Associations (OOM)

An association represents a structural relationship between classes or between a class and an interface.

An association can be created in the following diagrams:

Class Diagram
Composite Structure Diagram

It is drawn as a solid line between the pair of objects.

In addition to naming the association itself, you can specify a role name for each end in order to describe the function of a class as viewed by the opposite class. For example, a person considers the company where he works as an employer, and the company considers this person as an employee.

Reflexive Association

A reflexive association is an association between a class and itself.

In the following example, the association Supervise expresses the fact that an employee can, at the same time, be a manager and someone to manage.

In the Dependencies tab of the class, you can see two identical occurrences of the association, this is to indicate that the association is reflexive and serves as origin and destination for the link.

Aggregation

An aggregation is a special type of association in which one class represents a larger thing (a whole) made of smaller things (the parts). This is sometimes known as a "has-a" link, and allows you to represent the fact that an object of the whole has objects of the part. In the following example, the family is the whole that can contain children.

You can create an aggregation directly using the Aggregation tool in the Palette. The aggregation symbol in a diagram is the following:

Composition

A composition is a special type of aggregation in which the parts are strongly tied to the whole. In a composition, an object may be a part of only one composite at a time, and the composite object manages the creation and destruction of its parts. In the following example, the frame is a part of a window. If you destroy the window object, the frame part also disappears.

You can create a composition directly using the Composition tool in the Palette. The composition symbol in a diagram is the following:

You can define one of the roles of an association as being either an aggregation or a composition. The Container property needs to be defined to specify which of the two roles is an aggregation or a composition.

Creating an Association
You can create an association in any of the following ways:
Associations Properties
You can modify an object's properties from its property sheet. To open an association property sheet, double-click its diagram symbol or its Browser entry in the Associations folder. The following sections detail the property sheet tabs that contain the properties most commonly entered for associations.
Association Implementation
Associations describe structural relationships between classes that become links between the instances of these classes. These links represent inter-object navigation that is to say the fact that one instance can retrieve another instance through the navigable link.
Creating an Association Class
You can add properties to an association between classes or interfaces by creating an association class. It is used to further define the properties of an association by adding attributes and operations to the association.
Migrating Association Roles in a Class Diagram
You can migrate association roles and create attributes before generation. This is very convenient for many reasons including data type customization and the ability to change the attribute order in the list of attributes. The last feature is especially important in XML.
Rebuilding Data Type Links
If a classifier data type is not linked to its original classifier, you can use the Rebuild Data Type Links feature to restore the link. This feature looks for all classifiers of the current model and links them, if needed, to the original classifier.
Linking an Association to an Instance Link
You can drag an association node from the Browser and drop it into the communication or object diagrams. This automatically creates two objects, and an instance link between them. Both objects are instances of the classes or interfaces, and the instance link is an instance of the association.

Parent topic: Building Structural Diagrams