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5.3 Structural models 129

Object-oriented requirements analysis

In object-oriented requirements analysis, you model real-world entities using object classes. You may create
different types of object model, showing how object classes are related to each other, how objects are
aggregated to form other objects, how objects interact with other objects, and so on. These each present
unique information about the system that is being specified.

http://www.SoftwareEngineering-9.com/Web/OORA/

5.3 Structural models

Structural models of software display the organization of a system in terms of the
components that make up that system and their relationships. Structural models may
be static models, which show the structure of the system design or dynamic models,
which show the organization of the system when it is executing. These are not the
same things—the dynamic organization of a system as a set of interacting threads
may be very different from a static model of the system components.
You create structural models of a system when you are discussing and designing
the system architecture. Architectural design is a particularly important topic in soft-
ware engineering and UML component, package, and deployment diagrams may all
be used when presenting architectural models. I cover different aspects of software
architecture and architectural modeling in Chapters 6, 18, and 19. In this section, I
focus on the use of class diagrams for modeling the static structure of the object
classes in a software system.

5.3.1 Class diagrams
Class diagrams are used when developing an object-oriented system model to show
the classes in a system and the associations between these classes. Loosely, an object
class can be thought of as a general definition of one kind of system object. An asso-
ciation is a link between classes that indicates that there is a relationship between
these classes. Consequently, each class may have to have some knowledge of its
associated class.
When you are developing models during the early stages of the software engi-
neering process, objects represent something in the real world, such as a patient, a
prescription, a doctor, etc. As an implementation is developed, you usually need to
define additional implementation objects that are used to provide the required sys-
tem functionality. Here, I focus on the modeling of real-world objects as part of the
requirements or early software design processes.
Class diagrams in the UML can be expressed at different levels of detail. When you
are developing a model, the first stage is usually to look at the world, identify the
essential objects, and represent these as classes. The simplest way of writing these is to
write the class name in a box. You can also simply note the existence of an association