Re: Entity and Identity
Date: Tue, 21 Jul 2009 06:39:21 -0700 (PDT)
Message-ID: <48646900-6823-4904-a04f-0ffd322778cb_at_v20g2000yqm.googlegroups.com>
On Jul 21, 5:04 am, David BL <davi..._at_iinet.net.au> wrote:
> On Jul 21, 1:47 am, Brian <br..._at_selzer-software.com> wrote:
>
> > First, it doesn't matter if objects can conceal part of their states
> > provided that the references to those objects can be used to
> > distinguish between them, and second, if two objects are identical in
> > state, then they cannot differ in location, for that would constitute
> > a difference in state.
>
> When you say 'object' do you mean in the OO sense? Usually the OO
> community use 'object' to mean an identifiable state machine located
> at some address and don't regard the location to be part of its state.
> Furthermore usually the identity of an object is determined *only* by
> its location and has nothing at all to do with its current state.
I disagree with your use of the terms 'location' and 'identity.' In the OO world, objects are instances of reference types. The location of an object can change over its lifetime, but what is used to reference each object, the object identifier, doesn't. It may be splitting hairs, but there is a distinct difference between 'identity' and 'the identity' in that 'identity' is a binary relation between objects in the universe that denotes /is identical to/, but 'the identity' of an object is that essential property (unary relation) which distinguishes it from all other objects (its haecceity) and which is embodied by an object identifier or by a proper name (in the logical sense). The identity of an object is determined (functionally) by its object identifier but can also be determined by its current state in the same way that a relation schema can have more than one key. An object representing a particular serialized part can be identified by its object identifier as well as by the part's serial number, or by its position on the assembly line relative to all other similar parts on the line, which could change over time (for example, the part in front of it may have been scrapped). Received on Tue Jul 21 2009 - 15:39:21 CEST