Transcript CM402 Lecture Week 2 - Computing at Northumbria

IS0514 Lecture Week 5
Introduction to Object Orientation
IS0514
Slide 1
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
IS0514
Slide 2
Today's Lecture
• Introduction to object orientation concepts
–
–
–
–
–
IS0514
Object
Class
Method
Attribute
Relationships
• Inheritance
Slide 3
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!”
IS0514
Slide 4
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.
IS0514
Slide 5
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!
IS0514
Ideas
• The lecture itself
• Exercise 1
• Lecture slide
Slide 6
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
IS0514
• Example – A Car
– Attributes
• Make
• Model
• Engine Size
• Current Speed
– Methods
• break()
• accelerate()
• changeGear()
• honkHorn()
Slide 7
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.
IS0514
Slide 8
Exercise 2 - Solution
Attributes
• Name
• Registration Number
• Date of Birth
• Gender
• Address
• Programme
• etc
IS0514
Methods
• AttendLecture()
• AttendSeminar()
• CompleteAssignment()
• SitExam()
• etc
Slide 9
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
IS0514
Slide 10
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.
IS0514
Slide 11
Exercise 3 - Possible Classes
Physical
• Person
• Lecturer
• Student
• Seat
• Furniture
• Handout
• Computer
• Projector
IS0514
Ideas
• Lecture
• Exercise
Slide 12
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)
IS0514
Slide 13
Classes and Objects
IS0514
Slide 14
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
IS0514
Slide 15
Messages and Methods
IS0514
Slide 16
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
IS0514
Slide 17
Class Hierarchy
IS0514
Slide 18
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.
IS0514
Slide 19
Exercise 4 - Class Hierarchy
Person
Lecturer
Student
PartTimeStudent
IS0514
FullTimeStudent
Slide 20
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?
IS0514
Slide 21
Inheritance
IS0514
Slide 22
Basic Characteristics of Object
Oriented Systems
•
•
•
•
Classes and Objects
Methods / Messages
Inheritance
THE BASICS OF OBJECT ORIENTATION
• Next Week: Class Diagrams
IS0514
Slide 23
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) ObjectOriented Systems Analysis and Design using UML,
2nd Edition, McGraw-Hill
Chapter 4
IS0514
Slide 24