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umlaggregationclass-diagram

Proper modeling on UML class diagram

In an UML class diagram:

a) Do you have to state attributes that are aggregated? Or is it enough with the arrows indicating aggregation?

b) Do I have to add "id" as an attribute or is it a given?

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Thanks.

Solution

  • You are using a shared aggregation in the picture. That does not have any defined semantics as per UML 2.5 (see p. 110). If you need a composite aggregation the diamond must be filled. In that case the aggregated object will be deleted along with the aggregating one (the latter must assure that constraint). In your model it makes no sense. No employee aggregates a department. Even vice versa I would have doubts or at least reason for discussion.

    An id is only needed if it has a business purpose (e.g. an article number). If you transform your model to a database you introduce an artificial id for technical reasons. But on an abstract business level they are not modeled.

    Your models only differ in the use of attributes for associated classes. The B variant is preferred. But you need to place the attributes as role names towards the associated classes (as -department and -branch). What you have placed in the middle of the connectors is rather the association name. Badly chosen with the + in front. Naming associations is rarely needed. So get rid of that. Role names shall be placed near the class that takes the role. Also it's a good idea to use the dot-notation to show that the roles represent owned properties. Just place a small black dot near the left hand side of both (near where the role names should go).

    As for the dot-notation UML 2.5 states on p. 18:

    Dot notation is used to denote association end ownership, where the dot shows that the Class at the other end of the line owns the Property whose type is the Class touched by the dot. See 11.5.4 for details of Association notation and 11.5.5 for examples.

    enter image description here

    Also as JimL. commented the A-version uses associations plus attributes which introduces redundancy. This is not illegal but likely not intended and at least leads to confusion.