随机生成4位验证码

package com.hanqi;
import java.util.*;
public class yanzhengma {

    public static void main(String[] args) {
        // TODO 自动生成的方法存根
        String str="abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
        int ary[]=new int[4];
        for (int i = 0; i <4; i++)
        {
            Random r=new Random();
            //一共62个元素，ary[i]取值为0-61，是各个元素的索引,ary[i]+1取值1-62，确保了62个数都可以随机到
            ary[i]=r.nextInt(61);
            System.out.print(str.substring(ary[i],ary[i]+1));
        }    

        }

    }