Why don't enums allow generics since Java 5? (was "Re: 2 questions about enums and the closed/withdrawn JEP 301")

[David Alayachew]

Hello Maurizio,

Thank you for your response!

> sadly I don’t know the answer to your question
> as I wasn’t around when enums were added to the
> language ;-)

Lol, fair enough. I actually poked around the enum class itself [1] and saw
that it was originally made by at least 2 folks - Josh Bloch and Neal
Gafter. Maybe they read these mailing lists on occasion?

> I agree with all Brian said. I’d also add that
> the failure modes we discovered when we worked
> on generic enums are rather obscure, as they
> are a result of *three* factors: (i) you need to
> have some class Foo that is transitioning from
> being non-generic to being generic and (ii) you
> have to have some existing utterances of Foo
> nested inside some other parameterized type
> (e.g. Box<Foo>) and (iii) the declaration of
> that second generic class has to be
> sufficiently strict (e.g. Box<X extends Foo<X>>).
> If *all* these conditions are met, then you end
> up with cases where utterances of Box<Foo>
> were valid before generification of Foo, but
> invalid afterwards. Something like this was
> (and still is!) pretty hard to spot.

Thank you so much for the context. This really clarifies a lot and helps me
to understand even more the hesitation to push forward. Not only is it
going to break code, but the scope at which it will is difficult to
measure, so we can't accurately judge the cost to the benefit.

> One might argue (given what we know now) that
> it would have been perhaps better not to make
> the Enum class itself generic. But the static
> type checking you get on stuff like
> EnumSet.noneOf(Foo.class) is hard to pass up: if
> Enum wasn’t a generic class, then there’d be no
> way to tie the type of the class literal to the
> type of the returned EnumSet. So I think the
> Enum generic declaration we ended up with is the
> result of a forced move (type-system wise).

One thing being on this mailing list has taught me is that it is all about
tradeoffs when trying to figure out whether or not to put stuff into the
language, let alone how. I definitely see what you mean now.

Thank you for the insight and context!
David Alayachew
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