code: public double[][] toDenseArray() {
          double[][] result = new double[vectors.size()][columns];
          int row = 0;
          for (DoubleVector vector : vectors) {
              for (int i = 0; i < vector.length(); ++i)
                  result[row][i] = vector.get(i);
              row++;
          }
          return result;
      }
docs: {@inheritDoc}