MELJUN CORTES Vb.net handout object-oriented programming concepts

Visual Basic .NET Programming

Object Oriented Programming Concepts * Property of STI Page 1 of 11

TOPIC TITLE: Object-Oriented Programming Concepts Specific Objectives: At the end of the topic session, the students are expected to: Cognitive:

1. Explain constructor. 2. Describe the Sub New() sub routine. 3. Explain destructor. 4. Explain how to initialize an object. 5. Explain the garbage collection concept. 6. Explain what Inheritance is. 7. Discuss the different keywords in overriding. 8. Explain overloading. 9. Differentiate MyBase from MyClass keyword. 10. Explain what an Interface is and identify the use of Interface

keyword. 11. Explain what polymorphism is.

Affective:

1. Listen to others with respect. 2. Participate in class discussions actively.

MATERIALS/EQUIPMENT:

o topic slides o OHP

TOPIC PREPARATION:

o Have the students research on the following: � Inheritance � Polymorphism

o It is imperative for the instructor to incorporate various kinds of teaching strategies while discussing the suggested topics. The instructor may use the suggested learning activities below to facilitate a thorough and creative discussion of the topic.

o Prepare the slides to be presented in the class.

TOPIC PRESENTATION: The topic will revolve around the overview of classes and objects. This will be the suggested flow of discussion for the course topic:

1. Introduce to the students the topics to be covered in this session.

2. Explain constructor. 3. Explain the Sub New() sub routine and provide example. 4. Explain destructor and give an example. 5. Explain how to initialize an object by providing some examples. 6. Explain the garbage collection concept. Also, give an example

on how to use the Dispose method. 7. Ask the students to share their research about Inheritance. 8. Explain what Inheritance is. Enumerate the different Inheritance

keywords and provide examples. 9. Explain what overriding is and discuss the different overriding



keywords. 10. Explain overloading by providing an example. 11. Explain the MyBase and MyClass keywords by providing

examples on how to use these keywords. 12. Explain what an Inheritance is. 13. Ask the students to share their research about Polymorphism. 14. Explain what polymorphism is and give an example.



Constructor Page 1 of 20

Object-Oriented Programming Concepts


* Property of STIPage 1 of 20

ConstructorConstructorConstructorConstructor

� a special procedure used in initializing new objects

� Sub New constructor is used to perform the object initialization

Constructor In Visual Basic 6.0, the initialization of a class is placed in the Class_Initialize event of the class. This event is executed when the object is instantiated and it is used to set initial values of local variables, to open resources and to instantiate other objects. In Visual Basic .NET, the constructor is a special procedure used in initialization of new objects. The Class_Initialize event is replaced by the Sub New constructor to perform the object initialization. This procedure

is always executed once before any other code in a class. The Sub

New constructor can be explicitly invoked in the first line of code of

another constructor from either the same class or a derived class using the MyBase keyword.

[Constructor, Page 1 of 20]

Sub New Example Page 2 of 20




Sub New ExampleSub New ExampleSub New ExampleSub New Example

� Default Constructor

Public Class Student

Public Sub New()

‘default constructor

‘start of initialization

intCnt = 1

End Sub

End Class

Sub New Example Page 3 of 20




Sub New ExampleSub New ExampleSub New ExampleSub New Example

� Overloaded Constructor

Public Class Student

Public Sub New()

‘default constructor


intCnt = 1

End Sub

Public Sub New(ByVal x As

Integer)

‘overloaded constructor


intCnt = 1

End Sub

End Class

Sub New Example Public Class Student

Public Sub New()


intCnt = 1

End Sub

End Class

Another feature of using constructor is that it can be overloaded. Meaning, the New subroutine can be created as many as required. An

example of overloaded constructor is shown below: Public Class Student

Public Sub New()


intCnt = 1

End Sub

Public Sub New(ByVal x As Integer)

‘start of complex initialization

intCnt = x

End Sub

Public Sub New(ByVal x As Integer, ByVal m As

String)


intCnt = x

strMsg = m

End Sub



End Class

[Sub New Example, Pages 2-3 of 20]

