Object Oriented Design

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Object Oriented Design

• What is it?
• Why do it?Simon Woodworth, 19 August 2004

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Introduction

• Purpose of this presentation
• Explain what Object-Oriented Design (OOD) is
• Explain why it is important as a design and
  programming methodology
• OOD is important because
• It allows complex programming projects to be
  broken down into more mangeable sections
• It reduces the cost of maintaining software

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What is Object-Oriented Design?

• It promotes thinking about software in a way that
  models how we think about the real world
• It organises program code into classes of objects

Class Dog

• Every object is an instance of a class
• Every object has attributes and behaviours

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What is a Class?

• A class is a collection of things (objects) with
  similar attributes and behaviours.
• For example
• Dogs

• Attributes Four legs, a tail
• Behaviours Barking
• Cars

• Attributes Four wheels, engine, 3 or 5 doors
• Behaviours Acceleration, braking, turning

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What is an Object?

• An object is a specific instance of a class
• For example
• Volkswagen is an instance of the Car class

• Attributes (what it looks like)
• Blue
• Front wheel drive
• Max speed 115 mph.
• Behaviours (what it does)
• Accelerate
• Turn
• Stop

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In programming terms

• A class is a section of program code that defines
  how to create and use a specific group of
  objects.
• Its a template for creating new objects
• An object is a specific instance of a class and
  has its own place in computer memory
• Its attributes are stored as data values
• Its behaviours are defined by program code
• Every object is different even if they look
  exactly the same!
• For example My blue Volkswagen is not the same
  as your blue Volkswagen

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Classes and ObjectsAn Example
Object Alices Account AttributesName
AliceNumber 04701Balance 1003.22 Behaviours
WithdrawLodge
Class Account Attributes NameNumberBalance Beh
avioursWithdrawLodge
Object Bobs Account AttributesName
BobNumber 28804Balance 81.66 BehavioursWi
thdrawLodge
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Why is this important?

• It models how we view the real world
• Humans tend to categorise what they see into
  groups
• Even very small children do it
• Reduces cost of software development
• Large complex projects can be subdivided into
  classes
• Reliability the workings of objects cant be
  interfered with
• Classes can be reused in other projects
• Classes can be maintained without rewriting other
  program code
• Introduces new programming concepts
• Encapsulation
• Inheritance

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Object Oriented Design Concepts

• Encapsulation
• We can hide how an object works from other parts
  of the program
• Increases reliability because the internal
  workings of objects are protected from
  interference
• Allows us to reuse objects in other programs
  because their behaviour and attributes are
  well-defined
• Allows us to change the way an object works on
  the inside without changing the way it looks on
  the outside
• Example Car Stereo
• Standard case size and fittings, regardless of
  features
• Can be upgraded without affecting rest of car

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Object Oriented Design Concepts

• Inheritance
• We can create new classes of objects by
  inheriting attributes and behaviours from
  existing classes and then extending them
• No need to reproduce or duplicate work done in
  creating original class
• We can build hierarchies (family trees) of
  classes
• Example Rally Car
• Inherits properties of class Car
• and extends class Car by adding a rollcage,
  racing brakes, fire extinguisher, etc.

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Conclusion

• Object Oriented Programming Languages
• Java, C, Visual Basic .NET, C
• Object Oriented Design allows us to create
  program code that is
• Reusable
• Adaptable
• Maintainable
• Reliable