/*
  Sample code for Java Threads Lecture, based on ParticleApplet example in CPJ2e.
  RCS Info: $Id: ParticleAppletA.java,v 1.2 2001/04/25 13:33:30 bsl Exp $
*/


import java.util.*;
import java.applet.*;
import java.awt.*;


class Particle extends Thread {
    protected int x;
    protected int y;
    protected final Random rng = new Random(this.hashCode());
 
    public Particle(int initialX, int initialY) { 
	x = initialX;
	y = initialY;
    }

    public void run() {
	try {
	    for(;;) {
		move();
		sleep(100); // 100msec is arbitrary
	    }
	}
	catch (InterruptedException e) { 
	    return; 
	}
    }

    public synchronized void move() {
	x += (rng.nextInt() % 10);
	y += (rng.nextInt() % 10);
    }

    public void draw(Graphics g) {
	int lx, ly;
	synchronized (this) { lx = x; ly = y;}
	g.drawRect(lx, ly, 10, 10);
    }
}

class ParticleCanvas extends Canvas implements Runnable {

    private Particle[] particles = new Particle[0]; 

    public ParticleCanvas(int size) {
	setSize(new Dimension(size, size));
    }
 
    public void run() {
	try {
	    for(;;) {
		repaint();
		Thread.sleep(100); // 100msec is arbitrary
	    }
	}
	catch (InterruptedException e) { 
	    return; 
	}
    }

    // Called by the applet
    synchronized void setParticles(Particle[] ps) {
	if (ps == null) 
	    throw new IllegalArgumentException("Cannot set null");

	particles = ps; 
    }

    protected synchronized Particle[] getParticles() { 
	return particles; 
    }
 
    public void paint(Graphics g) { // override Canvas.paint
	Particle[] ps = getParticles();

	for (int i = 0; i < ps.length; ++i) 
	    ps[i].draw(g);

    }
}

public class ParticleAppletA extends Applet {

    protected final ParticleCanvas canvas = new ParticleCanvas(400);
    protected Particle[] particles; // null when not running
    protected Thread canvasThread;
 
    public void init() { add(canvas); }

    public synchronized void start() {
	int n = 10; // just for demo

	if (particles == null) { // bypass if already started
	    particles = new Particle[n];
	    for (int i = 0; i < n; ++i) {
		particles[i] = new Particle(200, 200);
		particles[i].setName("Particle Thread " + i);
		particles[i].start();
	    }
	    canvas.setParticles(particles);
	    canvasThread = new Thread(canvas);
	    canvasThread.setName("Canvas Thread");
	    canvasThread.start();
	}
    }

    public synchronized void stop() {
	if (particles != null) { // bypass if already stopped
	    for (int i = 0; i < particles.length; ++i)
		particles[i].interrupt();
	    particles = null;
	    canvasThread.interrupt();
	    canvasThread = null;
	}
    }
}