Why Synchronize in this program??

what is the importance of synchronize key word in this program,what will happen if we remove that from each method(that use it,specially getParticles(),setParticles())??

Particle {
 protected int x;
 protected int y;
 protected final Random rng = new Random();
 
 public Particle(int initialX, int initialY) {
  x = initialX;
  y = initialY;
 }    

 public synchronized void move() {
  x += rng.nextInt(10) - 5;
  y += rng.nextInt(20) - 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 {

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

 ParticleCanvas(int size) {
  setSize(new Dimension(size, size));
 }
 
 // intended to be called by applet
 protected 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 ParticleApplet extends Applet {

 protected Thread[ ] threads = null; // null when not running

 protected final ParticleCanvas canvas
                   = new ParticleCanvas(100);

 public void init() { add(canvas); }

 protected Thread makeThread(final Particle p) { // utility
  Runnable runloop = new Runnable() {
   public void run() {
    try {
     for(;;) {
      p.move();
      canvas.repaint();
      Thread.sleep(100); // 100msec is arbitrary
     }
    }
    catch (InterruptedException e) { return; }
   }
  };
  return new Thread(runloop);
 }

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

  if (threads == null) { // bypass if already started
   Particle[ ] particles = new Particle[n];
   for (int i = 0; i < n; ++i)
    particles[i] = new Particle(50, 50);
   canvas.setParticles(particles);

   threads = new Thread[n];
   for (int i = 0; i < n; ++i) {
    threads[i] = makeThread(particles[i]);
    threads[i].start();
   }
  }
 }

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