* <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);
      }

   * 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

            Serialization.getFieldSetter(ImmutableSetMultimap.class, "emptySet");
  }

  @GwtIncompatible // java.io.ObjectInputStream
  @J2ktIncompatible
  // Serialization type safety is at the caller's mercy.
  @SuppressWarnings("unchecked")
  private void readObject(ObjectInputStream stream) throws IOException, ClassNotFoundException {
    stream.defaultReadObject();

      @SuppressWarnings({"rawtypes", "unchecked"})
      Entry<Object, V>[] entries = new Entry[size];
      for (int i = 0; i < size; i++) {
        // requireNonNull is safe because the first `2*size` elements have been filled in.
        Object key = requireNonNull(alternatingKeysAndValues[2 * i]);
        @SuppressWarnings("unchecked")
        V value = (V) requireNonNull(alternatingKeysAndValues[2 * i + 1]);

  static <E extends @Nullable Object> Collection<E> filterCollection(
      Collection<E> collection, Predicate<? super E> predicate) {
    if (collection instanceof Set) {
      return Sets.filter((Set<E>) collection, predicate);
    } else {
      return Collections2.filter(collection, predicate);
    }
  }

  @Override
  public boolean containsKey(@CheckForNull Object key) {
    return asMap().get(key) != null;
  }

  @Override