The Java Boutique: Java Tutorial: Java by Example: Input Devices

import java.awt.*; import java.applet.*; class Ball { //the boundaries of the window and the diameter of our ball //we tell our balls the window size by constructor, so we can //change the applet's size easily int width, height; static final int diameter=30; //coordinates and value of increment double x, y, xinc, yinc; Color color; Graphics g; //the constructor public Ball(int w, int h, int x, int y, double xinc, double yinc, Color c) { width=w; height=h; this.x=x; this.y=y; this.xinc=xinc; this.yinc=yinc; color=c; } public void move() { x+=xinc; y+=yinc; //when the ball bumps against a boundary, it bounces off if(x<0 || x>width-diameter) { xinc=-xinc; x+=xinc; } if(y<0 || y>height-diameter) { yinc=-yinc; y+=yinc; } } public void paint(Graphics gr) { g=gr; g.setColor(color); //the coordinates in fillOval have to be int, so we cast //explicitly from double to int g.fillOval((int)x,(int)y,diameter,diameter); } } public class Project32 extends Applet implements Runnable { Thread runner; Image Buffer; Graphics gBuffer; Ball ball[]; //how many balls? static final int MAX=5; public void init() { Buffer=createImage(size().width,size().height); gBuffer=Buffer.getGraphics(); ball=new Ball[MAX]; int w=size().width; int h=size().height; //our balls have different start coordinates, increment values //(speed, direction) and colors ball[0]=new Ball(w,h,50,20,1.5,2.0,Color.blue); ball[1]=new Ball(w,h,60,100,2.0,-3.0,Color.red); ball[2]=new Ball(w,h,15,70,-2.0,-2.5,Color.green); ball[3]=new Ball(w,h,150,60,-2.7,-2.0,Color.cyan); ball[4]=new Ball(w,h,210,30,2.2,-3.5,Color.magenta); } public void start() { if (runner == null) { runner = new Thread (this); runner.start(); } } public void stop() { if (runner != null) { runner.stop(); runner = null; } } public void run() { while(true) { //Thread sleeps for 15 milliseconds here try {runner.sleep(15);} catch (Exception e) { } //move our balls around for(int i=0;i<MAX;i++) ball[i].move(); repaint(); } } public void update(Graphics g) { paint(g); } public void paint(Graphics g) { //draw a gray background and a grid of lines gBuffer.setColor(Color.gray); gBuffer.fillRect(0,0,size().width,size().height); gBuffer.setColor(Color.lightGray); for(int x=0;x<size().width;x+=50) gBuffer.drawLine(x,0,x,size().height); for(int y=0;y<size().height;y+=50) gBuffer.drawLine(0,y,size().width,y); //paint our balls for(int i=0;i<MAX;i++) ball[i].paint(gBuffer); g.drawImage (Buffer,0,0, this); } }