do {
      delegate.add(checkNotNull(elements.next()));
    } while (elements.hasNext());

    return unsafeDelegate(delegate);
  }

  // Factory methods that skips the null checks on elements, only used when
  // the elements are known to be non-null.
  static <E> ImmutableSet<E> unsafeDelegate(Set<E> delegate) {
    switch (delegate.size()) {
      case 0:
        return of();
      case 1: