EVENT-DRIVEN PROGRAMMING - Introduction to Java Programming - page 607

6. To avoid compile errors, the CirclePanel class (lines 38-53) now is also defined as
an inner class in ControlCircle , since another CirclePanel class is already
defined in Listing 16.2.

L ISTING 16.3 ControlCircle.java
1 import javax.swing.*;
2 import java.awt.*;
3 import java.awt.event.*;
4
5 public class ControlCircle extends JFrame {
6

VideoNote

Listener and its registration

private JButton jbtEnlarge = new JButton( "Enlarge" );

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

8 private CirclePanel canvas = new CirclePanel();

9
10 public ControlCircle() {
11 JPanel panel = new JPanel(); // Use the panel to group buttons
12 panel.add(jbtEnlarge);
13 panel.add(jbtShrink);
14
15

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

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

17
18
19 }
20
21 /** Main method */
22 public static void main(String[] args) {
23 JFrame frame = new ControlCircle();
24 frame.setTitle( "ControlCircle" );
25 frame.setLocationRelativeTo( null ); // Center the frame
26 frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
27 frame.setSize( 200 , 200 );
28 frame.setVisible( true );
29 }
30
31

jbtEnlarge.addActionListener( new EnlargeListener());

create/register listener

class EnlargeListener implements ActionListener {

listener class

// Inner class

32 @Override
33
34
35
36
37
38

public void actionPerformed(ActionEvent e) {

canvas.enlarge();

}

}

class CirclePanel extends JPanel { // Inner class

CirclePanel class

39 private int radius = 5 ; // Default circle radius

40
41

/** Enlarge the circle */

42
43
44
45
46
47 @Override
48 protected void paintComponent(Graphics g) {
49 super .paintComponent(g);
50 g.drawOval(getWidth() / 2 - radius, getHeight() / 2 - radius,
51

public void enlarge() { enlarge method

radius++;

repaint();

}

2 * radius, 2 * radius);

52 }
53 }
54 }