Computer Science 252
Problem Solving with Java

Spring 2016, The College of Saint Rose

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BallBounceAround BlueJ Project

Click here to download a BlueJ project for BallBounceAround.

BallBounceAround Source Code

The Java source code for BallBounceAround is below. Click on a file name to download it.

BallBounceAround.java

import objectdraw.*;
import java.awt.*;

/*
 * Example BallBounceAround: demonstration of the effect of
 * gravity on a bouncing ball
 *
 * Jim Teresco, The College of Saint Rose, CSC 252, Spring 2016
 *
 * $Id: template.java 2329 2014-02-27 02:46:26Z terescoj $
 */

public class BallBounceAround extends WindowController {

    
    public void begin() {
        
        new Text("Press to launch bouncy balls!", 0, 0, canvas);
	
    }
    
    public void onMousePress(Location point) {
	
        // each time the mouse is pressed on the canvas, a new
        // bouncy ball appears at that location
        new BouncyBall(point, canvas);
    }
}

BouncyBall.java

import objectdraw.*;
import java.awt.*;
import java.util.Random;

/**
 * 
 * Example BallBounceAround: the bouncy ball that bounces all around the
 * canvas.  The ball starts being stationary in y at the given point and is
 * subject to gravity.  It is given a random x speed.
 * 
 * Jim Teresco, The College of Saint Rose, CSC 252, Spring 2016
 */
public class BouncyBall extends ActiveObject {

    // the size of the ball
    private static final double SIZE = 20;
    
    // range of speeds in the x direction
    private static final double MAX_X_SPEED = 10;
    
    // animation delay time
    private static final int PAUSE_TIME = 33;
    
    // amount to add to ySpeed each frame to simulate gravity
    private static final double GRAVITY = 0.1;
    
    // we'll create a single instance of Random to be shared among
    // all instances of BouncyBall
    private static Random r = new Random();
    
    // standard instance variables to remember the canvas
    // and the ball itself
    private DrawingCanvas canvas;
    private FilledOval ball;

    public BouncyBall(Location point, DrawingCanvas c) {
        
        // remember the canvas, which we'll need down in run
        canvas = c;
        
        // the ball, centered at the specified point, making sure we're not 
        // creating partially off the canvas
        double xPos = point.getX() - SIZE/2;
        double yPos = point.getY() - SIZE/2;
        if (xPos > canvas.getWidth() - SIZE) {
            xPos = canvas.getWidth() - SIZE;
        }
        if (yPos > canvas.getHeight() - SIZE) {
            yPos = canvas.getHeight() - SIZE;
        }
        
        ball = new FilledOval(xPos, yPos, SIZE, SIZE, c);
                              
        // start it up!
        start();
    }
    
    public void run() {
        
        // choose a random x speed
        double xSpeed = r.nextDouble() * 2 * MAX_X_SPEED - MAX_X_SPEED;
        // and start y at 0, and let gravity do its thing
        double ySpeed = 0.0;
        
        // the ball bounces forever!
        while (true) {

            // move
            ball.move(xSpeed, ySpeed);
            
            // check for bouncing off the floor, but only reverse if still going down to 
            // avoid getting "stuck" along the floor
            if ((ball.getY() + SIZE >= canvas.getHeight()) && (ySpeed > 0)) {
                ball.moveTo(ball.getX(), canvas.getHeight() - SIZE);
                ySpeed = -ySpeed;
            }
            
            // and check for bouncing off the side walls
            if ((ball.getX() <= 0) && (xSpeed < 0)) {
                xSpeed = -xSpeed;
            }
            
            if ((ball.getX() + SIZE >= canvas.getWidth()) && (xSpeed > 0)) {
                xSpeed = -xSpeed;
            }
            
            // acceleration due to gravity
            ySpeed += GRAVITY;
            
            // color code the ball based on its y speed: color component is 20*speed.
            // shades of red for down, blue for up
            Color newColor;
            if (ySpeed > 0) {
                int red = (int)(20 * ySpeed);
                if (red > 255) red = 255;
                newColor = new Color(red, 0, 0);
            }
            else {
                int blue = (int)(-20 * ySpeed);
                if (blue > 255) blue = 255;
                newColor = new Color(0, 0, blue);                
            }
            ball.setColor(newColor);
            
            pause(PAUSE_TIME);
        }
    }
}