Multithread applet Java balles rebondissantes

Je suis en train de créer une applet Java qui rebondit plusieurs balles dans une applet fenêtre, chacun avec son propre thread. Avec le code ci-dessous toutes les boules sont tirées, mais seule la première personne se déplace. Ce que je fais mal?

import java.applet.Applet;
import java.awt.Color;
import java.awt.Graphics;
import java.util.ArrayList;
import java.util.List;
import java.util.Random;
import static java.awt.Color.*;
public class BouncingBalls extends Applet implements Runnable {
List<Ball> ballList = new ArrayList(); //holds Ball objects
Color[] colors = new Color[]{BLACK, GRAY, WHITE, PINK, RED, ORANGE, YELLOW,
GREEN, BLUE, CYAN}; //array holding available colors
static int width, height; //variables for applet dimensions
int ballCount; //number of balls to be created, set by html parameter
Random random = new Random(); //random number generator
public void init() {
//get window dimensions
width = getSize().width;
height = getSize().height;
//get number of balls from html
String ballCountString = this.getParameter("ballCount");
try {
ballCount = Integer.parseInt(ballCountString);
} catch (NumberFormatException e) {
ballCount = 10; //set to 10 by default
}
for (int i = 0; i < ballCount; i++) {
//randomly assign ballDiameter between 1 and 20
int ballDiameter = random.nextInt(20) + 1;
//create and add balls to ballList
ballList.add(new Ball(
random.nextInt(width - ballDiameter), //set x coordinate
random.nextInt(height - ballDiameter), //set y coordinate
ballDiameter, //set ballDiameter
random.nextInt(ballDiameter) + 1, //deltaX <= ballDiameter
random.nextInt(ballDiameter) + 1, //deltaY <= ballDiameter
colors[i % 10] //use remainder to choose colors[] element
)
);
} //end for
} //end init
public void start() {
for (Ball ball: ballList) {
Thread t;
t = new Thread(this);
t.start();
} //end for
} //end start
public void run() {
for (Ball ball : ballList) {
//infinite loop: ball moves until applet is closed
while (true) {
ball.move();
repaint(); //call paint method to draw circle in new location
//set ball repaint delay using Thread sleep method
try {
Thread.sleep(20); //wait 20 msec before continuing
} catch (InterruptedException e) {
return;
}
} //end while
} //end for
} //end run
public void paint(Graphics g) {
super.paint(g);
for (Ball ball : ballList) {
//set current color
g.setColor(ball.ballColor);
//draw filled oval using current x and y coordinates and diameter
g.fillOval(ball.x, ball.y, ball.diameter, ball.diameter);
} //end for
} //end paint
}
class Ball {
int x, y, //coordinates of upper-left corner of circle
diameter, //circle diameter
deltaX, deltaY; //number of pixels ball moves each time it's repainted
Color ballColor;
public Ball(int x, int y, int diameter, int deltaX, int deltaY,
Color ballColor) {
this.x = x;
this.y = y;
this.diameter = diameter;
this.deltaX = deltaX;
this.deltaY = deltaY;
this.ballColor = ballColor;
} //end Ball constructor
public void move() {
//update x and y coordinates using delta values
x += deltaX;
y += deltaY;
//reverse x direction when ball reaches boundary
if (x >= BouncingBalls.width - diameter || x <= 0){
deltaX = -deltaX;
} //end if
//reverse y direction when ball reaches boundary
if (y >= BouncingBalls.height - diameter || y <= 0) {
deltaY = -deltaY;
} //end if
} //end move
} //end BouncingBalls