Destructor Page 4 of 20




DestructorDestructorDestructorDestructor

� In Visual Basic .NET, the destruction of

objects can now be controlled using

destructors

� The Finalize destructor is executed

when the object is destroyed

� Example

Protected Overrides Sub Finalize()

‘close connection of other

resources

conn.Close

End Sub

Destructor In Visual Basic 6.0, objects are destroyed using the Class_Terminate event. In Visual Basic .NET, the destruction of objects can now be controlled using destructors. The Finalize destructor replaces the

Class_Terminate event. The Finalize destructor is executed when

the object is destroyed. Usually, this subroutine is used to perform clean up of open resources such as database connections and to release other objects. Finalize Example


‘close connection of other resources

conn.Close

End Sub

[Destructor, Page 4 of 20]

Initializing Objects Page 5 of 20




Initializing ObjectsInitializing ObjectsInitializing ObjectsInitializing Objects

� An example of default constructor

Dim x As TestClass = New

TestClass()

� An example of overloaded constructor


TestClass(100)

� Another example of overloaded

constructor


TestClass(100, “Hello”)

Initializing Objects To initialize an object, you use the New keyword and invoke either the

default constructor or the overloaded constructor. An example of using the default constructor is shown below:

Dim x As TestClass = New TestClass()

An example of using the overloaded constructor is shown below:

Dim x As TestClass = New TestClass(100)

Another example of overloaded constructor:

Dim x As TestClass = New TestClass(100,“Hello”)

[Initializing Objects, Page 5 of 20]

Garbage Collection Garbage collection is a new feature in Visual Basic .NET. Garbage collection is a background process that traces object references, cleans up unused variables and destroys those that cannot be reached by executing code. To enable garbage collection, set x = Nothing. When enabled, the

garbage collection introduces a time delay between when the last reference to an object is removed and when the collector destroys the object and reclaims the memory. This time delay is significant for very complex and sensitive applications. Thus, to force garbage collection you can use the Garbage Collector (GC) class and invoke the

class’s Collect method.



Garbage Collection Page 6 of 20




Garbage CollectionGarbage CollectionGarbage CollectionGarbage Collection

� a background process that traces object

references, cleans up unused variables

and destroys those that cannot be

reached by executing code

� To enable garbage collection, set:

x = Nothing

� Dispose method provides flexibility in

releasing resources manually

Garbage Collection Page 7 of 20




Garbage CollectionGarbage CollectionGarbage CollectionGarbage Collection

� Dispose method Example:

Public Sub Dispose()

‘close a database connection

conn.Close

End Sub


‘optional call to Dispose

Dispose()

End Sub

‘Client code

Dim x as TestClass = New

TestClass()

‘call the object’s dispose method

x.Dispose()

GC.Collect()

However, the use of the Collect method is not recommended because

it also collects other unused objects resulting to poor performance. It should be used with precaution.

An alternative way to release resources is by using the Dispose

method. This method provides flexibility in releasing resources manually. When objects resources are no longer needed, a client code can directly call coded placed in the Dispose method of the object. If

the client code does not call the Dispose method explicitly before

garbage collection occurs, the Finalize method of the class can also

call the Dispose method.

Example on how to use the Dispose method is given below:

Public Sub Dispose()

Conn.Close ‘close a database connection

End Sub


Dispose() ‘optional call to Dispose

End Sub

‘Client code

Dim x as TestClass = New TestClass()

x.Dispose() ‘call the object’s dispose method

[Garbage Collection, Pages 6-7 of 20]

Inheritance Page 8 of 20




InheritanceInheritanceInheritanceInheritance

� backbone of object-oriented programming

� enables programmers to create a

hierarchy among a group of classes that

have similar characteristics

� allows creating a class from an existing

base class

� Inheritance keywords

� Inherits

� NotInheritable

� MustInherit

� Protected

Inheritance

Inheritance Inheritance is the backbone of object-oriented programming. It enables programmers to create a hierarchy among a group of classes that have similar characteristics. We can have one class that defines all the attributes and behavior common to a specific group of classes. All classes that are part of this specific group can inherit the common attributes and behavior from this common class. Let us understand what inheritance is by taking a real world example. Assume that we want to create a class called SUV, which has the same features as the class called Vehicle. One way of doing this is to re-declare the member variables called attributes and the methods (behavior) already available in the Vehicle class in the SUV class also and then add the special features of the SUV. Inheritance allows creating a class from an existing base class. The class is also called as the derived class. The derived class can inherit all the base class properties, methods, members, events and event handlers. Thus, inheritance makes it easy to reuse the base class throughout an application.



