生成不重复的随机码

public static void main(String[] args) {          Random rand = new Random();          char[] letters=new char[]{'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','r',                  '0','1','2','3','4','5','6','7','8','9'};          String str = "";          int index;          boolean[] flags = new boolean[letters.length];//默认为false          for(int i=0;i<5;i++){              do{                  index = rand.nextInt(letters.length);               }while(flags[index]==true);              char c = letters[index];              str += c;              flags[index]=true;          }          System.out.println(str);      }