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Object Oriented Programming

Using Visual Studio

Objectives

To :

Learn about Classesand Objects

CreateYour Own Classes

Learn about Enumerated Types

To Focus on Program Design and Problem Solving:BankTeller Application

To undertakeManual Software Testing

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Classes and Objects

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Object Oriented Programming

A way of designing and coding applications thatfocuses on objects and entities in real-worldapplications.

Objects

Reusable software components that model items inthe real world.

e.g. wecanmodel a GPA calculator,or a tax calculator

Self-contained modules that combine data andprogram code which pass strictly defined messagesto one another.

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Objects

An object can also be defined as acontainer formemberssuch as properties, fields, methods, andevents

Often represents an entity in a real-worldapplicatione.g.:

If creating an car dealership application, the entitiesmight have names like vehicle, customer, salesperson,manager,and vehicle inventory.

If creating a GUI, the objectsmight be button, text box,list box, label,and radio button.

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Forms and Controls as Objects

Twokindsof objects you may use in VBare:

Formsand

Controls

Aform is a virtual blank space to design the userinterfacefor a VBapplication

Controls are the tools that you use to construct theuser interface e.g.

command button

textbox

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VB objects naming practice:

Start the name witha standard object abbreviation

cmd = command button, txt = text box, frm = form.

Finish the name with a descriptive word of theobjectspurpose e.g.

cmdCancel

frmMain.BackColor = vbRed

txtState.Text= ""

Spaces and special characters are not allowed inanobjectsname.

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Features of Objects

The programmer can manipulatethe object through the use of threekey object features:

properties/ attributes

methods/ behaviour

events

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Objects

Anobject

hasattributes: characteristicsthat cantakeonvalues

a vehicle objectsattributesinclude make, model, and colour

hasbehaviours:actionsthat can be carried out on theobject

A vehicle object might have behaviours such as start, stop,and turn.

canraiseevents: notif icationsof a change of state i.e.events representresponses by the object to externalactions.

A Button object in .NET raises a Click event when the user

clicksthe button. OOP

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Object Properties

A property isa named attributeof an object.

Using ananalogy to English grammar, if an object isthought of as anoun, then apropertymay bethoughtof asanadjective.

Used to change the appearance of objects.

An example of the relationship between objectsand propertiesusing an everyday object:

shirt.colour = Green

shirt.wash = Clean

We use dot (.) notation OOP

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Setting Values of Properties

Properties may be set during design time,in the PropertiesWindow

Design Time: when you are designing theproject and adding code

Some properties may be set or modifiedduring runtime.

RunTime: whenyouclick the Run icon

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Object Event

Notificationsof a change of state.

Representstheresponse by the object to an external action.

An actiontakenby the object whennotif ied by a message

User actions takenon the object that provokesa response f romthe object.

Examples

A football: kicking, throwing, holding etc.

Cat: feeding,petting, calling etc.

mouse click, formload, or key press.

VBexamplePrivate Sub cmdSubmit_Click()

End SubOOP

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frommsdn.microsoft.com

See also the document titledAPII 3 OOP -Methods vs Events, an excerpt from JohnSweeneys Visual Basic for Testersbook

Properties vsMethods vsEvents

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Closer Look: UnderstandingProperties, Methods, and Events All objects in the Visual Basic language have their own

properties,methods,andevents.

Properties can be thought of as theattributes of an object,methodsasitsactions, and eventsasitsresponses.

An everyday object such as a helium balloon also hasproper ties,methods, and events.

Aballoon'spropertiesinclude

visible attributessuchasitsheight, diameter, and colour.

Other propertiesdescribeitsstate (inflatedor deflated),or

attributesthat are not visible, suchasits age.

All balloonshave these properties, although the valuesof these

propertiesmay differ fromoneballoonto another.OOP

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Closer Look: UnderstandingProperties, Methods, and EventsA balloon also has knownmethodsor actionsthat it can

perform. It has

an inflatemethod (filling it with helium),

a deflatemethod(expelling its contents),and

a risemethod (letting go of it).

a MakeNoisemethod

Again, all balloonscanperformthese methods.

Balloonsalsohaveresponsestocertainexternalevents.

For example,a balloonrespondsto

theeventof being puncturedby deflating,or to

theeventof being releasedby r ising.

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Closer Look: UnderstandingProperties, Methods, and EventsProperties

If you could program a balloon, the Visual Basic code mightresemble the following "code," which sets a balloon'sproperties.

