Bouncing balls collision

Question

I am creating a program that has two balls which produce a sound when bouncing off the walls and each other. I am having trouble when they collide with each other, as my value for when they hit each other does not seem to be working. Here is my source code :
import java.applet.Applet;
import java.awt.Graphics;
import java.awt.*;
import java.applet.AudioClip;
import java.awt.event. *;
public class BallBouncing extends Applet implements Runnable {
        int x_pos = 20;
        int y_pos = 100;
        int x_speed = 1;
        int y_speed = 1;
        int x1_pos = 70;
        int y1_pos = 130;
        int x1_speed = 1;
        int y1_speed = 1;
        int appletsize_x = 300;
        int appletsize_y = 200;
        int radius = 20;
        int pos = (x_pos - x1_pos)*(x_pos - x1_pos) + (y_pos - y1_pos)*(y_pos - y1_pos);
        double pos1 = Math.sqrt(pos);       
        public AudioClip sound1, sound2;       
        Thread t;
        Button b1 = new Button("Reverse Direction");
        public void init() {
            b1.addActionListener(new B1());           
            add(b1);
        class B1 implements ActionListener {
        public void actionPerformed(ActionEvent e) {
        x_speed = +3;
        y_speed = -3;
        public void start() {
                        t = new Thread(this);
                        t.start();
        public void stop() {
        public void paint(Graphics g) {
                g.setColor (Color.blue);
                g.fillOval(x_pos - radius, y_pos - radius, 2 * radius, 2 * radius);
                g.fillOval(x1_pos - radius, y1_pos - radius, 2 * radius, 2 * radius);
        public void run() {
                sound1 = getAudioClip( getDocumentBase(), "Audio1.au" );
                sound2 = getAudioClip( getDocumentBase(), "Audio2.au" );
                while (true) {
                    try {
                        Thread.sleep(20);
                    } catch (InterruptedException e) {}
                    if (x_pos > appletsize_x - radius)
                        x_speed = -3;
                        sound2.play();
                    else if (x_pos < radius)
                        x_speed = +3;
                        sound2.play();
                    else if (y_pos > appletsize_y - radius)
                        y_speed = -3;
                        sound2.play();
                    else if (y_pos < radius)
                        y_speed = +3;
                        sound2.play();
                    else if (x1_pos > appletsize_x - radius)
                        x1_speed = -3;
                        sound2.play();
                    else if (x1_pos < radius)
                        x1_speed = +3;
                        sound2.play();
                    else if (y1_pos > appletsize_y - radius)
                        y1_speed = -3;
                        sound2.play();
                    else if (y1_pos < radius)
                        y1_speed = +3;
                        sound2.play();
                    else if (pos1 < 40)
                        x_speed = -3;
                        x1_speed = +3;
                        y_speed = -3;
                        y1_speed = +3;
                        sound1.play();
                        x_pos += x_speed;
                        y_pos += y_speed;
                        x1_pos += x1_speed;
                        y1_pos += y1_speed;
                        repaint();
}Any help would be appreciated, thanks.

