Problem in generics

[Paras Jain]

Hi,
I am not able to understand why I am not able to add instance of Top class to an arrayslist declared as List <? super Middle>

Please see the following code

    
import java.util.ArrayList;
import java.util.List;
class Top{}
class Middle extends Top{}
class test {
    public static void main( String args[] ) {
        List <? super Middle> list = new ArrayList <Middle> ();
        list.add(new Middle());
        list.add(new Top()); //Compiler failure
    }
}

Thanks
Paras

[Naveen Zed]

In a type safe Middle collection you cannot add a Top.Thats all.

[Paras Jain]

But I have declard the Collection type as
List <? super Middle>

not

List <Middle>

[Alina Petra]

If you have a collection declared as List<? super X> where X is a certain class, you are allowed to add to the collection only objects that pass IS-A X relationship.

[Jae Stryker]

The problem is as the other guys said - You've declared an ArrayList of Middles and are trying to add a object type of Top to it, which does fit the IS-A structure i.e. Top is not a Middle - though Middle is a Top so if you were to change your code to

List <? super Top> list = new ArrayList <Top> ();

it would compile.

[prashanth kumar]

Originally posted by Jae Stryker:
The problem is as the other guys said - You've declared an ArrayList of Middles and are trying to add a object type of Top to it, which does fit the IS-A structure i.e. Top is not a Middle - though Middle is a Top so if you were to change your code to

List <? super Top> list = new ArrayList <Top> ();

it would compile.

This question often causes me confusion.
Here is an extract that i found out in a website.

   
private final Collection<? extends T> values;
 
The ? is known as the wild card and ? extends T is known as a bounded wild card (with an upper bound) and it means - any class that implements T (if T were an interface) or extends T (if T were a class). Therefore, this collection can hold any object of type T or any subtype of T (either class or interface).
 
private final Comparable<? super T> value;.
 
This ? super T is known as a bounded wild card with lower bound. and it means that value could be any instance of T or any of its supertype. 
 
 

So even though "? super T" is a lower bound,it depends on final instatntiation of reference
Ex:

List <? super Top> list = new ArrayList <Top> ();

Even though the above code says,List can take any objects of Top anmd Super types,you can only add objects of Top and Subtypes..

Am i right?
Can anyone explain it bit more

Many Thanks