Java Bouncing Balls Threads problem?

Question

Hello,
I am working on a homework assignment to represent a java applet with some bouncing balls inside. So far so good. The balls bounce and behave as they are supposed. The only thing is that I want to make 2 buttons, Start and Stop (this is not part of the assignment, but my free will to provide some extra stuff :) ) . I am implementing Runnable for the animation and ActionListener for the buttons. I did research on threading, but somehow I am still not getting quite the result I want. The applet is not displaying my buttons (I guess I am not implementing them correctly) and I dont know whether I have synchronized the threads correctly as well. So, I am asking for some guidance how can I do this? Thanks in advance!
As a remark, I am new to Java, as I am just starting to learn it and this is my first assignment.
import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.FlowLayout;
import java.awt.Graphics;
import java.awt.Image;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JApplet;
import javax.swing.JButton;
import javax.swing.JPanel;
public class Balls extends JApplet implements Runnable, ActionListener
     Thread runner = null;     
     Image img;
    Graphics gr;          
    BallCollision ball[];
    Balls can;
    JButton stopButton;
    JButton startButton;
    JPanel controls;
    boolean stop,start;
    //field for 10 balls
    static final int MAX=10;
     public void init()
          setSize(800,600);
          img = createImage(size().width,size().height);
          gr = img.getGraphics();     
          startButton = new JButton("Start");
          stopButton = new JButton("Stop");
          stopButton.addActionListener(this);
          startButton.addActionListener(this);
          controls = new JPanel();
          controls.setLayout(new FlowLayout());
          controls.add(startButton);
          controls.add(stopButton);
          //new Thread(this).start();
          ball = new BallCollision[MAX];
          int w=size().width;
          int h=size().height;          
          //creation of balls, which have different coordinates,
          //speed, direction and colors
          ball[0] = new BallCollision(w,h,50,20,1.5,7.5,Color.orange);
        ball[1] = new BallCollision(w,h,60,210,2.0,-3.0,Color.red);
        ball[2] = new BallCollision(w,h,15,70,-2.0,-2.5,Color.pink);
        ball[3] = new BallCollision(w,h,150,30,-2.7,-1.0,Color.cyan);
        ball[4] = new BallCollision(w,h,210,30,2.2,-12.5,Color.magenta);
          ball[5] = new BallCollision(w,h,360,170,2.2,-1.5,Color.yellow);
          ball[6] = new BallCollision(w,h,210,180,-1.2,-2.5,Color.blue);
          ball[7] = new BallCollision(w,h,330,30,-2.2,-1.8,Color.green);
          ball[8] = new BallCollision(w,h,180,220,-2.2,-1.8,Color.white);
          ball[9] = new BallCollision(w,h,330,130,-2.2,9.0,Color.gray);     
     public void actionPerformed(ActionEvent e)
          if(e.getSource() == startButton) start = true;
               can.start();
          if(e.getSource() == stopButton) start = false;
               can.stop();
     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)
               try {Thread.sleep(15);}
                 catch (Exception e) { }               
               //move our balls around
               for(int i=0;i<MAX;i++)
                    ball.move();
               handleCollision();
               repaint();     
     boolean collide(BallCollision b1, BallCollision b2)
          double wx=b1.getCenterX()-b2.getCenterX();
          double wy=b1.getCenterY()-b2.getCenterY();
          //the distance between 2 colling balls' centres is
          //calculated by the theorem of Pythagoras
          double distance=Math.sqrt(wx*wx+wy*wy);
          if(distance<b1.diameter)
               return true;          
               return false;     
     private void handleCollision()
          //ecah ball is checked for possible 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)
          gr.setColor(Color.black);
          gr.fillRect(0,0,size().width,size().height);          
          //paint the balls
          for(int i=0;i<MAX;i++)
                    ball[i].paint(gr);          
          g.drawImage (img,0,0, this);                    
class BallCollision
     int width, height;
     int diameter=30;
     //balls' coordinates and values to be incremented for directions
     double x, y, xIncremented, yIncremented, coll_x, coll_y;
     boolean collide;
     Color color;
     Graphics g;
     //constructor
     public BallCollision(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.xIncremented=xInc;
          this.yIncremented=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((xIncremented>0 && xvect>0) || (xIncremented<0 && xvect<0))
                    xIncremented=-xIncremented;
               if((yIncremented>0 && yvect>0) || (yIncremented<0 && yvect<0))
                    yIncremented=-yIncremented;
               collide=false;
          x+=xIncremented;
     y+=yIncremented;
          //if the ball reaches a wall, it bounces to the opposite direction
     if(x<1 || x>width-diameter)
          xIncremented=-xIncremented;
               x+=xIncremented;
          if(y<1 || y>height-diameter)
               yIncremented=-yIncremented;
               y+=yIncremented;
     public void hit(BallCollision b)
          if(!collide)
               coll_x=b.getCenterX();
               coll_y=b.getCenterY();
               collide=true;
     public void paint(Graphics graphics)
          g=graphics;
          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);

Answer 1

well i didnt arrive at this point without reading tutorials and researching.... sometimes other people can spot your mistakes a lot easier than you can, that's why I asked for help. 10x anyway for the interest!