生成不重复的随机数

要求：此随机数中只允许出现0-9的数字，a-z和A-Z

package com.study.pratice03;

import java.security.SecureRandom;
import java.util.HashSet;
import java.util.Set;

class RandomString
{
    private static final String POSSIBLE_CHARS = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
    
    public String getRandomString(int length)
    {
        StringBuffer sbf=new StringBuffer();
        SecureRandom random = new SecureRandom();
        for (int i = 0; i < length; i++)
        {
            sbf.append(POSSIBLE_CHARS.charAt(random.nextInt(POSSIBLE_CHARS
                    .length())));
        }
        return sbf.toString();
    }
}
public class RandomStringTest
{
    public static void main(String[] args)
    {
        // TODO Auto-generated method stub
        System.out.println(new RandomString().getRandomString(10));
        //测试
        /*for (int i = 0; i < 2000000; i++)
        {
            String s = new RandomString().getRandomString(10);
            Set<String> check = new HashSet<String>();
            if (check.contains(s))
            {
                throw new IllegalStateException("重复字符被发现: " + s);
            }
            else
            {
                System.out.println("产生第"+(i+1)+"个字符:"+s);
                check.add(s);
            }
        }*/
    }

}