Balloon.Colour = Red

Balloon.Diameter = 10

Balloon.Inflated = True

Notice the order of the code - the object (Balloon), followed bythe property (Colour), followed by the assignment of the value

(= Red). You could change the balloon's colour by substituting a

diff erent value.OOP

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Closer Look: UnderstandingProperties, Methods, and EventsMethods

A balloon'smethodsare calledasfollows.Balloon.Inflate

Balloon.Deflate

Balloon.Rise(5)

The order resembles that of a property- the object (a noun),followed by the method (a verb).

In the third method, there is an additional item, calledanargument,whichspecifiesthe distancethe balloonwill rise.

Some methods wil l have one or more arguments to furtherdescribethe actionto be performed.

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Closer Look: UnderstandingProperties, Methods, and EventsEvents

The balloon might respond to an event asfollows.

Sub Balloon_Puncture()

Balloon.MakeNoise("Bang")

Balloon.Deflate

Balloon.Inflated = False

End Sub

The code describes the balloon's behaviour when aPunctureeventoccurs.

call the MakeNoise method with an argument of "Bang" (the type ofnoise to make), thencall the Deflate method.

Since the balloon is no longer inflated, the Inflated property is setto False. OOP

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Event Handlers

NB:

Event handlersare also methods, but theyhave a special role - to respond to eventmessages passed to your application f rom theoperating system.

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Object-Oriented Event-DrivenProgramming (OOED)

OOED uses objects in the program and runs onlyafter the eventsoccur

OOEDiseasy to work with

Users can combine multiple objects to create newsystemsor extend existing ones

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Classes

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What is a Class?

Classes are the basic elements of object-orientedprogramming, which in turn makesit possible for programmersto build rich, robust applications.

Aclassis a designtemplatefor objects

Defineswhich properties, methods, and eventscan be applied to itsobjects

Objectsare also knownas instances

A classisdefined using the Class keyword.

E.g. every form that you add to an applicationis defined by a class,suchasthefollowing:

Public Class Form1

End Class

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Class Examples

Each control in the Visual Studio Toolbox window wasdefinedby a class.

For instance,the Buttoncontrol wasdesigned asa class

Whenyoudrag a button fromthe ToolBox onto a form, itbecomesanobject.EachButtonobject canbe assigned unique property valuese.g.:

ForeColor: Blue

Text:ClickMe

A TextBoxcontrol haspropertiessuchas:Name, Text, V isible, andForeColor.

All TextBox objectshave theseproperties.

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Namespaces

The Microsoft .NET Framework contains a largelibrary ofclasses that make it possible to write applicationsfor desktopcomputing, mobile applications, and the Web.

The classesare grouped by similarity into namespacesto makeit easier to find them.

Anamespace is a logical container that holds classes ofsimilar types.

For example,

theSystem.Collections namespace contains classes related to

building collections(arrays,lists, dictionaries, sets).The System.Windows.Formsnamespace contains classes related to

building desktopapplicationsfor Windows.OOP

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Creating Objects

A class must already be defined before you cancreate one or moreobjectsof the class type (akainstancesof the class, orclass instances).

Use the Newoperator, as in:

Dim fresher AsNewStudent

The NewoperatortellsVBto createan object inmemory.

It is required whencreating an object (thisrule applies to allobjectsexceptStringobjects).String objects are a special case of reference types declared as

follows:

Dim strName As String

Dim strCity As String = Mombasa

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Creating Objects

Dim fresher As New Student

Thislineof codeanbe separated into twosteps:

Dim fresher As Student

Thisvariabledoesnotreferenceanyobjectat thispoint - itonly hasa data type.

fresher = New Student

creates an instance of the class (i.e. a new object) andassignsit to the variable.

The variable containsa referenceto the object.

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Visual Studio Controls

Visual Studio creates instancesof controls when you drag them from the ToolBoxonto a form.

For example, the following code waswritten to a formsdesigner file whena buttonwascreatedand certainpropertieswere set in thedesigner window:

Me.btnOk = New Button()

Me.btnOk.Location = New System.Drawing.Point(43, 48)

Me.btnOk.Name = btnOk

Me.btnOk.Size = New System.Drawing.Size(75, 23)

Me.btnOk.Text = OK

Notice how the first line uses theNew operator to create an instance of the Buttonclass.

Thenvariousproperty valuesare assigned to the button (Location, Name, Size, andText).

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Nothing

When a reference type variable has not beeninitialised it has a null value indicated by the

Nothing keyword.

