Databases Reference
In-Depth Information
Hardware
component
Software
component
Operating
system
PC server
Network
Code
Multiplexer
Drivers
Figure 6.1
The logical relationship of the components of ABC Corporations telephony
system.
entity. That means each variant could have different attributes and methods.
Hierarchical relationships infer a supertype or subtype association between
two or more objects. In the case of ABC Corporation, the pattern is clear that
we will be modeling systems, parts of systems, and parts of parts.
As a starting point for our modeling example (and a fundamental
construct in object-oriented methodology), we will create a base class, which
eventually leads us to a class structure. A base class equates to what is often
termed a generalization. The base class contains the high-level properties and
supporting functions common to all object classes. In object-oriented terms,
the class is a logical abstraction that describes the characteristics that an
object type will assume. Classes do not contain or represent actual instances
of objects. Instead, objects take on the properties of classes under which they
have been defined.
Inheritance is the essential ingredient that allows object-oriented pro-
grammers to create class hierarchies. The integration of true inheritance has
never been possible before in the relational environment, because the physi-
cal
representations
of
class
objects
in
the
relational
structure
lacked
persistence.
To practice the concept of inheritance, we must establish one or more
derived classes to our base class. The underlying concept behind the derived
class is that of a specific base class implementation. The derived class retains
all attributes and functions of the base class, but it can also specify additional
properties and functions of its own. Derived class definitions can also over-
ride default properties stipulated in the default class so long as those proper-
ties are public. The relationship between the base class and the newly created
