Generic collection & wildcard in java

Question

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

public void exampleMethod() {
    //         


        

Answer 1

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

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