Java: Example - Bouncing Ball

// File: animation/bb/BBDemo.java
// Description: Illustrates animation with a ball bouncing in a box
//              Possible extensions: faster/slower button,
// Author: Fred Swartz
// Date:   February 2005 ...

import javax.swing.*;

/////////////////////////////////////////////////////////////// BBDemo
public class BBDemo extends JApplet {
    
    //============================================== applet constructor
    public BBDemo() {
        add(new BBPanel());
    }
    
    //============================================================ main
    public static void main(String[] args) {
        JFrame win = new JFrame("Bouncing Ball Demo");
        win.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        
        win.setContentPane(new BBPanel());
        
        win.pack();
        win.setVisible(true); 
    }
}//endclass BBDemo

// File:  animation/bb/BBPanel.java
// Description: Panel to layout buttons and graphics area.
// Author: Fred Swartz
// Date:   February 2005

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

/////////////////////////////////////////////////////////////////// BBPanel
class BBPanel extends JPanel {
    BallInBox m_bb;   // The bouncing ball panel
    
    //========================================================== constructor
    /** Creates a panel with the controls and bouncing ball display. */
    BBPanel() {
        //... Create components
        m_bb = new BallInBox();        
        JButton startButton = new JButton("Start");        
        JButton stopButton  = new JButton("Stop");
        
        //... Add Listeners
        startButton.addActionListener(new StartAction());
        stopButton.addActionListener(new StopAction());
        
        //... Layout inner panel with two buttons horizontally
        JPanel buttonPanel = new JPanel();
        buttonPanel.setLayout(new FlowLayout());
        buttonPanel.add(startButton);
        buttonPanel.add(stopButton);
        
        //... Layout outer panel with button panel above bouncing ball
        this.setLayout(new BorderLayout());
        this.add(buttonPanel, BorderLayout.NORTH);
        this.add(m_bb       , BorderLayout.CENTER);
    }//end constructor
    
    
    ////////////////////////////////////// inner listener class StartAction
    class StartAction implements ActionListener {
        public void actionPerformed(ActionEvent e) {
            m_bb.setAnimation(true);
        }
    }
    
    
    //////////////////////////////////////// inner listener class StopAction
    class StopAction implements ActionListener {
        public void actionPerformed(ActionEvent e) {
            m_bb.setAnimation(false);
        }
    }
}//endclass BBPanel

// File:   animation/bb/BouncingBall.java
// Description: This Graphics panel simulates a ball bouncing in a box.
//         Animation is done by changing instance variables
//         in the timer's actionListener, then calling repaint().
//         * Flicker can be reduced by drawing into a BufferedImage, 
//           and/or using a clip region.
//         * The edge of the oval could be antialiased (using Graphics2).
// Author: Fred Swartz
// Date:   February 2005

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.event.*;

/////////////////////////////////////////////////////////////// BouncingBall
public class BallInBox extends JPanel {
    //============================================== fields
    //... Instance variables representing the ball.
    private Ball m_ball         = new Ball(0, 0, 2, 3);
    
    //... Instance variables for the animiation
    private int   m_interval  = 35;  // Milliseconds between updates.
    private Timer m_timer;           // Timer fires to anmimate one step.

    //========================================================== constructor
    /** Set panel size and creates timer. */
    public BallInBox() {
        setPreferredSize(new Dimension(200, 80));
        setBorder(BorderFactory.createLineBorder(Color.BLACK));
        m_timer = new Timer(m_interval, new TimerAction());
    }
    
    //========================================================= setAnimation
    /** Turn animation on or off.
     *@param turnOnOff Specifies state of animation.
     */
    public void setAnimation(boolean turnOnOff) {
        if (turnOnOff) {
            m_timer.start();  // start animation by starting the timer.
        } else {
            m_timer.stop();   // stop timer
        }
    }

    //======================================================= paintComponent
    public void paintComponent(Graphics g) {
        super.paintComponent(g);  // Paint background, border
        m_ball.draw(g);           // Draw the ball.
    }
    
    //////////////////////////////////// inner listener class ActionListener
    class TimerAction implements ActionListener {
        //================================================== actionPerformed
        /** ActionListener of the timer.  Each time this is called,
         *  the ball's position is updated, creating the appearance of
         *  movement.
         *@param e This ActionEvent parameter is unused.
         */
        public void actionPerformed(ActionEvent e) {
            m_ball.setBounds(getWidth(), getHeight());
            m_ball.move();  // Move the ball.
            repaint();      // Repaint indirectly calls paintComponent.
        }
    }
}//endclass

// File: animation/bb/BallModel.java
// Description: The logic / model of a ball.
// Author: Fred Swartz
// Date:   February 2005

import java.awt.*;

///////////////////////////////////////////////////////////////// BallModel
public class Ball {
    //... Constants
    final static int DIAMETER = 21;
    
    //... Instance variables
    private int m_x;           // x and y coordinates upper left
    private int m_y;
    
    private int m_velocityX;   // Pixels to move each time move() is called.
    private int m_velocityY;
    
    private int m_rightBound;  // Maximum permissible x, y values.
    private int m_bottomBound;
    
    //======================================================== constructor
    public Ball(int x, int y, int velocityX, int velocityY) {
        m_x = x;
        m_y = y;
        m_velocityX = velocityX;
        m_velocityY = velocityY;
    }
    
    //======================================================== setBounds
    public void setBounds(int width, int height) {
        m_rightBound  = width  - DIAMETER;
        m_bottomBound = height - DIAMETER;
    }
    
    //============================================================== move
    public void move() {
        //... Move the ball at the give velocity.
        m_x += m_velocityX;
        m_y += m_velocityY;        
        
        //... Bounce the ball off the walls if necessary.
        if (m_x < 0) {                  // If at or beyond left side
            m_x         = 0;            // Place against edge and
            m_velocityX = -m_velocityX; // reverse direction.
            
        } else if (m_x > m_rightBound) { // If at or beyond right side
            m_x         = m_rightBound;    // Place against right edge.
            m_velocityX = -m_velocityX;  // Reverse direction.
        }
        
        if (m_y < 0) {                 // if we're at top
            m_y       = 0;
            m_velocityY = -m_velocityY;
            
        } else if (m_y > m_bottomBound) { // if we're at bottom
            m_y       =  m_bottomBound;
            m_velocityY = -m_velocityY;
        }
    }
    
    //============================================================== draw
    public void draw(Graphics g) {
        g.fillOval(m_x, m_y, DIAMETER, DIAMETER);
    }
    
    //============================================= getDiameter, getX, getY
    public int  getDiameter() { return DIAMETER;}
    public int  getX()        { return m_x;}
    public int  getY()        { return m_y;}
    
    //======================================================== setPosition
    public void setPosition(int x, int y) {
        m_x = x;
        m_y = y;
    }
}