Re: Entity and Identity

From: Brian <brian_at_selzer-software.com>
Date: Tue, 21 Jul 2009 21:57:02 -0700 (PDT)
Message-ID: <0696521c-85e8-4ebd-b500-09d1a4cc97d4_at_m11g2000yqh.googlegroups.com>


On Jul 21, 11:15 pm, David BL <davi..._at_iinet.net.au> wrote:
> On Jul 21, 9:39 pm, Brian <br..._at_selzer-software.com> wrote:
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> > On Jul 21, 5:04 am, David BL <davi..._at_iinet.net.au> wrote:
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> > > On Jul 21, 1:47 am, Brian <br..._at_selzer-software.com> wrote:
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> > > > 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.
>
> You're assuming "location" means physical address in memory.
>
> In all OO systems there is a concept of an address space for object
> references.  Using the street address metaphor we might say that the
> objects are located at these addresses.  That is what I meant by
> "location".
>
> > 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.
>
> Agreed, but only assuming "can" is qualified with "sometimes" (not
> "always").

Yes. Not every relation has more than one key.

>
> In your original statement you implied that location was part of an
> object's state.  That was the part I disagreed with.- Hide quoted text -

I stated that a difference in location constitutes a difference in state. My line of thinking is that what is referenced by each object identifier is a particular object's state and that each object can have exactly one state at a time, so when there is more than one location at a given time, there is more than one state and therefore there must be more than one object. The eight nodes in your cube of 1- ohm resistors are distinct even though they can only be distinguished relative to one another, and in the same way states at different locations are distinct, even if all of the fields in each state contain identical values.

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> - Show quoted text -
Received on Wed Jul 22 2009 - 06:57:02 CEST

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