Question

This code

static void writeTo(List<? super Apple> apples) {
        apples.add(new Apple());
        apples.add(new Jonathan());
    }

The author of this code stated that

The argument apples is a List of some type that is the base type of Apple; thus you know that it is safe to add an Apple or a subtype of Apple. Since the lower bound is Apple,

Jonathan is a subclass of Apple.

But when I tried this

    List<Jonathan> loj = new ArrayList<Jonathan>();
    listSuper(loj);

It gave me this error

The method listSuper(List<? super Apple>) in the type Basket<T> is not applicable for the arguments (List<Jonathan>)

Where listSuper looks like this

static void listSuper (List<? super Apple> cont) {}

How does the two differ?

Also what confuses me on the first code that I posted is that
I thought ? super T means that any base type of T. but from the looks of it he added a subtype of T. I am confused.

Answer 1

List<? super Apple> means a List you can add an Apple to (and since Jonathan is an Apple, you can put Jonathans into a List of that type as well).

It can be List<Apple>, List<Fruit> or List<Object>, but not List<Jonathan>, since you cannot put arbitrary Apples into List<Jonathan>. As you can see, in this case ? can be an Apple or any of its superclasses.

List<? extends Apple> means a List you can get an Apple from. It can be List<Apple> or List<Jonathan>, but not List<Fruit>, since List<Fruit> is not guaranteed to contain only Apples.

This explanation is known as “producer – extends, consumer – super” rule: if parameter acts as a consumer of elements, it should be declared with super, and vice versa.