JAVA: Anonymous Listeners

There is no need to define a named class simply to add a listener object to a button. Java has a somewhat obscure syntax for creating an anonymous innner class listener that implements an interface. There is no need to memorize the syntax; just copy and paste it each time. For example,

class myPanel extends JPanel {
. . .
   public MyPanel() {
       . . .  //in the constructor
       JButton b1 = new JButton("Hello");
       b1.addActionListener(
           new ActionListener() {
               public void actionPerformed(ActionEvent e) {
                   // do something for button b1
               }
           }
       );

Creates a class

The above example creates a subclass of Object that implements the ActionListener interface. The compiler creates names for anonymous classes. The JDK typically uses the enclosing class name followed by $ followed by a number, eg, you may see a MyPanel$1.class file generated by the compiler.

ActionListener Example Using Anonymous Inner Classes
-------------------------------------------------------------

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

/****
 *
 * @author Sabah u Din Irfan
 * @Description ActionListener Example Using Anonymous Inner Classes
 */
public class ActionListnerDemo extends JFrame{

    JLabel lbl ;
    JTextField txt1;
    JTextField txt2;
    JTextField txt3;
    JButton btnAdd;
    JButton btnSub;
    
    ActionListnerDemo()
    {
        // Set Frame Window properties
        super("Action Listner Demo!!");
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        // Get Container
        Container container = getContentPane();
        container.setLayout(new FlowLayout());
        // Create Components
        lbl = new JLabel(" Please Enter two values in text fields:");
        txt1 = new JTextField(15);
        txt2 = new JTextField(15);
        txt3 = new JTextField(15);
        btnAdd = new JButton("Add");
        btnSub = new JButton("Sub");
        // Add Components
        
        container.add(lbl);
        container.add(txt1);
        container.add(txt2);
        container.add(txt3);
        container.add(btnAdd);
        container.add(btnSub);
        
        // Add Action Listners.. Use Annonymous Inner Classes
        
        btnAdd.addActionListener( new ActionListener() {
            
            public void actionPerformed(ActionEvent e) {
                int a = Integer.parseInt(txt1.getText());
                int b = Integer.parseInt(txt2.getText());
                
                int c = a+b;
                
                txt3.setText(c+"");
                
            }
        }
        );
        
        ////////////////////
        btnSub.addActionListener( new ActionListener() {
            
            public void actionPerformed(ActionEvent e) {
                int a = Integer.parseInt(txt1.getText());
                int b = Integer.parseInt(txt2.getText());
                
                int c = a-b;
                
                txt3.setText(c+"");
                
            }
        }
        );
        
        // Make frame visible and set its size
        setVisible(true);
        pack();
    }
    /**
     * @param args
     */
    public static void main(String[] args) {
        // TODO Auto-generated method stub
            ActionListnerDemo frame = new ActionListnerDemo();
            
    }

}