13.5.1. The Promise of Java, or Why You Might Want to Use Java

All this may not sound too exciting to you. There are lots of object-oriented programming languages, after all, and with Netscape plug-ins you can download executable programs from web servers and execute them on your local machine.

This distinction, which makes Java both a compiled and an interpreted language, makes it possible for you to write and compile your Java program and distribute it to someone else, and no matter what hardware and operating system she has, she will be able to run your program as long as a Java interpreter is available for it.

[50]One of us does all his financial business with his bank via a Java applet that his bank provides when browsing a special area of its web server.

If you decide that Java is something for you, we recommend that you get a copy of Thinking in Java by Bruce Eckel (Prentice Hall). It covers most of the things you need to know in the Java world and also teaches you general programming principles. Other Java titles that are well worth looking into include Learning Java by Pat Niemeyer and Jonathan Knudsen (O'Reilly) and Core Java by Cay Horstmann and Gary Cornell (Prentice Hall).

13.5.2. Getting Java for Linux

The "official" Java implementation JDK contains a compiler, an interpreter, and several related tools. Other kits are also available for Linux, often in the form of open source software. We'll cover the JDK here, though, because that's the standard. There are other Linux implementations, including a very good one from IBM, as well; you might even have them on your distribution CDs.

One more note: most distributions already contain the JDK for Linux, so it might be easier for you to simply install a prepackaged one. However, the JDK is moving fast, and you might want to install a newer version than the one your distribution contains.

13.5.3. A Working Example of Java

The following program is a complete Java program that can run as a standalone application as well as an applet. It is a small painting program that lets you scribble on a virtual canvas. It also utilizes some GUI elements like a push button and an option menu.

Enough talk now. Here is the code for the little scribbling program:

import java.applet.*;
import java.awt.*;
import java.awt.event.*;
/** An applet that can also run as a standalone application */
public class StandaloneScribble extends Applet {
  /**
   * The main( ) method.  If this program is invoked as an application,
   * this method will create the necessary window, add the applet to it,
   
   * and call init( ), below.  Note that Frame uses a PanelLayout by
   * default.              
   */
  public static void main(String[  ] args) {
    Frame f = new Frame( );                     // Create a window
    Applet a = new StandaloneScribble( );       // Create the applet panel
    f.add(a, "Center");                        // Add applet to window
    a.init( );                                  // Initialize the applet
    f.setSize(400, 400);                       // Set the size of the 
                                               // window
    f.show( );                                  // Make the window visible
    f.addWindowListener(new WindowAdapter( ) {  // Handle window close 
                                               // requests
      public void windowClosing(WindowEvent e) { System.exit(0); }
    });
  }
  /**
   * The init( ) method.  If the program is invoked as an applet, the 
   * browser allocates screen space for it and calls this method to set 
   * things up.
   */
  public void init( ) {
    // Define, instantiate, and register a MouseListener object.
    this.addMouseListener(new MouseAdapter( ) {
      public void mousePressed(MouseEvent e) {
        lastx = e.getX( );
        lasty = e.getY( );
      }
    });
    // Define, instantiate, and register a MouseMotionListener object.
    this.addMouseMotionListener(new MouseMotionAdapter( ) {
      public void mouseDragged(MouseEvent e) {
        Graphics g = getGraphics( );
        int x = e.getX( ), y = e.getY( );
        g.setColor(Color.black);
        g.drawLine(lastx, lasty, x, y);
        lastx = x; lasty = y;
      }
    });
    // Create a clear button.
    Button b = new Button("Clear");
    // Define, instantiate, and register a listener to handle button
    // presses.
    b.addActionListener(new ActionListener( ) {
      public void actionPerformed(ActionEvent e) {  // clear the scribble
        Graphics g = getGraphics( );
        g.setColor(getBackground( ));
        g.fillRect(0, 0, getSize( ).width, getSize( ).height);
      }
    });

    // And add the button to the applet.
    this.add(b);
  }
  protected int lastx, lasty;  // Coordinates of last mouse click.
}

Save this code in a file named StandaloneScribble.java. The name is important; it must be the same as the name of the class implemented in the file with .java attached. To compile this code, issue the following command:

$tigger javac StandaloneScribble.java

Most of the remaining code is the method init( ). It is called either from main( ) when run standalone or directly from the web browser when run as an applet. In the latter case, main( ) is not executed.