code: public Collection<Cell> getCells() {
          List<Cell> res = new ArrayList<>();
  
          for (Map.Entry<String, List<Cell>> entry : cells.entrySet()) {
              res.addAll(entry.getValue());
          }
  
          return Collections.unmodifiableList(res);
      }
docs: Returns an immutable collection of all the Cell objects contained in this Cells. Beware that internally each list
  of cells is associated to the table owning those cells, this method flattens the lists of cells known to this
  object to just one list, thus losing the table information.
  
  @return the request list of Cell objects.