[sealed] Sealed local classes?

Maurizio Cimadamore maurizio.cimadamore at oracle.com
Fri Oct 11 10:59:41 UTC 2019

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


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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