随机验证码

案例介绍：模拟登录时需要输入验证码功能;

案例设计：

①
   证码由4个字符组成;

②
   验证码只能由数字和大写字母组成;

③
   每次输入的字符各不相同，不能重复;

import java.util.Random;
public class RandomGenDemo{
    public static void main(String []args){
        System.out.println("验证码如下："+RandomGen.codeGen());
    }
}

class RandomGen{
    //生成4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','0','1','2','3','4','5','6','7','8','9'};
        Random random=new Random();
        StringBuilder sb=new StringBuilder();//动态字符串
        int count=0;
        while(true){
            //随机产生一个下标，通过下标取出字符数组中对应的字符
            char c= codeSequence[random.nextInt(codeSequence.length)];
            //假设取出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();
    }
}