生成四位随机不重复验证码

    //生成一个四位不重复的随机码
    public static  String get4Codes(){
        StringBuffer sb = new StringBuffer();
        String codes = "qwertyuiopasdfghjklzxcvbnm0123456789";
        Random random = new Random();
        for (int i =0; i< codes.length(); i++){
            char c = codes.charAt(random.nextInt(codes.length()));
            if (sb.length() == 4) break;
            if (!sb.toString().contains(c+"")) sb.append(c);
        }
        return sb.toString();
    }