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Programming with Visual Basic .NET

Creating and Using Classes

Lecture # 8

Tariq Ibn Aziz

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Classes

• A class may contain three type of items:

– Variables (state)

– methods (behavior)

– constructors

• variable store the values

• methods provides the logic, that constitutes the behavior of class

• Constructor initialize the state of a new instance of a class

Class Members

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Method

• A method is essentially a set of program statement

• Each method exist as part of a class

• Visual Basic program contain one or more classes, each may contain one or more method

• Method may call other methods in the same or a different class

• No program code can exist outside a method and no method can exist outside a class

• Note: Main() is also a method

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Shared and Instance Members

• There are two ways that an item can be a member of a class– Shared member

– Instance member

• Shared methods are accessed through the class. • Non Shared methods, also called instance

methods, are accessed through instances of the class.

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Shared Method and Variable

• A Shared method is associated with a class, so no need to create an instance of that class.

• To call Shared method, use the following syntax:

className.mthName(args)

• Shared variable is associated with a class, so no need to create an instance of that class.

• To call Shared variable, use the following syntax:

className.variable1

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Object-Oriented Programming

• In OOP classes are Types

• Objects are Instances of classes

• Objects may have member data or member methods or both

• The mechanism to create a new Object is called instantiation

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Three Principles of OOP

• Three language features are fundamental to object-oriented programming

– Encapsulation is a combination of Data and Behavior in one package and hiding the implementation of data from the user

or

– Encapsulation is the mechanism that associate data with code that manipulate it

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Three Principles of OOP

– Inheritance, The original class is often called the base class or the super class, and the new class is often called the derived class or the subclass. Inheritance is often referred to as extending the base class or super class.

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Three Principles of OOP

– Polymorphism (from the Greek, meaning "many forms", or something similar) is the quality that allows one name to be used for more than one (hopefully related) but technically different purpose

– Method overriding occurs when a class declare a method having same types of signature as a method by one of its super class

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Class in Visual Basic

• The following class is a valid class although its empty

Public Class Person

End ClassEmpty Class, i.e.

no members

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Data in a Class

• A class containing Data (state)class Students

Dim ID As Integer

Dim Name As String

Dim Email As String

End class

Class with member data

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Data and Method in a Class

• A class containing Data (state) and Method (Behavior)class Students

Dim ID As Integer

Dim Name As String

Dim Email As String

Sub print()

System.Console.Write (ID)

System.Console.Write (Name)

System.Console.Write (Email)

End Sub

End class

Class with member data and member method

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Creating and Using Class

class PersonPublic Dim Name As StringSub print()

System.Console.Write (Name)End Sub

End class

Public Module M1 Sub Main()

Dim v As New Personv.Name = "Tariq Aziz"v.print()

End SubEnd Module

Class with member data and member method

Using class in MODULE program

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Creating and Using Class

class PersonPublic Dim Name As StringSub print()

System.Console.Write (Name)End Sub

End class

Class M1 Shared Sub Main()

Dim v As New Personv.Name = "Tariq Aziz"v.print()

End SubEnd Class

Class with member data and member method

Using class in another class

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Instance Variables in a Class

• Each object has its own copy of all instance variables defined by its class

Dim v As New Person

Public Dim Name As String

Sub print()

System.Console.Write (Name)

End Sub

v

Person class Object

Reference to Object Instance variable

& method

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Instance Variables in a Class

• Instance variables are initialized to defaults values during the creation of object. Variable of boolean set to false. Numbers are set to zeros. Any variable that acts as an object reference is set to null

class Person

Public ID As Integer

Public Name As String

Public Married As Boolean

End class

Dim As New Person

Public ID As Integer=0

Public Name As String=null

Public Married As Boolean=Falsev

Person class Object

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Instance Variables Example class A

Public Dim x As Double

Public Dim y As Boolean

Public Dim s As String

End Class

class B

shared Sub Main()

Dim p As New A

System.Console.WriteLine(" p.x = " & p.x)

System.Console.WriteLine(" p.y = " & p.y)

System.Console.WriteLine(" p.s = " & p.s)

End Sub

End class

Output:

p.x = 0

p.y = False

p.s =

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Instance method Example (Contd.)

