Abstract Classes

Abstract Classes

An abstract class is the one that is not used to create objects. An abstract class is designed to act as a base class (to be inherited by other classes). Abstract class is a design concept in program development and provides a base upon which other classes are built. Abstract classes are similar to interfaces. After declaring an abstract class, it cannot be instantiated on it's own, it must be inherited. Like interfaces, abstract classes can specify members that must be implemented in inheriting classes. Unlike interfaces, a class can inherit only one abstract class. Abstract classes can only specify members that should be implemented by all inheriting classes.

Creating Abstract Classes

In Visual Basic .NET we create an abstract class by using the MustInherit keyword. An abstract class like all other classes can implement any number of members. Members of an abstract class can either be Overridable (all the inheriting classes can create their own implementation of the members) or they can have a fixed implementation that will be common to all inheriting members. Abstract classes can also specify abstract members. Like abstract classes, abstract members also provide no details regarding their implementation. Only the member type, access level, required parameters and return type are specified. To declare an abstract member we use the MustOverride keyword. Abstract members should be declared in abstract classes.

Implementing Abstract Class

When a class inherits from an abstract class, it must implement every abstract member defined by the abstract class. Implementation is possible by overriding the member specified in the abstract class. The following code demonstrates the declaration and implementation of an abstract class.

Module Module1

Public MustInherit Class AbstractClass
'declaring an abstract class with MustInherit keyword
Public MustOverride Function Add() As Integer
Public MustOverride Function Mul() As Integer
'declaring two abstract members with MustOverride keyword
End Class

Public Class AbstractOne
Inherits AbstractClass
'implementing the abstract class by inheriting
Dim i As Integer = 20
Dim j As Integer = 30
'declaring two integers

Public Overrides Function Add() As Integer
Return i + j
End Function
'implementing the add method

Public Overrides Function Mul() As Integer
Return i * j
End Function
'implementing the mul method

End Class

Sub Main()
Dim abs As New AbstractOne()
'creating an instance of AbstractOne
WriteLine("Sum is" & " " & abs.Add())
WriteLine("Multiplication is" & " " & abs.Mul())
'displaying output
Read()
End Sub

End Module


The output of above code is the image below.

Interfaces

Interfaces

Interfaces allow us to create definitions for component interaction. They also provide another way of implementing polymorphism. Through interfaces, we specify methods that a component must implement without actually specifying how the method is implemented. We just specify the methods in an interface and leave it to the class to implement those methods. Visual Basic .NET does not support multiple inheritance directly but using interfaces we can achieve multiple inheritance. We use the Interface keyword to create an interface and implements keyword to implement the interface. Once you create an interface you need to implement all the methods specified in that interface. The following code demonstrates the use of interface.

Imports System.Console
Module Module1

Sub Main()
Dim OneObj As New One()
Dim TwoObj As New Two()
'creating objects of class One and Two
OneObj.disp()
OneObj.multiply()
TwoObj.disp()
TwoObj.multiply()
'accessing the methods from classes as specified in the interface
End Sub

End Module

Public Interface Test
'creating an Interface named Test
Sub disp()
Function Multiply() As Double
'specifying two methods in an interface
End Interface

Public Class One
Implements Test
'implementing interface in class One

Public i As Double = 12
Public j As Double = 12.17

Sub disp() Implements Test.disp
'implementing the method specified in interface
WriteLine("sum of i+j is" & i + j)
Read()
End Sub

Public Function multiply() As Double Implements Test.Multiply
'implementing the method specified in interface
WriteLine(i * j)
Read()
End Function

End Class

Public Class Two
Implements Test
'implementing the interface in class Two

Public a As Double = 20
Public b As Double = 32.17

Sub disp() Implements Test.disp
WriteLine("Welcome to Interfaces")
Read()
End Sub

Public Function multiply() As Double Implements Test.Multiply
WriteLine(a * b)
Read()
End Function

End Class


Output of above code is the image below.

Polymorphism

Polymorphism

Polymorphism is one of the crucial features of OOP. It means "one name, multiple forms". It is also called as Overloading which means the use of same thing for different purposes. Using Polymorphism we can create as many functions we want with one function name but with different argument list. The function performs different operations based on the argument list in the function call. The exact function to be invoked will be determined by checking the type and number of arguments in the function.

The following code demonstrates the implementation of Polymorphism.

