18.2. How is I-O Handled in Java | Introduction to Java Programming-Comprehensive Version (6th Edition)

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16.8. (Optional) Case Study: Bouncing Ball

This section presents an applet that displays a ball bouncing in a panel. Use two buttons to suspend and resume the movement, and use a scroll bar to control the bouncing speed, as shown in Figure 16.13.

Figure 16.13. The ball's movement is controlled by the Suspend and Resume buttons and the scroll bar.

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Here are the major steps to complete this example.

1.	Create a subclass of JPanel named Ball to display a ball bouncing, as shown in Listing 16.8.
2.	Create a subclass of JPanel named BallControl to contain the ball with a scroll bar and two control buttons Suspend and Resume , as shown in Listing 16.9.
3.	Create an applet named BounceBallApp to contain an instance of BallControl and enable the applet to run standalone, as shown in Listing 16.10.

The relationship among these classes is shown in Figure 16.14.

Figure 16.14. BounceBallApp contains BallControl , and BallControl contains Ball .

Listing 16.8. Ball.java

(This item is displayed on pages 555 - 556 in the print version)

 1   import   javax.swing.Timer; 2   import   java.awt.*; 3   import   javax.swing.*; 4   import   java.awt.event.*; 5 6   public class   Ball   extends   JPanel { 7    private int   delay =   10   ;  8 9  // Create a timer with delay 1000 ms  10    protected   Timer timer =   new   Timer(delay,   new   TimerListener());  11 12   private int   x =     ;   private int   y =     ;  // Current ball position  13   private int   radius =   5   ;  // Ball radius  14   private int   dx =   2   ;  // Increment on ball's x-coordinate  15   private int   dy =   2   ;  // Increment on ball's y-coordinate  16 17   public   Ball() { 18  timer.start();  19 } 20 21    private class   TimerListener   implements   ActionListener {  22  /** Handle the action event */  23   public void   actionPerformed(ActionEvent e) { 24  repaint();  25 } 26 }

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 27 28    protected void   paintComponent(Graphics g) {  29   super   .paintComponent(g); 30 31 g.setColor(Color.red); 32 33  // Check boundaries  34   if   (x < radius) dx = Math.abs(dx); 35   if   (x > getWidth() - radius) dx = -Math.abs(dx); 36   if   (y < radius) dy = Math.abs(dy); 37   if   (y > getHeight() - radius) dy = -Math.abs(dy); 38 39  // Adjust ball position  40 x += dx; 41 y += dy; 42 g.fillOval(x - radius, y - radius, radius *   2   , radius *   2   ); 43 } 44 45   public void   suspend() { 46 timer.stop();  // Suspend timer  47 } 48 49   public void   resume() { 50 timer.start();  // Resume timer  51 } 52 53   public void   setDelay(   int   delay) { 54   this   .delay = delay; 55 timer.setDelay(delay); 56 } 57 }

Using Timer to control animation was introduced in §14.6, "Animation Using the Timer Class." Ball extends JPanel to display a moving ball. The timer listener implements ActionListener to listen for ActionEvent (line 21). Line 10 creates a Timer for a Ball . The timer is started in line 18 when a Ball is constructed . The timer fires an ActionEvent at a fixed rate. The listener responds in line 24 to repaint the ball to animate ball movement. The center of the ball is at ( x , y ), which changes to ( x + dx , y + dy ) on the next display. The suspend and resume methods (lines 45 “51) can be used to stop and start the timer. The setDelay(int) method (lines 53 “56) sets a new delay.

Listing 16.9. BallControl.java

(This item is displayed on pages 556 - 557 in the print version)

 1   import   javax.swing.*; 2   import   java.awt.event.*; 3   import   java.awt.*; 4 5   public class   BallControl   extends   JPanel { 6   private   Ball ball =   new   Ball(); 7    private   JButton jbtSuspend =   new   JButton(   "Suspend"   );  8    private   JButton jbtResume =   new   JButton(   "Resume"   );  9    private   JScrollBar jsbDelay =   new   JScrollBar();  10 11   public   BallControl() { 12  // Group buttons in a panel  13 JPanel panel =   new   JPanel(); 14 panel.add(jbtSuspend); 15 panel.add(jbtResume);

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 16 17  // Add ball and buttons to the panel  18 ball.setBorder(   new   javax.swing.border.LineBorder(Color.red)); 19 jsbDelay.setOrientation(JScrollBar.HORIZONTAL); 20 ball.setDelay(jsbDelay.getMaximum()); 21 setLayout(   new   BorderLayout()); 22 add(jsbDelay, BorderLayout.NORTH); 23 add(ball, BorderLayout.CENTER); 24 add(panel, BorderLayout.SOUTH); 25 26  // Register listeners  27  jbtSuspend.addActionListener(   new   ActionListener()  { 28   public void   actionPerformed(ActionEvent e) { 29  ball.suspend();  30 } 31 }); 32  jbtResume.addActionListener(   new   ActionListener() {  33   public void   actionPerformed(ActionEvent e) { 34  ball.resume();  35 } 36 }); 37  jsbDelay.addAdjustmentListener(   new   AdjustmentListener() {  38   public void   adjustmentValueChanged(AdjustmentEvent e) { 39  ball.setDelay(jsbDelay.getMaximum() - e.getValue());  40 } 41 }); 42 } 43 }

The BallControl class extends JPanel to display the ball with a scroll bar and two control buttons. When the Suspend button is clicked, the ball's suspend() method is invoked to suspend the ball movement (line 29). When the Resume button is clicked, the ball's resume() method is invoked to resume the ball movement (line 34). The bouncing speed can be changed using the scroll bar.

Listing 16.10. BounceBallApp.java

 1   import   java.awt.*; 2   import   java.awt.event.*; 3   import   java.applet.*; 4   import   javax.swing.*; 5 6   public class    BounceBallApp   extends   JApplet  { 7   public   BounceBallApp() { 8  add(   new   BallControl());  9 } 10 }

The BounceBallApp class simply places an instance of BallControl in the applet. The main method is provided in the applet (not displayed in the listing for brevity) so that you can also run it standalone.