As in use case modelling, it is often very helpful to be able to annotate a diagram to show particular features. The notation is exactly the same:

Figure 5.8. The notation for notes

You can, and should, apply notes wherever you think they will be helpful to the reader, but they should not be viewed as a substitute for proper documentation. A class model is not complete until all classes, attributes, operations, and associations, have been documented. Notes are useful for pointing out specific details on diagrams.

Constraints are additional information about a class that are not associated with any particular attribute or operation. A typical constraint might say, for example, that one attribute will have a value which depends on some other attribute. It is customary to write simple constraints below the class in braces, {like this}.

Constraints are becoming increasingly important in object-oriented modelling, as there is an increasing interest in formalising the technique. A scheme called the object constraint language is under development to support formalised constraints, but this is beyond the scope of this course.