Sommerville-Software-Engineering-10ed

7.1 ■ Object-oriented design using the UML 205
However, if a plan-based development process is used, you may need more
detailed models. When the links between requirements engineers, designers, and programmers
are indirect (e.g., where a system is being designed in one part of an organization
but implemented elsewhere), then precise design descriptions are needed for
communication. Detailed models, derived from the high-level abstract models, are
used so that all team members have a common understanding of the design.
An important step in the design process, therefore, is to decide on the design models
that you need and the level of detail required in these models. This depends on the type
of system that is being developed. A sequential data-processing system is quite different
from an embedded real-time system, so you need to use different types of design models.
The UML supports 13 different types of models, but, as I discussed in Chapter 5, many
of these models are not widely used. Minimizing the number of models that are produced
reduces the costs of the design and the time required to complete the design process.
When you use the UML to develop a design, you should develop two kinds of
design model:
1. Structural models, which describe the static structure of the system using object
classes and their relationships. Important relationships that may be documented
at this stage are generalization (inheritance) relationships, uses/used-by
relationships, and composition relationships.
2. Dynamic models, which describe the dynamic structure of the system and show
the expected runtime interactions between the system objects. Interactions that
may be documented include the sequence of service requests made by objects
and the state changes triggered by these object interactions.
I think three UML model types are particularly useful for adding detail to use
case and architectural models:
1. Subsystem models, which show logical groupings of objects into coherent subsystems.
These are represented using a form of class diagram with each subsystem
shown as a package with enclosed objects. Subsystem models are structural models.
2. Sequence models, which show the sequence of object interactions. These are
represented using a UML sequence or a collaboration diagram. Sequence models
are dynamic models.
3. State machine models, which show how 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 that 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 present the subsystems in the weather mapping system. As well as subsystem models,
you may also design detailed object models, showing the objects in the systems and their
associations (inheritance, generalization, aggregation, etc.). However, there is a danger