Smalltalk and Object Orientation: an Introduction - Free

18. UML: Dynamic Modeling And Deployment
18.1 Introduction
The basic class and association notations of the object model in the Unified Modeling Language (or
UML for short) were presented in the last chapter. This chapter presents the sequence, collaboration
diagrams and state diagrams as part of the dynamic modeling facilities in the UML. We then consider
deployment diagrams.
18.2 Dynamic modeling facilities
18.2.1 Sequence diagrams
A scenario shows a particular series of interactions among objects in a s ingle execution of a system.
That is, it is a history of how the system behaved from one start state to a single termination state. This
differs from an envisionment which is a description of all system behaviors from all start states to all
end states. En visionments thus contain all possible histories (although they may also contain paths
which the system is never intended to take).
Scenarios can be presented in two different ways: Sequence Diagrams and Collaboration Diagrams.
Both these diagrams present t he same information although they stress different aspects of this
information. For example, sequence diagrams stress the timing aspects of the interactions amongst the
objects, where as the collaboration diagrams stress the structure between these objects (which helps in
understanding the requirements of the underlying software structure).
Figure 18.1 illustrates the basic structure of a sequence diagram. The objects involved in the
exchange of messages are represented as vertical lines (which are labeled with the object’s name). For
example, Caller, Phone Line and Callee are all objects involved in the scenario of dialing 999 (the
Emergency services’ number in the UK). Time proceeds vertically down the diagram, as indic ated by
the dashed line arrow. The horizontal arrows indicate an event or message sent from one object to
another. The arrow indicates the direction in which the event or message is sent. That is, the receiver is
indicated by the head of the arrow. Normall y return values are not shown on these diagrams. However,
if they are significant, they can be illustrated by return events annotated with the type of value returned
etc.
Time proceeds vertically and can be made more explicit by additional timing marks. T hese timing
marks indicate how long the gap between messages should be or how long a message or event should
take to get from the sender to the receiver.
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