Java: Example - Rolling Dice

The applet / application to the left rolls two dice. It is divided into three source files.

  • RollDice.java is both an application (it defines main()) and an applet (it subclasses JApplet).
  • RollDicePanel.java is a subclass of JPanel that creates the GUI interface.
  • Die.java defines a component as a subclass of JComponent, and provides a graphical view of the die face. It implements logic with a roll() method that randomly "rolls" the die.

RollDice.java - the main program / applet

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// File:   rolldice/RollDice.java
// Description: Main program and applet display two dice and roll them.
// Tag: <applet code="rolldice.RollDice.class" archive="rolldice.jar" 
//            width="140" height="117"></applet>
// Author: Fred Swartz - 2006-11-30 - Placed in public domain.

package rolldice;

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

///////////////////////////////////////////////////////// class RollDice
public class RollDice extends JApplet {
    
    //=============================================== applet constructor
    /** Applet constructor requires putting the panel in applet.*/
    public RollDice() {
        this.setContentPane(new RollDicePanel());
    }
    
    //====================================================== method main
    /** Create JFrame and set content pane to a RollDicePanel. */
    public static void main(String[] args) {
        JFrame window = new JFrame("Dice Demo");
        window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        window.setContentPane(new RollDicePanel());
        window.pack();
        //System.out.println(window.getContentPane().getSize());
        window.setLocationRelativeTo(null);
        window.setVisible(true);
    }
}

RollDicePanel.java provides the GUI to this demo program

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// File:   rolldice/RollDicePanel.java
// Description: Panel of GUI, shows button and two dice.
// Author: Fred Swartz - 2006-11-30 - Placed in public domain.

package rolldice;

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

//////////////////////////////////////////////////////////// class RollDicePanel
public class RollDicePanel extends JPanel {
    //======================================================= instance variables
    private Die _leftDie;     // component for one die 
    private Die _rightDie;

    //============================================================== constructor
    /** Create border layout panel with one button and two dice. */
    RollDicePanel() {
        //... Create the dice
        _leftDie  = new Die();
        _rightDie = new Die();
        
        //...Create the button to roll the dice
        JButton rollButton = new JButton("New Roll");
        rollButton.setFont(new Font("Sansserif", Font.PLAIN, 24));
        
        //... Add listener.
        rollButton.addActionListener(new RollListener());
        
        //... Layout components
        this.setLayout(new BorderLayout(5, 5));
        this.add(rollButton, BorderLayout.NORTH);
        this.add(_leftDie , BorderLayout.WEST);
        this.add(_rightDie, BorderLayout.EAST);
        
        this.setBorder(BorderFactory.createEmptyBorder(5,5,5,5));
    }
    
    
    ////////////////////////////////////////// inner listener class RollListener
    private class RollListener implements ActionListener {
        public void actionPerformed(ActionEvent e) {
            _leftDie.roll();
            _rightDie.roll();
        }
    }
}

Die.java implements the graphics and logic of a die

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// File:   rolldice/Die.java
// Description: Models / displays one die.
// Author: Fred Swartz - 2006-11-30 - Placed in public domain.

package rolldice;

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

////////////////////////////////////////////////////////////////////// class Die
public class Die extends JComponent {
    //================================================================ constants
    private static final int SPOT_DIAM = 9;  // Diameter of spots
    
    //======================================================= instance variables
    private int _faceValue;     // value that shows on face of die
    
    //============================================================== constructor
    /** Initialize to white background and 60x60 pixels. Initial roll.*/
    public Die() {
        //-- Preferred size is set, but layout may change it.
        setPreferredSize(new Dimension(60,60));
        roll();  // Set to random initial value
    }
    
    //============================================================== 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;
    }
    
    //========================================================== method getValue
    /** Returns result of last roll.*/
    public int getValue() {
        return _faceValue;
    }
    
    //========================================================== method setValue
    /** Sets the value of the Die.  Causes repaint().
     * @param spots Number from 1-6.
     */
    public void setValue(int spots) {
        _faceValue = spots;
        repaint();    // Value has changed, must repaint
    }
    
    //==================================================== method paintComponent
    /** Draws spots of die face. */
    @Override public void paintComponent(Graphics g) {
        int w = getWidth();  // Get height and width
        int h = getHeight();
        
        //... Change to Graphic2D for smoother spots.
        Graphics2D g2 = (Graphics2D)g;  // See note below
        g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
                RenderingHints.VALUE_ANTIALIAS_ON);
        
        //... Paint background
        g2.setColor(Color.WHITE);
        g2.fillRect(0, 0, w, h);
        g2.setColor(Color.BLACK);
        
        g2.drawRect(0, 0, w-1, h-1);  // Draw border
        
        switch (_faceValue) {
            case 1:
                drawSpot(g2, w/2, h/2);
                break;
            case 3:
                drawSpot(g2, w/2, h/2);
                // Fall thru to next case
            case 2:
                drawSpot(g2, w/4, h/4);
                drawSpot(g2, 3*w/4, 3*h/4);
                break;
            case 5:
                drawSpot(g2, w/2, h/2);
                // Fall thru to next case
            case 4:
                drawSpot(g2, w/4, h/4);
                drawSpot(g2, 3*w/4, 3*h/4);
                drawSpot(g2, 3*w/4, h/4);
                drawSpot(g2, w/4, 3*h/4);
                break;
            case 6:
                drawSpot(g2, w/4, h/4);
                drawSpot(g2, 3*w/4, 3*h/4);
                drawSpot(g2, 3*w/4, h/4);
                drawSpot(g2, w/4, 3*h/4);
                drawSpot(g2, w/4, h/2);
                drawSpot(g2, 3*w/4, h/2);
                break;
        }
    }
    
    //========================================================== method drawSpot
    /** Utility method used by paintComponent(). */
    private void drawSpot(Graphics2D g2, int x, int y) {
        g2.fillOval(x-SPOT_DIAM/2, y-SPOT_DIAM/2, SPOT_DIAM, SPOT_DIAM);
    }
}
Copyleft 2006 Fred Swartz MIT License