RFR: 8188055: (ref) Add Reference::refersTo predicate [v5]

[Mandy Chung]

On Fri, 16 Oct 2020 18:27:31 GMT, Peter Levart <plevart at openjdk.org> wrote:

>> That sounds reasonable to me. Thanks for looking into it.
>
> I just want to note that if you have a `Reference<SomeType> ref` at hand, you can not just do:
> Referemce<Object> r = (Reference<Object>) ref;
> ...since those generic types are not related. You have to do something like:
> 
> @SuppressWarnings({"unchecked", "rawtypes"})
> Referemce<Object> r = (Reference) ref;
> which is very unfortunate. Comparing this method with for example `Collection.contains(Object element)`, you can see
> that Collection API has made a decision to not bother with T here. That was also due to keeping old code compatible
> when migrating from pre-generics Java to generified Collection, but as @dfuch noted, we have a migration story here
> too. We will be migrating `obj == ref.get()` to `ref.refersTo(obj)` ... Mind you that this is a boolean expression
> fragment which might be written inline surrounded with other parts of expression. So you'll be forced to split that
> into assignment with @SuppressWarnings and an expression or you will have to force the whole expression or method to
> @SuppressWarnings. I don't know if type "safety" is forth it here.

Reference instances should not be leaked and so I don't see very common that caller of `Reference::get` does not know
the referent's type.   It also depends on the `refersTo` check against `null` vs an object.  Any known use case would
be helpful if any (some existing code that wants to call `refersTo` to compare a `Reference` of raw type with an object
of unknown type).

FWIW, when converting a few use of `Reference::get` to `refersTo` in JDK, there is only one case (`equals(Object o)`
method that needs the cast.

http://cr.openjdk.java.net/~mchung/jdk15/webrevs/8188055/jdk-use-refersTo/index.html

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