Question

Keep the ball in bounds by adding conditional statements after the TODO comments in the paintComponent() method to check when the ball has hit the edge of the window.

1. When the ball reaches an edge, you will need to reverse the corresponding delta direction and position the ball back in bounds.

2. Make sure that your solution still works when you resize the window. Your bounds checking should not depend on a specific window size

This is the java code that I'm currently working with:

import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Toolkit;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.Random;

import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.Timer;

/**
 * Animated program with a ball bouncing off the program boundaries
 * @author CS121 Instructors
 * @author mvail
 * @author yourname
 */
@SuppressWarnings("serial")
public class BouncyBall extends JPanel
{
        private final int INIT_WIDTH = 600;
        private final int INIT_HEIGHT = 400;
        private final int DELAY = 60;       // milliseconds between Timer events

        private Random random;              // random number generator
        private Color color;                // initial ball color

        private int x, y;                   // ball anchor point coordinates
        private int xDelta, yDelta;         // change in x and y from one step to the next

        private int radius = 10;            // ball radius

        /**
         * Draws a filled bouncy ball that stays within the bounds of the screen.
         *
         * @param g Graphics context
         */
        public void paintComponent(Graphics g)
        {
                int width = getWidth();
                int height = getHeight();

                // Clear canvas
                g.setColor(getBackground());
                g.fillRect(0, 0, width, height);

                // Calculate new x anchor point value
                x += xDelta;
                // TODO: Add conditional statements to keep ball in bounds
                //       in the x axis (e.g. left and right edges)


                // Calculate new y anchor point value
                y += yDelta;
                // TODO: Add conditional statements to keep ball in bounds
                //       in the y axis (e.g. top and bottom edges)


                
                // Paint the ball at the calculated anchor point
                g.setColor(color);
                g.fillOval(x-radius, y-radius, 2*radius, 2*radius);

                // Makes the animation smoother
                Toolkit.getDefaultToolkit().sync();
        }

        /**
         * Constructor for the display panel initializes necessary variables.
         * Only called once, when the program first begins.
         * This method also sets up a Timer that will call paintComponent() 
         * with frequency specified by the DELAY constant.
         */
        public BouncyBall()
        {
                setPreferredSize(new Dimension(INIT_WIDTH, INIT_HEIGHT));
                setDoubleBuffered(true);
                setBackground(Color.black);

                 // Instantiate instance variable for reuse in paintComponent()
                random = new Random();

                // Initialize ball color
                // TODO: replace with random starting color.
                color = Color.RED;

                // Initialize ball anchor point within panel bounds
                // TODO: replace centered starting point with a random
                // position anywhere in-bounds - the ball should never
                // extend out of bounds, so you'll need to take RADIUS
                // into account. Use INIT_WIDTH and INIT_HEIGHT as the
                // screen width and height.
                x = INIT_WIDTH/2;
                y = INIT_HEIGHT/2;

                // Initialize deltas for x and y
                xDelta = 5;
                yDelta = 5;

                //Start the animation - DO NOT REMOVE
                startAnimation();
        }

        /**
         * Create an animation thread that runs periodically. DO NOT MODIFY.
         */
        private void startAnimation()
        {
                ActionListener taskPerformer = new ActionListener() {
                        public void actionPerformed(ActionEvent evt) {
                                repaint();
                        }
                };
                new Timer(DELAY, taskPerformer).start();
        }

        /**
         * Starting point for the BouncyBall program. DO NOT MODIFY.
         * @param args unused
         */
        public static void main (String[] args)
        {
                JFrame frame = new JFrame ("Bouncy Ball");
                frame.setDefaultCloseOperation (JFrame.EXIT_ON_CLOSE);
                frame.getContentPane().add(new BouncyBall());
                frame.pack();
                frame.setVisible(true);
        }
}

Answer 1

import java.awt.Color;

import java.awt.Dimension;

import java.awt.Graphics;

import java.awt.Toolkit;

import java.awt.event.ActionEvent;

import java.awt.event.ActionListener;

import java.util.Random;

import javax.swing.JFrame;

import javax.swing.JPanel;

import javax.swing.Timer;

/**

* Animated program with a ball bouncing off the program boundaries

* @author CS121 Instructors

* @author mvail

* @author yourname

*/

@SuppressWarnings("serial")

public class BouncyBall extends JPanel{

       private final int INIT_WIDTH = 600;

     private final int INIT_HEIGHT = 400;

     private final int DELAY = 60;       // milliseconds between Timer events

     private Random random;              // random number generator

     private Color color;                // initial ball color

     private int x, y;                   // ball anchor point coordinates

     private int xDelta, yDelta;         // change in x and y from one step to the next

     private int radius = 10;            // ball radius

     /**

      * Draws a filled bouncy ball that stays within the bounds of the screen.

