Page 162 - DCAP308_OBJECT_ORIENTED_ANALYSIS_AND_DESIGN

Object Oriented Analysis and Design

Notes Refinement of association between classes (if necessary)
Refinement of class hierarchy and design with inheritance (if necessary)
It also includes iteration and refinement work to be done again.

13.1 Design Axioms

An axiom is a fundamental truth that always is observed to be valid and for which there is no
counter example or exception. They cannot be proven or derived but they can be invalidated by
counter examples or exceptions.
A theorem is a proposition that may not be self-evident but can be proved from accepted
axioms. A corollary is a proposition that follows from an axiom or another proposition that has
been proven.
Axioms of OOD are as follows:
The axiom 1 of object-oriented design deals with relationships between system components
(such as classes, requirements and software components) and axiom 2 deals with the complexity
of design.

Axiom 1: The independence axiom. Maintain the independence of components. According
to axiom 1, each component must satisfy its requirements without affecting other
requirements. For example, let us design a refrigerator door which can provide access to
food and the energy lost should be minimised when the door is opened and closed.
Opening the door should be independent of losing energy.

Axiom 2: The information axiom. Minimise the information content of the design. It is
concerned with simplicity. In object-oriented system, to minimise complexity use
inheritance and the system’s built in classes and add as little as possible to what already is
there.

13.1.1 Corollaries

Corollaries may be called Design rules, and all are derived from the two basic axioms.

The origin of corollaries is shown in Figure 13.1. Corollaries 1, 2 and 3 are from both axioms,
whereas corollary 4 is from axiom 1 and corollaries 5 and 6 are from axiom 2.

Figure 13.1: Origin of Corollaries

Corollary 4

Corollary 1
Axiom 1
Corollary 2

Axiom 2

Corollary 3

Corollary 6 Corollary 5

Source: Bahrami, Ali. “Object Oriented Systems Development,” Tata McGraw-Hill Education, 2004

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