Need help with union-by-size method

Question

Hello :)
I'm having a problem with a union-by-size method.
It was originally intended for a university assignment, but as I'm getting random errors by using it, I've changed to using union-by-heigth instead (which works flawlessly :-).
But I'm still very much interested in finding out what I did wrong with the union-by-size implementation (as I belive it'd be the better method since I use path compression and a lot of find(x)).
So if anyone can tell me what I'm doing wrong with the following code, I'd really appreciate it. As it is it is I'm getting ArrayIndexOutOfBoundsException with a positive much higher number than the array contains (ie. 1606 with a 900 long array)...
(I've also gotten a few stackoverflows in main, but I guess that is more with too much printouts... )
int[] union-by-size-array = new int[n*n] //implemented in other part of the code
private void union (int root1, int root2){
        int temporaryInteger;
        if(union-by-size-array[root2] < union-by-size-array[root1]){
            temporaryInteger = union-by-size-array[root2];
            union-by-size-array[root1] = root2;
            union-by-size-array[navn2] += temporaryInteger;
        }else {
            temporaryInteger = union-by-size-array[root1];
            union-by-size-array[root2] = root1;
            union-by-size-array[root1] += temporaryInteger;
    }

Answer 1

Ok, I'll supplement.
n is given by the user as an argument.
int nrOfDisJointSets = 1;
int[][] board = new int[n][n];
int[] ] union-by-size-array = new int[n+1];
    private void createNewNodeBoard(){
        for(int i = 0; i < n; i++){
            for(int j = 0; j <n; j++){
                board[i][j] = createNode();
    private Node createNode(){
        Node newNode = new Node(nrOfDisJointSetst);
        union-by-size-array[nrOfDisJointSets] = -1;
        nrOfDisJointSets++;
        return newNode;
    }the union method is called by the following line
union(find(x), find(y));navn2 is supposed to be root2 btw..