How UML constrains subsetted properties

Classes are by far the most commonly encountered UML modeling element. As most users of the language have heard hundreds of times, classes do not exist in isolation for any given object system. Classes are often related to other classes by an association element. At each end of an association is a Property element. These property elements are generally not modeled directly but exist in the UML meta-model. In the context of a class associated with another class, these Property elements correlate to attributes of the two classes. These properties can be subsets of other properties and classes. In the UML model, this would be specified by applying the subsets tagged-value along with the context for the value. This values both modelers and software tools a better clue about how the design is organized. The UML specifies constraints that must be satisfied for a Property subset to be considered valid.

The following is an illustration of what takes place when the constraints of a subsetted property are validated.



First, the subsetting property and the subsetting context are both checked for emptiness. If so, the subset is not valid. Next, the validation will iterate through each subsetting context element and each subsetting property element. It is in this iteration that the validation process will ensure that the elements of both sets conform with one another.