[sealed] Sealed local classes?

Remi Forax forax at univ-mlv.fr
Fri Oct 11 13:51:38 UTC 2019

----- Mail original -----
> De: "Maurizio Cimadamore" <maurizio.cimadamore at oracle.com>
> À: "Tagir Valeev" <amaembo at gmail.com>
> Cc: "amber-spec-experts" <amber-spec-experts at openjdk.java.net>
> Envoyé: Vendredi 11 Octobre 2019 12:59:41
> Objet: Re: [sealed] Sealed local classes?

> I think an enum declaration is 'morally final' in the sense that, while
> it can't really be marked with ACC_FINAL (because there might be
> constants which extend from it), the user cannot subclass the enum.
> Everything weird you can do with an enum, remains _inside_ the enum
> declaration bubble, which I think makes mixing enums and sealed
> interface pretty safe. It is also lucky that we can't say 'final enum' -
> meaning that I would also extend it to the other keywords - that is, you
> can't put sealed, non-sealed on an enum.

i agree with you that you can not declared an enum final but at the same time, javac will add final automatically in the classfile of a flat enum so javac should add sealed automatically for an enum with subclasses.

> Regarding the 'anonymous enum constant' issue you raise how is that
> different from:
> sealed interface Y permits Bar, Baz {}
> class Bar implements Y {}
> ... new Bar() {}
> In this case, I don't think you break exhaustiveness in the same way you
> do if you allow anonymous implementations of Y.
> Clients will be assuming that Y is either a Bar or a Baz, and the fact
> that some of the Bars are anonymous instance is immaterial to this.
> Unless I misunderstood what you were trying to say. If not, I think my
> reasoning here would be to:
> 1) allow enums to implement sealed interfaces
> 1b) do not allow sealed, non-sealed modifiers on an enum (e.g. do the
> same as with final)
> 2) allow anonymous enum constants inside the enums in (1) - as they
> can't break exhaustiveness for clients
> Maurizio


> On 11/10/2019 04:02, Tagir Valeev wrote:
>> Hello!
>> Sorry if this was already discussed, but what about enums extending
>> sealed interfaces? E.g.:
>> sealed interface X permits Foo {}
>> enum Foo implements X {  // can we do this?
>>    A {}, // and what about this? Here we have an additional subclass at
>> runtime. Or we should explicitly declare "non-sealed enum Foo" to
>> allow this?
>>    B,
>>    C
>> }
>> With best regards,
>> Tagir Valeev.
>> On Thu, Oct 10, 2019 at 3:46 PM Maurizio Cimadamore
>> <maurizio.cimadamore at oracle.com> wrote:
>>> On 10/10/2019 01:50, Brian Goetz wrote:
>>>> Right. We already restrict anon and lambda instances of the sealed
>>>> type. Not only can't we stably write down their types in the PS
>>>> attribute, but even if we could, it's so easy to accidentally lose
>>>> exhaustiveness.
>>> This is a very good point; if I have type T = A | B | C, but then I have
>>> 'anonymous' Ts flying around, all switches assuming A|B|C are no longer
>>> exhaustive.
>>> Maurizio

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