assertTrue(queue.add("hi"));
    assertEquals("hi", queue.element());
    assertEquals("hi", queue.peek());
    assertEquals(1, queue.size());
    assertEquals(0, queue.remainingCapacity());

    assertTrue(queue.add("there"));
    assertEquals("there", queue.element());
    assertEquals("there", queue.peek());
    assertEquals(1, queue.size());
    assertEquals(0, queue.remainingCapacity());

    public boolean remove(Object object) {
      assertTrue(Thread.holdsLock(mutex));
      return delegate.remove(object);
    }

    @Override
    public @Nullable E peek() {
      assertTrue(Thread.holdsLock(mutex));
      return delegate.peek();
    }

    @Override
    public E element() {
      assertTrue(Thread.holdsLock(mutex));
      return delegate.element();
    }

    @Override

   *     Iterators.peekingIterator(Iterators.forArray("a", "b"));
   * String a1 = peekingIterator.peek(); // returns "a"
   * String a2 = peekingIterator.peek(); // also returns "a"
   * String a3 = peekingIterator.next(); // also returns "a"
   * }</pre>
   *
   * <p>Any structural changes to the underlying iteration (aside from those performed by the

      ReferenceEntry<?, ?> originalHead = segment.accessQueue.peek();
      @SuppressWarnings("unchecked")
      ReferenceEntry<Integer, Integer> entry = (ReferenceEntry<Integer, Integer>) originalHead;
      operation.accept(entry);
      drainRecencyQueue(segment);

      assertNotSame(originalHead, segment.accessQueue.peek());
      assertEquals(cache.size(), accessQueueSize(cache));
    }
  }

      this.queue = new ArrayDeque<>();
      queue.add(root);
    }

    @Override
    public boolean hasNext() {
      return !queue.isEmpty();
    }

    @Override
    public T peek() {
      return queue.element();
    }

    @Override
    public T next() {
      T result = queue.remove();
      Iterables.addAll(queue, children(result));
      return result;
    }
  }

    } finally {
      monitor.leave();
    }
  }

  @CanIgnoreReturnValue // pushed down from class to method
  @Override
  public @Nullable E peek() {
    final Monitor monitor = this.monitor;
    monitor.enter();
    try {
      return q.peek();
    } finally {
      monitor.leave();
    }
  }

  /**

    BlockingQueue<String> queue = new ArrayBlockingQueue<>(999);
    putUninterruptibly(queue, "");
    assertTimeNotPassed(stopwatch, LONG_DELAY_MS);
    assertEquals("", queue.peek());
  }

  public void testPutNoInterrupt() {
    TimedPutQueue queue = TimedPutQueue.createWithDelay(20);
    queue.putSuccessfully();
    assertNotInterrupted();
  }

  }

  /** Checks that #drain() invocations behave correctly for a drained (empty) queue. */
  private void assertDrained(BlockingQueue<Object> q) {
    assertNull(q.peek());
    assertInterruptibleDrained(q);
    assertUninterruptibleDrained(q);
  }

  private void assertInterruptibleDrained(BlockingQueue<Object> q) {
    // nothing to drain, thus this should wait doing nothing

      drainRecencyQueue();

      ReferenceEntry<K, V> e;
      while ((e = writeQueue.peek()) != null && map.isExpired(e, now)) {
        if (!removeEntry(e, e.getHash(), RemovalCause.EXPIRED)) {
          throw new AssertionError();
        }
      }
      while ((e = accessQueue.peek()) != null && map.isExpired(e, now)) {
        if (!removeEntry(e, e.getHash(), RemovalCause.EXPIRED)) {

    monitor.enterWhen(notEmpty);
    try {
      return extract();
    } finally {
      monitor.leave();
    }
  }

  @CanIgnoreReturnValue
  @Override
  public @Nullable E peek() {
    final Monitor monitor = this.monitor;
    if (monitor.enterIf(notEmpty)) {
      try {
        return items[takeIndex];
      } finally {
        monitor.leave();
      }
    } else {