List and List are incompatible types in java [duplicate]

Answer 1

    List<? extends List> a = new ArrayList();
    List<? extends List<?>> b = new ArrayList();
    a = b;

works

Set<Class<? extends GenericTypeHere<?>>>

would imply Set<Class<YourClass extends AnotherClass<YourClass>>> as mentioned in the above wildcard answer

What stops you from using

Set<Class<YourClass extends AnotherClass>> to propagate throughout your app?

Answer 2

Okay, so this is due to a subtle semantic difference.

List

This is the raw type of List, which equates to T being of type Object. So it's the same as saying:

List<Object>

Now, the Compiler knows for a fact, that whatever happens, this is a subclass of type Object. And if you do..

List myList = new ArrayList();
myList.add(new Object());

It will work fine! This is because Object is the same or it is some derivation of the type.

List<?>

This is literally a list of unknown (Java Docs). We don't even know that the subclass of the things in here are of type Object. In fact, the ? type is an unknown type all on its own. It has nothing to do with Object! This is why when you try and do..

List<?> myList = new ArrayList<?>();
myList.add(new Object());

You get a compile time error!