On 26 Jan 2022, at 16:36, Dan Smith wrote:
An instance of a class is also an instance of (and carries the
properties of) its superclasses. Value objects are instances of the
class Object.
I can imagine a design in which we say that instances of Object may be
either identity or value objects, but direct instances of the class
are always identity objects. But this is not how we've handled the
property anywhere else, and it breaks some invariants. We've gotten
where we are because it seemed less disruptive to introduce a subclass
of Object that can behave like a normal identity class.
And yet there is also a second way a class can be an instance; it can be
*exactly an instance of C*, when `x.getClass()==C.class`. That’s the
condition which can be teased apart here, if we allow ourselves to use
something other than marker interfaces. But marker interfaces (as I
said) are committed to ignoring the “exactly an instance” condition,
because they inherit.
