends Thread {
public void run() {
for (int i = 0; i < numberOfIncrements; i++) {
counter.inc();
}
}
}

Note that since the counter is a global static variable, it is used by all threads of type IncrementerThread. It is a shared resource.

In the main() routine, after getting the number of threads and the number of increments from the user, we need to create the threads, start them, and wait for them to terminate. We have to do the part about starting all the threads before we get to the part about waiting for threads to terminate, so we need an array to hold the threads:

IncrementerThread[] workers = new IncrementerThread[numberOfThreads];
counter = new Counter();
for (int i = 0; i < numberOfThreads; i++)
workers[i] = new IncrementerThread();
for (int i = 0; i < numberOfThreads; i++)
workers[i].start();

/* Wait for all threads to terminate. */

for (int i = 0; i < numberOfThreads; i++) {
try {
workers[i].join();
}
catch (InterruptedException e) {
}
}

(I hope that it's clear that you can't start a thread, wait for it to finish, start another thread, wait for it to finish, and so on. If you did that, you'd only have one thread running at any given time, and you would certainly not see any errors in the counter!)

My program actually runs in a loop, so the user can run several experiments without restarting the program every time.

In my own experiments, I saw a lot of errors when the number of increments per thread was one million, even with just two threads. For small numbers of increments, less than about 1000, I never saw any errors. (This might be because the threads didn't run long enough to actually get more than one running at a time.) I saw a few errors when I used a large number of threads with 1000 increments per thread.

Making inc() into a synchronized method completely solves the problem. If you try that, you should find that the program takes noticeably longer to run, because of the extra overhead associated with the synchronization. Simply declaring the variable count to be volatile does not fix the problem, since the race condition still exists. (In fact, curiously, I found that the number of errors increased when I made count volatile, possibly because accessing the volatile variable can cause a thread to block briefly and give another thread the chance to jump in.)

This exercise shows that synchronization errors can happen. In many programs, they would be very rare, but serious when they occur. Unfortunately, when trying to track down a bug that is due to a synchronization error, it might be very difficult to reproduce the error, since such errors depend on accidents of timing between two threads that are running independently. This makes it all the more important to analyze your program carefully and get it right when you are writing it in the first place, rather than count on your ability to debug it later.

The Solution

import java.util.Scanner;

/**
* A demo program to see how hard it is to get an
* error while using an unsynchronized counter with
* several threads.  The program runs one or more
* threads.  Each thread increments the counter a
* specified number of times.  After all the threads
* have completed, the value of the counter is
* printed out so it can be compared to the correct
* value.  The user specifies the number of threads
* and the number of times each thread increments the
* counter.  (To see an error, the number of increments
* probably has to be pretty large.  Try 1000000.)
*/
public class UnsynchronizedCounterTest {

/**
* A class representing a counter with a method for
* incrementing the counter.  No synchronization is
* used, so this counter is not "thread-safe".
*/
static class Counter {
int count;
void inc() {
count = count+1;
}
int getCount() {
return count;
}
}

static Counter counter;          // The counter that will be incremented.
static int numberOfIncrements;   // Number of times each thread will increment it.

/**
* The class that defines one of the threads.  The thread
* simply increments counter numberOfIncrements times, in
* a for loop.
*/
static class IncrementerThread extends Thread {
public void run() {
for (int i = 0; i < numberOfIncrements; i++) {
counter.inc();
}
}
}

/**
* The main program runs in a loop until the user wants to exit.
* Each time through the loop, it runs one experiment.  It gets
* the number of threads and the number of increments per thread
* from the user.  It creates and starts the threads, and then
* waits for them all to finish.  It prints the final value of
* the counter, as well as the expected value.  The program ends
* when the user enters a number less than or equal to zero as
* the number of threads.
*/
public static void main(String[] args) {

Scanner in = new Scanner(System.in);  // For reading the user's inputs.

while (true) {

/* Get number of threads and number of increments per thread
* from the user.  Exit if number of threads is <= 0. */

System.out.println();
System.out.print("How many threads do you want to run (Enter 0 to end)? ");
int numberOfThreads = in.nextInt();
if (numberOfThreads <= 0)
break;

do {
System.out.println();
System.out.println("How many times should each thread increment the counter? ");
numberOfIncrements = in.nextInt();
if (numberOfIncrements < 1) {
System.out.println("Number of increments must be positive.");
numberOfIncrements = 1;
}
} while (numberOfIncrements <= 0);

System.out.println();
System.out.println("Using " + numberOfThreads + " threads.");
System.out.println("Each thread increments the counter "
+ numberOfIncrements + " times.");

/* Create the threads and start them. */

System.out.println();
System.out.println("Working...");
System.out.println();
IncrementerThread[] workers = new IncrementerThread[numberOfThreads];
counter = new Counter();
for (int i = 0; i < numberOfThreads; i++)
workers[i] = new IncrementerThread();
for (int i = 0; i < numberOfThreads; i++)
workers[i].start();

/* Wait for all threads to terminate. */

for (int i = 0; i < numberOfThreads; i++) {
try {
workers[i].join();
}
catch (InterruptedException e) {
}
}

/* Display the results. */

System.out.println("The final value of the counter should be "
+ (numberOfIncrements*numberOfThreads));
System.out.println("Actual final value of counter is: " + counter.getCount());
System.out.println();
System.out.println();

} // end while

} // end main()

} // end class UnsynchronizedCounterTest

Solution for Programming Exercise 12.2

Exercise 12.2:

Exercise 3.2 asked you to find the integer in the range 1 to 10000 that has the largest number of divisors. Now write a program that uses multiple threads to solve the same problem, but for the range 1 to 100000 (or less, if you don't have a fast computer). By using threads, your program will take less time to do the computation when it is run on a multiprocessor computer. At the end of the program, output the elapsed time, the integer that has the largest number of divisors, and the number of divisors that it has. The program can be modeled on the sample prime-counting program ThreadTest2.java from Subsection 12.1.3. For this exercise, you should simply divide up the problem into parts and create one thread to do each part.

Discussion

Pseudocode for solving this problem without using threads is:

maxDivisors = 0    // Maximum number of divisors seen so far
for N from 1 to 100000:
Let count be the number of divisors of N
if (count > maxDivisors):
maxDivisors = count  // New maximum number of divisors
whichInt = N         // The integer that gave the maximum

To turn this into a multithreaded program, we have to divide the integers between 1 and 100000 into groups and assign each group of integers to a thread. In its run method, each thread finds the maximum number of divisors for integers in its assigned group. This is very similar to what was done in the prime-counting program ThreadTest2.java. The only problem is combining the final results of all the threads. In ThreadTest2.java, each thread counted some primes and recorded its results by calling a method

synchronized private static void addToTotal(int x)

We can do something similar here. The thread actually has two pieces of data to report, the maximum number of divisors that it found and the integer that had that many divisors. The data from the thread has to be combined with the overall information about the maximum found by any thread so far. So, the reporting method becomes:

/**
* This method is called by a thread when it has completed its computation,
* to report the largest number of divisors that it found in its assigned
* range of integers.  The information is used to update the variables
* maxDivisorCount and intWithMaxDivisorCount.
* @param maxCountFromThread largest divisor count in the thread's assigned
*          range
* @param intWithMaxFromThread the integer that had the maximum number
*          of divisors
*/
synchronized private static void report(int maxCountFromThread,
int intWithMaxFromThread) {
if (maxCountFromThread > maxDivisorCount) {
maxDivisorCount = maxCountFromThread;
intWithMaxDivisorCount = intWithMaxFromThread;
}
}

The class that defines the threads, CountDivisorsThread, is very similar to the thread class from ThreadTest2.java. You can see it in the complete solution below.

In the program ThreadTest2.java, the number of integers to be processed was chosen to be evenly divisible by the number of threads. This made it easier to divide the integers among the threads, since every thread can be assigned exactly the same number of integers to process. For this program, I decided to drop the assumption that the number of integers is divisible by the number of threads. This makes the program more general, but it's a little harder to decide how to assign the integers to the threads.

If there are T threads and N integers, then each thread gets at least N/T integers (where N/T means the integer quotient, discarding any remainder). But after assigning N/T integers to each thread, there might still be a few integers left over. (The number of leftovers is actually N % T.) In my program, I just add these extra integers onto the group of integers assigned to the last thread, so the last thread might have more integers to process than the other threads. I do this by making sure that the last number assigned to the last thread is 100000.)

int integersPerThread = 100000/numberOfThreads;
int start = 1;  // Starting point of the range of ints for first thread.
int end = start + integersPerThread - 1;   // End point of the range of ints.
for (int i = 0; i < numberOfThreads; i++) {
if (i == numberOfThreads - 1) {
end = 100000;  // Make sure that the last thread's range goes all
// the way up to 100000.  Because of rounding, this
// is not automatic.
}
worker[i] = new CountDivisorsThread( start, end );
start = end+1;    // Determine the range of ints for the NEXT thread.
end = start + integersPerThread - 1;
}

As one final change, instead of using the literal number 100000 as the number of integers to be processed in my program, I use a constant named MAX to make it easier to change the range of integers that is considered.

The Solution

/**
* This program finds the number in the range 1 to some maximum that has the
* largest number of divisors.  It prints that number and the number of divisors
* that it has.  Note that there might be several numbers that have the maximum
* number of divisors.  Only one of them is output.
*
* The program's work is divided among one to ten threads.  The number
* of threads is chosen by the user.
*/
public class CountDivisorsUsingThreads {

/**
* The upper limit of the range of integers that is to be tested.
*/
private final static int MAX = 100000;

/**
* The largest number of divisors found so far.  (Note: This is
* volatile since it is referenced in unsynchronized code in the
* countDivisorsWithThreads() method.)
*/
private volatile static int maxDivisorCount = 0;

/**
* An integer that has the maximum number of divisors seen so far.
*/
private volatile static int intWithMaxDivisorCount;

/**
* This method is called by a thread when it has completed its computation,
* to report the largest number of divisors that it found in its assigned
* range of integers.  The information is used to update the variables
* maxDivisorCount and intWithMaxDivisorCount.
* @param maxCountFromThread largest divisor count in the thread's assigned
*          range
* @param intWithMaxFromThread the integer that had the maximum number
*          of divisors
*/
synchronized private static void report(int maxCountFromThread,
int intWithMaxFromThread) {
if (maxCountFromThread > maxDivisorCount) {
maxDivisorCount = maxCountFromThread;
intWithMaxDivisorCount = intWithMaxFromThread;
}
}

/**
* A thread belonging to this class counts the number of divisors for all
* the integers in an assigned range of integers.  The range is specified
* in the constructor.  The thread finds the integer in the range that
* has the largest number of divisors, and a number that has that many
* divisors.  At the end of its computation, the thread reports its answer
* by calling the report() method.
*/
private static class CountDivisorsThread extends Thread {
int min, max;
public CountDivisorsThread(int min, int max) {
this.min = min;
this.max = max;
}
public void run() {
//         System.out.println("Thread " + this + " testing range " +
//               min + " to " + max);  // For testing.
//         long startTime = System.currentTimeMillis();
int maxDivisors = 0;
int whichInt = 0;
for (int i = min; i < max; i++) {
int divisors = countDivisors(i);
if (divisors > maxDivisors) {
maxDivisors = divisors;
whichInt = i;
}
}
//         long elapsedTime = System.currentTimeMillis() - startTime;
//         System.out.println("Thread " + this + " used " +
//               (elapsedTime/1000.0) + " seconds.");   // for testing.
report(maxDivisors,whichInt);
}
}

/**
* Finds the number in the range 1 to MAX that has the largest number of
* divisors, dividing the work among a specified number of threads.
*/
private static void countDivisorsWithThreads(int numberOfThreads) {
System.out.println("\nCounting divisors using " +
numberOfThreads + " threads...");
long startTime = System.currentTimeMillis();
CountDivisorsThread[] worker = new CountDivisorsThread[numberOfThreads];
int integersPerThread = MAX/numberOfThreads;
int start = 1;  // Starting point of the range of ints for first thread.
int end = start + integersPerThread - 1;   // End point of the range of ints.
for (int i = 0; i < numberOfThreads; i++) {
if (i == numberOfThreads - 1) {
end = MAX;  // Make sure that the last thread's range goes all
// the way up to MAX.  Because of rounding, this
// is not automatic.
}
worker[i] = new CountDivisorsThread( start, end );
start = end+1;    // Determine the range of ints for the NEXT thread.
end = start + integersPerThread - 1;
}
maxDivisorCount = 0;
for (int i = 0; i < numberOfThreads; i++)
worker[i].start();
for (int i = 0; i < numberOfThreads; i++) {
// Wait for each worker thread to die, because the results
// are not complete until all threads have completed and
// reported their results.
while (worker[i].isAlive()) {
try {
worker[i].join();
}
catch (InterruptedException e) {
}
}
}
long elapsedTime = System.currentTimeMillis() - startTime;
System.out.println("\nThe largest number of divisors " +
"for numbers between 1 and " + MAX + " is " + maxDivisorCount);
System.out.println("An integer with that many divisors is " +
intWithMaxDivisorCount);
System.out.println("Total elapsed time:  " +
(elapsedTime/1000.0) + " seconds.\n");
}

/**
* Gets the number of threads from the user and calls countDivisorsWithThreads()
* to do the actual work.
*/
public static void main(String[] args) {
int numberOfThreads = 0;
while (numberOfThreads < 1 || numberOfThreads > 10) {
System.out.print("How many threads do you want to use  (1 to 10) ?  ");
numberOfThreads = TextIO.getlnInt();
if (numberOfThreads < 1 || numberOfThreads > 10)
System.out.println("Please enter a number from 1 to 10 !");
}
countDivisorsWithThreads(numberOfThreads);
}

/**
* Finds the number of divisors of the integer N.  Note that this method does
* the counting in a stupid way, since it tests every integer in the range
* 1 to N to see whether it evenly divides N.
*/
public static int countDivisors(int N) {
int count = 0;
for (int i = 1; i <= N ; i++) {
if ( N % i == 0 )
count ++;
}
return count;
}

}

Solution for Programming Exercise 12.3

Exercise 12.3:

In the previous exercise, you divided up a large task into a small number of large pieces and created a thread to execute each task. Because of the nature of the problem, this meant that some threads had much more work to do than others -- it is much easier to find the number of divisors of a small number than it is of a big number. As discussed in Subsection 12.3.1, a better approach is to break up the problem into a fairly large number of smaller problems. Subsection 12.3.2 shows how to use a thread pool to execute the tasks: Each thread in the pool runs in a loop in which it repeatedly takes a task from a queue and carries out that task. Implement a thread pool strategy for solving the same maximum-number-of-divisors problem as in the previous exercise.

To make things even more interesting, you should try a new technique for combining the results from all the tasks: Use two queues in your program. Use a queue of tasks, as usual, to hold the tasks that will be executed by the thread pool (Subsection 12.3.2). But also use a queue of results produced by the threads. When a task completes, the result from that task should be placed into the result queue. The main program can read results from the second queue as they become available, and combine all the results to get the final answer. The result queue will have to be a blocking queue (Subsection 12.3.3), since the main program will have to wait for results to become available. Note that the main program knows the exact number of results that it expects to read from the queue, so it can do so in a for loop; when the for loop completes, the main program knows that all the tasks have been executed.

Discussion

I ran the solution to Exercise 12.2 on a computer with four processors, using four threads. The four threads took widely varying amounts of time to complete: 2.6 seconds, 6.4 seconds, 12.9 seconds, and 20.4 seconds. The program ran for just over 20.4 seconds. For a lot of that time, there were only one or two threads running, leaving several processors idle. By using a thread pool as required by the current exercise, I was able to bring the total time for solving the problem down to 10.9 seconds. With the thread pool, the job was divided more evenly among the processors, and none of the processors were idle for very long.

In my solution, I decided to use a ConcurrentLinkedQueue for the task queue and a LinkedBlockingQueue for the result queue. Both of these classes are discussed in Section 12.3, and a ConcurrentLinkedQueue is used for the task queue in the sample program MultiprocessingDemo2.java from that section. (I could have used a blocking queue for the task queue too, as I'll discuss below.)

private static ConcurrentLinkedQueue<Task> taskQueue;
private static LinkedBlockingQueue<Result> resultQueue;

We need objects to put in the queues, to represent the tasks and the results. I decided to write a nested class to represent tasks and another nested class to represent results. A task is assigned a range of integers and has to find the largest number of divisors for any integer in that range. We also need to know which integer gave that maximum number. When the task is completed, the results from the task have to be added to the result queue. Here are the Task and Result classes:

private static class Task {
int min, max; // Start and end of the range of integers for this task.
Task(int min, int max) {
this.min = min;
this.max = max;
}
public void compute() {
int maxDivisors = 0;
int whichInt = 0;
for (int i = min; i < max; i++) {
int divisors = countDivisors(i);
if (divisors > maxDivisors) {
maxDivisors = divisors;
whichInt = i;
}
}
resultQueue.add( new Result(maxDivisors, whichInt) );
}
}

private static class Result {
int maxDivisorFromTask;  // Maximum number of divisors found.
int intWithMaxFromTask;  // Which integer gave that maximum number.
Result(int maxDivisors, int whichInt) {
maxDivisorFromTask = maxDivisors;
intWithMaxFromTask = whichInt;
}
}

The big idea when using a thread pool and load balancing is to divide up the overall problem into a fairly large number of fairly small (but not too small) subtasks. In my program, each task is assigned a range of 1000 integers to work on. Since there are 100000 integers in the overall problem, there will be 100 subtasks. The question of how big the subtasks should be does not have a definite answer, but this choice seemed to work well, since the work was divided pretty evenly among the processors.

Once the tasks have been created and the threads have been started, the main thread has to collect and process all the results from the tasks. As each task completes, it places its result into the result queue. A result can be removed from the queue by calling resultQueue.take(), which will block if the queue is empty until a result becomes available. So the for loop that processes results won't be able to complete until all the tasks have been completed and all the threads have terminated:

int maxDivisorCount = 0;         // Overall maximum found by any task.
int intWithMaxDivisorCount = 0;  // Which integer gave that maximum?
for (int i = 0; i < numberOfTasks; i++) {
try {
Result result = resultQueue.take();
if (result.maxDivisorFromTask > maxDivisorCount) { // new maximum.
maxDivisorCount = result.maxDivisorFromTask;
intWithMaxDivisorCount = result.intWithMaxFromTask;
}
}
catch (InterruptedException e) { // Won't happen in this program!
}
}

Once the for loop ends, the final answers, taking into account the results from every thread, are in the variables maxDivisorCount and intWithMaxDivisorCount.

My solution uses a ConcurrentLinkedQueue for the task queue. This is a little touchy, since we have to be careful not to start the threads until the tasks are in the queue. This is because the threads are programmed to terminate as soon as they see an empty queue:

private static class CountDivisorsThread extends Thread {
public void run() {
while (true) {
Task task = taskQueue.poll();
if (task == null)
break; // queue is empty; terminate
task.compute();
}
}
}

If the queue is empty when the thread starts, it will terminate immediately. Furthermore, terminating the threads after the job is done is fine in this program, but often when we use a thread pool, we want to keep the threads around to work on more than one job.

One solution is to use a LinkedBlockingQueue for the task queue, instead of a ConcurrentLinkedQueue. When a blocking queue is faced with an empty queue, it waits for an item to become available in the queue. We can then start the queue whenever we want, and the threads will wait, if necessary, for tasks to be added to the queue. If we had more jobs for the thread pool, the threads would just wait around between jobs and would become active again as soon as tasks from a new job are enqueued. Here is a version of the thread class that will work with a blocking queue of tasks:

private static class CountDivisorsThread extends Thread {
CountDivisorsThread() {
setDaemon(true);
}
public void run() {
while (true) {
try {
Task task = blockingTaskQueue.take();
task.compute();
}
catch (InterruptedException e) {
}
}
}
}

Note that I've made the thread into a daemon thread. This will allow the Java Virtual Machine to exit, even though the thread still exists, as long as all the non-daemon threads have exited. For this exercise, if the threads were not daemon threads, the program would not actually end after completing the job, since the non-daemon thread-pool threads would still be around. (However, you could still make the program end by calling System.exit().)

