EVENT-DRIVEN PROGRAMMING - Introduction to Java Programming - page 606

L ISTING 16.2 ControlCircleWithoutEventHandling.java
1 import javax.swing.*;
2 import java.awt.*;
3
4 public class ControlCircleWithoutEventHandling extends JFrame {
5 private JButton jbtEnlarge = new JButton( "Enlarge" ); buttons

6 private JButton jbtShrink = new JButton( "Shrink" );

7
8
9 public ControlCircleWithoutEventHandling() {
10 JPanel panel = new JPanel(); // Use the panel to group buttons
11 panel.add(jbtEnlarge);
12 panel.add(jbtShrink);
13
14

private CirclePanel canvas = new CirclePanel(); circle canvas

this .add(canvas, BorderLayout.CENTER); // Add canvas to center

15 this .add(panel, BorderLayout.SOUTH); // Add buttons to the frame

16 }
17
18 /** Main method */
19 public static void main(String[] args) {
20 JFrame frame = new ControlCircleWithoutEventHandling();
21 frame.setTitle( "ControlCircleWithoutEventHandling" );
22 frame.setLocationRelativeTo( null ); // Center the frame
23 frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
24 frame.setSize( 200 , 200 );
25 frame.setVisible( true );
26 }
27 }
28
29 class CirclePanel extends JPanel {
30

CirclePanel class

private int radius = 5 ; // Default circle radius

31
32 @Override /** Repaint the circle */
33

protected void paintComponent(Graphics g) {

paint the circle

34 super .paintComponent(g);

35
36
37 }
38 }

g.drawOval(getWidth() / 2 - radius, getHeight() / 2 - radius,

2 * radius, 2 * radius);

How do you use the buttons to enlarge or shrink the circle? When the Enlarge button is clicked,
you want the circle to be repainted with a larger radius. How can you accomplish this? You can
expand the program in Listing 16.2 into Listing 16.3 with the following features:

second version

1. Define a listener class named EnlargeListener that implements ActionListener
(lines 31-36).

2. Create a listener and register it with jbtEnlarge (line 18).

3. Add a method named enlarge() in CirclePanel to increase the radius, then repaint
the panel (lines 42-45).

4. Implement the actionPerformed method in EnlargeListener to invoke
canvas.enlarge() (line 34).

5. To make the reference variable canvas accessible from the actionPerformed
method, define EnlargeListener as an inner class of the ControlCircle class
(lines 31-36). ( Inner classes are defined inside another class. We will introduce inner
classes in the next section.)

inner class