code: public HttpMediaType setSubType(String subType) {
      Preconditions.checkArgument(
          TYPE_REGEX.matcher(subType).matches(), "Subtype contains reserved characters");
      this.subType = subType;
      cachedBuildResult = null;
      return this;
    }
docs: Sets the sub media type, for example {@code "plain"} when using {@code "text"}.
  
  @param subType sub media type