@Override
    public String toDescription() {
      if (lower == null && upper == null) {
        return null;
      }
      if (lower != null && upper != null) {
        return "Expects value between " + lower + " and " + upper;
      }
      if (lower != null) {
        return "Expects value bigger than " + lower;
      }
      return "Expects value smaller than " + upper;
    }
validate value for a ConfVar, return non-null string for fail message
common/src/java/org/apache/hadoop/hive/conf/Validator.java