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Article Version : 2.03
Introduction
Most of beginner programmer may not know and use the Object Oriented facilities in their codes , although they can write bunch of lines ; the codes are not readable for the next developers and fall them in trouble.
one of the facility in OOP is interface , "An interface contains only the signatures of methods, properties, events or indexers.
A class or struct that implements the interface must implement the
members of the interface that are specified in the interface definitionvoid." as Microsoft said.
In this article I explain the simple example of using interface and combine with inheritance concept in OOP.
the example will be useful for those novice VB.NET programmer who had problem with understanding to use Interface and Inheritance beside each other .
The article Codes written in Visual Basic.Net language and might be useful in Windows Application , Web Application (ASP.NET) , Mobile Application and etc .
Background
before you continue reading this article please be aware I assume you know about the theoretical concept of OOP programming , Such as Class ,Methods , Property , Inheritance .
For more information about the concepts of Object Oriented Programming such as class , Methods , Property , Inheritance , Access modifier please search on codeproject.com or Microsoft MSDN.
Scenario
Imagine you got order from your software project manager and you should prepare three different classes for three types of users in the system , some users have common properties such as NAME , LAST NAME , AGE , TELEPHONE , ADDRESS and etc .
you provide a parent class and put all of the common Properties inside that then create three child classes .
if we assume we have "SALES" users , "Managers" users , "Developers" users ; finally we should have three class with their names , and one parent class name "EmployeeBase" , that are very obvious so far .
One of the method which your Software Project Manager wants is "GetSalary()" , and which mean the classes should get the salary from database according to Users Types , and any time you need to use "GetSalary()" method you should understand first the "current user type" then create appropriate instance of the user class then call "GetSalery()"
If UserType = "Developer" Then
Dim objectDeveloper As New EmployeeDeveloper
objectDeveloper.GetSalery()
End If
If UserType = "Manager" Then
Dim objectManager As New EmployeeManager
objectManager.GetSalery()
End If
the above codes are completely ugly and not acceptable by All professional software developers seniors .
Using the code
For solving the problem I try to lead you deep into the OOP concept and after four iteration we will have clean , nice , OOP based Code and finally we have a piece of codes which are acceptable for most Software Seniors and Software Team Leaders .
Here Interface concept help us to make a change in the codes and make them better and I will explain the evolution steps of codes .
Iteration 1 :
Public Interface IEmployee
Function GetSalary() As Double
Function GetEmpType() As String
End Interface
Public Class EmployeeManager
Inherits EmployeeBase
Implements IEmployee
Sub New()
End Sub
Public Function GetSalary() As Double Implements IEmployee.GetSalary
Return 2300.0
End Function
Public Function GetEmpType() As String Implements IEmployee.GetEmpType
Return "I am an Employee >> 'Manager' "
End Function
End Class
then we have tow other classes which same implementation (Refer to source Code File)
When We need to use it :
Module Main_Module
Sub Main()
Dim oEmp As IEmployee
oEmp = New EmployeeDeveloper
Console.WriteLine(oEmp.GetEmpType())
Console.WriteLine("The Salary is : " & oEmp.GetSalary())
oEmp = New EmployeeManager
Console.WriteLine(oEmp.GetEmpType())
Console.WriteLine("The Salary is : " & oEmp.GetSalary())
End Sub
End Module
Then Result gonna be like this :
after the first iteration our problem has solved so far and you have learned
how to use interface and why we need interface ,you might ask Is that
all ?
The second problem shows itself when your Software
Project Manager wants you to Calculate the each Employee's salary according the work
time and their wage percent , then you should forget GetSalery() method and
improve your codes to get better result and It is better we create
another class who has duty in calculate the salary and I called it
"Accounting" Class .
Iteration 2 :
I added another function signature in the interface
Public Interface IEmployee
Function GetSalary() As Double
Function GetEmpType() As String
Function GetWorkTime() As Integer
Function GetExtraWagePercent() As Short
End Interface
I have Added GetExtraWagePercent() method in All three child class
Public Class EmployeeManager
Inherits EmployeeBase
Implements IEmployee
Public Function GetWorkTime() As Integer Implements IEmployee.GetWorkTime
Return 210
End Function
Public Function GetExtraWagePercent() As Short Implements IEmployee.GetExtraWagePercent
Return 1.3
End Function
Public Function GetEmpType() As String Implements IEmployee.GetEmpType
Return "I am an Employee >> 'Manager' "
End Function
End Class
and another two classes also follow the same implementation but diffrent Wage Percent Amount and etc
(Reffer to Source Code of this project )
I also create an "Acounting" Class
Public Class Accounting
Public Function CalculateSalery(employee As IEmployee) As Double
Return employee.GetWorkTime * 5 * employee.GetExtraWagePercent
End Function
you should connect these classes to "Accounting" Class and the class
should provide you CalculateSalery() methods which are be able to calculate the salery with out the class does not need to know about another class functions.
