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();

    }
  }

package verificationcode;

public class RandomGenDemo {

  public static void main(String[] args) {
    // TODO Auto-generated method stub
   System.out.println(RandomGen.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'};