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

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

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

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

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

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

  public @Nullable K lowerKey(K key) {
    return keyOrNull(lowerEntry(key));
  }

  @Override
  public @Nullable Entry<K, V> floorEntry(K key) {
    return headMap(key, true).lastEntry();
  }

  @Override
  public @Nullable K floorKey(K key) {
    return keyOrNull(floorEntry(key));
  }

  @Override
  public @Nullable Entry<K, V> ceilingEntry(K key) {
    return tailMap(key, true).firstEntry();

    }
    {
      Entry<Integer, String> entry = unmod.lowerEntry(1);
      assertThat(entry).isNull();
    }
    {
      Entry<Integer, String> entry = unmod.floorEntry(2);
      assertThrows(UnsupportedOperationException.class, () -> entry.setValue("four"));
    }
    {
      Entry<Integer, String> entry = unmod.ceilingEntry(2);

    }
    {
      Entry<Integer, String> entry = unmod.lowerEntry(1);
      assertThat(entry).isNull();
    }
    {
      Entry<Integer, String> entry = unmod.floorEntry(2);
      assertThrows(UnsupportedOperationException.class, () -> entry.setValue("four"));
    }
    {
      Entry<Integer, String> entry = unmod.ceilingEntry(2);