BouncingBall.java

Below is the syntax highlighted version of BouncingBall.java.

It differs from the version in Booksite 1.5 in the initial position of the ball, and random factors for the velocity and radius.

/*************************************************************************
 *  Compilation:  javac BouncingBall.java
 *  Execution:    java BouncingBall
 *  Dependencies: StdDraw.java
 *  Implementation of a 2-d bouncing ball in the box from (-1, -1) to (1, 1).
 *************************************************************************/
public class BouncingBall { 
    public static void main(String[] args) {

        // set the scale of the coordinate system
        StdDraw.setXscale(-1.0, 1.0);
        StdDraw.setYscale(-1.0, 1.0);

        // initial values, random velocity and size
        double rx = 0.0, ry = 0.0;                    // position
        double vx = 0.015 - Math.random() * 0.03;     // x velocity
        double vy = 0.015 - Math.random() * 0.03;     // y velocity
        double radius = 0.025 + Math.random() * 0.05; // size

        // main animation loop
        while (true)  { 
            // bounce off wall according to law of elastic collision
            if (Math.abs(rx + vx) > 1.0 - radius) vx = -vx;
            if (Math.abs(ry + vy) > 1.0 - radius) vy = -vy;

            // update position
            rx = rx + vx; 
            ry = ry + vy; 

            // clear the background
            StdDraw.setPenColor(StdDraw.GRAY);
            StdDraw.filledSquare(0, 0, 1.0);

            // draw ball on the screen
            StdDraw.setPenColor(StdDraw.BLACK); 
            StdDraw.filledCircle(rx, ry, radius); 

            // display and pause for 20 ms
            StdDraw.show(20); 
        } 
    } 
}