随机四位验证码

import java.util.Random;

public class Homework0511 {

public static void main(String[] args) {

      Random rand=new Random();
        String str="abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";

        System.out.print("验证码是（区分大小写）：");
        for(int i=0;i<4;i++)
        {
            int p=rand.nextInt(61);
            System.out.print(str.substring(p, p+1)+"  ");
        }
      }}  

运行结果：