/*
 *  =============================================================
 *  BouncePanel.java : Bounce circles around on the screen.
 *
 *  Written by : Vasilios Lagakos                     March, 2001
 *  ==============================================================
 */

import javax.swing.*;
import java.awt.*;

public class BouncePanel extends JComponent implements Runnable {

   private MovingCircle circle;
   private int width, height;
   private Thread animationThread = null;  
  

   public BouncePanel() {
	

   }

   //----------------------------------------------------
   /*** Skip the usual screen-clearing step of update
      *  so that there is no "flicker" between each
      *  drawing step.
      */

   public void update(Graphics g){
	paint(g);
   }

   //----------------------------------------------------

   /***  Draws the circle in its new location
      */
   public void paint(Graphics g) {

      width = getSize().width;
      height = getSize().height;

     /* If you do not check if "circle" is not null before using it to draw a circle 
      * you will get a NullPointerException, a Runtime exception that occurs when a 
      * method tries to access a member of an object through a null reference. This 
      * NullPointerException occurs because when the JApplet starts it automatically
      * calls this paint method and executes everything in the paint method. However, 
      * when the JApplet starts we have not pressed the start button yet and thus
      * have not created the MovingCircle object, "circle". Therefore "circle" tries
      * to access a method through a null reference.
      */
      if(circle != null) {
         circle.move(width, height);
         circle.draw(g, width, height);
      }
   }

   //----------------------------------------------------
   /*** When the "start" button is pressed, start the
      * animation thread if it is not already started.
      * Create a new MovingCircle.
      */ 
   public void start() {

      int radius = 25;
      int x = radius + randomInt(width - 2 * radius);
      int y = radius + randomInt(height - 2 * radius);
      int deltaX = 1 + randomInt(10);


      int deltaY = 1 + randomInt(10);
      circle = new MovingCircle(x, y, radius, deltaX, deltaY);

      animationThread = new Thread(this);
      animationThread.start();

   }

   //----------------------------------------------------
   /***  When the "stop" button is pressed, stop
      *  the thread.
      */
   public void stop() {   
	if (animationThread != null) {	   
         animationThread = null;
      }
   }


   //----------------------------------------------------
   /** Each time around the loop, call paint and then
    *  take a short pause. The paint method will
    *  move the circles and draw them.
    */
   public void run() {
	Thread myThread = Thread.currentThread();
      // Really while animationThread not null
      while(animationThread==myThread) { 
         repaint();
         pause(100);
      }
  }



   //----------------------------------------------------
   // Returns an int from 0 to max (inclusive),
   // yielding max + 1 possible values.
   private int randomInt(int max) {
	double x = Math.floor((double)(max + 1) * Math.random());
      return((int)(Math.round(x)));
   }

   //----------------------------------------------------
   // Sleep for the specified amount of time.
   private void pause(int milliseconds) {
      try {
         Thread.sleep((long)milliseconds);
      } catch(InterruptedException ie) {}
   }
} // end of class BouncePanel