下界通配符方法

Question

Could anyone please let me understand why this piece of code is getting compiler error:

List<? super A> superList = new ArrayList<>();
superList.add(new Object());

While this one is perfectly correct:

setSuper(new ArrayList<Object>());

public void superMethod(List<? super A> list) {
    //...
}

I understand that lower bound narrows available types to A or their descendants, but why this is fine for compiler when it comes to methods arguments?

Answer 1

An ArrayList<Object> is a valid value of type List<? super A> List<? super A> .

An ArrayList<A> is also a valid value of type List<? super A> List<? super A> .

But you can't add new Object() to an ArrayList<A> . So not all List<? super A> List<? super A> values can add an object, so the compiler correctly errors.