public static <E extends @Nullable Object> Collection<E> filter(
      Collection<E> unfiltered, Predicate<? super E> predicate) {
    if (unfiltered instanceof FilteredCollection) {
      // Support clear(), removeAll(), and retainAll() when filtering a filtered
      // collection.
      return ((FilteredCollection<E>) unfiltered).createCombined(predicate);
    }

    return new FilteredCollection<>(checkNotNull(unfiltered), checkNotNull(predicate));

  public static <E extends @Nullable Object> Collection<E> filter(
      Collection<E> unfiltered, Predicate<? super E> predicate) {
    if (unfiltered instanceof FilteredCollection) {
      // Support clear(), removeAll(), and retainAll() when filtering a filtered
      // collection.
      return ((FilteredCollection<E>) unfiltered).createCombined(predicate);
    }

    return new FilteredCollection<>(checkNotNull(unfiltered), checkNotNull(predicate));