Use case diagrams

 

Definition

Use case diagrams describe the most important services rendered by the system.

 

Starting with actors, external participants who interact with the system, use cases represent the most important cases of system operation.  A use case can then be sub-divided into sequence diagrams, each of which details a different function of one use case.

 

Use case diagrams (an example is shown below) are created in packages, classes, interfaces, actors or components.

 

An example of a use case diagram

 

 

Creating use cases

Use cases are created in use case diagrams by clicking on the  "Create a use case" icon in the diagram palette and then clicking in the diagram over the spot where you wish to create the new use case.

 

 

Graphic representation of use cases

Use cases can be graphically represented in the following forms:

·         elliptical form (the use case is presented as an ellipse with its name appearing in the center)

·         in box form (the use case is presented as a rectangle with its name appearing in the top section of the rectangle)

·         in stereotype form (the use case is graphically represented by the use case stereotype with its name below

 

In a use case diagram, a use case never appears on its own.  It is always displayed inside its parent element.  This parent element is referred to in one of two ways:

·         either as the use case's subject, in which case it is graphically represented as a box with a name label inside the box on the top left corner

·         or as the parent element, in which case it is presented as a box with a name label outside the box on the top left corner

 

 

Creating actors

Actors are created in use case diagrams by clicking on the  "Create an actor" icon in the diagram palette and then clicking in the diagram over the spot where you wish to create the new actor.

 

 

Creating extension points

Extension points can be created inside a use case to identify specific locations within the use case where the behavior of the given use case can be augmented with elements of the behavior of another (extending) use case.

 

Extension points are created in use case diagrams by clicking on the  icon in the diagram palette and then clicking in a use case.

 

 

Creating inclusion and extension relationships

Inclusion relationships are used to denote that a given use case contains another use case's functionality as part of its normal processing.  Inclusion relationships are created in use case diagrams by clicking on the "Create an inclusion relationship" icon in the diagram palette, and then clicking on the two use cases you want to link.

 

Extension relationships are used to denote that a given use case extends the behavior of another use case.  Extension relationships are created in use case diagrams by clicking on the  "Create an extension relationship" icon in the diagram palette, and then clicking on the two use cases you want to link.

 

 

Creating communication links

Communication links represent the interaction that a user who is external to the system can have with the system in specific use modes.  Communication links are created in use case diagrams by clicking on the  icon in the diagram palette, and then clicking first on an actor and then on a use case. 

 

 

Migrating UML 1.4 use case diagrams to UML 2 use case diagrams

If your model contains use case diagrams created using an earlier version of Objecteering, these UML 1.4 use case diagrams are automatically migrated to UML 2 through the automatic creation of a UML 2 use case diagram when the UML 1.4 use case diagram is opened.  This UML 2 diagram has the same name as the earlier diagram, belongs to the same model element and is directly displayed.

 

The new diagram retains the following information from the migration of the earlier diagram:

 

Association labels are positioned using the new intelligent positioning system implemented in UML 2 diagrams.

 

Elements not managed by diagram migration are as follows:

 

Since the earlier UML 1.4 use case diagram is conserved, it can still be opened after migration in the usual way (either by double-clicking on it in the Objecteering explorer, or through the "Edit" command in the context menu).  The diagram is then opened in read-only mode, meaning that only the zoom and grab functions are active.

 

We recommend that the UML 1.4 use case diagram be deleted once it has been migrated.  If the new UML 2 use case diagram is renamed, the migration procedure will once again be run the next time the UML 1.4 use case diagram is opened.