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What is an Association?

Smartpedia: A UML association describes a relationship between classifiers, e.g. bidirectional relationships between classes or interfaces.

Association as a linking of elements or aspects

The term association is used in many areas. In psychology the association describes the conscious or unconscious linking of thoughts, in Marximus a union of workers and in chemistry a grouping of several molecules to form larger associations. In the Unified Modeling Language (UML) as the graphical modeling language of software and system development, an association describes a relationship between classifiers.

A Relationship between UML Classifiers

Associations in UML usually express relationships between

  • two classes,
  • two interfaces or even
  • two use cases.

An association with more than two model elements is called an n-ary association and rarely occurs in the practice of modeling software or systems.

An association that refers to the same model element is reflexive.

Graphically, an association is represented by a line between the two model elements and is referred to as a link at the instance level. In the case of an n-ary association, the visualisation is carried out using a diamond and adjoining lines. The ends of an association are also independent model elements; they are modeled with the model element Property and can have a name, a multiplicity, a visibility and a navigability.

Association in UML

Navigability in the UML Association

Basically, there are three types of navigability:

  • The permitted navigation from one model element to another; visualised with an arrow at the end of the association.
  • The unauthorized navigation from one model element to another, displayed with a small x at the end of the association.
  • No specific statement.

Associations can also be classified as unidirectional or bidirectional: An association that allows only one direction is unidirectional, a two-way navigation correspondingly bidirectional. With aggregation and composition there are two special cases of association.

What does t2informatik do?

Was does t2informatik do? One click and you'll know it.


Here you find a video about Association and Multiplicity.

And here you will find additional information from our Smartpedia section:

Smartpedia: What is an UML Composition?

What is an UML Composition?

Smartpedia: What is an Aggregation?

What is an Aggregation?