NullPointerException when trying to set a double array in java

Question

This is the relevant code I think

class Sudoku {
    int[][] grid;     

    void generateSudokuFromInput()
    {

      grid = new int[][]   <--- java.lang.NullPointerException at Sudoku.generateSudokuFromInput(Sudoku.java:309)
      {
         (...)
      }
    }

    void solveIt() {

      generateSudokuFromInput(); 

    }

}

Keep getting NullPointer error

Answer 1

When declaring multidimensional array, you must specify all the dimensions except the last one.

Eg

grid = new int [3][];

This will work nicely.

Answer 2

As an alternative to declaring the array size later (as described in other answers), you can also just initialize your array when declaring it:

class Sudoku {
    int[][] grid = {
        {0, 0, 0, 0, 0, 0, 0, 0, 0},
        {0, 0, 0, 0, 0, 0, 0, 0, 0},
        {0, 0, 0, 0, 0, 0, 0, 0, 0},
        {0, 0, 0, 0, 0, 0, 0, 0, 0},
        {0, 0, 0, 0, 0, 0, 0, 0, 0},
        {0, 0, 0, 0, 0, 0, 0, 0, 0},
        {0, 0, 0, 0, 0, 0, 0, 0, 0},
        {0, 0, 0, 0, 0, 0, 0, 0, 0},
        {0, 0, 0, 0, 0, 0, 0, 0, 0}
    };

    void generateSudokuFromInput() {
        // just set the values on grid, e.g. grid[0][0] = 9
    }

    void solveIt() {
        generateSudokuFromInput();
    }
}

Answer 3

To create an array you must have to specify the subscript value.

grid = new int[2][];
grid[0]=new int[2];
grid[1]=new int[5];

or

grid=new int[3][3];

To learn more about arrays refer this document.

Answer 4

You need to provide size for the first dimension:

grid = new int[3][];

Else, how do you expect the second dimension to work? A 2-dimensions array [i][j] basically means "My array has i arrays, each of them have j values". If there's no i, it doesn't mean anything.