The Solution

import java.util.concurrent.ConcurrentLinkedQueue;
import java.util.concurrent.LinkedBlockingQueue;

/**
* This program finds the number in the range 1 to some maximum that has the
* largest number of divisors.  It prints that number and the number of divisors
* that it has.  Note that there might be several numbers that have the maximum
* number of divisors.  Only one of them is output.
*
* The program's work is divided into a large number of tasks that are executed
* by a thread pool.  Each task consists of finding the maximum number of
* divisors among a sequence of 1000 integers.
*/
public class CountDivisorsUsingThreadPool {

/**
* The upper limit of the range of integers that is to be tested.
* (This must be a fairly large multiple of 1000 for the thread
* pool load-balancing strategy to be effective.)
*/
private final static int MAX = 100000;

/**
* A queue to hold the tasks.  Tasks are represented as objects
* of type Task, a nested class that is defined below.  Note
* that queue operations must be synchronized because the
* queue is used by multiple threads.  A ConcurrentLinkedQueue
* handles synchronization internally.
*/
private static ConcurrentLinkedQueue<Task> taskQueue;

/**
* A queue to hold the results from the tasks.  Results
* are defined by the nested class, Result, which is defined
* below.  This is a blocking queue since the thread
* that takes results from the queue should block when
* the queue is empty until a result becomes available.
* (Note:  The Task class could have been used to represent
* results as well; I am using a separate Result class
* for clarity in this example.)
*/
private static LinkedBlockingQueue<Result> resultQueue;

/**
* A class to represent the task of finding the number in
* a given range of integers that has the largest number of
* divisors.  The range is specified in the constructor.
* The task is executed when the compute() method is
* called.  At the end of the compute() method, a Result
* object is created to represent the results from this
* task, and the result object is added to resultQueue.
*/
private static class Task {
int min, max; // Start and end of the range of integers for this task.
Task(int min, int max) {
this.min = min;
this.max = max;
}
public void compute() {
int maxDivisors = 0;
int whichInt = 0;
for (int i = min; i < max; i++) {
int divisors = countDivisors(i);
if (divisors > maxDivisors) {
maxDivisors = divisors;
whichInt = i;
}
}
resultQueue.add( new Result(maxDivisors, whichInt) );
}
}

/**
* A class to represent the result from one task.  The
* result consists of the maximum number of divisors in
* the range of integers assigned to that task, and the
* integer in the range that gave the maximum number of
* divisors.
*/
private static class Result {
int maxDivisorFromTask;  // Maximum number of divisors found.
int intWithMaxFromTask;  // Which integer gave that maximum number.
Result(int maxDivisors, int whichInt) {
maxDivisorFromTask = maxDivisors;
intWithMaxFromTask = whichInt;
}
}

/**
* A thread belonging to this class counts the number of divisors for all
* the integers in an assigned range of integers.  The range is specified
* in the constructor.  The thread finds the integer in the range that
* has the largest number of divisors, and a number that has that many
* divisors.  At the end of its computation, the thread reports its answer
* by calling the report() method.
*/
private static class CountDivisorsThread extends Thread {
public void run() {
while (true) {
Task task = taskQueue.poll();
if (task == null)
break;
task.compute();
}
}
}

/**
* Finds the number in the range 1 to MAX that has the largest number of
* divisors, dividing the work into tasks that will be executed by threads
* in a thread pool.  This method creates the task and result queues.
* It adds all the tasks to the task queue.  Then it creates the threads
* for the thread pool and starts them.  (Note that this must be done
* AFTER all the tasks are in the task queue, since the threads exit
* when they see an empty queue.)  Finally, it reads results from
* the result queue and combines them to get the overall answer.
* @param numberOfThreads the number of threads in the thread pool.
*/
private static void countDivisorsWithThreads(int numberOfThreads) {

System.out.println("\nCounting divisors using " +
numberOfThreads + " threads...");

/* Create the queues and the thread pool, but don't start
* the threads yet. */

long startTime = System.currentTimeMillis();
resultQueue = new LinkedBlockingQueue<Result>();

taskQueue = new ConcurrentLinkedQueue<Task>();
CountDivisorsThread[] workers = new CountDivisorsThread[numberOfThreads];
for (int i = 0; i < workers.length; i++)
workers[i] = new CountDivisorsThread();

/* Create the tasks and add them to the task queue.  Each
* task consists of a range of 1000 integers, so the number of
* tasks is (MAX+999)/1000.  (The "+999"  gives the correct number
* of tasks when MAX is not an exact multiple of 1000.  The last
* task in that case will consist of the last (MAX%1000)) ints. */

int numberOfTasks = (MAX + 999) / 1000;
for (int i = 0; i < numberOfTasks; i++) {
int start = i*1000 + 1;
int end = (i+1)*1000;
if (end > MAX)
end = MAX;
//System.out.println(start + " " + end);  // for testing
taskQueue.add( new Task(start,end) );
}

/* Now that the tasks are in the task queue, start the threads. */

for (int i = 0; i < numberOfThreads; i++)
workers[i].start();

/* The threads will execute the tasks and results will be placed
* into the result queue.  This method now goes on to read all
* the results from the result queue and combine them to give
* the overall answer. */

int maxDivisorCount = 0;         // Over maximum found by any task.
int intWithMaxDivisorCount = 0;  // Which integer gave that maximum?
for (int i = 0; i < numberOfTasks; i++) {
try {
Result result = resultQueue.take();
if (result.maxDivisorFromTask > maxDivisorCount) { // new maximum.
maxDivisorCount = result.maxDivisorFromTask;
intWithMaxDivisorCount = result.intWithMaxFromTask;
}
}
catch (InterruptedException e) {
}
}

/* Report the results. */

long elapsedTime = System.currentTimeMillis() - startTime;
System.out.println("\nThe largest number of divisors " +
"for numbers between 1 and " + MAX + " is " + maxDivisorCount);
System.out.println("An integer with that many divisors is " +
intWithMaxDivisorCount);
System.out.println("Total elapsed time:  " +
(elapsedTime/1000.0) + " seconds.\n");

} // end countDivisorsWithThreads()

/**
* The main() routine just gets the number of threads from the user and
* calls countDivisorsWithThreads() to do the actual work.
*/
public static void main(String[] args) {
int numberOfThreads = 0;
while (numberOfThreads < 1 || numberOfThreads > 10) {
System.out.print("How many threads do you want to use  (1 to 10) ?  ");
numberOfThreads = TextIO.getlnInt();
if (numberOfThreads < 1 || numberOfThreads > 10)
System.out.println("Please enter a number from 1 to 10 !");
}
countDivisorsWithThreads(numberOfThreads);
}

/**
* Finds the number of divisors of the integer N.  Note that this method does
* the counting in a stupid way, since it tests every integer in the range
* 1 to N to see whether it evenly divides N.
*/
private static int countDivisors(int N) {
int count = 0;
for (int i = 1; i <= N ; i++) {
if ( N % i == 0 )
count ++;
}
return count;
}

} // end CountDivisorsUsingThreadPool

Solution for Programming Exercise 12.4

Exercise 12.4:

In Exercise 11.3, you wrote a network server program that can send text files from a specified directory to clients. That program used a single thread, which handled all the communication with each client. Modify the program to turn it into a multithreaded server. Use a thread pool of connection-handling threads and use an ArrayBlockingQueue to get connected sockets from the main() routine to the threads. The sample program DateServerWithThreads.java from Subsection 12.4.3 is an example of a multithreaded server that works in this way. Your server program will work with the same client program as the original server. You wrote the client program as the solution to Exercise 11.4.

Discussion

This is not a difficult exercise, given the model in DateServerWithThreads.java. My solution to Exercise 11.3, FileServer,java, already has a method

private static void handleConnection(File directory, Socket connection)

to handle a connection with one client. The connection in this method is represented by a Socket. The File in this method represents the directory that contains the file server's files; it is set at the beginning of the program and does not change after that. So, the only thing that we have to send in the queue from the main program to the connection-handling threads is the socket that represents a given connection. So, the queue can be a queue of Sockets:

private static ArrayBlockingQueue<Socket> connectionQueue;

A connection-handling thread runs in an infinite loop in which it takes a socket from this queue and uses it to communicate with the client. I could have moved the code from the handleConnection() method into the run method of the thread, but I decided it would be easier to have the run() method call the method that already exists. I also need to pass the directory to the handleConnection() method, so the thread needs some way of knowing the directory. I pass the directory to the thread in the thread's constructor. (An alternative would have been to make it a global variable, as I did for the queue.) Here is the class that defines the connection-handling threads:

private static class ConnectionHandler extends Thread {
File directory;  // The directory that contains the files
// that are made available on this server.
ConnectionHandler(File directory) {
this.directory = directory;
setDaemon(true);
}
public void run() {
while (true) {
try {
Socket connection = connectionQueue.take();
handleConnection(directory,connection);
}
catch (Exception e) {
}
}
}
}

The main program creates and starts several threads belonging to this class, before it starts listening for connection requests. Other changes are straightforward. You can see the details in the complete solution, given below.

The Solution

Changes from the solution of Exercise 11.3 are shown in red italic:

import java.net.*;
import java.io.*;
import java.util.Scanner;
import java.util.concurrent.ArrayBlockingQueue;

/**
* This program is a very simple network file server.  The
* server has a list of available text files that can be
* downloaded by the client.  The client can also download
* the list of files.  When the connection is opened, the
* client sends one of two possible commands to the server:
* "index" or "get <file-name>".  The server replies to
* the first command by sending the list of available files.
* It responds to the second with a one-line message,
* either "ok" or "error".  If the message is "ok", it is
* followed by the contents of the file with the specified
* name.  The "error" message indicates that the specified
* file does not exist on the server. (The server can also
* respond with the message "unknown command" if the command
* it reads is not one of the two possible legal commands.)
*
* The server program requires a command-line parameter
* that specifies the directory that contains the files
* that the server can serve.  The files should all be
* text files, but this is not checked.  Also, the server
* must have permission to read all the files.
*
* This version of the program defines a multithreaded
* server that uses a thread pool.  The threads handle
* all communication with the clients.  The main program
* simply accepts connections and puts them into a queue.
* The connection-handling threads in the thread pool remove
* connections from the queue as they become available.
*/
public class ThreadedFileServer {

static final int LISTENING_PORT = 3210;

/**
* The number of threads in the thread pool.
*/
private static final int THREAD_POOL_SIZE = 10;

/**
* The length of the ArrayBlockingQueue of connections.
* This should not be too big, since connections in the
* queue are waiting for service and hopefully won't
* spend too long in the queue.
*/
private static final int CONNECTION_QUEUE_SIZE = 5;

/**
* The queue that is used to send connections from the
* main program to the connection-handling threads.
* A connection is represented by a connected Socket.
*/
private static ArrayBlockingQueue<Socket> connectionQueue;

/**
* Main program creates the thread pool, then opens a
* server socket to listen for connection requests.
*/
public static void main(String[] args) {

File directory;        // The directory from which the server
//    gets the files that it serves.

ServerSocket listener; // Listens for connection requests.

Socket connection;     // A socket for communicating with a client.

/* Check that there is a command-line argument.
If not, print a usage message and end. */

if (args.length == 0) {
System.out.println("Usage:  java FileServer <directory>");
return;
}

/* Get the directory name from the command line, and make
it into a file object.  Check that the file exists and
is in fact a directory. */

directory = new File(args[0]);
if ( ! directory.exists() ) {
System.out.println("Specified directory does not exist.");
return;
}
if (! directory.isDirectory() ) {
System.out.println("The specified file is not a directory.");
return;
}

/* Create the connection queue.  We want to do this before
* creating the threads, which need to use the queue. */

connectionQueue = new ArrayBlockingQueue<Socket>(CONNECTION_QUEUE_SIZE);

/* Create the thread pool and start the threads.  The directory
* that contains the files is passed to each thread as a
* parameter to its constructor.  Note that there is no
* need to keep references to the threads, since we don't have
* to do anything with them in this program after they have
* been started.*/

for (int i = 0; i < THREAD_POOL_SIZE; i++) {
ConnectionHandler worker = new ConnectionHandler(directory);
worker.start();
}

/* Listen for connection requests from clients.  For each
* connection, add the connected socket to the connection
* queue.  The server runs until the program is terminated,
* for example by a CONTROL-C. */

try {
listener = new ServerSocket(LISTENING_PORT);
System.out.println("Listening on port " + LISTENING_PORT);
while (true) {
connection = listener.accept();
connectionQueue.add(connection);
}
}
catch (Exception e) {
System.out.println("Server shut down unexpectedly.");
System.out.println("Error:  " + e);
return;
}

} // end main()

/**
* The class that defines the connection-handling threads in the
* thread pool.  The thread runs in an infinite loop in which
* it removes a connected socket from the connection queue and
* calls the handleConnection() method for that socket.
*/
private static class ConnectionHandler extends Thread {
File directory;  // The directory that contains the files
// that are made available on this server.
ConnectionHandler(File directory) {
this.directory = directory;
setDaemon(true);
}
public void run() {
while (true) {
try {
Socket connection = connectionQueue.take();
handleConnection(directory,connection);
}
catch (Exception e) {
}
}
}
}

/**
* This method processes process the connection with one client.
* It creates streams for communicating with the client,
* reads a command from the client, and carries out that
* command.  The connection is also logged to standard output.
* An output beginning with ERROR indicates that a network
* error occurred.  A line beginning with OK means that
* there was no network error, but does not imply that the
* command from the client was a legal command.
*/
private static void handleConnection(File directory, Socket connection) {
Scanner incoming;       // For reading data from the client.
PrintWriter outgoing;   // For transmitting data to the client.
String command = "Command not read";
try {
incoming = new Scanner( connection.getInputStream() );
outgoing = new PrintWriter( connection.getOutputStream() );
command = incoming.nextLine();
if (command.equals("index")) {
sendIndex(directory, outgoing);
}
else if (command.startsWith("get")){
String fileName = command.substring(3).trim();
sendFile(fileName, directory, outgoing);
}
else {
outgoing.println("unsupported command");
outgoing.flush();
}
System.out.println("OK    " + connection.getInetAddress()
+ " " + command);
}
catch (Exception e) {
System.out.println("ERROR " + connection.getInetAddress()
+ " " + command + " " + e);
}
finally {
try {
connection.close();
}
catch (IOException e) {
}
}
}

/**
* This is called by the run() method in response to an "index" command
* from the client.  Send the list of files in the server's directory.
*/
private static void sendIndex(File directory, PrintWriter outgoing) throws Exception {
String[] fileList = directory.list();
for (int i = 0; i < fileList.length; i++)
outgoing.println(fileList[i]);
outgoing.flush();
outgoing.close();
if (outgoing.checkError())
throw new Exception("Error while transmitting data.");
}

/**
* This is called by the run() command in response to "get <fileName>"
* command from the client.  If the file doesn't exist, send the message "error".
* Otherwise, send the message "ok" followed by the contents of the file.
*/
private static void sendFile(String fileName, File directory, PrintWriter outgoing) throws Exception {
File file = new File(directory,fileName);
if ( (! file.exists()) || file.isDirectory() ) {
// (Note:  Don't try to send a directory, which
// shouldn't be there anyway.)
outgoing.println("error");
}
else {
outgoing.println("ok");
BufferedReader fileIn = new BufferedReader( new FileReader(file) );
while (true) {
// Read and send lines from the file until
// an end-of-file is encountered.
String line = fileIn.readLine();
if (line == null)
break;
outgoing.println(line);
}
}
outgoing.flush();
outgoing.close();
if (outgoing.checkError())
throw new Exception("Error while transmitting data.");
}

} //end class ThreadedFileServer

Solution for Programming Exercise 12.5

Exercise 12.5:

It is possible to get an estimate of the mathematical constant π by using a random process. The idea is based on the fact that the area of a circle of radius 1 is equal to π, and the area of a quarter of that circle is π/4. Here is a picture of a quarter of a circle of radius 1, inside a 1-by-1 square:

The area of the whole square is one, while the area of the part inside the circle is π/4. If we choose a point in the square at random, the probability that it is inside the circle is π/4. If we choose N points in the square at random, and if C of them are inside the circle, we expect the fraction C/N of points that fall inside the circle to be about π/4. That is, we expect 4*C/N to be close to π. If N is large, we can expect 4*C/N to be a good estimate for π, and as N gets larger and larger, the estimate is likely to improve.

We can pick a random point in the square by choosing numbers x and y in the range 0 to 1 (using Math.random()). Since the equation of the circle is x*x+y*y=1, the point lies inside the circle if x*x+y*y is less than 1. One trial consists of picking x and y and testing whether x*x+y*y is less than 1. To get an estimate for π, you have to do many trials, count the trials, and count the number of trials in which x*x+y*y is less than 1,

For this exercise, you should write a GUI program that does this computation and displays the result. The computation should be done in a separate thread, and the results should be displayed periodically. The program can use JLabels to the display the results. It should set the text on the labels after running each batch of, say, one million trials. (Setting the text after each trial doesn't make sense, since millions of trials can be done in one second, and trying to change the display millions of times per second would be silly.

Your program should have a "Run"/"Pause" button that controls the computation. When the program starts, clicking "Run" will start the computation and change the text on the button to "Pause". Clicking "Pause" will cause the computation to pause. The thread that does the computation should be started at the beginning of the program, but should immediately go into the paused state until the "Run" button is pressed. Use the wait() method in the thread to make it wait until "Run" is pressed. Use the notify() method when the "Run" button is pressed to wake up the thread. Use a boolean signal variable running to control whether the computation thread is paused. (The wait() and notify() methods are covered in Subsection 12.3.4.)

Here is a picture of the program after it has run many trials:

You might want to start with a version of the program with no control button. In that version, the computation thread can run continually from the time it is started. Once that is working, you can add the button and the control feature.

To get you started, here is the code from the thread in my solution that runs one batch of trials and updates the display labels:

for (int i = 0; i < BATCH_SIZE; i++) {
double x = Math.random();
double y = Math.random();
trialCount++;
if (x*x + y*y < 1)
inCircleCount++;
}
double estimateForPi = 4 * ((double)inCircleCount / trialCount);
countLabel.setText(      " Number of Trials:   " + trialCount);
piEstimateLabel.setText( " Current Estimate:   " + estimateForPi);

The variables trialCount and inCircleCount are of type long in order to allow the number of trials to be more than the two billion or so that would be possible with a variable of type int.

(I was going to ask you to use multiple computation threads, one for each available processor, but I ran into an issue when using the Math.random() method in several threads. This method requires synchronization, which causes serious performance problems when several threads are using it to generate large amounts of random numbers. A solution to this problem is to have each thread use its own object of type java.util.Random to generate its random numbers (see Subsection 5.3.1). My on-line solution to this exercise discusses this problem further.)

Discussion

I will present three versions of the solution: one without a control button, one with the button and a single computation thread, and one that uses multiple threads. But first, you might be interested in how I set up the JLabels:

countLabel =      new JLabel(" Number of Trials:   0");
piEstimateLabel = new JLabel(" Current Estimate:   (none)");
JLabel piLabel =  new JLabel(" Actual value of pi: " + Math.PI + "  ");
Font bigMonospace = new Font("Monospaced", Font.PLAIN, 20);
countLabel.setFont(bigMonospace);
piEstimateLabel.setFont(bigMonospace);
piLabel.setFont(bigMonospace);
countLabel.setOpaque(true);
piEstimateLabel.setOpaque(true);
piLabel.setOpaque(true);

I used a monospaced font (in which all the characters are the same width) so that the characters on the three labels would line up neatly. I set the labels to be opaque to make sure that they would use their own background color rather than let the color of the panel show through.

For the first version of the program, I simply wrapped the code given in the exercise for doing one batch of trials inside an infinite while loop, and made that the run() method of the thread:

private class ComputationThread extends Thread {
final int BATCH_SIZE = 1000000;  // Number of trials between updates of the display.
long trialCount;     // Total number of trials that have been performed.
long inCircleCount;  // Number of trials in which x*x+y*y is less than 1.
public ComputationThread() {
setDaemon(true);
setPriority(Thread.currentThread().getPriority() - 1);
}
public void run() {
while (true) {
for (int i = 0; i < BATCH_SIZE; i++) {
double x = Math.random();
double y = Math.random();
trialCount++;
if (x*x + y*y < 1)
inCircleCount++;
}
double estimateForPi = 4 * ((double)inCircleCount / trialCount);
countLabel.setText(      " Number of Trials:   " + trialCount);
piEstimateLabel.setText( " Current Estimate:   " + estimateForPi);
}
}
}

The thread is created and started in the constructor of the panel. It's quite possible that several batches of trials are done before the labels ever appear on the screen. That's OK because it's OK to call a label's setText() method even before the label appears.

The first version of the program is pretty short and simple, and it's not too hard to add the control button to get the second version. First of all, I added the volatile boolean variable, running to the program, and I added the following code at the beginning of the while loop in the thread to make the thread pause when the value of running is false:

synchronized(this) {
while ( ! running ) { // wait for running to be true
try {
wait();
}
catch (InterruptedException e) {
}
}
}

When running is false, this code calls the wait() method to go to sleep. It will require a call to notify() elsewhere in the program to wake the thread so that it can continue. Note that the synchronization is on the object "this", which refers here to the thread object. (Recall that obj.wait() must always be called in code that is synchronized on obj. In this case, this plays the role of obj because a simple call to wait() is really a call to this.wait().) A while loop is used in this code rather than an if statement to be absolutely sure that running is true before continuing.

When the thread is first started at the beginning of the program, the value of running is false. The thread sees this value and calls wait() as soon as it starts, and the thread remains in this waiting state until the user clicks the "Run" button. The actionPerformed() method that responds to the button has to set the value of running to true. It also has to call notify() to wake up the thread so it can see the new value. The notify() method must be called on the same object as the thread used to call wait() -- that is, on the thread object itself. The program has a variable named runner that refers to the thread, so the thread is woken with the code:

synchronized(runner) {
running = true;
runner.notify();
}

It's important, by the way, that the statement that sets running to true is inside the synchronized statement here, and that the code in the thread that tests the value of running is also inside a statement that is synchronized on the same object. This avoids a race condition: Suppose these statements are not synchronized. Then it is possible that the thread might test the value of running, find it to be false, and decide to call wait(). However, before it actually calls wait(), the event-handling thread might set running to true and call notify(). So it's possible for the notify() to come before the wait(), which means the thread will be left waiting for a notification that was already given! Proper synchronization prevents the thread from being interrupted between the time it decides to call wait() and the time it actually does so.

As I mentioned in the statement of the exercise, I actually wanted to use several computation threads -- one for each available processor. When I wrote that program, however, I was dismayed to find that it was much slower than the single-threaded program. After trying for a while to track down the reason, I eventually remembered that the call to Math.random() has to be synchronized internally (because there is a race condition in the way that the next random number in a sequence is generated). There is a significant overhead involved in synchronization, which can really slow down a program when it is done too often. In this case, when Math.random() was called millions of times per second by several different threads, the slowdown was by a factor of about 100!

Behind the scenes, Math.random() uses an object of type java.util.Random to generate the random numbers. By having each thread create and use its own object of this type, instead of calling Math.random(), I avoided the synchronization problem. (I am honestly not sure how Java avoids the overhead of a synchronized statement when there is only one thread that is synchronizing on a given object.)

Aside from that problem, the main issue introduced by using multiple threads is how to combine the results from the different threads. In the first two versions of the program, the thread keeps track of the total number of trials and the number of trials where x*x+y*y is less than 1. It uses this information to figure out what to put on the display labels. However, when there are two or more threads, the data that is needed to update the display is not available to any one thread. So, someone else has to collect the data from the threads and combine all the data to get the overall total number of trials and the overall total number of trials in which x*x+y*y<1.

There are several ways to do this. I decided to use a queue to transport the results from the computation threads to another thread that combines the results and updates the display accordingly. In fact, the role of the other thread is played by the event-handling thread. There is a timer that goes off every 1/10 of a second. The actionPerformed() method that is called in response to the timer removes all the data that have been collected in the queue and uses it to update the display.

Trials are run in batches of 1,000,000. Each batch produces one result. The result is simply the number of trials in that batch for which x*x+y*y<1. The result is an integer, so for the result queue I use a queue of Integers. Each result in the queue corresponds to 1,000,000 trials, so each time I process a result, I add 1000000 to the total number of trials and add the number from the queue to the number or trials in which x*x+y*y<1.

For the queue, I used a LinkedBlockingQueue. Since the queue is being used by several threads, I need a queue where the operations are properly synchronized. In fact, I don't need the blocking behavior of the blocking queue, but a LinkedBlockingQueue has a nice, synchronized method, drainTo(), for retrieving all the items from a queue in one step. I would have used ConcurrentLinkedQueue if it had a similar method.

I won't discuss the third version of the program further. You can see the source code, which is well commented, below.

The Solution

The first version of the program, with no control button:

import java.awt.*;
import javax.swing.*;

/**
* This program uses a probabilistic technique to estimate the
* value of the mathematical constant pi.  The technique is to
* choose random numbers x and y in the range 0 to 1, and to
* compute x*x + y*y.  The probability that x*x + y*y is less than
* 1 is pi/4.  If many trials are performed, and the number of
* trials in which x*x+y*y is less than 1 is divided by the total
* number of trials, the result is an approximation for pi/4.
* Multiplying this by 4 gives an approximation for pi.
*
* The program shows the estimate produced by this procedure, along
* with the number of trials that have been done and, for comparison,
* the actual value of pi.  These values are shown in three JLabels.
* The computation is done by a separate thread that updates the
* contents of the labels after every millionth trial.
*
* In this version of the program, the computation thread runs
* continually from the time the program is started until it
* ends.  It is run at a reduced priority so that it does not
* interfere with the GUI thread.
*/
public class EstimatePi_1 extends JPanel {

/**
* Main program just creates a window to show the panel that
* contains the JLabels.
*/
public static void main(String[] args) {
JFrame window = new JFrame("Estimating Pi Probabilistically");
EstimatePi_1 content = new EstimatePi_1();
window.setContentPane(content);
window.pack();
window.setResizable(false);
window.setLocation( 300, 200 );
window.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
window.setVisible(true);
}

private JLabel piEstimateLabel;  // A label for showing the current estimate of pi.
private JLabel countLabel;       // A label for showing the number of trials.

private ComputationThread runner;  // The thread that does the computation.

