lecture 01 oop concepts

8/8/2019 Lecture 01 OOP Concepts

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The Robert Gordon University

Object Oriented Programming

(CMM504)

Lecture#1

Object-Oriented ProgrammingConcepts


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The Robert Gordon University K. Hui 2008-2009 2

Content Object-Oriented Programming

Concepts components of an object

class-level vs object-level

Main features of Object-OrientedProgramming


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Object-Oriented Programming

Concepts OOP models the world as a collection of

objects

e.g.you, your classmates, your dog, your mobilephone

objects may relate to/interact with eachothers e.g. you have a dog, you make a call with your

mobile, you talk to your classmates interaction does something

change of state, side-effect of interaction (e.g.display on screen, file operation), etc.


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How to Program in the OO

Way? Model your world (i.e. problem domain)

as objects

What objects do you needed?

How are these objects related to eachother? (i.e. relationships among them)

Do these relationships make sense? How do these objects interact to achieve

what you want?


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The Concept ofObject a self-contained entity

has 2 components:attributes &

methods attributes are also called instance variables

may or may not be visible to the outside

world (i.e.other objects)

i.e. can be hidden

interaction with other objects iscontrolled/ confined by its interface


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Attribute & Method attributes

the datacomponentofan object

define the descriptive characteristicsofan object

e.g.your dog is brown in colour, your mobile is branded Nokia

methods the procedural componentof an object

define the behaviour of an object

e.g.a dog walks, a mobile sends SMS

an attribute/method must belong tosomething (setting its context) you will not find an attribute/method that belongs to nobody


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Interface of an Object an object can hide its internal components

from the outside world

why hiding? the set of visible components defines the

object's interface to the outside world an object's interface defines how it can

interact with others NOTE: some time later, you will see the term

"interface" in Java, which is NOT the samething


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Calling vs Invoking a Method in OOP, you dont really call a method

for A to invoke a method in B:

A sends a message to B, asking B to invoke amethod

e.g. you ask your dog to give you its paw

B receives the message and decides how to

respond invoking the appropriate method

e.g. your dog and cat may give different responses tothe same request


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The Concept ofClass a template for creating objects

e.g. the dog class defines the attributes

& methods of a dog, but the dog class isNOT an object.

an object is created from a class an instance of a class

e.g. you are an instance ofthe studentclass, your dog isan instance ofthe dogclass


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Objects & Classes objects of the same class

have the same set of attributes & methods

e.g. each student has a student no. attribute,each dog has a colour attribute

may have different attribute values

each object has

its

own copy of the attribute e.g. differentstudentsmay have different

"student no." values, different dogs may havedifferent "colour" values


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The Context of Attributes &

Methods an attribute/ method must be attached/

belong to something

its context

when you refer to anattribute/method, you must

specify its context whose attribute/method is it?

e.g. Lassies colour, tell Peter to write an email


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Class-level vs Object-level

Context attribute/ method can be defined at the class-

levelor object-level (i.e.instance-level)

e.g. the bank account classmay have a class-levelattribute interest rate which isshared by allbank account objects

each object has itsown local copy of an

object-levelattribute a class-level attribute is shared by all

objects of that class, only 1 single copy


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When to Make it Object/Class

Level? object/instance-level attribute:

if you want each object to have its own copy ofthe attribute (i.e. not shared)

e.g. each dog object has its own copy of thecolour attribute (probably of different value)

class-level attribute if you want all objects of the same class to share

the same copy of the attribute e.g. The Dog class has a class-level attributedefault_num_of_legs which is shared by alldog objects.


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Object-level vs Class-level

Method object-level method:

when it makes sense that the behaviour should

occur in individual objects e.g. deposit of money into a bank account

you must specify which bank account to deposit, not toall objects in the same class!

class-level method: usually as utility methods e.g. the String class has a method parseInt()

that converts a string into an int value


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How to Program in OOP? (Re-

Revisited) design classes

design their internals & interfaces

take other peoples classes know their interfaces

i.e. what are visible to the outside world

create objects

let objects interact via their interfaces i.e. visible attributes/methods

achieve some tasks


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OOP Features 4 major features in OOP

encapsulation information hiding

inheritance

overloading


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Encapsulation an object encapsulates both its

attributes & methods

implications:

an attribute/ method is attached to an object/class

when you mention an attribute/ methods, youhave to specify which object/ class it comes from

why encapsulation? when you get hold of an object, you also get hold

of its data & behaviour components good for reuse


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Information Hiding an object can hide its internal details

e.g. you dont know how your mobiles electronics

works except punching the buttons can selectively show some details to the

outside world e.g. your mobile only shows the number it dials

defines an interface to interact with theoutside world e.g. your mobile interacts with your through the

buttons & screen


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Why Information Hiding? the object can have a complex internal but

simple interface making interaction with the outside world easier

you dont need to know about the internal ofan object only the interface is important i.e. how to interact with it

facilitate code reuse hiding any internal change from the outside world

by keeping the interface unchanged


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Inheritance a class may be similar to another

class but being more specialised

e.g. the class student is similar to theclass person but student is morespecialised

a person has attributes like:

sex, age, name a student has all these + a student no.


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Inheritance (contd) a subclass

extends/ specialises a superclass inheritsattributes& methods from itssuperclass may have more attributes/ methods than its

superclass may change the content/ procedure of an

inherited method i.e.same method name/ signature but different

behaviour

why inheritance? reuse existing class definition customise/ specialise if needed


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Overloading different functions/ procedures/ methods can

have the same name

provided that the parameters are of differenttypes giving a unique signature

the system will figure out which one to invoke

e.g. you can have 2 procedures, both named

call, taking a dog or person object asparameter respectively. Depending on you give ita dog or person object as the parameter, Javawill know which one to use.


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Why Overloading? you can call the same method (name)

but invoke different behaviour

dynamic binding of method

which method to invoke is determined atruntime

code reuse in term of the calling code


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Summary OOP models the world as objects

classes are templates

objects are instances of classes contain attributes (data) + methods

(procedures)

attributes & methods are attached to aclass or object

OOP focuses on code reuse

lecture 01 oop concepts

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