隨機產生數碼

 public String getRandom(){
  //隨機生成優惠碼
     Random  random  = new Random();
        String  str = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
        StringBuffer  pomot = new StringBuffer();
        int length = random.nextInt(9);
         //優惠碼 限制長度6-8位
         while(length<6)
         {
          length = random.nextInt(9);
         }
         for(int i=0;i<length;i++)
         {
          //0-61
        int beginIndex = Math.abs(random.nextInt())%62;
        String st = str.substring(beginIndex, beginIndex+1);
        pomot.append(st);
      }
         return pomot.toString();
 }