Copying an Object from Array to Array in Java

Question

I'm trying to copy an Object I've created from one array to another of the same type in Java. When I run my program I receive a NullPointerException.

The relevant class functions are:

private int mState;
public Cell(int pRow, int pColumn, int pState) {
    //other things
    setState(pState);
}

public void setState(int pNewState) {
    mState = pNewState;
}

public void setDead() {
    mState = DEAD;
}

and the line in which the error occurs:

mFutureGeneration[i][j].setDead();

That array is defined as

private Cell [][] mFutureGeneration;

then dimensioned as

mFutureGeneration = new Cell[100][100];

It receives its contents from:

Cell [][] vSeedArray = new Cell[100][100];

which is filled as

for (int i = 0; i<100; i++) {
    for (int j = 0; j<100; j++) {
        int vNewState = mGenerator.nextInt(2) - 1;
        vSeedArray[i][j] = new Cell(i,j,vNewState);
    }
}

I think the problem is happening in the copy, but I was always under the impression Java copied by reference, so I can't see why it would be failing.

I copy the contents across with a loop

for(int i = 0; i<vSeedArray.length; i++) {
    for(int j=0; j<vSeedArray[i].length; j++) {
            mCurrentGeneration[i][j] = vSeedArray[i][j];
    }
}

Any help would be appreciated.

Thanks.

I suggest that you start by using your debugger (or adding SOP statements). Check the values of i, j, and mFutureGeneration[i][j] at the line with mFutureGeneration[i][j].setDead();. — Code-Apprentice
– Code-Apprentice, Commented Oct 31, 2012 at 1:02
@EliAlgranti That was the problem. I have two array's mFutureGeneration and mCurrentGeneration I've been refactoring from using a simple 0 and 1 in the array to having an object, and forgot to actually create objects in the mFutureGeneration array. Thanks! — xyzjace
– xyzjace, Commented Oct 31, 2012 at 1:09
@xyzjace - no prob sometimes you just need someone to ask you "Are you sure you turned it on?" :-) — Eli Algranti
– Eli Algranti, Commented Oct 31, 2012 at 11:52

paulsm4 · Accepted Answer · 2012-10-31 01:02:46Z

3

I'm not sure you're ever allocating any "Cell" objects - just the array to hold them.

If so, that's probably the cause of your NullPointer exception.

Good link:

http://www.willamette.edu/~gorr/classes/cs231/lectures/chapter9/arrays2d.htm

Scroll down to the section "Array of Objects".

'Hope that helps!

answered Oct 31, 2012 at 1:02

paulsm4

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