110-I 1

Object Terminology Review• Object - like a noun, a thing

– Buttons, Text Boxes, Labels• Properties - like an adjective, characteristics of

object– Text, ForeColor, Checked, Visible, Enabled

• Methods - like a verb, an action or behavior, something the object can do or have done to it– ShowDialog, Focus, Clear, ToUpper, ToLower

• Events - object response to user action or other events– Click, Enter, Activate

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So Far . . .

• Since Chapter 1 we have been using objects

• Up until now the classes for all objects used have been predefined

• We have created new objects for these classes by using the controls in the Toolbox

• VB allows programmers to create their own object types by creating a Class Module

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• The class is the blueprint for the object. It describes– The data contained in the object

(properties)– The actions that the object can perform

(methods, event procedures)

An object is an instance of a class.

What is a Class?

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• Defining a class does not create an object, only a definition of what that type of object looks like and behaves.

• Example: the Button class. You can have many buttons in your project (many instances of the Button class). Each button will have its own properties; different text, different name, different code for the event procedures.

A Class vs. an Object (1)

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A Class vs. an Object (2)

• When we add a button object from the button tool in the toolbox to the form we are creating an Instance of the Button Class

• Every button on the form is an Instance of the Button Class

• Defining your own Class is like creating a new tool for the Toolbox

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"Cookie Analogy"

• Class = Cookie cutter• Instantiate = Making a cookie using the

cookie cutter• Instance = Newly made cookie• Properties of the Instance may have

different values– Icing property can be True or False– Flavor property could be Lemon or Chocolate

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"Cookie Analogy" (cont.)

• Methods = Bake

• Events = Cookie crumbling when you step on it accidentally

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Encapsulation• Combination of characteristics of an object along

with its behavior in "one package"• Cannot make object do anything it doesn't already

"know" how to do• Cannot make up new properties, methods, or events• Sometimes referred to as data hiding;an object can

expose only those data elements and procedures that it wishes

• The details of the implementation of the class are not relevant (they are hidden) to the user of the class.

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Reusability

• The main purpose behind OOP and Inheritance in particular

• New classes created with Class Module can be used in multiple projects

• Each object created from the class can have its own properties

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Class Design - Analyze:

• Characteristics of your new objects– Characteristics will be properties– Define the properties as module level variables

in the class module

• Behaviors of your new objects– Behaviors will be methods– Define the methods as sub procedures and

functions in the class module

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Create a New Class

Add Class Module to your project

• 1. Go to New Item dialog, choose Class

• 2. Name the Class

• 3. Define the Class properties

• 4 .Code the methods

STEPS:

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Properties of a Class

• Define variables inside the Class Module by declaring them as Private at module level.

• Do not make them Public-that would violate Encapsulation (each object should be in charge of its own data)

• Private mintPatientNum as Integer

• Private mdtmDate as DateTime

• Private mstrLastName as String

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Assign Values to Properties• Write special property procedures to

– Pass the values to the class module– Return values from the class module

• Name used for property procedure is the name of the property seen by the outside world

• Property Get– Retrieves property values from a class– Like a function must return a value

• Property Set– Sets or assigns values to properties

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Property Procedure General Form

Private ClassVariable As DataType[Public] Property PropertyName As DataType

GetReturn ClassVariable

End Get

Set (ByVal Value As DataType)[statements, such as validation]ClassVariable = Value

End SetEnd Property

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Read-Only Properties

• In some instances a value for a property should only be retrieved by an object and not changed– Create a read-only property by using the

ReadOnly modifier– Write only a Get portion of the property

procedure

[Public] ReadOnly Property PropertyName As DataType

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Code a Method

• Create methods by adding sub procedures and functions for the behaviors to the class module (just like we have up to now).

• The sub procedures and functions should be defined as Public unless there are going to be used for implementation purposes.

