Trouble with arrays.......

[jcarosella10]

Hey guys, I'm writing a program for class to show inheritance, and i think im doing it wrong. Could you help out? I'm not asking for a the answer, just a little debugging.

// CLASS COIN:
public class Coin
{
    private final int HEADS = 0;
    private final int TAILS = 1;
    
    private int face;
    
    //--------------------------------------------------------------------------------------------------
    //  Sets up the coin by flipping it initially
    //--------------------------------------------------------------------------------------------------
    public Coin()
    {
        flip();
    }
    //--------------------------------------------------------------------------------------------------
    //  Flips the coin by randomly choosing a face value
    //--------------------------------------------------------------------------------------------------
    public void flip()
    {
        face  = (int) (Math.random() * 2);
    }
    
    //--------------------------------------------------------------------------------------------------
    //  Returns true if the current face of the coin is heads
    //--------------------------------------------------------------------------------------------------
    public boolean isHeads()
    {
        return (face == HEADS);
    }
    
    //--------------------------------------------------------------------------------------------------
    //  Returns the current face of the coin as a string
    //--------------------------------------------------------------------------------------------------
    public String toString()
    {
        String faceName;
        if (face == HEADS)
            faceName = "Heads";
        else
            faceName = "Tails";
        return faceName;
    }
}

// CLASS MONETARYCOIN:
import java.util.Random;
public class MonetaryCoin extends Coin
{
    private int value;
    private int[] list = {1, 5, 10, 25};
    private Random generator = new Random();
    //--------------------------------------------------------------------------------------------------------------------
    // Calls the Coin constructor and adds a value
    //--------------------------------------------------------------------------------------------------------------------
    public MonetaryCoin()
    {
        super();
        value = list[generator.nextInt(4)];
    }
    //--------------------------------------------------------------------------------------------------------------------
    // lets user input a value for the coin
    //--------------------------------------------------------------------------------------------------------------------
    public void setValue(int newvalue)
    {
        value = newvalue;
    }
    //--------------------------------------------------------------------------------------------------------------------
    // returns the value of the coin
    //--------------------------------------------------------------------------------------------------------------------
    public int getValue()
    {
        return value;
    }
}

// DRIVER:

import javax.swing.*;
public class driver
{
    public static void main(String[] args)
    {
        MonetaryCoin myCoin[] = new MonetaryCoin[34];       //makes an array of 34 monetary coins called MyCoin
        int sum = 0;                                        //sets the base value of the sum counter
        for(int counter = 0; counter < 34; counter++)       //runs through all 34 objects of myCoin
        {
            sum += myCoin[counter].getValue();              //adds current MonetaryCoins value to sum
        }
        JOptionPane.showMessageDialog(null, "The total of all 34 coins is: " + sum);    //outputs the value of sum in a dialog box
    }
}

I'm getting a NullPointerException at sum += myCoin[counter].getValue(); Any ideas why?

[jcarosella10]

oh by the way the Coin class is out of a book, not my code

[eRaaaa]

a)list[generator.nextInt(4) - 1];
nextInt(4) can generate zero. 0-1 = -1 -> ArrayIndexOutOfBoundsException
b) sum += myCoin[counter].getValue();
you never create myCoin objects! you have created a array, but myCoin[0] = null
add the line myCoin[counter] = new MonetaryCoin(); before

[jcarosella10]

ohhh. I figured out the arrayoutofbounds but i knew i was forgetting something. Thanks!