Switching on class literals

[Tagir Valeev]

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