title: Simple bouncing ball program in Java
body: <p>I am trying to code a simple bouncing ball program. It has a rectangle and two balls in it. The balls have to bounce off the walls of the rectangle (I have coded that succesfull) and they also have to bounce off of each other. This is where I need your help. When I give the balls the same speed, they bounce fine and the program works, but I have to give the balls random speeds. When I do this the balls won't bounce anymore and just move through each other. </p>
  
  <pre><code>import java.applet.*; 
  import java.awt.*; 
  import javax.swing.*;
  import javax.swing.event.*;
  
  public class BallApplet2 extends Applet implements Runnable 
  { 
    // Begin variabelen
    int x_pos1 = 150; 
    int y_pos1 = 200; 
    int radius1 = 20; 
  
    int x_pos2 = 250; 
    int y_pos2 = 200; 
    int radius2 = 20;
  
    private float ballspeedx1 = -3;   
    private float ballspeedy1 = 0;
  
    private float ballspeedx2 = 3;   
    private float ballspeedy2 = 0; 
  
  
    public void init() {} 
  
    public void start() { 
    Thread th = new Thread (this); 
     th.start (); } 
     public void stop() {} 
    public void destroy() {} 
    public void run () {
  Thread.currentThread().setPriority(Thread.MIN_PRIORITY); 
  while (true) 
  { 
    x_pos1 += ballspeedx1;
    y_pos1 += ballspeedy1;
    x_pos2 += ballspeedx2;
    y_pos2 += ballspeedy2;  
  
  
    repaint();
    // als x_pos &lt; 100 is draait de richting van de bal om
    if (x_pos1  &lt; 100) {
      ballspeedx1 = -ballspeedx1; 
      x_pos1 = 100; 
    } 
    if (x_pos2  &lt; 100) {
      ballspeedx2 = -ballspeedx2; 
      x_pos2 = 100; 
    }  
    // als x_pos &gt; 300 is draait de richting van de bal om
    if (x_pos1  &gt; 300) {
      ballspeedx1 = -ballspeedx1; 
      x_pos1 = 300; 
    } 
    if (x_pos2  &gt; 300) {
      ballspeedx2 = -ballspeedx2; 
      x_pos2 = 300; 
    }                                       
    // als de x van de rode bal gelijk is aan de x en twee keer de straal van de blauwe bal, draaien beide ballen om.
    if (x_pos1 == x_pos2 + 40) {
      ballspeedx1 = -ballspeedx1;
      ballspeedx2 = -ballspeedx2;
    }
    // als de x van de blauwe bal gelijk is aan de x van de rode bal, draaien beide ballen om.
    if (x_pos2 == x_pos1 ) {
      ballspeedx1 = -ballspeedx1;
      ballspeedx2 = -ballspeedx2;
    }
    // als de x en twee keer de straal van de rode bal gelijk is aan de x van de blauwe bal, draaien beide ballen om.
    if (x_pos1 + 40 == x_pos2) {
      ballspeedx1 = -ballspeedx1;
      ballspeedx2 = -ballspeedx2;  
    }
    // als de x en twee keer de straal    
    if (x_pos2 + 40 == x_pos1) {
      ballspeedx1 = -ballspeedx1;
      ballspeedx2 = -ballspeedx2;  
    }
  
    try { Thread.sleep (20); } 
  
  
  
    catch (InterruptedException ex) {} 
  
  Thread.currentThread().setPriority(Thread.MAX_PRIORITY); }} 
  
  public void paint (Graphics g) {
  
  g.setColor (Color.red); 
  g.fillOval (x_pos1 - radius1, y_pos1 - radius1, 2 * radius1, 2 * radius1); 
  
  g.setColor (Color.blue); 
  g.fillOval (x_pos2 - radius2, y_pos2 - radius2, 2 * radius2, 2 * radius2); 
  
  
  g.setColor(Color.black);
  g.drawLine(80,80,80,320); // lijn links
  g.drawLine(320,80,320,320); // lijn rechts
  g.drawLine(80,80,320,80);  // lijn boven
  g.drawLine(80,320,320,320); // lijn onder
    }
  
  
  
  
    // Einde eventmethoden
  
  
  } 
  </code></pre>
  
  <p>Does anyone have a solution? If so try to keep it as simple as possible please :)</p>