}

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

  @Override
  public ImmutableSet<C> columnKeySet() {

  private ArrayTable(Iterable<? extends R> rowKeys, Iterable<? extends C> columnKeys) {
    this.rowList = ImmutableList.copyOf(rowKeys);
    this.columnList = ImmutableList.copyOf(columnKeys);
    checkArgument(rowList.isEmpty() == columnList.isEmpty());

    /*
     * TODO(jlevy): Support only one of rowKey / columnKey being empty? If we
     * do, when columnKeys is empty but rowKeys isn't, rowKeyList() can contain

                    List<Integer> columnKeys = Lists.newArrayList();
                    for (Object element : elements) {
                      @SuppressWarnings("unchecked")
                      Cell<String, Integer, Character> cell =
                          (Cell<String, Integer, Character>) element;
                      columnKeys.add(cell.getColumnKey());
                    }

                    List<Integer> columnKeys = Lists.newArrayList();
                    for (Object element : elements) {
                      @SuppressWarnings("unchecked")
                      Cell<String, Integer, Character> cell =
                          (Cell<String, Integer, Character>) element;
                      columnKeys.add(cell.getColumnKey());
                    }

        @ParametricNullness C columnKey,
        @ParametricNullness V value) {
      this.rowKey = rowKey;
      this.columnKey = columnKey;
      this.value = value;
    }

    @Override
    @ParametricNullness
    public R getRowKey() {
      return rowKey;
    }

    @Override
    @ParametricNullness
    public C getColumnKey() {
      return columnKey;
    }

    @Override

      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));
      V existingValue = values[rowIndex][columnIndex];
      checkNoDuplicate(rowKey, columnKey, existingValue, cell.getValue());

      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));
      V existingValue = values[rowIndex][columnIndex];
      checkNoDuplicate(rowKey, columnKey, existingValue, cell.getValue());

      return (Table<R, C, V>) super.delegate();
    }

    @Override
    public boolean contains(@CheckForNull Object rowKey, @CheckForNull Object columnKey) {
      synchronized (mutex) {
        return delegate().contains(rowKey, columnKey);
      }
    }

    @Override
    public boolean containsRow(@CheckForNull Object rowKey) {
      synchronized (mutex) {

      return (Table<R, C, V>) super.delegate();
    }

    @Override
    public boolean contains(@CheckForNull Object rowKey, @CheckForNull Object columnKey) {
      synchronized (mutex) {
        return delegate().contains(rowKey, columnKey);
      }
    }

    @Override
    public boolean containsRow(@CheckForNull Object rowKey) {
      synchronized (mutex) {

   * referencing the type variables seem worthwhile.
   */
  final void checkNoDuplicate(R rowKey, C columnKey, @CheckForNull V existingValue, V newValue) {
    checkArgument(
        existingValue == null,
        "Duplicate key: (row=%s, column=%s), values: [%s, %s].",
        rowKey,
        columnKey,
        newValue,
        existingValue);
  }

  // redeclare to satisfy our test for b/310253115