Classifier Roles

A classifier role is a model element that describes a specific role played by a classifier participating in a collaboration without specifying an exact instance of a classifier. A classifier role is neither a class nor an object. Instead, it is a model element that specifies the kind of object that must ultimately fulfill the role in the collaboration. The classifier role limits the kinds of classifier that can be used in the role by referencing a base classifier. This reference identifies the operations and attributes that an instance of a classifier will need in order to fulfill its responsibilities in the collaboration.

Classifier roles are commonly used in collaborations that represent patterns. For example, a subject-observer pattern may be used in a system. One classifier role would represent the subject, and one would represent the observer. Each role would reference a base class that identifies the attributes and operations that are needed to participate in the subject-observer collaboration. When you use the pattern in the system, any class that has the specified operations and behaviors can fill the role.

Shape

A classifier role appears as a rectangle. Its name is prefixed with a slash and is not underlined. In sequence diagrams, a lifeline (a dashed, vertical line) is attached to the bottom of a classifier role to represent its life over a period of time. For details about lifelines, see Lifelines.


Classifier Role	Classifier Role with Lifeline

Using Classifier Roles

Classifier roles can appear on a model to represent the following:

In models depicting role-based interactions, a classifier role represents an instance in an interaction. Using classifier roles instead of objects can provide two advantages: First, a class can serve as the base classifier for multiple classifier roles. Second, instances of a class can realize multiple classifier roles in one or more collaborations.
In models depicting patterns, a classifier role specifies the kind of object that must ultimately fulfill a role in the pattern. The classifier role shows how the object will participate in the pattern, and its reference to a base class defines the attributes and operations that are required for participation in the pattern. When the pattern is used in the model, classes are bound to the collaboration to identify the type of objects that realize the classifier roles.

The classifier roles in a model are usually contained in a collaboration and usually appear in sequence diagrams.

Naming Conventions

The name of a classifier role consists of a role name and base class name. You can omit one of the names. The following table identifies the variations of the naming convention.


Convention	Example	Description
/rolename:baseclass	/courseOffering:course	The courseOffering role is based on the course class.
/rolename	/courseOffering	Role name. The base class is hidden or is not defined.
:baseclass	:course	Unnamed role based on the course class.