public static int diagonalDifference(List<List<Integer>> arr) {

    int size = arr.size();
    int mainDiagonalSum = 0;
    int theOtherDiagonalSum = 0;
    for (int i = 0; i < arr.size(); i++) {
        for (int j = 0; j < arr.get(i).size(); j++) {
            int current = arr.get(i).get(j);
            // check if number is on main diagonal
            if (i == j) {
                mainDiagonalSum += current;
            }
            if (i + j == size - 1) {
                theOtherDiagonalSum += current;
            }
        }
    }
    return Math.abs(mainDiagonalSum - theOtherDiagonalSum);

    }