Question

I am having problems getting my head around generics in the following situation, see inline comments below for my questions:

public void exampleMethod() {
    //Intuitively I would expect this to mean that test is set containing objects 
    //that subclass AbstractGroup
    Set<? extends AbstractGroup> test;

    //Yet the compiler complains here and I do not understand why?

    test.add(new AnyAbstractGroupSubGroup());

    //I would guess that a method call such as this at runtime

    test = new HashSet<SubGroupA>()

    //would mean that only objects of subgroupA can be added to the collection, but then
    //what is the point in using the wildcard in the first place?  
}

Answer 1

//Intuitively I would expect this to mean that test is set containing objects 
//that subclass AbstractGroup
Set<? extends AbstractGroup> test;

Nope, it means that it’s a set of one specific ? which extends AbstractGroup. And neither you nor the Compiler have any way of knowing what that ? is, so there’s no way you can add anything to that Set.

You can assign the set’s values to variables of type AbstractGroup, but not the other way around.

Instead, you need this:

Set<? super AbstractGroup> test;

This principle is sometimes called PECS and explained well in this answer.