Sommerville-Software-Engineering-10ed

7.1 ■ Object-oriented design using the UML 203
identifier
WeatherStation
reportWeather ( )
reportStatus ( )
powerSave (instruments)
remoteControl (commands)
reconfigure (commands)
restart (instruments)
shutdown (instruments)
WeatherData
airTemperatures
groundTemperatures
windSpeeds
windDirections
pressures
rainfall
collect ( )
summarize ( )
Figure 7.6 Weather
station objects
Ground
thermometer
gt_Ident
temperature
get ( )
test ( )
Anemometer
an_Ident
windSpeed
windDirection
get ( )
test ( )
Barometer
bar_Ident
pressure
height
get ( )
test ( )
such as offices, organizational units such as companies, and so on (Wirfs-Brock,
Wilkerson, and Weiner 1990).
3. Use a scenario-based analysis where various scenarios of system use are identified
and analyzed in turn. As each scenario is analyzed, the team responsible for
the analysis must identify the required objects, attributes, and operations (Beck
and Cunningham 1989).
In practice, you have to use several knowledge sources to discover object classes.
Object classes, attributes, and operations that are initially identified from the informal
system description can be a starting point for the design. Information from application
domain knowledge or scenario analysis may then be used to refine and extend the initial
objects. This information can be collected from requirements documents, discussions
with users, or analyses of existing systems. As well as the objects representing
entities external to the system, you may also have to design “implementation objects”
that are used to provide general services such as searching and validity checking.
In the wilderness weather station, object identification is based on the tangible
hardware in the system. I don’t have space to include all the system objects here, but
I have shown five object classes in Figure 7.6. The Ground thermometer,
Anemometer, and Barometer objects are application domain objects, and the
WeatherStation and WeatherData objects have been identified from the system
description and the scenario (use case) description:
1. The WeatherStation object class provides the basic interface of the weather station
with its environment. Its operations are based on the interactions shown in
Figure 7.3. I use a single object class, and it includes all of these interactions.
Alternatively, you could design the system interface as several different classes,
with one class per interaction.