Stuck on your typical Die Class example with a Gui Interface..HELP!!!

[Kburge03]

Hi!!

I've been working on this assingment for class where I have to design and implement an application that displays two Die objects, a button, and a label. Every time the button is pushed, the following should happen: the two Die objects should be rolled, the face value of the two Die objects should be displayed and the label should display the sum of the two dice. I got so close so far, but now the gui won't even pop up with the driver anymore! Can anyone help!! I am putting my 3 programs below : the DIE CLASS, DIE PANEL and DIE DRIVER.

When I try to run the program I get the following error:
Exception in thread "main" java.lang.NullPointerException
at DiePanel.<init>(DiePanel.java:29)
at DieDriver.main(DieDriver.java:17)

Any advice???








[DIE CLASS]

//Author: Sue McFarland Metzger
//Purpose: Die Class
 
   import javax.swing.*;
 
    public class Die
   {
   //constants
      final static ImageIcon D1 = new ImageIcon( "d1.gif");
      final static ImageIcon D2 = new ImageIcon( "d2.gif");
      final static ImageIcon D3 = new ImageIcon( "d3.gif");
      final static ImageIcon D4 = new ImageIcon( "d4.gif");
      final static ImageIcon D5 = new ImageIcon( "d5.gif");
      final static ImageIcon D6 = new ImageIcon( "d6.gif");
      final static ImageIcon D7 = new ImageIcon( "dice.gif");
      final static int DEFAULT_SIDES = 6;
 
   //instance data
      private int faceValue;
      private int sides;
 
   //the constructor
       public Die(int numSides)
      {
         sides = numSides;
         faceValue = -1;
      }
 
   //another Constructor
   //this is an example of method overloading
       public Die()
      {
         sides = DEFAULT_SIDES;
         faceValue = -1;
      }
 
   //toString method used to display an object
       public String toString()
      {
         return sides + " sided die";
      }
 
   //roll die - a setter method
       public int roll()
      {
         faceValue = (int)(Math.random() * sides) + 1;      
         return faceValue;
      }
 
   //getter method
       public int getFaceValue()
      {
         return faceValue;
      }
 
   //setter method
       public void setSides( int sides )
      {
         this.sides = sides;
         return;
      }
 
   //display the die as an Icon   
       public ImageIcon getIcon( )
      {
         ImageIcon icon;
 
         switch (faceValue)
         {
            case 1:
               icon = D1;
               break;
            case 2:
               icon = D2;
               break;
            case 3:
               icon = D3;
               break;
 
            case 4:
               icon = D4;
               break;
 
            case 5:
               icon = D5;
               break;
 
            case 6:
               icon = D6;        
               break;
 
            default:
               icon = D7;
         }
         return icon;
      }
 
   }//end of the class

[DIE PANEL]

//import GUI classes
import javax.swing.*;
import javax.swing.JLabel;
import java.awt.*;
import java.awt.event.*;
 
public class DiePanel extends JPanel
{
    //instance data
    private JLabel lab1, lab2, sum;
    private JButton roll1, roll2;
    Die d1, d2;
    int totalSum;
 
    //constructor
    public DiePanel()
    {
 
        //set the panel's color
        this.setBackground( Color.YELLOW );
 
        //instantiate components
        lab1 = new JLabel (d1.getIcon ());
        lab2 = new JLabel (d2.getIcon ());
        sum = new JLabel ("Sum:" + totalSum);
        roll1 = new JButton( "Roll" );
        roll1.addActionListener (new ButtonListener ());
        roll2 = new JButton ("Roll");
        roll2.addActionListener (new ButtonListener ());            
 
 
        //add components to the panel
      this.add (roll1);
      this.add (lab1);
      this.add (roll2);
      this.add (lab2);
        this.add (sum);
 
        //set size of the panel
        this.setPreferredSize( new Dimension( 200,200 ) );
 
    }
 
    //supporting methods - inner class (ActionListeners)
        public class ButtonListener implements ActionListener
        {
            public void actionPerformed (ActionEvent event)
            {
 
                //Declare Variabes
                d1 = new Die ();
                d2 = new Die ();
                int totalsum;
 
                //Process
                d1.roll();
                lab1.setIcon (d1.getIcon ());
                d2.roll();
                lab2.setIcon (d2.getIcon ());
 
 
            }
        }
 
}

[DIE DRIVER]

//import GUI class for Frames
import javax.swing.*;
 
public class DieDriver
{
    public static void main( String[] args)
    {
 
        //construct a new container - the Frame
        JFrame frame = new JFrame("Die Application");
        frame.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
 
        //construct the primary panel 
        DiePanel newPanel = new DiePanel();
 
        //add the primary panel to the frame
        frame.add( newPanel );
 
        //compress the frame and make it visible
        frame.pack();        
        frame.setVisible( true );    //make frame visible to user
 
    }
}

[r035198x]

private JButton roll1;

does not create a JButton.
At this point roll1 is null.
So if you do roll1.setWhatever or roll1.addWhatever, you'll get a nullpointer exception because roll1 is not initialized.
To initialize roll1 you'd use

roll1 = new JButton("Maybe some text here");