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Can requirements be expressed in class-diagrams?

For example:

A student can enroll in a class. There are different classes. Some classes have dependencies, e.g. you can't go to Spanish III before you went to Spanish II and Spanish I.

How to model that?

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    Usually requirements are expressed as user stories. Sometimes the problem is clear enough to start writing classes but we usually want to keep that conceptual at the beginning. (Look up CRC cards). Class Responsibility Collaborator cards are a way to think about what classes you may need before writing the details of their implementations.
    – John Douma
    Commented Mar 25, 2020 at 2:19

2 Answers 2

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No, most requirements can not be fully expressed in a class diagram.

A class diagram shows the static structure of how classes are related to each other, but many requirements also have a dynamic aspect to them. However, requirements do drive the structure of the class diagram.

For example, to be able to fulfil the requirement that a Student must have followed the Class "Spanish I" before being allowed to enroll in "Spanish II", the class diagram must show an optional relation of Class with itself to record the possible prerequisites for a class.

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Requirements can be expressed either via requirement elements you can create on your own with a profile (there are already a couple of profiles offering that kind of element). However, more quickly you can just add constraints derived from requirements and plaster them in your class diagram - linked up to where they shall belong. A constraint can be represented as a {note with text in curly brackets}. Eventually you can stereotype that with <<pre-condition>>, <<post-condition>> or <<invariant>>.

With respect to the requirements you listed: they are related to runtime and are a better fit for use cases. The above is more for things like "student names may not exceed 20 chars or the like.

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