If you're using NSGA2, your custom fitness class must be a subclass of NSGA2MultiObjectiveFitness. Furthermore, NSGA2 has its own way of handling fitness comparisons, so overriding betterThan will deviate from how NSGA2 works: you need to make sure you know what you're doing. The only reason you'd subclass directly from MultiObjectiveFitness is if you're implementing your own multiobjective optimization algorithm. Sean > On Dec 9, 2018, at 4:35 PM, Adrián Romero Cáceres <[log in to unmask]> wrote: > > Hello! > > I have implemented a new betterThan operator into a CustomFitness class, which is an extended class of MultiObjectiveFitness. I wrote pop.subpop.0.species.fitness = ec.multiobjective.CustomFitness inside my experiment param file. Once the execution starts, after the first generation fails, returning the next error: > > Exception in thread "main" java.lang.ClassCastException: ec.multiobjective.CustomFitness cannot be cast to ec.multiobjective.nsga2.NSGA2MultiObjectiveFitness > at ec.multiobjective.nsga2.NSGA2Breeder.assignFrontRanks(NSGA2Breeder.java:185) > at ec.multiobjective.nsga2.NSGA2Breeder.buildArchive(NSGA2Breeder.java:121) > at ec.multiobjective.nsga2.NSGA2Breeder.loadElites(NSGA2Breeder.java:82) > at ec.simple.SimpleBreeder.breedPopulation(SimpleBreeder.java:225) > at ec.multiobjective.nsga2.NSGA2Breeder.breedPopulation(NSGA2Breeder.java:113) > at ec.simple.SimpleEvolutionState.evolve(SimpleEvolutionState.java:104) > at ec.EvolutionState.run(EvolutionState.java:482) > at ec.Evolve.main(Evolve.java:771) > > I forgot something? Is it the right way to extend the betterThan operator? > > Thanks in advance, > Adrián.