Input Devices
A Bouncing Balls Applet
The following applet is a little more advanced. We use a separate class to represent the balls and create 5 objects of it. To make it more interesting, we not only assign different colors, but also different initial movement direction (clockwise / counterclockwise), speed and angle. The coordinates of the balls (x, y) must be stored in floating point values here, to provide a smooth and exact movement. To avoid casting, we use the double type. The variables xinc and yinc are added to x and y in every new frame, in order to move the balls around. The value of these variables and their sign (+ / -) is responsible for speed and direction.
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//Sourcecode
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);
}
}
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