Implementing an interface in anonymous class

[Chris Allen]

I was going through some of the excellent mock exams on the Dan Chisholm site and came across a question I need to see some code to understand. The answer indicates that while anonymous classes can not use the implements keyword (which I can understand), they can implement interfaces. Can someone provide a simple example on how you can implement an interface without specifying the interface you are implementing and why you would do so? The only thing I can think of is something like the following (albeit a bit contrived):

abstract class AnonymousInterface implements Runnable {
  public abstract void run();
}
 
class B {
  static AnonymousInterface b = new AnonymousInterface() {
    public void run() {
      System.out.println("In B.run method");
    }
  };
 
  public static void main(String[] args) {
    b.run();
  }
}

In this case, the anonymous class is not using the implements keyword but is implementing the Runnable interface by specifying a method body to the run method.

[Alex Collins]

The best example is when using Gui's, and when creating a button (for example) supplying the implementation of it's ActionListener in an anonymous class using the ActionListener interface from java.awt.
Study this code:
----------------------------

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
 
/**
 *@author Alex Collins - [e-mail]
 *
 */
public class AnonymousClassTest extends JFrame{
    static char A = 'a';
    public AnonymousClassTest(){
createGui();
    }
 
    public void createGui(){
Button okButton = new Button("OK");
okButton.addActionListener( 
new ActionListener() {//Anonymous Class for button action
int i = 0;
    public void actionPerformed( ActionEvent e ){
    i++;
    System.out.println( AnonymousClassTest.A );
    System.out.println( "Button Clicked"+i );
}
    }
);
addWindowListener( 
new WindowAdapter(){
    public void windowClosing( WindowEvent e ){
System.exit( 0 );
    }
});
 

getContentPane().setLayout( new BorderLayout() );
getContentPane().add( okButton,BorderLayout.CENTER );
setBounds( 200,200,200,200 );
setVisible( true );
    }
 
    public static void main(String[] args){
new AnonymousClassTest();
    }
}  

Do you understand?
Using the ActionPerformed interface in the addActionListener of the button component, i created an anonymous class (implementing the ActionListener interface) to System.out "Button Clicked" and a number each time.

I hope that helps.