> On Sep 9, 2020, at 12:15 PM, Brian Goetz <brian.go...@oracle.com> wrote:
>
>> Remi points out that the System class (or, say, Objects) is a place we could
>> put a factory method if we want one but don't want to risk adding it to
>> Object.
>
> Looking at it from the other direction, I am not sure having a nominal
> SimpleIdentityObject class adds much value. You can of course write a
> do-nothing identity class yourself:
>
> class WednesdayIdentityClass extends Object { }
>
> Will anyone want to extend SIO? There's no point, since any concrete class
> that extends Object already becomes an IdentityObject, so they'll just extend
> Object.
>
> Will anyone want to use it as a parameter type / type bound? I doubt it;
> they'll use IdentityObject.
>
> Will anyone want to use it as a return type? I don't see much point, because
> there's nothing a SIO can do that Object can't. The only reason we're
> exposing it is as a replacement for `new Object()`, which we've deemed to be
> an idiom only for getting a unique identity. If we did nothing, people could
> replace:
>
> Object lock = new Object();
>
> with
>
> Object lock = new Object() { };
>
> (and someone would brand it the "identity brace trick".) But that's a little
> too obscure, so recommending
>
> Object lock = Objects.newIdentity();
>
> says exactly what you mean. The precise typestate of the result is an
> implementation detail; the key is that a new identity is dispensed.
Yes, this is all good from the language perspective. The outstanding bit is
"what does the instruction 'new java/lang/Object' do?" The simple thing is for
JVMS to say "an instance of java/lang/SimpleIdentityObject is created on the
heap".
If the class is private, the JVM implementation still knows what it is, so
that's probably okay, but what does JVMS say? I don't want to phrase it in a
way that, say, lets a JVM implementation interpret 'new java/lang/Object' as
'new java/lang/String' or something stupid like that.
