code: private void advance() throws IOException {        
          if (entry >= nzEntriesInMatrix) {
              next = null;
          }
          else {
              // Reads off a new vector from the data file.
              next = new SparseHashDoubleVector(rows);
              int nzInCol = data.getInt();
              for (int i = 0; i < nzInCol; ++i, ++entry) {
                  int row = data.getInt();
                  double value = data.getFloat();
                  next.set(row, value);          
              }
              curCol++;
          }
      }
docs: Sets {@code next} to be the next row in the data matrix.