An object variable that is null must be initialisedbefore it canbe used

Thiscode generatesa runtimeerror:myObject = Nothing

myObject.Print( )

Asdo these linesof code:Dim fresher As Student

fresher.PrintCourses() OOP

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Nothing

If your code needs to know whether a variable hasbeen initialised, you can compare the variable to thekeywordNothing.

If fresher Is Nothing Then

' must initialise the variable

fresher = New Student

End If

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Value Type

There are two general categoriesof Visual Basic data types:

valuetypesand

reference types.

Valuetypes include all the number types, such as Integer andDecimal, aswell asDouble and Boolean.

These typesuse a standard-size storage location.

Thevariableholdsitsowndatainasinglememorylocation

Valuetypesdonotrequireanyinitialisation.

Assoonas youdeclare them, they haveimmediate storage.

Easy to use, consume little memory, and are the simplest tounderstand whenusing the assignment operator (=).

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Value Types

Example:

Dim Count As Integer

Count = 5

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Value Types

When you assign one value type to another using theassignment operator (=), a copy is made of the datain the variable on the right-hand side.

The data is copied into the variable on the left-handside.Dim mCount As Integer = 25

Dim temp As Integer = mCount

mCount is copied to temp

If a new value is later assigned to temp, mCount is notaffected:

temp = 40' mCount still equals 25OOP

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Reference Type

Whenever you create an instance of a class(an object) and assign it to a variable, yourvariable isa referencetype.

A variable declared as a reference type doesnotdirectlyholditsdata.

Instead the variable points to (holds areference or address of) an object storedsomewhere else in memory

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Reference Type

Classesand Arraysare examplesof reference types.

When an object is created by invoking theNewoperator, the .NET runtime reserves space in memoryfor the object.

Theaddress of the object is stored in areferencevariable.

Takesmoreprocessing time thanfor value types, but

Allows .NETto reclaim the storage used by the objectwhenitisnolongerneededbytheprogram.

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Reference Type ExampleWhat does the following piece of code do?

Dim P As New Person

creates a Person object

assigns the objects reference to P

P.Name = "Fred Omar

assigns a value to the objects Name

property

Omar

Pcontainsa reference to the data, not the data itself.If at any time in the future, the Person object is no

longer needed, we can assign a value of Nothing to P

i.e. P = NothingAssuming that no other references to the same Person

object existed, a special utility in the .NET runtime

called the garbage collectorwould eventually remove

the objectfrom memory.OOP

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Assigning Objects

The assignment operator, (=),copies a reference,not a value:

Dim Y As New Account

creates an Account object

Dim X As Account

creates object/reference type variable

whose data type is Account. It is

uninitialised (has no reference)

X = YNow, Both X and Y reference the same

objectOOP

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Assigning ObjectsArray Example

The following code creates an array ofintegers namedtests, fills the array, andassigns the array to the variable namedscores:

Dim scores() As Integer

Dim tests() As Integer =

{80, 95, 88, 76, 54}

scores = testsOOP

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Assigning ObjectsArray Example

After this code executes, the same array isreferenced by bothscoresandtests

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Array Example

The following code can be used to show that thetwoarraysshare thesame memory.

scores(2) = 11111

MessageBox.Show(tests(2).ToString())

' displays 11111

By assigning a new value to scores(2), weautomatically assign thesame valueto tests(2):

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Assigning Objects

This type of dual reference can lead to a commontype of programming error known asa side effect

As it causes unwanted effects by changing variables in away that canfool a programmer.

Assigning one reference type to another using the=operator leads to both variables referencing thesame object.

Thusa change to one variable will cause a change to

the other (this is the side effect)Code containing side eff ects is very dif ficult to debug.

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More on ArraysUsing a Loop to Copy an Array

If you want to copy the contents of one array toanother, you can use a loop to copy the individualelements.

First, youreserve spacein the scoresarray.

Thenyoucopy the data.

Dim scores(tests.Length - 1) As Integer

For i As Integer = 0 To tests.Length - 1

scores(i) = tests(i)

NextOOP

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The result af ter copying the array.

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The following code showsthat the two arraysdo notshare thesame memory.

scores(2) = 11111

MessageBox.Show(tests(2).ToString())

displays 88

When a new value is assigned to scores(2), thevalueof tests(2) isunchanged:

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Object.Clone

The Object.Clone method is a more general way ofcopying the data fromany referencetypeto another.

It

Copiesanobject or anarray

ReturnsanObject

The following statement copies the array from ourscoresand testsarraysexample:

scores = CType(tests.Clone(), Integer())

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CType Function

Returns the result of explicitly converting anexpression to a specified data type, object,structure,class, or interface.

