求一个3*3矩阵对角线元素之和

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public static void main(String[] args) {         int[][] a={{1,2,3,},{4,5,6},{7,8,9}};         System.out.println(a.length);         int sum=0;         for (int i = 0; i < a.length; i++) {             for (int j = 0; j < a[i].length; j++) {                 if(i==j || i+j==a.length-1){                     sum+=a[i][j];                 }                   System.out.print("  "+a[i][j]);             }             System.out.println();         }         System.out.println(sum);     }