Complete GUI Program

An event-handling program must register its event listener with the component that generates the events or with a container that holds that component. With a JFrame component, use addWindowListener() to register a listener. Here is a complete GUI program, including the listener object. The parts in blue are additions to the example program of the previous chapter:

import java.awt.*; 
import java.awt.event.*;
import javax.swing.*; 
          
class myFrame extends JFrame
{
  public void paint ( Graphics g )
  {
    g.drawString("Click the close button", 10, 50 );  
  }
} 
 
class WindowQuitter extends WindowAdapter
{
  public void windowClosing( WindowEvent e )
  {
    System.exit( 0 );  // what to do for this event
  }
} 
          
public class GUItester
{
  public static void main ( String[] args )
  {
    myFrame frm = new myFrame();    // construct a myFrame object

    WindowQuitter wquit             // construct a listener 
        = new WindowQuitter();      // for the frame
    frm.addWindowListener( wquit ); // register the listener

    frm.setSize( 150, 100 );     
    frm.setVisible( true );      
          
  } 
}

Notice how the program registers the listener object with the event-generating GUI object. You can think of registering a listener object as establishing a channel of communication between the GUI object and the listener.