java - Why List<? super Cat> list can't add Animal?

Question

I am a novice, I met a problem, do not understand, as follows:

class Animal {}

class Cat extends Animal {}

class Garfield extends Cat {}

public class Demo {
    public static void main(String[] args) {

        List<? super Cat> list = new ArrayList<>(); // list can include cat and animal

        list.add(new Cat());      // perfect !!
        list.add(new Garfield()); // wait... why can ??
        list.add(new Animal());   // wait... why can't ?? 
    }
}

Thank you very much!

Answer 1

To a collection defined with ? super T as the type parameter, you can add objects of type T and it's descendants: ? super Cat means "Cat and it's superclasses, up to Object". In your case, List<? super Cat> may be a List<Cat>, and you can't add, for instance, Animals, to a collection of Cats, since Animal is-not-a Cat (the reverse is true).