Question

According to the Generics trial,

This section states:

Given two concrete types A and B (for example, Number and Integer), MyClass<A> has no relationship to MyClass<B>, regardless of whether or not A and B are related. The common parent of MyClass<A> and MyClass<B> is Object.

Yet, here we’re told,

Although Integer is a subtype of Number, List<Integer> is not a subtype of List<Number> and, in fact, these two types are not related. The common parent of List<Number> and List<Integer> is List<?>.

Why isn’t the parent of MyClass<A> / MyClass<B> in the first example MyClass<?>? What is the distinction?

Answer 1

I think the answer is rather trivial. While the correct parent of MyClass<A> and MyClass<B> is indeed MyClass<?>, the tutorial did a small simplification there, as wildcards have not been introduced yet.

The point of saying

The common parent of MyClass<A> and MyClass<B> is Object.

was just to make it clear that none of the two types is the parent of the other, regardless of the relationship between A and B.

This is confirmed by the following comment right below your first quote:

For information on how to create a subtype-like relationship between two generic classes when the type parameters are related, see Wildcards and Subtyping.

as well as by the introduction of the chapter Wildcards and Subtyping:

As described in Generics, Inheritance, and Subtypes, generic classes or interfaces are not related merely because there is a relationship between their types. However, you can use wildcards to create a relationship between generic classes or interfaces.