delegate().addLast(e);
  }

  @Override
  public Iterator<E> descendingIterator() {
    return delegate().descendingIterator();
  }

  @Override
  @ParametricNullness
  public E getFirst() {
    return delegate().getFirst();
  }

  @Override
  @ParametricNullness
  public E getLast() {
    return delegate().getLast();
  }

  @CanIgnoreReturnValue // TODO(cpovirk): Consider removing this?
  @Override

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

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

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

    @Override

    Iterable<String> iterable = Collections.singletonList("foo");
    assertEquals("foo", Iterables.getFirst(iterable, "bar"));
  }

  public void testGetFirst_withDefault_empty() {
    Iterable<String> iterable = Collections.emptyList();
    assertEquals("bar", Iterables.getFirst(iterable, "bar"));
  }

  public void testGetFirst_withDefault_empty_null() {

      return new Traversal<N>(graph) {
        @Override
        @CheckForNull
        N visitNext(Deque<Iterator<? extends N>> horizon) {
          Iterator<? extends N> top = horizon.getFirst();
          while (top.hasNext()) {
            N element = top.next();
            // requireNonNull is safe because horizon contains only graph nodes.
            /*

   * forward to this implementation.
   */
  @CheckForNull
  protected Entry<K, V> standardFirstEntry() {
    return Iterables.<@Nullable Entry<K, V>>getFirst(entrySet(), null);
  }

  /**
   * A sensible definition of {@link #firstKey} in terms of {@code firstEntry}. If you override
   * {@code firstEntry}, you may wish to override {@code firstKey} to forward to this

   * forward to this implementation.
   */
  @CheckForNull
  protected Entry<K, V> standardFirstEntry() {
    return Iterables.<@Nullable Entry<K, V>>getFirst(entrySet(), null);
  }

  /**
   * A sensible definition of {@link #firstKey} in terms of {@code firstEntry}. If you override
   * {@code firstEntry}, you may wish to override {@code firstKey} to forward to this

  /** @since 12.0 */
  @Override
  @CheckForNull
  public E ceiling(E e) {
    return Iterables.<@Nullable E>getFirst(tailSet(e, true), null);
  }

  /** @since 12.0 */
  @GwtIncompatible // NavigableSet
  @Override
  @CheckForNull
  public E higher(E e) {
    return Iterables.<@Nullable E>getFirst(tailSet(e, false), null);
  }

  @Override
  public E first() {
    return iterator().next();
  }

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

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

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

    @Override

    Iterable<String> iterable = Collections.singletonList("foo");
    assertEquals("foo", Iterables.getFirst(iterable, "bar"));
  }

  public void testGetFirst_withDefault_empty() {
    Iterable<String> iterable = Collections.emptyList();
    assertEquals("bar", Iterables.getFirst(iterable, "bar"));
  }

  public void testGetFirst_withDefault_empty_null() {

    delegate().addLast(e);
  }

  @Override
  public Iterator<E> descendingIterator() {
    return delegate().descendingIterator();
  }

  @Override
  @ParametricNullness
  public E getFirst() {
    return delegate().getFirst();
  }

  @Override
  @ParametricNullness
  public E getLast() {
    return delegate().getLast();
  }

  @CanIgnoreReturnValue // TODO(cpovirk): Consider removing this?
  @Override