Module Module1

Sub Main()
Dim two As New One()
WriteLine(two.add(10))
'calls the function with one argument
WriteLine(two.add(10, 20))
'calls the function with two arguments
WriteLine(two.add(10, 20, 30))
'calls the function with three arguments
Read()
End Sub

End Module

Public Class One
Public i, j, k As Integer

Public Function add(ByVal i As Integer) As Integer
'function with one argument
Return i
End Function

Public Function add(ByVal i As Integer, ByVal j As Integer) As Integer
'function with two arguments
Return i + j
End Function

Public Function add(ByVal i As Integer, ByVal j As Integer, ByVal k As Integer) As Integer
'function with three arguments
Return i + j + k
End Function

End Class


Output of the above code is shown in the image below.

Inheritance

Inheritance

A key feature of OOP is reusability. It's always time saving and useful if we can reuse something that already exists rather than trying to create the same thing again and again. Reusing the class that is tested, debugged and used many times can save us time and effort of developing and testing it again. Once a class has been written and tested, it can be used by other programs to suit the program's requirement. This is done by creating a new class from an existing class. The process of deriving a new class from an existing class is called Inheritance. The old class is called the base class and the new class is called derived class. The derived class inherits some or everything of the base class. In Visual Basic we use the Inherits keyword to inherit one class from other. The general form of deriving a new class from an existing class looks as follows:

Public Class One
---
---
End Class

Public Class Two
Inherits One
---
---
End Class


Using Inheritance we can use the variables, methods, properties, etc, from the base class and add more functionality to it in the derived class. The following code demonstrates the process of Inheritance in Visual Basic.

Imports System.Console
Module Module1

Sub Main()
Dim ss As New Two()
WriteLine(ss.sum())
Read()
End Sub

End Module

Public Class One
'base class
Public i As Integer = 10
Public j As Integer = 20

Public Function add() As Integer
Return i + j
End Function

End Class

Public Class Two
Inherits One
'derived class. class two inherited from class one
Public k As Integer = 100

Public Function sum() As Integer
'using the variables, function from base class and adding more functionality
Return i + j + k
End Function

End Class


Output of above code is sum of i, j, k as shown in the image below.

Constructors in VB.NET

Constructors in VB.NET:::

A constructor is a special member function whose task is to initialize the objects of it's class. This is the first method that is run when an instance of a type is created. A constructor is invoked whenever an object of it's associated class is created. If a class contains a constructor, then an object created by that class will be initialized automatically. We pass data to the constructor by enclosing it in the parentheses following the class name when creating an object. Constructors can never return a value, and can be overridden to provide custom intitialization functionality. In Visual Basic we create constructors by adding a Sub procedure named New to a class. The following code demonstrates the use of constructors in Visual Basic.

Module Module1

Sub Main()

Dim con As New Constructor(10)
WriteLine(con.display())
'storing a value in the constructor by passing a value(10) and calling it with the
'display method
Read()

End Sub

End Module

Public Class Constructor
Public x As Integer

Public Sub New(ByVal value As Integer)
'constructor
x = value
'storing the value of x in constructor
End Sub

Public Function display() As Integer
Return x
'returning the stored value
End Function

End Class


Destructors

A destructor, also know as finalizer, is the last method run by a class. Within a destructor we can place code to clean up the object after it is used, which might include decrementing counters or releasing resources. We use Finalize method in Visual Basic for this and the Finalize method is called automatically when the .NET runtime determines that the object is no longer required. When working with destructors we need to use the overrides keyword with Finalize method as we will override the Finalize method built into the Object class. We normally use Finalize method to deallocate resources and inform other objects that the current object is going to be destroyed. Because of the nondeterministic nature of garbage collection, it is very hard to determine when a class's destructor will be called. The following code demonstrates the use of Finalize method.

Module Module1

Sub Main()
Dim obj As New Destructor()
End Sub

End Module

Public Class Destructor

Protected Overrides Sub Finalize()
Write("hello")
Read()
End Sub

End Class


When you run the above code, the word and object, obj of class, destructor is created and "Hello" is displayed. When you close the DOS window, obj is destroyed.

Classes n Objects VB Language

Visual Basic .NET is Object-Oriented. Everything we do in Visual Basic involves objects in some way or other and everything is based on the Object class. Controls, Forms, Modules, etc are all types of classes. Visual Basic .NET comes with thousands of built-in classes which are ready to be used. Let's take a closer look at Object-Oriented Programming in Visual Basic. We will see how we can create classes, objects, how to inherit one class from other, what is polymorphism, how to implement interfaces and so on. We will work with Console Applications here as they are simple to code.

