Java 8 solution

public static int diagonalDifference(List<List<Integer>> arr) {
// Write your code here
        int sumElementLeftToRightDiagonal= IntStream
            .range(0,arr.size())
            .mapToObj(i->arr.get(i).get(i))
            .collect(toList())
            .stream()
            .reduce(0,Integer::sum);

    int sumElementRightToLeftDiagonal=IntStream
            .range(0, arr.size())
            .mapToObj(i->arr.get(i).get(arr.size()-1-i))
            .collect(toList())
            .stream()
            .reduce(0,Integer::sum);

    return Math.abs(sumElementLeftToRightDiagonal-sumElementRightToLeftDiagonal);

}