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On Mon, Feb 23, 2026 at 6: 26 AM Maurizio Cimadamore <maurizio. cimadamore@ oracle. com> wrote: Hi Clement On 22/02/2026 18: 21, Clement Cherlin wrote: > Greetings, > > I have been working on the problem of generic enums off and on</div>
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<div dir="ltr"><div dir="ltr"><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Mon, Feb 23, 2026 at 6:26 AM Maurizio Cimadamore <<a href="mailto:maurizio.cimadamore@oracle.com" target="_blank">maurizio.cimadamore@oracle.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">Hi Clement<br>
<br>
On 22/02/2026 18:21, Clement Cherlin wrote:<br>
> Greetings,<br>
><br>
> I have been working on the problem of generic enums off and on for <br>
> quite some time (see <br>
> <a href="https://github.com/Mooninaut/cursed-generic-enum-java" rel="noreferrer" target="_blank">https://github.com/Mooninaut/cursed-generic-enum-java</a> for a previous <br>
> dead-end attempt).<br>
><br>
> I think I have an actual solution this time, and it doesn't require <br>
> any changes to Java's type system. It only requires a slight tweak to <br>
> the "extends" clause of generic enums: Replace "extends <br>
> Enum<SomeEnum<T>>" with "extends Enum SomeEnum<?>>".<br><br>I'd need more time to think about the consequences of this. That said, <br>
this has a good property, in that it only affects generic enum <br>
declarations. Non-generic enum would stay exactly the same.<br><br>
> There is one slightly awkward issue. It's not possible to simply pass <br>
> an enum class literal to EnumSet's static factory methods without a <br>
> raw cast.<br>
><br>
> EnumSet.allOf((Class<Option<?>>) (Class) Option.class);<br>
<br>
This makes me more worried -- because this is such a common idiom that <br>
(I think) has to work like for any other enum.<br>
<br>
In the spirit of your earlier suggestion, a possible way out would be to <br>
also redefine what Foo.class mean when Foo is a generic enum -- e.g. not <br>
just Class<Foo> (class-of-raw) but Class<Foo<?>> (class-of-wildcard).<br>
<br>
All such changes would indeed help making generic enums more useful <br>
(and interoperable with EnumSet and friends), but whether all such <br>
changes are actually "sound", that's a different story.<br>
<br>
For instance, code like:<br>
<br>
MyEnumSet<Option> set = MyEnumSet.allOf(Option.class)<br>
<br>
Would _still_ fail. You need the LHS to be _exactly_ <br>
MyEnumSet<Option<?>> for this to work. Which means graduating an enum <br>
into a generic enum would still be a source incompatible change </blockquote><div><div> </div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">Cheers<br>
Maurizio</blockquote></div><div><br></div><div>That's an excellent point. Doing the migration in a single step is source incompatible.</div><div><br></div><div>However, it is possible to perform the migration in multiple steps, each step source</div><div>compatible with the previous step, allowing for gradual migration. This would</div><div>require multiple major releases of a library to complete. The result would be</div><div>that at each step, clients can update the library without incompatibility, as long</div><div>as they complete the required migration steps before upgrading to a major version</div><div>that takes the next migration step.</div><div><br></div><div>See <a href="https://github.com/Mooninaut/java-generic-enums/tree/main/src/main/java/org/duckdns/mooninaut/migration">https://github.com/Mooninaut/java-generic-enums/tree/main/src/main/java/org/duckdns/mooninaut/migration</a></div><div>for a step-by-step example.</div><div><br></div><div>Given a library that has an enum, which can usefully be migrated to a generic enum:</div><div><br></div><div>Step 1:</div><div>* Library changes method parameters of type "EnumSet<X>" (if any) to</div><div> "EnumSet<? extends X>". This change is source-compatible for clients.</div><div>* Casts from "EnumSet<? extends X>" to "EnumSet<X>" may be</div><div> required inside library methods.</div><div><div>* Suggestion: Some sort of @MigratingToGenericEnum annotation to prompt</div><div> clients to update references, ideally with tool support.</div><br></div></div><div class="gmail_quote">Step 2:</div><div class="gmail_quote"><div>* Clients update library, and change variables of type "EnumSet<X>" to "EnumSet<? extends X>"<br>* Both "EnumSet<X>" and "EnumSet<? extends X>" compile.</div><div><br></div><div>Step 3:</div><div>* Library makes X generic, updates methods that return "EnumSet<X>" to</div><div> return "EnumSet<X<?>>" This is source-compatible for clients that have completed</div><div> step 2, with an unchecked warning on "EnumSet.allOf(X.class)" and related methods.<br>* Library methods may require casts from "EnumSet<? extends X>" to "EnumSet<X<?>>"<br>* Library changes "EnumSet.allOf(X.class)" to "EnumSet.allOf(X.CONSTANT.getDeclaringClass())"</div><div>* Library removes @MigratingToGenericEnum.</div><div><br></div><div>Step 4:</div><div>* Clients update library, change usages of "EnumSet<? extends X>" to "EnumSet<X<?>>"</div><div> and change "EnumSet.allOf(X.class)" to "EnumSet.allOf(X.CONSTANT.getDeclaringClass())"</div><div>* Usages of "EnumSet<X>" now produce compilation errors.</div><div>* Both "EnumSet<? extends X> set = EnumSet.allOf(X.class)" and</div><div> "EnumSet<X<?>> set = EnumSet.allOf(X.CONSTANT.getDeclaringClass())" compile.</div><div>* Tools should prompt these changes when seeing use of a generic enum with old syntax.</div><div><br></div><div>Step 5 (optional):</div><div>* Library replaces "EnumSet<? extends X>" with "EnumSet<X<?>>" in method parameter types.<br>* Library methods can no longer be called with values of type "EnumSet<? extends X>".<br>* This is source-compatible with clients that have completed step 4.<br>* No further client changes are required.<br>* Casts to EnumSet<X<?>> are no longer required.<br>* This step is optional. The only benefit is removing remaining casts.</div><div> Libraries can delay this step for as long as desired to allow clients time to complete step 4.</div><div><br></div><div><br></div><div><div>This process is analogous to the gradual transition from a deprecated API to its replacement</div><div>over several major versions. The non-generic syntax becomes "deprecated" at step 1, but</div><div>still compiles, possibly with warnings, until step 4.</div><br></div><div>Cheers,</div><div>Clement </div></div></div>
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