Re: Entity and Identity
Date: Tue, 21 Jul 2009 03:07:48 -0700 (PDT)
Message-ID: <f806f72e-b25c-474d-b403-3cda6fb09304_at_v23g2000pro.googlegroups.com>
On Jul 20, 11:51 pm, "Walter Mitty" <wami..._at_verizon.net> wrote:
> Anyway, my view of identity (or of identification, if you prefer) is that an
I find that confusing because it's not clear when you're talking about
your view of identity versus the OO one.
In OO, object identity is usually regarded as determined by object
location and is independent of object state. That of course is very
different to your first sentence above where you say you prefer to use
the object's state as the basis for identification.
However in the context of composing complex state machines out of
simpler ones it is entirely appropriate to identify state machines
independently of their current state. More to the point it wouldn't
make sense to do otherwise. For example two stack objects (i.e.
simple state machines that support push and pop operations) used for
entirely different purposes within a containing state machine may
occasionally have the same state. It wouldn't make sense to say there
is only one stack object whenever that happens. In fact the
containing state machine will normally specify exactly which stack
> object's state is all we have to go on as the basis for identification. In
> particular, an object's location (as specified by a pointer) or its
> trajectory (a history of pointers over time) are unavailable for purposes of
> identification. This view of identity fits pretty comfortably into the
> relional model, but it runs afoul of object oriented thinking at least two
> important ways. Frst, if an object can conceal part of its state
> (encapsulation), then it necessarily can conceal some of what needs to be
> known to identify it. Second, if two objects are identical in state, then
> they are the same object, even if they differ in location (at the same point
> in time). I'll call this the "Doppelganger effect".
I find it hard to see how one could define "object" such that object identity is determined by state not location. You appear to be thinking about eternal, abstract mathematical values, but it doesn't make much sense to say that values have state (because that suggests a value can change) or location (as though a value exists in time and space). Received on Tue Jul 21 2009 - 12:07:48 CEST