code: private static Data checkData(final Data data) {
  		if (Data.EMPTY.equals(data)) {
  			throw new IllegalStateException("Argument 'data' must not be empty.");
  		}
  		return data;
  	}
docs: Runtime check that the passed instance of {@link Data} is not empty (respectively {@link Data#EMPTY}).
  
  @param data
  instance of {@code Data}
  @throws IllegalStateException
  if the passed instance is empty