NavigableMap<String, Integer> map = create();
    NavigableMap<String, Integer> descendingMap = map.descendingMap();
    assertTrue(descendingMap instanceof SynchronizedNavigableMap);
    assertSame(mutex, ((SynchronizedNavigableMap<String, Integer>) descendingMap).mutex);
  }

  public void testFirstEntry() {
    create().firstEntry();
  }

  public void testFirstKey() {

                NavigableMap<String, String> delegate = populate(new TreeMap<>(), entries);
                return Collections.synchronizedNavigableMap(delegate);
              }
            })
        .named("synchronizedNavigableMap/TreeMap, natural")
        .withFeatures(
            MapFeature.GENERAL_PURPOSE,
            MapFeature.ALLOWS_NULL_VALUES,

    return new SynchronizedNavigableMap<>(navigableMap, mutex);
  }

  @GwtIncompatible // NavigableMap
  @VisibleForTesting
  static final class SynchronizedNavigableMap<
          K extends @Nullable Object, V extends @Nullable Object>
      extends SynchronizedSortedMap<K, V> implements NavigableMap<K, V> {

    SynchronizedNavigableMap(NavigableMap<K, V> delegate, @CheckForNull Object mutex) {

    return new SynchronizedNavigableMap<>(navigableMap, mutex);
  }

  @GwtIncompatible // NavigableMap
  @VisibleForTesting
  static final class SynchronizedNavigableMap<
          K extends @Nullable Object, V extends @Nullable Object>
      extends SynchronizedSortedMap<K, V> implements NavigableMap<K, V> {

    SynchronizedNavigableMap(NavigableMap<K, V> delegate, @CheckForNull Object mutex) {

   * descendingMap}, {@code subMap}, {@code headMap} or {@code tailMap} views.
   *
   * <pre>{@code
   * NavigableMap<K, V> map = synchronizedNavigableMap(new TreeMap<K, V>());
   *
   * // Needn't be in synchronized block
   * NavigableSet<K> set = map.navigableKeySet();
   *
   * synchronized (map) { // Synchronizing on map, not set!

   * descendingMap}, {@code subMap}, {@code headMap} or {@code tailMap} views.
   *
   * <pre>{@code
   * NavigableMap<K, V> map = synchronizedNavigableMap(new TreeMap<K, V>());
   *
   * // Needn't be in synchronized block
   * NavigableSet<K> set = map.navigableKeySet();
   *
   * synchronized (map) { // Synchronizing on map, not set!