Question

I am currently working on a simple GUI application that rolls two dice. I am currently using two classes, one called “Game” and the other “DiceRoll”. For meeting the assignment criteria, we have to use multiple classes in order for the program to work. It would be much easier just using one class…Anyways, I am successfully calling my “roll()” method in my “DiceRoll” class, which uses a switch statement and implements the “Graphics.drawImage()” method in order to draw the specified “.png” images. Everything looks fine, and I have no errors before I run it. When I run the program, the GUI frame pops up and all of the buttons/menus that I have created are working. However, once I press the “Roll Dice” button, I keep receiving multiple run-time errors.
This is what I have so far:

Game Class

import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JMenu;
import javax.swing.JMenuBar;
import javax.swing.JMenuItem;
import javax.swing.JOptionPane;
import javax.swing.JPanel;

public class Game extends JFrame implements ActionListener 
{
JPanel mainPanel;
JPanel optionPanel;
JPanel dicePanel;
JButton rollDice;
JMenu options;
JMenuItem quit;
JMenuItem explanation;
JMenuBar menuBar;
JLabel diceLabel;
JLabel diceLabel2;
DiceRoll dr;
Graphics die1;
Graphics die2;

public Game()
{
    setTitle("Rollin' Dice");
    this.setPreferredSize(new Dimension(600,600));

    mainPanel = new JPanel();
    optionPanel = new JPanel();
    dicePanel = new JPanel();
    rollDice = new JButton("Roll Dice");
    options = new JMenu("Options");
    quit = new JMenuItem("Quit");
    explanation = new JMenuItem("Explanation");
    menuBar = new JMenuBar();
    dr = new DiceRoll();
    diceLabel = new JLabel();
    diceLabel2 = new JLabel();

    options.add(quit);
    options.add(explanation);

    menuBar.add(options);

    optionPanel.add(menuBar);
    optionPanel.setPreferredSize(new Dimension(600,100));

    dicePanel.add(rollDice);

    dicePanel.add(diceLabel);
    dicePanel.add(diceLabel2);

    mainPanel.setPreferredSize(new Dimension(600,600));
    mainPanel.add(optionPanel);
    mainPanel.add(dicePanel);


    quit.addActionListener(this);
    explanation.addActionListener(this);
    rollDice.addActionListener(this);

    this.getContentPane().add(mainPanel);

    this.pack();
    this.setVisible(true);
}


public void actionPerformed(ActionEvent e) {
    if (e.getSource()== quit)
        System.exit(0);

    if (e.getSource() == explanation)
    {
        JOptionPane.showMessageDialog(mainPanel,
                "Win: Roll a sum that is an even number \nLose: Roll a sum that is an odd number" + dicePanel, "Rules", JOptionPane.INFORMATION_MESSAGE); 
    }

    if (e.getSource() == rollDice)
    {

        dr.roll(die1);
        dr.roll(die2);

        diceLabel.updateUI();

        dicePanel.updateUI();
    }

}

public static void main (String []args)
{
    Game dg = new Game();
}

}

DiceRoll Class

    import java.awt.Graphics;
    import java.awt.image.BufferedImage;
    import javax.imageio.*;
    import java.io.File;
    import java.io.IOException;
    import javax.swing.JComponent;

public class DiceRoll extends JComponent {

private BufferedImage die1;
private BufferedImage die2;
private BufferedImage die3;
private BufferedImage die4;
private BufferedImage die5;
private BufferedImage die6;


public DiceRoll()
{
    try {
        die1 = (ImageIO.read(new File("die1.png")));
        die2 = ImageIO.read(new File("die2.png"));
        die3 = ImageIO.read(new File("die3.png"));
        die4 = ImageIO.read(new File("die4.png"));
        die5 = ImageIO.read(new File("die5.png"));
        die6 = ImageIO.read(new File("die6.png"));
    } catch (IOException ex){
        System.err.println("That is invalid");
    }
}


public Graphics roll(Graphics g)
{
    int dieResult = (int)(6 * Math.random());

    switch(dieResult){
    case 1: 
        g.drawImage(die1, 0, 0, null);
        break;
    case 2:
        g.drawImage(die2, 0, 0, null);
        break;
    case 3:
        g.drawImage(die3, 0, 0, null);
        break;
    case 4:
        g.drawImage(die4, 0, 0, null);
        break;
    case 5:
        g.drawImage(die5, 0, 0, null);
        break;
    case 6:
        g.drawImage(die6, 0, 0, null);
        break;
    }
    return g;
}

