RFR: 8188055: (ref) Add Reference::refersTo predicate [v3]

[Mandy Chung]

On Tue, 13 Oct 2020 08:49:23 GMT, Daniel Fuchs <dfuchs at openjdk.org> wrote:

>> @dfuch I would expect the users of `refersTo` should know the type of the referent to be compared, i.e. it should know
>> the parameter type of a Reference object.   Do you have any specific use case in mind that you have a `Reference<?>
>> ref` but wants to compare a referent of unknown type?
>
> @mlchung  I have often used a `Reference<?>` in tests - but my main usage there would be to call `ref.refersTo(null)`
> which works in all cases. My main concern here however is that using `T` in `refersTo` seems to go against the
> advertised usage of the method - I mean - if I have an object `obj` of type unknown, I can always do `obj == ref.get()`
> whatever the parameter type of `ref` is. But I won't be able to call `ref.refersTo(obj)` unless I use raw types and
> suppress warnings. So I just wanted to check that this was intentional.

For the common cases, the application should know the type of the referent and using `T` in `refersTo` will benefit
from the compiler type checking.   For the unknown type case, cast to `Reference<Object>` is not ideal but reasonable?
something like this:

Reference<Object> r = (Reference<Object>) ref;
r.refersTo(obj);

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PR: https://git.openjdk.java.net/jdk/pull/498