请教前辈们~怎么用JAVA写验证码那块程序~~

import java.util.Random;

class Random_number {
String str = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";

Random ran = new Random();

char[] chr = str.toCharArray();

int chrlong = chr.length;

String rnumber = "";

public Random_number() {
}

public String randomNumber(int n) {
int k;
for (int i = 0; i < n; i++) {
k = ran.nextInt(35);
if (rnumber.indexOf(chr[k]) == -1) {//判读验证新生成的验证码是否与前面的重复
// System.out.println(chr[k]);
rnumber = rnumber + chr[k];
} else {//如果重复重新生成
i--;
continue;
}
}
return rnumber;
}

}
//测试类
public class test {
public static void main(String[] args) {
Random_number number = new Ran