code: public static String createRandomNum(int length) {
          if (length < 0) {
              throw new IllegalArgumentException("长度不能小于0");
          }
          char[] chars = new char[length];
          for (int i = 0; i < length; i++) {
              chars[i] = (char) ((Math.random() * 10) + 48);
          }
          return new String(chars);
      }
docs: 生成随机字符串（只包含数字）
  
  @param length 长度
  @return 随机字符串