public void testDescendingMap() {
    NavigableMap<String, Integer> map = create();
    NavigableMap<String, Integer> descendingMap = map.descendingMap();
    assertTrue(descendingMap instanceof SynchronizedNavigableMap);
    assertThat(((SynchronizedNavigableMap<String, Integer>) descendingMap).mutex)
        .isSameInstanceAs(mutex);
  }

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

   * descendingMap}, {@code subMap}, {@code headMap} or {@code tailMap} views.
   *
   * {@snippet :
   * 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.
   *
   * {@snippet :
   * 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!