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

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

    private static final long serialVersionUID = 0;
  }

  @AndroidIncompatible // test-suite builders
  public static TestSuite suite() {

    return navigableKeySet();
  }

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

  @Override
  public K lastKey() {
    return delegate.lastKey();
  }

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

  @Override
  public @Nullable K lowerKey(K key) {

      return standardFirstKey();
    }

    @Override
    public SortedMap<K, V> headMap(K toKey) {
      return standardHeadMap(toKey);
    }

    @Override
    public K lastKey() {
      return standardLastKey();
    }

    @Override
    public SortedMap<K, V> tailMap(K fromKey) {
      return standardTailMap(fromKey);
    }
  }

    return navigableKeySet();
  }

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

  @Override
  public K lastKey() {
    return delegate.lastKey();
  }

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

  @Override
  public @Nullable K lowerKey(K key) {

    @Override
    @ParametricNullness
    public K firstKey() {
      return sortedMap().firstKey();
    }

    @Override
    @ParametricNullness
    public K lastKey() {
      return sortedMap().lastKey();
    }

    @Override
    public SortedMap<K, Collection<V>> headMap(@ParametricNullness K toKey) {
      return new SortedAsMap(sortedMap().headMap(toKey));
    }

      return transformEntries(fromMap().headMap(toKey), transformer);
    }

    @Override
    @ParametricNullness
    public K lastKey() {
      return fromMap().lastKey();
    }

    @Override
    public SortedMap<K, V2> subMap(@ParametricNullness K fromKey, @ParametricNullness K toKey) {

      return transformEntries(fromMap().headMap(toKey), transformer);
    }

    @Override
    @ParametricNullness
    public K lastKey() {
      return fromMap().lastKey();
    }

    @Override
    public SortedMap<K, V2> subMap(@ParametricNullness K fromKey, @ParametricNullness K toKey) {

    return comparator;
  }

  @Override
  public @Nullable K firstKey() {
    return sortedDelegate.firstKey();
  }

  @Override
  public @Nullable K lastKey() {
    return sortedDelegate.lastKey();
  }

  @Nullable K higher(K k) {
    Iterator<K> iterator = keySet().tailSet(k).iterator();
    while (iterator.hasNext()) {
      K tmp = iterator.next();

    assertThat(sortedTable.rowMap().firstKey()).isEqualTo("bar");
  }

  public void testRowMapLastKey() {
    sortedTable = create("foo", 1, 'a', "bar", 1, 'b', "foo", 3, 'c');
    assertThat(sortedTable.rowMap().lastKey()).isEqualTo("foo");
  }

  public void testRowKeyMapHeadMap() {
    sortedTable = create("foo", 1, 'a', "bar", 1, 'b', "foo", 3, 'c');
    Map<String, Map<Integer, Character>> map = sortedTable.rowMap().headMap("cat");

    assertThrows(NoSuchElementException.class, () -> assertNull(navigableMap.lastKey()));
  }

  @CollectionSize.Require(ONE)
  public void testSingletonMapFirst() {
    assertEquals(a.getKey(), navigableMap.firstKey());
  }

  @CollectionSize.Require(ONE)
  public void testSingletonMapLast() {
    assertEquals(a.getKey(), navigableMap.lastKey());
  }

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