Не проходит валидацию

public class Solution {
    public static void main(String[] args) {
        Set<Cat> cats = createCats();
        Set<Dog> dogs = createDogs();

        Set<Object> pets = join(cats, dogs);
        printPets(pets);

        removeCats(pets, cats);
        printPets(pets);
    }

    public static Set<Cat> createCats() {
        HashSet<Cat> result = new HashSet<Cat>();

        result.add(new Cat());
        result.add(new Cat());
        result.add(new Cat());
        result.add(new Cat());

        return result;
    }

    public static Set<Dog> createDogs() {
        HashSet<Dog> dogs = new HashSet<>();
        dogs.add(new Dog());
        dogs.add(new Dog());
        dogs.add(new Dog());

        return dogs;
    }

    public static Set<Object> join(Set<Cat> cats, Set<Dog> dogs) {
        HashSet<Object> catDogs = new HashSet<>();
        catDogs.addAll(cats);
        catDogs.addAll(dogs);
        return catDogs;
    }

    public static void removeCats(Set<Object> pets, Set<Cat> cats) {
        Iterator<Object> iterator = pets.iterator();
        while (iterator.hasNext()){
            if (iterator.next() instanceof Cat)iterator.remove();
        }


    }

    public static void printPets(Set<Object> pets) {
        for (Object c : pets){
            System.out.println(c);
        }
    }

   public static class Cat{}
   public static class Dog{}
}