object oriented software development 2009-2010 object oriented principles supplementary lecture:...

Object Oriented Software

Development2009-2010

Object Oriented PrinciplesSupplementary Lecture: Week 2

Brian Farrimond

Derived from Computing Concepts course for IT students

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What is an object? An object is a piece of software that can receive

and act on messages. The piece of software that is an object is a model of

an entity, either physical, conceptual or software.

•Physical entity

•Conceptual entity

•Software entity

Truck

Chemical Process

This is a Microsoft Word document block, bold and italicised and centred in a

box layout

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A Microsoft Word object Demo….

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A Microsoft Word object Demo…. The block of text is a software object It can receive and act on messages In Windows, we use mouse clicks on buttons

etc to send messages. The set of messages an object can

understand is called its protocol Messages can be “simple” requiring no extra

information e.g. Make bold – Exercise 1 Messages can be “complicated” requiring

extra information e.g. Set font size – Exercise 1

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Objects can hold informationAn object has data or attributes (state) An implementation of a protocol

(functionality expressed as responses to messages)

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Objects respond to messagesAn object has data or attributes (state) An implementation of a protocol

(functionality expressed as responses to messages)

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Objects ……An object has data or attributes (state) An implementation of a protocol

(functionality expressed as responses to messages)

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The state of an object The information an object holds at any

one time is known as its state. What kind of information does a block of

text hold - Exercise 2 What is the state of the following block of

text? - Exercise 3

Mary had a little lamb

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Object attributes Each piece of information an object

holds (state) is called an attribute Some of the attributes of a block of text

are:

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Object attributes Each piece of information an object

holds (state) is called an attribute Some of the attributes of a block of text are:

Text Colour Justification Font type Font size Bold Italic Underline Strikethrough

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Its fleece as white as snow

Exercise 4

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Exercise 5 What is the only way we can change

the state of an object?

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Picture objects Demo… Exercise 6

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Table objects Exercise 6

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Conclusions Block of text, picture and table are

different kinds of objects. They have some similarities but

some differences: in the way they respond to messages in the information they hold

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Conclusions We borrow from the world of nature

where we recognise different kinds of animals

They have some similarities but some differences: Cats and dogs both have four legs Cats purr, dogs do not

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Conclusions In biology we refer to species – Dog, Cat,

Mouse etc Rover and Spot are examples of the Dog

species Felix and Tiddles are examples of the Cat

species In object oriented terms we say that

Rover and Spot are instances of the Dog class Felix and Tiddles are instances of the Cat class

An object is an instance of a class

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Objects belong to classes All objects are of some class of

objects. E.g. “That red Nissan Primera in my driveway is a car.”

In object oriented language we say: That red Nissan Primera in my driveway is an instance of class car.

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Objects belong to classes In object oriented programming

we write code to create classes. The code for each class we write identifies: Attributes - the information about

objects in the class we want to use Methods - how the objects respond to

messages they receive

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Identifying attributes A class is to model books, storing the

books’ title, author and retail price. Suggest a suitable set of attributes.

Rules for names No spaces – you can join words together Lower case apart from initial letters of second

word, third word etc Attributes are:

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Identifying attributes A class is to model books, storing the

books’ title, author and retail price. Suggest a suitable set of attributes.

Rules for names No spaces – you can join words together Lower case apart from initial letters of second

word, third word etc Attributes are:

title author retailPrice

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Object diagrams We can represent objects showing their state

and protocol in an object diagram

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Object diagrams Here is a particular green object named Pie1

in the top left corner facing east.

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Exercise 14

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Identifying attributes A class is to model customers, storing

name, address, current balance, overdraft limit.

Rules for names No spaces – you can join words together Lower case apart from initial letters of second word, third word etc

Attributes are:

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Identifying attributes A class is to model customers, storing

name, address, current balance, overdraft limit.

Rules for names No spaces – you can join words together Lower case apart from initial letters of second word, third word etc

Attributes are: name address currentBalance overdraftLimit

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Identifying the protocol Messages return an answer Messages need a name Messages may carry information

Complicated messages like setting Font size

Protocol needs to document these in a signature.

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Messages return an answer Messages need a name Messages may carry information

A message signature:void setFont(String fontName)

Answer type

Message name

Parameter list containing information

Identifying the protocol in C++

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Messages return an answer Messages need a name Messages may carry information

A message signature:String getAddress()

Answer type

Message name

Parameter list containing information

(empty in this case)

Identifying the protocol in C++

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A message signature:void setFont(String fontName)

Using the protocol

Message name Parameter list

containing information(the name of the desired

font)

Answer type(void means

no answer will be given)

Example messages:

setFont(“Times New Roman”)

setFont(“Arial”)

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A message signature:void setFontSize(int fontSize)

Using the protocol

Message name Parameter list containing

information(the desired size of the

font)

Answer type(void means

no answer will be given)

Example messages:

setFontSize(12)

setFontSize(32)

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A message signature:void addTeam(String name, int points)

Using the protocol

Message name Parameter list containing

information(the desired team name and number of points)

Answer type(void means

no answer will be given)

Example messages:

addTeam(“Tranmere Rovers”, 23)

addTeam(“Chester”, 32)

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Sending messages to an object

textBlock1.setFont(“Courier New”)

Receiver of

message

The full

stop

The message

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General Message sends

Receiver of

message

Object . Message

The message

An Object “DOT” a message name

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Attribute names in C++ In C++, attributes are referred to as

member variables Consequently, the convention is to

begin their names with m_ E.g. m_name, m_dataOfBirth

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Example on Page 58 - 59 A C++ software developer has been

commissioned to create a hospital administration application in which wards are to be modelled. He will need to be able to store for each ward the following information: the name of the ward, the name of the ward sister, the total number of beds on the ward and the number of patients currently on the ward.

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Attributes He will need to be able to store for each

ward the following information: the name of the ward, the name of the ward sister, the total number of beds on the ward and the number of patients currently on the ward.

Attributes?

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Attributes He will need to be able to store for each

ward the following information: the name of the ward, the name of the ward sister, the total number of beds on the ward and the number of patients currently on the ward.

Attributes? m_wardName m_wardSisterName m_numberBeds m_numberPatients

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Protocol He will want to send messages to the ward

object to make it carry out the following actions: set the total number of beds in the ward to a given value admit a patient to the ward discharge a patient from the ward set the name of the ward sister to a given value reply with the name of the ward sister

Protocol

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Protocol He will want to send messages to the ward object to

make it carry out the following actions: set the total number of beds in the ward to a given value admit a patient to the ward discharge a patient from the ward set the name of the ward sister to a given value reply with the name of the ward sister

Protocolvoid setNumberBeds(int numberOfBeds)void admitPatient()void dischargePatient()void setWardSisterName(String name)String getWardSisterName()

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Using the protocol Suppose the user of the hospital administration

program carries out the following sequence of actions on an object of this class called Ward1.

Admit a patient Discharge a patient Make the total number of beds 30

Set the ward sister name to Mary Jones

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Using the protocol Suppose the user of the hospital administration program

carries out the following sequence of actions on an object of this class called Ward1.

Admit a patient Discharge a patient Make the total number of beds 30 Set the ward sister name to Mary Jones

Ward1.admitPatient(); Ward1.dischargePatient(); Ward1.setNumberBeds(30); Ward1.setWardSisterName(“Mary Jones”);

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Heading towards C++ C++ programming is mainly about

creating classes of objects The C++ code that is executed

when an object is created or when a message arrives creates and manipulates other objects