[sealed] Sealed local classes?

Brian Goetz brian.goetz at oracle.com
Fri Oct 11 14:07:11 UTC 2019

This seems reasonable to me.  So, spec consequences:

 - sealed, non-sealed illegal on enums
 - enums can implement sealed types
 - said permission to extend pushes down to constants, including the anonymous classes of nontrivial constants

> On Oct 11, 2019, at 6:59 AM, Maurizio Cimadamore <maurizio.cimadamore at oracle.com> wrote:
> 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.
> 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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