624 Chapter 18 Applets and Multimedia

59frame.setLocationRelativeTo(null); // Center the frame

60frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

61frame.setVisible(true);

62}

63

64/** Get command-line parameters */

65private void getCommandLineParameters(String[] args) {

66// Check usage and get x, y and message

67if (args.length != 3) {

68System.out.println(

69"Usage: java DisplayMessageApp x y message");

70System.exit(0);

71}

72else {

73x = Integer.parseInt(args[0]);

74y = Integer.parseInt(args[1]);

75message = args[2];

76}

77}

78}

When you run the program as an applet, the main method is ignored. When you run it as an application, the main method is invoked. Sample runs of the program as an application and as an applet are shown in Figure 18.8.

FIGURE 18.8 The DisplayMessageApp class can run as an application and as an applet.

The main method creates a JFrame object frame and creates a JApplet object applet, then places the applet applet into the frame frame and invokes its init method. The application runs just like an applet.

The main method sets isStandalone true (line 45) so that it does not attempt to

18.8 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 18.9.

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 18.7.

53public void setDelay(int delay) {

54this.delay = delay;

55timer.setDelay(delay);

56}

57}

The use of Timer to control animation was introduced in §16.12, “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 (lines 40–41). 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 18.8 BallControl.java