Question

I have following code:

public class GameCanvas extends JPanel {

private GridField[][] grid;
private int x, y;
private int fieldSize;

public GameCanvas(int rows, int cols, int fieldSize)
{
    this.grid = new GridField[cols][rows];
    this.x = cols;
    this.y = rows;
    this.fieldSize = fieldSize;
}

...
}

Here is the definition of the GridField class:

public class GridField {

private FieldType fieldType;    

public GridField() {
    fieldType = FieldType.EMPTY;
}

public FieldType getFieldType() {
    return fieldType;
}

public void setFieldType(FieldType fieldType) {
    this.fieldType = fieldType;
    }
}

The problem is, when I try to access the “grid” object, the compiler says it’s null, although I have initialized it in the constructor of the class.

I did a little check:

if(grid[xSize][ySize] == null) {
  System.out.println("Grid[x][y] is null");
}

It printed out exactly what I expected – null.

I’m coming to Java from C# background so I might have missed something. I believe, it’s a trivial mistake, but I can’t find it.

Thanks in advance, for any hint.

Answer 1

this.grid = new GridField[cols][rows];

The above code only initializes the array. You also need to initialize each array elements with objects of GridField.

Probably something like this: –

for (int i = 0; i < cols; i++) {
    for (int j = 0; j < rows; j++) {
        grid[i][j] = new GridField();
    }
}

I’m coming to Java from C# background so I might have missed
something.

This way of initialization is no different in C#. So, if you initialized arrays like this in C#, then probably you missed something there also. Note the comments from @JonSkeet.