Java 8:

public static int diagonalDifference(List<List<Integer>> arr) {
int sum1 = 0;
int sum2 = 0;
    int j = arr.size() - 1;
    for (int i = 0; i < arr.size(); i++) {
        sum1 += arr.get(i).get(i);
        sum2 += arr.get(i).get(j);
        j--;
    }
    return Math.abs(sum1 - sum2);
}