From: Brian Candler <B.Candler@...>
Date: 2003-05-17T17:57:32+09:00
Subject: Re: [OO design] Objects VS Datastructures

On Sat, May 17, 2003 at 05:43:37PM +0900, henq <henq _ replace 0 by o wrote:
> Validation and formatting gives you more than enough reasons to write smart
> getters and setters. Some ideas for you:
> - If country is USA, than only one of the 52 (?) states are allowed
> - If country is USA tha format phone in standard US style
> - International address labels are complex. Streetnumber before or after
> street, for example.
> - If no postal address given, give visit address instead, and the other way
> around, (the latter unless p.o. box involved)

That imposes orderings in which the attributes can be set (e.g. you must set
the country before setting the phone number). It might be a better solution
for the Addressbook itself to perform validation at the point where you
insert a new or updated entry: that way, when you check out an address and
modify it, you can make a series of changes (some of which temporarily leave
the entry in an inconsistent state, like country=UK but postcode=US zip
code), until you write it back.

Alternatively, if you want addresses to be self-validating, it might be
better to have a separate class for each country of interest:

    class Address               # general purpose rest-of-world address
    class USAddress < Address   # US zip-code and phone number
    class UKAddress < Address   # UK postcode and phone number
    ... etc

where method 'format_address_label' does the right thing in each case.

Then you'd have a factory method for creating new addresses, which would
force you to provide the country code first:

    MakeAddress('US')   => returns a USAddress
    MakeAddress('BE')   => returns an Address

You still wouldn't be able to modify someone's address from country X to
country Y: you'd have to create a brand new Address object to replace the
existing Address object. In which case for consistency maybe all Address
objects should be immutable?

Regards,

Brian.