Is <? extends ...> exclusivity of method parameters?

Question

Let's say I have two classes A and B which extends A. With the following method I can print a Collection of A or something extending A:

private static void print(Collection<? extends A> collection) {

    for (A element : collection) {
        System.out.println(element);
    }

}

It's great, I can do something like:

public static void main(String[] args) {

    List<A> l1 = new ArrayList<>();
    l1.add(new A());
    l1.add(new B());
    print(l1);

    List<B> l2 = new ArrayList<>();
    l2.add(new B());
    l2.add(new B());
    print(l2);

}

Now my question is why in my method main (or somewhere else) I can write this

List<? extends A> l3 = new ArrayList<>();

but not this

l3.add(new A());
l3.add(new B());

I do understand why I can not add an instance of A or B in l3. But why the first statement which seems useless is authorized ?

EDIT

I ask with another words because I think my question is misunderstood.

What can I do with my empty list List<? extends A> l3 = new ArrayList<>(); ? I know it's not possible to add anything because the type of the list is unknown.

Answer 1

List<? extends A> l3 means that l3 might be a List<A> or it might be a List<B>. <? extends A> means the compiler doesn't know and cannot safely assume any more than that.

Since it might be List<B>, l3.add(new A()) is not safe. The List would be corrupt, since it would not truly contain only instances of B.