confusing output

So, the good new is the code works exactly as intended. Here is what you are noticing:

Set<Object> pets = join(cats, dogs);
printPets(pets); //<-- this prints the full list of 7 Cat/Dog Objects

removeCats(pets, cats); //< -- this removes the 4 Cat Objects
printPets(pets);  //<-- This prints the remaining 3 Dog Objects

If you look at your output, the first 7 are a mix of Cat and Dog objects from the first printPets() method call, then the last 3 are all Dog() objects from the second printPets() method. So you are seeing the result of both the printPets() methods being called. The computer works so fast that it happens instantly and looks like one output. You could add this into the code between those lines to separate the final output:

// first printPets()
System.out.println();
// second printPets()

You won't get in trouble for posting the solution, they may delete your comment at the worst, however you can modify your comment to only show the main method if you want to make it more acceptable. I personally don't like seeing the solution to tasks posted publicly, it doesn't help people learn programming by just copying other people's solution; but with that being said, honestly, the people that just copy other people's solutions are going to do so no matter what. It really matters not either way.

Aah I see. Now I feel stupid for asking ;) Thanks again a lot Guadalupe! You are really helping me to understand! :)

Sometimes when I stare on the code for too long, some lines get invisible :D

Don't feel that you are any worse than anyone else first learning, we all go through the learning curve. I know all about not seeing the very obvious. Even now, I can sit at my code for over an hour trying to find what I think is a huge problem in my code. When I actually find the issue it ends up being something small like I was off by one (a very common error). There is a saying in computer programming: "The 3 most common errors are 1) naming errors; and 2) off by one errors".

That one made me laugh :D That makes me feel better ;) I'll just keep going and going. CodeGym definitely helps me to train my frustration tolerance, which is a good thing i guess. Thanks again !!! :)