All Possible Outcomes Of Dice Roll Using Recursion

Here is what I have so far:

/**
* This class encapsulates a simple dice game. The number of dice and number of sides on those dice are given by instance variables. The outcomes ArrayList holds a list of all possible outcomes of throwing that number of dice with that number of sides.
*
* If there were 2 dice, each with 6 sides, then possible outcomes would include 1 1, 1 2, 1 3, 1 4, 1 5, 1 6, 2 1, 2 2, 2 3, and so on.
*
* Your task is to complete the methods that calculate the possible outcomes. One method calculates outcomes allowing for repeated numbers. One method calculates the outcomes of a fictional dice game where repeated numbers cannot occur.
*
* You must use recursion. This is a variation on the permutations problem from the book.
*/

public class Dice
{
private static int numberOfSides;
private static int numberOfDice;
public ArrayList<String> outcomes;

[code]...

I manage to calculate the numberOfOutcomes correctly, but then get a nullPointerException. Also, is there a way that I can use getNumberOf Outcomes without making it static? The testing code that the teacher provided is using it in another class and if I make it static, then it doesn't work in that file.