assertEquals(c, navigableMap.higherEntry(a.getKey()));
    assertEquals(c.getKey(), navigableMap.higherKey(a.getKey()));
    assertEquals(c, navigableMap.higherEntry(b.getKey()));
    assertEquals(c.getKey(), navigableMap.higherKey(b.getKey()));
    assertEquals(null, navigableMap.higherEntry(c.getKey()));
    assertEquals(null, navigableMap.higherKey(c.getKey()));
  }

  @CollectionSize.Require(SEVERAL)
  public void testLast() {

      assertTrue(Thread.holdsLock(mutex));
      return delegate().higherEntry(key);
    }

    @Override
    public @Nullable K higherKey(K key) {
      assertTrue(Thread.holdsLock(mutex));
      return delegate().higherKey(key);
    }

    @Override
    public @Nullable Entry<K, V> lastEntry() {
      assertTrue(Thread.holdsLock(mutex));
      return delegate().lastEntry();
    }

  }

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

  @Override
  public @Nullable K higherKey(K key) {
    return delegate.higherKey(checkValid(key));
  }

  @Override
  public boolean isEmpty() {
    return delegate.isEmpty();
  }

  @Override
  public NavigableSet<K> keySet() {

  }

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

  @Override
  public @Nullable K higherKey(K key) {
    return delegate.higherKey(checkValid(key));
  }

  @Override
  public boolean isEmpty() {
    return delegate.isEmpty();
  }

  @Override
  public NavigableSet<K> keySet() {

    public Entry<K, V2> higherEntry(@ParametricNullness K key) {
      return transformEntry(fromMap().higherEntry(key));
    }

    @Override
    @CheckForNull
    public K higherKey(@ParametricNullness K key) {
      return fromMap().higherKey(key);
    }

    @Override
    @CheckForNull
    public Entry<K, V2> lastEntry() {
      return transformEntry(fromMap().lastEntry());
    }

    @Override

    public Entry<K, V2> higherEntry(@ParametricNullness K key) {
      return transformEntry(fromMap().higherEntry(key));
    }

    @Override
    @CheckForNull
    public K higherKey(@ParametricNullness K key) {
      return fromMap().higherKey(key);
    }

    @Override
    @CheckForNull
    public Entry<K, V2> lastEntry() {
      return transformEntry(fromMap().lastEntry());
    }

    @Override

        return nullableSynchronizedEntry(delegate().higherEntry(key), mutex);
      }
    }

    @Override
    @CheckForNull
    public K higherKey(K key) {
      synchronized (mutex) {
        return delegate().higherKey(key);
      }
    }

    @Override
    @CheckForNull
    public Map.Entry<K, V> lastEntry() {
      synchronized (mutex) {

        return nullableSynchronizedEntry(delegate().higherEntry(key), mutex);
      }
    }

    @Override
    @CheckForNull
    public K higherKey(K key) {
      synchronized (mutex) {
        return delegate().higherKey(key);
      }
    }

    @Override
    @CheckForNull
    public Map.Entry<K, V> lastEntry() {
      synchronized (mutex) {

  }

  @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() {
    return isEmpty() ? null : entrySet().asList().get(0);
  }

  }

  @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() {
    return isEmpty() ? null : entrySet().asList().get(0);
  }