Error Message

[S. Stacey]

Hello All,
I am VERY new to Java and have come across this error message and I am not sure what it means or how to fix it.
I can complie both my class and test class and when I go to run the program I get the following error:
Exception in thread "main" java.lang.NullPointerException at Game.throwDie(Game.java:20)
at TestGame.main (TestGame.java:18)
Can some one please help me out?
Thanks
Stacey

[Maulin Vasavada]

hi Stacey
can u please post ur code if its not much big? it says that u get NullPointerException at line no:18.
NullPointerException means that - "the code tried to invoke a method on a object reference which was null". if you are famaliar with C/C++ then you can consider this NullPointerException (NPE written in short most of the time for ease) as "NULL" pointer usage.
regards
maulin

[S. Stacey]

Here is my code for the class:

import java.util.Random;
public class Game
{
/**
Creates a Game object
*/
public Game()
{
Random generator = new Random ();
hits = 0;
}
/**
Method used to generate the random numbers that represent the 
die faces and count the number of winning hits when the two 
faces add to 7
*/
public void throwDie()
{
int numberRolled = (1 + generator.nextInt(6)) + (1 + generator.nextInt(6));
if (numberRolled == 7) hits++;
}
/**
Get the number of hits
@return hits number of hits
*/
public int getHits()
{
return hits;
}
/**
instance fields
*/
private Random generator; // an object of the class Random
private int hits; // counts the number of hits
}
Here is my code for the test class:
import javax.swing.JOptionPane; //Import the class JOptionPane
public class TestGame
{
public static void main (String[] args)
{ 

int tries =Integer.parseInt(JOptionPane.showInputDialog( "How many times would you like to play the game?"));

Game userplay = new Game ();
{

for (int i = 1; i<= tries; i++)

{
userplay.throwDie();
}
}
System.out.println ("You threw " + tries + " times and you won " + userplay.getHits());
System.exit (0);
}
}

Any help would be greatly appreciated.
Thanks
Stacey
[ July 03, 2003: Message edited by: Stacey Young ]
[ July 03, 2003: Message edited by: Stacey Young ]
[ July 03, 2003: Message edited by: Stacey Young ]
[ July 03, 2003: Message edited by: Stacey Young ]
[ July 03, 2003: Message edited by: Stacey Young ]
[ July 04, 2003: Message edited by: Stacey Young ]

[Bear Bibeault]

Stacey, if you place your code between UBB CODE tags, the formatting will be preserved and it will be a lot easier for all of us here to help you out.
hth,
bear
P.S. Welcome to the Ranch!
P.P.S. You can edit your previous post to add the code tags.

[Joel McNary]

Your problem is that you are "hiding" your generator in your constructor. Instead of

public Game(){
    Random generator = new Random();
...

use

public Game(){
    this.generator = new Random();
...

The difference? You have already declared generator as a class member; by including the type again, you are telling the compiler that you are creating a reference that is local to the constructor. When you create an object for that reference, you are not creating one for the class reference. That is never assigned a value, so it takes the default, which is null. (While the "this." is technically optional in my example above, I placed it there to help show the concept of local vs. class reference)

[S. Stacey]

WOO HOOO!!! that worked, THanks Joel!!!

and Bear I am not sure what you mean by the UBB CODE tag....can you explain it a little more to me.

[Larry Cha Cha]

nd Bear I am not sure what you mean by the UBB CODE tag

When replying or creating a new topic there are buttons below the Instant UBB Code label, one of which is CODE. It simply adds an open and close tags which you should place your code in to format it correctly.

[Dirk Schreckmann]

Stacy, welcome to JavaRanch!
Take a look at the UBB Code explanation document for a good idea of what Bear was talking about.