生成一个四位数的随机验证码

/**
 * 生成随机验证码
 * @author Administrator
 *
 */
public class RandomGendemo {
    public static void main(String[] args) {
        System.out.println("生成的随机验证码:"+RandomGen.codeGen());
    }
}
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',
                '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();
        //动态字符串
        StringBuilder sb = new StringBuilder();
        int count = 0;
        while(true){
            //随机产生一个下标，通过下标取出字符数组中内容
            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，代表没有重复的
            if(sb.indexOf(c+" ") == -1){
                //追加到动态字符串中
                sb.append(c);
                count++;
                if(count == 4){
                    break;
                }
            }
        }
        return sb.toString();
    }
}