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Subsections
Built-in Subroutines and Functions
Operations on Strings
Introduction to Enums

Section 2.3

Strings, Objects, Enums, 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 the String type: 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.

Another reason for considering classes and objects at this point is so that we can introduce enums. An enum is a data type that can be created by a Java programmer to represent a small collection of possible values. Technically, an enum is a class and its possible values are objects. Enums will be our first example of adding a new type to the Java language. We will look at them later in this section.

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. 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 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.

Classes in Java have two very different functions. First of all, a class can group together variables and subroutines that 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", just like the main() routine of a program. However, classes have a second function. They are used to describe objects. In this role, the class of an object specifies what subroutines and variables are contained in that object. The class is a type -- in the technical sense of a specification of a certain type of data value -- and the object is a value of that type. For example, String is actually the name of a class that is included as a standard part of the Java language. String is also a type, and literal strings such as "Hello World" represent values of type String.

So, every subroutine is contained either in a class or in an object. Classes contain subroutines, which are called static member subroutines. Classes also describe objects and the subroutines that are contained in those objects.

This dual use can be confusing, and in practice most classes are designed to perform primarily or exclusively in only one of the two possible roles. For example, although the String class does contain a few rarely-used static member subroutines, it exists mainly to specify a large number of subroutines that are contained in objects of type String. Another standard class, named Math, exists entirely to group together a number of static member subroutines that compute various common mathematical functions.

To begin to get a handle on all of this complexity, let's look at the subroutine System.out.print as an example. As you have seen earlier in this chapter, this subroutine is used to display information to the user. For example, System.out.print("Hello World") displays the message, Hello World.

System is one of Java's standard classes. One of the static member variables in this class is named out. Since this variable is contained in the class System, its full name -- which you have to use to refer to it in your programs -- is System.out. The variable System.out refers to an object, and that object in turn contains a subroutine named print. The compound identifier System.out.print refers to the subroutine print in the object out in the class System.

(As an aside, I will note that 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 print subroutine that can be used to send information to that destination. The object System.out is just one possible destination, and System.out.print is the 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 print subroutine. The PrintStream class expresses the commonalities among all these objects.)

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 (with one very technical exception). The same is not true for other programming languages. Nevertheless, the term "method" is mostly used in the context of object-oriented programming, and until we start doing real object-oriented programming in Chapter 5, 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.

Classes can contain static member subroutines, as well as static member variables. For example, the System class contains a subroutine named exit. In a program, of course, this subroutine must be referred to as System.exit. Calling this subroutine will terminate the program. You could use it if you had some reason to terminate the program before the end of the main routine. For historical reasons, this subroutine takes an integer as a parameter, so the subroutine call statement might look like "System.exit(0);" or "System.exit(1);". (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. But in practice, the value of the parameter is usually ignored.)

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.

You are familiar with the mathematical function that computes the square root of a number. Java has a corresponding function 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:

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 in Greenwich Mean Time, 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 second of a computer's time.

/**
 * This program performs some mathematical computations and displays
 * the results.  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() );
      
      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

And here is an applet that simulates this program. If you run it several times, you should see a different random number in the output each time, and you might see different run times.

2.3.2  Operations on Strings

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 string variable str, the value of str.length() is an int equal to the number of characters in the string that is the value of str. 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 refer to values of type String:

For the functions s1.toUpperCase(), s1.toLowerCase(), and s1.trim(), note that the value of s1 is not modified. 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 it is concatenated onto the 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.

2.3.3  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). Enums are a recent addition to Java. They were only added in Version 5.0. Many programming languages have something similar, and many people believe that enums should have been part of Java from the beginning.

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. Enum values are not variables. Each value is a constant that always has the same value. In fact, 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() in our example.

Right now, it might not seem to you that enums are all that useful. As you work though the rest of the book, you should be convinced that they are. For now, you should at least 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.");
              // You can concatenate enum values onto Strings!
         
    }
   
}

You can run the following applet version of this program to see what the output actually looks like:.

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