code: @NonNull
      Collection<TObj> getAll() {
          Collection<TObj> result = new LinkedList<>();
          for (int countR = mData.size(), i = 0; i < countR; i++) {
              int rowKey = mData.keyAt(i);
              SparseArrayCompat<TObj> columns = mData.get(rowKey);
              for (int countC = columns.size(), j = 0; j < countC; j++) {
                  int key = columns.keyAt(j);
                  result.add(columns.get(key));
              }
          }
          return result;
      }
docs: Get all matrix's items
  
  @return Collection with column's Objects