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I'm pretty new at UML modelling so bear with me.

Say I have a class with an attribute students that is a list of Student instances. I have set an aggregation relation to the Student class which is also in the diagram. Does this relation replace an attribute entry such as #students : Student[] in the container class, or can the attribute and relation co-exist?

Thanks in advance for any answers!

Victor
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3 Answers3

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Short answer: Can the attribute and relationship both exist? No...

But if your classes are software classes you may have many choices for showing them:

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But if your classes are not sofware classes and you model the domain you should be more carefully to show an entity as an attribute or class. Generally if the entity is complex type, it is shown as seperate class. But that is not a rule: depends on context.

enter image description here

Novalis
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An attribute helps to identify the structure of the object. Aggregation gives a high level overview that the object will contain one (or more) of another type. Neither replaces, but instead shows the relationship between the object types.

Suroot
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You generally do not have attributes of a user-defined (or any complex) class. You certainly don't show an attribute AND an association for the same thing in UML. If I understand your situation, you're in need of an association (or aggregation/composition)... And play with the multiplicities to get it right.

Patrick87
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  • Thanks for the reponse. Why is it that one generally does not have attributes of a user-defined class? – Victor Jul 23 '11 at 18:38
  • Because that is exactly what associations are for. UML has separate notation for user-defined types. – Patrick87 Jul 23 '11 at 20:05