29 求一个3*3矩阵对角线元素之和

题目:求一个3*3矩阵对角线元素之和
 1    public class _029MatrixSum {
 2 
 3     public static void main(String[] args) {
 4         matrixSum();
 5     }
 6 
 7     private static void matrixSum() {
 8         Scanner scanner = new Scanner(System.in);
 9         int[][] a = new int[3][3];
10         System.out.println("请输入9个整数:");
11 
12         for (int i = 0; i < 3; i++) {
13             for (int j = 0; j < 3; j++) {
14                 a[i][j] = scanner.nextInt();
15             }
16         }
17 
18         System.out.println("输入的3*3矩阵是:");
19         for (int i = 0; i < 3; i++) {
20             for (int j = 0; j < 3; j++) {
21                 System.out.print(a[i][j] + " ");
22             }
23             System.out.println();
24         }
25 
26         int sum = 0;
27         for (int i = 0; i < 3; i++) {
28             for (int j = 0; j < 3; j++) {
29                 if (i == j) {
30                     sum += a[i][j];
31                 }
32             }
33         }
34 
35         System.out.println("对角线之和是:" + sum);
36     }
37 }