void updateWholeRowField() {
      if (wholeRow == null || (wholeRow.isEmpty() && backingMap.containsKey(rowKey))) {
        wholeRow = (SortedMap<C, V>) backingMap.get(rowKey);
      }
    }

    @Override
    @CheckForNull
    SortedMap<C, V> computeBackingRowMap() {
      updateWholeRowField();
      SortedMap<C, V> map = wholeRow;
      if (map != null) {