Java: Example - Rolling Dice

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

///////////////////////////////////////////////////////// class RollDice
/** RollDice.java -- Program to display two dice and roll them.
   @author Fred Swartz
   @version 2004-04-20
This can run as an application because it has a main method.
    It can also be used as an applet because it extends JApplet.
*/
// <applet code="RollDice.class" height="110" width="160"></applet>
public class RollDice extends JApplet {
    //=============================================== applet constructor
    /** Applet initialization requires putting the panel in applet.*/
    public RollDice() {
        this.setContentPane(new RollDicePanel());
    }//end constructor
    
    //====================================================== method main
    /** Create JFrame and set content pane to a RollDicePanel. */
    public static void main(String[] args) {
        JFrame window = new JFrame();
        window.setTitle("Dice Demo");
        window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        window.setContentPane(new RollDicePanel());
        window.pack();
        window.show();
    }//end main
    
}//endclass

// RollDice.java -- Program to display two dice and roll them.
// Fred Swartz - 2003-04-30

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

///////////////////////////////////////////////////////// class RollDice
/** RollDicePanel.java - Panel to build GUI for RollDice application/applet.
    It subclasses JPanel to contain two dice and a button to roll them.
    @author Fred Swartz
    @version 2003-04-20
*/
public class RollDicePanel extends JPanel {
    //=============================================== instance variables
    private Die _left;     // component for one die
    private Die _right;
    //end instance variables

    //====================================================== constructor
    /** Create border layout panel with two Die's and one button. */
    RollDicePanel() {
        //--- Create the dice
        _left = new Die();
        _right = new Die();
        
        //--- Create the button to roll the dice
        JButton rollButton = new JButton("Roll");
        rollButton.setFont(new Font("Sansserif", Font.PLAIN, 24));
        rollButton.addActionListener(new RollListener());

        //--- Create panel for two dice
        JPanel dicePanel = new JPanel();
        dicePanel.setLayout(new GridLayout(1, 2, 4, 0));
        dicePanel.add(_left);
        dicePanel.add(_right);
        
        //--- Add components to content pane
        this.setLayout(new BorderLayout());
        this.add(rollButton, BorderLayout.NORTH);
        this.add(dicePanel , BorderLayout.CENTER);
    }//end constructor
    
    
    /////////////////////////////////// inner listener class RollListener
    /** Inner listener class for Roll button. */
    private class RollListener implements ActionListener {
        public void actionPerformed(ActionEvent e) {
            _left.roll();
            _right.roll();
        }
    }
    
}//endclass

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

////////////////////////////////////////////////////////////// class Die
/** Die.java - Component to display one die (dice).  This class
    subclasses JPanel to display the face, and it provides functions
    to "roll", giving different random values. 
    Possible improvements:
    <ul>
    <li>Use Graphics2 anti-aliasing to smooth the dot edges.</li>
    <li>Add a border.</li>
    </ul>
  @author Fred Swartz
  @version 2003-04-30
*/
public class Die extends JPanel {
    
    //=============================================== instance variables
    private int _value;     // value that shows on face of die
    private int _diam = 9;  // Diameter of spots
    //end instance variables
    
    //====================================================== constructor
    /** Initialize to white background and 60x60 pixels. Initial roll.*/
    public Die() {
        setBackground(Color.white);
        //-- Preferred size is set, but layout may change it.
        setPreferredSize(new Dimension(60,60));
        roll();  // Set to random initial value        
    }//end constructor
    
    //====================================================== method roll
    /** Produce random roll in range 1-6.  Causes repaint().
        @return Result of roll (1-6).
    */
    public int roll() {
        int val = (int)(6*Math.random() + 1);   // Range 1-6
        setValue(val);
        return val;
    }//end roll

    //================================================== method getValue
    /** Returns result of last roll.*/
    public int getValue() {
        return _value;
    }//end setValue

    //================================================== method setValue
    /** Sets the value of the Die.  Causes repaint().
        @param spots Number from 1-6.
    */
    public void setValue(int spots) {
        _value = spots;
        repaint();    // Value has changed, must repaint
    }//end setValue

    //============================================ method paintComponent
    /** Draws spots of die face. */
    public void paintComponent(Graphics g) {
        super.paintComponent(g);  // required
        
        int w = getWidth();
        int h = getHeight(); // should use to resize spots too.
        
        switch (_value) {
            case 1: drawSpot(g, w/2, h/2);
                    break;
            case 3: drawSpot(g, w/2, h/2);
                    // Fall thru to next case
            case 2: drawSpot(g, w/4, h/4);
                    drawSpot(g, 3*w/4, 3*h/4);
                    break;
            case 5: drawSpot(g, w/2, h/2);
                    // Fall thru to next case  
            case 4: drawSpot(g, w/4, h/4);
                    drawSpot(g, 3*w/4, 3*h/4);
                    drawSpot(g, 3*w/4, h/4);
                    drawSpot(g, w/4, 3*h/4);
                    break;
            case 6: drawSpot(g, w/4, h/4);
                    drawSpot(g, 3*w/4, 3*h/4);
                    drawSpot(g, 3*w/4, h/4);
                    drawSpot(g, w/4, 3*h/4);
                    drawSpot(g, w/4, h/2);
                    drawSpot(g, 3*w/4, h/2);
                    break;
        }
    }//end paintComponent
    
    /** Utility method used by paintComponent(). */
    //================================================== method drawSpot
    private void drawSpot(Graphics g, int x, int y) {
        g.fillOval(x-_diam/2, y-_diam/2, _diam, _diam);
    }//end drawSpot
}//end class