code: public static DoubleVector addUnmodified(DoubleVector vector1,
                                               DoubleVector vector2) {
          if (vector2.length() != vector1.length())
              throw new IllegalArgumentException(
                      "Vectors of different sizes cannot be added");
          DoubleVector finalVector = Vectors.copyOf(vector1);
          // If vector is a sparse vector, simply get the non zero values and
          // add them to this instance.
          if (vector2 instanceof SparseVector)
              addSparseValues(finalVector, vector2);
          else {
              // Otherwise, inspect all values of vector, and only add the non
              // zero values.
              int length = vector2.length();
              for (int i = 0; i < length; ++i) {
                  double value = vector2.get(i);
                  finalVector.add(i, value);
              }
          }
          return finalVector;
      }
docs: Returns a new {@code DoubleVector} which is the summation of {@code
  vector2} and {@code vector1}.
  
  @param vector1 The first vector to used in a summation.
  @param vector2 The second vector to be used in a summation.
  @return The summation of {code vector1} and {@code vector2}.