This program creates a beautiful animation of colorful bouncing balls. The finished product could definitely make a solid screen saver.

We'll have to generate 50 circles and place them randomly about the canvas. Note that in addition to creating the balls, we'll also need to generate initial x and y velocities for each ball. These velocities should be random. Importantly, we will store the balls and velocities in three ArrayLists. We can decompose all of this into a method. On each iteration of the animation while loop, we'll also need to check for bounces on all of the balls using an inner for loop.

## Solution

/**
* Class: BouncingBalls
* -----------------
* A program that bounces many balls simultaneously around the screen.
*/
public class BouncingBalls extends GraphicsProgram {
private static final double BALL_SIZE = 20;
private static final double N_BALLS = 50;

private ArrayList balls;
private ArrayList ballVx;
private ArrayList ballVy;
private RandomGenerator rg = new RandomGenerator();

public void run() {
createBalls();
while(true) {
animateBalls();
pause(10);
}
}

private void animateBalls() {
for(int i = 0; i < N_BALLS; i++) {
GOval ball = balls.get(i);
if(ball.getX() + ball.getWidth() > getWidth() || ball.getX() < 0) {
reflectX(i);
}
if(ball.getY() + ball.getHeight() > getHeight() || ball.getY() < 0) {
reflectY(i);
}
ball.move(ballVx.get(i), ballVy.get(i));
}
}

private void reflectY(int i) {
ballVy.set(i, -ballVy.get(i));
}

private void reflectX(int i) {
ballVx.set(i, -ballVx.get(i));
}

private void createBalls() {
balls = new ArrayList();
ballVx = new ArrayList();
ballVy = new ArrayList();
for(int i = 0; i < N_BALLS; i++) {
double x = rg.nextDouble(0, getWidth() - BALL_SIZE);
double y = rg.nextDouble(0, getHeight()- BALL_SIZE);

double vx = rg.nextDouble(1, 7);
if(rg.nextBoolean()) {
vx *= -1;
}
double vy = rg.nextDouble(1, 7);
if(rg.nextBoolean()) {
vy *= -1;
}

GOval ball = new GOval(x, y, BALL_SIZE, BALL_SIZE);
ball.setFilled(true);
ball.setColor(rg.nextColor());