rowEntry = null;
      }
    }
  }

  @Override
  public Map<C, V> row(R rowKey) {
    return new Row(rowKey);
  }

  class Row extends IteratorBasedAbstractMap<C, V> {
    final R rowKey;

    Row(R rowKey) {
      this.rowKey = checkNotNull(rowKey);
    }

    @CheckForNull Map<C, V> backingRowMap;

    final void updateBackingRowMapField() {

  @Override
  public boolean contains(@CheckForNull Object rowKey, @CheckForNull Object columnKey) {
    Map<C, V> row = Maps.safeGet(rowMap(), rowKey);
    return row != null && Maps.safeContainsKey(row, columnKey);
  }

  @Override
  @CheckForNull
  public V get(@CheckForNull Object rowKey, @CheckForNull Object columnKey) {
    Map<C, V> row = Maps.safeGet(rowMap(), rowKey);
    return (row == null) ? null : Maps.safeGet(row, columnKey);

  /**
   * {@inheritDoc}
   *
   * @throws NullPointerException if {@code rowKey} is {@code null}
   */
  @Override
  public ImmutableMap<C, V> row(R rowKey) {
    checkNotNull(rowKey, "rowKey");
    return MoreObjects.firstNonNull(
        (ImmutableMap<C, V>) rowMap().get(rowKey), ImmutableMap.<C, V>of());
  }

  @Override
  public ImmutableSet<R> rowKeySet() {

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

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

  @Override
  public SortedMap<K, V> subMap(@ParametricNullness K fromKey, @ParametricNullness K toKey) {
    return delegate().subMap(fromKey, toKey);
  }

  @Override

  }

  public ImmutableSortedMap<K, V> headMap(K toKey) {
    checkNotNull(toKey);
    return newView(sortedDelegate.headMap(toKey));
  }

  ImmutableSortedMap<K, V> headMap(K toKey, boolean inclusive) {
    checkNotNull(toKey);
    if (inclusive) {
      K tmp = higher(toKey);
      if (tmp == null) {
        return this;
      }
      toKey = tmp;
    }
    return headMap(toKey);
  }

    }

    @Override
    public NavigableMap<K, Collection<V>> headMap(@ParametricNullness K toKey) {
      return headMap(toKey, false);
    }

    @Override
    public NavigableMap<K, Collection<V>> headMap(@ParametricNullness K toKey, boolean inclusive) {
      return new NavigableAsMap(sortedMap().headMap(toKey, inclusive));
    }

    @Override

 *
 * <p>Lookups by row key are often faster than lookups by column key, because the data is stored in
 * a {@code Map<R, Map<C, V>>}. A method call like {@code column(columnKey).get(rowKey)} still runs
 * quickly, since the row key is provided. However, {@code column(columnKey).size()} takes longer,
 * since an iteration across all row keys occurs.
 *

    for (int i = 0; i < cellList.size(); i++) {
      Cell<R, C, V> cell = cellList.get(i);
      R rowKey = cell.getRowKey();
      C columnKey = cell.getColumnKey();
      // The requireNonNull calls are safe because we construct the indexes with indexMap.
      int rowIndex = requireNonNull(rowKeyToIndex.get(rowKey));
      int columnIndex = requireNonNull(columnKeyToIndex.get(columnKey));

    return delegate.get(checkValid(key));
  }

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

  @Override
  public NavigableMap<K, V> headMap(K toKey, boolean inclusive) {
    return new SafeTreeMap<>(delegate.headMap(checkValid(toKey), inclusive));
  }

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

    return delegate.get(checkValid(key));
  }

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

  @Override
  public NavigableMap<K, V> headMap(K toKey, boolean inclusive) {
    return new SafeTreeMap<>(delegate.headMap(checkValid(toKey), inclusive));
  }

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