@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);

  @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);