Enthuware

Posted: **Wed Dec 11, 2019 11:38 am**

Please, can you elaborate again these two statements with more explanations?

Hierarchy 2 : A<T> <<< A<? super T> <<< A<? super S> Example: List<Number> is a subtype of List<? super Number> and List<? super Number> is a subtype of A<? super Integer> Thus, if an overridden method returns List<? super Number>, the overriding method can return List<Number> but not List<Integer> or List<? super Integer>.

It is important to understand that List<Integer> is not a subtype of List<Number> even though Integer is a subtype of Number.

I can understand easily

Hierarchy 1 : A<S> <<< A<? extends S> <<< A<? extends T> Example: Since Integer is a subtype of Number, List<Integer> is a subtype of List<? extends Integer> and List<? extends Integer> is a subtype of A<? extends Number>. Thus, if an overridden method returns List<? extends Integer>, the overriding method can return List<Integer> but not List<Number> or List<? extends Number>.

But I can't understand why:
1 - "List<? super Number> is a subtype of A<? super Integer>" instead of super type
2 - List<Integer> is not a subtype of List<Number> even though Integer is a subtype of Number

Posted: **Wed Dec 11, 2019 9:36 pm**

Please go through Java Generics FAQs - Frequently Asked Questions by Angelika Langer. It will solve all your doubts about generics. Remember that it is a bit advanced for OCA/OCP Part 1 but you will need to learn it anyway later.

Posted: **Wed Apr 14, 2021 6:01 pm**

This is also true
public ArrayList<? extends Number> getList(){

class A {
public List<Number> getList() {
return null;
}

}

class B extends A {
@Override
public ArrayList<? extends Number> getList() {
return new ArrayList<Integer>();
}
}

Enthuware

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