code: protected final void assertContainsOnly(@NotNull Object... objects) {
      isNotNull();
      checkNotNull(objects);
      Set<Object> copy = actualAsSet();
      List<Object> notFound = notFoundInCopy(copy, newLinkedHashSet(objects));
      if (!notFound.isEmpty()) {
        throw failureIfExpectedElementsNotFound(notFound);
      }
      if (copy == null || copy.isEmpty()) {
        return;
      }
      throw failureIfUnexpectedElementsFound(copy);
    }
docs: Verifies that the actual group of objects contains the given objects <strong>only</strong>, in any order.
  
  @param objects the objects to look for.
  @throws AssertionError       if the actual group of objects is {@code null}.
  @throws NullPointerException if the given group of objects is {@code null}.
  @throws AssertionError       if the actual group of objects does not contain the given objects, or if the actual
  group of objects contains elements other than the ones specified.