Page 9 of 20





� Inherits keyword

Public Class HumanResources

Inhertis Department

…

End Class

� NotInheritable keyword

Public Class NotInheritable Shape

…

End Class

Public Class Circle

Inherits Shape

‘it will generate a compile

error

End Class

Inheritance Page 10 of 20





� MustInherit keyword

Public Class MustInherit People

…

End Class

‘client code

Dim emp1 As New People()

‘generates a compile error

� Protected keyword

Public Class Subject

‘ accessible anywhere

Public subCode As String

‘accessible only in this

class or a ‘derived class

Protected subDesc As String

End Class

Inheritance keywords:

• Inherits

• NotInheritable

• MustInherit

• Protected The Inherits keyword is used to define a derived class that will inherit

from an existing base class. An example is shown below: Public Class DerivedClass

Inherits BaseClass

…

End Class

Another example: Public Class HumanResources

Inherits Department

…

End Class

The NotInheritable keyword is used to define a class that cannot be

used as a base for inheritance. A compile error will be generated if another class attempts to inherit from the NotInheritable defined class. Example implementation is shown below. Public Class NotInheritable Shape

…

End Class

Public Class Circle

Inherits Shape

‘it will generate a compile error

End Class

The MustInherit keyword on the other hand is used to define a class

that is not intended to be used directly as instantiated objects. In other words, MustInherit keyword is used to create an abstract class. An abstract class is used to lay a foundation (in terms of behavior) for other classes. An example code is shown below: Public Class MustInherit People

…

End Class

‘client code

Dim emp1 As New People() ‘generates a compile error

The above code generates an error because the New keyword cannot be used to instantiate a MustInherit class. The Protected keyword is used to define special relationship among a

base class and its derived classes. It is used to allow a base class to create members that can only be accessed by its derived classes. An example is shown below: Public Class Subject

‘ accessible anywhere

Public subCode As String

‘accessible only in this class or a derived class

Protected subDesc As String



End Class

[Inheritance, Pages 8-10 of 20]

Overriding Page 11 of 20




OverridingOverridingOverridingOverriding

� used to create specific implementation

code not found in the base class

� Overriding Keywords

� Overridable

� MustOverride

� Overrides

� NotOverridable






� Overridable keyword

Public Overridable Sub

MethodOverride()

MsgBox(“Base Class

MethodOverrirde”)

End Sub

� MustOverride keyword

Public MustOverride Sub

CalculateGPA()

End Sub




* Property of STI

Page 13 of 20


� Overriden keyword

Public Overrides Sub

MethodOverride()

MsgBox(“Derived Class

MethodOverride”)

End Sub

� NotOverridable keyword

Public NotOverridable Overrides

Sub MethodOverride()

MsgBox(“Derived Class

MethodOverride”)

End Sub

Overriding Overriding is a technique used in derived classes to create specific implementation code not found in the base class rather than using the inherited methods. The following are keywords that can be used in implementing overriding:

• Overridable

• MustOverride

• Overrides

• NotOverridable To make a property, function or sub routine in the base class overridable, the Overridable keyword is specified. This will allow the

derive class to create specific implementation of the class members. An example is shown below: Public Overridable Sub MethodOverride()

MsgBox(“Base Class MethodOverride”)

End Sub

The MustOverride keyword is used only in a base class that is marked

as MustInherit. This keyword is used to create a base class member

that must be overridden in all derived classes. It is created in the base class with no implementation. An example is shown below: Public MustOverride Sub CalculateGPA()

End Sub

The Overriden keyword is used to specify that a derived class method

overrides the implementation of the base class method. A compile error occurs if an Overridden method is defined in the derived class and is

not specified in the base class as Overridable. The method signature

must exactly match the method being overridden except for the parameter names. An example is shown below: Public Overrides Sub MethodOverride()

MsgBox(“Derived Class MethodOverride”)

End Sub

The NotOverridable keyword is used in the derived class to stop

subsequent inheritance from overriding the method. The following example shows how to declare a derived class method that overrides the base class implementation but does not allow any further overriding. Public NotOverridable Overrides Sub MethodOverride()

MsgBox(“Derived Class MethodOverride”)

End Sub

[Overriding, Pages 11-13 of 20]



Overloading Page 14 of 20



* Property of STI

Page 14 of 20

OverloadingOverloadingOverloadingOverloading

� creating a function or sub routine with the

same name as defined in the base class

but with different signatures

� uses Overloads keyword

� Example:

Public Overloads Sub Display(ByVal

x As Integer)

MsgBox(“Derive Class Display –

Overloaded method”)

End Sub

Overloading Overloading is creating a function or sub routine with the same name as defined in the base class but with different signatures. The Overloads

keyword is used for this purpose. The signature must include different parameters or parameter types. An example is shown below: Public Overloads Sub Display(ByVal x As Integer)

MsgBox(“Derive Class Display – Overloaded

method”)

End Sub

[Overloading, Page 14 of 20]

Using MyBase and MyClass Keywords Page 15 of 20



* Property of STI

Page 15 of 20

Using Using Using Using MyBaseMyBaseMyBaseMyBase and and and and MyClassMyClassMyClassMyClass KeywordsKeywordsKeywordsKeywords

� MyBase keyword

Public Class DerivedClass

Inherits BaseClass

Public Sub New()

MyBase.New() ‘a call to the

constructor of the base class

intCtr = 1

End Sub


OverrideMethod()

MsgBox(“Derived

OverrideMethod”)

MyBase.OverrideMethod()

‘ call the original method

End Sub

End Class

Using MyBase and MyClass Keywords Page 16 of 20



* Property of STI

Page 16 of 20

Using Using Using Using MyBaseMyBaseMyBaseMyBase and and and and MyClassMyClassMyClassMyClass KeywordsKeywordsKeywordsKeywords

� MyClass keyword

Public Class Computer

Public Overridable Sub

CalculateMemory()

MsgBox(“BaseOverrideMethod”)

End Sub

Public Sub Display()

MyClass.CalculateMemory()

‘will call base method

End Sub

End Class

Public Class Desktop

Inherits Computer


CalculateMemory()

MsgBox(“Derivex

OverrideMethod”)

End Sub

End Class

‘client code

Dim comp1 As Desktop = New Desktop()

comp1.Display()

Using MyBase and MyClass Keywords

The MyBase keyword is used to access the immediate base class from

which a derived class is inheriting. It cannot be used to access classes higher in the class hierarchy. A common practice is to use MyBase in constructors and destructors where you want to access the code in the overridden method. Public Class DerivedClass

Inherits BaseClass

Public Sub New()

MyBase.New() ‘a call to the constructor

of the base class

intCtr = 1

End Sub

Public Overrides Sub OverrideMethod()

MsgBox(“Derived OverrideMethod”)

MyBase.OverrideMethod()

‘ call the original method

End Sub

End Class

The MyClass keyword is used to invoke the Overridable method in

the base class rather than the overridden method in the derived class. It ensures that the base class implementation of an overridable method is called. An example is shown below: Public Class Computer

Public Overridable Sub CalculateMemory()

MsgBox(“Base OverrideMethod”)

End Sub

Public Sub Display()

MyClass.CalculateMemory()

‘will call base method

End Sub

End Class

Public Class Desktop

Inherits Computer



Public Overrides Sub CalculateMemory()

MsgBox(“Derived OverrideMethod”)

End Sub

End Class

‘client code

Dim comp1 As Desktop = New Desktop()

comp1.Display()

The above example will invoke the CalculateMemory() sub routine of

the class Computer. Note that the CalculateMemory() sub routuine

is an overridden method. The use of MyClass keyword specifies that

the method to be called is the method defined in the base class.

