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188 Chapter 7 Design and implementation

Controlled

Operation
shutdown( ) remoteControl( )

restart( ) reportStatus( )
Shutdown Running Testing
transmission done
reconfigure( ) configuration done test complete
powerSave( )
clock collection
done Transmitting

Configuring Collecting reportWeather( )
weather
summary
complete
Summarizing

4. If a reportStatus() message is received, the system moves to the Testing state,
Figure 7.8 Weather
station state diagram then the Transmitting state, before returning to the Running state.
5. If a signal from the clock is received, the system moves to the Collecting state,
where it collects data from the instruments. Each instrument is instructed in turn
to collect its data from the associated sensors.
6. If a remoteControl() message is received, the system moves to a controlled state
in which it responds to a different set of messages from the remote control
room. These are not shown on this diagram.

State diagrams are useful high-level models of a system or an object’s operation.
You don’t usually need a state diagram for all of the objects in the system. Many of
the objects in a system are relatively simple and a state model adds unnecessary
detail to the design.

7.1.5 Interface specification

An important part of any design process is the specification of the interfaces between
the components in the design. You need to specify interfaces so that objects and sub-
systems can be designed in parallel. Once an interface has been specified, the devel-
opers of other objects may assume that interface will be implemented.
Interface design is concerned with specifying the detail of the interface to an
object or to a group of objects. This means defining the signatures and semantics of