Clever Geek Handbook

Association in UML

How I read Appendix 1 to Software Development from Roger Pressman's book, which

a relationship between two classes means there is a structural relationship between them

what structural relationships mean?

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The UML differentiates between "structural" and "behavioral" models. Class diagrams, package diagrams and some other aspects of structural aspects. State / Sequence / Activity Diagrams capture behavioral aspects.

"Structural" means that it lasts over time. For example, the relationship between Order and OrderLines ("An order consists of 1 or more OrderLines / OrderLine is part of exactly one order"). Or Dog and Human ("A Dog belongs to only one Human / Human, to whom many Dogs belong"). Used well, associations capture invariant rules from the problem domain. To use the Dog example: the association says that a dog can never belong to more than one Person at any given time. It doesn't matter if the dog is working, sitting or riding: it must have exactly one Owner. Please note that the owner can change over time: but at any time there cannot be more than one.



An alternative is to think of associations as something that can be captured using foreign keys in a relational database.

NTN.

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