/**
* Constructor creates the three display labels and adds them to this
* panel.  It also creates and starts the computation thread.
*/
public EstimatePi_1() {

setLayout(new GridLayout(0, 1, 2, 2));
setBackground(Color.BLUE);
setBorder(BorderFactory.createLineBorder(Color.BLUE,2));

countLabel =      new JLabel(" Number of Trials:   0");
piEstimateLabel = new JLabel(" Current Estimate:   (none)");
JLabel piLabel =  new JLabel(" Actual value of pi: " + Math.PI + "  ");
Font bigMonospace = new Font("Monospaced", Font.PLAIN, 20);
countLabel.setFont(bigMonospace);
piEstimateLabel.setFont(bigMonospace);
piLabel.setFont(bigMonospace);
countLabel.setOpaque(true);
piEstimateLabel.setOpaque(true);
piLabel.setOpaque(true);
add(piLabel);
add(piEstimateLabel);
add(countLabel);

runner = new ComputationThread();
runner.start();

} // end constructor

/**
*  This class defines the thread that does the computation.
*  The thread runs in an infinite loop in which it performs
*  batches of 1000000 trials and then updates the display labels.
*/
private class ComputationThread extends Thread {
final int BATCH_SIZE = 1000000;  // Number of trials between updates of the display.
long trialCount;     // Total number of trials that have been performed.
long inCircleCount;  // Number of trials in which x*x+y*y is less than 1.
public ComputationThread() {
setDaemon(true);
setPriority(Thread.currentThread().getPriority() - 1);
}
public void run() {
while (true) {
for (int i = 0; i < BATCH_SIZE; i++) {
double x = Math.random();
double y = Math.random();
trialCount++;
if (x*x + y*y < 1)
inCircleCount++;
}
double estimateForPi = 4 * ((double)inCircleCount / trialCount);
countLabel.setText(      " Number of Trials:   " + trialCount);
piEstimateLabel.setText( " Current Estimate:   " + estimateForPi);
}
}
}

} // end class EstimatePi_1

The second version of the program, with a control button and just one thread. Significant changes from the first version are shown in red italic:

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

/**
* This program uses a probabilistic technique to estimate the
* value of the mathematical constant pi.  The technique is to
* choose random numbers x and y in the range 0 to 1, and to
* compute x*x + y*y.  The probability that x*x + y*y is less than
* 1 is pi/4.  If many trials are performed, and the number of
* trials in which x*x+y*y is less than 1 is divided by the total
* number of trials, the result is an approximation for pi/4.
* Multiplying this by 4 gives an approximation for pi.
*
* The program shows the estimate produced by this procedure, along
* with the number of trials that have been done and, for comparison,
* the actual value of pi.  These values are shown in three JLabels.
* The computation is done by a separate thread that updates the
* contents of the labels after every millionth trial.
*
* In this version of the program, there is a "Run"/"Pause" button
* that controls the computation thread.  Clicking the button once
* starts the thread; clicking it again pauses it.  Initially, the
* thread is paused.
*/
public class EstimatePi_2 extends JPanel implements ActionListener {

/**
* Main program just creates a window to show the panel that
* contains the JLabels and the Run/Pause button.
*/
public static void main(String[] args) {
JFrame window = new JFrame("Estimating Pi Probabilistically");
EstimatePi_2 content = new EstimatePi_2();
window.setContentPane(content);
window.pack();
window.setResizable(false);
window.setLocation( 300, 200 );
window.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
window.setVisible(true);
}

private JLabel piEstimateLabel;  // A label for showing the current estimate of pi.
private JLabel countLabel;       // A label for showing the number of trials.

private JButton runPauseButton;  // Button to control the thread.  Clicking this
// button will pause the thread if it is running
// and will restart it if it is paused.

private ComputationThread runner;  // The thread that does the computation.

private volatile boolean running;  // Control variable for signaling the thread to
// run or pause.  Initially, this is false, so
// the thread pauses as soon as it is created,
// until the user clicks the "Run" button.

/**
* Constructor creates the three display labels and adds them to this
* panel.  It adds a "Run"/"Pause" button below the labels.
* It also creates and starts the computation thread.
*/
public EstimatePi_2() {

setLayout(new GridLayout(4, 1, 2, 2));
setBackground(Color.BLUE);
setBorder(BorderFactory.createLineBorder(Color.BLUE,2));

countLabel =      new JLabel(" Number of Trials:   0");
piEstimateLabel = new JLabel(" Current Estimate:   (none)");
JLabel piLabel =  new JLabel(" Actual value of pi: " + Math.PI + " ");
Font bigMonospace = new Font("Monospaced", Font.PLAIN, 20);
countLabel.setFont(bigMonospace);
piEstimateLabel.setFont(bigMonospace);
piLabel.setFont(bigMonospace);
countLabel.setOpaque(true);
piEstimateLabel.setOpaque(true);
piLabel.setOpaque(true);
add(piLabel);
add(piEstimateLabel);
add(countLabel);

JPanel bottom = new JPanel();
add(bottom);
runPauseButton = new JButton("Run");
bottom.add(runPauseButton);
runPauseButton.addActionListener(this);

runner = new ComputationThread();
runner.start();

} // end constructor

/**
* This method responds to clicks on the button, by
* toggling the value of the signal variable from true
* to false or from false to true.  The text on the
* button is changed to match the state.  When
* running is set to true, notify() is called to wake
* up the thread.
*/
public void actionPerformed(ActionEvent evt) {
if (running) {
runPauseButton.setText("Run");
running = false;
}
else {
runPauseButton.setText("Pause");
synchronized(runner) {
running = true;
runner.notify();
}
}
}

/**
*  This class defines the thread that does the computation.
*  The thread runs in an infinite loop in which it performs
*  batches of 1000000 trials and then updates the display labels.
*  Just after it starts and between batches, the thread tests
*  the value of the signal variable, running.  If this variable
*  is false, then the thread sleeps until the value of running
*  is set to true.
*/
private class ComputationThread extends Thread {
final int BATCH_SIZE = 1000000;  // Number of trials between updates of the display.
long trialCount;     // Total number of trials that have been performed.
long inCircleCount;  // Number of trials in which x*x+y*y is less than 1.
public ComputationThread() {
setDaemon(true);
setPriority(Thread.currentThread().getPriority() - 1);
}
public void run() {
while (true) {
synchronized(this) {
while ( ! running ) { // wait for running to be true
try {
wait();
}
catch (InterruptedException e) {
}
}
}
for (int i = 0; i < BATCH_SIZE; i++) {
double x = Math.random();
double y = Math.random();
trialCount++;
if (x*x + y*y < 1)
inCircleCount++;
}
double estimateForPi = 4 * ((double)inCircleCount / trialCount);
countLabel.setText(      " Number of Trials:   " + trialCount);
piEstimateLabel.setText( " Current Estimate:   " + estimateForPi);
}
}
}

}  // end class EstimatePi_2

The third version of the program, with a control button and multiple threads. Significant changes from the previous versions are shown in red italic

import java.awt.*;
import java.awt.event.*;
import java.util.ArrayList;
import java.util.Random;
import java.util.concurrent.LinkedBlockingQueue;
import javax.swing.*;

/**
* This program uses a probabilistic technique to estimate the
* value of the mathematical constant pi.  The technique is to
* choose random numbers x and y in the range 0 to 1, and to
* compute x*x + y*y.  The probability that x*x + y*y is less than
* 1 is pi/4.  If many trials are performed, and the number of
* trials in which x*x+y*y is less than 1 is divided by the total
* number of trials, the result is an approximation for pi/4.
* Multiplying this by 4 gives an approximation for pi.
*
* The program shows the estimate produced by this procedure, along
* with the number of trials that have been done and, for comparison,
* the actual value of pi.  These values are shown in three JLabels.
* The computation is done by several separate threads.  Periodically,
* these threads place their results into a queue.  Another thread,
* the event-handling thread in a method called by a Timer, removes
* results from the queue and applies them to the labels every
* 1/10 second.
*
* In this version of the program, there is a "Run"/"Pause" button
* that controls the computation threads.  Clicking the button once
* starts the threads; clicking it again pauses them.  Initially, the
* threads are paused.
*/
public class EstimatePi_3 extends JPanel implements ActionListener {

public static void main(String[] args) {
JFrame window = new JFrame("Estimating Pi Probabilistically");
EstimatePi_3 content = new EstimatePi_3();
window.setContentPane(content);
window.pack();
window.setResizable(false);
window.setLocation( 300, 200 );
window.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
window.setVisible(true);
}

private final static int BATCH_SIZE = 1000000;  // This is the number of trials
// in a batch.  A computation thread runs this
// many trials in a fast for loop, without checking
// the value of running and without reporting its
// results.  After the for loop, the thread puts
// the result from that batch into the queue.  It
// then checks the value of running and will pause
// if running is false.

private long totalTrialCount;    // Total number of trials considered so far.
private long totalInCircleCount; // Number of those trials for which x*x+y*y < 1.

private JLabel piEstimateLabel;  // A label for showing the current estimate of pi.
private JLabel countLabel;       // A label for showing the number of trials.

private JButton runPauseButton;  // Button to control the threads.  Clicking this
// button will pause the threads if they are running
// and will restart it if it is paused.

private volatile boolean running;  // Control variable for signaling the threads to
// run or pause.  Initially, this is false, so
// the thread pauses as soon as it is created,
// until the user clicks the "Run" button.

private LinkedBlockingQueue<Integer> resultsQueue; // Results from the computation
// threads are placed into this queue.  Every number
// in the queue represents the results from running
// a batch of trials, of size BATCH_SIZE.  The number
// in the queue is the number of trials in that batch
// that resulted in x*x+y*y being less than 1.  (Note
// that I use a blocking queue rather than a
// ConcurrentLinkedQueue only because the blocking
// queue has a convenient drainTo() method for getting
// all the items out of the queue at once, with correct
// synchronization.)

private Timer resultsTimer;  // While the computation is running, this timer is
// also running.  Every 1/10 second, it grabs the
// results from the queue and applies them to the
// display labels.  (Note that some results can be
// left in the queue while the timer and threads are
// paused.  This seems harmless.)

/**
* Constructor creates the three display labels and adds them to this
* panel.  It adds a "Run"/"Pause" button below the labels.
* It also creates and starts the computation threads and the timer
* that takes the results from the threads and updates the labels
* every 1/10 second.
*/
public EstimatePi_3() {

setLayout(new GridLayout(4, 1, 2, 2));
setBackground(Color.BLUE);
setBorder(BorderFactory.createLineBorder(Color.BLUE,2));

countLabel =      new JLabel(" Number of Trials:   0");
piEstimateLabel = new JLabel(" Current Estimate:   (none)");
JLabel piLabel =  new JLabel(" Actual value of pi: " + Math.PI + " ");
Font bigMonospace = new Font("Monospaced", Font.PLAIN, 20);
countLabel.setFont(bigMonospace);
piEstimateLabel.setFont(bigMonospace);
piLabel.setFont(bigMonospace);
countLabel.setOpaque(true);
piEstimateLabel.setOpaque(true);
piLabel.setOpaque(true);
add(piLabel);
add(piEstimateLabel);
add(countLabel);

JPanel bottom = new JPanel();
add(bottom);
runPauseButton = new JButton("Run");
bottom.add(runPauseButton);
runPauseButton.addActionListener(this);

resultsQueue = new LinkedBlockingQueue<Integer>();
int threadCount = Runtime.getRuntime().availableProcessors();
for (int i = 0; i < threadCount; i++) {
ComputationThread runner = new ComputationThread();
runner.start();
}

resultsTimer = new Timer(100, new ActionListener() {
public void actionPerformed(ActionEvent e) {
grabResults();
}
});

} // end constructor

/**
* This method responds to clicks on the button, by
* toggling the value of the signal variable from true
* to false or from false to true.  The text on the
* button is changed to match the state.  The timer
* that grabs the results is also started or stopped,
* depending on whether the computation is running.  When
* running is set to true, notifyAll() is called to wake
* up all the threads.  (Note that the synchronization
* for controlling the threads is on the panel object,
* this.  The threads have to synchronize on the same
* object when testing the value of running.)
*/
public void actionPerformed(ActionEvent evt) {
if (running) {
resultsTimer.stop();
runPauseButton.setText("Run");
running = false;
}
else {
runPauseButton.setText("Pause");
resultsTimer.start();
synchronized(this) { // IMPORTANT: Synchronization is now on this JPanel object!
running = true;
notifyAll(); // IMPORTANT: Use notifyAll(), not notify(), to wake all computation threads
}
}
}

/**
* This method is called by the timer, every 1/10 second while the
* computation is running.  It grabs the entire contents of the
* queue that is used to send results from the threads to
* this method.  Each value in the queue represents the number of
* trials, out of a batch of size BATCH_SIZE, in which x*x+y*Y was
* less than 1.  This method updates the total number of trials that
* have been performed and the total number of trials for which
* x*x+y*y was less than 1.  It then updates the display labels
* with the new data.
*/
private void grabResults() {
ArrayList<Integer> results = new ArrayList<Integer>();
resultsQueue.drainTo(results);  // Get entire contents of queue.
// Using this method avoids having to synchronize
// the entire process of removing items from the
// queue one at a time.  (And doing that without
// synchronization would introduce a race condition.)
for (int inCircleCount : results) {
totalTrialCount += BATCH_SIZE;
totalInCircleCount += inCircleCount;
}
double estimateOfPi = 4 * ((double)totalInCircleCount / totalTrialCount);
countLabel.setText(      " Number of Trials:   " + totalTrialCount);
piEstimateLabel.setText( " Current Estimate:   " + estimateOfPi);
// System.out.println("Got " + results.size() + " results.");  // for testing
}

/**
*  This class defines the thread that does the computation.
*  The thread runs in an infinite loop in which it performs
*  batches of 1000000 trials and places the result in the queue.
*  Just after it starts and between batches, the thread tests
*  the value of the signal variable, running.  If this variable
*  is false, then the thread sleeps until the value of running
*  is set to true.  Note that this method creates and uses its
*  own object of type Random to generate random numbers. (Because
*  access to Math.random() has to be synchronized, using it
*  in multiple threads slowed things down immensely.)
*  Synchronization in this thread, as in the rest of the program,
*  is on the panel object, which is referred to here as
*  "EstimatePi_3.this".  The previous version used the thread
*  object for synchronization, but in this version there can
*  be multiple threads, so it seemed more natural to use the panel.
*/
private class ComputationThread extends Thread {
public ComputationThread() {
setDaemon(true);
setPriority(Thread.currentThread().getPriority() - 1);
}
public void run() {
Random myRandom = new Random();
while (true) {
synchronized(EstimatePi_3.this) {
while ( ! running ) {
try {
EstimatePi_3.this.wait();
}
catch (InterruptedException e) {
}
}
}
int inCircleCount = 0;
for (int i = 0; i < BATCH_SIZE; i++) {
double x = myRandom.nextDouble();
double y = myRandom.nextDouble();
if (x*x + y*y < 1)
inCircleCount++;
}
resultsQueue.add(inCircleCount);
}
}
}

} // end class EstimatePi_3

Solution for Programming Exercise 12.6

Exercise 12.6:

The chat room example from Subsection 12.5.2 can be improved in several ways. First, it would be nice if the participants in the chat room could be identified by name instead of by number. Second, it would be nice if one person could send a private message to another person that would be seen just by that person rather than by everyone. Make these two changes. You can start with a copy of the package netgame.chat. You will also need the package netgame.common, which defines the netgame framework.

To make the first change, you will have to implement a subclass of Hub that can keep track of client names as well as numbers. To get the name of a client to the hub, you can override the extraHandshake() method both in the Hub subclass and in the Client subclass. The extraHandshake() method is called as part of setting up the connection between the client and the hub. It is called after the client has been assigned an ID number but before the connection is considered to be fully connected. It should throw an IOException if some error occurs during the setup process. Note that any messages that are sent by the hub as part of the handshake must be read by the client and vice versa. The extraHandshake() method in the Client is defined as:

protected void extraHandshake(ObjectInputStream in, ObjectOutputStream out)
throws IOException

while in the Hub, there is an extra parameter that tells the ID number of the client whose connection is being set up:

protected void extraHandshake(in playerID, ObjectInputStream in,
ObjectOutputStream out) throws IOException

In the ChatRoomWindow class, the main() routine asks the user for the name of the computer where the server is running. You can add some code there to ask the user their name. You will have to decide what to do if two users want to use the same name. You might consider having a list of users who are allowed to join the chat room. You might even assign them passwords.

For the second improvement, personal messages, I suggest defining a PrivateMessage class. A PrivateMessage object would include both the string that represents the message and the ID numbers of the player to whom the message is being sent and the player who sent the message. The hub will have to be programmed to know how to deal with such messages. A PrivateMessage should only be sent by the hub to the client who is listed as the recipient of the message. You need to decide how the user will input a private message and how the user will select the recipient of the message.

If you attempt this exercise, you are likely to find it quite challenging.

Discussion

My solution defines a subclass, NewChatRoomHub, of Hub to make it possible to manage user names. I also wrote NewChatRoomWindow class, but not as a subclass; instead it is a modified copy of ChatRoomWindow. the Hub class, users are identified by ID numbers. In some ways, it would have been easier to modify Hub, instead of creating a subclass. However, I wanted to build on the package netgame.common, without modifying the existing classes. The NewChatRoomHub class has an instance variable

private TreeMap<Integer,String> nameMap = new TreeMap<Integer,String>();

to keep track of client names by associating each client's name to that client's ID number. That is, nameMap.get(id) is the name of the client whose ID number is id. There has to be a way to add a user name to nameMap when a new client connects, and there has to be a way to remove a client from nameMap when that client disconnects. There also has to be a way for a client to get a name in the first place.

It might have been nice to have a list of authorized users, and give each authorized user a name and a password. The hub would then require a name and a password before it would let a client connect. The list of names and passwords could be stored in a file or, if there are a lot of them, in a database. However, I decided to keep things simple. Clients choose their names when they connect. The client's selected name is sent to the hub as part of the extraHandshake() method. To avoid having duplicate names, the hub will modify the client's name if there is already someone in the chat room using that name. For example, if the client wants to be "Fred" and there is already a Fred, then the hub will assign the name "Fred#2" to the client. Since the hub can modify the name, it sends the client's name back to the client, possibly with some modification. The client reads the returned name as part of the handshake. To implement this idea, the extraHandshake() method in the client simply sends the client's requested name to the hub by writing the name to the connection's output stream. It then reads the possibly modified name from the connection's input stream:

protected void extraHandshake(ObjectInputStream in, ObjectOutputStream out) throws IOException {
try {
out.writeObject(myName);  // Send user's name request to the server.
myName = (String)in.readObject();  // Get the actual, possibly modified, name from the server.
}
catch (Exception e) {
throw new IOException("Error while setting up connection: " + e);
}
}

(This code is from the ChatClient class, which is a nested subclass of NewChatRoomWindow.)

The corresponding extraHandshake() method in the NewChatRoomHub class has to read the name that was sent by the client, possibly modify the name, and write the name back to the client. It also adds the client to nameMap, which contains the names of all connected clients. My code is very careful to produce a name that is non-null, non-empty, and unique:

protected void extraHandshake(int playerID,
ObjectInputStream in, ObjectOutputStream out) throws IOException {
try {
String name = (String)in.readObject(); // Read requested name from client.
if (name == null)
name = "noname";
if (name.length() > 15)
name = name.substring(0,15).trim();
if (name.equals(""))
name = "noname";
synchronized(nameMap) {
// Synchronized to be absolutely sure that there will be
// no duplicate names.
if (nameMap.containsValue(name)) {
String approvedName = name;
int num = 2;
while (nameMap.containsValue(approvedName)) {
approvedName = name + "#" + num;
num++;
}
name = approvedName;
}
}
out.writeObject(name);      // Send actual name to client.
nameMap.put(playerID,name); // Add client's name to nameMap.
}
catch (Exception e) {
throw new IOException("Error while setting up connection: " + e);
}
}

In order to get private messages from one player to another, I defined a PrivateMessage class, as suggested in the exercise. An object pm of type PrivateMessage has public instance variables pm.senderID and pm.recipientID to hold the ID numbers of the sender and the recipient, and it has pm.message to hold the message itself. The hub has to know what to do with such a message. Messages are processed in the hub by the messageReceived() method. I overrode this method in NewChatRoomHub to handle PrivateMessage and to pass messages of other types on to the superclass. One interesting point is that the hub sets the senderID in the message to be the ID number of the client that actually sent the message. This is done to prevent clients from sending forged messages that appear to be from someone else. Here is the messageReceived() method from NewChatRoomHub:

/**
* This method is overridden to provide support for PrivateMessages.
* If a PrivateMessage is received from some client, this method
* will set the senderID field in the message to be the ID number
* of the client who sent the message.  It will then send the
* message on to the specified recipient.  If some other type
* of message is received, it is handled by the messageReceived()
* method in the superclass (which will wrap it in a ForwardedMessage
* and send it to all connected clients).
*/
protected void messageReceived(int playerID, Object message) {
if (message instanceof PrivateMessage) {
PrivateMessage pm = (PrivateMessage)message;
pm.senderID = playerID;
sendToOne(pm.recipientID, pm);
}
else
super.messageReceived(playerID, message);
}

It is easy enough to add some extra inputs to the original window from ChatRoomWindow to make it possible for the user to send private messages. Just add a text input box for the message, a button to send the message, and a JComboBox that contains a list of possible recipients. The user enters a message, selects the recipient from the list, and clicks the button to send. The program constructs a PrivateMessage containing the message and the ID number of the recipient, and it sends the PrivateMessage to the hub, which forwards it on to the intended recipient. This code can be found in NewChatRoomWindow.

At least, it would be easy if not for the fact that I want users of the chat room to be identified by name, not ID number. The whole point of using names is that they provide a more meaningful way to refer to users. Each client already gets a list of ID numbers of all connected clients, but I want each client to have a list of names of connected clients. The hub has that information. I just needed a way to get that information to the clients. Since the client list changes every time a client connects or disconnects, the hub will have to send new information to the clients every time that happens. I was able to program this behavior by overriding the playerConnected() and playerDisconnected() methods from the Hub class. My NewChatRoomHub class includes the following definitions for those methods (which do nothing in class Hub):

/**
*  This method is called when a new client connects.  It is called
*  after the extraHandshake() method has been called, so that the
*  client's name has already been added to nameMap.  This method
*  creates a ClientConnectedMessage and sends it to all connected
*  clients to announce the new participant in the chat room.
*/
protected void playerConnected(int playerID) {
resetOutput(); // Reset the output stream before resending nameMap.
sendToAll(new ClientConnectedMessage(playerID,nameMap));
}

/**
* This method is called when a client has been disconnected from
* this hub.  It removes the client from the nameMap and sends
* a ClientDisconnectMessage to all connected players to
* announce the fact that the client has left the chat room.
*/
protected void playerDisconnected(int playerID) {
String name = nameMap.get(playerID); // Get the departing player's name.
nameMap.remove(playerID);  // Remove the player from nameMap.
resetOutput(); // Reset the output stream before resending nameMap.
sendToAll(new ClientDisconnectedMessage(playerID, name, nameMap));
}

It actually took me quite a while to get this right. To simplify the processing, I ended up defining new message types, ClientConnectedMessage and ClientDisconnectedMessage, to hold the information that I wanted to send to the clients. A ClientConnectedMessage includes the ID number of the client who has just connected, as well as the nameMap that contains the ID numbers and names of all connected clients. When a client receives this message, it can inform the user that someone has entered the chat room, and it can use the nameMap to construct the list of names of possible recipients of private messages. ClientDisconnectedMessages are similar.

Not for the first time, I introduced a bug by forgetting that an ObjectOuputStream has to be reset if it is used to send the same object twice, with modifications between the two transmissions. This is necessary in this example because the same object, nameMap, is transmitted as part of every ClientConnectedMessage and every ClientDisconnectedMessage. When I didn't reset the output, the changes that I made to that object were not seen by the clients that received those messages. In my code, the method resetOuput() will cause the output stream leading to every client to be reset.

