1. 编写程序，求一个三阶方阵的对角线上各元素之和。

 1 package stringDiagonal;
 2 
 3 import java.util.Scanner;
 4 
 5 public class Diagonal {
 6     public static void main(String[] args) {
 7         
 8         int[][] data=new int[3][3];
 9         int mainSum=0;
10         int sideSum=0;
11         
12         Scanner scan=new Scanner(System.in);
13 
14         for(int i=0;i<3;i++) {
15             for(int j=0;j<3;j++){
16                 data[i][j]=scan.nextInt();
17             }
18         }
19         scan.close();
20         
21         for(int i=0;i<3;i++) {
22             for(int j=0;j<3;j++){
23                 if(i==j){
24                     mainSum+=data[i][j];
25                 }
26                 if(i+j==2) {
27                     sideSum+=data[i][j];
28                 }
29             }
30         }
31         
32         System.out.println("The main diagonal: "+mainSum);
33         System.out.println("The side diagonal: "+sideSum);
34 
35     }
36 }

2. 编写程序，从键盘上输入一个字符串和子串开始位置与长 度，截取该字符串的子串并输出。

 1 package srtringSunstring;
 2 
 3 import java.util.Scanner;
 4 
 5 public class Substring {
 6     public static void main(String[] args) {
 7         
 8         System.out.println("Enter <STRING BEGINNUMBER LENGTH>:");
 9         
10         Scanner scan=new Scanner(System.in);
11         String s=scan.nextLine();
12         int begin=scan.nextInt();
13         int length=scan.nextInt();
14         scan.close();
15         
16         String subS=s.substring(begin, begin+length);
17         
18         System.out.println(subS);        
19         
20     }
21 }

3. 编写程序，统计用户从键盘输入的字符串中包含的字母、数 字和其他字符的个数。

 1 package stringAnalysis;
 2 
 3 import java.util.Scanner;
 4 
 5 public class Analysis {
 6     public static void main(String[] args) {
 7     
 8         System.out.println("Enter <STRING>:");
 9         
10         Scanner scan=new Scanner(System.in);
11         String s=scan.nextLine();
12         scan.close();
13         
14         char[] ss=s.toCharArray();
15         
16         int letter=0;
17         int number=0;
18         int blank=0;
19         int other=0;
20         
21         int len=s.length();
22         for(int i=0;i<len;i++){
23             if(Character.isLetter(ss[i])) {
24                 letter++;
25             }else if(Character.isDigit(ss[i])) {
26                 number++;
27             }else if(Character.isSpaceChar(ss[i])) {
28                 blank++;
29             }else {
30                 other++;
31             }        
32         }
33         
34         System.out.println("letter: " + letter);
35         System.out.println("number: " + number);
36         System.out.println("blank: " + blank);
37         System.out.println("other: " + other);
38         
39     }
40 }

来支舞吧！