Extend relationships

In UML modeling, you can use an extend relationship to specify that one use case (extension) extends the behavior of another use case (base). This type of relationship reveals details about a system or application that are typically hidden in a use case.

The extend relationship specifies that the incorporation of the extension use case is dependent on what happens when the base use case executes. The extension use case owns the extend relationship. You can specify several extend relationships for a single base use case.

While the base use case is defined independently and is meaningful by itself, the extension use case is not meaningful on its own. The extension use case consists of one or several behavior sequences (segments) that describe additional behavior that can incrementally augment the behavior of the base use case. Each segment can be inserted into the base use case at a different point, called an extension point.

The extension use case can access and modify the attributes of the base use case; however, the base use case is not aware of the extension use case and, therefore, cannot access or modify the attributes and operations of the extension use case.

You can add extend relationships to a model to show the following situations:

Extend relationships do not have names.

As the following figure illustrates, an extend relationship is displayed in the diagram editor as a dashed line with an open arrowhead pointing from the extension use case to the base use case. The arrow is labeled with the keyword «extend».

Two use cases, called Base use case and Extension use case, are connected by a dashed line. An open arrowhead points from the Extension use case to the Base use case. The word extend, enclosed by double angle brackets, is displayed on the dashed line.

Example

You are developing an e-commerce system in which you have a base use case called Place Online Order that has an extending use case called Specify Shipping Instructions. An extend relationship points from the Specify Shipping Instructions use case to the Place Online Order use case to indicate that the behaviors in the Specify Shipping Instructions use case are optional and only occur in certain circumstances.

Parent topic: Relationships between model elements

Related tasks
Extending the behavior of use cases
Adding extend relationships to modeling diagrams
Creating an extension point for a use case

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