Saturday, March 2, 2013

MS - Sem1 - OO Analysis and Design - post 1

OO principles (The four fundamentals)

Abstraction (a simplified description)
                “An abstraction denotes the essential characteristics of an object that distinguish it from all other kind of objects and thus provide crisply defined conceptual boundaries, relative to the perspective of the viewer”
                In general, encapsulation is one of the four fundamentals of OOP (object-oriented programming). Encapsulation is to hide the variables or something inside a class, preventing unauthorized parties to use. So the public methods like getter and setter access it and the other classes call these methods for accessing.
•Information hiding              •Interface –Implementation                                •Behavior & Data
In his book on object-oriented design, Grady Booch defined encapsulation as "the process of compartmentalizing the elements of an abstraction that constitute its structure and behavior; encapsulation serves to separate the contractual interface of an abstraction and its implementation."

“Hierarchy is a ranking of abstractions”
Inheritance:           expresses “is-a” or “Kind-of” relationship (Extensibility & Reusability)
                Part-of:                   expresses that object is an aggregate of another


Hiding alternative procedures behind a Common Interface
Send a Message to an object -Polymorphism guarantees that the correct/proper implementations invoked.
Sender does not know specific class of receiver

v  Benefits of OO Paradigm
       Models System using Objects
       Understanding the system is easier
       No change to your existing code (almost) –If designed properly, of course!

v  Object –
§  “Concept, abstraction, or thing with crisp boundary & meaning for a problem”
§  An Object has state and behavior
§  Objects receive stimuli/messages & respond
§  Receiving a stimulus, Object may change state
§  Class is blueprint of objects.
§  Each object knows its class.

v  Structured Programming
§  Look at the problem
§  Design a collection of functions
o    –Break down the functions into smaller pieces; Functional Decomposition
§  Identify the characteristics or data
o    –Define them globally
§  Map the functions with the required data

Ripple effect:
Upon introducing a local change in a method or a class in an object oriented program, the ripple effect gives a measure of the amount by which this change may affect other methods/classes. That is, the ripple effect shows maintainers the ramifications of any change they may make before that change is actually implemented. We need to propose techniques for measuring the ripple effect in object oriented programs and use it to compute an indicator of logical stability. First, we investigate object oriented dependences and illustrate how these dependences and some metrics can be used to determine change impact and propagation. Then, we use matrix arithmetic techniques for computing the ripple effect in object oriented programs and show their application at the intra-class, inter-class and architectural levels.

v  Structured Programming: Issues
§  System becomes too complex
§  Data and functions are scattered throughout
§  Accommodate change in requirements is a pain
§  Maintenance and debugging is an issue

v  Object Oriented Programming
§  Lessens the impact of the problem
§  Combines data and functions into the same module
§  Modules can talk to each other directly
§  Small gap between reality and model
§  Modifications are localized
o    Confined to a module
§  Module is the Object

The key difference between Structured Programming and OOP is that the focus of Structured Programming is to structure the program in to a hierarchy of subprograms while, the focus of OOP is to break down the programming task in to objects, which encapsulate data and methods. OOP is considered more flexible than structured programming, because OOP separates a program in to a network of subsystems rather than structuring the program in to a hierarchy. Even though structuring provides certain clarity, a small change to a very large structured program may cause a ripple effect of having to change multiple subprograms.

v  Object -Oriented Development
§  Functions and data highly integrated
§  More understandable
§  More flexible to modifications
§  Stable -modifications are localized

v  Object -Oriented Analysis
§  Activities:
o    –Finding the objects
o    –Organizing the objects
o    –Describing object interaction
o    –Defining operations on the objects
o    –Defining the object’s internals
§  Obtain understanding of system based on functional requirements
§  Considers functional requirements & data as integrated objects

v  OO Design & Implementation
§  Analysis model Designed & Implemented
§  Take care of restrictive demands
o    Memory, response time, etc.
§  Traceability of Objects
§  Create & use Components

v  OOA vs OOD
§  OOA is the act of determining the abstractions which underlay the requirements. It is the backwards reasoning that starts with the requirements and ends up with a set of objects that express the abstractions; and messages that support the required behaviors.

§  OOD embodies the set of decisions that determines how those objects and methods will be implemented. Typically class inheritance and composition hierarchies are among those design decisions

Analysis and Design should not be separated into broad phases. Nor should the analysis model be frozen before the design modeling begins. The analysis model cannot be completed in the absence of the design model. And the design model cannot be completed in the absence of the analysis model.  -RobertMartin

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