Java solution to check if a Sudoku solution is valid
Your setting of count = 0 should be immediately before the appropriate for loop, not after it, and I would actually call that variable sum (renaming your sum to target) since it's not really a "count" at all, e.g:

private boolean rowCheck(){
    int target = size * (size + 1) / 2;
    for (int i = 0; i &lt;size; i++) {
        int sum = 0;        // declaration moved, too
        for (int j = 0; j &lt; size ; j++) {
            sum += board[i][j];
        }
        if (sum != target) return false;
    }
    return true;
}


The point being that you should be setting the variable in advance of the loop, not resetting it afterwards (that being a pointless operation on the final iteration).

I would probably also move target into a class member, calculated in the constructor.