code: public AttributeVector response() {
          double[] y = new double[data.size()];
          for (int i = 0; i < y.length; i++) {
              y[i] = data.get(i).y;
          }
          return new AttributeVector(response, y);
      }
docs: Returns the response attribute vector. null means no response
  variable in this dataset.
  @return the response attribute vector. null means no response
  variable in this dataset.