Syntax:

CType(expression, typename)

You can also perform a conversion to a specificdata type using functions

CByte,

CDbl, and

CInt. OOP

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Object.Clone

scores = CType(tests.Clone(), Integer())

The Clone methodreturns anObject, so the returnvalue must be cast into an Integer arraywhenOption Strictis ineffect.

asscoresis an integer array.

Option Strictprevents program from automaticvariable conversions, i.e. implicit data typeconversions

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Object.Clone

Heresanother example, using two Personobjects:

Copying a Personobject:

Dim P As New Person

P.LName = Omari

Dim S As Person

S = CType(P.Clone(), Person)

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Comparing Standard .NET Objects

All standard .NET objectscan be compared forequality by by using the= operator or calling theEqualsmethod.

Thisis thecase for strings:

Dim A As String = abcde

Dim B As String = "abcde"

If A = B Then ...' result: True

If A.Equals(B) Then ...' result: True

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You can also comparestrings by calling theCompareTomethod

If X < Y, CompareTo returns a negative value

If X = Y, CompareTo returns zero

If X > Y, CompareTo returns a nonzero positive value

Example:

Dim A As String = "abcde"

Dim B As String = "abd"

Dim result As Integer = A.CompareTo(B)

' result is assigned a negative value

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Dim A As String = abf

Dim B As String = abd


' result = positive value

Dim A As String = abd

Dim B As String = abd


' result = zero

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Comparing User Class Types

Your own classes, by default, will not use EqualsandCompareToeffectively.

Consider the following comparison of two Studentobjects:

Dim s1 As New Student(1001)

Dim s2 As New Student(1001)

If s1.Equals(s2) Then ...

The call to Equals will return

False, even though the students

apparently have the same ID numberOOP

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Comparing User Class Types

Similarly, calling s1.CompareTo(s2) below is notmeaningful:

Dim result As Integer = s1.CompareTo(s2)

This meansthat youcannot effectively sort an arrayof Students.

The following 2 slides showsan example of how tocompare user classtypes

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Comparing User Class TypesUsing CompareToClass Student

Implements IComparable

class implements the IComparable interface. An

interface defines a set of methods and properties

that can be implemented by other classes.

Public Property Id As String

Public Property LastName As String

Public SubNew(ByVal pId As String, ByVal

pName As String)

Id = pId

LastName = pName

End Sub OOP

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continued on next slide

Comparing User Class TypesUsing CompareToPublic Function CompareTo(ByVal obj As Object) As Integer

_Implements IComparable.CompareTo

The CompareTo method in this class implements the

CompareTo method specified in the Icomparable Interface

Dim S As Student = CType(obj, Student)

casts the obj parameter into a Student object, allowing

us to refer to the Id field in the next line:

Return Me.Id.CompareTo(S.Id)

The Id value of the current student (identified by Me)is

compared to the Id value of the student who was passed as

the parameter to this method.

End Function

End Class

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Creating Your Own Classes55

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Creating Your Own Classes

The.NETFramework comeswith its ownset of classes.

Youcan also design and build your ownuser-defined(orcustom) classes.

You create a class in Visual Basic by coding aclassdefinition.

Syntax

Public Class ClassName

class members hereEnd Class

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FromtheMenu:

Select Projectonthe menubar, then

select AddClass.

TheAdd New Itemdialog box appears.

Or

IntheSolutionExplorerwindow:

Right-click the Project name,

select Add

select Class.

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The keywordPublicisanaccess specifier.

It tells VB that the class will be visiblefromall partsof your application.

i.e. itwill bepossibletocreateobjectsthatusethisclassname.

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Class-Level Variables

Class-levelVariablesare those variablesdeclaredinside a classbut outside of any methodsin the class

They are thusvisible to all methodsin theclass.

(Local variablesare declared inside methods and arevisibleonly inside themethod)

Syntax of a Class-level Variable:AccessSpecifier name As DataType

Example:Private myIdNumber As String

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Class-Level Variables

Public Class Student

Private mIdNumber As String

Private mLastName As String

Private mTestAverage As Double

End Class

NB:

A class definition does not, by itself, create an instance

of the class. It establishes a blueprint for the classs organisation, which

makesit possible for you to write other code that creates anobject of this type OOP

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Encapsulation

The capability of an object tohide its internalworkingsfromother objects.

The programmer needsNOTknow what is going oninside the object, but only how to work with theobjectspropertiesand methods.

In object-oriented programming, the encapsulationprinciple says that you should bundleattributesandbehavioursinsideaclass.

