*/
  // TODO(kevinb): how can we omit that Iterables link when building gwt
  // javadoc?
  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.

   * <p><b>{@code Stream} equivalent:</b> {@link Stream#filter}.
   */
  public static <T extends @Nullable Object> Iterable<T> filter(
      final Iterable<T> unfiltered, final Predicate<? super T> retainIfTrue) {
    checkNotNull(unfiltered);
    checkNotNull(retainIfTrue);
    return new FluentIterable<T>() {
      @Override
      public Iterator<T> iterator() {
        return Iterators.filter(unfiltered.iterator(), retainIfTrue);
      }

                unfiltered.add("zzz");
                unfiltered.add("abc");
                return Sets.filter(
                    Sets.filter(unfiltered, Collections2Test.LENGTH_1),
                    Collections2Test.NOT_YYY_ZZZ);
              }
            })
        .named("Sets.filter, filtered input")
        .withFeatures(
            CollectionFeature.SUPPORTS_ADD,

   * removed from the underlying multiset.
   *
   * <p>The returned multiset isn't threadsafe or serializable, even if {@code unfiltered} is.
   *
   * <p>Many of the filtered multiset's methods, such as {@code size()}, iterate across every
   * element in the underlying multiset and determine which elements satisfy the filter. When a live

    Enumeration<Integer> enumer = enumerate(1, 2, 3);
    Iterator<Integer> iter = Iterators.forEnumeration(enumer);

    assertTrue(iter.hasNext());
    assertEquals(1, (int) iter.next());
    assertTrue(iter.hasNext());
    assertEquals(2, (int) iter.next());
    assertTrue(iter.hasNext());
    assertEquals(3, (int) iter.next());
    assertFalse(iter.hasNext());
  }

  public void testAsEnumerationEmpty() {

    assertThat(stream(FluentIterable.of(1, 2, 3)).filter(n -> n > 1)).containsExactly(2, 3);
  }

  @SuppressWarnings("deprecation")
  public void testStream_collection() {
    assertThat(stream(Arrays.asList())).isEmpty();
    assertThat(stream(Arrays.asList("a"))).containsExactly("a");
    assertThat(stream(Arrays.asList(1, 2, 3)).filter(n -> n > 1)).containsExactly(2, 3);
  }

   */
  // TODO(kevinb): how can we omit that Iterables link when building gwt
  // javadoc?
  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.

  }

  /**
   * Combines this Bloom filter with another Bloom filter by performing a bitwise OR of the
   * underlying data. The mutations happen to <b>this</b> instance. Callers must ensure the Bloom
   * filters are appropriately sized to avoid saturating them.
   *
   * @param that The Bloom filter to combine this Bloom filter with. It is not mutated.
   * @throws IllegalArgumentException if {@code isCompatible(that) == false}

   * java.util.stream.Stream#filter}. This method is not being deprecated, but we gently encourage
   * you to migrate to streams.
   */
  // TODO(kevinb): how to omit that last sentence when building GWT javadoc?
  public static <E extends @Nullable Object> Set<E> filter(
      Set<E> unfiltered, Predicate<? super E> predicate) {
    if (unfiltered instanceof SortedSet) {
      return filter((SortedSet<E>) unfiltered, predicate);

 * relevant methods below.
 *
 * <ul>
 *   <li>This class is serializable; {@code java.util.Optional} is not.
 *   <li>{@code java.util.Optional} has the additional methods {@code ifPresent}, {@code filter},
 *       {@code flatMap}, and {@code orElseThrow}.
 *   <li>{@code java.util} offers the primitive-specialized versions {@code OptionalInt}, {@code