GRAPHICAL USER INTERFACE (GUI) AND OBJECT-ORIENTED DESIGN (OOD) - Java Programming: From Problem Analysis to Program Design - page 352

We can now repeat these four steps to create and register the action listener with the
JButton exitB .
private class ExitButtonHandler implements ActionListener
{

public void actionPerformed(ActionEvent e)
{

System.exit(0);

}

}
The following statements create the action listener object for the button exitB :

ExitButtonHandler ebHandler;

ebHandler = new ExitButtonHandler();
exitB.addActionListener(ebHandler);
The interface ActionListener is contained in the package java.awt.event . There-
fore, to use this interface to handle events, your program must include the statement:

import java.awt.event.*;
or:

import java.awt.event.ActionListener;

The complete program to calculate the perimeter and area of a rectangle is:
//Given the length and width of a rectangle, this Java
//program determines its area and perimeter.

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

public class RectangleProgram extends JFrame
{

private JLabel lengthL, widthL, areaL, perimeterL;

private JTextField lengthTF, widthTF, areaTF, perimeterTF;

private JButton calculateB, exitB;

private CalculateButtonHandler cbHandler;
private ExitButtonHandler ebHandler;

private static final int WIDTH = 400;
private static final int HEIGHT = 300;

public RectangleProgram()
{

//Create the four labels
lengthL = new JLabel("Enter the length: ",
SwingConstants.RIGHT);