1. Unit - 3 OBJECT ORIENTED DESIGN PROCESS AND AXIOMS

2. Design Analysis- What needs to be done
Objects discovered serve as the framework for design
Identified objects, attributes, methods, associations must be designed for implementation as a data type expressed in the implementation language
Emphasis from application domain to implementation and computer concepts (user interface, access layer)
Good design simplifies the implementation and maintenance of a project
Axiomatic approach is to formalize the design process and assist in establishing a scientific foundation for the OOD process

3. THE OOD PROCESS
The classes are viewed in the implementation point of view. New classes and attributes can be added if required
The following activities are followed
Apply design axioms to design classes, their attributes, methods, associations, structures and protocols
1.1 Refine and complete the static UML class diagram
1.1.1 Refine attributes.
1.1.2 Design methods and protocols using UML activity diagram
1.1.3 Refine the associations
1.1.4 Refine class hierarchy and design with inheritance
1.2 Iterate and refine again

4. Design the access layer
2.1 Create mirror classes for the access layer as business layer
2.2 Identify access layer class relationship
2.3 Simplify classes and their relationships
2.3.1 Avoid redundant classes
2.3.2 Delete classes that have only 1 or 2 methods. Try to add in other classes
2.4 Iterate and refine again

5. Design the view layer
3.1 Design the macro level user interface, identifying view layer objects
3.2 Design the micro level user interface, which includes the following activities
3.2.1 Design the view layer objects by applying the design axioms and corollaries
3.2.2 Build the prototype of the view layer
3.3 Test usability and user satisfaction
3.4 Iterate and refine
Iterate and refine the whole design. Reapply the design axioms and repeat the preceding steps.

6. OOD Axioms
Axiom is a fundamental truth that is always observed to be valid and for which there is no counterexample or exception. They cannot be proven or derived.
A theorem is a preposition that may not be self evident but can be proven by accepted axioms. It is a law or principle. Theorem is valid if its referent axioms and deductive steps are valid.

7. There are two axioms followed.
Axiom 1: The independence Axiom. (relationships between components)
Maintain the independence of components
Axiom 2: The Information Axiom. ( complexity of design)
Minimize the information content of the design

8. Axiom 1: The independence Axiom
This Axiom states that, during the design process, as we go from requirements to a system component, each component must satisfy that requirement without affecting other requirements
E.g.., designing a refrigerator door with two requirements:
Door should provide access to the food
Minimal loss of energy on opening and closing it (opening the door is independent of loss of energy)
Requirements are coupled
Vertical door – coupled
This can be designed by providing horizontally as like chest-type freezers. Which satisfies both the requirements, without violating the first axiom

9. Axiom 2: The Information Axiom.
This Axiom is concerned with simplicity
Each fact should be with a minimum amount of complexity and maximum simplicity and straightforwardness.
Minimal complexity produces the most easily maintained and enhance application
OOS- to use inheritance, system’s built in classes to minimize complexity

10. COROLLARIES
A Corollary is a proposition that follows from an axiom or another proposition that has been proven
Can be valid or invalid as theorems
They are also called as design rules.
Derived from design axioms
Useful in making design decisions , since they are applied to actual situations more easily than original axioms

11. From the two axioms, the following corollaries are formed
Uncoupled design with less information content
Highly cohesive objects can improve coupling because only a minimal amount of information need to be passed between objects
Single purpose
Each Class must have a single, clearly defined purpose
Large number of simple classes
Allows reusability
Strong mapping
Strong association between the analysis object and design object
Standardization
Promote standardization by designing interchangeable components and reusing existing classes or components
Design with inheritance

12. Corollary 1: Uncoupled design with less information content
The main goal here is to maximize objects cohesiveness among objects and software components in-order to improve coupling
Coupling
This is the measure of strength of association established by a connection from one object or software component to another
Change to one component of the system should have minimal impact on the other system.

13. Strong coupling among the objects complicates the system.
The degree of coupling is a function of
How complicated the connection is
Whether the connection refers to the object itself or something inside it
What is being sent or received
OOD – Interaction and Inheritance coupling
Interaction Coupling involves the amount and complexity of the messages between the components
Hence its better to keep the messages simple
Also reduce the number of messages sent & received by an object
(infrequent as possible)
Inheritance coupling: coupling between super and sub classes
A subclass is coupled with its super class in terms of attributes and methods
High inheritance coupling is desirable
If the sub class is overriding all of the methods (low inheritance coupling)

14. Types of coupling among different objects or components
Content coupling – degree is very high
Common coupling – degree is high
Control coupling – degree is medium
Stamp coupling – degree is low
Data coupling – degree is very low

15. Refers to the “single-purposeness” of an object
Cohesion
This is the measure of strength of interaction within a single object or a software component
Refers to the “single-purposeness” of an object
Highly cohesive object have minimal coupling, because it needs only minimum amount of messages to be passed between others.
Types of cohesion
Method cohesion
Method should carry only one function
Multiple functions is undesirable
Class cohesion
Class’s methods and attributes must be highly cohesive
Used by internal methods or derived classes
Inheritance cohesion
How interrelated are the classes?
Does specialization portray or arbitrary?

16. Corollary 2: Single purpose
Each class must have a purpose
During documentation, it should be possible to write the purpose of the class within one or two sentence
If it’s not possible, then think such that, responsibilities can be divided into different classes.
Method must provide only one service
Keep the classes simple

17. Corollary 3: Large number of simple classes
The classes created should be simple and more general
This is useful for reusing the classes for the future
If the class is complex, try to divide the class, such that there are large number of small classes
Focus on reusability (higher productivity)

18. Corollary 4: Strong Mapping
This is used to link the classes analyzed during the analysis phase and the classes designed during the design phase

19. Corollary 5: Standardization
The class libraries used in several object oriented programming must in a standard form to enable reusability.
Store it in repository
Provide easy search through the repository

20. Corollary 6: Designing with inheritance
When a class is implemented, it is necessary to determine its ancestors. What attribute it will have, and what messages it will understand.
Inheritance is used in-order to minimize the amount of coding in the program