Java Generic with ArrayList <? extends A> add element

Question

I have classes A, B, C and D where B extends A, C extends A and

Answer 1

Did I use the correctly?

List list; means that the 'list' refers to an object implemented interface List, and the list can hold elements inherited from class A (including A). In other words the following statements are correct:

List listA1 = new ArrayList();
List listB1 = new ArrayList();
List listC1 = new ArrayList();
List listD1 = new ArrayList();

Java generics are a compile-time strong type checking. The compiler uses generics to make sure that your code doesn't add the wrong objects into a collection.

You tried to add C in ArrayList

listB1.add(new C());

If it were allowed the object ArrayList would contain different object types (B,C).
That is why the reference listB1 works in 'read-only' mode. Additionally you could take a look at this answer.

how to resolve this?

List list = new ArrayList();