Back in the NewChatRoomWindow class, the chat room client has to be prepared to receive PrivateMessages, ClientConnectedMessages, and ClientDisconnectedMessages. It can also receive ForwardedMessages which are used, as in the original chat room application, to broadcast a message from one client to all connected clients. In each case, the client will add an appropriate message to the transcript. It uses clientNameMap -- the local copy of nameMap -- to translate ID numbers from the messages into the names of the corresponding chat room users:

protected void messageReceived(Object message) {
if (message instanceof ForwardedMessage) {
ForwardedMessage fm = (ForwardedMessage)message;
String senderName = clientNameMap.get(fm.senderID);
addToTranscript(senderName + " SAYS:  " + fm.message);
}
else if (message instanceof PrivateMessage) {
PrivateMessage pm = (PrivateMessage)message;
String senderName = clientNameMap.get(pm.senderID);
addToTranscript("PRIVATE MESSAGE FROM " + senderName + ":  " + pm.message);
}
else if (message instanceof ClientConnectedMessage) {
ClientConnectedMessage cm = (ClientConnectedMessage)message;
addToTranscript('"' + cm.nameMap.get(cm.newClientID) + "\" HAS JOINED THE CHAT ROOM.");
newNameMap(cm.nameMap);
}
else if (message instanceof ClientDisconnectedMessage) {
ClientDisconnectedMessage dm = (ClientDisconnectedMessage)message;
addToTranscript('"' + clientNameMap.get(dm.departingClientID) + "\" HAS LEFT THE CHAT ROOM.");
newNameMap(dm.nameMap);
}
}

The newNameMap() method, which is called twice in the above code, will assign the nameMap from the message to clientNameMap. It also updates the JComboBox, , which stores the list of names of connected users (leaving out the name of the user who is using the program, since he won't want to send a private message to himself):

private void newNameMap(final TreeMap<Integer,String> nameMap) {
SwingUtilities.invokeLater(new Runnable() {
public void run() {
clientNameMap = nameMap;
clientList.removeAllItems();
boolean someoneIsThere = false;
for (String str: nameMap.values()) {
if (!str.equals(myName)) {
clientList.addItem(str);
someoneIsThere = true;
}
}
privateMessageInput.setEditable(someoneIsThere);
privateMessageInput.setEnabled(someoneIsThere);
sendPrivateButton.setEnabled(someoneIsThere);
if (!someoneIsThere)
clientList.addItem("(no one available)");
}
});
}

The complete source code for the new chat room application is shown below. It consists of six classes, which I defined in a package named netgame.newchat. These classes depend on the classes from the netgame.common package.

The Solution

NewChatRoomServer is a short class with a main routine that simply creates a hub that acts as the server for the application:

package netgame.newchat;

import java.io.IOException;

/**
* This class contains just a small main class that creates a NewChatRoomHub
* and starts it listening on port 37830.  This port is used
* by the NewChatRoomWindow application.  This program should be run
* on the computer that "hosts" the chat room.  See the NewChatRoomWindow
* class for more details.  Once the server starts listening, it
* will listens for connection requests from clients until the
* NewChatRoomServer program is terminated (for example by a
* Control-C).
*/
public class NewChatRoomServer {

private final static int PORT = 37830;

public static void main(String[] args) {
try {
new NewChatRoomHub(PORT);
}
catch (IOException e) {
System.out.println("Can't create listening socket.  Shutting down.");
}
}

}

There are three short classes that define various types of messages which are used in the application:

package netgame.newchat;

import java.io.Serializable;

/**
* Represents a string sent as a message from one client
* to another client.  Note:  The ChatRoomHub will set the
* senderID of a PrivateMessage to be the ID number of the
* client who actually sent the message, in order to avoid
* the possibility of rogue clients that try to forge
* messages that appear to come from other clients.
*/
public class PrivateMessage implements Serializable {

public int senderID;    // The ID number of the sender.
public int recipientID; // The ID number of the recipient.
public String message;  // The message.

/**
*  Create a private message from one user to another.
*  The senderID of the message will be set by the hub.
*/
public PrivateMessage(int recipientID, String message) {
this.recipientID = recipientID;
this.message = message;
}

}

package netgame.newchat;

import java.io.Serializable;
import java.util.TreeMap;

/**
* A message of this type will be sent by the hub to all
* connected clients when a new client joins the chat room
*/
public class ClientConnectedMessage implements Serializable {

public int newClientID;  // The ID number of the client who has connected.
public TreeMap<Integer,String> nameMap;  // Map of all connected client IDs to their names.

public ClientConnectedMessage(int newClientID, TreeMap<Integer,String> nameMap) {
this.newClientID = newClientID;
this.nameMap = nameMap;
}

}

package netgame.newchat;

import java.io.Serializable;
import java.util.TreeMap;

/**
* A message of this type will be sent by the hub to all
* remaining connected clients when a client leaves the
* chat room.
*/
public class ClientDisconnectedMessage implements Serializable {

public int departingClientID;  // The ID number of the client who has left the chat room.
public String departingClientName;  // The name of the departing client
public TreeMap<Integer,String> nameMap;  // Map of all connected client IDs to their names.
//  (Note that the departing client is not included.)

public ClientDisconnectedMessage(int departingClientID,
String departingClientName, TreeMap<Integer,String> nameMap) {
this.departingClientID = departingClientID;
this.departingClientName = departingClientName;
this.nameMap = nameMap;
}

}

The subclass of Hub that defines the server for the application:

package netgame.newchat;

import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.util.TreeMap;

import netgame.common.*;

/**
*  This class defines the "hub" that acts as a server for the
*  chat room application.  It extends the basic Hub class in
*  order to support names for clients, as well as ID numbers.
*/
public class NewChatRoomHub extends Hub {

/**
* This map keeps track of the names of all connected clients.
* It maps client ID numbers to client names.
*/
private TreeMap<Integer,String> nameMap = new TreeMap<Integer,String>();

/**
* Create a NewChatRoomHub, which will listen for connections on
* a specified port.
* @param port the port on which to listen for connections
* @throws IOException if it is not possible to create a listening socket
*/
public NewChatRoomHub(int port) throws IOException {
super(port);
}

/**
* This method is called as part of the connection setup between this hub
* and a client that has requested a connection.  It is overridden in this
* class so that a name can be assigned to the client as part of the setup
* process.  This method works in cooperation with the extraHandshake()
* method in the client class (which is defined as a nested class inside
* NewChatRoomWindow).  In this method, the Hub reads a string from the
* user that contains the name that the client wants to use.  The name
* can be modified to make sure that it is non-null, 15 characters or less.
* The resulting name is further modified by adding a suffix such as
* "#2" or "#3" if the name is already in use by another client.  Finally,
* the possibly modified name is sent back to the client, which will use
* the returned value as the name that identifies the client in the chat
* room.
*/
protected void extraHandshake(int playerID,
ObjectInputStream in, ObjectOutputStream out) throws IOException {
try {
String name = (String)in.readObject();
if (name == null)
name = "noname";
if (name.length() > 15)
name = name.substring(0,15).trim();
if (name.equals(""))
name = "noname";
synchronized(nameMap) {
if (nameMap.containsValue(name)) {
String approvedName = name;
int num = 2;
while (nameMap.containsValue(approvedName)) {
approvedName = name + "#" + num;
num++;
}
name = approvedName;
}
}
out.writeObject(name);
nameMap.put(playerID,name);
}
catch (Exception e) {
throw new IOException("Error while setting up connection: " + e);
}
}

/**
* This method is overridden to provide support for PrivateMessages.
* If a PrivateMessage is received from some client, this method
* will set the senderID field in the message to be the ID number
* of the client who sent the message.  It will then send the
* message on to the specified recipient.  If some other type
* of message is received, it is handled by the messageReceived()
* method in the superclass (which will wrap it in a ForwardedMessage
* and send it to all connected clients).
*/
protected void messageReceived(int playerID, Object message) {
if (message instanceof PrivateMessage) {
PrivateMessage pm = (PrivateMessage)message;
pm.senderID = playerID;
sendToOne(pm.recipientID, pm);
}
else
super.messageReceived(playerID, message);
}

/**
*  This method is called when a new client connects.  It is called
*  after the extraHandshake() method has been called, so that the
*  client's name has already been added to nameMap.  This method
*  creates a ClientConnectedMessage and sends it to all connected
*  clients to announce the new participant in the chat room.
*/
protected void playerConnected(int playerID) {
resetOutput(); // Reset the output stream before resending nameMap.
sendToAll(new ClientConnectedMessage(playerID,nameMap));
}

/**
* This method is called when a client has been disconnected from
* this hub.  It removes the client from the nameMap and sends
* a ClientDisconnectedMessage to all connected players to
* announce the fact that the client has left the chat room.
*/
protected void playerDisconnected(int playerID) {
String name = nameMap.get(playerID); // Get the departing player's name.
nameMap.remove(playerID);  // Remove the player from nameMap.
resetOutput(); // Reset the output stream before resending nameMap.
sendToAll(new ClientDisconnectedMessage(playerID, name, nameMap));
}

}

And finally, the class that defines the clients, NewChatRoomWindow. This class is a modified version of ChatRoomWindow from the original chat room application. Significant changes from the original are shown in red italic.

package netgame.newchat;

import java.awt.*;
import java.awt.event.*;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.util.TreeMap;

import javax.swing.*;
import netgame.common.*;

/**
* This class represents a client for a "chat room" application.  The chat
* room is hosted by a server running on some computer.  The user of this
* program must know the host name (or IP address) of the computer that
* hosts the chat room.  When this program is run, it asks for that
* information and for the name that the user wants to use in the chat
* room.  Then, it opens a window that has an input box where the
* user can enter messages to be sent to the chat room.  The message is
* sent when the user presses return in the input box or when the
* user clicks a Send button.  There is also a text area that shows
* a transcript of all messages from participants in the chat room.
* <p>The user can also send private messages to individual users.
* The user selects the recipient's name from a pop-up list of
* connected users.
* <p>Participants in the chat room are represented by ID numbers
* that are assigned to them by the server when they connect. They
* also have names which they select.
*/
public class NewChatRoomWindow extends JFrame {

private final static int PORT = 37830; // The ChatRoom port number; can't be
// changed here unless the ChatRoomServer
// program is also changed.

/**
* Gets the host name (or IP address) of the chat room server from the
* user, gets the user's name, and opens a NewChatRoomWindow.  The program ends
* when the user closes the window.
*/
public static void main(String[] args) {
String host = JOptionPane.showInputDialog(
"Enter the host name of the\ncomputer that hosts the chat room:");
if (host == null || host.trim().length() == 0)
return;
String name = JOptionPane.showInputDialog(
"Enter that name that you want\nto use in the chat room.");
if (name == null || name.trim().length() == 0)
return;
NewChatRoomWindow window = new NewChatRoomWindow(host,name.trim());
window.setLocation(200,100);
window.setVisible(true);
window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
}

/**
* A ChatClient connects to the Hub and is used to send messages to
* and receive messages from a Hub.  Four types of message are
* received from the Hub.  A ForwardedMessage represents a message
* that was entered by some user and sent to all users of the
* chat room.  A PrivateMessage represents a message that was
* sent by another user only to this user.  A ClientConnectedMessage
* is sent when a new user enters the room.  A ClientDisconnectedMessage
* is sent when a user leaves the room.
*/
private class ChatClient extends Client {

/**
* Opens a connection the chat room server on a specified computer.
*/
ChatClient(String host) throws IOException {
super(host, PORT);
}

/**
* Responds when a message is received from the server.
*/
protected void messageReceived(Object message) {
if (message instanceof ForwardedMessage) {
ForwardedMessage fm = (ForwardedMessage)message;
String senderName = clientNameMap.get(fm.senderID);
addToTranscript(senderName + " SAYS:  " + fm.message);
}
else if (message instanceof PrivateMessage) {
PrivateMessage pm = (PrivateMessage)message;
String senderName = clientNameMap.get(pm.senderID);
addToTranscript("PRIVATE MESSAGE FROM " + senderName + ":  " + pm.message);
}
else if (message instanceof ClientConnectedMessage) {
ClientConnectedMessage cm = (ClientConnectedMessage)message;
addToTranscript('"' + cm.nameMap.get(cm.newClientID) + "\" HAS JOINED THE CHAT ROOM.");
newNameMap(cm.nameMap);
}
else if (message instanceof ClientDisconnectedMessage) {
ClientDisconnectedMessage dm = (ClientDisconnectedMessage)message;
addToTranscript('"' + clientNameMap.get(dm.departingClientID) + "\" HAS LEFT THE CHAT ROOM.");
newNameMap(dm.nameMap);
}
}

/**
* This method is part of the connection set up.  It sends the user's selected
* name to the hub by writing that name to the output stream.  The hub will
* respond by sending the name back to this client, possibly modified if someone
* is the chat room is already using the selected name.
*/
protected void extraHandshake(ObjectInputStream in, ObjectOutputStream out) throws IOException {
try {
out.writeObject(myName);  // Send user's name request to the server.
myName = (String)in.readObject();  // Get the actual, possibly modified, name from the server.
}
catch (Exception e) {
throw new IOException("Error while setting up connection: " + e);
}
}

/**
* Called when the connection to the client is shut down because of some
* error message.  (This will happen if the server program is terminated.)
*/
protected void connectionClosedByError(String message) {
addToTranscript("Sorry, communication has shut down due to an error:\n     " + message);
sendButton.setEnabled(false);
messageInput.setEnabled(false);
messageInput.setEditable(false);
messageInput.setText("");
sendPrivateButton.setEnabled(false);
privateMessageInput.setEnabled(false);
privateMessageInput.setEditable(false);
connected = false;
connection = null;
}

// Note:  the methods playerConnected() and playerDisconnected(), which where present here
// in ChatRoomWindow, were removed, since their functionality (to announce arrivals
// and departures) has been taken over by ClientConnectedMessage and ClientDisconnectedMessage.

} // end nested class ChatClient

private volatile String myName;   // The name that this client uses in the chat room.
// Originally selected by the user, but might be modified
// if there is already a client of the same name connected
// to the Hub.

private volatile TreeMap<Integer,String> clientNameMap = new TreeMap<Integer, String>();
// The clientNameMap maps client ID numbers to the names that they are
// using in the chat room.  Every time a client connects or disconnects,
// the Hub sends a new, modified name map to each connected client.  When
// that message is received, the clientNameMap is replaced with the new value,
// and the content of the clientList is replaced with info from the nameMap.

private JComboBox clientList;      // List of connected client names, where the user can select
//   the client who is to receive the private message.

private JTextField messageInput;   // For entering messages to be sent to the chat room
private JButton sendButton;        // Sends the contents of the messageInput.
private JButton quitButton;        // Leaves the chat room cleanly, by sending a DisconnectMessage

private JTextField privateMessageInput;  // For entering messages to be set to individual clients.
private JButton sendPrivateButton; // Sends the contents of privateMesssageInput to the user selected
//   in the clientList.

private JTextArea transcript;      // Contains all messages sent by chat room participant, as well
// as a few additional status messages, such as when a new user arrives.

private ChatClient connection;      // Represents the connection to the Hub; used to send messages;
// also receives and processes messages from the Hub.

private volatile boolean connected; // This is true while the client is connected to the hub.

/**
* Constructor creates the window and starts the process of connecting
* to the hub; the actual connection is done in a separate thread.
* @param host  The IP address or host name of the computer where the server is running.
*/
private NewChatRoomWindow(final String host, String clientName) {
super("Chat Room");
myName = clientName;
setBackground(Color.BLACK);
setLayout(new BorderLayout(2,2));
transcript = new JTextArea(30,60);
transcript.setLineWrap(true);
transcript.setWrapStyleWord(true);
transcript.setMargin(new Insets(5,5,5,5));
transcript.setEditable(false);
add(new JScrollPane(transcript), BorderLayout.CENTER);
sendButton = new JButton("send to all");
quitButton = new JButton("quit");
sendPrivateButton = new JButton("send to one");
messageInput = new JTextField(40);
messageInput.setMargin(new Insets(3,3,3,3));
privateMessageInput = new JTextField(40);
privateMessageInput.setMargin(new Insets(3,3,3,3));
clientList = new JComboBox();
clientList.addItem("(No one available)");
ActionHandler ah = new ActionHandler();
sendButton.addActionListener(ah);
quitButton.addActionListener(ah);
sendPrivateButton.addActionListener(ah);
messageInput.addActionListener(ah);
privateMessageInput.addActionListener(ah);
sendButton.setEnabled(false);
sendPrivateButton.setEnabled(false);
messageInput.setEditable(false);
messageInput.setEnabled(false);
privateMessageInput.setEditable(false);
privateMessageInput.setEnabled(false);
JPanel bottom = new JPanel();
bottom.setBackground(Color.LIGHT_GRAY);
bottom.add(new JLabel("You say:"));
bottom.add(messageInput);
bottom.add(sendButton);
bottom.add(Box.createHorizontalStrut(30));
bottom.add(quitButton);
JPanel bottom2 = new JPanel();
bottom2.setBackground(Color.LIGHT_GRAY);
bottom2.add(new JLabel("Say:"));
bottom2.add(privateMessageInput);
bottom2.add(new JLabel(" to: "));
bottom2.add(clientList);
bottom2.add(sendPrivateButton);
JPanel panel = new JPanel();
panel.setLayout(new GridLayout(2,1,5,5));
panel.add(bottom);
panel.add(bottom2);
add(panel,BorderLayout.SOUTH);
pack();
addWindowListener( new WindowAdapter() { // calls doQuit if user closes window
public void windowClosing(WindowEvent e) {
doQuit();
}
});
new Thread() {
// This is a thread that opens the connection to the server.  Since
// that operation can block, it's not done directly in the constructor.
// Once the connection is established, the user interface elements are
// enabled so the user can send messages.  The Thread dies after
// the connection is established or after an error occurs.
public void run() {
try {
addToTranscript("Connecting to " + host + " ...");
connection = new ChatClient(host);
connected = true;
messageInput.setEditable(true);
messageInput.setEnabled(true);
sendButton.setEnabled(true);
messageInput.requestFocus();
setTitle("Chat Room: Connected as " + myName);
}
catch (IOException e) {
addToTranscript("Connection attempt failed.");
addToTranscript("Error: " + e);
}
}
}.start();
}

/**
* Adds a string to the transcript area, followed by a blank line.
*/
private void addToTranscript(String message) {
transcript.append(message);
transcript.append("\n\n");
// The following line is a nasty kludge that was the only way I could find to force
// the transcript to scroll so that the text that was just added is visible in
// the window.  Without this, text can be added below the bottom of the visible area
// of the transcript.
transcript.setCaretPosition(transcript.getDocument().getLength());
}

/**
* Called when the user clicks the Quit button or closes
* the window by clicking its close box.
*/
private void doQuit() {
if (connected)
connection.disconnect();  // Sends a DisconnectMessage to the server.
dispose();
try {
Thread.sleep(1000); // Time for DisconnectMessage to actually be sent.
}
catch (InterruptedException e) {
}
System.exit(0);
}

/**
* This method is called when a ClientConnectedMessage or ClientDisconnectedMessage
* is received from the hub.  Its job is to save the nameMap that is part of the
* message and use it to rebuild the contents of the JComboBox, clientList, where
* the user selects the recipient of a private message.  It also enables or
* disables the private message input box and send button, depending on whether
* there are any possible message recipients.
* @param nameMap the new nameMap, which will replace the value of clientNameMap.
*/
private void newNameMap(final TreeMap<Integer,String> nameMap) {
SwingUtilities.invokeLater(new Runnable() {
public void run() {
clientNameMap = nameMap;
clientList.removeAllItems();
boolean someoneIsThere = false;
for (String str: nameMap.values()) {
if (!str.equals(myName)) {
clientList.addItem(str);
someoneIsThere = true;
}
}
privateMessageInput.setEditable(someoneIsThere);
privateMessageInput.setEnabled(someoneIsThere);
sendPrivateButton.setEnabled(someoneIsThere);
if (!someoneIsThere)
clientList.addItem("(no one available)");
}
});
}

/**
* Defines the object that handles all ActionEvents for the program.
*/
private class ActionHandler implements ActionListener {
public void actionPerformed(ActionEvent evt) {
Object src = evt.getSource();
if (src == quitButton) {  // Disconnect from the server and end the program.
doQuit();
}
else if (src == sendButton || src == messageInput) {
// Send the string entered by the user as a message
// to the Hub, using the ChatClient that handles communication
// for this ChatRoomWindow.  Note that the string is not added
// to the transcript here.  It will get added after the Hub
// receives the message and broadcasts it to all clients,
// including this one.
String message = messageInput.getText();
if (message.trim().length() == 0)
return;
connection.send(message);
messageInput.selectAll();
messageInput.requestFocus();
}
else if (src == sendPrivateButton || src == privateMessageInput) {
// Send a private message to a specified recipient.
// If the private message inputbox is empty, nothing is done.
String message = privateMessageInput.getText();
if (message.trim().length() == 0)
return;
String recipient = (String)clientList.getSelectedItem(); // name of recipient.
int recipientID = -1;  // The ID number of the recipient
for (int id : clientNameMap.keySet()) {
// Search the clientNameMap to find the ID number
// corresponding to the specified recipient name.
if (recipient.equals(clientNameMap.get(id))) {
recipientID = id;
break;
}
}
if (recipientID == -1) {
JOptionPane.showMessageDialog(NewChatRoomWindow.this,
"Funny... The selected recipient\ndoesn't seem to exit???");
return;
}
connection.send(new PrivateMessage(recipientID,message));
addToTranscript("Sent to " + recipient + ":  " + message);
}
}
}

}

Solution for Programming Exercise 13.1

Exercise 13.1:

The sample program PaintWithOffScreenCanvas.java from Section 13.1 is a simple paint program. Make two improvements to this program: First, add a "File" menu that lets the user open an image file and save the current image in either PNG or JPEG format. Second, add a basic one-level "Undo" command that lets the user undo the most recent operation that was applied to the image. (Do not try to make a multilevel Undo, which would allow the user to undo several operations.)

When you read a file into the program, you should copy the image that you read into the program's off-screen canvas. Since the canvas in the program has a fixed size, you should scale the image that you read so that it exactly fills the canvas.

Discussion

The new menu commands must be added to the menu bar in the getMenuBar() method, and they must be implemented in the actionPerformed() method inside the nested class MenuHandler. I won't discuss any further how to do this. See Subsection 6.7.1 for a discussion of menus and menubars. I used two "Save" commands in the "File" menu, "Save PNG File" and "Save JPEG File", for saving the image in the two possible formats. Since there didn't seem to be a good place in the existing menus for an "Undo" command, I added an "Edit" menu and put the "Undo" command there. I also moved the "Fill With Color" command from the "Color" menu to the "Edit" menu (so that the "Undo" command would not be so lonely). Finally, I also added a "Quit" command to the "File" menu since most users would probably look for it there.

The "File" commands are easy to implement. A method for saving a BufferedImage to a file is given at the end of Section 13.1. That doSaveFile() method can be used almost directly. The doSaveFile() method has a parameter, format, that specifies the image format for the file. When the user selects the "Save PNG File" command, I call doSaveFile("PNG"); when the user selects "Save JPEG File", I call doSaveFile("JPEG").

