Object models

An object-oriented approach to the whole software development process is now
commonly used, particularly for interactive systems development. This means expressing
the systems requirements using an object model, designing using objects anddeveloping the system in an object-oriented programming language such as Java
or C++.
Object models that you develop during requirements analysis may be used
to represent both system data and its processing. In this respect. they combine
some of the uses of data-flow and semantic data models. They are also useful
for showing how entities in the system may be classified and composed of other
entities.
For some classes of system. object models are natural ways of reflecting the realworld
entities that are manipulated by the system. This is particularly true when the
system processes information about tangible entities. such as cars. aircraft or books.
which have clearly identifiable attributes. More abstract. higher-level entities, such
as the concept of a library. a medical record system or a word processor, are harder
to model as object classes. They do not necessarily have a simple interface consisting
of independent attributes and operations.
Developing object models during requirements analysis usually simplifies the transition
to object-oriented design and programming. However. I have found that endusers
of a system often find object models unnatural and difficult to understand.
They may prefer to adopt a more functional, data-processing view. Therefore. it is
sometimes helpful to supplement object models with data-flow models that show
the end-to-end data processing in the system.
An object class is an abstraction over a set of objects that identifies common
attributes (as in a semantic data model) and the services or operations that are provided
by each object. Objects are executable entities with the attributes and services
of the object class. Objects are instantiations of the object class, and many
objects may be created from a class. Generally, the models developed using analysis
focus on object classes and their relationships.
In object-oriented requirements analysis, you should model real-world entities
using object classes. You should not include details of the individual objects
(instantiations of the class) in the system. You may create different types of object
models, 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.The analysis process for identifying objects and object classes is recognised as
one of the most difficult areas of object-oriented development. Object identification
is basically the same for analysis and design. The methods of object identification
covered in Chapter 14. which discusses object-oriented design, may be used.
I concentrate here on some of the object models that might be generated during the
analysis process.
Various methods of object-oriented analysis were proposed in the 1990s (Coad and
Yourdon. 1990; Rumbaugh, et al., 1991; Jacobsen, et aI., 1993; Booch, 1994). These
methods had a great deaI in common, and three of the key developers (Booch.
Rumbaugh, and Jacobsen) decided to integrate their approaches to produce a unified
method (Rumbaugh et aI., 1999b). The Unified Modeling Language (UML) used in this
unified method has become a standard for object modelling. The UML includesnotations for different types of system models. We have already seen use-case models
and sequence diagrams in earlier chapters and state machine models earlier in this chapter.
An object class in UML, as illustrated in the examples in Figure 8.10, is represented
as a vertically oriented rectangle with three sections:
I. The name of the object class is in the top section.
2. The class attributes are in the middle section.
3. The operations associated with the object class are in the lower section of the
rectangle.
I don't have space to cover all of the UML, so I focus here on object models
that show how objects can be classified and can inherit attributes and operations
from other objects, aggregation models that show how objects are composed, and
simple behavioural models, which show object interactions.