code: public static <T extends Iterable<?>> T checkContentsNotNull(
        T iterable, Object errorMessage) {
      if (containsOrIsNull(iterable)) {
        throw new NullPointerException(String.valueOf(errorMessage));
      }
      return iterable;
    }
docs: Ensures that an {@code Iterable} object passed as a parameter to the
  calling method is not null and contains no null elements.
  
  @param iterable the iterable to check the contents of
  @param errorMessage the exception message to use if the check fails; will
  be converted to a string using {@link String#valueOf(Object)}
  @return the non-null {@code iterable} reference just validated
  @throws NullPointerException if {@code iterable} is null or contains at
  least one null element