2 questions about enums and the closed/withdrawn JEP 301

[Archie Cobbs]

On Wed, Mar 29, 2023 at 12:53 PM Maurizio Cimadamore <
maurizio.cimadamore at oracle.com> wrote:

> The following is legal:
>
> Box<? extends Foo>
>
> But that is also a type that has significantly different characteristics
> from the ones above - e.g. you can’t call Box::set on it. So, even before
> getting to generic methods accepting Foos, we have a much more fundamental
> problem: we can't even express the type of a variable of type Box<Foo> in
> the type-system (!!).
>
Thanks Maurizio...  and of course if I had actually tried what I was
suggesting I would have seen the problem... !

You *can* come close though... you just have to pay for your offense with
@SuppressWarnings("unchecked"):

import java.util.*;
public class GenericEnum {

    public static class Enum<T extends Enum<T>> {
    }

    public static class Foo<S> extends Enum<Foo<S>> {
    }

    public abstract static class EnumSet<T extends Enum<T>> implements
Set<T> {

        public static <T extends Enum<T>> EnumSet<T> noneOfGeneric(Class<?
extends T> c) {
            return null;
        }
    }

    @SuppressWarnings("unchecked")   // this is still necessary, too bad
    public static void main(String[] args) {
        Set<Foo<?>> fooSet = EnumSet.noneOfGeneric(Foo.class);
    }
}

Personally, I would still be happy to have generic enums even without any
accomodation by EnumSet/EnumMap.

Then all I would have to homebrew would be a few wrapper methods around the
EnumSet/EnumMap factory methods :)

-Archie

-- 
Archie L. Cobbs
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