Answer 1

Hi, here is a solution to your problem I hope. I have also included some extra features and brought your code up to a higher standard in terms of java conventions (however the {} on the next line is my convention, from C# :p).
A few things:
- For the balls deflecting at the proper angles then you will need to learn vector maths and I am afraid that my knowledge of that is rather sketchy (still 1st year uni student, more on that next year :p)
- I used Graphics2D purely because I was lazy so I could write g2.draw(bounds); instead of using g.drawRect(x, y, height, width); - both methods give the same result
- You will need to look up how to do drawing in applets to remove that flickering, I'm not sure if you do the same thing as normal swing/awt components (ie double buffering), as applets aren't my strong point.
- I removed the button because it was unneccessary, it will take you 60 seconds to add it back in if you want.
- You really REALLY should write a 'Ball' class and make some Ball objects instead of having separate variables for each ball. I will leave that up to you :)
- I STRONGLY recommend the use of comments throughout any program you use, not because of convention but because it not only helps other developers to understand your code and what it does, but also to refresh your memory when you come back from a break and try to keep writing. (trust me, do not underestimate the power of comments!) . I have put a few in to show you what they should look like; they should be brief and concise, explain what a section of code does, and how it does it.
- Enjoy!
package sunforum_bounceball;
import java.applet.*;
import java.awt.*;
public class BallBouncing extends Applet implements Runnable
     private static final long serialVersionUID = 1L;
     // ball 1
     private int x1_pos;
     private int y1_pos;
     private int x1_speed;
     private int y1_speed;
     private int b1_radius;
     // ball 2
     private int x2_pos;
     private int y2_pos;
     private int x2_speed;
     private int y2_speed;
     private int b2_radius;
     // other variables
     private Rectangle bounds;
     private Dimension appletSize;
     private boolean hasCollision;
     private boolean debugCollisionTesting;
     private AudioClip sound1, sound2;       
     private Thread t;
     public BallBouncing()
          // ball 1
          x1_pos = 100;
          y1_pos = 109;
          x1_speed = 2;
          y1_speed = 1;
          b1_radius = 40;
          // ball 2
          x2_pos = 100;
          y2_pos = 237;
          x2_speed = 1;
          y2_speed = 1;
          b2_radius = 20;
          hasCollision = false;
          // This is just for your help; turn this on to disable the balls changing direction when they collide.
          // This will enable you to check that when the balls are touching, we are detecting it.
          debugCollisionTesting = false;
          // Variables for the size of the applet and the bounding rectangle for the balls.
          appletSize = new Dimension(400, 400);
          bounds = new Rectangle(0, 0, 400, 400);
     public void start()
          t = new Thread(this);
          t.start();
     public void stop()
     public void paint(Graphics g)
          Graphics2D g2 = (Graphics2D)g;
          if (hasCollision)
          { g2.setColor(Color.RED); }
          else
          { g2.setColor(Color.BLUE); }
          g2.fillOval(x1_pos - b1_radius, y1_pos - b1_radius, 2 * b1_radius, 2 * b1_radius);
          g2.fillOval(x2_pos - b2_radius, y2_pos - b2_radius, 2 * b2_radius, 2 * b2_radius);
          g2.setColor(Color.BLACK);
          g2.draw(bounds);
     public void run()
          // +1 just so we can see the border of the collision area. Try taking out the +1's and see what happens.
          this.setSize(appletSize.width + 1, appletSize.height + 1);
          sound1 = getAudioClip( getDocumentBase(), "Audio1.au" );
          sound2 = getAudioClip( getDocumentBase(), "Audio2.au" );
          // Used to hold the distance between the balls.
          double p;
          while (true)
               try
               { Thread.sleep(20); }
               catch (InterruptedException e)
               // ball 1 x coordinate.
               if (x1_pos > (bounds.x + bounds.width) - b1_radius || x1_pos < bounds.x + b1_radius)
                    x1_speed *= -1;
                    sound2.play();
               // ball 1 y coordinate.
               else if (y1_pos > (bounds.y + bounds.height) - b1_radius || y1_pos < bounds.y + b1_radius)
                    y1_speed *= -1;
                    sound2.play();
               // ball 2 x coordinate.
               if (x2_pos > (bounds.x + bounds.width) - b2_radius || x2_pos < bounds.x + b2_radius)
                    x2_speed *= -1;
                    sound2.play();
               // ball 2 y coordinate.
               else if (y2_pos > (bounds.y + bounds.height) - b2_radius || y2_pos < bounds.y + b2_radius)
                    y2_speed *= -1;
                    sound2.play();
               // Checks the distance between the balls. If it is less than the sum of the radii, then they are colliding.
               p = Math.sqrt(Math.pow((x1_pos - x2_pos), 2) + Math.pow((y1_pos - y2_pos), 2));
               if (p < (b1_radius + b2_radius))
                    // To check there is a collision. Useful for debugging.
                    // System.out.println("Collision");
                    hasCollision = true;
                    // see declaration for details
                    if (!debugCollisionTesting)
                         x1_speed *= -1;
                         x2_speed *= -1;
                         y1_speed *= -1;
                         y2_speed *= -1;
                         sound1.play();
               else
               { hasCollision = false; }
               // Move both balls.
               x1_pos += x1_speed;
               y1_pos += y1_speed;
               x2_pos += x2_speed;
               y2_pos += y2_speed;
               // Repaint the scene
               repaint();
}Cheers
Sutasman
Edited by: Sutasman on Dec 6, 2009 1:17 AM