Java 随机产生四位验证码

import java.util.Random;

public class RandGenDemo {
    public static void main(String[] args) {
        
        System.out.println("请输入验证码："+VerificationCode.codeFen());
    }
}
class VerificationCode{
    public static String codeFen(){
    String str = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
    
    char[] VerificationCodeArray = str.toCharArray();
    Random random = new Random();
    int count = 0;
    StringBuilder stringBuilder = new StringBuilder();
    while(true) {
        
        int index = random.nextInt(VerificationCodeArray.length);
        
        char c = VerificationCodeArray[index];
        
        if (stringBuilder.indexOf(c + "") == -1) {
            
            stringBuilder.append(c);
            
            count++;
        }
        
        if (count == 4) {
            break;
            }
        }
        
        return stringBuilder.toString();
    }
}