android/guava/src/com/google/common/collect/Collections2.java
public static <E> 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<E>(checkNotNull(unfiltered), checkNotNull(predicate));
  }
public static <E> 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<E>(checkNotNull(unfiltered), checkNotNull(predicate));
  }
javadoc?