Qualifiers on association ends

In UML, qualifiers are properties of binary associations and are an optional part of association ends. A qualifier holds a list of association attributes, each with a name and a type. Association attributes model the keys that are used to index a subset of relationship instances.

A qualifier is visually represented as a rectangle attached to the qualified end of the association relationship. The list of association attributes is displayed in the qualifier box.

Qualifiers identify subsets of related instances in association navigations; they provide a model of indices or keys for association ends. It is rare to see qualifiers on both ends of an association, because only the unqualified element typically varies in multiplicity.

In a navigation context, qualifiers are used to select a specific object pair from the set of all related objects in that association. In an implementation context, each qualifier value points to a unique target object. Generally, if an application requires the retrieval of data based on search keys, the model should use qualified associations.

Example

In a banking application, a class called Bank represents a banking institution and has an association with a class called Person, which represents an individual. Each individual is associated with the bank through several bank accounts. The account number qualifies the association, and it enables the indexing of many associations between the Person and Bank classes.

As the following figure illustrates, the qualifier is attached to the association end that corresponds to the Bank class.

This image shows a qualified association between a Bank class and a Person class

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