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What is a Classifier?

Smartpedia: A classifier is a metaclass in UML that can be used to describe common properties of model elements such as class, interface, and component.

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.


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In the current UML 2.5 description of the OMG, you will find a whole series of classifier descriptions in chapter 7.8.

Here you will find additional information from our Smartpedia section:

Smartpedia: What is a Structure Diagram?

What is a Structure Diagram?

Smartpedia: What is a Behavior Diagram?

What is a Behavior Diagram?