[JSR308] annotation for wildcard types

[Chin Wei Ngan]

> Another question, is annotation types allowed for wildcards ?
> List<? extends @Existing File> and
> List<? super @Existing File>

You raise an interesting question.

I think "File" is meant as a lower/upper bound in
the above typing, rather than the type field
itself. I suppose annotation (if any) ought to
be for the whole of (? extends File), e.g
in pre-fix form:

     List<@nonnull (? extends File)>

The "? extends" or "? super" can be viewed as a kind of
type annotation too but this is already part of Java type system,
so it beyond this JSR. I say this because wildcard
type is based on variant parametric type (VPT) which were
originally written as:

  List<? extends t> is just a sugar for List<+t>

to help support covariant subtyping. Similarly,

  List<? super t> is just a sugar for List<-t>

to support contravariant subtyping. It is tempting to
think of +/- are variance annotations.