Subsection 13.1.5 does not give a complete method for reading an image from a user-selected file, but it does describe the ImageIO.read() method, which can be used to read the image after the file has been selected. The outline for a general file output routine, readFile(), can be found in Subsection 11.2.3; we just have to add the code for reading an image from a selected file. I chose to use the version of ImageIO.read() that takes an InputStream as a parameter.

The input stream is a FileInputStream that is created to read from the file selected by the user, using:

stream = new FileInputStream(selectedFile);

A BufferedImage is then created by reading from the stream with ImageIO.read(stream). If the selected file is not an image file, then the return value of this method is null:

BufferedImage image = ImageIO.read(stream);
if (image == null)
throw new Exception("File does not contain a recognized image format.");

Finally, if there is no error, the image is copied onto the program's off-screen canvas, OSC:

Graphics g = OSC.createGraphics();
g.drawImage(image,0,0,OSC.getWidth(),OSC.getHeight(),null);
g.dispose();
repaint(); // Repaint the panel to show the new image

The version of g.drawImage() that is used here specifies the height and width of the rectangle in which the image is to be drawn. OSC.getWidth() and OSC.getHeight() are used as the width and height to specify that the image should exactly fill the entire off-screen canvas. For the complete doOpenFile() method, look near the end of the solution that is given below.

For the "Undo" command, we need to keep an extra copy of the image, and we need an extra off-screen canvas for this. I use a BufferedImage named imageSavedForUndo. This buffered image is created at the same time as the main off-screen canvas, OSC, and is the same size. Just before performing any operation that will change the image, a copy of the current image is copied from OSC to imageSavedForUndo. Since this will have to be done at several points in the program, I wrote a little routine to do it:

private void saveUndoImage() {
Graphics g = imageSavedForUndo.createGraphics();
g.drawImage(OSC,0,0,null);
g.dispose();
}

This method is called in the mousePressed() routine to save a copy of the image before it is modified by the user's mouse action; this allows the program to undo basic drawing operations. It is called in the actionPerformed() method just before carrying out the "Fill With Color" command; this makes it possible to restore the image that is about to be erased with the fill command. And it is called in the doOpenFile() command before writing the image from the file to OSC; this makes it possible to restore the previous image.

To implement the "Undo" command itself, we could just copy the image from imageSavedForUndo into OSC. However, think about the question of what happens when a second "Undo" command is given just after another "Undo" command. What should the second "Undo" do? If an "Undo" command just copies imageSavedForUndo to OSC, then the second "Undo" will have no visible effect, since it will just copy the same image as the first "Undo". A better alternative would be to make the second "Undo" command "undo the undo." That is, the second "Undo" acts as a "Redo" that restores the image to state that it was in before the first "Undo". We can implement this by swapping the images in OSC and imageSavedForUndo in response to an "Undo" command, instead of simply copying the image from OSC to imageSavedForUndo. That way, we keep both images around, and a second swap operation will restore both images to the state they were in before the first swap. To swap the images, we just have to swap the values of the variables OSC and imageSavedForUndo. So, the code that implements the "Undo" command is:

BufferedImage temp = OSC;
OSC = imageSavedForUndo;
imageSavedForUndo = temp;
repaint();  // Show the new OSC on the screen.

The Solution

Significant changes from PaintWithOffScreenCanvas.java are shown in red italic:

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.imageio.ImageIO;
import java.awt.image.BufferedImage;
import java.io.*;
import java.util.EnumSet;

/**
* A simple paint program with File and undo commands.
* Note that the way that the off-screen canvas is used
* in this class requires that the panel be non-resizable; this
* is because the size of the off-screen canvas does not change
* when the panel changes size.
*/
public class PaintWithFiles extends JPanel {

/**
* The main routine simply opens a window that shows a PaintWithFiles panel.
*/
public static void main(String[] args) {
JFrame window = new JFrame("PaintWithFiles");
PaintWithFiles content = new PaintWithFiles();
window.setContentPane(content);
window.setJMenuBar(content.getMenuBar());
window.pack();
window.setResizable(false);
Dimension screenSize = Toolkit.getDefaultToolkit().getScreenSize();
window.setLocation( (screenSize.width - window.getWidth())/2,
(screenSize.height - window.getHeight())/2 );
window.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
window.setVisible(true);
}

/**
* The possible drawing tools in this program.  (The CURVE tool allows the
* user to sketch a free-hand curve, while the LINE tool draws a line
* between two points.  The SMUDGE tool lets the user "spread paint around"
* with the mouse.  The ERASE tool erases with a 10-by-10 pixel rectangle.)
*/
private enum Tool { CURVE, LINE, RECT, OVAL, FILLED_RECT, FILLED_OVAL, SMUDGE, ERASE }

/**
* The set of Tools that represent "shapes."  Shapes are handled differently
* during dragging than other tools, since they are drawn "on top of" the
* current picture during a mouse drag and are only added permanently to the
* picture on mouse release.
*/
private final static EnumSet<Tool> SHAPE_TOOLS = EnumSet.range(Tool.LINE, Tool.FILLED_OVAL);

/**
* The currently selected drawing tool.  Initially Tool.CURVE.  Can be
* changed by the user with commands in the "Tool" menu.
*/
private Tool currentTool = Tool.CURVE;

/**
* The current drawing color.  Initially Color.BLACK.  Can be changed
* by the user with the "Select Drawing Color" command in the "Color" menu.
*/
private Color currentColor = Color.BLACK;

/**
* The background color that is used to fill the off-screen canvas when
* it is created.  If the user selects the "Fill With Color", the fill
* color changes, and the canvas is filled with the new fill color,
* erasing whatever was there before.
*/
private Color fillColor = Color.WHITE;

/**
* The off-screen canvas.  This is not created until the first time
* paintComponent() is called.  If the size of the component changes,
* a new OSC is created (and the picture in the old one is lost).
*/
private BufferedImage OSC;

/**
* This is set to true when the user is dragging the mouse.
*/
private boolean dragging;

/**
* The start position of the mouse during a mouse drag.
*/
private int startX, startY;

/**
* The current position of the mouse during a mouse drag.
*/
private int currentX, currentY;

/**
* File dialog for implementing the File menu commands.
*/
private JFileChooser fileDialog;

/**
* An extra copy of the image that is used for the Undo command.
* Before any change is made to the image, the image is copied
* to this variable.  The Undo command swaps the saved image
* with the image that is shown on screen.  (Thus, a second Undo
* will effectively act as a Redo.)
*/
private BufferedImage imageSavedForUndo;

/**
* The constructor sets the preferred size of the panel to
* 640-by-480.  It also sets up a mouse listener.
*/
public PaintWithFiles() {
setPreferredSize(new Dimension(640,480));
MouseHandler mouseHandler = new MouseHandler();
addMouseListener(mouseHandler);
addMouseMotionListener(mouseHandler);
}

/**
* The paintComponent() method copies the off-screen canvas to the screen
* (first creating it, if necessary).  If a mouse drag is in progress,
* then the current tool is not Tool.CURVE, then the shape that the user
* is drawing is drawn over the off-screen canvas.  (This is to avoid
* making the shape a permanent part of the picture until after the user
* releases the mouse.  The effect is a "rubber band cursor" in which
* the shape changes as the user drags the mouse, but the picture under
* the shape is not affected.)
*/
public void paintComponent(Graphics g) {

/* First create the off-screen canvas, if it does not already exist. */

if (OSC == null)
createOSC();

/* Copy the off-screen canvas to the panel.  Since we know that the
image is already completely available, the fourth "ImageObserver"
parameter to g.drawImage() can be null.  Since the canvas completely
fills the panel, there is no need to call super.paintComponent(g). */

g.drawImage(OSC,0,0,null);

/* If the user is currently dragging the mouse to draw a line, oval,
or rectangle, draw the shape on top the image from the off-screen
canvas, using the current drawing color.  (This is not done if the
user is drawing a curve or using the smudge tool.) */

if (dragging && SHAPE_TOOLS.contains(currentTool)) {
g.setColor(currentColor);
putCurrentShape(g);
}

}

/**
* This method creates the off-screen canvas and fills it with the current
* fill color.  The image that is used to implement the Undo command is
* also created here.
*/
private void createOSC() {
OSC = new BufferedImage(getWidth(),getHeight(),BufferedImage.TYPE_INT_RGB);
imageSavedForUndo = new BufferedImage(getWidth(),getHeight(),BufferedImage.TYPE_INT_RGB);
Graphics osg = OSC.createGraphics();
osg.setColor(fillColor);
osg.fillRect(0,0,getWidth(),getHeight());
osg.dispose();
saveUndoImage();
}

/**
* Copies the current image into the image that is saved for the Undo
* command.  This method is called before any change is made to the image.
*/
private void saveUndoImage() {
Graphics g = imageSavedForUndo.createGraphics();
g.drawImage(OSC,0,0,null);
g.dispose();
}

/**
* A utility method to draw the current shape in a given graphics context.
* It draws the correct shape for the current tool in a rectangle whose
* corners are given by the starting position of the mouse and the current
* position of the mouse.  This is used by paintComponent() during a
* mouse drag.  It is also used to make the shape a permanent part of the
* off-screen canvas when the mouse is released.
* This method is not used when the current tool is Tool.CURVE or Tool.SMUDGE.
*/
private void putCurrentShape(Graphics g) {
switch (currentTool) {
case LINE:
g.drawLine(startX, startY, currentX, currentY);
break;
case OVAL:
putOval(g,false,startX, startY, currentX, currentY);
break;
case RECT:
putRect(g,false,startX, startY, currentX, currentY);
break;
case FILLED_OVAL:
putOval(g,true,startX, startY, currentX, currentY);
break;
case FILLED_RECT:
putRect(g,true,startX, startY, currentX, currentY);
break;
}
}

/**
* Draws a filled or unfilled rectangle with corners at the points (x1,y1)
* and (x2,y2).  Nothing is drawn if x1 == x2 or y1 == y2.
* @param g the graphics context where the rectangle is drawn
* @param filled tells whether to draw a filled or unfilled rectangle.
*/
private void putRect(Graphics g, boolean filled, int x1, int y1, int x2, int y2) {
if (x1 == x2 || y1 == y2)
return;
if (x2 < x1) {  // Swap x1,x2 if necessary to make x2 > x1.
int temp = x1;
x1 = x2;
x2 = temp;
}
if (y2 < y1) {  // Swap y1,y2 if necessary to make y2 > y1.
int temp = y1;
y1 = y2;
y2 = temp;
}
if (filled)
g.fillRect(x1,y1,x2-x1,y2-y1);
else
g.drawRect(x1,y1,x2-x1,y2-y1);
}

/**
* Draws a filled or unfilled oval in the rectangle with corners at the
* points (x1,y1) and (x2,y2).  Nothing is drawn if x1 == x2 or y1 == y2.
* @param g the graphics context where the oval is drawn
* @param filled tells whether to draw a filled or unfilled oval.
*/
private void putOval(Graphics g, boolean filled, int x1, int y1, int x2, int y2) {
if (x1 == x2 || y1 == y2)
return;
if (x2 < x1) {  // Swap x1,x2 if necessary to make x2 > x1.
int temp = x1;
x1 = x2;
x2 = temp;
}
if (y2 < y1) {  // Swap y1,y2 if necessary to make y2 > y1.
int temp = y1;
y1 = y2;
y2 = temp;
}
if (filled)
g.fillOval(x1,y1,x2-x1,y2-y1);
else
g.drawOval(x1,y1,x2-x1,y2-y1);
}

/**
* Calls the repaint() method of this panel for the rectangle with corners
* at the points (x1,y1) and (x2,y2).  An extra one-pixel border is added
* to the area that is repainted; this allows for the size of the "pen"
* that is used to draw lines and unfilled ovals and rectangles.
*/
private void repaintRect(int x1, int y1, int x2, int y2) {
if (x2 < x1) {  // Swap x1,x2 if necessary to make x2 >= x1.
int temp = x1;
x1 = x2;
x2 = temp;
}
if (y2 < y1) {  // Swap y1,y2 if necessary to make y2 >= y1.
int temp = y1;
y1 = y2;
y2 = temp;
}
x1--;
x2++;
y1--;
y2++;
repaint(x1,y1,x2-x1,y2-y1);
}

/**
* Creates a menu bar for use with this panel, with "File", "Edit",
* "Color", and "Tool" menus.
*/
public JMenuBar getMenuBar() {
JMenuBar menubar = new JMenuBar();
JMenu fileMenu = new JMenu("File");
JMenu editMenu = new JMenu("Edit");
JMenu colorMenu = new JMenu("Color");
JMenu toolMenu = new JMenu("Tool");
menubar.add(fileMenu);
menubar.add(editMenu);
menubar.add(colorMenu);
menubar.add(toolMenu);
ActionListener listener = new MenuHandler();
JMenuItem item;
item = new JMenuItem("Open Image File...");
item.addActionListener(listener);
fileMenu.add(item);
item = new JMenuItem("Save PNG File...");
item.addActionListener(listener);
fileMenu.add(item);
item = new JMenuItem("Save JPEG File...");
item.addActionListener(listener);
fileMenu.add(item);
fileMenu.addSeparator();
item = new JMenuItem("Quit");
item.addActionListener(listener);
fileMenu.add(item);
item = new JMenuItem("Undo");
item.addActionListener(listener);
editMenu.add(item);
editMenu.addSeparator();
item = new JMenuItem("Fill With Color...");
item.addActionListener(listener);
editMenu.add(item);
item = new JMenuItem("Draw With White");
item.addActionListener(listener);
colorMenu.add(item);
item = new JMenuItem("Draw With Red");
item.addActionListener(listener);
colorMenu.add(item);
item = new JMenuItem("Draw With Green");
item.addActionListener(listener);
colorMenu.add(item);
item = new JMenuItem("Draw With Blue");
item.addActionListener(listener);
colorMenu.add(item);
item = new JMenuItem("Draw With Yellow");
item.addActionListener(listener);
colorMenu.add(item);
item = new JMenuItem("Select Drawing Color...");
item.addActionListener(listener);
colorMenu.add(item);
item = new JMenuItem("Curve");
item.addActionListener(listener);
toolMenu.add(item);
toolMenu.addSeparator();
item = new JMenuItem("Line");
item.addActionListener(listener);
toolMenu.add(item);
item = new JMenuItem("Rectangle");
item.addActionListener(listener);
toolMenu.add(item);
item = new JMenuItem("Oval");
item.addActionListener(listener);
toolMenu.add(item);
item = new JMenuItem("Filled Rectangle");
item.addActionListener(listener);
toolMenu.add(item);
item = new JMenuItem("Filled Oval");
item.addActionListener(listener);
toolMenu.add(item);
toolMenu.addSeparator();
item = new JMenuItem("Smudge");
item.addActionListener(listener);
toolMenu.add(item);
item = new JMenuItem("Erase");
item.addActionListener(listener);
toolMenu.add(item);
return menubar;
}

/**
* This nested class defines the ActionListener that responds when the
* user selects a command from one of the menus.  It is used in the
* getMenuBar() method.
*/
private class MenuHandler implements ActionListener {
public void actionPerformed(ActionEvent evt) {
String command = evt.getActionCommand();
if (command.equals("Select Drawing Color...")) {
Color newColor = JColorChooser.showDialog(PaintWithFiles.this,
"Select Drawing Color", currentColor);
if (newColor != null)
currentColor = newColor;
}
else if (command.equals("Fill With Color...")) {
Color newColor = JColorChooser.showDialog(PaintWithFiles.this,
"Select Fill Color", fillColor);
if (newColor != null) {
fillColor = newColor;
saveUndoImage();
Graphics osg = OSC.createGraphics();
osg.setColor(fillColor);
osg.fillRect(0,0,OSC.getWidth(),OSC.getHeight());
osg.dispose();
PaintWithFiles.this.repaint();
}
}
else if (command.equals("Draw With Black"))
currentColor = Color.BLACK;
else if (command.equals("Draw With White"))
currentColor = Color.WHITE;
else if (command.equals("Draw With Red"))
currentColor = Color.RED;
else if (command.equals("Draw With Green"))
currentColor = Color.GREEN;
else if (command.equals("Draw With Blue"))
currentColor = Color.BLUE;
else if (command.equals("Draw With Yellow"))
currentColor = Color.YELLOW;
else if (command.equals("Curve"))
currentTool = Tool.CURVE;
else if (command.equals("Line"))
currentTool = Tool.LINE;
else if (command.equals("Rectangle"))
currentTool = Tool.RECT;
else if (command.equals("Oval"))
currentTool = Tool.OVAL;
else if (command.equals("Filled Rectangle"))
currentTool = Tool.FILLED_RECT;
else if (command.equals("Filled Oval"))
currentTool = Tool.FILLED_OVAL;
else if (command.equals("Smudge"))
currentTool = Tool.SMUDGE;
else if (command.equals("Erase"))
currentTool = Tool.ERASE;
else if (command.equals("Open Image File..."))
doOpenFile();
else if (command.equals("Save PNG File..."))
doSaveFile("PNG");
else if (command.equals("Save JPEG File..."))
doSaveFile("JPEG");
else if (command.equals("Quit"))
System.exit(0);
else if (command.equals("Undo")) {
BufferedImage temp = OSC;
OSC = imageSavedForUndo;
imageSavedForUndo = temp;
repaint();
}
}
} // end nested class MenuHandler

/**
* This nested class defines the object that listens for mouse and
* mouse motion events on the panel.  It is used in the constructor.
*/
private class MouseHandler implements MouseListener, MouseMotionListener {

int prevX, prevY;  // Previous position of mouse during a drag.

double[][] smudgeRed, smudgeGreen, smudgeBlue;  // data for smudge tool

/**
* When the ERASE or SMUDGE tools are used and the mouse jumps
* from (x1,y1) to (x2,y2), the tool has to be applied to a
* line of pixel positions between the two points in order to
* cover the entire line that the mouse moves along.  The change
* is made to the off-screen canvas, and repaint() is called to
* copy the changes to the screen.
*/
void applyToolAlongLine(int x1, int y1, int x2, int y2) {
Graphics g = OSC.createGraphics();
g.setColor(fillColor);    // (for ERASE only)
int w = OSC.getWidth();   // (for SMUDGE only)
int h = OSC.getHeight();  // (for SMUDGE only)
int dist = Math.max(Math.abs(x2-x1),Math.abs(y2-y1));
// dist is the number of points along the line from
// (x1,y1) to (x2,y2) at which the tool will be applied.
double dx = (double)(x2-x1)/dist;
double dy = (double)(y2-y1)/dist;
for (int d = 1; d <= dist; d++) {
// Apply the tool at one of the points (x,y) along the
// line from (x1,y1) to (x2,y2).
int x = (int)Math.round(x1 + dx*d);
int y = (int)Math.round(y1 + dy*d);
if (currentTool == Tool.ERASE) {
// Erase a 10-by-10 block of pixels around (x,y)
g.fillRect(x-5,y-5,10,10);
repaint(x-5,y-5,10,10);
}
else {
// For the SMUDGE tool, blend some of the color from
// the smudgeRed, smudgeGreen, and smudgeBlue arrays
// into the pixels in a 7-by-7 block around (x,y), and
// vice versa.  The effect is to smear out the color
// of pixels that are visited by the tool.
for (int i = 0; i < 7; i++)
for (int j = 0; j < 7; j++) {
int r = y + j - 3;
int c = x + i - 3;
if (!(r < 0 || r >= h || c < 0 || c >= w || smudgeRed[i][j] == -1)) {
int curCol = OSC.getRGB(c,r);
int curRed = (curCol >> 16) & 0xFF;
int curGreen = (curCol >> 8) & 0xFF;
int curBlue = curCol & 0xFF;
int newRed = (int)(curRed*0.7 + smudgeRed[i][j]*0.3);
int newGreen = (int)(curGreen*0.7 + smudgeGreen[i][j]*0.3);
int newBlue = (int)(curBlue*0.7 + smudgeBlue[i][j]*0.3);
int newCol = newRed << 16 | newGreen << 8 | newBlue;
OSC.setRGB(c,r,newCol);
smudgeRed[i][j] = curRed*0.3 + smudgeRed[i][j]*0.7;
smudgeGreen[i][j] = curGreen*0.3 + smudgeGreen[i][j]*0.7;
smudgeBlue[i][j] = curBlue*0.3 + smudgeBlue[i][j]*0.7;
}
}
repaint(x-3,y-3,7,7);
}
}
g.dispose();
}

/**
* Start a drag operation.
*/
public void mousePressed(MouseEvent evt) {
startX = prevX = currentX = evt.getX();
startY = prevY = currentY = evt.getY();
dragging = true;
saveUndoImage();
if (currentTool == Tool.ERASE) {
// Erase a 10-by-10 block around the starting mouse position.
Graphics g = OSC.createGraphics();
g.setColor(fillColor);
g.fillRect(startX-5,startY-5,10,10);
g.dispose();
repaint(startX-5,startY-5,10,10);
}
else if (currentTool == Tool.SMUDGE) {
// Record the colors in a 7-by-7 block of pixels around the
// starting mouse position into the arrays smudgeRed,
// smudgeGreen, and smudgeBlue.  These arrays hold the
// red, green, and blue components of the colors.
if (smudgeRed == null) {
// Create the arrays, if they have not already been created.
smudgeRed = new double[7][7];
smudgeGreen = new double[7][7];
smudgeBlue = new double[7][7];
}
int w = OSC.getWidth();
int h = OSC.getHeight();
int x = evt.getX();
int y = evt.getY();
for (int i = 0; i < 7; i++)
for (int j = 0; j < 7; j++) {
int r = y + j - 3;
int c = x + i - 3;
if (r < 0 || r >= h || c < 0 || c >= w) {
// A -1 in the smudgeRed array indicates that the
// corresponding pixel was outside the canvas.
smudgeRed[i][j] = -1;
}
else {
int color = OSC.getRGB(c,r);
smudgeRed[i][j] = (color >> 16) & 0xFF;
smudgeGreen[i][j] = (color >> 8) & 0xFF;
smudgeBlue[i][j] = color & 0xFF;
}
}
}
}

/**
* Continue a drag operation when the user drags the mouse.
* For the CURVE tool, a line is drawn from the previous mouse
* position to the current mouse position in the off-screen canvas,
* and the repaint() method is called for a rectangle that contains
* the line segment that was drawn.  For shape tools, the off-screen
* canvas is not changed, but the repaint() method is called so
* that the paintComponent() method can redraw the picture with
* the user's shape in the new position.  For the SMUDGE and
* ERASE tools, the tool is applied along a line from the previous
* mouse position to the current position;
*/
public void mouseDragged(MouseEvent evt) {
currentX = evt.getX();
currentY = evt.getY();
if (currentTool == Tool.CURVE) {
Graphics g = OSC.createGraphics();
g.setColor(currentColor);
g.drawLine(prevX,prevY,currentX,currentY);
g.dispose();
repaintRect(prevX,prevY,currentX,currentY);
}
else if (SHAPE_TOOLS.contains(currentTool)) {
// Repaint the rectangles occupied by the previous position of
// the shape and by its current position.
repaintRect(startX,startY,prevX,prevY);
repaintRect(startX,startY,currentX,currentY);
}
else {
// Tool has to be ERASE or SMUDGE
applyToolAlongLine(prevX,prevY,currentX,currentY);
}
prevX = currentX;
prevY = currentY;
}

