code: private void fillEmptyMatrixCells() {
          try {
              for (int i=0; i < rows.size(); i++) {
                  for (int j=0; j < columns; j++) {
                      if (!((Row) rows.get(i)).isReserved(j)) {
                          addCell(defaultCell, new Point(i, j));
                      }
                  }
              }
          }
          catch(BadElementException bee) {
              throw new ExceptionConverter(bee);
          }
      }
docs: adds new<CODE>Cell</CODE>'s to empty/null spaces.