Java: NullExceptionPointer in object[]

Question

I am trying to check the state of cells for a true value. If the state is true, then I am going to save the X and Y of the element (2D Array). Here is the method I am using where c is the counter that counts how many cells are true:

public void checkState(){
        System.out.println("Checking State");
        int c = 0;

        for(int y = 0; y < GRID_Y; y++){
            for(int x = 0;x < GRID_X; x++){
                if(state[x][y]==true){
                    //Error starts after this line
                    coords[c].setX(x);
                    coords[c].setY(y);
                    c++;

                    //DEBUG

                        System.out.println(coords.getX());
                        System.out.println(coords.getY());
                }

            }
        }
        System.out.println(c);
    }
}

The coords object is declared like this:

private Coords[] coords = new Coords[totalCells];

(Tried changing totalCells to a small integer value like 10, still didn't work)

Where Coords class is:

public class Coords {
    int x,y;

    public Coords(){
        x = 0;
        y = 0;
    }

    public void setX(int xIn){
        x = xIn;
    }

    public void setY(int yIn){
        y = yIn;
    }

    public int getX(){
        return x;
    }

    public int getY(){
        return y;
    }

}

I am getting the following error:

Checking State
java.lang.NullPointerException
    at game.GameOfLifeGUI.checkState(GameOfLifeGUI.java:116)
    at game.GameOfLifeGUI.<init>(GameOfLifeGUI.java:57)
    at game.GameOfLifeGUI$1.run(GameOfLifeGUI.java:32)
    at java.awt.event.InvocationEvent.dispatch(Unknown Source)
    at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
    at java.awt.EventQueue.access$200(Unknown Source)
    at java.awt.EventQueue$3.run(Unknown Source)
    at java.awt.EventQueue$3.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
    at java.awt.EventQueue.dispatchEvent(Unknown Source)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.run(Unknown Source)

I don't get how a null value is being passed as I am storing the x and y of the for loops.

Thanks for your time.

private Coords[] coords = new Coords[totalCells]; creates an array of nulls. That's why you get the NPE error. You need to fill it after the creation. — Danstahr
– Danstahr, Commented Apr 4, 2014 at 13:57
Yeah that was the problem. I had a brain fart for a moment.Thanks. — Trifecta
– Trifecta, Commented Apr 4, 2014 at 14:06

JajaDrinker · Accepted Answer · 2014-04-04 14:03:31Z

3

After this : private Coords[] coords = new Coords[totalCells];

Add :

for(int index = 0; index < totalCells-1; index++){
    coords[index] = new Coords();
}

That should do the trick

answered Apr 4, 2014 at 14:03

JajaDrinker

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3 Comments

Trifecta Over a year ago

Yeah I always forget to do this when I am working with arrays of objects.

JajaDrinker Over a year ago

Your totally right. But lets hope he have a gridX*gridY == totalCell. Or he will be posting again for an ArrayOutOfBoundExecption :D

Trifecta Over a year ago

Yeah yeah I do have that :)

m09 · Accepted Answer · 2014-04-04 14:19:51Z

1

While @user3497004's answer is fine for java < 8, you should prefer:

Arrays.setAll(coords, n -> new Coords());

for java 8 (and probably above, let's be futuristic).

answered Apr 4, 2014 at 14:19

m09

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2 Comments

Trifecta Over a year ago

What does n represent? Never used the Arrays class tbh.

m09 Over a year ago

n is the index of the particular cell being initialized

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