/**
* Finish a mouse drag operation.  Nothing is done unless the current tool
* is a shape tool.  For shape tools, the user's shape is drawn to the
* off-screen canvas, making it a permanent part of the picture, and
* then the repaint() method is called to show the modified picture
* on the screen.
*/
public void mouseReleased(MouseEvent evt) {
dragging = false;
if (SHAPE_TOOLS.contains(currentTool)) {
Graphics g = OSC.createGraphics();
g.setColor(currentColor);
putCurrentShape(g);
g.dispose();
repaint();
}
}

public void mouseMoved(MouseEvent evt) { }
public void mouseClicked(MouseEvent evt) { }
public void mouseEntered(MouseEvent evt) { }
public void mouseExited(MouseEvent evt) { }

} // end nested class MenuHandler

/**
* Opens an image file selected by the user.  If the image is read
* successfully, it replaces the image in the off-screen canvas.
* (The new image is scaled to fit the canvas size exactly.)
*/
private void doOpenFile() {
if (fileDialog == null)
fileDialog = new JFileChooser();
fileDialog.setDialogTitle("Select File to be Opened");
fileDialog.setSelectedFile(null);  // No file is initially selected.
int option = fileDialog.showOpenDialog(this);
if (option != JFileChooser.APPROVE_OPTION)
return;  // User canceled or clicked the dialog's close box.
File selectedFile = fileDialog.getSelectedFile();
FileInputStream stream;
try {
stream = new FileInputStream(selectedFile);
}
catch (Exception e) {
JOptionPane.showMessageDialog(this,
"Sorry, but an error occurred while trying to open the file:\n" + e);
return;
}
try {
BufferedImage image = ImageIO.read(stream);
if (image == null)
throw new Exception("File does not contain a recognized image format.");
saveUndoImage();
Graphics g = OSC.createGraphics();
g.drawImage(image,0,0,OSC.getWidth(),OSC.getHeight(),null);
g.dispose();
repaint();
}
catch (Exception e) {
JOptionPane.showMessageDialog(this,
"Sorry, but an error occurred while trying to read the image:\n" + e);
}
}

/**
* Attempts to save the current image to a file selected by the user.
* @param format the format of the image, either "PNG" or "JPEG" in this program
*/
private void doSaveFile(String format) {
if (fileDialog == null)
fileDialog = new JFileChooser();
fileDialog.setSelectedFile(new File("image." + format.toLowerCase()));
fileDialog.setDialogTitle("Select File to be Saved");
int option = fileDialog.showSaveDialog(null);
if (option != JFileChooser.APPROVE_OPTION)
return;  // User canceled or clicked the dialog's close box.
File selectedFile = fileDialog.getSelectedFile();
if (selectedFile.exists()) {  // Ask the user whether to replace the file.
int response = JOptionPane.showConfirmDialog( null,
"The file \"" + selectedFile.getName()
+ "\" already exists.\nDo you want to replace it?",
"Confirm Save",
JOptionPane.YES_NO_OPTION,
JOptionPane.WARNING_MESSAGE );
if (response != JOptionPane.YES_OPTION)
return;  // User does not want to replace the file.
}
try {
boolean hasFormat = ImageIO.write(OSC,format,selectedFile);
if ( ! hasFormat )
throw new Exception(format + " format is not available.");
}
catch (Exception e) {
System.out.println("Sorry, but an error occurred while trying to save the image.");
e.printStackTrace();
}
}

} // end class PaintWithFiles

Solution for Programming Exercise 13.2

Exercise 13.2:

For this exercise, you should continue to work on the program from the previous exercise. Add a "StrokeWidth" menu that allows the user to draw lines of varying thicknesses. Make it possible to use different colors for the interior of a filled shape and for the outline of that shape. To do this, change the "Color" menu to "StrokeColor" and add a "FillColor" menu. (My solution adds two new tools, "Stroked Filled Rectangle" and "Stroked Filled Oval", to represent filled shapes that are outlined with the current stroke.) Add support for filling shapes with transparent color. A simple approach to this is to use a JCheckboxMenuItem to select either fully opaque or 50% opaque fill. (Don't try to apply transparency to stokes -- it's very difficult to make transparency work correctly for the Curve tool, and in any case, shape outlines look better if they are opaque.) Finally, make the menus more user friendly by adding keyboard accelerators to some commands and by using JRadioButtonMenuItems where appropriate, such as in the color and tool menus. This exercise takes quite a bit of work to get it all right, so you should tackle the problem in pieces.

Discussion

Most of the individual things that you have to do for this exercise are not very difficult, but there are a lot of them. First of all, although it was not required by the project, I decided to use antialiasing, since it produces nicer pictures. So, before drawing in a graphics context g, I say

Graphics2D g2 = (Graphics2D)g;
g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
RenderingHints.VALUE_ANTIALIAS_ON);

I added these lines at several points in the program, both for the graphics context in the paintComponent() method and for graphics contexts for drawing in the off-screen canvas.

There are a lot of changes in the getMenuBar() method, which creates the menus that are used in the program. First of all, I changed the colorMenu to strokeColorMenu, and I added two new menus named strokeWidthMenu and fillColorMenu. The contents of the strokeColorMenu are the same as the old colorMenu, but the items in that menu have changed from simple JMenuItems to JRadioButtonMenuItems. All the items are added to the same ButtonGroup so that only one of the items in the menu can be selected. The only real point of this is to give the user some visual feedback about which color is currently selected in the menu -- this is just the sort of touch that can make a program much more usable. Items could be added to the menu as follows:

ButtonGroup group = new ButtonGroup();  // Will contain all the radio buttons.

JRadioButtonMenuItem radioItem;
radioItem = new JRadioButtonMenuItem("Draw With Black");
strokeColorMenu.add( radioItem );
group.add( radioItem );
radioItem.setSelected( true );  // Black is currently selected.
radioItem.addActionListener( listener );

JRadioButtonMenuItem radioItem;
radioItem = new JRadioButtonMenuItem("Draw With White");
strokeColorMenu.add( radioItem );
group.add( radioItem );
radioItem.addActionListener( listener );
.
.
.

However, having so much repetitive code becomes annoying, so I decided to write a method to do the work. The method makes an entire set of JRadioButtonMenuItems and adds them to a menu. I pass the names of all the menu items to the method as an array of Strings. My first version of this method looked like this:

/**
* Adds a set of JRadioButtonMenuItems to a JMenu.  All the items are in
* the same button group and have the same ActionListener.  Initially,
* the first item is selected.
* @param menu  the menu to which the items will be added
* @param commandNames  the names of the items
* @param listener  the ActionListener for the items
*/
private void createRadioGroup(JMenu menu, String[] commandNames,
ActionListener listener) {
ButtonGroup group = new ButtonGroup();
for ( int i = 0; i < commandNames.length; i++ )  {
JRadioButtonMenuItem item = new JRadioButtonMenuItem(commandNames[i]);
menu.add(item);
group.add(item);
if ( i == 0 ) // First item is the one that is currently selected.
item.setSelected(true);
}
}

With this method available, I could fill the strokeColorMenu with one command:

createRadioGroup( strokeColorMenu,
new String[] {
"Draw With Black", "Draw With White", "Draw With Red",
"Draw With Green", "Draw With Blue", "Draw With Yellow",
"Custom Drawing Color..."
}, listener);

I also used createRadioGroup() to add entries to fillColorMenu and strokeWidthMenu. When I came to the toolMenu, I wanted to add separators between some of the menu items, but the createRadioGroup() method just adds all the items to the menu, without separators. To solve this problem, I came up with the idea of using a null value in the array of commandNames to indicate a position where a separator should be inserted instead of a menu item.

It was not until the end of the project that I added accelerator keys to the menus. Once again, my createRadioGroup() caused a problem, since it provided no way to add accelerator keys to the menu items that it creates. At first, this stumped me, and I thought I might have to go back to creating individual menu items. However, I came up with the idea of coding the accelerator key into the command name in the commandName array. For example, to specify the accelerator keystroke "ctrl L" for the "Line" command, I used "Line/ctrl L" in the array. The final version of the createRadioGroup() method became:

/**
* Adds a set of JRadioButtonMenuItems to a JMenu.  All the items are in
* the same button group and have the same ActionListener.
* @param menu  the menu to which the items will be added
* @param commandNames  the names of the items; null values in this array
*    become separators in the menu.  The name can contain the character "/".
*    In that case the actual item name is the part of the name that
*    precedes the "/" and the remainder of the name specifies a
*    KeyStroke that will be used as the keyboard accelerator key
*    for the item.
* @param listener  the ActionListener for the items
*/
private void createRadioGroup(JMenu menu, String[] commandNames,
ActionListener listener) {
ButtonGroup group = new ButtonGroup();
for ( int i = 0; i < commandNames.length; i++) {
if (commandNames[i] == null)
menu.addSeparator();
else {
String name = commandNames[i];
String accel = null;
if (name.indexOf("/") > 0) {
int pos = name.indexOf("/");  // Extract accelerator key and name.
accel = name.substring(pos+1);
name = name.substring(0,pos);
}
JRadioButtonMenuItem item = new JRadioButtonMenuItem(name);
menu.add(item);
group.add(item);
item.addActionListener(listener);
if (accel != null)
item.setAccelerator(KeyStroke.getKeyStroke(accel));
if (i == 0)
item.setSelected(true);
}
}
}

With this method available, I could fill the "Tool" menu with:

createRadioGroup( toolMenu,
new String[] {
"Curve/ctrl U", null, "Line/ctrl L", "Rectangle/ctrl R", "Oval/ctrl V",
"Filled Rectangle/ctrl alt R", "Filled Oval/ctrl alt V",
"Stroked Filled Rectangle/ctrl shift R", "Stroked Filled Oval/ctrl shift V",
null, "Smudge/ctrl M", "Erase/ctrl E"
}, listener);

The only other menu command that remains to be added is the one that controls transparency. For that, I needed a JCheckBoxMenuItem. Whenever a filled shape is drawn, the setting of this item has to be checked to determine whether or not to use a transparent color. Since I need to have the item available for use in more than one method, it's an instance variable in the class:

private JCheckBoxMenuItem transparent = new JCheckBoxMenuItem("Translucent Fill");

I decided to add this item to the end of the "FillColor" menu, since it only affects colors that are used for filling shapes.

The new menu commands had to be implemented in the actionPerformed() method that responds when the user selects a menu command. This was straightforward. I did have to introduce a new instance variable to represent the fill color, since previously there was only a single color, used for both filling and drawing. Note that no response is programmed for the JCheckBoxMenuItem, transparent, since the program can check the status of the checkbox itself whenever it needs to know the status.

Turning to the implementation of all the new menu commands, consider the problem of setting the properties of a graphics context before drawing. Previously, it was just a matter of setting a color. Now, there is the possibility of using transparency (when filling a shape) or using a non-default BasicStroke (when drawing a line or outlining a shape). To make it easier to take these possibilities into account, I wrote the following two methods:

/**
* Set the graphics context g to use the current stroke color and
* lineWidth.
*/
private void applyStrokeProperties(Graphics g) {
if (lineWidth > 1) {
Graphics2D g2 = (Graphics2D)g;
g2.setStroke( new BasicStroke(lineWidth,
BasicStroke.CAP_ROUND, BasicStroke.JOIN_MITER) );
}
g.setColor(currentStrokeColor);
}

/**
* Set the graphics context g to use the current fill color, taking
* into account the setting of transparency.  If transparency is on,
* the RGB components are obtained from the current fill color, and
* the alpha component is set to 125.
*/
private void applyFillColor(Graphics g) {
Color c = currentFillColor;
if (transparent.isSelected())
g.setColor(new Color( c.getRed(), c.getGreen(), c.getBlue(), 125 ));
else
g.setColor(c);
}

I used BasicStroke.CAP_ROUND for the BasicStroke because the default cap gave really ugly results when drawing a thick curve. Note that to implement transparency, I just use a transparent version of the current color. The currentColor variable always holds a fully opaque color.

Drawing ovals and rectangles has become more complicated, since there are now three different possibilities for each shape: fill only, outline only, or both fill and outline. To implement this, I modified the putRect() method to have two boolean parameters, one to tell whether to fill the rectangle and one to tell whether to outline it:

/**
* Draws a rectangle with corners at the points (x1,y1)
* and (x2,y2).  Nothing is drawn if x1 == x2 or y1 == y2.  The rectangle
* can be either stroked (that is, the outline is drawn) or filled or both.
* @param g the graphics context where the rectangle is drawn
* @param stroked tells whether to draw an outline of the rectangle.
* @param filled tells whether to draw a filled or unfilled rectangle.
*/
private void putRect(Graphics g, boolean stroked, boolean filled,
int x1, int y1, int x2, int y2) {
if (x1 == x2 || y1 == y2)
return;
if (x2 < x1) {  // Swap x1,x2 if necessary to make x2 > x1.
int temp = x1;
x1 = x2;
x2 = temp;
}
if (y2 < y1) {  // Swap y1,y2 if necessary to make y2 > y1.
int temp = y1;
y1 = y2;
y2 = temp;
}
if (filled) {
applyFillColor(g);
g.fillRect(x1,y1,x2-x1,y2-y1);
}
if (stroked) {
applyStrokeProperties(g);
g.drawRect(x1,y1,x2-x1,y2-y1);
}
}

And putOval() was modified in the same way. When I call these methods (in the putShape() method), the value that I used for stroked and filled depend on which tool is currently selected.

There is one further really tricky point. The method repaintRect() is defined to call repaint() for a rectangle with corners (x1,y1) and (x2,y2). I call this method when part of the offscreen canvas is modified, so that that part of the canvas can be copied to the screen. For example, after drawing a rectangle or oval with corners (x1,y1) and (x2,y2), I call repaintRect(x1,y1,x2,y2). The problem is that the drawing operation can actually modify parts of the canvas that are outside the specified rectangle. When you outline the figure with a wide stroke, part of the stroke will lie outside the boundary of the rectangle, and that part will not be copied to the screen. A quick solution would be to simply call repaint() to repaint the entire component. Repainting just a rectangle is an optimization. It's more efficient, but it probably doesn't affect the performance of the program all that much. However, another solution is simply to make the rectangle that is repainted big enough to contain the entire area that was modified in the canvas. That's the approach that I take in my solution.

The Solution

Significant changes from the solution to Exercise 13.1 are shown in red italic

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.imageio.ImageIO;
import java.awt.image.BufferedImage;
import java.io.*;
import java.util.EnumSet;

/**
* An improved paint program with File and undo commands.
* Note that the way that the off-screen canvas is used
* in this class requires that the panel be non-resizable; this
* is because the size of the off-screen canvas does not change
* when the panel changes size.
*/
public class PaintWithOSCFinal extends JPanel {

/**
* The main routine simply opens a window that shows a PaintWithOSCFinal panel.
*/
public static void main(String[] args) {
JFrame window = new JFrame("PaintWithOSCFinal");
PaintWithOSCFinal content = new PaintWithOSCFinal();
window.setContentPane(content);
window.setJMenuBar(content.getMenuBar());
window.pack();
window.setResizable(false);
Dimension screenSize = Toolkit.getDefaultToolkit().getScreenSize();
window.setLocation( (screenSize.width - window.getWidth())/2,
(screenSize.height - window.getHeight())/2 );
window.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
window.setVisible(true);
}

/**
* The possible drawing tools in this program.  (The CURVE tool allows the
* user to sketch a free-hand curve, while the LINE tool draws a line
* between two points.  The SMUDGE tool lets the user "spread paint around"
* with the mouse.  The ERASE tool erases with a 10-by-10 pixel rectangle.)
*/
private enum Tool { CURVE, LINE, RECT, OVAL, FILLED_RECT, FILLED_OVAL,
STROKED_FILLED_RECT, STROKED_FILLED_OVAL, SMUDGE, ERASE }

/**
* The set of Tools that represent "shapes."  Shapes are handled differently
* during dragging than other tools, since they are drawn "on top of" the
* current picture during a mouse drag and are only added permanently to the
* picture on mouse release.
*/
private final static EnumSet<Tool> SHAPE_TOOLS =
EnumSet.range(Tool.LINE, Tool.STROKED_FILLED_OVAL);

/**
* The currently selected drawing tool.  Initially Tool.CURVE.  Can be
* changed by the user with commands in the "Tool" menu.
*/
private Tool currentTool = Tool.CURVE;

/**
* The current drawing color for drawing lines.  Initially Color.BLACK.  Can be
* changed by the user using the "StrokeColor" menu.
*/
private Color currentStrokeColor = Color.BLACK;

/**
* The current drawing color for filling shapes.  Initially Color.BLACK.  Can be
* changed by the user using the "FillColor" menu. This is a fully opaque color,
* even if the user has selected transparent  drawing; transparency is applied when
* the color is used.
*/
private Color currentFillColor = Color.BLACK;

/**
* The background color that is used to fill the off-screen canvas when
* it is created.  If the user selects the "Fill With Color", the fill
* color changes, and the canvas is filled with the new fill color,
* erasing whatever was there before.
*/
private Color fillColor = Color.WHITE;

/**
* The width of the stroke that is used to draw lines and curves.
*/
private int lineWidth = 1;

/**
* Tells whether to fill shapes with fully opaque colors or to use colors that are
* 50% transparent.
*/
private JCheckBoxMenuItem transparent = new JCheckBoxMenuItem("Translucent Fill");

/**
* The off-screen canvas.  This is not created until the first time
* paintComponent() is called.  If the size of the component changes,
* a new OSC is created (and the picture in the old one is lost).
*/
private BufferedImage OSC;

/**
* This is set to true when the user is dragging the mouse.
*/
private boolean dragging;

/**
* The start position of the mouse during a mouse drag.
*/
private int startX, startY;

/**
* The current position of the mouse during a mouse drag.
*/
private int currentX, currentY;

/**
* File dialog for implementing the File menu commands.
*/
private JFileChooser fileDialog;

/**
* An extra copy of the image that is used for the Undo command.
* Before any change is made to the image, the image is copied
* to this variable.  The Undo command swaps the saved image
* with the image that is shown on screen.  (Thus, a second Undo
* will effectively act as a Redo.)
*/
private BufferedImage imageSavedForUndo;

/**
* The constructor sets the preferred size of the panel to
* 640-by-480.  It also sets up a mouse listener.
*/
public PaintWithOSCFinal() {
setPreferredSize(new Dimension(640,480));
MouseHandler mouseHandler = new MouseHandler();
addMouseListener(mouseHandler);
addMouseMotionListener(mouseHandler);
}

/**
* The paintComponent() method copies the off-screen canvas to the screen
* (first creating it, if necessary).  If a mouse drag is in progress,
* then the current tool is not Tool.CURVE, then the shape that the user
* is drawing is drawn over the off-screen canvas.  (This is to avoid
* making the shape a permanent part of the picture until after the user
* releases the mouse.  The effect is a "rubber band cursor" in which
* the shape changes as the user drags the mouse, but the picture under
* the shape is not affected.)
*/
public void paintComponent(Graphics g) {

/* First create the off-screen canvas, if it does not already exist. */

if (OSC == null)
createOSC();

/* Copy the off-screen canvas to the panel.  Since we know that the
image is already completely available, the fourth "ImageObserver"
parameter to g.drawImage() can be null.  Since the canvas completely
fills the panel, there is no need to call super.paintComponent(g). */

g.drawImage(OSC,0,0,null);

/* If the user is currently dragging the mouse to draw a line, oval,
or rectangle, draw the shape on top the image from the off-screen
canvas, using the current drawing color.  (This is not done if the
user is drawing a curve or using the smudge tool.) */

if (dragging && SHAPE_TOOLS.contains(currentTool)) {
Graphics2D g2 = (Graphics2D)g;
g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
RenderingHints.VALUE_ANTIALIAS_ON);
putCurrentShape(g);
}

}

/**
* This method creates the off-screen canvas and fills it with the current
* fill color.  The image that is used to implement the Undo command is
* also created here.
*/
private void createOSC() {
OSC = new BufferedImage(getWidth(),getHeight(),BufferedImage.TYPE_INT_RGB);
imageSavedForUndo = new BufferedImage(getWidth(),getHeight(),BufferedImage.TYPE_INT_RGB);
Graphics osg = OSC.createGraphics();
osg.setColor(fillColor);
osg.fillRect(0,0,getWidth(),getHeight());
osg.dispose();
saveUndoImage();
}

/**
* Copies the current image into the image that is saved for the Undo
* command.  This method is called before any change is made to the image.
*/
private void saveUndoImage() {
Graphics g = imageSavedForUndo.createGraphics();
g.drawImage(OSC,0,0,null);
g.dispose();
}

/**
* A utility method to draw the current shape in a given graphics context.
* It draws the correct shape for the current tool in a rectangle whose
* corners are given by the starting position of the mouse and the current
* position of the mouse.  This is used by paintComponent() during a
* mouse drag.  It is also used to make the shape a permanent part of the
* off-screen canvas when the mouse is released.
* This method is not used when the current tool is Tool.CURVE or Tool.SMUDGE.
*/
private void putCurrentShape(Graphics g) {
switch (currentTool) {
case LINE:
applyStrokeProperties(g);
g.drawLine(startX, startY, currentX, currentY);
break;
case OVAL:
putOval(g,true,false,startX, startY, currentX, currentY);
break;
case RECT:
putRect(g,true,false,startX, startY, currentX, currentY);
break;
case FILLED_OVAL:
putOval(g,false,true,startX, startY, currentX, currentY);
break;
case FILLED_RECT:
putRect(g,false,true,startX, startY, currentX, currentY);
break;
case STROKED_FILLED_OVAL:
putOval(g,true,true,startX, startY, currentX, currentY);
break;
case STROKED_FILLED_RECT:
putRect(g,true,true,startX, startY, currentX, currentY);
break;
}
}

/**
* Set the graphics context g to use the current stroke color and
* lineWidth.
*/
private void applyStrokeProperties(Graphics g) {
if (lineWidth > 1) {
Graphics2D g2 = (Graphics2D)g;
g2.setStroke( new BasicStroke(lineWidth,
BasicStroke.CAP_ROUND, BasicStroke.JOIN_MITER) );
}
g.setColor(currentStrokeColor);
}

/**
* Set the graphics context g to use the current fill color, taking
* into account the setting of transparency.  If transparency is on,
* the RGB components are obtained from the current fill color, and
* the alpha component is set to 125.
*/
private void applyFillColor(Graphics g) {
Color c = currentFillColor;
if (transparent.isSelected())
g.setColor(new Color( c.getRed(), c.getGreen(), c.getBlue(), 125 ));
else
g.setColor(c);
}

/**
* Draws a rectangle with corners at the points (x1,y1)
* and (x2,y2).  Nothing is drawn if x1 == x2 or y1 == y2.  The rectangle
* can be either stroked (that is, the outline is drawn) or filled or both.
* @param g the graphics context where the rectangle is drawn
* @param stroked tells whether to draw an outline of the rectangle.
* @param filled tells whether to draw a filled or unfilled rectangle.
*/
private void putRect(Graphics g, boolean stroked, boolean filled,
int x1, int y1, int x2, int y2) {
assert stroked || filled;  // It doesn't make sense for both to be false,
//    since then nothing would be drawn.
if (x1 == x2 || y1 == y2)
return;
if (x2 < x1) {  // Swap x1,x2 if necessary to make x2 > x1.
int temp = x1;
x1 = x2;
x2 = temp;
}
if (y2 < y1) {  // Swap y1,y2 if necessary to make y2 > y1.
int temp = y1;
y1 = y2;
y2 = temp;
}
if (filled) {
applyFillColor(g);
g.fillRect(x1,y1,x2-x1,y2-y1);
}
if (stroked) {
applyStrokeProperties(g);
g.drawRect(x1,y1,x2-x1,y2-y1);
}
}

