Classifier – generalisation of UML model elements
Classifier as a term is used in various areas. In the context of machine learning, a classifier is, for example, a special case of a hypothesis that assigns a label to a data point (e.g. when an e-mail is classified as spam or non-spam or when document properties and tags are automatically recognised and assigned). In UML, a classifier is a so-called metaclass. A metaclass is the class of a class and therefore also the basis for the instance of a class.
As a metaclass, the Classifier is particularly useful for authors of UML – the Object Management Group (OMG) – because it can be used to describe common properties of UML model elements such as class, interface, component, attribute, method, activity, or inheritance. Each model element is thus a spezialisation of the classifier.
For UML users, a classifier is merely an intellectual construct that they cannot use in any of the seven structure diagrams
- class diagram,
- composition structure diagram,
- component diagram,
- distribution diagram,
- object diagram or
- package diagram,
or in any of the three behavior diagrams
- activity diagram,
- use case diagram or
- state (machine) diagram,
or in one of the four interaction diagrams
- sequence diagram
- interaction overview diagram,
- communication diagram or
- timing diagram.
In the current UML 2.5 description of the OMG, you will find a whole series of classifier descriptions in chapter 7.8.
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