code: public void buildNewReferenceSet1() {
      DoubleSolution individual;
      strengthRawFitness.computeDensityEstimator(getPopulation());
      Collections.sort(getPopulation(), fitnessComparator);
  
      for (int i = 0; i < referenceSet1Size; i++) {
        individual = getPopulation().get(0);
        getPopulation().remove(0);
        marked.setAttribute(individual, false);
        referenceSet1.add(individual);
      }
    }
docs: Build the referenceSet1 by moving the best referenceSet1Size individuals, according to
  a fitness comparator, from the population to the referenceSet1