Think of a class as a container that encapsulateseverything inside for easy transportingand usage.

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Encapsulation

Encapsulation is the processof integrating dataand code into one entity: anobject.

i.e. the object comprises all the data it usesand itsfunctionality.

E.g.:

Data for an employee includes

name,date hired,and title

Thefunctionality includes

methodsfor adding and removing employees.

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Encapsulation

Your application, as well as external applications,could then work with the employee data through aconsistent interface - the Employee objectsinterface.

An interface is a set of exposed functionality -basically, code routines that define methods,properties, and events.

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Information Hiding

Theinformation hiding principle,which is closelyrelated to encapsulation, says that certain classmembersshouldbevisible only to methodsinsidetheclass.

Usually, this applies tovariables, which are labeledasPrivate.

A class-level variable could be declared Public, socode anywhere in an application (e.g. any classes)

could accessit directly.But doing so would violate the information hiding

principle.OOP

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Information Hiding

Instead, we usepublic methods and propertiestodefinean interface, or public view of a class.

Other information, such as variables, remain hiddenby using the Private keyword.

A variable isusually declared asPrivate.

It may be accessed only by statements inside methodsbelonging to thesame class.

i.e. hidden (private) memberscan be accessed only by othermethodsinthesameclass.

This is a good idea because it leads to more reliableprogramsthat are easier to debug.

NB:Many software engineersconsiderencapsulationand informationhiding to be thesame.

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Object Methods

An action (or named executable unit) that implements somebehaviour of a class.

Simply put, it is asetof predefinedactivities that an objectcancarry out.

i.e. a method isa verbthat can be carried out by theobject.

The syntax for using an objectsmethod is:object.method

For the various VB objects, there are usually several methodsalready available.

Advanced programmerscancreate their ownmethods.

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Methods Example

Real life example

dog.eat

dog.bark

dog.run

A Visual Basic example frmMain.hide

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Methods

There are twotypesof methods:

Instancemethod called using an instance of theclass(object)

Dim S As New Student

S.Print()

Sharedmethod called using the classname

Dim arrayScores() As Integer = {62, 45, 89}

Array.Sort(arrayScores)

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Properties

In VB,a property is a special type ofmethod thatuses the same member name forgetting and settingavalue

Whereas methods are the implementation of classbehaviours, properties are implementationsof classattributes.

Button objects, for example, have a number ofproperties that are listed in the Properties window inVisual Studio.

A Property name lookslike a variable to usersof the

class OOP

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Declaring a Property

Public Property PropertyName() As

DataType The brackets after

PropertyName are optional

Get

'(code that returns data)

End Get

Set(value As DataType)

'(code that assigns value to a class

variable)

End Set

End Property OOP

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Property Definition Code Example

class Student

Private mLastName As String


Get

Return mLastName

End Get

Set(ByVal value As String)

mLastName = value

End Set

End Property

End Class OOP

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Auto-Implemented Property

A property that is defined by only a single line of code

Getsand setsvalue of a hidden private field

Youdonthavetocreateaprivatememberfieldtoholdthepropertydata.

There are two general formats:Public Property PropertyName As DataType

Public Property PropertyName As DataType = InitialValue

You can follow each property name with optional parentheses:PropertyName()

Example:Public Property IdNumber As String

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Public Property PropertyName As DataType = InitialValue

Init ialValueisan optional value that youcan assign tothe property when it is created.

When you declare an auto-implemented property,Visual Studio automatically creates a hidden privatefield called a backing field that contains theproperty value.

The backing fields name is an underscore and theproperty name.

E.g., if you declare anautoimplemented property named ID,

itsbacking f ield isnamed _ID. OOP

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Examples

auto-implemented properties that could be used in theStudent class:

Public Property IdNumber As String


Public Property TestAverage As Double = 0.0

Now that we have auto-implemented properties, whydo we need to create the longer property definitions?

Because they allow usto include range checking and othervalidationsondata assigned to the property.

A ReadOnly property must be f ully codedit cannot be

auto-implemented. OOP

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Getting and Setting PropertyValues

Before accessing a property, you mustdeclareaninstanceof theclass (object) that containsthe property.

We could place the following statementanywhere in the program outsidetheStudentclass:

Dim fresher As New Student

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The Set section of the property procedureexecutes when a value is assigned to theproperty.

The following statement sets the value ofLastName:

fresher.LastName = Odhis

Thus executing the following statement inside

the property procedure:

_LastName = valueOOP

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TheGetsection of a property procedure executeswhen a program needs to have a copy of theLastName.