/**
* Draws an oval in the rectangle with corners at the points (x1,y1)
* and (x2,y2).  Nothing is drawn if x1 == x2 or y1 == y2.  The oval
* can be either stroked (that is, the outline is drawn) or filled or both.
* @param g the graphics context where the oval is drawn
* @param stroked tells whether to draw an outline of the oval.
* @param filled tells whether to draw a filled or unfilled oval.
*/
private void putOval(Graphics g, boolean stroked, boolean filled,
int x1, int y1, int x2, int y2) {
assert stroked || filled;  // It doesn't make sense for both to be false,
//    since then nothing would be drawn.
if (x1 == x2 || y1 == y2)
return;
if (x2 < x1) {  // Swap x1,x2 if necessary to make x2 > x1.
int temp = x1;
x1 = x2;
x2 = temp;
}
if (y2 < y1) {  // Swap y1,y2 if necessary to make y2 > y1.
int temp = y1;
y1 = y2;
y2 = temp;
}
if (filled) {
applyFillColor(g);
g.fillOval(x1,y1,x2-x1,y2-y1);
}
if (stroked) {
applyStrokeProperties(g);
g.drawOval(x1,y1,x2-x1,y2-y1);
}
}

/**
* Calls the repaint() method of this panel for the rectangle with corners
* at the points (x1,y1) and (x2,y2).  An extra border is added
* to the area that is repainted; this allows for the size of the "pen"
* that is used to draw lines and stroked ovals and rectangles.
*/
private void repaintRect(int x1, int y1, int x2, int y2) {
if (x2 < x1) {  // Swap x1,x2 if necessary to make x2 >= x1.
int temp = x1;
x1 = x2;
x2 = temp;
}
if (y2 < y1) {  // Swap y1,y2 if necessary to make y2 >= y1.
int temp = y1;
y1 = y2;
y2 = temp;
}
x1 -= lineWidth + 1;
x2 += lineWidth + 1;
y1 -= lineWidth + 1;
y2 += lineWidth + 1;
repaint(x1,y1,x2-x1,y2-y1);
}

/**
* Adds a set of JRadioButtonMenuItems to a JMenu.  All the items are in
* the same button group and have the same ActionListener.
* @param menu  the menu to which the items will be added
* @param commandNames  the names of the items; null values in this array
*    become separators in the menu.  The name can contain the character "/".
*    In that case the actual item name is the part of the name that
*    precedes the "/" and the remainder of the name specifies a
*    KeyStroke that will be used as the keyboard accelerator key
*    for the item.
* @param listener  the ActionListener for the items
*/
private void createRadioGroup(JMenu menu, String[] commandNames,
ActionListener listener) {
ButtonGroup group = new ButtonGroup();
for ( int i = 0; i < commandNames.length; i++) {
if (commandNames[i] == null)
menu.addSeparator();
else {
String name = commandNames[i];
String accel = null;
if (name.indexOf("/") > 0) {
int pos = name.indexOf("/");
accel = name.substring(pos+1);
name = name.substring(0,pos);
}
JRadioButtonMenuItem item = new JRadioButtonMenuItem(name);
menu.add(item);
group.add(item);
item.addActionListener(listener);
if (accel != null)
item.setAccelerator(KeyStroke.getKeyStroke(accel));
if (i == 0)
item.setSelected(true);
}
}
}

/**
* Creates a menu bar for use with this panel, with "File", "Edit",
* "StrokeColor", "FillColor", and "Tool" menus.
*/
public JMenuBar getMenuBar() {
JMenuBar menubar = new JMenuBar();
JMenu fileMenu = new JMenu("File");
JMenu editMenu = new JMenu("Edit");
JMenu strokeWidthMenu = new JMenu("StrokeWidth");
JMenu strokeColorMenu = new JMenu("StrokeColor");
JMenu fillColorMenu = new JMenu("FillColor");
JMenu toolMenu = new JMenu("Tool");
menubar.add(fileMenu);
menubar.add(editMenu);
menubar.add(toolMenu);
menubar.add(strokeWidthMenu);
menubar.add(strokeColorMenu);
menubar.add(fillColorMenu);
ActionListener listener = new MenuHandler();
JMenuItem item;
item = new JMenuItem("Open Image File...");
item.setAccelerator(KeyStroke.getKeyStroke("ctrl O"));
item.addActionListener(listener);
fileMenu.add(item);
item = new JMenuItem("Save PNG File...");
item.setAccelerator(KeyStroke.getKeyStroke("ctrl S"));
item.addActionListener(listener);
fileMenu.add(item);
item = new JMenuItem("Save JPEG File...");
item.addActionListener(listener);
fileMenu.add(item);
fileMenu.addSeparator();
item = new JMenuItem("Quit");
item.setAccelerator(KeyStroke.getKeyStroke("ctrl Q"));
item.addActionListener(listener);
fileMenu.add(item);
item = new JMenuItem("Undo");
item.setAccelerator(KeyStroke.getKeyStroke("ctrl Z"));
item.addActionListener(listener);
editMenu.add(item);
editMenu.addSeparator();
item = new JMenuItem("Clear to Color...");
item.setAccelerator(KeyStroke.getKeyStroke("ctrl K"));
item.addActionListener(listener);
editMenu.add(item);

createRadioGroup( strokeWidthMenu,
new String[] {
"Thickness = 1/ctrl 1", "Thickness = 2/ctrl 2", "Thickness = 3/ctrl 3",
"Thickness = 4/ctrl 4",
"Thickness = 5/ctrl 5", "Thickness = 7", "Thickness = 10", "Thickness = 15",
"Thickness = 20", "Thickness = 25"
}, listener );

createRadioGroup( strokeColorMenu,
new String[] {
"Draw With Black/ctrl B", "Draw With White/ctrl W", "Draw With Red",
"Draw With Green", "Draw With Blue", "Draw With Yellow",
"Custom Drawing Color.../ctrl C"
}, listener);

createRadioGroup( fillColorMenu,
new String[] {
"Fill With Black/ctrl shift B", "Fill With White/ctrl shift W", "Fill With Red",
"Fill With Green", "Fill With Blue", "Fill With Yellow",
"Custom Fill Color.../ctrl shift C"
}, listener);

fillColorMenu.addSeparator();
fillColorMenu.add(transparent);
transparent.setAccelerator(KeyStroke.getKeyStroke("ctrl T"));

createRadioGroup( toolMenu,
new String[] {
"Curve/ctrl U", null, "Line/ctrl L", "Rectangle/ctrl R", "Oval/ctrl V",
"Filled Rectangle/ctrl alt R", "Filled Oval/ctrl alt V",
"Stroked Filled Rectangle/ctrl shift R", "Stroked Filled Oval/ctrl shift V",
null, "Smudge/ctrl M", "Erase/ctrl E"
}, listener);

return menubar;
}

/**
* This nested class defines the ActionListener that responds when the
* user selects a command from one of the menus.  It is used in the
* getMenuBar() method.
*/
private class MenuHandler implements ActionListener {
public void actionPerformed(ActionEvent evt) {
String command = evt.getActionCommand();
if (command.equals("Custom Drawing Color...")) {
Color newColor = JColorChooser.showDialog(PaintWithOSCFinal.this,
"Select Drawing Color", currentStrokeColor);
if (newColor != null)
currentStrokeColor = newColor;
}
else if (command.equals("Custom Fill Color...")) {
Color newColor = JColorChooser.showDialog(PaintWithOSCFinal.this,
"Select Fill Color", currentFillColor);
if (newColor != null)
currentFillColor = newColor;
}
else if (command.equals("Clear to Color...")) {
Color newColor = JColorChooser.showDialog(PaintWithOSCFinal.this,
"Select Background Color", fillColor);
if (newColor != null) {
fillColor = newColor;
saveUndoImage();
Graphics osg = OSC.createGraphics();
osg.setColor(fillColor);
osg.fillRect(0,0,OSC.getWidth(),OSC.getHeight());
osg.dispose();
PaintWithOSCFinal.this.repaint();
}
}
else if (command.startsWith("Thickness = "))
lineWidth = Integer.parseInt(command.substring(12));
else if (command.equals("Draw With Black"))
currentStrokeColor = Color.BLACK;
else if (command.equals("Draw With White"))
currentStrokeColor = Color.WHITE;
else if (command.equals("Draw With Red"))
currentStrokeColor = Color.RED;
else if (command.equals("Draw With Green"))
currentStrokeColor = Color.GREEN;
else if (command.equals("Draw With Blue"))
currentStrokeColor = Color.BLUE;
else if (command.equals("Draw With Yellow"))
currentStrokeColor = Color.YELLOW;
else if (command.equals("Fill With Black"))
currentFillColor = Color.BLACK;
else if (command.equals("Fill With White"))
currentFillColor = Color.WHITE;
else if (command.equals("Fill With Red"))
currentFillColor = Color.RED;
else if (command.equals("Fill With Green"))
currentFillColor = Color.GREEN;
else if (command.equals("Fill With Blue"))
currentFillColor = Color.BLUE;
else if (command.equals("Fill With Yellow"))
currentFillColor = Color.YELLOW;
else if (command.equals("Curve"))
currentTool = Tool.CURVE;
else if (command.equals("Line"))
currentTool = Tool.LINE;
else if (command.equals("Rectangle"))
currentTool = Tool.RECT;
else if (command.equals("Oval"))
currentTool = Tool.OVAL;
else if (command.equals("Filled Rectangle"))
currentTool = Tool.FILLED_RECT;
else if (command.equals("Filled Oval"))
currentTool = Tool.FILLED_OVAL;
else if (command.equals("Stroked Filled Rectangle"))
currentTool = Tool.STROKED_FILLED_RECT;
else if (command.equals("Stroked Filled Oval"))
currentTool = Tool.STROKED_FILLED_OVAL;
else if (command.equals("Smudge"))
currentTool = Tool.SMUDGE;
else if (command.equals("Erase"))
currentTool = Tool.ERASE;
else if (command.equals("Open Image File..."))
doOpenFile();
else if (command.equals("Save PNG File..."))
doSaveFile("PNG");
else if (command.equals("Save JPEG File..."))
doSaveFile("JPEG");
else if (command.equals("Quit"))
System.exit(0);
else if (command.equals("Undo")) {
BufferedImage temp = OSC;
OSC = imageSavedForUndo;
imageSavedForUndo = temp;
repaint();
}
}
} // end nested class MenuHandler

/**
* This nested class defines the object that listens for mouse and
* mouse motion events on the panel.  It is used in the constructor.
*/
private class MouseHandler implements MouseListener, MouseMotionListener {

int prevX, prevY;  // Previous position of mouse during a drag.

double[][] smudgeRed, smudgeGreen, smudgeBlue;  // data for smudge tool

/**
* When the ERASE or SMUDGE tools are used and the mouse jumps
* from (x1,y1) to (x2,y2), the tool has to be applied to a
* line of pixel positions between the two points in order to
* cover the entire line that the mouse moves along.  The change
* is made to the off-screen canvas, and repaint() is called to
* copy the changes to the screen.
*/
void applyToolAlongLine(int x1, int y1, int x2, int y2) {
Graphics g = OSC.createGraphics();
Graphics2D g2 = (Graphics2D)g;
g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
RenderingHints.VALUE_ANTIALIAS_ON);
g.setColor(fillColor);    // (for ERASE only)
int w = OSC.getWidth();   // (for SMUDGE only)
int h = OSC.getHeight();  // (for SMUDGE only)
int dist = Math.max(Math.abs(x2-x1),Math.abs(y2-y1));
// dist is the number of points along the line from
// (x1,y1) to (x2,y2) at which the tool will be applied.
double dx = (double)(x2-x1)/dist;
double dy = (double)(y2-y1)/dist;
for (int d = 1; d <= dist; d++) {
// Apply the tool at one of the points (x,y) along the
// line from (x1,y1) to (x2,y2).
int x = (int)Math.round(x1 + dx*d);
int y = (int)Math.round(y1 + dy*d);
if (currentTool == Tool.ERASE) {
// Erase a 10-by-10 block of pixels around (x,y)
g.fillRect(x-5,y-5,10,10);
repaint(x-5,y-5,10,10);
}
else {
// For the SMUDGE tool, blend some of the color from
// the smudgeRed, smudgeGreen, and smudgeBlue arrays
// into the pixels in a 7-by-7 block around (x,y), and
// vice versa.  The effect is to smear out the color
// of pixels that are visited by the tool.
for (int i = 0; i < 7; i++)
for (int j = 0; j < 7; j++) {
int r = y + j - 3;
int c = x + i - 3;
if (!(r < 0 || r >= h || c < 0 || c >= w || smudgeRed[i][j] == -1)) {
int curCol = OSC.getRGB(c,r);
int curRed = (curCol >> 16) & 0xFF;
int curGreen = (curCol >> 8) & 0xFF;
int curBlue = curCol & 0xFF;
int newRed = (int)(curRed*0.7 + smudgeRed[i][j]*0.3);
int newGreen = (int)(curGreen*0.7 + smudgeGreen[i][j]*0.3);
int newBlue = (int)(curBlue*0.7 + smudgeBlue[i][j]*0.3);
int newCol = newRed << 16 | newGreen << 8 | newBlue;
OSC.setRGB(c,r,newCol);
smudgeRed[i][j] = curRed*0.3 + smudgeRed[i][j]*0.7;
smudgeGreen[i][j] = curGreen*0.3 + smudgeGreen[i][j]*0.7;
smudgeBlue[i][j] = curBlue*0.3 + smudgeBlue[i][j]*0.7;
}
}
repaint(x-3,y-3,7,7);
}
}
g.dispose();
}

/**
* Start a drag operation.
*/
public void mousePressed(MouseEvent evt) {
startX = prevX = currentX = evt.getX();
startY = prevY = currentY = evt.getY();
dragging = true;
saveUndoImage();
if (currentTool == Tool.ERASE) {
// Erase a 10-by-10 block around the starting mouse position.
Graphics g = OSC.createGraphics();
Graphics2D g2 = (Graphics2D)g;
g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
RenderingHints.VALUE_ANTIALIAS_ON);
g.setColor(fillColor);
g.fillRect(startX-5,startY-5,10,10);
g.dispose();
repaint(startX-5,startY-5,10,10);
}
else if (currentTool == Tool.SMUDGE) {
// Record the colors in a 7-by-7 block of pixels around the
// starting mouse position into the arrays smudgeRed,
// smudgeGreen, and smudgeBlue.  These arrays hold the
// red, green, and blue components of the colors.
if (smudgeRed == null) {
// Create the arrays, if they have not already been created.
smudgeRed = new double[7][7];
smudgeGreen = new double[7][7];
smudgeBlue = new double[7][7];
}
int w = OSC.getWidth();
int h = OSC.getHeight();
int x = evt.getX();
int y = evt.getY();
for (int i = 0; i < 7; i++)
for (int j = 0; j < 7; j++) {
int r = y + j - 3;
int c = x + i - 3;
if (r < 0 || r >= h || c < 0 || c >= w) {
// A -1 in the smudgeRed array indicates that the
// corresponding pixel was outside the canvas.
smudgeRed[i][j] = -1;
}
else {
int color = OSC.getRGB(c,r);
smudgeRed[i][j] = (color >> 16) & 0xFF;
smudgeGreen[i][j] = (color >> 8) & 0xFF;
smudgeBlue[i][j] = color & 0xFF;
}
}
}
}

/**
* Continue a drag operation when the user drags the mouse.
* For the CURVE tool, a line is drawn from the previous mouse
* position to the current mouse position in the off-screen canvas,
* and the repaint() method is called for a rectangle that contains
* the line segment that was drawn.  For shape tools, the off-screen
* canvas is not changed, but the repaint() method is called so
* that the paintComponent() method can redraw the picture with
* the user's shape in the new position.  For the SMUDGE and
* ERASE tools, the tool is applied along a line from the previous
* mouse position to the current position;
*/
public void mouseDragged(MouseEvent evt) {
currentX = evt.getX();
currentY = evt.getY();
if (currentTool == Tool.CURVE) {
Graphics g = OSC.createGraphics();
Graphics2D g2 = (Graphics2D)g;
g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
RenderingHints.VALUE_ANTIALIAS_ON);
applyStrokeProperties(g);
g.drawLine(prevX,prevY,currentX,currentY);
g.dispose();
repaintRect(prevX,prevY,currentX,currentY);
}
else if (SHAPE_TOOLS.contains(currentTool)) {
// Repaint the rectangles occupied by the previous position of
// the shape and by its current position.
repaintRect(startX,startY,prevX,prevY);
repaintRect(startX,startY,currentX,currentY);
}
else {
// Tool has to be ERASE or SMUDGE
applyToolAlongLine(prevX,prevY,currentX,currentY);
}
prevX = currentX;
prevY = currentY;
}

/**
* Finish a mouse drag operation.  Nothing is done unless the current tool
* is a shape tool.  For shape tools, the user's shape is drawn to the
* off-screen canvas, making it a permanent part of the picture, and
* then the repaint() method is called to show the modified picture
* on the screen.
*/
public void mouseReleased(MouseEvent evt) {
dragging = false;
if (SHAPE_TOOLS.contains(currentTool)) {
Graphics g = OSC.createGraphics();
Graphics2D g2 = (Graphics2D)g;
g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
RenderingHints.VALUE_ANTIALIAS_ON);
putCurrentShape(g);
g.dispose();
repaint();
}
}

public void mouseMoved(MouseEvent evt) { }
public void mouseClicked(MouseEvent evt) { }
public void mouseEntered(MouseEvent evt) { }
public void mouseExited(MouseEvent evt) { }

} // end nested class MenuHandler

/**
* Opens an image file selected by the user.  If the image is read
* successfully, it replaces the image in the off-screen canvas.
* (The new image is scaled to fit the canvas size exactly.)
*/
private void doOpenFile() {
if (fileDialog == null)
fileDialog = new JFileChooser();
fileDialog.setDialogTitle("Select File to be Opened");
fileDialog.setSelectedFile(null);  // No file is initially selected.
int option = fileDialog.showOpenDialog(this);
if (option != JFileChooser.APPROVE_OPTION)
return;  // User canceled or clicked the dialog's close box.
File selectedFile = fileDialog.getSelectedFile();
FileInputStream stream;
try {
stream = new FileInputStream(selectedFile);
}
catch (Exception e) {
JOptionPane.showMessageDialog(this,
"Sorry, but an error occurred while trying to open the file:\n" + e);
return;
}
try {
BufferedImage image = ImageIO.read(stream);
if (image == null)
throw new Exception("File does not contain a recognized image format.");
saveUndoImage();
Graphics g = OSC.createGraphics();
g.drawImage(image,0,0,OSC.getWidth(),OSC.getHeight(),null);
g.dispose();
repaint();
}
catch (Exception e) {
JOptionPane.showMessageDialog(this,
"Sorry, but an error occurred while trying to read the image:\n" + e);
}
}

/**
* Attempts to save the current image to a file selected by the user.
* @param format the format of the image, either "PNG" or "JPEG" in this program
*/
private void doSaveFile(String format) {
if (fileDialog == null)
fileDialog = new JFileChooser();
fileDialog.setSelectedFile(new File("image." + format.toLowerCase()));
fileDialog.setDialogTitle("Select File to be Saved");
int option = fileDialog.showSaveDialog(null);
if (option != JFileChooser.APPROVE_OPTION)
return;  // User canceled or clicked the dialog's close box.
File selectedFile = fileDialog.getSelectedFile();
if (selectedFile.exists()) {  // Ask the user whether to replace the file.
int response = JOptionPane.showConfirmDialog( null,
"The file \"" + selectedFile.getName()
+ "\" already exists.\nDo you want to replace it?",
"Confirm Save",
JOptionPane.YES_NO_OPTION,
JOptionPane.WARNING_MESSAGE );
if (response != JOptionPane.YES_OPTION)
return;  // User does not want to replace the file.
}
try {
boolean hasFormat = ImageIO.write(OSC,format,selectedFile);
if ( ! hasFormat )
throw new Exception(format + " format is not available.");
}
catch (Exception e) {
System.out.println("Sorry, but an error occurred while trying to save the image.");
e.printStackTrace();
}
}

} // end class PaintWithOSCFinal

Solution for Programming Exercise 13.3

Exercise 13.3:

The StopWatchLabel component from Subsection 13.4.5 displays the text "Timing..." when the stop watch is running. It would be nice if it displayed the elapsed time since the stop watch was started. For that, you need to create a Timer. (See Subsection 6.4.1.) Add a Timer to the original source code, StopWatchLabel.java, to drive the display of the elapsed time in seconds. Create the timer in the mousePressed() routine when the stop watch is started. Stop the timer in the mousePressed() routine when the stop watch is stopped. The elapsed time won't be very accurate anyway, so just show the integral number of seconds. You only need to set the text a few times per second. For my Timer method, I use a delay of 200 milliseconds for the timer.

Discussion

This one is almost too easy. (The hardest part is that I referred to the stop watch as a "timer" in my program, and this could be confused with a Timer object, so I changed some of the old references to "timer" to "stop watch".)

An instance variable named timer of type Timer is added to the class. The timer is started when the stop watch is started and is stopped when the stop watch is stopped. Both of these things happen in the mousePressed() method. I could have created a new timer each time the stop watch is started, but I decided to reuse a single timer. The first time the stop watch is started, a Timer object is created and started. After that, the same Timer object is simply restarted. I can tell the difference between these two cases since the first time, the timer will be null:

if (timer == null) {
timer = new Timer(200,this);
timer.start();
}
else
timer.restart();

The class is declared to implement ActionListener so that it can respond to events from the timer. (As usual, it would probably be better style to create another object to do the listening.) The actionPerformed() method just has to set the text on the label to show how much time has passed since the stop watch was started. The starting time of the stop watch is in the instance variable startTime. The current time is given by System.currentTimeMillis(), so the elapsed time, in milliseconds, is just System.currentTimeMillis() - startTime. This has to be divided by 1000 to give the number of seconds. (Remember that dividing an integer by an integer always gives an integer. The answer is truncated by dropping the fractional part, giving the integral number of seconds.) The actionPerformed() method simply does this calculation and then sets the text of the label.

The complete source code is shown below, followed by the source code for the little program that tests the component. The program sets the fonts and colors of the stop watch component.

The Solution

The timer component, with changes from the original shown in red italic:

import java.awt.event.*;
import javax.swing.*;

/**
* A custom component that acts as a simple stop-watch.  When the user clicks
* on it, this component starts timing.  When the user clicks again,
* it displays the time between the two clicks.  Clicking a third time
* starts another timer, etc.  While it is timing, the label displays
* the integral number of seconds since it was started.
*/
public class StopWatchLabel2 extends JLabel
implements MouseListener, ActionListener {

private long startTime;   // Start time of timer.
//   (Time is measured in milliseconds.)

private boolean running;  // True when the timer is running.

private Timer timer;      // Generates events that cause the component
// to be repainted periodically while the
// stop watch is running.

