Sommerville-Software-Engineering-10ed

146 Chapter 5 ■ System modeling
Register
patient
Unregister
patient
Medical
receptionist
View patient
info.
Transfer data
Figure 5.5 Use cases
involving the role
“Medical receptionist”
Contact
patient
in which the actor “Medical Receptionist” is involved. Each of these should be
accompanied by a more detailed description.
The UML includes a number of constructs for sharing all or part of a use case in
other use case diagrams. While these constructs can sometimes be helpful for system
designers, my experience is that many people, especially end-users, find them difficult
to understand. For this reason, these constructs are not described here.
5.2.2 Sequence diagrams
Sequence diagrams in the UML are primarily used to model the interactions between
the actors and the objects in a system and the interactions between the objects themselves.
The UML has a rich syntax for sequence diagrams, which allows many different
kinds of interaction to be modeled. As space does not allow covering all
possibilities here, the focus will be on the basics of this diagram type.
As the name implies, a sequence diagram shows the sequence of interactions that
take place during a particular use case or use case instance. Figure 5.6 is an example
of a sequence diagram that illustrates the basics of the notation. This diagram models
the interactions involved in the View patient information use case, where a medical
receptionist can see some patient information.
The objects and actors involved are listed along the top of the diagram, with a
dotted line drawn vertically from these. Annotated arrows indicate interactions
between objects. The rectangle on the dotted lines indicates the lifeline of the object
concerned (i.e., the time that object instance is involved in the computation). You
read the sequence of interactions from top to bottom. The annotations on the arrows
indicate the calls to the objects, their parameters, and the return values. This example
also shows the notation used to denote alternatives. A box named alt is used with the