Each use case actor is also a system actor. We
could calculate most of the associations between the system and the actors from
the use case model. To this end, we can use the UML construct of a derived association
, which is denoted by a slash ( Figure 2.42 ).
Unfortunately, in practice we often have to deal with the problem that derived
associations are poorly or not at all supported by modeling tools. On the other
hand, since the redundancy is controllable it is not dangerous to leave it in the
model. Bear in mind that redundancy occurs in the model only later. We don ??™ t
have use cases yet in the fi rst step of modeling the system context.
In Section 3.5 you will fi nd more techniques for actor modeling, such as generalization
of actors, the OR relationship, and multiplicities in the actor/use case
relationship that we haven ??™ t used here.
FIGURE 2-42
A derived association.
bdd [package] System context [derived association]
Customer
?«system?»
On-board computer
/
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We can communicate directly with the domain experts about the use cases just
as we do about the system context model.
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