@Override
  Cell<R, C, V> getCell(int index) {
    int rowIndex = cellRowIndices[index];
    int columnIndex = cellColumnIndices[index];
    R rowKey = rowKeySet().asList().get(rowIndex);
    C columnKey = columnKeySet().asList().get(columnIndex);
    // requireNonNull is safe because we use indexes that were populated by the constructor.
    V value = requireNonNull(values[rowIndex][columnIndex]);
    return cellOf(rowKey, columnKey, value);

  /**
   * Returns a set of column keys that have one or more values in the table. Changes to the set will
   * update the underlying table, and vice versa.
   *
   * @return set of column keys
   */
  Set<C> columnKeySet();

  /**
   * Returns a collection of all values, which may contain duplicates. Changes to the returned
   * collection will update the underlying table, and vice versa.
   *
   * @return collection of values

  /**
   * Returns a set of column keys that have one or more values in the table. Changes to the set will
   * update the underlying table, and vice versa.
   *
   * @return set of column keys
   */
  Set<C> columnKeySet();

  /**
   * Returns a collection of all values, which may contain duplicates. Changes to the returned
   * collection will update the underlying table, and vice versa.
   *
   * @return collection of values

      synchronized (mutex) {
        return set(delegate().rowKeySet(), mutex);
      }
    }

    @Override
    public Set<C> columnKeySet() {
      synchronized (mutex) {
        return set(delegate().columnKeySet(), mutex);
      }
    }

    @Override
    public Collection<V> values() {
      synchronized (mutex) {

      synchronized (mutex) {
        return set(delegate().rowKeySet(), mutex);
      }
    }

    @Override
    public Set<C> columnKeySet() {
      synchronized (mutex) {
        return set(delegate().columnKeySet(), mutex);
      }
    }

    @Override
    public Collection<V> values() {
      synchronized (mutex) {