Apply Inheritance and Polymorphism

7/28/2019 Apply Inheritance and Polymorphism

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Chapter 4.2

APPLY INHERITANCE AND POLYMORPHISM


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What is Inheritance?

Inheritance is the property in which a derived class

acquires the attributes of its base class.

Recall that classes contain

Fields

Properties

Event

Methods


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What is Inheritance?

Inheritance implies at least two classes

Class play role of parent (superclass)

Class play role of child (subclass) To implement inheritance, use the Inherits keyword.


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Basic Inheritance Statement

Simplest inheritance relationship

Public Class Parent From listing you can see

End Class Inherits Parent is all thatwe must add to indicate

Public Class Child inheritance of

Inherits Parent child from parent.

End Class


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Inheritance

When user create class, new class can be based onanother class.

User can create new class inherit from one of

existing .NET classes or from one of user own

classes. Example:

Partial Public Class Form1

Inherits System.Windows.Forms.Form

The Inherits statement must follow the class header priorto any comments.

Public class NewClass

Inherits BaseClass

C B C


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Ca ng t e Base C ass

Constructor

Constructor in Inheritance Subclass cannot inherit constructor from base class.

Each class must have its own constructor.

Calling base class constructor

With this statemen MyBase.NEW()

Example:

Sub New(ByVal title As String, ByVal price By Decimal)

assign property values

call the base class constructor

MyBase.NEW(title, price)

End Sub


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Important Of Inheritance

Provides re-usability of code.

Help in reducing the code size and better project

management.

Use Inherits keyword to inherit a class into otherclass.

Old class called Base class and new class called

Derived class

Use mybase keyword to reference the mambers of

base class.


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Polymorphism

Polymorphism is one of the crucial features of OOP.

An object can be treated as if it were a different kind

of object provided common sense prevails. 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 withdifferent argument list.


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Example:

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


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Implement Polymorphism

Class ParentPublic Overridable Sub p1()

Console.Write("Parent.p1")

End Sub

Public Sub p2()MyClass.p1()

'Implementing keyword MyClass here tells all

derived classes to refer to the base / abstract

class wherein the keyword MyClass appearsEnd Sub

End Class


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Implement Polymorphism

Implementing the keyword MyClass in the baseclass tells all derived classes to use the method in

the base class itself and, therefore, a stack overflow

error is prevented.

p.p2() 'stack overflow error is preventedc = New Child()

c.p1() 'stack overflow error is prevented

c.p2() 'stack overflow error is prevented

End Sub


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Chapter 4.3

Construct String


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Declare and Manipulate char type

The String Class represents character strings.

The String data type comes from the System.Str ing

class

The String type represents a string ofUnicode

Characters .

The String class is a sealed class , so you cannot

inherit another class from the String class.


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String References as Parameters

What is Parameters? A constructor that contains an argument list; as opposed

to an empty constructor.

VB.NET String.Format()

VB.NET String Format method replace the argumentObject into a text equivalent System.String.

System .Format(ByVal form at As String, ByVal arg0 As

Object) As String

Parameters:String format : The format String


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VB.NET String.Length()

The Length() function in String Class returned the

number of characters occurred in a String.

Sys tem.String .Leng th() As Integer

Returns: Integer : The number of characters in the specified String

For exmple:

"This is a Test".Length() returns 14.


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VB.NET String.Concat()

Concat in VB.NET String Class using for concat twospecified String Object

System.Str ing .Concat(ByVal str1 As Str ing,

ByVal str2 As Str ing ) As Str ing

String Concat method returns a new String

Parameters:

String str1 : Parameter String

String str2 : Parameter String

Returns:

String : A new String retrun with str1 Concat with str2


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VB.NET String.substring()

Substring in Vb.Net String Class returns a new string

that is a substring of this string. The substring begins atthe specified given index and extended up to the givenlength.

Publ ic Func t ion Substr ing (ByVal star t Index As

Integer, ByVal leng th As Integer) As Str ing

Parameters: startIndex: The index of the start of the substring.

length: The number of characters in the substring.

Returns:

The specified substring. Exceptions:

System.ArgumentOutOfRangeException: the beginIndex orlength less than zero, or the begin index + length not within thespecified string


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Chapter 4.4

Exception Handling


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Exception Handling in VB.NET

What is exception?

An error that occurs at run time.

Exception Handling?

Exception handling is an in built mechanism in .NET

framework to detect and handle run time errors.

Cause of error?

Logic errors.

System errors.

VB.NET provides three keywords try, catch andfinally to do exception handling.


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The general form try-catch

Try

statement that may cause error

Catch [VariableName As ExceptionType]

statement for action when exception occurs

[Finallystatements that always execute before exit of

Try block]

End Try


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Example

Public Sub MyException(ByVal a As Int32, ByVal b As Int32)

Dim Result As Int32Try

'The try block from where an exception will thrown

Result = a / b

textBox3.Text = Result.ToString()Catch e As System.DivideByZeroException

'The catch block where Exception will be handle

'Here I am using e.Message

MessageBox.Show(e.Message)End Try

End Sub

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Apply Inheritance and Polymorphism