Suppose thatoutsidetheStudentclass, we wrote thefollowing statement, which copies the studentsLastName value to a TextBox:

txtLastName.Text = fresher.LastName

The following statement inside the propertyprocedure would execute:

Return _LastNameOOP

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Input Validation in Properties

A property can be very useful whenvalidating valuesassigned to it.

In the foll owing example, which implementsthe TestAverage property, the value assigned totheproperty must be between0.0 and 100.0:

Public Property TestAverage As Double

Get

Return mTestAverage

End Get

Set(ByVal value As Double)

If value >= 0.0 And value

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Object Initialisers

The following statement creates and initialises aStudentobject using control values:

Dim aStudent As New Student With

{

.IdNumber = txtIdNumber.Text,

.LastName = txtLastName.Text,

.TestAverage = CDbl(txtAvg.Text)

}

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Assigning Object Variables

There are well-defined rules for assigningvaluesofstandarddatatypesto eachother.

E.g. we can assign an Integer expression to aDoublevariable, because VB automaticallyexpands the integer expression to typeDouble.

There are similar but more restrictive rules forassigningclassobjects to eachother.

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Assigning Object Variables

In nearly all cases, you must perform a cast fromone type to anotherexcept in the following twocircumstances

1. Twovariableshavethe same classtypeDim student1 As new Student

Dim student2 As Student = student1

2. The twovariables are of different types, but thevariableonthe left istypeObjectDim student1 As New Student

Dim obj As Object = student1

The second opt ion is allowed because Object is a very general type that a ccepts any type of a ssignment.

83

Not ice here we are assigning one reference variable (not object) to another.

Assigning Object VariabesNote:

The expression on the right side of the =operator doesnot have to be a variable;

it might be a property name or method call.

For example, a method namedGet-Studentreturns a Student object, which cannot beassigned directly to a String variable:

Dim temp As String = GetStudent("12345")'error

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Assigning Object VariabesNote:

We can assign the Student object to a string variableif we call the objectsToString method:

Dim temp As String = GetStudent("12345").ToString()' ok

Converting any object to a string is easy because allclassesimplicitly contain aToStringmethod.

But if you want to convert to some other type, youmay have to call the CType function.

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Assigning Object Variabes

In nearly all cases, you must perform a cast from one

typeto another

E.g., the ListBox controlsSelectedItem propertyreturnsan object.

If you want to assign this object to a Studentvariable,youmust call theCType function:

The variable onthe right istype ObjectDim selStudent As Student = lstStudents.SelectedItem

Will NOT compile if Opt ion Stric t is turned on

Dim selStudent As Student =

CType(lstStudents.SelectedItem, Student)OOP

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Three-Tier Application Model

The basic design that most businessapplicationstoday follow.

Each tier contains classes that callmethodsin the tier below it.

PresentationTier objectsthat interactwith theuser

Middle Tier core information, such as calculations and

decisionmakingData AccessTier interactdirectly withdata source

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The Presentation Tier(User Interface Tier/User Services Layer)

Consistsofall objectsthatinteractwiththeuser.

Visual Basic usesa classto define a form, aswell asthe variouscontrolson a form.

Code that you write inside the form of anapplicationbelongshere e.g. all the formclasss:

eventhandler procedures,

class-level variables, and

other subprocedures.

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The Middle Tier(Business Logic Tier / Business Services Layer)

Consists of classes that provide core information tothe application, such as essential calculations anddecisionmaking.

They often embody the business rules of anorganisation Including operational principles that are common to

multipleapplications.

Theseclassesdonotinteractwiththeuser. Instead, they contain methods and properties that are

called by classesin thepresentationtier.

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The Data Access Tier(Data Services Layer)

Contains classes that interact directly with a datasource.

Later we will create classes for this tier that readand writeto databases.

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Constructors

A methodthatrunsautomaticallywhenaninstanceoftheclassiscreated.

In Visual Basic, a constructor is always namedNew.

Constructors typically initialise classmember variablesto default values, but they can also be used toperformany required classinitialisation.

E.g. if a class is connected to a network connection, the

constructor could be used to open a connection to aremote computer.

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Constructors

A default constructoris a constructor with noparameters.

To create a simple one for the Student class thatassigns a default value to themIdNumber datamember:

Public Sub New()

mIdNumber = "999999"

End Sub

With this constructor in place, if a client programcreates a new Student object, we know for certainwhat value theobjectsmIdNumberwill contain.OOP

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ParameterisedConstructor

A classmay contain more than one constructor.

Thus apart from the default constructor, youmay want to create a parameterisedconstructor(a constructor with parameters).