Class A

Public Dim x As Double

Public Dim y As Boolean

Public Dim s As String

Sub Display()

System.Console.WriteLine( "x=" & x)

System.Console.WriteLine( "y=" & y)

System.Console.WriteLine( "s=" & s)

End Sub

End Class

Class B

shared Sub main()

Dim p As New A

p.Display()

End Sub

End class

Output:

x = 0

y = False

s =

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Shared Variables in a Class

• A Shared variable is shared by all objects of its class and thus relates to the class itself

• Shared var1 As Type• Shared variables are initialized to defaults values

when the class is loaded into memory. Variable of boolean set to false. Numbers are set to zeros. Any variable that acts as an object reference is set to null

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Shared Variables Example class A

Public Shared Dim x As Double

Public Shared Dim y As Boolean

Public Shared Dim s As String = "T"

End Class

class B

shared Sub main()

System.Console.WriteLine ("x = " & A.x)

System.Console.WriteLine ("y = " & A.y)

System.Console.WriteLine ("s = " & A.s)

End Sub

End class

Output:

x = 0

y = False

s = T

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Shared Method Example

class APublic Shared Dim x As Integer = 10Public Shared Dim y As Boolean = TruePublic Shared Dim s As String ="CA121"Shared Sub Display() System.Console.WriteLine(" x = " & x) System.Console.WriteLine(" y = " & y) System.Console.WriteLine(" s = " & s)End Sub

End Classclass B

shared Sub main() A.Display()End Sub

End class

Output:

x = 10

y = True

s = CA121

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Exercise (Classes)

• Write an application that declares a class named Person. It should have instance variable to record name, age, and salary. These should be of types String, integer, and Single. Use the new operator to create a Person object. Set and display its instance variable.

• Write an application that declares a class named Sphere. It should have instance variables to record its radius and the coordinates of its center. The should be of type Double. Use the new operator to create a Sphere object. Set and display its instance variable.

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Polymorphism

• A class may have several Method with same name, it is known as Method overloading/Polymorphism

• Each Method can have same name, but parameters list must be different

• The println() is a good example of this concept. It has several overloaded forms, Each of these accepts one argument of a different type. The type may be Boolean, Char, Integer, Long, Single, Double, String or Object. It is much convenient for programmer to remember one method name rather than several different ones

• Method overloading provides an easy way to handle default parameter values

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Polymorphism in Instance Method Example 1

Class Point3DPublic Dim x, y As Integer

Sub move(a As Integer)x = a

End Sub Sub move(a As Integer, b As Integer)

x = ay = b

End SubEnd Classclass Point3DConstructorShared Sub Main()

Dim p As new Point3Dp.move(100)

System.Console.WriteLine(p.x & " " & p.y) p.move(200,300) System.Console.WriteLine(p.x & " " & p.y)End SubEnd Class

OUTPUT:

100 0

200 300

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Polymorphism in Instance Method Example 2

class Point3DPublic Dim x, y As Doublesub Point3D(a as Double, b as Double) x=a y=b System.Console.WriteLine ("x=" & x & "y=" & y)End Subsub Point3D(a as Double) x=a y=200 System.Console.WriteLine ("x=" & x & "y=" & y)End Sub

Polymorphism Example

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Polymorphism in Instance Method Example 2

Shared Sub main()

Dim p As New Point3D

p.point3d(1.0,2.0)

p.point3d(100.0)

p.x = 1.1

p.y = 3.4

System.Console.WriteLine(" p.x = " & p.x)

System.Console.WriteLine(" p.y = " & p.y)

End Sub

End class

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Polymorphism in Constructor Method

class Point3DPublic Dim x, y As IntegerSub New()

x = 100End Sub Sub New( a As Integer, b As Integer )

x = ay = b

End SubEnd Classclass Point3DConstructor

Shared Sub Main()Dim p As new Point3DSystem.Console.WriteLine( p.x & " " & p.y)Dim q As New Point3D(200,300)System.Console.WriteLine( q.x & " " & q.y)

End SubEnd Class

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Exercise

• Write an application that defines a Circle class with two Method. The first form accepts a double value that represents the radius of the circle. This method assumes that the circle is centered at the origin. The second form accepts three double values. The first two arguments defines the coordinates of the center, and the third argument defines the radius