/**
* Constructor sets initial text on the label to
* "Click to start timer." and sets up a mouse listener
* so the label can respond to clicks.
*/
public StopWatchLabel2() {
super("  Click to start timer.  ", JLabel.CENTER);
addMouseListener(this);
}

/**
* Tells whether the timer is currently running.
*/
public boolean isRunning() {
return running;
}

/**
* This will be called when an event from the timer is received.  It just
* sets the stop watch to show the amount of time that it has been running.
* Time is rounded down to the nearest second.
*/
public void actionPerformed(ActionEvent evt) {
long time = (System.currentTimeMillis() - startTime) / 1000;
setText("Running:  " + time + " seconds");
}

/**
* React when the user presses the mouse by starting
* or stopping the stop watch and changing the text that
* is shown on the label.
*/
public void mousePressed(MouseEvent evt) {
if (running == false) {
// Record the time and start the stop watch.
running = true;
startTime = evt.getWhen();  // Time when mouse was clicked.
setText("Running:  0 seconds");
if (timer == null) {
timer = new Timer(100,this);
timer.start();
}
else
timer.restart();
}
else {
// Stop the stop watch.  Compute the elapsed time since the
// stop watch was started and display it.
timer.stop();
running = false;
long endTime = evt.getWhen();
double seconds = (endTime - startTime) / 1000.0;
setText("Time: " + seconds + " sec.");
}
}

public void mouseReleased(MouseEvent evt) { }
public void mouseClicked(MouseEvent evt) { }
public void mouseEntered(MouseEvent evt) { }
public void mouseExited(MouseEvent evt) { }

}

The code for the program that tests the component:

import java.awt.*;
import javax.swing.*;

/**
* A trivial program that tests the StopWatchLabel2 component.
* The program just creates a window to show a StopWatchLabel2.
*/

public class TestStopWatch2 {

public static void main(String[] args) {

StopWatchLabel2 watch = new StopWatchLabel2();
watch.setFont( new Font("SansSerif", Font.BOLD, 24) );
watch.setBackground(Color.WHITE);
watch.setForeground( new Color(180,0,0) );
watch.setOpaque(true);

JFrame window = new JFrame("Stop Watch");
window.setContentPane(watch);
window.pack();
window.setResizable(false);
window.setLocation(100,80);
window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
window.setVisible(true);

}

}

Solution for Programming Exercise 13.4

Exercise 13.4:

The custom component example MirrorText.java, from Subsection 13.4.5, uses an off-screen canvas to show mirror-reversed text in a JPanel. An alternative approach would be to draw the text after applying a transform to the graphics context that is used for drawing. (See Subsection 13.2.5.) With this approach, the custom component can be defined as a subclass of JLabel in which the paintComponent() method is overridden. Write a version of the MirrorText component that takes this approach. The solution is very short, but tricky. Note that the scale transform g2.scale(-1,1) does a left-right reflection through the left edge of the component.

Discussion

The original version of MirrorText was defined as a subclass of JPanel, so it inherited only the methods defined in that class. Since JPanel has no setText() or getText() method, I had to define them myself. To make the custom component behave like a normal component, I had to define a getPreferredSize() method, and I had to call revalidate() in the setText() method. And my component still does not behave exactly like a standard component. For example, a JLabel generates an event when its text is changed.

By defining the new version of MirrorText as a subclass of JLabel, we avoid these problems, since the custom component will automatically have all the methods and behaviors of a JLabel. The main thing that has to be changed is the paintComponent() method. I also defined a constructor that sets the default alignment of the text in the label to centered rather than left-justified. In the paintComponent() method, I used

Graphics2D g2 = (Graphics2D)g.create();

to get a Graphics2D graphics context for drawing on the label. This is required since transforms are only defined in Graphics2D class. I used (Graphics2D)g.create() rather than simply (Graphics2D)g because I wanted to avoid changing the original graphics context, which might be used for further drawing after the paintComponent() method finishes. (See Section 13.2). To apply a mirror reversal to the label, I used g2.scale(-1,1), as suggested in the exercise. However, this reflects the contents of the label through the left edge of the label. If that were the only thing that I did, the text would end up on the other side of the left edge, and that means it wouldn't appear on the label at all. To fix this, before scaling, I translate the text to the left by an amount equal to the width of the label, with the command g2.translate(getWidth(),0). This puts the text to the left of the label, and then the scale transform reflects it back onto the label. Finally, I call super.paintComponent(g2) to call the paint method from the JLabel class. This will draw the text of the label, but the drawing will be subject to the transforms that have been applied to g2.

The way I did it is not the only way to apply the transforms. One alternative is to use g.scale(-1,1) followed by g.translate(-getWidth(),0). After the scale, the text lies to the left of the label. To translate it back onto the label, it must be moved to the right by getWidth() pixels. However, that direction is now the direction of the negative x-axis, so the translation is actually -getWidth(), with a minus sign to indicate that the motion is in the negative direction. (As I said before, it takes a fair amount of study to fully understand transforms.)

The Solution

import java.awt.*;
import javax.swing.*;

/**
* A component for displaying a mirror-reversed line of text.
* The text will be centered in the available space.  This component
* is defined as a subclass of JLabel.
*
* This version uses a transform in a Graphics2D graphics context
* to achieve the mirror-reversal, and is defined as a simple
* subclass of JLabel.
*/
public class MirrorText2 extends JLabel {

/**
* Define the constructor so that the default alignment of
* the text will be centered, instead of left-justified.
* @param text The string that is to be shown on the label.
*/
public MirrorText2(String text) {
super(text,JLabel.CENTER);
}

/**
* The paintComponent method calls super.paintComponent() after
* applying a left/right mirror transform.
*/
public void paintComponent(Graphics g) {
Graphics2D g2 = (Graphics2D)g.create();
g2.translate(getWidth(),0);
g2.scale(-1,1);
super.paintComponent(g2);
g2.dispose();
}

}  // end MirrorText2

Here is a little program for testing the component:

import java.awt.*;
import javax.swing.*;

/**
* A trivial program that tests the StopWatchLabel2 component.
* The program just creates a window to show a StopWatchLabel2.
*/

public class TestMirrorText2 {

public static void main(String[] args) {

MirrorText2 mtext = new MirrorText2("Let me out of here!");
mtext.setFont( new Font("SansSerif", Font.BOLD, 48) );
mtext.setBackground(Color.BLACK);
mtext.setForeground(Color.RED);
mtext.setOpaque(true);
mtext.setBorder(BorderFactory.createEmptyBorder(10,10,10,10));

JFrame window = new JFrame("Stop Watch");
window.setContentPane(mtext);
window.pack();
window.setResizable(false);
window.setLocation(100,80);
window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
window.setVisible(true);

}

}

Solution for Programming Exercise 13.5

Exercise 13.5:

The sample program PhoneDirectoryFileDemo.java from Subsection 11.3.2 keeps data for a "phone directory" in a file in the user's home directory. Exercise 11.5 asked you to revise that program to use an XML format for the data. Both programs have a simple command-line user interface. For this exercise, you should provide a GUI interface for the phone directory data. You can base your program either on the original sample program or on the modified XML version from the exercise. Use a JTable to hold the data. The user should be able to edit all the entries in the table. Also, the user should be able to add and delete rows. Include either buttons or menu commands that can be used to perform these actions. The delete command should delete the selected row, if any. New rows should be added at the end of the table. For this program, you can use a standard DefaultTableModel.

Your program should load data from the file when it starts and save data to the file when it ends, just as the two previous programs do. For a GUI program, you can't simply save the data at the end of the main() routine, since main() terminates as soon as the window shows up on the screen. You want to save the data when the user closes the window and ends the program. There are several approaches. One is to use a WindowListener to detect the event that occurs when the window closes. Another is to use a "Quit" command to end the program; when the user quits, you can save the data and close the window (by calling its dispose() method), and end the program. If you use the "Quit" command approach, you don't want the user to be able to end the program simply by closing the window. To accomplish this, you should call

frame.setDefaultCloseOperation(JFrame.DO_NOTHING_ON_CLOSE);

where frame refers to the JFrame that you have created for the program's user interface. When using a WindowListener, you want the close box on the window to close the window, not end the program. For this, you need

frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);

When the listener is notified of a window closed event, it can save the data and end the program.

Most of the JTable and DefaultTableModel methods that you need for this exercise are discussed in Subsection 13.4.3, but there are a few more that you need to know about. To determine which row is selected in a JTable, call table.getSelectedRow(). This method returns the row number of the selected row, or returns -1 if no row is selected. To specify which cell is currently being edited, you can use:

table.setRowSelectionInterval(rowNum, rowNum);  // Selects row number rowNum.
table.editCellAt( rowNum, colNum ); // Edit cell at position (rowNum,colNum).
phoneTable.getEditorComponent().requestFocus();  // Put input cursor in cell.

One particularly troublesome point is that the data that is in the cell that is currently being edited is not in the table model. The value in the edit cell is not put into the table model until after the editing is finished. This means that even though the user sees the data in the cell, it's not really part of the table data yet. If you lose that data, the user would be justified in complaining. To make sure that you get the right data when you save the data at the end of the program, you have to turn off editing before retrieving the data from the model. This can be done with the following method:

private void stopEditing() {
if (table.getCellEditor() != null)
table.getCellEditor().stopCellEditing();
}

This method must also be called before modifying the table by adding or deleting rows; if such modifications are made while editing is in progress, the effect can be very strange.

Discussion

There are many, many ways to organize the program. I will discuss just one. The main GUI class in my program is a subclass of JPanel, named PhoneDirectoryPanel. This panel holds the table and two buttons that are used for adding and deleting rows. In the main() routine, the data from the file is read into a variable of type TreeMap<String,String>. Somehow, that data has to get into the panel. I decided to pass it as a parameter to a constructor of the form

public PhoneDirectoryPanel(TreeMap<String,String> initialPhoneBook)

When the program ends, the data has to be gotten back out of the panel object so it can be saved to the file. I defined a method in the PhoneDirectoryPanel class that can be used to get the data:

public TreeMap<String, String> getPhoneBook()

Aside from the constructor, some instance variables, and the getPhoneBook() method, the only other thing in the panel class is the stopEditing() method that was mentioned in the exercise. While the program is running, the data is managed by the table.

In the command-line version, the code for saving the data is at the end of the main() routine. I moved it into a static method

private static void saveData(TreeMap<String,String> newPhoneBook)

This method has to be called when the program ends. I decided to use a WindowListener to save the data when the window is closed. The main() routine creates the window, puts a PhoneDirectoryPanel in the window, and adds a listener to the window:

JFrame window = new JFrame("PhoneBook");
final PhoneDirectoryPanel panel = new PhoneDirectoryPanel(phoneBook);
window.setContentPane( panel );
window.pack();
Dimension screenSize = Toolkit.getDefaultToolkit().getScreenSize();
window.setLocation( (screenSize.width - window.getWidth())/2, 80 );
window.setVisible(true);
window.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
window.addWindowListener( new WindowAdapter() {
// When the user clicks the close box of the window,
// the window will be disposed (that is, closed), and the
// windowClosed method in this WindowListener will be
// called.  This method saves the phone book data and
// calls System.exit() to terminate the program.
public void windowClosed(WindowEvent evt) {
saveData(panel.getPhoneBook());
System.exit(0);
}
});

After it does this, the main() routine ends, and the window takes over. The constructor of the panel class has to create a table and a table model using the data from the TreeMap, initialPhoneBook. To do this, it copies the data into a two-dimensional array of Strings that can be used in the constructor for the DefaultTableModel:

int entryCount = initialPhoneBook.size();
String[][] entries;
if (entryCount == 0)
entries = new String[1][2]; // Represents an empty row.
else {
entries = new String[entryCount][2];
int index = 0;
for (Map.Entry<String, String> entry : initialPhoneBook.entrySet()) {
entries[index][0] = entry.getKey();
entries[index][1] = entry.getValue();
index++;
}
}
String[] columnHeads = new String[] { "Name", "Number" };

model = new DefaultTableModel(entries,columnHeads);
table = new JTable(model);

Note that if there are no entries in the phone book, I place one empty row in the table, since the user would have to do that anyway before they could do anything else with the table. (Also, I need a real array when I create the model, since the number of columns is taken from the size of the array.) I also did some customization of the table's appearance. The constructor also creates an "Add Row" button and a "Delete Row" button. It's worth looking at the ActionListener for the "Add Row" button:

JButton addRowButton = new JButton("Add Row");
addRowButton.addActionListener( new ActionListener() {
public void actionPerformed(ActionEvent evt) {
// Add a row at the end of the table.  Also, select
// that row and set the first cell in that row to
// be the cell that is currently being edited.
stopEditing();
model.addRow( new String[] { null, null } );
int newRow = model.getRowCount() - 1;  // Number of the row that was added.
table.setRowSelectionInterval(newRow, newRow);
table.editCellAt( newRow, 0 );
Component c = table.getEditorComponent();
if (c != null) // (Should not be null.)
c.requestFocus();
}
});

The statement model.addRow( new String[] { null, null } ); adds a row at the end of the table. The data that is to go into the cells in that row is passed to the addRow() method as an array. In this case, the null values mean that both cells will initially be empty. Before adding the row, I call stopEditing(), as suggested in the exercise, and after adding the row, I select the first cell in the row for editing, again following the exercise.

The only other thing to do in the panel class is to define the getPhoneBook() method. This method has to get the data out of the table model and add it to a TreeMap. One question was, what to do about empty cells in the table? A row with one or two empty rows represents missing or incomplete data. One possibility is just to ignore all such rows. However, I decided that having a name in the phone book with no number attached to it would be OK. On the other hand, it doesn't make any sense to have a number without a name, so my policy is to ignore any row in which the "name" cell is empty. You can see how it's done in the solution shown below.

One other issue that came up is what to do if an error occurs while trying to read or write the data file. The command-line version of the program simply prints a message to standard output. For a GUI program, however, the user probably won't even see standard output, so sending the output there does no good. A much better way to report errors in a GUI program is to use the JOptionPane class, so I converted error reports from the original program to use JOptionPane instead of System.out.

The Solution

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.table.DefaultTableModel;

import java.io.*;

import java.util.Map;
import java.util.TreeMap;

import javax.xml.parsers.*;
import org.w3c.dom.*;

/**
* This program lets the user keep a persistent "phone book" that
* contains names and phone numbers.  The data for the phone book
* is stored in a file in the user's home directory.  This is
* a GUI version of the program, with the phonebook displayed
* in a table that the user can edit.  The data files for this
* version of the program are in XML format.
*
* WARNING:  This program will save a file named ".phone_book_xml_demo"
* in the home directory of the user who runs it.  On some computers,
* this will be a "hidden" file.
*/
public class PhoneDirectoryGUI {

/**
* The name of the file in which the phone book data is kept.  The
* file is stored in the user's home directory.  The "." at the
* beginning of the file name means that the file will be a
* "hidden" file on Unix-based computers, including Linux and
* Mac OS X.
*/
private static String DATA_FILE_NAME = ".phone_book_xml_demo";

/**
* A File object created from the absolute path name of the file.
*/
private static File dataFile;

/**
* Holds the phone book data as it was read from the data file
* when the program started.  The data in this map is not modified
* after it has been read.
*/
private static TreeMap<String,String> phoneBook;

public static void main(String[] args) {

phoneBook = new TreeMap<String,String>();
File userHomeDirectory = new File( System.getProperty("user.home") );
dataFile = new File( userHomeDirectory, DATA_FILE_NAME );

/* If the data file already exists, then the data in the file is
* read and is used to initialize the phone directory.  The user
* is informed before the file is created and is given a chance to
* exit the program immediately.
*/

if ( ! dataFile.exists() ) {
int response = JOptionPane.showConfirmDialog(null,
"No phone book data file found.  To create a new one,\n"
+ "click OK.  To exit the program now, click CANCEL.\n\n"
+ "(The name of the file will be:\n      "
+ dataFile.getAbsolutePath() + ")",
"Create phonebook?", JOptionPane.OK_CANCEL_OPTION);
if (response == JOptionPane.CANCEL_OPTION)
System.exit(1);
}
else {
System.out.println("Reading phone book data...");
try {
DocumentBuilder docReader =
DocumentBuilderFactory.newInstance().newDocumentBuilder();
Document xmldoc = docReader.parse(dataFile);
Element root = xmldoc.getDocumentElement();
if (! root.getTagName().equals("phone_directory"))
throw new Exception();
NodeList nodes = root.getChildNodes();
for (int i = 0; i < nodes.getLength(); i++) {
if ( nodes.item(i) instanceof Element ) {
Element entry = (Element)nodes.item(i);
if (! entry.getTagName().equals("entry"))
throw new Exception();
String entryName = entry.getAttribute("name");
String entryNumber = entry.getAttribute("number");
if (entryName.length() == 0)
throw new Exception();
phoneBook.put(entryName,entryNumber);
}
}
}
catch (Exception e) {
JOptionPane.showMessageDialog(null,
"An error occurred while trying to read\n"
+ "the phone directory from the file:\n   "
+ dataFile.getAbsolutePath()
+"\n\nThis program cannot continue.");
System.exit(1);
}
}

/* The phone book has been read successfully (if it existed).  Open
* a window where the user can view and edit the phone directory.
*/

JFrame window = new JFrame("PhoneBook");
final PhoneDirectoryPanel panel = new PhoneDirectoryPanel(phoneBook);
window.setContentPane( panel );
window.pack();
Dimension screenSize = Toolkit.getDefaultToolkit().getScreenSize();
window.setLocation( (screenSize.width - window.getWidth())/2, 80 );
window.setVisible(true);
window.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
window.addWindowListener( new WindowAdapter() {
// When the user clicks the close box of the window,
// the window will be disposed (that is, closed), and the
// windowClosed method in this WindowListener will be
// called.  This method saves the phone book data and
// calls System.exit() to terminate the program.
public void windowClosed(WindowEvent evt) {
saveData(panel.getPhoneBook());
System.exit(0);
}
});

} // end main()

/**
* Before ending the program, write the current contents of the
* phone directory, but only if some changes have been made to
* the directory.  This is called by a window listener when the
* window is closed.  If an error occurs while the data is being
* saved, a pop-up box will inform the user (but this is unlikely).
* @param newPhoneBook the phone book data that has possibly been
*    edited by the user.  If this is the same as the data that
*    was read from the file originally, this method does not
*    re-save the unchanged data.  If the data has changed, the
*    new data is written to the file.
*/
private static void saveData(TreeMap<String,String> newPhoneBook) {
if (phoneBook.equals(newPhoneBook) )
System.out.println("No changes to phone book.");
else{
System.out.println("Saving phone directory changes to file " +
dataFile.getAbsolutePath() + " ...");
PrintWriter out;
try {
out = new PrintWriter( new FileWriter(dataFile) );
}
catch (IOException e) {
JOptionPane.showMessageDialog(null,"Whoops!  Some error occurred while\n"
+ "trying to save your phone book.  Sorry.");
return;
}
out.println("<?xml version=\"1.0\"?>");
out.println("<phone_directory>");
for ( Map.Entry<String,String> entry : newPhoneBook.entrySet() ) {
out.print("  <entry name='");
out.print(entry.getKey());
out.print("' number='");
out.print(entry.getValue());
out.println("'/>");
}
out.println("</phone_directory>");
out.close();
if (out.checkError()) {
JOptionPane.showMessageDialog(null,"Whoops!  Some error occurred while\n"
+ "trying to save your phone book.  Sorry.");
}
}
}

/**
* This class defines the GUI for viewing and editing the phone
* book.  The main program adds an object of this type to a frame.
*/
private static class PhoneDirectoryPanel extends JPanel {

private JTable phoneTable;        // For showing/editing the phone book.
private DefaultTableModel model;  // The model for the table.  (This is
//   where the data is actually stored.)

/**
* Constructor creates the table and shows it with an "Add row"
* and a "Delete Row" button beneath it.
* @param initialPhoneBook contains the phone book data that is initially
*    added to the table.  (This comes from the main program and
*    contains the data that was read from the file.)
*/
public PhoneDirectoryPanel(TreeMap<String,String> initialPhoneBook) {

/* First create the arrays that hold the data for the table
* and the names of its columns.  These arrays are used to
* create the table model.
*/

int entryCount = initialPhoneBook.size();
String[][] entries;
if (entryCount == 0)
entries = new String[1][2];
else {
entries = new String[entryCount][2];
int index = 0;
for (Map.Entry<String, String> entry : initialPhoneBook.entrySet()) {
entries[index][0] = entry.getKey();
entries[index][1] = entry.getValue();
index++;
}
}
String[] columnHeads = new String[] { "Name", "Number" };

/* Create the table model from the data and column name arrays,
* and use it to create the JTable.  After this, the official
* phone book data is what's in the table, and the arrays and
* initialPhoneBook are no longer used.
*/

model = new DefaultTableModel(entries,columnHeads);
phoneTable = new JTable(model);

/* Customize the appearance of the table.
*/

phoneTable.setRowHeight(27);
phoneTable.setShowGrid(true);
phoneTable.setGridColor(Color.BLACK);
phoneTable.getTableHeader().setReorderingAllowed(false);

/* Create the two buttons, and set up listeners to respond
* when the user clicks them.
*/

JButton addRowButton = new JButton("Add Row");
addRowButton.addActionListener( new ActionListener() {
public void actionPerformed(ActionEvent evt) {
// Add a row at the end of the table.  Also, select
// that row and set the first cell in that row to
// be the cell that is currently being edited.
stopEditing();
model.addRow( new String[] { null, null } );
int newRow = model.getRowCount() - 1;
phoneTable.setRowSelectionInterval(newRow, newRow);
phoneTable.editCellAt( newRow, 0 );
Component c = phoneTable.getEditorComponent();
if (c != null) // (Should not be null.)
c.requestFocus();
}
});

JButton deleteRowButton = new JButton("Delete Row");
deleteRowButton.addActionListener( new ActionListener() {
public void actionPerformed(ActionEvent evt) {
// Delete the selected row, if there is one.  If more than
// one cell is selected, the first selected row is deleted.
stopEditing();
int selectedRow = phoneTable.getSelectedRow();
if (selectedRow >= 0)
model.removeRow(selectedRow);
}
});

/* Build the layout for the panel.
*/

setLayout(new BorderLayout(2,2));
setBackground(Color.GRAY);
setBorder(BorderFactory.createLineBorder(Color.GRAY,2));
add( new JScrollPane(phoneTable), BorderLayout.CENTER );
JPanel buttonBar = new JPanel();
buttonBar.add(addRowButton);
buttonBar.add(deleteRowButton);
add( buttonBar, BorderLayout.SOUTH );

} // end constructor

/**
* This method is called before modifying the table or getting
* the data out of the table.  If a cell is currently being
* edited, it turns off editing of that cell and changes the
* data model to match the current content of the editor.  (Note
* that the table model does not change while the cell is being
* edited.)
*/
private void stopEditing() {
if (phoneTable.getCellEditor() != null)
phoneTable.getCellEditor().stopCellEditing();
}

/**
* This method is called when the program ends to get the phone
* book data, which might have been edited by the user.  The
* data is in the table model.  This method gets the data from
* the table model and puts it into a TreeMap, which is then
* returned as the value of the method.  Note that if a row in
* the table contains an empty name, it is ignored.  However,
* if there is an empty number for a non-empty name, the number
* is changed to "(unknown)" and the row is added to the TreeMap.
* Note that by using a TreeMap, we allow only one number for
* a given name.  If the user has used the same name in more than
* one row, the first row with that name will be lost;
* it would probably be better to warn the user about this or to
* take some other, more reasonable answer (like adding a number
* to the end of the duplicate name).
*/
public TreeMap<String, String> getPhoneBook() {
stopEditing();
TreeMap<String,String> phoneBook = new TreeMap<String,String>();
for (int row = 0; row < model.getRowCount(); row++) {
String name = (String)model.getValueAt(row, 0);
String number = (String)model.getValueAt(row, 1);
if (number == null || number.trim().length() == 0)
number = "(unknown)";
if (name != null && name.trim().length() > 0)
phoneBook.put(name,number);
}
return phoneBook;
}

}

}