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Analyze the characteristics (properties) and behaviors (methods) of your object

Write Property Procedures to assign and retrieve the properties of the classIt allows us to declare all module variables (properties) private

Write Sub Procedures and Functions for the behaviors of the class

Summary:Defining a Class(1)

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Writing Property Procedures

(they are public by default)

Private mstrLastName As String

Property LastName( ) As String

Get

Return mstrLastName

End Get

Set (ByVal Value As String)

mstrLastName= Value

End Set

End Property

Declaration of property

Property P

rocedure

Returns value of property

Sets value of property (needs argument)

Summary:Defining a Class(2)

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Read Only Property Procedures

Example:

Private mstrLastName As String

ReadOnly Property LastName( ) As String

Get

Return mstrLastName

End Get

End Property

Declaration of property

Property P

rocedure

Returns value of property

Summary:Defining a Class(3)

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BookSale class• Three private instance variables

- mstrTitle

- mintQuantity

- mdecPrice• Three public property Procedures

– Title– Quantity– Price

• One public method– ExtendedPrice

The class diagram is incomplete (e.g., no type for the instance variables, no information about the method)

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Public Class BookSale ‘ Class Name: BookSale

Private mstrTitle As String

Private mintQuantity As Integer

Private mdecPrice As Decimal

‘Property Procedures

Property Title( ) As String

Get

Return mstrTitle

End Get

Set (ByVal Value As String)

mstrTitle= Value

End Set

End Property

A complete class code (1)

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Property Quantity( ) As Integer

Get

Return mintQuantity

End Get

Set (ByVal Value As Integer)

mintQuantity= Value

End Set

End Property

Property Price( ) As Decimal

Get

Return mdecPrice

End Get

Set (ByVal Value As Decimal)

mdecPrice= Value

End Set

End Property

A complete class code (2)

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‘write the methods

Public Function ExtendedPrice() As Decimal

Return CDec(mintQuantity* mdecPrice)

End Function

End Class

A complete class code (3)

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Creating a New Object Using a Class

Steps

• Declare a variable for the new object with datatype of the class

• Then, instantiate the object using the New keyword

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Instantiating An Object

• Creating a new object based on a class

• Create an instance of the class by using the New keyword and specify the class

New className ( )

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Examples of InstantiatingAn Object

Dim fntMyFont = New Font ("Arial", 12)lblMsg.Font = fntMyFont

OR

lblMsg.Font = New Font ("Arial", 12)

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Best Practices

• You may declare and instantiate an object at the same time but it is not the best practice

• Should declare the variable separately in the Declarations section

• Instantiate the object– Only when(if) it is needed– Inside a Try/Catch block for error handling

(Try/Catch block must be inside a procedure)

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Example using our class Public Class frmBookSale : Inherits System.Windows.Forms.Form

Private mBookSale As BookSale ‘declare module level variable

Private Sub mnuFileCalculateSale(..)

Try

mBookSale = New BookSale() 'instantiate the object

'Set the properties of the booksale object

mBookSale.Title = txtTitle.Text

mBookSale.Quantity = CInt(txtQuantity.Text)

mBookSale.Price = CDec(txtPrice.Text)

‘Display the book title and calculate and format the result

lblTitle.Text=mBookSale.Title

lblExtendedPrice.Text = FormatCurrency(mBookSale.ExtendedPrice())

Catch 'display message if there is an error

MessageBox.Show("Please make sure your entries are correct”)

End Try

End Sub

End Class

Calling the object method

Calling “Set” property procedures

Creating the object

Calling “Get”

property procedure

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Instance Members And Shared MembersMembers include both properties and methods

A shared member has ONE copy for ALL objects of the class

No need to instantiate an object to access the shared members (property or method), just use the Class Name

ex. Color.Red

A instance member has ONE copy for EACH object or instance of the class

Class Name

Property Name

Use the Shared Keyword to create a Shared Member

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Instance versus Shared Members

• Instance Members– Separate memory location for each instance

of the object

• Shared Members– Single memory location that is available for

ALL objects of a class– Can be accessed without instantiating an object

of the class– Use the Shared keyword to create

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Creating Shared Members

Use the Shared Keyword to create a Shared Member

Example.

