package Zuoye1;import java.util.*;public class Choujiang {        public static void main(String[]args)    {                String str = "0123456789qwertyuiopasdfghjklzxcvbnmQWERTYUIOPASDFGHJKLZXCVBNM";        Random yan=new Random();        for(int i=0; i<4; i++)             {             int j = yan.nextInt(str.length()-1);                          String s = (str.substring(j, j+1));                          System.out.print(s);             }    }}