//Animation Applet
//M.Dubson, Feb. 1999, ENVD 3252

import java.awt.*;
import java.applet.Applet;
import java.awt.event.*;
import java.lang.Thread;

public class Animation extends Applet implements Runnable
{ 
  Thread theThread;
  Image offscreen;
  int w,h; //width and height of screen
  int a;  //acceleration of gravity
  int nbrBalls; //Number of balls
  int x[], y[]; // x-, y-coordinates of ball position
  int vx[], vy[]; // x-, y-coordinates of ball velocity
  int r1; //radius of ball

  

  public void init()
  {
    nbrBalls = 8;
    a = 1;
    w = getSize().width; h = getSize().height;
    x = new int[nbrBalls];
    y = new int[nbrBalls];
    vx = new int[nbrBalls];
    vy = new int[nbrBalls];
    for(int i = 0; i < nbrBalls; i++)
    {
      x[i] = w/2 + (int)(Math.random()*w/5 - w/10);
      y[i] = h/2 + (int)(Math.random()*h/5 - h/10);
      vx[i] = (int)(Math.random()*11 - 20); 
      vy[i] = (int)(Math.random()*8);
      r1 = w/30;
    }
  }
  
  public void update( Graphics g )
  {
    if (offscreen == null || 
      offscreen.getHeight(this) != h || offscreen.getWidth(this) != w)
      { offscreen = createImage(w, h);}
    Graphics g2 = offscreen.getGraphics();
    paint(g2);
    g.drawImage( offscreen, 0, 0, this);
      
  }


  public void step()
  {
    Graphics g = getGraphics();
             
    for (int i=0; i<nbrBalls; i++)    
    {
      if( x[i] < 2*r1 || x[i] > (w-2*r1))
      {
        vx[i] *= -1;
      }
                     
      if( (vy[i] <0 && y[i] <= 2*r1) || (vy[i]>0 && y[i] >= h - 2*r1) )
      {
        vy[i] *= -1;
      }
        vy[i] += a;             
        x[i] += vx[i];
        y[i] += vy[i] ;
        
    }   
    repaint();
  
  }//End of step method

  public void paint(Graphics g)
  {
    g.setColor(Color.white);
    g.fillRect(0,0,w,h);
    for (int i = 0; i < nbrBalls; i++)
    {
      int red = i*128/nbrBalls +127;
      g.setColor(new Color(red, 0, 0));
      g.fillOval( x[i]-r1, y[i] - r1, 2*r1, 2*r1);
    }
  }//end of paint method

  public void start()
      {
         if( theThread == null ){theThread = new Thread( this );}
         theThread.start();
      }
             
  public void run()
  {
    while( true )
      {
        step();
        try
        {
          theThread.sleep( 40 );
        }
        catch( Exception e ) {;}

      }
  }//end of run method
             
  public void stop()
  {
    if( theThread != null && theThread.isAlive() )
    {
      theThread.stop();
    }
      theThread = null;
  }


} // End of class Animation