return nullableSynchronizedEntry(delegate().pollFirstEntry(), mutex);
      }
    }

    @Override
    public Map.@Nullable Entry<K, V> pollLastEntry() {
      synchronized (mutex) {
        return nullableSynchronizedEntry(delegate().pollLastEntry(), mutex);
      }
    }

    @Override
    public NavigableMap<K, V> subMap(

        return nullableSynchronizedEntry(delegate().pollFirstEntry(), mutex);
      }
    }

    @Override
    public Map.@Nullable Entry<K, V> pollLastEntry() {
      synchronized (mutex) {
        return nullableSynchronizedEntry(delegate().pollLastEntry(), mutex);
      }
    }

    @Override
    public NavigableMap<K, V> subMap(

    assertThat(map.descendingKeySet()).containsExactly("two", "three", "one").inOrder();

    assertEquals(mapEntry("one", 3), map.pollFirstEntry());
    assertEquals(mapEntry("two", 3), map.pollLastEntry());
    assertEquals(1, map.size());
  }

  @GwtIncompatible // NavigableMap
  public void testAsMapNavigableReadsThrough() {
    NavigableSet<String> strings = Sets.newTreeSet();

   * @deprecated Unsupported operation.
   */
  @CanIgnoreReturnValue
  @Deprecated
  @Override
  @DoNotCall("Always throws UnsupportedOperationException")
  public final @Nullable Entry<K, V> pollLastEntry() {
    throw new UnsupportedOperationException();
  }

  @Override
  public ImmutableSortedMap<K, V> descendingMap() {
    // TODO(kevinb): The descendingMap is never actually cached at all. Either:

   * @deprecated Unsupported operation.
   */
  @CanIgnoreReturnValue
  @Deprecated
  @Override
  @DoNotCall("Always throws UnsupportedOperationException")
  public final @Nullable Entry<K, V> pollLastEntry() {
    throw new UnsupportedOperationException();
  }

  @Override
  public ImmutableSortedMap<K, V> descendingMap() {
    // TODO(kevinb): The descendingMap is never actually cached at all. Either: