MELJUN CORTES Visual Basic 2005 Object-Oriented Programming

Visual Basic 2005Visual Basic 2005V. OBJECT-ORIENTED PROGRAMMING

MELJUN CORTESMELJUN CORTES

Introduction to Classes and ObjectsIntroduction to Classes and ObjectsObjectives

OBJECT-ORIENTED PROGRAMMING 2

In this chapter you will learn:•What classes, objects, methods, instance variables and properties are.•How to declare a class and use it to create an object.•How to implement a class's behaviors as methods.•How to implement a class's attributes as instance variables and properties.•How to call an object's methods to make them perform their tasks.•The differences between instance variables of a class and local variables of a method.•How to use a constructor to ensure that an object's attributes are initialized when the object is created.•The differences between value types and reference types.•How to use properties to ensure that only valid data is placed in attributes

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OverviewOverview

Object orientation uses classes to encapsulate instance variables (data) and methods (behaviors)

Objects have the ability to hide their implementation from other objects

Objects are unaware of how other objects are implemented


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Introduction to Classes and Objects

Classes vs ObjectsClasses vs Objects

A class is a type of somethingYou can think of a class as being the

blueprint out of which objects can be constructed


Car Blueprint (Class)

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An object is an instance of a classFor example, you may have several

instances of a class named CarEach instance of Car is an object and may

be given its own name


Actual Cars (Objects)

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Class vs. Object


Car Blueprint (Class) Actual Cars (Objects)

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Creating a ClassCreating a Class

You can add a class from Project menu and selecting the Add Class item or by right-clicking the project from the Solution Explorer and selecting Add then Class


Click Add Class menu item to

create a class in your project

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Class declaration with one method


' Class declaration with one method.Public Class GradeBook

' display a welcome message to the GradeBook user Public Sub DisplayMessage() Console.WriteLine("Welcome to the Grade Book!") End Sub ' DisplayMessage

End Class

' Class declaration with one method.Public Class GradeBook

' display a welcome message to the GradeBook user Public Sub DisplayMessage() Console.WriteLine("Welcome to the Grade Book!") End Sub ' DisplayMessage

End Class

Class keyword DisplayMessage is a method of Class

GradeBook

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Creating an object of class GradeBook


Module ModGradeBookTest ' Main begins program execution Sub Main() ' initialize gradeBook to refer to a new GradeBook object Dim gradeBook As New GradeBook()

' call gradeBook's DisplayMessage method gradeBook.DisplayMessage() End Sub ' MainEnd Module

Module ModGradeBookTest ' Main begins program execution Sub Main() ' initialize gradeBook to refer to a new GradeBook object Dim gradeBook As New GradeBook()

' call gradeBook's DisplayMessage method gradeBook.DisplayMessage() End Sub ' MainEnd Module

Object variable gradeBook

Class GradeBookNew keyword will create an instance of class GradeBook

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Instance Variables and PropertiesInstance Variables and Properties

Variables declared in the body of a particular method are local variables and can be used only in that method

When a method terminates, the values of its local variables are lost

In contrast, an object's attributes are carried with the object as it is used in a program


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GradeBook class with instance variable and property


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' Class declaration with one method.Public Class GradeBook Private courseNameValue As String ' course name for this GradeBook

' property CourseName Public Property CourseName() As String Get ' retrieve courseNameValue Return courseNameValue End Get

Set(ByVal value As String) ' set courseNameValue courseNameValue = value ' store the course name in the object End Set End Property ' CourseName

' display a welcome message to the GradeBook user Public Sub DisplayMessage() ' use property CourseName to display the ' name of the course this GradeBook represents Console.WriteLine("Welcome to the grade book for " _ & vbCrLf & CourseName & "!") End Sub ' DisplayMessageEnd Class




' display a welcome message to the GradeBook user Public Sub DisplayMessage() ' use property CourseName to display the ' name of the course this GradeBook represents Console.WriteLine("Welcome to the grade book for " _ & vbCrLf & CourseName & "!") End Sub ' DisplayMessageEnd Class

Property CourseName

Instance variable courseNameValue



Most instance variable declarations are preceded with the access modifier Private

Declaring instance variables with access modifier Private is known as information hiding

When a program creates (instantiates) an object of class GradeBook, variable courseNameValue is encapsulated (hidden) in the object and can be accessed only by methods and properties of the object's class


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The Get accessor begins with the keyword Get and ends with the keywords End Get

The Get accessor returns the value to the client code that references the property

The Get accessor's body contains a Return statement


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Get accessor



The Set accessor begins with the keyword Set and ends with the keywords End Set

The Set accessor stores the value into the instance variable from a client code that references the property


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Set accessor


Initializing Objects with ConstructorsInitializing Objects with Constructors

Each class you declare can provide a constructor that can be used to initialize an object of the class when the object is created

The New keyword calls the class's constructor to perform the initialization

The constructor call is indicated by the class name followed by parentheses


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GradeBook class with a constructor


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Public Sub New(ByVal name As String) CourseName = name End Sub



. . .End Class


Public Sub New(ByVal name As String) CourseName = name End Sub



. . .End Class

Class constructor



Constructor used to initialize GradeBook objects


Module GradeBookTest ' Main begins program execution Sub Main() ' create GradeBook object Dim gradeBook1 As New GradeBook( _ "CS101 Introduction to Visual Basic Programming") Dim gradeBook2 As New GradeBook( _ "CS102 Data Structures in Visual Basic")

' display initial value of CourseName for each GradeBook Console.WriteLine( _ "gradeBook1 course name is: " & gradeBook1.CourseName) Console.WriteLine( _ "gradeBook2 course name is: " & gradeBook2.CourseName) End Sub ' Main End Module ' GradeBookTest

Module GradeBookTest ' Main begins program execution Sub Main() ' create GradeBook object Dim gradeBook1 As New GradeBook( _ "CS101 Introduction to Visual Basic Programming") Dim gradeBook2 As New GradeBook( _ "CS102 Data Structures in Visual Basic")

' display initial value of CourseName for each GradeBook Console.WriteLine( _ "gradeBook1 course name is: " & gradeBook1.CourseName) Console.WriteLine( _ "gradeBook2 course name is: " & gradeBook2.CourseName) End Sub ' Main End Module ' GradeBookTest


Object-Oriented Programming: Object-Oriented Programming: InheritanceInheritanceObjectives


In this chapter you will learn:•What inheritance is and how it promotes software reusability.•The notions of base classes and derived classes.•To use keyword Inherits to create a class that inherits attributes and behaviors from another class.•To use the access modifier Protected in a base class to give derived class methods access to base class members.•To access base class members from a derived class with MyBase.•How constructors are used in inheritance hierarchies.•To access the current object with Me and MyClass.•The methods of class Object the direct or indirect base class of all classes in Visual Basic.

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OverviewOverview

Inheritance is a form of software reuse in which a new class is created by absorbing an existing class's members and embellishing them with new or modified capabilities

With inheritance, you can save time during program development by reusing proven and debugged high-quality software

19OBJECT-ORIENTED PROGRAMMING

Object-Oriented Programming: Inheritance

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OverviewOverview

The existing class is called the base class (superclass in Java)

The new class is the derived class (subclass in Java)

Derived class can become the base class for future derived classes

A derived class is more specific than its base class and represents a more specialized group of objects


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Base Classes and Derived ClassesBase Classes and Derived Classes

Often, an object of one class is an object of another class as well

For example, in geometry, a rectangle is a quadrilateral

Thus, in Visual Basic, class Rectangle can be said to inherit from class Quadrilateral

In this context, class Quadrilateral is a base class and class Rectangle is a derived class


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Inheritance examples:


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Base Class Derived Classes

Student GraduateStudent, UndergraduateStudent

Shape Circle, Triangle, Rectangle

Loan CarLoan, HomeImprovementLoan, MortgageLoan

Employee Faculty, Staff

BankAccount CheckingAccount, SavingsAccount



Inheritance hierarchy


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ShapeShape

TwoDimensionalShapeTwoDimensionalShape ThreeDimensionalShapeThreeDimensionalShape

SquareSquareCircleCircle TriangleTriangle CubeCubeSphereSphere TetrahedronTetrahedron


ProtectedProtected Members Members

Protected access offers an intermediate level of access between Public and Private access

A base class's Protected members can be accessed only by members of that base class and by members of its derived classes

All Public and Protected base class members retain their original access modifier when they become members of the derived class


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Relationship Between Base Classes and Relationship Between Base Classes and Derived ClassesDerived Classes

CommissionEmployee class with Protected instance variables


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' CommissionEmployee class represents a commission employee.Public Class CommissionEmployee

Protected firstNameValue As String ' first name Protected lastNameValue As String ' last name Protected socialSecurityNumberValue As String ' social security number Protected grossSalesValue As Decimal ' gross weekly sales Protected commissionRateValue As Decimal ' commission percentage

' five-argument constructor Public Sub New(ByVal first As String, ByVal last As String, _ ByVal ssn As String, ByVal sales As Decimal, _ ByVal rate As Decimal)

' implicit call to Object constructor occurs here FirstName = first LastName = last SocialSecurityNumber = ssn GrossSales = sales ' validate and store gross sales CommissionRate = rate ' validate and store commission rate End Sub ' New. . .continued

' CommissionEmployee class represents a commission employee.Public Class CommissionEmployee

Protected firstNameValue As String ' first name Protected lastNameValue As String ' last name Protected socialSecurityNumberValue As String ' social security number Protected grossSalesValue As Decimal ' gross weekly sales Protected commissionRateValue As Decimal ' commission percentage

' five-argument constructor Public Sub New(ByVal first As String, ByVal last As String, _ ByVal ssn As String, ByVal sales As Decimal, _ ByVal rate As Decimal)

' implicit call to Object constructor occurs here FirstName = first LastName = last SocialSecurityNumber = ssn GrossSales = sales ' validate and store gross sales CommissionRate = rate ' validate and store commission rate End Sub ' New. . .continued




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' property FirstName Public Property FirstName() As String Get Return firstNameValue End Get Set(ByVal first As String) firstNameValue = first ' no validation End Set End Property ' FirstName

' property LastName Public Property LastName() As String Get Return lastNameValue End Get Set(ByVal last As String) lastNameValue = last ' no validation End Set End Property ' LastName. . .continued

' property FirstName Public Property FirstName() As String Get Return firstNameValue End Get Set(ByVal first As String) firstNameValue = first ' no validation End Set End Property ' FirstName

' property LastName Public Property LastName() As String Get Return lastNameValue End Get Set(ByVal last As String) lastNameValue = last ' no validation End Set End Property ' LastName. . .continued




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' property SocialSecurityNumber Public Property SocialSecurityNumber() As String Get Return socialSecurityNumberValue End Get Set(ByVal ssn As String) socialSecurityNumberValue = ssn ' no validation End Set End Property ' SocialSecurityNumber

' property GrossSales Public Property GrossSales() As Decimal Get Return grossSalesValue End Get Set(ByVal sales As Decimal) If sales < 0.0 Then ' validate gross sales grossSalesValue = 0 Else grossSalesValue = sales End If End Set End Property ' GrossSales. . .continued

' property SocialSecurityNumber Public Property SocialSecurityNumber() As String Get Return socialSecurityNumberValue End Get Set(ByVal ssn As String) socialSecurityNumberValue = ssn ' no validation End Set End Property ' SocialSecurityNumber

' property GrossSales Public Property GrossSales() As Decimal Get Return grossSalesValue End Get Set(ByVal sales As Decimal) If sales < 0.0 Then ' validate gross sales grossSalesValue = 0 Else grossSalesValue = sales End If End Set End Property ' GrossSales. . .continued




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' property CommissionRate Public Property CommissionRate() As Decimal Get Return commissionRateValue End Get

Set(ByVal rate As Decimal) If rate > 0.0 AndAlso rate < 1.0 Then ' validate rate commissionRateValue = rate Else commissionRateValue = 0 End If End Set End Property ' CommissionRate

' calculate earnings Public Overridable Function CalculateEarnings() As Decimal Return commissionRateValue * grossSalesValue End Function ' CalculateEarnings. . .continued

' property CommissionRate Public Property CommissionRate() As Decimal Get Return commissionRateValue End Get

Set(ByVal rate As Decimal) If rate > 0.0 AndAlso rate < 1.0 Then ' validate rate commissionRateValue = rate Else commissionRateValue = 0 End If End Set End Property ' CommissionRate

' calculate earnings Public Overridable Function CalculateEarnings() As Decimal Return commissionRateValue * grossSalesValue End Function ' CalculateEarnings. . .continued




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' return String representation of CommissionEmployee object Public Overrides Function ToString() As String Return ("commission employee: " & firstNameValue & " " & _ lastNameValue & vbCrLf & "social security number: " & _ socialSecurityNumberValue & vbCrLf & "gross sales: " & _ String.Format("{0:C}", grossSalesValue) & vbCrLf & _ "commission rate: " & String.Format("{0:F}", _ commissionRateValue)) End Function ' ToStringEnd Class

' return String representation of CommissionEmployee object Public Overrides Function ToString() As String Return ("commission employee: " & firstNameValue & " " & _ lastNameValue & vbCrLf & "social security number: " & _ socialSecurityNumberValue & vbCrLf & "gross sales: " & _ String.Format("{0:C}", grossSalesValue) & vbCrLf & _ "commission rate: " & String.Format("{0:F}", _ commissionRateValue)) End Function ' ToStringEnd Class


BasePlusCommissionEmployee inherits Protected instance variables from CommissionEmployee


Public Class BasePlusCommissionEmployee Inherits CommissionEmployee

Private baseSalaryValue As Decimal ' base salary per week

' six-argument constructor Public Sub New(ByVal first As String, ByVal last As String, _ ByVal ssn As String, ByVal sales As Decimal, _ ByVal rate As Decimal, ByVal salary As Decimal)

' use MyBase reference to CommissionEmployee constructor explicitly MyBase.New(first, last, ssn, sales, rate) BaseSalary = salary ' validate and store base salary End Sub ' New. . .continued

Public Class BasePlusCommissionEmployee Inherits CommissionEmployee

Private baseSalaryValue As Decimal ' base salary per week

' six-argument constructor Public Sub New(ByVal first As String, ByVal last As String, _ ByVal ssn As String, ByVal sales As Decimal, _ ByVal rate As Decimal, ByVal salary As Decimal)

' use MyBase reference to CommissionEmployee constructor explicitly MyBase.New(first, last, ssn, sales, rate) BaseSalary = salary ' validate and store base salary End Sub ' New. . .continued


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' property BaseSalary Public Property BaseSalary() As Decimal Get Return baseSalaryValue End Get

Set(ByVal salary As Decimal) If salary < 0.0 Then ' validate base salary baseSalaryValue = 0 Else baseSalaryValue = salary End If End Set End Property ' BaseSalary

. . .continued

' property BaseSalary Public Property BaseSalary() As Decimal Get Return baseSalaryValue End Get

Set(ByVal salary As Decimal) If salary < 0.0 Then ' validate base salary baseSalaryValue = 0 Else baseSalaryValue = salary End If End Set End Property ' BaseSalary

. . .continued


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' calculate earnings Public Overrides Function CalculateEarnings() As Decimal Return baseSalaryValue + (commissionRateValue * grossSalesValue) End Function ' CalculateEarnings

' return String representation of BasePlusCommissionEmployee object Public Overrides Function ToString() As String Return ("base-plus-commission employee: " & firstNameValue & " " & _ lastNameValue & vbCrLf & "social security number: " & _ socialSecurityNumberValue & vbCrLf & "gross sales: " & _ String.Format("{0:C}", grossSalesValue) & vbCrLf & _ "commission rate: " & String.Format("{0:F}", _ commissionRateValue) & vbCrLf & "base salary: " & _ String.Format("{0:C}", baseSalaryValue)) End Function ' ToStringEnd Class

' calculate earnings Public Overrides Function CalculateEarnings() As Decimal Return baseSalaryValue + (commissionRateValue * grossSalesValue) End Function ' CalculateEarnings

' return String representation of BasePlusCommissionEmployee object Public Overrides Function ToString() As String Return ("base-plus-commission employee: " & firstNameValue & " " & _ lastNameValue & vbCrLf & "social security number: " & _ socialSecurityNumberValue & vbCrLf & "gross sales: " & _ String.Format("{0:C}", grossSalesValue) & vbCrLf & _ "commission rate: " & String.Format("{0:F}", _ commissionRateValue) & vbCrLf & "base salary: " & _ String.Format("{0:C}", baseSalaryValue)) End Function ' ToStringEnd Class


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Testing class BasePlusCommissionEmployee


Module BasePlusCommissionEmployeeTest Sub Main() ' instantiate BasePlusCommissionEmployee object Dim employee As New BasePlusCommissionEmployee( _ "Bob", "Lewis", "333-33-3333", 5000, 0.04D, 300)

' get base-salaried commission employee data Console.WriteLine("Employee information obtained by properties:" & _ vbCrLf & _ "First name is " & employee.FirstName & vbCrLf & _ "Last name is " & employee.LastName & vbCrLf & _ "Social Security Number is " & _ employee.SocialSecurityNumber)

Console.WriteLine("Gross sales is {0:C}", employee.GrossSales) Console.WriteLine("Commission rate is {0:F}", employee.CommissionRate) Console.WriteLine("Base salary is {0:C}", employee.BaseSalary)

. . .continued

Module BasePlusCommissionEmployeeTest Sub Main() ' instantiate BasePlusCommissionEmployee object Dim employee As New BasePlusCommissionEmployee( _ "Bob", "Lewis", "333-33-3333", 5000, 0.04D, 300)

' get base-salaried commission employee data Console.WriteLine("Employee information obtained by properties:" & _ vbCrLf & _ "First name is " & employee.FirstName & vbCrLf & _ "Last name is " & employee.LastName & vbCrLf & _ "Social Security Number is " & _ employee.SocialSecurityNumber)

Console.WriteLine("Gross sales is {0:C}", employee.GrossSales) Console.WriteLine("Commission rate is {0:F}", employee.CommissionRate) Console.WriteLine("Base salary is {0:C}", employee.BaseSalary)

. . .continued


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Testing class BasePlusCommissionEmployee


employee.BaseSalary = 1000 ' set base salary

' get new employee information Console.WriteLine(vbCrLf & _ "Updated employee information obtained by ToString: " & _ vbCrLf & employee.ToString() & vbCrLf)

' display the employee's earnings Console.WriteLine("Employee's earnings: $" & _ employee.CalculateEarnings()) Console.Read() End Sub ' MainEnd Module

employee.BaseSalary = 1000 ' set base salary

' get new employee information Console.WriteLine(vbCrLf & _ "Updated employee information obtained by ToString: " & _ vbCrLf & employee.ToString() & vbCrLf)

' display the employee's earnings Console.WriteLine("Employee's earnings: $" & _ employee.CalculateEarnings()) Console.Read() End Sub ' MainEnd Module


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Class ObjectClass Object

All classes inherit directly or indirectly from the Object class (namespace System)

So the Object’s seven methods are inherited by all other classes◦Equals()◦Finalize()◦GetHashCode()◦GetType()◦MemberwiseClone()◦ReferenceEquals()◦ToString()



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FriendFriend Members Members

Another intermediate level of member access is Friend access

A class's Friend members can be accessed only by code in the same assembly

Unlike Public access, any other programs that are declared outside the assembly cannot access these Friend members



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Showing accessibility of Friend members


Assembly 1

Assembly 2

BlogAccount

ClassInSamePackage~countEntries()aMethod()

SpecializedClassInAnotherPackageClassInAnotherPackage

countEntries is a Friend method


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ClassLibrary1 assembly

(DLL)

ClassLibrary2Assembly

(DLL)

FriendAccessTestAssembly

(EXE)


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Friend instance variable data1


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Accessing a Friend member within the same assembly is

allowed


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Attempting to access Friend member in another assembly

produces an error


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Object-Oriented Programming: Object-Oriented Programming: PolymorphismPolymorphismObjectives


In this chapter you will learn:•What polymorphism is.•To use overridden methods to effect polymorphism.•To distinguish between abstract and concrete classes.•To declare abstract methods to create abstract classes.•How polymorphism makes systems extensible and maintainable.•To determine an object's type at execution time.•To declare and implement interfaces.

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OverviewOverview

Polymorphism enables us to "program in the general" rather than "program in the specific”

In particular, polymorphism enables us to write programs that process objects that share the same base class in a class hierarchy as simply as if they were all objects of the base class


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Object-Oriented Programming: Polymorphism

OverviewOverview

With polymorphism, we can design and implement systems that are easily extensible new classes can be added with little or no modification to the general portions of the program


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AnimalAnimal

The base class animal implements a method eat.

Each of these animals implement their own way of

eating.

Demonstrating Polymorphic BehaviorDemonstrating Polymorphic Behavior

Implementing the Animal hierarchy


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Public Class Animal

Public Overridable Sub Eat() Console.WriteLine("Animal: Eat") End SubEnd Class

Public Class Animal

Public Overridable Sub Eat() Console.WriteLine("Animal: Eat") End SubEnd Class

Public Class Bird Inherits Animal

Public Overrides Sub Eat() Console.WriteLine("Bird: Eat") End SubEnd Class

Public Class Bird Inherits Animal

Public Overrides Sub Eat() Console.WriteLine("Bird: Eat") End SubEnd Class

Public Class Fish Inherits Animal

Public Overrides Sub Eat() Console.WriteLine("Fish: Eat") End SubEnd Class

Public Class Fish Inherits Animal

Public Overrides Sub Eat() Console.WriteLine("Fish: Eat") End SubEnd Class

Inherits keyword enables

inheritance

Overrides keyword

replaces the implementation

of the base class

Overridable keyword allows derived classes

to implement their own behavior


Implementing the Animal hierarchy


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Public Class Dog Inherits Animal

Public Overrides Sub Eat() Console.WriteLine("Dog: Eat") End SubEnd Class

Public Class Dog Inherits Animal

Public Overrides Sub Eat() Console.WriteLine("Dog: Eat") End SubEnd Class

Module ModPolymorphTest Sub Main() Dim Hayop As Animal Dim bantay As New Dog, tweety As New Bird, nemo As New Fish

Hayop = bantay ‘set Hayop to reference bantay Hayop.Eat()

Hayop = tweety ‘set Hayop to reference tweety Hayop.Eat()

Hayop = nemo ‘set Hayop to reference nemo Hayop.Eat() End SubEnd Module

Module ModPolymorphTest Sub Main() Dim Hayop As Animal Dim bantay As New Dog, tweety As New Bird, nemo As New Fish

Hayop = bantay ‘set Hayop to reference bantay Hayop.Eat()

Hayop = tweety ‘set Hayop to reference tweety Hayop.Eat()

Hayop = nemo ‘set Hayop to reference nemo Hayop.Eat() End SubEnd Module

Inherits keyword enables

inheritance


Output:


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Each of these strings is

generated by invoking different

implementations of the Eat

method referenced by

the Hayop object variable

Abstract Classes and MethodsAbstract Classes and Methods

Abstract classes are used to declare classes for which you never intend to instantiate objects

They are used only as base classes in inheritance hierarchies, we refer to them as abstract base classes

These classes cannot be used to instantiate objects, because abstract classes are incomplete


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Abstract Classes and MethodsAbstract Classes and Methods

The purpose of an abstract class is primarily to provide an appropriate base class from which other classes can inherit and thus share a common design

To define an abstract base class:


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‘Abstract class EmployeePublic MustInherit Class Employee

. . . ' abstract method overridden by derived class Public MustOverride Function CalculateEarnings() As Decimal

End Class

‘Abstract class EmployeePublic MustInherit Class Employee

. . . ' abstract method overridden by derived class Public MustOverride Function CalculateEarnings() As Decimal

End Class

MustInherit keyword

MustOverride keyword A MustOverride procedure doesn’t implement any code

NotOverridableNotOverridable Methods and Methods and NotInheritableNotInheritable Classes Classes

A method that is declared NotOverridable in a base class cannot be overridden in a derived class

A method that is declared Overridable in a base class can be declared NotOverridable in a derived class

Methods that are declared Private are implicitly NotOverridable


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NotOverridableNotOverridable Methods and Methods and NotInheritableNotInheritable Classes Classes

A class that is declared NotInheritable cannot be a base class

All methods in a NotInheritable class are implicitly NotOverridable

NotInheritable class is the opposite of a MustInherit class

Class String is a NotInheritable class


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Creating and Using InterfacesCreating and Using Interfaces

Interfaces define and standardize the ways in which things such as people and systems can interact with one another

For example, the controls on a radio serve as an interface between radio users and a radio's internal components

The controls allow users to perform only a limited set of operations


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Software objects also communicate via interfaces

A Visual Basic interface describes a set of methods that can be called on an object, to tell the object to perform some task or return some piece of information

An interface is often used in place of a MustInherit class when there is no default implementation to inherit


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An interface declaration begins with the keyword Interface:


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' IPayable interface declaration. Public Interface IPayable

Function GetPaymentAmount() As Decimal ' calculate payment Function ToString() As String ' display Payable object

End Interface ' IPayable

' IPayable interface declaration. Public Interface IPayable

Function GetPaymentAmount() As Decimal ' calculate payment Function ToString() As String ' display Payable object

End Interface ' IPayable

Interface members will not have any implementation

Interface keyword


IPayable interface hierarchy


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<<interface>>IPayable

<<interface>>IPayable

InvoiceInvoice EmployeeEmployee

SalariedEmployeeSalariedEmployee


Using the interface


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' IPayable interface declaration. Public Class Invoice Implements IPayable

Public Function GetPaymentAmount() As Decimal _ Implements IPayable.GetPaymentAmount ‘implementation here End Function

Public Function ToString1() As String _ Implements IPayable.ToString ‘implementation here End Function

. . .

End Class ' Invoice

' IPayable interface declaration. Public Class Invoice Implements IPayable

Public Function GetPaymentAmount() As Decimal _ Implements IPayable.GetPaymentAmount ‘implementation here End Function

Public Function ToString1() As String _ Implements IPayable.ToString ‘implementation here End Function

. . .

End Class ' Invoice

Implements keyword

Interface IPayable

Classes that implements the IPayable should include the methods defined in

the interface

ExerciseExerciseOBJECT-ORIENTED PROGRAMMING 57

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End of Object-Oriented End of Object-Oriented ProgrammingProgramming


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MELJUN CORTES Visual Basic 2005 Object-Oriented Programming

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