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Assigns values to each of the Student class-levelvariables:

Public Sub New(ByVal pIdNumber As

String, ByVal pLastName As

String,ByVal pTestAverage As Double)

mIdNumber = pIdNumber

mLastName = pLastName

mTestAverage = pTestAverage

End Sub

ParameterisedConstructorExample

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Will This Code Compile?

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Public Sub New(ByVal pIdNumber

As String, ByVal pLastName As

String, ByVal pTestAverage As

Double)

pIdNumber = IdNumber

pLastName = LastName

pTestAverage = TestAverage

End Sub

Will This Code Compile?

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The code compilescorrectly

However, the operands in the assignmentstatementsare reversed.

They copy the valuesfromthe propertiesto theparameters.

The result is that the constructor does not workproperly: the values passed to the constructor

are not assigned to the classproperties.

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When coding a constructor,DO NOT use the samename for the constructor parameters that you use forclasslevel variables and properties:

Public Sub New(ByVal IdNumber As String,

ByVal LastName As String, ByVal TestAverage

As Double)

IdNumber = IdNumber

LastName = LastName

TestAverage = TestAverage

End Sub

ParameterisedConstructorParametreNaming

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Default Constructors

If your class does not contain any constructors,VB creates an invisible empty defaultconstructorfor you.

This is for convenience, so you can declare anobject like this:

Dim secondYear As New Student

But if you add a parameterised constructor tothe class, a default constructor is NOTCREATEDautomatically for you.

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Default Constructors

Suppose thiswere the only constructor we had in theStudent class:


String, ByVal pLastName As String,

ByVal pTestAverage As Double)

' (lines omitted)

End Sub

Why would thefollowing statement not compile?Dim objStudent As New Student

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Constructors with OptionalParameters

An optional parameter does not require thecalling method to pass a correspondingargument value.

Sometimes you will want to create instances ofa classusing varying amountsof information.

You can declare optional parameters in anymethod (including constructors) using the

Optional keyword, as long as youassign eacha default value.

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String,Optional ByVal pLastName As

String = , Optional ByVal

pTestAverage As Double = 0.0)

the pLastName and pTestAverage

parameters are optional

IdNumber = pIdNumber

LastName = pLastName

TestAverage = pTestAverage

End Sub

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Dim A As New Student("200103")

Dim B As New Student("200103",

Odhis")

Dim C As New Student("200103",

Odhis", 86.4)

All three statements are valid ways of declaringStudent objects as the pLastName andpTestAverage parameters are

optional

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Constructors with OptionalParameters: Rules

1. Once a parameter is labeled optional, allsubsequent parameters in the methods parameterlist must also be labeled the same way.

So let the optional parametresbe the last inthe list

2. All optional parameters must be assigned defaultvalues.

When the Visual Studio editors Intellisense tooldisplays a methods parameter, optionalparametersappear inside square bracketse.g.:

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Do Labs:

1-1: Creating a Student Class

1-2: Adding a parameterised constructor

Lab104

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Lab 1-1

Creatinga Student Class

Two-tier application(Presentation& Middle)

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Lab 1-2

Adding a parameterised constructorto the Student classThree parameters:

1. ID number,

2. last name,

3. test average

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ReadOnly Property

A property that allows methods to get the currentproperty value, but not change it

Example:

Public ReadOnly Property Count As Integer

Get

Return mCount

End Get

End Property

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Enumerated Types

List of symbolic namesassociated with integer constants

Makesprogramsmore readable and maintainable

by giving namesto what would otherwise be integers.

E.g.

Consider an application that works with four differentaccount types,numbered 0, 1, 2, and 3.

It might be diff icult, when looking at programcode, to recallwhichinteger corresponded to eachtype of account.

Better to define an enumerated type that would provide thisinformation

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Enumerated TypesExampleEnum AccountType

Current

Savings

Trading

Annuity

End Enum

The enumerated typedefines, and thereforerestricts, the set ofvalues that can beassigned to variablesof its type.

Internally, the list ofAccountType valuesareassigned the integervalues0, 1, 2, and 3.

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Enumerated Types

When you press the dot after an Enum variable,Visual Studios Intellisense tool showsa list of all theEnumvaluesthe variable can hold.

Youdo not use the Newkeyword when declaring anenumeratedobject:

Dim acct As AccountType

If you declare an AccountType object, only valuesfromthe prescribed list should be assigned to it:

acct = AccountType.Current

acct = AccountType.TradingOOP

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Enumerated Types

The following statement is i llegal because integers are not assignment-compatible withEnum types:

acct = 1

In special cases, you can assign an integer into an AccountType, but youshould dothat only when noother optionis available.

