can help me ?!

package com.codegym.task.task08.task0820;

import java.util.*;

/*
Animal set

*/

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

    Set pets = join(cats, dogs);
    printPets(pets);

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

    public static Set createCats() {
        Cat cat1 = new Cat();
        Cat cat2 = new Cat();
        Cat cat3 = new Cat();
        Cat cat4= new Cat();
        Set set = new HashSet(Arrays.asList(cat1,cat2,cat3,cat4));
        return set;

        //write your code here


    }

    public static Set createDogs() {
        //write your code here

        Dog dog1 = new Dog();
        Dog dog2 = new Dog();
        Dog dog3 = new Dog();
        Set set = new HashSet(Arrays.asList(dog1,dog2,dog3));
        return set;
    }

    public static Set join(Set cats, Set dogs) {
        //write your code here
        HashSet result = new HashSet();
        Collections.addAll(result,cats);
        Collections.addAll(result,dogs);
        return result;
    }

    public static void removeCats(Set pets, Set cats) {
        //write your code here
        ArrayList list = new ArrayList(4);
        list.addAll(cats);
        Iterator itr = pets.iterator();
        while (itr.hasNext())
        {
            Object obj = itr.next();
            for (Object listObj : list)
            {
                if (obj.equals(listObj))
                {
                    itr.remove();
                }
            }
        }
    }

    public static void printPets(Set pets) {
        //write your code here
        pets.forEach(System.out::println);
    }

//write your code here
public static class Cat{
    public Cat(){}
}
public  static class Dog {
    public Dog (){}
}
}