Java: Example - Bouncing Ball

// animation/bb/BBDemo.java - ball bouncing in a box.
//  -- Fred Swartz, December 2004
// Possible extensions: faster/slower button, ...

import javax.swing.JFrame;

///////////////////////////////////////////////////////////////////// BBDemo
public class BBDemo {
    //================================================================= 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); 
    }//endmethod main
}//endclass BBDemo

// animation/bb/BBPanel.java - Panel with buttons and box.
// Fred Swartz - December 2004

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");
        
        //-- 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);
        
        //-- Add Listeners
        startButton.addActionListener(new StartAction());
        stopButton.addActionListener(new StopAction());
    }//end constructor
    
    
    ////////////////////////////////////// inner listener class StartAction
    class StartAction implements ActionListener {
        public void actionPerformed(ActionEvent e) {
            m_bb.setAnimation(true);
        }
    }//end inner listener class StartAction
    
    
    //////////////////////////////////////// inner listener class StopAction
    class StopAction implements ActionListener {
        public void actionPerformed(ActionEvent e) {
            m_bb.setAnimation(false);
        }
    }//end inner listener class StartAction
}//endclass BBPanel

// animation/bb/BouncingBall.java - A ball bouncing in a box.
//   Fred Swartz - December 2004
// Animation is done by changing instance variables
// in a timer's actionListener, then calling repaint().
// * This panel has a simple model for movement: changing
//   the x and y values each time the timer fires.
// * 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).

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);
    private int  m_ballDiameter = m_ball.getDiameter(); 
    
    //... Instance variables for the animiation
    private int   m_velocityX = 2;   // Initial x velocity (pixels/interval)
    private int   m_velocityY = 3;   // Initial y velocity
    private int   m_interval  = 30;  // Milliseconds between updates.
    private Timer m_timer;           // Ball moves every time 

    //========================================================== constructor
    /** Sets panel size and creates timer. */
    public BallInBox() {
        setPreferredSize(new Dimension(200, 80));
        setBorder(BorderFactory.createLineBorder(Color.BLACK));
        m_timer = new Timer(m_interval, new TimerListener());
    }//end constructor
    
    //========================================================= setAnimation
    /** Turns the 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
        }
    }//end setAnimation

    //======================================================= paintComponent
    public void paintComponent(Graphics g) {
        super.paintComponent(g);  // Paint background, border
        m_ball.draw(g);           // Draw the ball.
    }//end method paintComponent
    
    //////////////////////////////////// inner listener class ActionListener
    class TimerListener 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) {
            //... Get the the current size of the Panel, ball bounds.
            int rightBound = getWidth() - m_ballDiameter;
            int bottomBound= getHeight()- m_ballDiameter;
            
            //--- Compute new x position, check for passing over edge.
            int nextX = m_ball.getX() + m_velocityX; // Move in x direction.
            if (nextX < 0) {                // If at or beyond left side
                nextX       = 0;            // Place against edge and
                m_velocityX = -m_velocityX; // reverse direction.
                
            } else if (nextX > rightBound) { // If at or beyond right side
                nextX       = rightBound;    // Place against right edge.
                m_velocityX = -m_velocityX;  // Reverse direction.
            }
            
            //--- Compute new y position analogous to x above.
            int nextY = m_ball.getY() + m_velocityY;
            if (nextY < 0) {                 // if we're at top
                nextY       = 0;
                m_velocityY = -m_velocityY;
                
            } else if (nextY > bottomBound) { // if we're at bottom
                nextY       = bottomBound;
                m_velocityY = -m_velocityY;
            }
            
            //... Move ball to new position.
            m_ball.setPosition(nextX, nextY);
            
            // repaint() must be used to indirectly call paintComponent..
            repaint();     // Repaint the panel
        }//end actionPerformed
    }//end inner class listener
}//endclass

// animation/bb/BallModel.java - The logic / model of a ball.
// Fred Swartz - December 2004

import java.awt.*;

///////////////////////////////////////////////////////////////// BallModel
public class Ball {
    final static int DIAMETER = 21;
    int m_x;
    int m_y;
    
    //======================================================== constructor
    public Ball(int x, int y) {
        m_x = x;
        m_y = y;
    }
    
    //============================================================== draw
    public void draw(Graphics g) {
        g.fillOval(m_x, m_y, DIAMETER, DIAMETER);        
    }
    
    //============================================= getDiamter, 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;
    }
}