* 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

   * 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);

  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

 * Iterator#remove} operations. This utility takes the brute-force approach of trying <i>all</i>
 * possible sequences of these operations, up to a given number of steps. So, if the caller
 * specifies to use <i>n</i> steps, a total of <i>3^n</i> tests are actually performed.
 *
 * <p>For instance, if <i>steps</i> is 5, one example sequence that will be tested is:
 *
 * <ol>
 *   <li>remove();

    // be deleted.
    Map<K, @Nullable V> duplicates = null;
    int dupCount = 0;
    for (int i = 0; i < n; i++) {
      // requireNonNull is safe because the first `n` elements have been filled in.
      entryArray[i] = makeImmutable(requireNonNull(entryArray[i]));
      K key = entryArray[i].getKey();
      V value = entryArray[i].getValue();
      V oldValue = delegateMap.put(key, value);

    waiter.awaitWaiting();
    PollingThread poller = new PollingThread(future);
    poller.start();
    PollingThread poller2 = new PollingThread(future);
    poller2.start();
    PollingThread poller3 = new PollingThread(future);
    poller3.start();
    poller.awaitInLoop();
    poller2.awaitInLoop();
    poller3.awaitInLoop();

    // The waiter queue should be {poller x 3}->waiter1
    waiter.interrupt();

        // ignore
      }
    }
  }

  /**
   * Cleans up the given reference and any other references already in the queue. Catches and logs
   * all throwables.
   *
   * @return true if the caller should continue to wait for more references to be added to the
   *     queue, false if the associated FinalizableReferenceQueue is no longer referenced.
   */
  private boolean cleanUp(Reference<?> firstReference) {

   * the given class. If the object being tested is {@code null} this predicate evaluates to {@code
   * false}.
   *
   * <p>If you want to filter an {@code Iterable} to narrow its type, consider using {@link
   * com.google.common.collect.Iterables#filter(Iterable, Class)} in preference.
   *
   * <p><b>Warning:</b> contrary to the typical assumptions about predicates (as documented at

   * themselves their proper values under GWT, since GWT's EnumMap does need the Class instance.
   *
   * Note that sometimes these fields *do* have correct values under J2CL: They will if the caller
   * calls `create(Foo.class)`, rather than `create(map)`. That's fine; we just shouldn't rely on
   * it.
   */
  transient Class<K> keyTypeOrObjectUnderJ2cl;
  transient Class<V> valueTypeOrObjectUnderJ2cl;

  /**

   * themselves their proper values under GWT, since GWT's EnumMap does need the Class instance.
   *
   * Note that sometimes these fields *do* have correct values under J2CL: They will if the caller
   * calls `create(Foo.class)`, rather than `create(map)`. That's fine; we just shouldn't rely on
   * it.
   */
  transient Class<K> keyTypeOrObjectUnderJ2cl;
  transient Class<V> valueTypeOrObjectUnderJ2cl;

  /**