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How can I specify a conditions for method parameters in a UML class diagram?

For example the following method:

+grade(n : Integer)

The value for n should be between 1 and 6. Is it possible to do this? Or is there a different way like:

"+grade(n oneOf [1,2,3,4,5,6]"

Thx for your help.

Mr. Moose
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  • A further way apart from the already mentioned ones, would be using a data type. I.e., you define a data type and use this as the type of your parameter. The underlying primitive type and the value constraints can be defined in the type itself. This especially helpful when the consistency between several usages is required. – sim May 26 '21 at 16:00

2 Answers2

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Your 2nd approach is not allowed from UML's syntax. If you are in a certain domain you have control over you could document the use of that and it would be ok. However, the better approch would be to attach a constraint to that operation. You can use free text inside curly brackets attached as note. Or, if you're more daring, you can write a proper OCL. This from my POV only desirable in academic circles.

qwerty_so
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To elaborate on qwerty_so's answer:

The UML constraint could be expressed, just next to the operation in the class:

  • in plain-text, like: { n between 1 and 6 }

  • using OCL syntax, like: { n>0 and n<=6 }

If it is a longer expression, you can also put it in a note symbol that is attached to the operation (in UML we say "operation" rather than "method").

More complex constraints could also be expressed in plain OCL, indicating a context to which it applies (e.g. a specific class, property or operation). The constraint can express invariants (inv:), and in the case of operations, also preconditions (pre:) and post-conditions (post:). In your case, it would look like :

 context: MyClass::grade(n : Integer)
   pre: n>0 and n<=6     -- expression could involve properties using prefix 'self.'

This could be expressed in an UML note attached to the class. But since it can be really complex, especially if you want to document the contract for each operation, it could also be held separately (e.g. in a constraint specification windows of your modelling tool, or as a text in complement to a diagram or a model).

Christophe
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