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IS0514 Lecture Week 5

Introduction to Object Orientation

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Why Object Oriented?• Based on domain analysis / subject matter analysis• Close match to real world• Nearly all Modern Technologies are Object Oriented

– Java / C# / C++ / PHP (object extension) / Delphi– Oracle – Object relational database– => closer match to end solution

• Widely accepted ( most common approach )• Extensible / Maintainable• Reusable

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Today's Lecture

• Introduction to object orientation concepts– Object

– Class

– Method

– Attribute

– Relationships

• Inheritance

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What is the world?

• The world can be seen to consist:– Things

– Relationships between things

• A thing can be:– Physical / Tangible

– Idea / Concept / Non-Tangible

• In object orientation a thing is called an Object• “Thing-Oriented Analysis!”

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Exercise 1

Think about the objects in this room.• Try to identify the objects in this room

– What “things” are here• Physical• Conceptual / Ideas

• In groups of 3-4 spend 5 minutes discussing what objects there are. At the end of that time you will be asked to share your thoughts with the rest of the class.

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Exercise 1 - Possible Objects

Physical • The lecturer• Each of the students• Each of the seats• The handouts• Computer• Projector• Crisps / Chocolate / Cans• Each of the Mobile phone

– switched off or silent!

Ideas• The lecture itself• Exercise 1• Lecture slide

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Attributes and behaviour

• Objects have properties called attributes– Attributes are the data associated

with an object

• Objects have behaviour – we call these methods – These are the tasks an object will

do (typically when we ask it to)

– These may also be called operations / member functions but we will try and call them methods

• Example – A Car– Attributes

• Make• Model• Engine Size• Current Speed

– Methods• break()• accelerate()• changeGear()• honkHorn()

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Exercise 2

• Think about the object “a student”• Try an identify its

– Attributes– Methods

• Think about which of these are relevant as regard to the university?

• In groups of 3-4 spend 5 minutes discussing this. At the end of that time you will be asked to share your thoughts with the rest of the class.

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Exercise 2 - Solution

Attributes

• Name

• Registration Number

• Date of Birth

• Gender

• Address

• Programme

• etc

Methods

• AttendLecture()

• AttendSeminar()

• CompleteAssignment()

• SitExam()

• etc

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Categories of things• Things naturally fall into categories (Car, Furniture, House)• In object orientation, a category of things is called a class

– an object is an instance of a class – my car => object Car => class

• Objects share behaviour as defined by their class – respond to the same set of methods– For example all cars can accelerate– Some value(s) can affect what a method does

• Objects share Class attributes e.g. every car has a colour• Object attributes will have different values

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Exercise 3

Think about the objects in the room and try to identify what classes these objects belong to.

• In groups of 3-4 spend 5 minutes discussing this. At the end of that time you will be asked to share your thoughts with the rest of the class.

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Exercise 3 - Possible Classes

Physical

• Person

• Lecturer

• Student

• Seat

• Furniture

• Handout

• Computer

• Projector

Ideas

• Lecture

• Exercise

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Classes and Objects –more formally

• Class – Template to define specific instances or objects

• Object – Instantiation of a class• Attributes – Describes each object• Behaviors – Specify what each object can do

– Methods (a.k.a. operations)

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Classes and Objects

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Methods and Messages

• Methods implement objects’ behavior– Analogous to a function or procedure

• Messages are sent to trigger methods– Procedure call from one object to another object

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Messages and Methods

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Relationships between Classes

• Classes / Objects can be related– Student completes Assignment

– Student is registered for a Module

– Student has access to the Blackboard

• Some relationships are stronger than others– Student is a kind of Person

– Lecturer is a kind of Person

• Inheritance– Is a generalisation / specialisation relationship between two classes

– Is “a kind of” relationship

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Class Hierarchy

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Exercise 4

Think about the class hierarchy between Person, Student, Lecturer, Full Time Student and Part Time Student.

• Draw the class hierarchy • In groups of 3-4 spend 5 minutes discussing this. At

the end of that time you will be asked to share your thoughts with the rest of the class.

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Exercise 4 - Class Hierarchy

Person

Lecturer Student

PartTimeStudent FullTimeStudent

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Inheritance – some terms

• Superclasses or general classes are at the top of a hierarchy of classes

• Subclasses or specific classes are at the bottom• Subclasses inherit attributes and methods from

classes higher in the hierarchy• In our tree, which are the subclasses and

superclasses?

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Inheritance

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Basic Characteristics of Object Oriented Systems

• Classes and Objects• Methods / Messages• Inheritance• THE BASICS OF OBJECT ORIENTATION

• Next Week: Class Diagrams

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This weeks reading

ESSENTIAL READING

Dennis A, Wixom B, and Tegarden D (2005) System Analysis and Design with UML version 2 second edition, Wiley

Pages 23-29

Further reading

Bennett, S., McRobb, S. and Farmer, R. (2002) Object-Oriented Systems Analysis and Design using UML, 2nd Edition, McGraw-Hill

Chapter 4