code: private Object writeGrid(SerIterator itemIterator) {
          List<Object> result = new ArrayList<>();
          result.add(itemIterator.dimensionSize(0));
          result.add(itemIterator.dimensionSize(1));
          while (itemIterator.hasNext()) {
              itemIterator.next();
              Integer outputKey = (Integer) itemIterator.key();
              Integer outputCol = (Integer) itemIterator.column();
              Object outputValue = writeObject(itemIterator.valueType(), itemIterator.value(), itemIterator);
              result.add(Arrays.asList(outputKey, outputCol, outputValue));
          }
          return result;
      }
docs: write grid using sparse approach