stephenson college dp 96 1 object-orientation by derek peacock

Stephenson College DP 96 1

Object-Orientation

by Derek Peacock


Types of Objects

Tangible things Cars, Cards, Books

Roles Employer, Teacher, Student

Incidents Flight, Transactions


Types of Objects cont...

Interactions Electrical circuit, a contract

Specifications Architectural Design Engineering blueprint


Objects in Programming

Direct representation of the application Graphical Playing Cards Documents Accounts

Artefact of Implementation Card Deck Unit stacks, queues


What are Objects?

Attributes, States Methods, Behaviour Composed of other objects


Program Objects

Object = (private) Data + (public) Processes

Object Name

Public Processes

Private Data


Encapsulation

Data & Processes linked

Public interface Information is hidden private data private internal

processes

Bank Account

Account numberaccount holdercurrent balance

get current balancemake depositmake withdrawal


Traditional Modularity

Process Driven Functional

Decomposition Data structured to

suite the process code re-use difficult

Data Driven Data Decomposition Process constrained by

the structure of data code re-use difficult


Object Oriented Modularity

Data & Process tightly bound together (encapsulated)

Internal data and processes hidden Public interface can remain fixed while

internal workings are changed Can be extended without altering original

code


‘Good’ Modules

Reusability Extensibility Decomposability Composability Understandability Continuity Protection

Software components

New components from old

breaking a system down

building a system up

easy to understand the partsmall changes lead to small effects

errors confined to module


Features of OOP

Encapsulation/Abstraction Inheritance Polymorphism Aggregation


Encapsulation/Abstraction

Abstraction is used to represent essential features of behaviours and states

Behaviours & States encapsulated into an abstract data typeabstract data type or classclass

Internal States not directly accessible Changes to internal implementation do not

effect external interface


Inheritance

One Class can inherit the characteristics of another class. One class can be a “Kind of” other class a text window is a kind of a kind of window

A classification hierarchy shows the inheritance relationships between classes


Polymorphism

Different objects can respond to the same message in different ways e.g. Square.Draw Circle.Draw

At runtime we might not know what object is present but for example Object.Draw will draw whatever object is available


Aggregation

Objects can be aggregated into new relationships

Objects of one class can be composed of objects of another class Object A is a part ofis a part of Object B Engine is a part ofis a part of Car


Comparing Approaches

Procedural Data Type

Variable Function/Procedure

Object-Oriented Abstract Data Type

or Class Object/Instance Method/Operation