The interfaces IdentityObject and ValueObject must die !

[Dan Smith]

On Jan 26, 2022, at 4:55 PM, John Rose <john.r.rose at oracle.com<mailto:john.r.rose at oracle.com>> wrote:

Independently of that, for the specific case of Object, having a query function Class.instanceKind, which returns “NONE” for abstracts else “VALUE” or “IDENTITY”, would encode the same information we are looking at with those marker interfaces.

Right, so you're envisioning a move in which, rather than 'obj instanceof ValueObject', the dynamic test is 'obj.getClass().instanceKind() == VALUE'.

For dynamic testing (my #3), sure, these are equivalent.


But the contract for a method is more flexible than the contract of a marker interface.

In particular, instanceKind is not required to report the same thing for T and U when T<:U but marker interfaces are forced to be consistent across T<:U. I think this is an advantage, precisely because it has more flexible structure, for the method rather than the marker interface.

I would expect that 'cls.instanceKind() == IDENTITY' has the exact same semantics as 'IdentityObject.class.isAssignableFrom(cls)': if a class claims to be an identity class, then all its instances (direct and via subclassing) are identity objects. I'm not seeing a sensible alternative.

How does this behave in the odd world in which direct instances of Object are identity objects?

'Object.class.instanceKind()' must return NONE, just as Object.class must not implement either IdentityObject or ValueObject.

Given one of these oddball objects, 'obj.getClass().instanceKind()' will, naturally, return NONE. Which is surprising, breaking the expectation that there are only two possible results of this expression. Just as these objects would break the expectation that every object is either 'instanceof IdentityObject' or 'instanceof ValueObject'.

I keep saying this: how we handle the 'new Object()' problem doesn't seem to me to have any impact on how we encode "I'm an identity class". It's not a discussion that belongs in this email thread.


If the marker interfaces also have little use as textual types (e.g., for bounds and method parameters) then I agree with Remi. Ditch ‘em.

I outlined many ways in which we're making use of these interfaces. Static types is just one. Getting rid of them isn't as easy as "ditch 'em", it would involve redesigning for all of those use cases (plus any I forgot), and coming up with something that is compellingly better. (This is an invitation, for anyone interested in proposing something specific...)