import java.util.Scanner;public class zuoye22_shixianstartWith {    public static void main(String[] args){        Scanner sc=new Scanner(System.in);        String str=sc.nextLine();        char start=str.charAt(0);        char end=str.charAt(str.length()-1);        if(start==‘a‘){            System.out.println("这个字符串以a开头！");        }        else{            System.out.println("这个字符串不以a开头！");        }        if(end==‘a‘){            System.out.println("这个字符串以a结尾！");        }        else{            System.out.println("这个字符串不以a结尾！");        }    }}

import java.util.Random;import java.util.Scanner;public class zuoye17_jiamiqi {    public static void main(String[] args)         {             System.out.println("请输入密码：");             Scanner sc = new Scanner(System.in);             String password1=sc.nextLine();             int[] num=new int[password1.length()];             int[] password2=new int[password1.length()];             char[] password3=new char[password1.length()];             Random a=new Random();             for(int i=0;i<password1.length();i++){                 int s=a.nextInt(20);                 num[i]=s;             }             if(password1!=""&&password1.length()>=6)             {                 System.out.print("加密后的密码是：");                     for(int i=0;i<password1.length();i++)                 {                     password2[i]=password1.charAt(i)+num[i];                     System.out.print(password2[i]);                 }                System.out.println();                 }else                {                    System.out.println("请输入合法密码！至少6位！");                  }        //解密             System.out.print("解密后的密码是：");                for(int i=0;i<password2.length;i++)                 {                    int index =    password2[i]-num[i];                    password3[i]=(char)(index);                    System.out.print(password3[i]);                }             }     }    

import java.util.Random;import java.util.Scanner;public class zuoye16_yanzhengma {    public static void main(String[] args){        Scanner sc=new Scanner(System.in);        Random s=new Random();        String shuru=new String();        String yanzhengma2=new String();        String num="0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";        int j=0;        do{            StringBuilder yanzhengma=new StringBuilder();            System.out.print("请输如验证码:");             shuru=sc.nextLine();             for(int i=0;i<4;i++){                 int a=s.nextInt(61);                 yanzhengma.append(num.substring(a, a+1));             }             yanzhengma2=yanzhengma.toString();             System.out.println("输入验证码错误，验证码是："+yanzhengma);             j++;             if(j==5)break;        }while(!yanzhengma2.equals(shuru));        if(yanzhengma2.equals(shuru)){            System.out.println("输入验证码正确！");        }    }}