Diagonal Sum of a Matrix
The following program illustrates Diagonal Sum of given Matrix 

public class DiagonalSum {

	public static void main(String[] args) {

		int[][] matrix = { { 1, 2, 3 }, { 4, 5, 6 }, { 7, 8, 9 } };

		System.out.println(new DiagonalSum().getDiagonalSum(matrix));
	}

	public int getDiagonalSum(int[][] matrix) {
		int sum = 0;

		for (int i = 0; i < matrix.length; i++) {
			for (int j = 0; j < matrix[i].length; j++) {
				if (i == j) {
					sum += matrix[i][j];
				}
			}

		}
		return sum;
	}
}

Result:
Ouput: 15