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132 Chapter 5 System modeling

Doctor

Hospital General
Doctor Practitioner

Consultant Team Doctor

Trainee Qualified
Figure 5.11 A generalization Doctor Doctor
hierarchy

these classes. This allows us to infer that different members of these classes have some
common characteristics (e.g., squirrels and rats are rodents). We can make general state-
ments that apply to all class members (e.g., all rodents have teeth for gnawing).
In modeling systems, it is often useful to examine the classes in a system to see
if there is scope for generalization. This means that common information will be
maintained in one place only. This is good design practice as it means that, if
changes are proposed, then you do not have to look at all classes in the system to
see if they are affected by the change. In object-oriented languages, such as Java,
generalization is implemented using the class inheritance mechanisms built into the
language.
The UML has a specific type of association to denote generalization, as illus-
trated in Figure 5.11. The generalization is shown as an arrowhead pointing up to
the more general class. This shows that general practitioners and hospital doctors
can be generalized as doctors and that there are three types of Hospital Doctor—
those that have just graduated from medical school and have to be supervised
(Trainee Doctor); those that can work unsupervised as part of a consultant’s team
(Registered Doctor); and consultants, who are senior doctors with full decision-
making responsibilities.
In a generalization, the attributes and operations associated with higher-level
classes are also associated with the lower-level classes. In essence, the lower-level
classes are subclasses inherit the attributes and operations from their superclasses.
These lower-level classes then add more specific attributes and operations. For
example, all doctors have a name and phone number; all hospital doctors have a staff
number and a department but general practitioners don’t have these attributes as they
work independently. They do however, have a practice name and address. This is
illustrated in Figure 5.12, which shows part of the generalization hierarchy that I
have extended with class attributes. The operations associated with the class Doctor
are intended to register and de-register that doctor with the MHC-PMS.