Mega Code Archive
Bouncing Circle
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/*
* Copyright (c) 2000 David Flanagan. All rights reserved.
* This code is from the book Java Examples in a Nutshell, 2nd Edition.
* It is provided AS-IS, WITHOUT ANY WARRANTY either expressed or implied.
* You may study, use, and modify it for any non-commercial purpose.
* You may distribute it non-commercially as long as you retain this notice.
* For a commercial use license, or to purchase the book (recommended),
* visit http://www.davidflanagan.com/javaexamples2.
*/
import java.applet.Applet;
import java.awt.Color;
import java.awt.Graphics;
import java.awt.Rectangle;
/** 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 */
public void stop() {
// Set the flag that causes the run() method to end
pleaseStop = true;
}
}
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