Answers to Self-Check Problems - Building Java Programs: A Back to Basics Approach - page 1090

public void paintComponent(Graphics g) {
super.paintComponent(g);
g.drawOval(10, 30, 50, 50);
}
}
17. A Timer is an object that can fire an ActionListener event at regular intervals.
A Timer can be used to animate a panel by having its ActionListener change
the state of the panel in some way and then repaint it.
18. // An animated panel that draws a red and blue circle.
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
public class RedCirclePanel2 extends JPanel {
private boolean red;
public RedCirclePanel2() {
red = true;
Timer timer = new Timer(1000, new ColorListener());
timer.start();
}
public void paintComponent(Graphics g) {
super.paintComponent(g);
if (red) {
g.setColor(Color.RED);
} else {
g.setColor(Color.BLUE);
}
g.drawOval(10, 30, 50, 50);
}
private class ColorListener implements ActionListener {
public void actionPerformed(ActionEvent e) {
red = !red;
repaint();
}
}
}