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## Answers for Quiz on Chapter 11

This page contains sample answers to the quiz on Chapter 11 of Introduction to Programming Using Java. Note that generally, there are lots of correct answers to a given question.

Question 1:

In Java, input/output is done using I/O streams. I/O streams are an abstraction. Explain what this means and why it is important.

Answer:

An I/O 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 I/O 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 I/O stream: 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 FileChooser class?

Answer:

An object of type FileChooser 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 two solutions use the standard classes BufferedReader and Scanner in the usual way. The third solution uses the stream API and the BufferedReader lines() method. 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, the Scanner solution will crash, the first BufferedReader solution will just print a null for each missing line, and the second BufferedReader solution will stop after it has read all the lines that the file contains.)

    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

    //-----------------------------------------------------------------

    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.*;
    public class TenLinesWithStreamAPI {

        public static void main(String[] args) {
    	    try {
    	         BufferedReader in = new BufferedReader( new FileReader(args[0]) );
    	         in.lines()     // Make a Stream<String> containing lines from file.
    	           .limit(10)   // Truncate the string after at most 10 items.
    	           .forEach( s -> System.out.println(s) );  // Print each line.
    	    }
    	    catch (Exception e) {
    	        System.out.println("Error: " + e);
    	    }
       }
    } // end class TenLinesWithStreamAPI

Note: I didn't close the files that were used in any of these solutions, although it would have been good form to do so. The easiest way to make sure that the files is closed is to use a try..catch statement with resources. For example:

    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);
    }

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