@Override
  protected void validateRequest() {
    if (!params().containsKey("placeid")) {
      throw new IllegalArgumentException("Request must contain 'placeId'.");
    }
  }
Specifies the field masks of the details to be returned by PlaceDetails.

@param fields The Field Masks of the fields to return.
@return Returns this {@code PlaceDetailsRequest} for call chaining.
src/main/java/com/google/maps/PlaceDetailsRequest.java