Learning Java, 4th Edition

HelloJava2: The Sequel

Now that we’ve got some basics down, let’s make our application a little more interactive. The following minor upgrade allows us to drag the message text around with the mouse.

We’ll call this example HelloJava2 rather than cause confusion by continuing to expand the old one, but the primary changes here and further on lie in adding capabilities to the HelloComponent class and simply making the corresponding changes to the names to keep them straight (e.g., HelloComponent2, HelloComponent3, and so on). Having just seen inheritance at work, you might wonder why we aren’t creating a subclass of HelloComponent and exploiting inheritance to build upon our previous example and extend its functionality. Well, in this case, that would not provide much advantage, and for clarity we simply start over.

Here is HelloJava2:

    //file: HelloJava2.java
    import java.awt.*;
    import java.awt.event.*;
    import javax.swing.*;

    public class HelloJava2
    {
      public static void main( String[] args ) {
        JFrame frame = new JFrame( "HelloJava2" );
        frame.add( new HelloComponent2("Hello, Java!") );
        frame.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
        frame.setSize( 300, 300 );
        frame.setVisible( true );
      }
    }

    class HelloComponent2 extends JComponent
        implements MouseMotionListener
    {
      String theMessage;
      int messageX = 125, messageY = 95; // Coordinates of the message

      public HelloComponent2( String message ) {
        theMessage = message;
        addMouseMotionListener(this);
      }

      public void paintComponent( Graphics