Re: UML-ish question

Responding to Jhinchey...

I came across something on a UML class diagram today that I have never
seen before and I'm wondering about the validity of it. The diagram
has a class which has an association with itself (a line coming out of
the class, making 2 90 degree turns and going back into itself).

Is that legal UML? If so, what does it mean? I think they are trying
to show an association between two instances of the class, which I
don't think belongs on the class diagram.

Yes, it is quite legal and fairly common. It is known as a reflexive relationship. The problem you are having is over the difference between static structure and individual participation in relationships.

For example, the organization of a POS web site's pages is typically in a hierarchical tree. One can express that very succinctly as:

[Web Site]
| 0..1
|
| R1
|
| rooted at
| 1 0..* parent of
[Page] ----------------+
| 0..1 |
| child of | R2
| |
+-------------------+

Basically this says that any web page fits into the hierarchy through child/parent relationships. The conditionality just reflects that leaf Pages don't have children and the root Page doesn't have a parent. [In a real POS web site one would probably use a more complex structure that would eliminate the conditionality, but this is just an example.]

The reason that this is valid is because the relationship defines that static structure of individual relationships between [Page] objects, but it doesn't describe who the objects are related to on an individual basis. So your second paragraph is close to the justification; it is instance based. For example, in a real POS site the tree might look like:

[Home]
/ \
/ \
[Computers] [Books]
/ \ / \
/ \ / \
[Laptops] [Desktops] [Fiction] [Nonfiction]

Here I am representing Page identity in the brackets, rather than classes. Thus every [...] represents a specific [Page] object.

The parent of [Computers] and [Books] happens to be [Home] in both cases here, but that would usually not be true for a tree with more levels. However, the children of [Computers] are entirely different pages than the children of [Books]. Thus at the level of the Computers and Books objects, their participation in the static structure is quite different.

So at the instance level the relationships have different participants. But all Pages participate in the same parent/child relationship structure.


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H. S. Lahman
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.

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