Learning Java - Chapter 7 : Java

In addition to the inner and anonymous classes, another useful type of class is the adapter class. Here this applies in particular to classes used to reduce the code for event listeners.

While the listener architecture has greatly improved the efficiency and capabilities of event handling, there are some complications and annoyances that come along with it. In particular, we find that the listener interfaces hold up to 6 methods that must be implemented.

Remember than an interface holds only abstract methods and its implementation requires that ALL of its methods be implemented, i.e. overridden with real methods. So for those methods that you are not using, you must still implement the methods with empty code bodies.

We illustrate this with the following example where we implement an anonymous MouseListener class but only need to use one of the methods.

See Java console for output as cursor moves over applet area.

AnonListenerApplet.java

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;

/** Demonstrate the use of an anonyomous listener object. **/
public class AnonListenerApplet extends JApplet
{
  public void init () {
    Container content_pane = getContentPane ();

    // Create an instance of a JPanel sub-class
    JPanel panel = new JPanel ();

    // Add an anonymous MouseListener object
    // to the panel
    panel.addMouseListener (
      new MouseListener () {
         public void mouseEntered (MouseEvent e) {
           System.out.println (e.toString () );
         }
         // Though we don't use these methods, we
         // we must override them with empty code bodies.
         public void mouseClicked (MouseEvent e) {}
         public void mousePressed (MouseEvent e) {}
         public void mouseReleased (MouseEvent e) {}
         public void mouseExited (MouseEvent e) {}
       }
    );

    // And add the panel to the JApplet panel.
    content_pane.add (panel);

  } // init

} // class AnonListenerApplet

We can avoid implementing all of these unneeded methods by taking advantage of the adapter classes in the java.awt.event package. For eight of the listener interfaces there is a corresponding adapter class for each. These adapters simply implement all of the the methods for the listener interface method with empty code bodes. Though the adapters are abstract classes, the methods are real so you only need to override the method(s) of interest.

See Java console for output as cursor moves over applet area.

AdapterApplet.java

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;

/** Demonstrate the use of an MouseAdapter. **/
public class AdapterApplet extends JApplet
{
  public void init () {
    Container content_pane = getContentPane ();

    // Create an instance of JPanel
    JPanel panel = new JPanel ();

    // Add a anonymous MouseAdapter object
    // to the panel
    panel.addMouseListener
    (
      new MouseAdapter ()
      {
        public void mouseEntered (MouseEvent e) {
          System.out.println (e.toString ());
        }
      }
    );

    // And a panel to the JApplet panel.
    content_pane.add (panel);

  } // ctor

} //class AdapterApplet

Note: Be careful that you properly spell the method that you are overriding in the adapter. Otherwise, you are just adding another method to the subclass. It can be hard to track down this bug in your program.

We don't have space to discuss them further, but here is the list of listener adapters with links to the Java 5.0 specifications pages: