"extends" versus "aggregates" in UML CRC

Hi,

This is just a trivial matter, I think the strictly technical answer is
"whichever you prefer", but I find it interesting and I'm googling for
brick walls so I'd like to hear what the general consensus is on this

I've been trudging through the PHP docs & UML 1.5 specification to try and
figure out whether it's correct to say "class a extends b" is an extension
or an aggregation, in the context of a CRC diagram.

I've been using extends up until now, but I have a feeling that aggregates
is what I should be using. The problem is that PHP has (or had) it's own
aggregation functions, so while it could be correct in other languages,
and perhaps in PHP 5, to call it aggregation, in PHP 4 I have no idea.

I'd appreciate your opinions on this - thanks in advance

Sincerely,
Kelvin

"Kelvin Mackay" <kelvin@nospam.sands-design.com> wrote in message
news:opslreaw06krqzhn@snoopy...
> Hi,
>
> This is just a trivial matter, I think the strictly technical answer is
> "whichever you prefer", but I find it interesting and I'm googling for
> brick walls so I'd like to hear what the general consensus is on this
>
> I've been trudging through the PHP docs & UML 1.5 specification to try and
> figure out whether it's correct to say "class a extends b" is an extension
> or an aggregation, in the context of a CRC diagram.
>
> I've been using extends up until now, but I have a feeling that aggregates
> is what I should be using. The problem is that PHP has (or had) it's own
> aggregation functions, so while it could be correct in other languages,
> and perhaps in PHP 5, to call it aggregation, in PHP 4 I have no idea.
>
> I'd appreciate your opinions on this - thanks in advance
>
> Sincerely,
> Kelvin

Kelvin,

In PHP, we say that class B "extends class A". In UML, I think we might
call this an "association". Take a look at
http://www.objectmentor.com/resource...ssDiagrams.pdf and see
what you think.

George

George King wrote:
> "Kelvin Mackay" <kelvin@nospam.sands-design.com> wrote in message
> news:opslreaw06krqzhn@snoopy...
>
>>Hi,
>>
>>This is just a trivial matter, I think the strictly technical answer is
>>"whichever you prefer", but I find it interesting and I'm googling for
>>brick walls so I'd like to hear what the general consensus is on this
>>
>>I've been trudging through the PHP docs & UML 1.5 specification to try and
>>figure out whether it's correct to say "class a extends b" is an extension
>>or an aggregation, in the context of a CRC diagram.
>>
>>I've been using extends up until now, but I have a feeling that aggregates
>>is what I should be using. The problem is that PHP has (or had) it's own
>>aggregation functions, so while it could be correct in other languages,
>>and perhaps in PHP 5, to call it aggregation, in PHP 4 I have no idea.
>>
>>I'd appreciate your opinions on this - thanks in advance
>>
>>Sincerely,
>>Kelvin
>
>
> Kelvin,
>
> In PHP, we say that class B "extends class A". In UML, I think we might
> call this an "association". Take a look at
> http://www.objectmentor.com/resource...ssDiagrams.pdf and see
> what you think.
>
> George
>
>

In my understanding of UML, an extension (or inheritance) is a separate
issue from composition (or aggregation).

- Extension/inheritance describes an "is-a" relationship. e.g.

class Animal
{
int age;

function think() { ... }
function eat() { ... }
}

class Dog extends Animal
{
string breed;

function wagTail() { ... }
}

A dog *is* an animal.

- Aggregation/composition describes "has-a" relationship. This is a type
of UML association. e.g.

class Office
{
Worker manager;
Worker employees[];
Desk desks[];
}

An Office *has* a manager, *has* some employees, and *has* some desks.

Two fundamentally different concepts.

--
Oli

On Sun, 06 Feb 2005 17:16:03 GMT, Oli Filth
<oli_filth@eatspam.coldmail.com> wrote:

> George King wrote:
>> "Kelvin Mackay" <kelvin@nospam.sands-design.com> wrote in message
>> news:opslreaw06krqzhn@snoopy...
>>
>>> Hi,
>>>
>>> This is just a trivial matter, I think the strictly technical answer
>>> is "whichever you prefer", but I find it interesting and I'm googling
>>> for brick walls so I'd like to hear what the general consensus is on
>>> this
>>>
>>> I've been trudging through the PHP docs & UML 1.5 specification to try
>>> and figure out whether it's correct to say "class a extends b" is an
>>> extension or an aggregation, in the context of a CRC diagram.
>>>
>>> I've been using extends up until now, but I have a feeling that
>>> aggregates is what I should be using. The problem is that PHP has (or
>>> had) it's own aggregation functions, so while it could be correct in
>>> other languages, and perhaps in PHP 5, to call it aggregation, in PHP
>>> 4 I have no idea.
>>>
>>> I'd appreciate your opinions on this - thanks in advance
>>>
>>> Sincerely,
>>> Kelvin
>> Kelvin,
>> In PHP, we say that class B "extends class A". In UML, I think we
>> might call this an "association". Take a look at
>> http://www.objectmentor.com/resource...ssDiagrams.pdf and
>> see what you think.
>> George
>
> In my understanding of UML, an extension (or inheritance) is a separate
> issue from composition (or aggregation).
>
> - Extension/inheritance describes an "is-a" relationship. e.g.
>
> class Animal
> {
> int age;
>
> function think() { ... }
> function eat() { ... }
> }
>
> class Dog extends Animal
> {
> string breed;
>
> function wagTail() { ... }
> }
>
> A dog *is* an animal.
>
> - Aggregation/composition describes "has-a" relationship. This is a type
> of UML association. e.g.
>
> class Office
> {
> Worker manager;
> Worker employees[];
> Desk desks[];
> }
>
> An Office *has* a manager, *has* some employees, and *has* some desks.
>
> Two fundamentally different concepts.
>


Thanks for your replies - the document & example have certainly made
things clearer.
In the PDF I noticed a relationship that I somehow hadn't considered -
"inherit". I think this most accurately describes the 'extends' operation,
so I'll use it from now on.
Thanks for your clarification of aggregation, I'll find it very useful now
that I know how to use it!


Kelvin