RFR: 8246778: Compiler implementation for Sealed Classes (Second Preview)

[Alan Bateman]

On Mon, 16 Nov 2020 13:30:06 GMT, Vicente Romero <vromero at openjdk.org> wrote:

> Please review the code for the second iteration of sealed classes. In this iteration we are:
> 
> - Enhancing narrowing reference conversion to allow for stricter checking of cast conversions with respect to sealed type hierarchies
> - Also local classes are not considered when determining implicitly declared permitted direct subclasses of a sealed class or sealed interface
> - renaming Class::permittedSubclasses to Class::getPermittedSubclasses, still in the same method, the return type has been changed to Class<?>[] instead of the previous ClassDesc[]
> - adding code to make sure that annotations can't be sealed
> - improving some tests
> 
> TIA
> 
> Related specs:
> [Sealed Classes JSL](http://cr.openjdk.java.net/~gbierman/jep397/jep397-20201104/specs/sealed-classes-jls.html)
> [Sealed Classes JVMS](http://cr.openjdk.java.net/~gbierman/jep397/jep397-20201104/specs/sealed-classes-jvms.html)
> [Additional: Contextual Keywords](http://cr.openjdk.java.net/~gbierman/jep397/jep397-20201104/specs/contextual-keywords-jls.html)

src/java.base/share/classes/java/lang/Package.java line 227:

> 225:      * This method reports on a distinct concept of sealing from
> 226:      * {@link Class#isSealed() Class::isSealed}.
> 227:      *

This API note will be very confusing to readers. I think the javadoc will need to be fleshed out and probably will need to link to a section the Package class description that defines the legacy concept of sealing.

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PR: https://git.openjdk.java.net/jdk/pull/1227