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7.2 Design patterns 189

«interface»
«interface» Remote Control
Reporting
startInstrument(instrument): iStatus
weatherReport(WS-Ident): Wreport stopInstrument(instrument): iStatus
statusReport(WS-Ident): Sreport
Figure 7.9 Weather collectData(instrument): iStatus
provideData(instrument): string
station interfaces

the services that are provided by the object or by a group of objects. Interfaces can be
specified in the UML using the same notation as a class diagram. However, there is
no attribute section and the UML stereotype ‹‹interface›› should be included in the
name part. The semantics of the interface may be defined using the object constraint
language (OCL). I explain this in Chapter 17, where I cover component-based soft-
ware engineering. I also show an alternative way to represent interfaces in the UML.
You should not include details of the data representation in an interface design,
as attributes are not defined in an interface specification. However, you should
include operations to access and update data. As the data representation is hidden, it
can be easily changed without affecting the objects that use that data. This leads to
a design that is inherently more maintainable. For example, an array representation
of a stack may be changed to a list representation without affecting other objects
that use the stack. By contrast, it often makes sense to expose the attributes in a
static design model, as this is the most compact way of illustrating essential charac-
teristics of the objects.
There is not a simple 1:1 relationship between objects and interfaces. The same
object may have several interfaces, each of which is a viewpoint on the methods that
it provides. This is supported directly in Java, where interfaces are declared sepa-
rately from objects and objects ‘implement’ interfaces. Equally, a group of objects
may all be accessed through a single interface.
Figure 7.9 shows two interfaces that may be defined for the weather station. The
left-hand interface is a reporting interface that defines the operation names that are
used to generate weather and status reports. These map directly to operations in the
WeatherStation object. The remote control interface provides four operations, which
map onto a single method in the WeatherStation object. In this case, the individual
operations are encoded in the command string associated with the remoteControl
method, shown in Figure 7.6.

7.2 Design patterns

Design patterns were derived from ideas put forward by Christopher Alexander
(Alexander et al., 1977), who suggested that there were certain common patterns of
building design that were inherently pleasing and effective. The pattern is a description
of the problem and the essence of its solution, so that the solution may be reused in