Enhanced Enums Redux (with working code)

[Chen Liang]

Hi Clement, I think the issue you've discovered is already considered in the original proposal, in the "Too raw?" section:
https://cr.openjdk.org/~mcimadamore/amber/enhanced-enums.html#too-raw
TLDR; a reifiable type doesn't work with class literals.

Regards
________________________________
From: amber-dev <amber-dev-retn at openjdk.org> on behalf of Clement Cherlin <ccherlin at gmail.com>
Sent: Sunday, February 22, 2026 12:21 PM
To: amber-dev at openjdk.org <amber-dev at openjdk.org>
Subject: Enhanced Enums Redux (with working code)

Greetings,

I have been working on the problem of generic enums off and on for quite some time (see https://github.com/Mooninaut/cursed-generic-enum-java for a previous dead-end attempt).

I think I have an actual solution this time, and it doesn't require any changes to Java's type system. It only requires a slight tweak to the "extends" clause of generic enums: Replace "extends Enum<SomeEnum<T>>" with "extends Enum SomeEnum<?>>".

I have a fully-worked example of this at https://github.com/Mooninaut/java-generic-enums implementing the example given in https://mail.openjdk.org/pipermail/amber-spec-experts/2017-May/000041.html, which was the stopping point for the original generic enums effort.

I included lightly modified copies of Enum, EnumSet and EnumSet's subclasses, and a Main class which demonstrates that the original problems with collections of enums are almost entirely resolved.

I also provide several generic enums with non-trivial type bounds to demonstrate that generic enums are no more limited than any other generic class.

I cannot say definitively that every conceivable generic type will work, but I haven't yet found any generic enum that wouldn't compile and run successfully.

There is one slightly awkward issue. It's not possible to simply pass an enum class literal to EnumSet's static factory methods without a raw cast.

EnumSet.allOf((Class<Option<?>>) (Class) Option.class);

But this can be worked around by taking advantage of the fact that getDeclaringClass casts to Class<E>

EnumSet.allOf(Option.D.getDeclaringClass());

Cheers,
Clement Cherlin
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