      *

      * @param g Graphics context

      */

    

     public void paintComponent(Graphics g)

     {

             int width = getWidth();

             int height = getHeight();

             // Clear canvas

             g.setColor(getBackground());

             g.fillRect(0, 0, width, height);

             // Calculate new x anchor point value

             x += xDelta;

             // TODO: Add conditional statements to keep ball in bounds

             //       in the x axis (e.g. left and right edges)

            

             // check if the left-most coordinate of the ball is beyond the left edge

             // or right most coordinate of the ball is beyond the right edge, then reverse xDelta

             if((x-radius) < 0 || (x+radius) > width)

            x -= xDelta;

             // Calculate new y anchor point value

             y += yDelta;

             // TODO: Add conditional statements to keep ball in bounds

             //       in the y axis (e.g. top and bottom edges)

            

             //check if the top-most coordinate of the ball is beyond the top edge

             // or bottom most coordinate of the ball is beyond the bottom edge, then reverse yDelta

             if((y-radius) < 0 || (y+radius) > height)

            y -= yDelta;

            

             // Paint the ball at the calculated anchor point

             g.setColor(color);

             g.fillOval(x-radius, y-radius, 2*radius, 2*radius);

             // Makes the animation smoother

             Toolkit.getDefaultToolkit().sync();

     }

     /**

      * Constructor for the display panel initializes necessary variables.

      * Only called once, when the program first begins.

      * This method also sets up a Timer that will call paintComponent()

      * with frequency specified by the DELAY constant.

      */

     public BouncyBall()

     {

             setPreferredSize(new Dimension(INIT_WIDTH, INIT_HEIGHT));

             setDoubleBuffered(true);

             setBackground(Color.black);

              // Instantiate instance variable for reuse in paintComponent()

             random = new Random();

             // Initialize ball color

             // TODO: replace with random starting color.

            

             // randomly select the ball color

             color = new Color(random.nextFloat(),random.nextFloat(),random.nextFloat());

             //color = Color.RED;

             // Initialize ball anchor point within panel bounds

             // TODO: replace centered starting point with a random

             // position anywhere in-bounds - the ball should never

             // extend out of bounds, so you'll need to take RADIUS

             // into account. Use INIT_WIDTH and INIT_HEIGHT as the

             // screen width and height.

             

             // randomly select the starting point of the ball

             // the ball must be within the bounds i.e x = [10,590] and y = [10,390] since radius = 10

             x = random.nextInt(INIT_WIDTH-(2*radius)+1)+radius;

             y = random.nextInt(INIT_HEIGHT-(2*radius)+1)+radius;

             //x = INIT_WIDTH/2;

             //y = INIT_HEIGHT/2;

             // Initialize deltas for x and y

             xDelta = 5;

             yDelta = 5;

             //Start the animation - DO NOT REMOVE

             startAnimation();

     }

     /**

      * Create an animation thread that runs periodically. DO NOT MODIFY.

      */

     private void startAnimation()

     {

             ActionListener taskPerformer = new ActionListener() {

                     public void actionPerformed(ActionEvent evt) {

                             repaint();

                     }

             };

             new Timer(DELAY, taskPerformer).start();

     }

     /**

      * Starting point for the BouncyBall program. DO NOT MODIFY.

      * @param args unused

      */

     public static void main (String[] args)

     {

             JFrame frame = new JFrame ("Bouncy Ball");

             frame.setDefaultCloseOperation (JFrame.EXIT_ON_CLOSE);

             frame.getContentPane().add(new BouncyBall());

             frame.pack();

             frame.setVisible(true);

     }

      

}

//end of program

Output: