code: public List<PermutationSolution<Integer>> execute(List<PermutationSolution<Integer>> parents) {
      if (null == parents) {
        throw new JMetalException("Null parameter") ;
      } else if (parents.size() != 2) {
        throw new JMetalException("There must be two parents instead of " + parents.size()) ;
      }
  
      return doCrossover(crossoverProbability, parents) ;
    }
docs: Executes the operation
  
  @param parents An object containing an array of two solutions