where are all the objects?

[Dan Smith]

On Jul 17, 2022, at 12:21 PM, Brian Goetz <brian.goetz at oracle.com<mailto:brian.goetz at oracle.com>> wrote:

Abstractly, my conception of “object” is that it is a bundle of type state, identity (optionally), and zero or more fields.  I think this most closely corresponds to Kevin’s notion of “compound value”, though it might only have zero or one fields — but since there is type state and potentially identity, it is still a composite.  Historically we touched objects only through references, and we can still do so for all objects, but we now have the ability that a variable can store some objects directly.

How we choose to see the formerly-primitive types is mostly a matter of choosing which fiction we prefer.  At the VM level, we have I/J/F/D carriers, which are definitely some sort of primitive value.  At the language level, we can tell ourselves that `int` is an identity-free direct instance of class Integer, if we like (though we still have to cut off the turtle-regress when declaring Integer.java.).


I think some of us would like to reserve the term “object” for something that has a header and a storage block in the heap.

I’m not in that camp.  The header and storage block is how we reassociate type state with a (non-flattened) object *reference*.  But a field of type C.val stores its typestate “statically”, in the field descriptor.  (Theoretically, a field of type C.ref could too, though we don’t currently make that optimization.). But these are all just implementation options a JVM has.

In the most idealized version of this world, values are either “bare objects” (bag of type state and fields) or references to objects.  In the slightly less idealized version, we would call out the legacy primitives as distinguished bare values.

Relevant to this: we talked about the "object" term in a January EG meeting; my summary at the time (subject "Terminology bikeshedding summary"):

"primitive value vs. object

We're trying to make a distinction between primitive values being "class instances" and calling them "objects", but for many developers, especially beginners, that sounds like meaningless pedantry. We might be over-rotating on the subtle differences that make these entities distinct, rather than acknowledging that, with their fields and methods, they will be commonly understood to be a kind of object."

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