Java using generics with lists and interfaces

Question

Ok, so here is my problem:

I have a list containing interfaces - List a - and a list of interfaces that extend that interface: Li

Answer 1

This works :

public class TestList {

    interface Record {}
    interface SubRecord extends Record {}

    public static void main(String[] args) {
        List l = new ArrayList();
        List l2 = new ArrayList();
        Record i = new Record(){};
        SubRecord j = new SubRecord(){};

        l = l2;
        Record a = l.get( 0 );
        ((List)l).add( i );       //<--will fail at run time,see below
        ((List)l).add( j );    //<--will be ok at run time

    }

}

I mean it compiles, but you will have to cast your List before adding anything inside. Java will allow casting if the type you want to cast to is a subclass of Record, but it can't guess which type it will be, you have to specify it.

A List can only contain Records (including subRecords), A List can only contain SubRecords.

But A List> is not a List has it cannot contains Records, and subclasses should always do what super classes can do. This is important as inheritance is specilisation, if List would be a subclass of List, it should be able to contain ` but it'S not.

A List and a List both are List. But in a List you can't add anything as java can't know which exact type the List is a container of. Imagine you could, then you could have the following statements :

List l = l2;
l.add( new Record() );

As we just saw, this is only possible for List not for any List such as List.

Regards, Stéphane