Why a generic declaration in a class prevent USE of methods with generic argument?

[Car Tola]

Greetings!, I've read the Generics stuff many times, and I also have searched the forum, but my question keep unanswered :-(:
Let's say you have declared these classes:

class Triquitraca<E> {
	public void unMetodo(E arg) {}
	public E otroMetodo() {return null;}
}

class Uno{}
class Dos extends Uno {}
class Tres extends Dos {}

so, why the following lines are INCORRECT?:

Triquitraca<? extends Dos> triqui = new Triquitraca<Dos>();
triqui.unMetodo(new Uno());
triqui.unMetodo(new Dos());
triqui.unMetodo(new Tres());

They all fail in compilation time, so, (again), what is the purpose of being able to declare a generic class with the metacharacter '?' if then you cannot use methods with arguments of the generic Type??? (because you CAN use otroMetodo without any problem). It could be to prevent modification of the Generic object received as argument? (It seems a too strong limitation to me!!). Anyway, IMHO, if you cannot USE these kind oh methods, the compiler should alert you in their DECLARATION (because they are useless!), not when you try to use them. Or am i missing something?. Any help/explanation would be very appreciated, thanks!

[Car Tola]

Ok, I've just discovered that the following is CORRECT:

triqui.unMetodo(null);

I understand even less