Question

Possible Duplicate:
java generics super keyword

I am not able to relate my knowledge with the below sample program. Please see the below sample program then my doubts are below that program.

import java.util.*;
class A { }
class B extends A { }
class C extends B { }

public class sampleprog {

    public static void main(String[] args) {

        List<? super A> list1 = new ArrayList<A>();
        list1.add(new A());//valid. ok
        list1.add(new B());//valid, why? it is not super to A?
        list1.add(new C());//valid, why? it is not super to A?

        List<? super B> list2 = new ArrayList<A>();
        list2.add(new A());//not valid. why? it is also super to B!!!
        list2.add(new B());
        list2.add(new C());

        List<? super C> list3 = new ArrayList<C>();
        list3.add(new A());//not valid, why? It is super to A so should be valid!!!
        list3.add(new B());//not valid, why? It is super to A so should be valid!!!
        list3.add(new C());
    }
}

My Doubts:
– As far as I know ? super T means any class you can add that is super to T but here output is different? Even subclass also added successfully that is totally confusing.
– output is not different with list initialization (List<? super C> list3 = new ArrayList<C>();) . In this initialization, I assigned list of A or B, output was same!

Please clear my doubts.

Answer 1

“? super T” means any class you can add that must be an instance of T or its sub class.

So List<? super C> list3 = new ArrayList<C>(); in the list3 either you can add instance of class C or its sub class instances (SomeClass extends C).

A parent object list can have its own, child and child of child but a List that has created for a child class can not have its parent object. Hope you are clear now.