E.g., suppose you were to read an integer from a TextBox, and the integer wassupposed to indicate a type of account.

TheCtypefunctionmust be used to cast theinteger intoAccountType:

Dim acct As AccountType

Dim N As Integer = CInt(txtAccountType.Text)

acct = CType(N, AccountType) No cast is required to assign an enumerated type to an integer:

Dim N As Integer = acct

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Using Boolean Expressions

Enumerated types are particularly useful when usedin Boolean expressionsthat involve comparisons.

Example1

To take a particular action if an account is an annuity:If acct = AccountType.Annuity Then

taxDeferred = True

End If

Sucha statement iseasier to read than:

If acctCode = 3 Then

taxDeferred = True

End IfOOP

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Using Boolean Expressions

Example2

TheSelect Casestatement can go through a list ofenumerated values and take a separate action foreachpossible value:

Select Case acct

Case AccountType.Annuity

lblResult.Text = "Plan payments for retirement"

Case AccountType.Checking

lblResult.Text = "Write checks to pay bills"

' etc.

End Select OOP

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Do Labs:

1-3: Enumerated Types

1-4: Bank Teller Application

Lab114

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Lab 1-3

Enumerated Account type

When the user selects an account type f rom a list box,the selected index is converted into an AccountTypeobject.

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1.4Bank Teller Application

Two-tier application that simulates an electronicbankteller

user looks up account, deposits & withdraws funds,viewsthebalance

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Requirements

1. User looks up existing account information byentering an Account number (ID).

2. If the account exists, the account name and balanceare displayed.

3. User can enter an amount to deposit into theaccount. Thisdeposit is added to the balance.

4. User can enter an amount to withdraw from theaccount. If the amount is

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Manual Software Testing

Importance of software testing

We must create programsthat performin theway they were intended.

We all like to use reliable software, andoftenour livesdepend on it.For example, the fl ight navigation system for an

aircraft must not fail, nor should medicaldevices.Search wikipedia for Therac-25, a famous software

failure. OOP

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Testing Modes

Manual testing

Automated testing

In manual testing, a human tester manually enters a variety ofinputs into an application.

The tester compares the actual outcomes produced by thesoftware to a set of expected outcomes.

Manual testing is often associated with the termblack boxtesting

where the tester isconcerned only with the programsinput and outputs.

The tester cannot see the code inside.

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Requires a lot of human labor, and therefore isexpensive.

Automated testing is performed by a computerprogram, whichexecutespart or all of an applicationin a way that requiresno manual intervention.

Testingplan

A list of tests that are run on an application to verifythat the application works asexpected

For each given user action or input value, the testingplan lists the expected output or action produced bytheapplication. OOP

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Requirements Specification

Before creating an application, we usually want toknowwhat it issupposed to do.

A req spec is a complete description of thebehaviour of anapplication

Used asa guide for software testing

Describes inputs and actions by the user and outputs,andhow they affect the application'sbehaviour

In the next slide is a sample requirementsspecification for a program that inputs an integerand displaysa corresponding colour

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Sample RequirementsSpecification

The application prompts the user with a range of acceptableinteger values.

The user inputs an integer N.

If N is a noninteger value, the application displays an errormessage.

If N is outside the range of acceptable values, the applicationdisplaysan error message.

If N is within the range of acceptable values, the applicationdisplays the name of a colour that matches N from thefollowing list:

0 = white,1 = yellow, 2 = green, 3 = red, 4 = blue, 5 = orange.

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Do the Lab

Lab1-5: Manually Testing IntegerInput

126

OOP

Lab 1-5

Manually testing integer input

Displays a string by using the input value as asubscript intoan array of strings

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Testing Plan

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Key Terms

accessspecifier

assignment operator (=)

attributes

auto-implemented property

automated testing

behaviours

class

classdefinition

classinstance

class-level variable

constructor

data accesstier

default constructor

encapsulation principle

enumerated type

information hiding principle

inheritance

instance

local variable

manual testing

method

middle tier

Microsoft .NETFramework

namespace

New operator

object

object behaviours

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More Key Terms

ob ject initializer

object-oriented programming (OOP)

optional parameter

parameterisedconstructor

presentat ion tier

property

property procedure

ReadOnlyproperty

reference type

reference variable

requirementsspecification

shared property

side effect

testing plan

three-tier application model

ToString method

user-defined class

value type

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Documents

APII 3 OOP