public void testPoll_empty() {
    assertNull("emptyQueue.poll() should return null", getQueue().poll());
    expectUnchanged();
  }

  @CollectionFeature.Require(SUPPORTS_REMOVE)
  @CollectionSize.Require(ONE)
  public void testPoll_size1() {
    assertEquals("size1Queue.poll() should return first element", e0(), getQueue().poll());
    expectMissing(e0());
  }

  /**
   * Constructs a new finalizable phantom reference.
   *
   * @param referent to phantom reference
   * @param queue that should finalize the referent
   */
  protected FinalizablePhantomReference(@CheckForNull T referent, FinalizableReferenceQueue queue) {
    super(referent, queue.queue);
    queue.cleanUp();
  }

  @CanIgnoreReturnValue
  public static Set<Feature<?>> addImpliedFeatures(Set<Feature<?>> features) {
    Queue<Feature<?>> queue = new ArrayDeque<>(features);
    while (!queue.isEmpty()) {
      Feature<?> feature = queue.remove();
      for (Feature<?> implied : feature.getImpliedFeatures()) {
        if (features.add(implied)) {
          queue.add(implied);
        }
      }
    }
    return features;
  }

  /**

     */
    @CheckForNull
    abstract N visitNext(Deque<Iterator<? extends N>> horizon);
  }

  /** Poor man's method reference for {@code Deque::addFirst} and {@code Deque::addLast}. */
  private enum InsertionOrder {
    FRONT {
      @Override
      <T> void insertInto(Deque<T> deque, T value) {
        deque.addFirst(value);
      }
    },
    BACK {
      @Override

import java.util.List;
import java.util.Queue;

/**
 * Create queue of strings for tests.
 *
 * @author Jared Levy
 */
@GwtCompatible
@ElementTypesAreNonnullByDefault
public abstract class TestStringQueueGenerator implements TestQueueGenerator<String> {
  @Override
  public SampleElements<String> samples() {
    return new Strings();
  }

  @Override
  public Queue<String> create(Object... elements) {

      }
    }
  }

  /** Invokes {@code queue.}{@link BlockingQueue#take() take()} uninterruptibly. */
  @J2ktIncompatible
  @GwtIncompatible // concurrency
  public static <E> E takeUninterruptibly(BlockingQueue<E> queue) {
    boolean interrupted = false;
    try {
      while (true) {
        try {
          return queue.take();
        } catch (InterruptedException e) {

    private final Queue<T> queue;

    UnIterableQueue(Queue<T> queue) {
      this.queue = queue;
    }

    @Override
    public Iterator<T> iterator() {
      throw new UnsupportedOperationException();
    }

    @Override
    protected Queue<T> delegate() {
      return queue;
    }
  }

  public void testIndexOf_empty() {

  private void weaklyReferenceQueue() {
    frq = new FinalizableReferenceQueue();
    queueReference = new WeakReference<>(frq.queue);

    /*
     * Queue and clear a reference for good measure. We test later on that
     * the finalizer thread stopped, but we should test that it actually
     * started first.
     */
    reference =

  private void weaklyReferenceQueue() {
    frq = new FinalizableReferenceQueue();
    queueReference = new WeakReference<>(frq.queue);

    /*
     * Queue and clear a reference for good measure. We test later on that
     * the finalizer thread stopped, but we should test that it actually
     * started first.
     */
    reference =

    WeakValueReferenceImpl(ReferenceQueue<V> queue, V referent, E entry) {
      super(referent, queue);
      this.entry = entry;
    }

    @Override
    public E getEntry() {
      return entry;
    }

    @Override
    public WeakValueReference<K, V, E> copyFor(ReferenceQueue<V> queue, E entry) {
      return new WeakValueReferenceImpl<>(queue, get(), entry);
    }
  }

  /**