NullPointerException when initialising a 2D array of BooleanProperty?

Question

I have a very simple constructor where I initialise a 2D array of type BooleanProperty to all false. However, I am getting a NullPointerException at the line grid[i][j].set(false). I am not sure why this is the case, as grid is not null? I think I must be using BooleanProperty incorrectly, but I'm not sure why.

public class Game {
    private BooleanProperty[][] grid;

    public Game() {
        grid = new BooleanProperty[10][10];
        for (int i = 0; i < 10; i++) {
            for (int j = 0; j < 10; j++) {
                grid[i][j].set(false);
            }
        }
    }

    // other methods
}

grid[i][j] = new BooleanProperty() ? – toootooo Nov 07 '19 at 01:28 — toootooo, Nov 07 '19 at 01:28

Shankha057 · Answer 1 · 2019-11-07T01:41:32.213

3

grid isn't null. Its contents are. So add this line and you won't even need the setter:

grid[i][j] = new SimpleBooleanProperty(false); // Or whatever constructor or builder or factory method you want to use

You have allocated memory for the grid object but it itself is made up of other objects, which themselves, haven't been allocated anything. So you have to allocate memory to them individually as well.

edited Nov 07 '19 at 01:41

answered Nov 07 '19 at 01:32

Shankha057

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When I tried that I got the error 'cannot instantiate the type BooleanProperty'? – scott Nov 07 '19 at 01:34
BooleanProperty is an abstract class – Nick Clark Nov 07 '19 at 01:36
Got it, SimpleBooleanProperty works. Thanks! – scott Nov 07 '19 at 01:37
@scott please find out about the correct constructor to use to instantiate `BooleanProperty` and use it. It will solve your problem – Shankha057 Nov 07 '19 at 01:37
1

@NickClark 's answer worked for me – scott Nov 07 '19 at 01:39
@scott if that's the case, please mark it as the correct solution – Nick Clark Nov 07 '19 at 01:40
@scott I've updated my answer. Please check. – Shankha057 Nov 07 '19 at 01:42

score 3 · Accepted Answer · answered Nov 07 '19 at 01:32

Even though you have created the array of BooleanProperty references, you need to initialize each one. Try this:

public class Game {
    private BooleanProperty[][] grid;

    public Game() {
        grid = new BooleanProperty[10][10];
        for (int i = 0; i < 10; i++) {
            for (int j = 0; j < 10; j++) {
                BooleanProperty p = new SimpleBooleanProperty();
                p.set(false);
                grid[i][j] = p;
            }
        }
    }

    // other methods
}

To follow up, BooleanProperty is an abstract class. The bare bones implementation class is SimpleBooleanProperty. For most cases, this does the trick. — Nick Clark, Nov 07 '19 at 01:40

NullPointerException when initialising a 2D array of BooleanProperty?

2 Answers2