Classes and Objects

Classes are types and Objects are instances of the Class. Classes and Objects are very much related to each other. Without objects you can't use a class. In Visual Basic we create a class with the Class statement and end it with End Class. The Syntax for a Class looks as follows:

Public Class Test

----- Variables
-----Methods
-----Properties
-----Events

End Class


The above syntax created a class named Test. To create a object for this class we use the new keyword and that looks like this: Dim obj as new Test(). The following code shows how to create a Class and access the class with an Object. Open a Console Application and place the following code in it.

Module Module1
Imports System.Console

Sub Main()
Dim obj As New Test()
'creating a object obj for Test class
obj.disp()
'calling the disp method using obj
Read()
End Sub

End Module

Public Class Test
'creating a class named Test
Sub disp()
'a method named disp in the class
Write("Welcome to OOP")
End Sub
End Class


Output of above code is the image below.



Fields, Properties, Methods and Events

Fields, Properties, Methods, and Events are members of the class. They can be declared as Public, Private, Protected, Friend or Protected Friend.

Fields and Properties represent information that an object contains. Fields of a class are like variables and they can be read or set directly. For example, if you have an object named House, you can store the numbers of rooms in it in a field named Rooms. It looks like this:

Public Class House
Public Rooms as Integer
End Class


Properties are retrieved and set like fields but are implemented using Property Get and Property Set procedures which provide more control on how values are set or returned.

Methods represent the object’s built-in procedures. For example, a Class named Country may have methods named Area and Population. You define methods by adding procedures, Sub routines or functions to your class. For example, implementation of the Area and Population methods discussed above might look like this

Public Class Country
Public Sub Area()
Write("--------")
End Sub
Public Sub population()
Write("---------")
End Sub
End Class


Events allow objects to perform actions whenever a specific occurrence takes place. For example when we click a button a click event occurs and we can handle that event in an event handler.

OOPS with VB

OOPS with VB

OOPS Basics

Visual Basic was Object-Based, Visual Basic .NET is Object-Oriented, which means that it's a true Object-Oriented Programming Language. Visual Basic .NET supports all the key OOP features like Polymorphism, Inheritance, Abstraction and Encapsulation. It's worth having a brief overview of OOP before starting OOP with VB.

Why Object Oriented approach?

A major factor in the invention of Object-Oriented approach is to remove some of the flaws encountered with the procedural approach. In OOP, data is treated as a critical element and does not allow it to flow freely. It bounds data closely to the functions that operate on it and protects it from accidental modification from outside functions. OOP allows decomposition of a problem into a number of entities called objects and then builds data and functions around these objects. A major advantage of OOP is code reusability.

Some important features of Object Oriented programming are as follows:



Emphasis on data rather than procedure

Programs are divided into Objects

Data is hidden and cannot be accessed by external functions

Objects can communicate with each other through functions

New data and functions can be easily added whenever necessary

Follows bottom-up approach

Concepts of OOP:


Objects

Classes

Data Abstraction and Encapsulation

Inheritance

Polymorphism

Briefly on Concepts:

Objects

Objects are the basic run-time entities in an object-oriented system. Programming problem is analyzed in terms of objects and nature of communication between them. When a program is executed, objects interact with each other by sending messages. Different objects can also interact with each other without knowing the details of their data or code.

Classes

A class is a collection of objects of similar type. Once a class is defined, any number of objects can be created which belong to that class.

Data Abstraction and Encapsulation

Abstraction refers to the act of representing essential features without including the background details or explanations. Classes use the concept of abstraction and are defined as a list of abstract attributes.

Storing data and functions in a single unit (class) is encapsulation. Data cannot be accessible to the outside world and only those functions which are stored in the class can access it.

Inheritance

Inheritance is the process by which objects can acquire the properties of objects of other class. In OOP, inheritance provides reusability, like, adding additional features to an existing class without modifying it. This is achieved by deriving a new class from the existing one. The new class will have combined features of both the classes.

Polymorphism

Polymorphism means the ability to take more than one form. An operation may exhibit different behaviors in different instances. The behavior depends on the data types used in the operation. Polymorphism is extensively used in implementing Inheritance.

Advantages of OOP

Object-Oriented Programming has the following advantages over conventional approaches:


OOP provides a clear modular structure for programs which makes it good for defining abstract datatypes where implementation details are hidden and the unit has a clearly defined interface.

OOP makes it easy to maintain and modify existing code as new objects can be created with small differences to existing ones.

OOP provides a good framework for code libraries where supplied software components can be easily adapted and modified by the programmer. This is particularly useful for developing graphical user interfaces.


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