[Using MyBase and MyClass Keywords, Pages 15-16 of 20]

Interface Page 17 of 20



* Property of STI

Page 17 of 20

InterfaceInterfaceInterfaceInterface

� captures the roles played by various

classes that implement the interfaces

� specifies the prototype or behavior of a

class

� defines signatures for procedures,

properties and events but contains no

implementation code

� Interface keyword

• used to explicitly create an interface

without creating a class

� Implements keyword

• used to implement the interface

Interface Page 18 of 20



* Property of STI

Page 18 of 20

InterfaceInterfaceInterfaceInterface

� Example:

Interface IShape

Function Method1(ByRef s As

String) As Boolean

Sub Method2()

Sub Method2(ByVal x As Integer)

End Interface

� Class Implementing IShape interface:

Public Class Polygon Implements

IShape

….

End Class

Interface An interface essentially captures the roles played by various classes that implement the interfaces. An interface specifies the prototype or behavior of a class. All methods declared in an interface are abstract methods which mean no implementation code. If a class agrees to implement an interface it must implement all of the properties and methods defined. Interface defines signatures for procedures, properties and events but contains no implementation code. The Interface keyword is used to

explicitly create an interface without creating a class. It can be defined in the Declaration section of the module. An interface can also inherit

another interface. The Implements keyword is used to implement the

interface. An example interface implementation is shown below: Interface IShape

Function Method1(ByRef s As String) As Boolean

Sub Method2()

Sub Method2(ByVal x As Integer)

End Interface

This means that if a class implements the IShape interface it will have to implement these routines. Here’s an example of a class implementing this interface: Public Class Polygon Implements IShape

….

End Class

[Interface, Pages 17-18 of 20]



Polymorphism Page 19 of 20



* Property of STI

Page 19 of 20

PolymorphismPolymorphismPolymorphismPolymorphism

� means “one name in multiple forms”

� in VB.Net, it can be achieved by using interfaces and inheritance

Polymorphism Page 20 of 20



* Property of STI

Page 20 of 20

PolymorphismPolymorphismPolymorphismPolymorphism

� Example:

‘IPerson interface definition

Interface IPerson

Property LastNameI() As String

Sub Display()

End Interface

Class Employee ‘Employee class definition

Implements IPerson

Private strName As String

Private strCompany As String

Public Sub Display() Implements IPerson.Display

MsgBox(strCompany & “ honors “ & strName)

End Sub

Private Property LastName() As String Implements IPerson.LastName()

Get

Return strName

End Get

Set (ByVal val As String)

End Set

End Property

End Class

Polymorphism Polymorphism is another pillar in object-oriented programming. It means “one name in multiple forms”. Using polymorphism, many functions or sub routines can be created but with different argument list. The exact function or sub routine to be invoked will be determined by checking the type and number of arguments in the function. Polymorphism in VB.Net can be achieved by using interfaces and inheritance. Interfaces are created explicitly by using a combination of the Interface and

Implements keywords. An example is shown below:

‘IPerson interface definition

Interface IPerson

Property LastNameI() As String

Sub Display()

End Interface

‘Employee class definition

Class Employee

Implements IPerson

Private strName As String

Private strCompany As String

Public Sub Display() Implements

IPerson.Display

MsgBox(strCompany & “ honors “ & strName)

End Sub

Private Property LastName() As String

Implements IPerson.LastName()

Get

Return strName

End Get

Set (ByVal val As String)

End Set

End Property

End Class

[Polymorphism, Pages 19-20 of 20]

EVALUATION/GENERALIZATION:

• A constructor is the primary method of a class. It allows the programmer to initialize a variable of a class when the class is instantiated.

• A class can have multiple constructors.

• Regardless the name of the class, the constructor in VB.NET is called New.

• The Finalize() method is automatically called when an

instance of a class is not needed anymore.

• Garbage collection is a background process that traces object references, cleans up unused variables and destroys those that cannot be reached by executing code.

• The derived class can inherit all the base class properties, methods, members, events and event handlers.



• An interface is a class that creates a foundation that new derived classes can use.

REFERENCES:

� Microsoft Official Course, (2002), 2373B: Programming with

Microsoft Visual Basic .NET, Microsoft Corporation � Holzner, Steven, (2003), Sams teach yourself Microsoft Visual

Basic.Net 2003 in 21 days, USA, Sams Publishing � Liberty, Jesse, (2003), Learning Visual Basic .NET, USA,

O'Reilly & Associates, Inc

Technology

MELJUN CORTES Vb.net handout object-oriented programming concepts