Unit 4

Object Oriented design process and design axioms

Object Oriented design process

Design calsses, methods, attributes,

associations

Refine UML classDiagrams

Apply design axioms Design

view /access layers and prototype

User satisfaction and usability test based on

use cases

Access layer

•Create mirror class•Identify access layer class relationship•Simplify class and relationship

•Remove redundant class•Method class

Object Oriented design axioms

• Axioms– Hypothesized from observation– Common truth– Always valid– No need to prove and derive

• Theorem– It can be proven from axioms

• Corollary– Follows from axioms and theorems

Axioms for OOAD• Axiom1

– Maintain the independence of component• While going from requirements and use case to system component, each

component must satisfy requirements without affecting other requirements• Ex:Fridge door

– Req» Access to food» Minimal energy loss

– Vertical door» Access to food» No Minimal energy loss

– Horizontal door» Provides both requiremnt

• Axiom 2– Minimize the information content of the design

corollary

• Called design rules

• Derived from axioms

corollaries

• Corollary 1– Uncoupled design with less information content

• Corollary 2– Single purpose

• Corollary 3– Large number of single classes

• Corollary 4– Strong mapping

• Corollary 5– standardization

• Corollary 6– Design with inheritance

Axiom1

Axiom2

Corollary 1

Corollary 2

Corollary 3

Corollary 6Corollary 5

Corollary 4

Corollary 1• Uncoupled design with less information content

– Goal• Maximize cohesiveness• Improve coupling• Less information transaction among objects

– Coupling• Deals interaction among objects

– Types of coupling• Interaction coupling

– Involves the amount and complexity of messages bw components– Tightly coupled objects leads to change in one object reflects changes

in others• Inheritance coupling

– Inheritance is form of coupling bw super and sub class– High Inheritance coupling is desirable

– Cohesion• Deals interaction within a single objects• Reflects single purpose ness• High cohesiveness can lower coupling

Corollary 2

• Single purpose– Each class must have a purpose – Clearly defined and necessary in the context

of achieving system’s goals

Corollary 3

• Large number of single classes, reusability– The less specialized the classes are, the

more likely future problems can be solved by a recombination of existing classes

– Smaller classes• less specialized• Chance of reusing

– Large and complex classes• More specialized• Less chances of reusing

Corollary 4

• Strong mapping– As the model progress from analysis to

implementation, more detail is added, but it remains the same

– Strong mapping links classes identified during analysis and design phase

Corollary 5

• Standardization– Documentation

• Knowledge of existing classes helps to identify new classes

Corollary 6

• Design with inheritance