return keyOrNull(ceilingEntry(key));
  }

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

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

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

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

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

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

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

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

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

    return keyOrNull(ceilingEntry(key));
  }

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

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

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