CSM18 VB OOP 1 Dr Terry Hinton 28/10/03

Visual Basic:An Object Oriented Approach

Programming is a Model of the Real World Object in Real World modelled as an

object in the program Hence Object Oriented Programming

CSM18 VB OOP 2 Dr Terry Hinton 28/10/03

Object Oriented Programming

Classes – templates for objects The “cookie cutter” idea

Class Definition Objects

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Object Oriented Programming

Classes – templates for objects Encapsulation – of Data and Code Attributes of an object - variables Methods – operations defined for a

class of objects Messages – interactions between

objects via Methods to send or receive data

CSM18 VB OOP 4 Dr Terry Hinton 28/10/03

Object Models

Real programs may involve large numbers of objects Will interact in various ways Will form logical structures

Key is in building an object model that provides a ‘world view’ of the system

One to one correspondence between object in the real world and object in the model

CSM18 VB OOP 5 Dr Terry Hinton 28/10/03

Use-Case Diagrams

Used to obtain a high level (user-level) picture of the system

Can be used in discussions with clients

Shows main interactions between system and users/external environment

ATM - Bank Machine

Accept Card

Verify PIN

Print Statement

WithdrawCash

DepositCash

User (Actor)

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Class Model Diagrams

Class Name

Attributes - Variables define state of object

Methods - Program code defines processes or behaviour of object methods interact with methods of other objects

CSM18 VB OOP 7 Dr Terry Hinton 28/10/03

Class Model based on Use-Case Diagram

Having identified classes, need to define… Interactions between

them Structure Class Interfaces - public

view of object

ATMObj

ValidateUserPrintStatementMakeDepositMakeWithdrawal

AccountObj

BalancePINOwner

DepositWithdrawStatement

ATM AccountAccount.Deposit(50)

CSM18 VB OOP 8 Dr Terry Hinton 28/10/03

Coding a Class Need to…

Determine storage needs - attributes or variables

Determine Methods and interaction between them

Consider mechanisms for getting data into and out of an object Define Methods & Functions to do this Data - Private Methods & Functions - Public

CSM18 VB OOP 9 Dr Terry Hinton 28/10/03

Example – Bank/ATM

Assume two classes only ATM Class models Automatic Teller Machine Account class models individual account in Bank

Start with Account class ATM accesses Account Account receives deposit, agree a withdrawal,

present state of balance, agree PIN number

CSM18 VB OOP 10 Dr Terry Hinton 28/10/03

Account Class

Private Balance As Currency ‘ very simple examplePrivate PIN As IntegerPrivate Owner As String

Public Function GetBalance() As Currency…Public Function GetPIN() As Integer…Public Sub AssignPIN(newValue As Integer)…Public Function GetOwner() As String…Public Sub AssignOwner(newValue As String)…Public Sub Deposit(amount As Currency)…Public Sub Withdraw(amount As Currency)…Public Function Statement() As String… ‘returns text about account

Private Balance As Currency ‘ very simple examplePrivate PIN As IntegerPrivate Owner As String

Public Function GetBalance() As Currency…Public Function GetPIN() As Integer…Public Sub AssignPIN(newValue As Integer)…Public Function GetOwner() As String…Public Sub AssignOwner(newValue As String)…Public Sub Deposit(amount As Currency)…Public Sub Withdraw(amount As Currency)…Public Function Statement() As String… ‘returns text about account

Methods are Subroutines & Functions

CSM18 VB OOP 11 Dr Terry Hinton 28/10/03

Account – Methods

Public Function GetBalance() As CurrencyGetBalance = Balance

End Function

Public Funtion GetPIN() As IntegerGetPin = PIN

End Function

Public Sub AssignPIN(newValue As Integer)PIN = newValue

End Sub

Public Sub Deposit(amount As Currency)Balance = Balance + amount

End Sub

Public Function GetBalance() As CurrencyGetBalance = Balance

End Function

Public Funtion GetPIN() As IntegerGetPin = PIN

End Function

Public Sub AssignPIN(newValue As Integer)PIN = newValue

End Sub

Public Sub Deposit(amount As Currency)Balance = Balance + amount

End Sub

Read-only

Read-only property

ReadWrite

Method definition

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Class - Object - Instances Define a Class of Objects but need an actual

example of an object Instance of a Class - Object Instance Each Object has the same set of attributes Each Object has its own values of these

attributes Values of Attributes set the state of the Object Only one copy of an Object Instance but have

any number of Place Holders for a reference or pointer to an Object - more later

CSM18 VB OOP 13 Dr Terry Hinton 28/10/03

Testing Account

Set Account = New AccountObjAccount.AssignOwner ( “John Smith”)Account.AssignPIN (1234)Account.Deposit (500.00)Account.StatementStatement for: John Smith Balance = £500.00Account .Withdraw (50)Account.StatementStatement for: John Smith Balance = £450.00‘ etc…

Set Account = New AccountObjAccount.AssignOwner ( “John Smith”)Account.AssignPIN (1234)Account.Deposit (500.00)Account.StatementStatement for: John Smith Balance = £500.00Account .Withdraw (50)Account.StatementStatement for: John Smith Balance = £450.00‘ etc…

In VB can use the Immediate Window to test a class

Create an object Execute methods Print attribute values (Using

Print)

Can also copy, the sequence of test statements, paste into Notepad or an editor, and save for re-testing if changes are made.

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Continue Development

Create next class ATM User-Interactions with ATM should

translate into ATM Methods and interactions with Account Methods

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Example ATM Method

Public Function ValidateUser(Account As AccountObj) As BooleanDim userPIN As Integer

userPIN = InputBox(“Enter PIN”)If TestAccount.PIN = userPIN Then

ValidateUser = TrueElse

ValidateUser = FalseEnd If

End Function

Public Function ValidateUser(Account As AccountObj) As BooleanDim userPIN As Integer

userPIN = InputBox(“Enter PIN”)If TestAccount.PIN = userPIN Then

ValidateUser = TrueElse

ValidateUser = FalseEnd If

End Function This method takes as its parameter an Object of type AccountObj. Returns a Boolean.

