return keyOrNull(ceilingEntry(key));
  }

  @Override
  public @Nullable Entry<K, V> higherEntry(K key) {
    return tailMap(key, false).firstEntry();
  }

  @Override
  public @Nullable K higherKey(K key) {
    return keyOrNull(higherEntry(key));
  }

  @Override
  public @Nullable Entry<K, V> firstEntry() {

    public SortedMap<K, V> headMap(K toKey) {
      return headMap(toKey, false);
    }

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

    @Override
    public @Nullable K higherKey(K key) {
      synchronized (mutex) {

    public SortedMap<K, V> headMap(K toKey) {
      return headMap(toKey, false);
    }

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

    @Override
    public @Nullable K higherKey(K key) {
      synchronized (mutex) {

    return keyOrNull(ceilingEntry(key));
  }

  @Override
  public @Nullable Entry<K, V> higherEntry(K key) {
    return tailMap(key, false).firstEntry();
  }

  @Override
  public @Nullable K higherKey(K key) {
    return keyOrNull(higherEntry(key));
  }

  @Override
  public @Nullable Entry<K, V> firstEntry() {

    assertThat(map.higherKey("r")).isEqualTo("three");
    assertThat(map.ceilingKey("r")).isEqualTo("three");
    assertThat(map.ceilingKey("one")).isEqualTo("one");
    assertEquals(mapEntry("three", 5), map.higherEntry("one"));
    assertEquals(mapEntry("one", 3), map.ceilingEntry("one"));
    assertThat(map.lowerKey("three")).isEqualTo("one");
    assertThat(map.lowerKey("r")).isEqualTo("one");