difference between extends and super

[rahul.ch]

List<? extends Integer> list = new ArrayList<Integer>();
            for (Integer element : list) { 
                System.out.println(element);

List<? super Integer> list = new ArrayList<Integer>();
            for (Integer element : list) { 
                System.out.println(element); 

For the first three lines as Case 1 using extends it compiles fine. But for Case 2 of next three lines, it says incompatible types. Found Object, required Integer. My confusion is ? extends Integer being anything that is an Integer or sub of it works.

But why in Case 2 where ? super Integer means anything that is an Integer or a superclass of Integer doesn't work when it's elements ar being given to an Integer element in the for loop??

[jalpesh_007]

It depends which way on the inheritance hierarchy it allows. Assume you a class "Child" which inherits from "Parent" which inherits from "Grandparent".

<T extends Parent> would accept either Parent or Child while <T super Parent> would accept either Parent or Grandparent.

Also go through:
http://objectmentor.com/resources/articles/The_Craftsman_44__Brown_Bag_I.pdf

[JamesCherrill]

Case 1 every element is an Integer or a subclass of Integer. All subclasses of Integer are Integers (can be cast to Integer), so the implicit cast to Integer is OK
Case 2 every element is an Integer or a superclass of Integer. Not all superclasses of Integer are Integers (eg Object is not a kind of Integer), so the implicit cast is not valid.

[rahul.ch]

Ok so in general the first line of both case which is instantiation says what compatible elements can be added into the list? if that's the case why is this code below wrong:

NavigableSet<? super String> set = new TreeSet<String>();
set.add(new Object());

Compiler says cannot find symbol: method add(java.lang.Object)
Isn't <? super String> should allow addition of Object since it is a superclass of String?

[rahul.ch]

Likewise for

NavigableSet<?> set = new TreeSet<Object>();
            set.add(new Object());

Saying same error as one on previous post.

[rahul.ch]

Why I asked that question was because I noticed with super wild card, that whatever is on the RHS(right hand side) of super keyword is only accepted or it's sub class.

For e.g.

NavigableSet<? super Object> set = new TreeSet<Object>();
            set.add(new Object()); 
            set.add(new String("Hello"));

is accepting both Object and String following my assumption of anything on RHS of super or it's subclass.

So going back to my very first post,

List<? super Integer> list = new ArrayList<Integer>();
            for (Integer element : list) { 
                System.out.println(element); 

this should accept and Integer object and hence in the for loop when list varible is assigned to another Integer wrraper itself, why complains of incompatible types ?? :( :(

[JamesCherrill]

Because it will also accept objects from Integer's superclass(es), and they cannot be assigned to an Integer variable

[rahul.ch]

Thanks again james!