Re: Yet another generics question: Needs unchecked conversion to
conform to ...
Michel T. wrote:
Tim, thank you very much for the feedback.
You have not and in general cannot verify that the object is of that
type. The signature is wrong. For instance, in your example ServiceX is
not a Service<ServiceComponent> but it is a Service<? extends
ServiceComponent>.
But since ServiceComponent is an abstract class or interface, in
practice, is'nt Service<? extends ServiceComponent> a "subset" of
Service<ServiceComponent> ? Therefore, is'nt the cast from the later
into the former safe?
No. The set of all objects that are of type Service<? extends
ServiceComponent> is a superset of all objects that are of type
Service<ServiceComponent> (gloss over erasure here...).
You had an interface:
public interface Service<T extends ServiceComponent> {
T createServiceComponent();
void doSomethingWith(T component);
}
Suppose ServiceComponentX extends ServiceComponent. Then
Service<ServiceComponentX> is a Service<? extends ServiceComponent>. Now
if we could assign that to Service<ServiceComponent> we have a problem.
Service<ServiceComponentX> a = new Service<ServiceComponentX>() {
public ServiceComponentX createServiceComponent() {
return new ServiceComponentX();
}
public void doSomethingWith(ServiceComponentX component) {
// We hope this is true.
System.err.println(component instanceof ServiceComponentX);
}
};
Service<? extends ServiceComponent> b = a;
Service<ServiceComponentX> c = b; // wrong
// Here ServiceComponent is implicitly casted to ServiceComponentX!
c.doSomething(new ServiceComponent());
On the other hand, it seems more work for the service implementation
(ServiceX) who would have to provide two interfaces, and matching
implementation classes. I may be missing a point though.
Two interfaces containing no definitions.
They are matching the types used by the client code. You could have more
than one service per type (otherwise what would the point of dynamically
loading them be?).
Tom Hawtin