Want a multidimensional array but get a null pointer exception

Question

  1 public class TestWin{
  2     public static void main(String[] args){
  3         int n;
  4         hexagon[][] board;
  5
  6         n = 4;
  7         board          


        

Answer 1

Spot on kwatford. All you have done with line 7 is to tell java to create space for n*n Hexagon objects in a 2 dimensional array.

You will still need to call new for each of these Hexagons

Essentially, you need to replace line 7 with something like:

board = new Hexagon[n][n];
for(int i=0; i<n; i++)
    for(int j=0; j<n; j++)
        board[i][j] = new Hexagon();

Answer 2

To expand on what kwatford said, initializing an array in java gives you nulls if the array type is an object. If you had a raw array, such as an array of doubles, you would start with 0 as the entry for each element of the array.

Answer 3

Short Answer:

As kwatford said what you need to do is this:

board[0][0] = new hexagon(); // or whatever its constructor is

Longer Explanation:

Just to expand further. Your 2D array is; an array of pointers (or references in Java). This is what one row of the array will look like immediately after this call board = new hexagon[n][n];:

    0      1      2      3      4      5       // column index, row index = 0
-------------------------------------------
|   |   |   |   |   |   |      |      |      |    // value
--- | ----- | ----- | ---------------------
    |       |       |      ...
    |       |       |
    |       |       |
    |       |       |
    |       |       v
    |       |       Null
    |       v       
    |       Null
    v
    Null (This means that it points to nothing)

---

You tried:

board[0][0].value = 'R';

which is the same as this:

null.value = 'R';

---

You have initialized your array using this line:

board = new Hexagon[n][n];

But you still need to initialize the elements in your array. This would initialize the first three:

board[0][0] = new hexagon(); // or whatever its constructor is
board[1][0] = new hexagon(); // or whatever its constructor is
board[2][0] = new hexagon(); // or whatever its constructor is

Which would result in an array that looks like this:

    0      1      2      3      4      5       // column index, row index = 0
-------------------------------------------
|   |   |   |   |   |   |      |      |      |    // value
--- | ----- | ----- | ---------------------
    |       |       |
    |       |       |
    |       |       |
    |       |       |
    |       |       v
    |       |       An instance of type Hexigoon (what you get when you type new Hexigon)
    |       v       
    |       An instance of type Hexigon (what you get when you type new Hexigon)
    v
    An instance of type Hexigon (what you get when you type new Hexigon)

I remember banging my head on the table with this exact problem two years ago. I love stackoverflow

Answer 4

It's been a while since I've so much as looked at Java, but have you tried doing this first?

board[0][0] = new hexagon(); // or whatever its constructor is