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186 Chapter 7 Design and implementation

Weather
Information System

:SatComms :WeatherStation :Commslink :WeatherData

request(report)

acknowledge
reportWeather( )
acknowledge get(summary) summarize( )

send(report)
acknowledge
reply(report)

acknowledge

2. Sequence models, which show the sequence of object interactions. These are
Figure 7.7 Sequence
diagram describing represented using a UML sequence or a collaboration diagram. Sequence
data collection models are dynamic models.

3. State machine model, which show how individual objects change their state in
response to events. These are represented in the UML using state diagrams.
State machine models are dynamic models.
A subsystem model is a useful static model as it shows how a design is organized into
logically related groups of objects. I have already shown this type of model in Figure 7.4
to show the subsystems in the weather mapping system. As well as subsystem models,
you may also design detailed object models, showing all of the objects in the systems
and their associations (inheritance, generalization, aggregation, etc.). However, there is
a danger in doing too much modeling. You should not make detailed decisions about the
implementation that really should be left to the system programmers.
Sequence models are dynamic models that describe, for each mode of interaction,
the sequence of object interactions that take place. When documenting a design, you
should produce a sequence model for each significant interaction. If you have devel-
oped a use case model then there should be a sequence model for each use case that
you have identified.
Figure 7.7 is an example of a sequence model, shown as a UML sequence dia-
gram. This diagram shows the sequence of interactions that take place when an
external system requests the summarized data from the weather station. You read
sequence diagrams from top to bottom:
1. The SatComms object receives a request from the weather information system
to collect a weather report from a weather station. It acknowledges receipt of