Bouncing Balls (with Solution)

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);
X