Java Reference
In-Depth Information
Example 11•4: BouncingCircle.java
package com.davidflanagan.examples.graphics;
import java.applet.*;
import java.awt.*;
/** An applet that displays a simple animation */
public class BouncingCircle extends Applet implements Runnable {
int x = 150, y = 50, r = 50; // Position and radius of the circle
int dx = 11, dy = 7; // Trajectory of circle
Thread animator; // The thread that performs the animation
volatile boolean pleaseStop; // A flag to ask the thread to stop
/** This method simply draws the circle at its current position */
public void paint(Graphics g) {
g.setColor(Color.red);
g.fillOval(x-r, y-r, r*2, r*2);
}
/**
* This method moves (and bounces) the circle and then requests a redraw.
* The animator thread calls this method periodically.
**/
public void animate() {
// Bounce if we've hit an edge.
Rectangle bounds = getBounds();
if ((x - r + dx < 0) || (x + r + dx > bounds.width)) dx = -dx;
if ((y - r + dy < 0) || (y + r + dy > bounds.height)) dy = -dy;
// Move the circle.
x += dx; y += dy;
// Ask the browser to call our paint() method to draw the circle
// at its new position.
repaint();
}
/**
* This method is from the Runnable interface. It is the body of the
* thread that performs the animation. The thread itself is created
* and started in the start() method.
**/
public void run() {
while(!pleaseStop) { // Loop until we're asked to stop
animate(); // Update and request redraw
try { Thread.sleep(100); } // Wait 100 milliseconds
catch(InterruptedException e) {} // Ignore interruptions
}
}
/** Start animating when the browser starts the applet */
public void start() {
animator = new Thread(this); // Create a thread
pleaseStop = false; // Don't ask it to stop now
animator.start(); // Start the thread.
// The thread that called start now returns to its caller.
// Meanwhile, the new animator thread has called the run() method
}
/** Stop animating when the browser stops the applet */
