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Java by Example - 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 the balls
for(int i=0;i<MAX;i++)
ball[i].paint(gBuffer);
g.drawImage (Buffer,0,0, this);
}
}
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The next applet enhances the last one for the feature of collision among the balls, not only from the borders! This is not easy at all, as you can see. I used the Theorem of Pythagoras algorithm, to detect the correct collision distance. The bouncing impact only reverses the current movement direction of the two colliding balls, which is not realistic, but easier to do.
We iterate through two nested loops, in order to check for collision of each and every ball with any other. The collision of two balls then has to be "reported back" to the two involved balls. Clicking the applet toggles the sound, which is played on every ball collision. To toggle the sound flag (the boolean sound), we use a bitwise shift operator: sound^=true;. This toggles sound to true if false and vice versa. This is more elegant and shorter than to write: if(sound)sound=false; else sound=true;.
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//Sourcecode
import java.awt.*;
import java.applet.*;
class CollideBall
{
int width, height;
public static final int diameter=30;
//coordinates and value of increment
double x, y, xinc, yinc, coll_x, coll_y;
boolean collide;
Color color;
Graphics g;
//the constructor
public CollideBall(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 double getCenterX() {return x+diameter/2;}
public double getCenterY() {return y+diameter/2;}
public void move()
{
if (collide)
{
double xvect=coll_x-getCenterX();
double yvect=coll_y-getCenterY();
if((xinc>0 && xvect>0) || (xinc<0 && xvect<0))
xinc=-xinc;
if((yinc>0 && yvect>0) || (yinc<0 && yvect<0))
yinc=-yinc;
collide=false;
}
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 hit(CollideBall b)
{
if(!collide)
{
coll_x=b.getCenterX();
coll_y=b.getCenterY();
collide=true;
}
}
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 Project33 extends Applet implements Runnable
{
Thread runner;
Image Buffer;
Graphics gBuffer;
CollideBall ball[];
AudioClip clickSound;
boolean sound;
//how many balls?
static final int MAX=5;
public void init()
{
Buffer=createImage(size().width,size().height);
gBuffer=Buffer.getGraphics();
ball=new CollideBall[MAX];
int w=size().width;
int h=size().height;
try{clickSound=getAudioClip(getCodeBase(),"click.au");}
catch (Exception e){}
//our balls have different start coordinates, increment values
//(speed, direction) and colors
ball[0]=new CollideBall(w,h,50,20,1.5,2.0,Color.orange);
ball[1]=new CollideBall(w,h,60,100,2.0,-3.0,Color.red);
ball[2]=new CollideBall(w,h,15,70,-2.0,-2.5,Color.pink);
ball[3]=new CollideBall(w,h,150,60,-2.7,-2.0,Color.cyan);
ball[4]=new CollideBall(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();
handleCollision();
repaint();
}
}
boolean collide(CollideBall b1, CollideBall b2)
{
double wx=b1.getCenterX()-b2.getCenterX();
double wy=b1.getCenterY()-b2.getCenterY();
//we calculate the distance between the centers two
//colliding balls (theorem of Pythagoras)
double distance=Math.sqrt(wx*wx+wy*wy);
if(distance<b1.diameter)
{
if(sound)clickSound.play();
return true;
}
return false;
}
public boolean mouseDown(Event evt,int x,int y)
{
sound^=true;
return true;
}
private void handleCollision()
{
//we iterate through all the balls, checking for collisions
for(int i=0;i<MAX;i++)
for(int j=0;j<MAX;j++)
{
if(i!=j)
{
if(collide(ball[i], ball[j]))
{
ball[i].hit(ball[j]);
ball[j].hit(ball[i]);
}
}
}
}
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);
gBuffer.setColor(Color.orange);
if(sound)
gBuffer.drawString("Click to turn sound OFF", 10, 20);
else
gBuffer.drawString("Click to turn sound ON", 10, 20);
//paint the balls
for(int i=0;i<MAX;i++)
ball[i].paint(gBuffer);
g.drawImage (Buffer,0,0, this);
}
}
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