Question

I’ve written a program to bounce a ball around a screen. The program as written below does not work (the ball just moves off screen).

However if I declare the boolean variables atHorizontalEdge and atVerticalEdge inside the while loop, it seems to work. Why is that the case? As the booleans are defined for the entire run() method, shouldn’t it be callable by the while loop even though its outside the while loop?

import acm.program.*;
import acm.graphics.*;
import java.awt.*;

public class BouncingBallv3 extends GraphicsProgram {
    public void run() {

        double x = (getWidth() - BALL_SIZE)/2 ;  //sets the starting position of ball at center
        double y = (getHeight() - BALL_SIZE)/2 ;


        GOval ball = new GOval (x, y, BALL_SIZE, BALL_SIZE ); // creates a red ball at center of screen
        ball.setFilled(true);
        ball.setColor(Color.red);
        add (ball);

        double dx = 1; //increments by which the ball moves
        double dy = 1;

        //declares boolean variables to test if ball position is at an edge
        boolean atHorizontalEdge =  (ball.getX() == getWidth() - BALL_SIZE) || ball.getX() == 0 ; 
        boolean atVerticalEdge = (ball.getY() == getHeight() - BALL_SIZE) || ball.getY() == 0 ;

        /* while loop keeps the ball moving in direction dx,dy
         * if ball reaches a position at any edge, the direction dx or dy changes
         */

        while (true) {

            if (atHorizontalEdge) {          //changes direction of ball if it hits a left/right wall
                dx = -dx;
            } else if (atVerticalEdge) {     //changes direction of ball if it hits a top/bottom wall
                dy = -dy;
            }
                ball.move(dx,dy); 
                pause (PAUSE_TIME);

        }



    }



    private static final double BALL_SIZE = 50;
    private static final int PAUSE_TIME = 5;
}

Answer 1

The issue is not that the declaration of the booleans is outside the while loop. It is that you are checking for your boundraies outside the while loop. Because of this, your condition is never updated and it only checks for the original state of the ball.