使用Java怎么生成一个随机验证码

package verificationcode;import java.util.Random;public class RandomGen {  //生成四位不重复的验证码  public static String codeGen(){    char [] codeSequence={'A','B','C','D','E','F','G','H','I','J',  'K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z',  '1','2','3','4','5','6','7','8','9'};    Random random =new Random();    StringBuilder sb=new StringBuilder();//动态字符串，String创建的字符串不能修改    int count=0;//计数器确定产生的是四位验证码    while(true){      //随机产生一个下标，通过下标取出字符数组对应的字符      char c=codeSequence[random.nextInt(codeSequence.length)];      //假设取出来的字符在动态字符串中不存在，代表没有重复      if (sb.indexOf(c+"")==-1) {        sb.append(c);//追加到动态字符串中        count++;        if (count==4) {          break;        }      }    }  return sb.toString();    }  }