I'm trying to create a 2D array of lists for a Sudoku. Essentially 81 lists each containing possible solutions to that box in the Sudoku grid. I've tried multiple declarations so far, but whenever I try to add values to a list it returns a null pointer exception. Here is an example, simply populating each of the lists with the numbers 1-9.

```
List<Integer>[][] sudoku = (List<Integer>[][]) new List[9][9];

for (int i = 0; i < 9; i++) {
        for (int j = 0; j < 9; j++) {
            for (int k = 1; k < 10; ) {
                sudoku[i][j].add(k);
            }
        }
}

```

I'm not even positive a 2D array of lists is the optimal way to go about this, but I've done everything from scratch (with a relatively low knowledge of java) so far so I'd like to follow through with this method. The original code looked as follows:

```
List[][] sudoku = new List[9][9];

```

Research quickly revealed that this wouldn't cut it.

Thank you in advanced for any help!