Switching on class literals

[Nir Lisker]

Hi Tagir,

I went through them, thanks (I'm not subbed to the specs lists). Looks like
I'm in time for the once-every-two-years discussion on this. There have
been quite a few advances since.

On Mon, Apr 11, 2022 at 10:52 AM Tagir Valeev <amaembo at gmail.com> wrote:

> Hello, Nir!
>
> To add some context, switch over class literals was already discussed
> at least twice. See the following threads:
>
> https://mail.openjdk.java.net/pipermail/amber-spec-experts/2018-April/000531.html
>
> https://mail.openjdk.java.net/pipermail/amber-spec-observers/2020-August/002515.html
>
> With best regards,
> Tagir Valeev
>
> On Mon, Apr 11, 2022 at 1:34 AM Nir Lisker <nlisker at gmail.com> wrote:
> >
> > Hi,
> >
> > Given the code example:
> >
> > public class ClassLiterals {
> >
> >     interface Vehicle {}
> >
> >     class Car implements Vehicle {}
> >
> >     class Boat implements Vehicle {}
> >
> >     static Class<? extends Vehicle> createVehicleClass() { return
> > Car.class; }
> >
> >     public static void main(String[] args) {
> >         Class<? extends Vehicle> vehicleClass = createVehicleClass();
> >
> >         boolean b = switch (vehicleClass) {
> >             case Car.class -> true;
> >             case Boat.class -> true;
> >             default -> false;
> >         };
> >
> >         boolean b2;
> >         if (vehicleClass == Car.class) {
> >             b2 = true;
> >         } else if (vehicleClass == Boat.class) {
> >             b2 = true;
> >         } else {
> >             b2 = false;
> >         }
> >     }
> > }
> >
> > The case labels in the switch expression give the error:
> > incompatible types: Class<ClassLiterals.Car> cannot be converted to
> > Class<CAP#1>
> >
> > Firstly, I understand that there is type erasure afoot. Secondly, the
> > if-else chain works, although I'm aware that there's some cheating
> involved
> > since it's not doing the parallel (illegal) instanceof check: if
> > (vehicleClass instanceof Class<Car>).
> > Using the == operator is guaranteed to work for class token comparison
> with
> > the caveat of being loaded with the same class loader, as far as I know.
> >
> > Is it correct to say that Class<...> comparisons (switching and equality
> > checks) are special compared to other generic type comparisons?
> > Is there a reasonable way to make switch work for class literals if the
> > above is true?
> >
> > Thanks,
> > Nir
>