[sealed-types] Draft Spec for JEP 360 Sealed Types (Preview)

Gavin Bierman gavin.bierman at oracle.com
Wed May 6 15:13:30 UTC 2020

We have made some presentational changes to the spec for JEP360 (Sealed Types), which are available at:


The only semantic change is a new error if the direct superclass or direct superinterface of a local class is `sealed`. A more complete set of changes to address all interactions between local and member classes and sealed types (see [1] for some of these) will come later, although perhaps not until JDK 16. 


[1] http://mail.openjdk.java.net/pipermail/amber-spec-experts/2020-May/002156.html

> On 20 Apr 2020, at 22:50, Gavin Bierman <gavin.bierman at oracle.com> wrote:
> The latest (and hopefully final) draft of JEP 360 (Sealed Types) is available at:
>    http://cr.openjdk.java.net/~gbierman/jep360/latest
> The changes since the last draft was circulated in February [1]:
> * Some minor typos have been corrected, including changing the title of 8.1.6.
> * We have make corrections in a number of places to make it clear that the name
> in a `permits` clause is not a type (and can not be annotated, for example).
> * We now require a functional interface to not be `sealed`, rather than imposing
> checks on target types of lambda expressions. 
> * We have removed the changes to narrowing reference conversion which allowed
> for stricter checking of cast conversions wrt sealed type hierarchies. We have
> decided to defer this feature until a later release to allow us to develop a
> broader treatment of "disjoint types" that can be used not just in cast
> conversion, but in other places such as bounds checking and pattern matching.
> The refined cast conversion was nice to have, but really only will make a
> difference when we get to patterns in switches, so it makes sense to spend some
> more time now considering our design rather than refining cast conversion in a
> piecewise manner. 
> Thanks,
> Gavin
> [1] https://mail.openjdk.java.net/pipermail/amber-spec-experts/2020-February/002031.html

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