What is the benefit of extending a generic by specifying the new type as actual type of generic

Question

I saw this pattern somewhere:

class A extends B {

}

This structure is a little unusual to extend a generic by specifying the new

Answer 1

Of course the OOP answer is that A is a B. If A were not a B than A should merely compose itself with a B to make use of B's functionality.

Presumably B also has some general implementations which take advantage of restrictions placed on the generic type.

Another use case would be for B to look something like:

abstract class B> {
    public T createCopy(T t);
}

Now subclasses can implement createCopy and client code can safely use it without having to cast... e.g.

class A extends B {
    public A createCopy(A t) {
        return new A(t); //copy constructor
    }
}

Compare the above to:

abstract class B {
    public B createCopy(B t);
}
class A extends B {
    public B createCopy(B t) { //Is the copy an A or a different subtype of B? We don't know.
        return new A(t); //copy constructor
    }
}