生成字母数字随机串

 private  List<String> genCodes(int length,long num){

   List<String> results=new ArrayList<String>();

   for(int j=0;j<num;j++){
      String val = "";

      Random random = new Random();
      for(int i = 0; i < length; i++)
      {
         String charOrNum = random.nextInt(2) % 2 == 0 ? "char" : "num"; // 输出字母还是数字

         if("char".equalsIgnoreCase(charOrNum)) // 字符串
         {
            int choice = random.nextInt(2) % 2 == 0 ? 65 : 97; //取得大写字母还是小写字母
            val += (char) (choice + random.nextInt(26));
         }
         else if("num".equalsIgnoreCase(charOrNum)) // 数字
         {
            val += String.valueOf(random.nextInt(10));
         }
      }
      val=val.toUpperCase();
      if(results.contains(val)){
         continue;
      }else{
         results.add(val);
      }
   }
   return results;


}