Here, I have an abstract class:

abstract class A<E extends A> {
    abstract void foo(E x);
}

Here's a class that extends A:

class B extends A<B>{
    void foo(B x){}
}

And here's another (E is B here on purpose):

class C extends A<B>{
    void foo(B x){}
}

Both of those classes are valid, and the reasoning for that makes sense to me.

However what confuses me is how this could possibly be valid:

class D extends A{
    void foo(A x){}
}

Since when are generics optional like that? I thought the extending class (subclass) of A would be required to specify an E?