Private Shared mdecSalesTax As Decimal

Shared ReadOnly Property SalesTax( ) As Decimal

Get

Return mdecSalesTax

End Get

End Property Make shared properties Read-Only, so their valuescan be retrieved but not set directly

[Public|Private] Shared Function FunctionName (ArgumentList) _As Datatype

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The KeyWord Me• An instance method of a class is always called using an

instance of the class (an object), Object.Method(Arguments);

• The instance object is not part of the actual parameter list. However, it is still passed to the method implicitly.

• It can't appear in the formal parameter list

• To refer to the object used to make the call, use the keyword Me.

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Using The KeyWord Me An example(1)• In an instance method, access the object implicitly passed to that

method with the me keyword. Public class SomeClass

private var As Integer Public Sub changeVar(n As Integer)

Me.var = n ‘OK, but superfluous End SubEnd Class

• Could also do Public Sub changeVar(var As Integer)

Me.var = var End Sub

var is local to changeVar. Since the name is the same as the instance variable var, we need to use Me to refer to the instance variable var (potentially confusing).The local variable shadows the instance variable.

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Using The KeyWord Me An example(2)• In an instance method, any reference to an instance variable or any call to another

instance method done without an explicit reference uses the implicit object(me). Public class Bar

Public Sub foo1() ‘ codeEnd SubPublic Sub foo2()

foo1(); ‘call the instance method foo1 ‘ of the implicit object.

‘ same as Me.foo1(); End SubEnd Class

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Constructors and Destructors• Constructor

– Method that automatically executes when an object is instantiated

– Create by writing a Public Sub New procedure

• Destructor– Method that automatically executes when an object is

destroyed

– Create by writing a Finalize procedure

– Usage discouraged by Microsoft

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Overloading

• Overloading means that 2 or more methods have the same name but a different list of arguments (the signature)

• Create by giving the same name to multiple procedures in your class module, each with a different argument list

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Parameterized Constructor

• Constructor that requires arguments

• Arguments are passed when an object is instantiated

• Can be used to assign initial property values

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More on Constructors

Sub New()

‘Constructor with empty argument List

‘Initialize Instance Variables

End Sub

Sub New( Arguments)

‘Constructor with argument List

‘Use Arguments to initialize instance variables

End Sub

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Example Using a Constructors

Sub New( ByVal Title As String, ByVal Quantity As Integer, ByVal Price As Decimal)

‘Use Arguments to initialize instance variables

Me.Title=Title

Me.Quantity=Quantity

Me.Price=Price

End Sub

Call to the Set Property Procedure

Set the value using the argument

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References and assignment (1)• Consider

Public Sub SomeSales() ‘ create two objects

mBookSale1 = New BookSale() mBookSale2 = New BookSale()

‘ Set title, quantity and price mBookSale1.Quantity=5 mBookSale2.Quantity=10 mBookSale1.Price=2 mBookSale2.Price=3

mBookSale2= mBookSale1‘ Calculate the cost of each sale and display it

lblDisplay1.Text= mBookSale1.ExtendedPrice() lblDisplay2.Text= mBookSale2.ExtendedPrice()

End Sub

• What happens?

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References and assignment (2)

• Before executing mBookSale2 = mBookSale1

mBookSale1:BookSale

mBookSale2:BookSale

mBookSale1 = mBookSale2

:BookSale

mBookSale2:BookSaleX

The object that mBookSale2 referred to can't be accessed anymore. The memory it uses is freed by the garbage collector.

The sale price is the same in both labels

Note: if 2 or more references refer to the same object, drop the object reference in the UML notation

mBookSale1

mBookSale2

mBookSale1

mBookSale2

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Garbage Collection• Feature of .NET Common Language Runtime

(CLR) that cleans up unused components• Periodically checks for un-referenced objects and

releases all memory and system resources used by the objects

• Microsoft recommends depending on Garbage Collection rather than Finalize procedures