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.

If you decide that Java is something for you, we would recommend that you get a copy of Thinking in Java by Bruce Eckel. 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 Exploring Java by Pat Niemeyer and Josh Peck and Core Java by Gary Cornell and Cay Horstmann.

13.6.2. 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.

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.

13.6.3. A Working Example of Java

The following program (which has been written for Java 1.1, but should also work with Java 1.2/2) is a complete Java program that can run as a stand-alone 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

Now you can run this program from the command-line. Simply issue the command:

$tigger java StandaloneScribble

If you have installed the JDK correctly, you should get a window to scribble in. Note that the argument passed to the Java command was StandaloneScribble without the .class extension. This is because, technically, the interpreter is not executing the file, but the class.

You can also run this program, in a web browser or in the appletviewer from the JDK. For this, you need a bit of HTML code. The following should be enough:

<APPLET code="StandaloneScribble.class" width=150 height=100>
</APPLET>

Save this code to a file and open it with either a web browser like Netscape Navigator or the appletviewer, and you'll see the program in the browser window.

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 at all.

13.6.4. Executing Java Programs Like Ordinary Programs