PreviewFeature on packages and modules

[Alex Buckley]

On 2/8/2021 8:00 AM, Nikita Eshkeev wrote:
> Additional questions:
> 1. If a module is annotated with PreviewFeature then javac raises a 
> compile-time errors/warnings when it's used in a requires statement of 
> another module. Is this true?

It should be.

> 2. If a module is not annotated with PreviewFeature then each export 
> statement of a package that is annotated with PreviewFeaturein in 
> module-info.java should lead to compile-time errors/warnings. Is this true?

Package annotations are a no-op. Don't expect any warnings/errors from 
javac if your module `exports` a package annotated @PreviewFeature.

> 3. If a module is not annotated with PreviewFeature then each provides 
> statement of a class that is annotated with PreviewFeature in 
> module-info.java should lead to compile-time errors/warnings. Is this true?

Yes, that counts as a source code reference to a preview API element.

> 4. If a class inherits/implements a class/interface that is marked as 
> PreviewFeature then the class is considered a PreviewFeature. Is this true?

No, that's an over-reading of JEP 12. Assuming that the 
inheriting/implementing class C is not "participating" for the preview 
class P, C's `extends`/`implements` clause will generate a warning/error 
and C.class will be marked as depending on preview features, just as if 
C had declared a field of type P or C's code had used a preview language 
feature. The javadoc for C will display that C's 
superclass/superinterface is a preview API.

Alex