}

Errors that I am receiving

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at DiceRoll.roll(DiceRoll.java:51)
at Game.actionPerformed(Game.java:89)
at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source)
at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source)
at javax.swing.DefaultButtonModel.setPressed(Unknown Source)
at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(Unknown Source)
at java.awt.Component.processMouseEvent(Unknown Source)
at javax.swing.JComponent.processMouseEvent(Unknown Source)
at java.awt.Component.processEvent(Unknown Source)
at java.awt.Container.processEvent(Unknown Source)
at java.awt.Component.dispatchEventImpl(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source)
at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source)
at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Window.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.EventQueue.dispatchEvent(Unknown Source)
at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.run(Unknown Source)

Answer 1

I think you’re making this much more complicated than is necessary. Why don’t you change your public Graphics roll(Graphics g) method to calculate the roll and update the JLabel, something like this…

public void roll(JLabel dieLabel) {
    int dieResult = (int)(6 * Math.random());
    dieLabel.setIcon(new ImageIcon("die" + dieResult + ".png"))
}

Then all you need to do is change your code here …

if (e.getSource() == rollDice){
    dr.roll(die1);
    dr.roll(die2);
    diceLabel.updateUI();
    dicePanel.updateUI();
}

to send the label you want to change, like this…

if (e.getSource() == rollDice){
    dr.roll(diceLabel);
    dr.roll(diceLabel2);
}

You pass the JLabel through to the roll() method, which calculates the roll and sets the image on that label. Much simpler.

This also means that you don’t need the 2 Graphics objects either (die1 and die2), so you can get rid of these. You also wouldn’t need the BufferedImages, because the loading of the image file is done by the ImageIcon class

Let me know if you need any further information about my suggestion, for learning purposes.

EDIT…

This is my re-writing of your Game() constructor, that shows how to add items to JPanels using GridLayout and BorderLayout as the layout managers…

public Game(){
    // Set the JFrame properties
    setTitle("Rollin' Dice");
    this.setPreferredSize(new Dimension(600,600));

    // Create the main JPanel to hold the interface
    mainPanel = new JPanel(new BorderLayout());


    // Build the Menu
    options = new JMenu("Options");

    quit = new JMenuItem("Quit");
    options.add(quit);

    explanation = new JMenuItem("Explanation");
    options.add(explanation);

    menuBar = new JMenuBar();
    menuBar.add(options);

    // Add the menu to the top of the main panel
    mainPanel.add(menuBar,BorderLayout.NORTH);


    // Create the dice
    dr = new DiceRoll();
    diceLabel = new JLabel();
    diceLabel2 = new JLabel();

    dicePanel = new JPanel(new GridLayout(2,1));
    dicePanel.add(diceLabel);
    dicePanel.add(diceLabel2);

    // Add the dicePanel to the center of the main panel
    mainPanel.add(dicePanel,BorderLayout.CENTER);


    // Add the rollDice button to the bottom of the main panel
    rollDice = new JButton("Roll Dice");
    mainPanel.add(rollDice,BorderLayout.SOUTH);


    // Add listeners to the menu items and buttons
    quit.addActionListener(this);
    explanation.addActionListener(this);
    rollDice.addActionListener(this);

    // Add the main panel to the JFrame
    this.getContentPane().add(mainPanel);

    // Show the JFrame
    this.pack();
    this.setVisible(true);
}