CSM18 VB OOP 16 Dr Terry Hinton 28/10/03

Testing an ATM Method

Set Account = New AccountObjAccount.AssignOwner (“John Smith”)Account.AssignPIN = 1234Set ATM = New ATMObjPrint ATM.ValidateUser (Account)‘ VB creates an InputBox() here to enter PIN number‘ ATM returns True or False depending on input to it.‘ Could now have ATM making deposits and‘ withdrawals etc. e.g…ATM.MakeDeposit (100.00)ATM.PrintStatement (Account)Statement for: John SmithBalance = £100.00

Set Account = New AccountObjAccount.AssignOwner (“John Smith”)Account.AssignPIN = 1234Set ATM = New ATMObjPrint ATM.ValidateUser (Account)‘ VB creates an InputBox() here to enter PIN number‘ ATM returns True or False depending on input to it.‘ Could now have ATM making deposits and‘ withdrawals etc. e.g…ATM.MakeDeposit (100.00)ATM.PrintStatement (Account)Statement for: John SmithBalance = £100.00

CSM18 VB OOP 17 Dr Terry Hinton 28/10/03

References

A Reference is a variable that acts as an alias for an object or pointer to an object

References are used for interacting with objects avoids copying objects

An object with no references to it is destroyed automatically - orphan

When an object instance is created it is given a name and allocated space for its attributes

Reference to that object can be made without copying the Object.

CSM18 VB OOP 18 Dr Terry Hinton 28/10/03

References and Objects

Dim Account As AccountObj, TempAccount As AccountObj

Set Account = New AccountObj

Set TempAccount = Account

Dim Account As AccountObj, TempAccount As AccountObj

Set Account = New AccountObj

Set TempAccount = AccountCopies reference for

Account to TempAccount

Object: Account

Creates two references of type AccountObjCreates object instance and sets aside space for variables

Only one instance of Account

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Collections

When we create a set of object instances they all have the same name!!

To avoid losing track of Objects we need a Collection which is a list of object references Therefore no need to give each object

instance a different name Acts as an unbounded array – no need to

indicate how many elements to accommodate

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Collection Methods

Four main methods Add – adds an object reference to the collection Remove – removes a reference Count – returns number of references in the

collection Item – allows access to individual object references

Indexing Objects can be retrieved by number Objects can be retrieved by value of an attribute Objects can be added with a Key (string), which acts

as a textual index

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A Collection

Set Queue = New CollectionObjSet Account = New AccountObjAccount.AssignOwner (“Fred”)Queue.Add (Account)Set Account = New AccountObjAccount.AssignOwner (“Mary”)Queue.Add (Account)Print Queue.Count2Print Queue(1).OwnerFredPrint Queue(2).OwnerMary

Set Queue = New CollectionObjSet Account = New AccountObjAccount.AssignOwner (“Fred”)Queue.Add (Account)Set Account = New AccountObjAccount.AssignOwner (“Mary”)Queue.Add (Account)Print Queue.Count2Print Queue(1).OwnerFredPrint Queue(2).OwnerMary

CSM18 VB OOP 22 Dr Terry Hinton 28/10/03

Collections and For Each…

Collections and objects introduce a new style of For..Next loop

For Each Account In Queue Print Account.OwnerNext

For Each Account In Queue Print Account.OwnerNext

FredMaryFredMary

CSM18 VB OOP 23 Dr Terry Hinton 28/10/03

Building Structures with Objects

Class CarObj Private ID As Integer, mileage As Single Private Passenger As PassengerObj Public Sub AcceptPass(Pass As

PassengerObj) Public Sub Journey CurrentName = Car.GetPass.GetName

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Structures in programs Ways of grouping objects using

Arrays, Collections Lists

Simple lists of objects Stacks (Last-in, First out) Queues (Last-in, Last-out)

Trees Hierarchical structures of objects - node contains

reference to next node Graphs

Arbitrarily interconnected objects

CSM18 VB OOP 25 Dr Terry Hinton 28/10/03

Logical and Physical Structure

Interconnect objects using references Forms of interconnection involve

Logical structure The model of interconnections we wish to

create Physical structure code

The actual mechanisms we use to create it

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Alternative Collections

Ordered Collection Items added in a specific order Can use binary search to insert and retrieve

items

Efficiency in searching (and therefore retrieval

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Queues and Stacks

Collections with limitations on insertions/retrievals

Item 5 Item 4 Item 3 Item 2 Item 1 Front of Queue(items leave here)

Rear of Queue (items join here)

Item 3

Item 2

Item 1

Item 2

Item 1

Item 4

Item 3

Item 2

Item 1

Item 3

Item 2

Item 1

Item 1 Item 2

Item 1

Item 1

ItemAdded

ItemAdded

ItemAdded

ItemRemoved

ItemRemoved

ItemRemoved

ItemAdded

CSM18 VB OOP 28 Dr Terry Hinton 28/10/03

Alternatives to Collections

Tree Structures Each item can contain references to

several other items Efficient for storage and retrieval Recursive

Me

My Father My Mother

My father’sfather

My father’sMother

My mother’s My mother’smotherfather

CSM18 VB OOP 29 Dr Terry Hinton 28/10/03

Graphs

Arbitrary interconnections Complex, and can be inefficient to code Require use of specialist algorithms to search and traverse

NodeA

NodeB

NodeE

NodeF

NodeD

NodeC