Then in main module we can use this as below :
Module Main_Module
Sub Main()
Dim oEmp As IEmployee
Dim oAccounting As New Accounting
oEmp = New EmployeeManager
Console.WriteLine(oEmp.GetEmpType())
Console.WriteLine("My Salary is : " & oAccounting.CalculateSalery(oEmp))
oEmp = New EmployeeSales
Console.WriteLine(oEmp.GetEmpType())
Console.WriteLine("My Salary is : " & oAccounting.CalculateSalery(oEmp))
oEmp = New EmployeeDeveloper
Console.WriteLine(oEmp.GetEmpType())
Console.WriteLine("My Salary is : " & oAccounting.CalculateSalery(oEmp))
End Sub
End Module
and then so on you can improve your codes.
Then Result gonna be like this :
Remember I use inheritance to access the common properties for Example : Name
Iteration 3 :
Now in third Iteration I want to Add method in Accounting class which be able to beside calculate the salary also be able write the employee name but I don't want connected to database in each class , Just one place and that is in Parent Class (EmployeeBase), hence I need to manipulate the EmployeeBase and just add one property "Name" As Example and another problem is parent class dot know about which Name should be return thus I need to pass the Employee Id from each Child Class to Parent Class and As you Know in OOP we can Acess to the Parent Class from the Child Constructor Like below example :
MyBase.New()
We Need to write Initialize Sub for All three classes which be able to get Employee ID from the User Interface (UI)(According to Klaus Luedenscheidt's Comment I improved this part) and then we just need to call Initialize Sub in Each Child Class Constructor. The Initialize Sub Can Get the data from the DataBase (XML-SQL Server - any ).
according the Employee Id which has given to new child Class Instance (During the calling , here this happened at the main module)
Notice : The Access Modifier of Initialize Sub Is Protected which means all child class can use this SUB routine , And Its obvious the EMP ID will send to Initialize class from the Child Class Constructor. Hence each child class have 2 Constructor :
First one : is for making an instance (NORMAL)
Second one : is used when we want to Work with available data
"Example for Second Manner : Imagine you just want to ADD Employee in the DB , Just need to Use the first Constructor , but when you decide to Delete or Modify the Employee you shall to use the second constructor."
MyBase.New()
Initialize(EmpId)
The blow code shows you the last changes on EmployeeBase Class
Public Class EmployeeBase
<pre> Private _name As String
Public Property Name As String
Get
Return _name
End Get
Set(value As String)
_name = value
End Set
End Property
Sub New()
Name = "-----"
End Sub
Protected Sub Initialize(ByVal EmpId As Integer)
Select Case EmpId
Case 1
Name = "Babak Manager"
Case 2
Name = "Ali Developer"
Case 3
Name = "Mike Sales"
Case Else
End Select
End Sub
End Class
And we need to write second Constructor for all "Child Classes" and also I write two function
singnature GetEmployeeNameAndSalery() , GetEmployeeName() in the interface IEmployee and then
I write the related functions in child clasess to show how Accounting Class can get EmployeeName
From the Child Class without direct access to them .
Public Class EmployeeSales
Inherits EmployeeBase
Implements IEmployee
Sub New()
End Sub
Sub New(ByVal EmpId As Integer)
MyBase.New()
Initialize(EmpId)
End Sub
Public Function GetWorkTime() As Integer Implements IEmployee.GetWorkTime
Return 142
End Function
Public Function GetExtraWagePercent() As Short Implements IEmployee.GetExtraWagePercent
Return 1.1
End Function
Public Function GetEmployeeNameAndSalery() As String Implements IEmployee.GetEmployeeNameAndSalery
Dim objAccounting As New Accounting
Return MyBase.Name & " - " & objAccounting.CalculateSalery(Me)
End Function
Public Function GetEmployeeName() As String Implements IEmployee.GetEmployeeName
Return MyBase.Name
End Function
Public Function GetEmpType() As String Implements IEmployee.GetEmpType
Return "I am an Employee >> 'Sales' "
End Function
End Class
Finally the IEmployee Interface will look like this :
Public Interface IEmployee
Function GetSalary() As Double
Function GetWorkTime() As Integer
Function GetExtraWagePercent() As Short
Function GetEmpType() As String
Function GetEmployeeNameAndSalery() As String
Function GetEmployeeName() As String
End Interface
and accounting class will looks like this :
Public Class Accounting
Private _employeeId As Short
Public Property EmployeeId As Short
Get
Return _employeeId
End Get
Set(value As Short)
_employeeId = value
End Set
End Property
Public Function CalculateSalery(employee As IEmployee) As Double
Return employee.GetWorkTime * 5 * employee.GetExtraWagePercent
End Function
Public Function CalculateSaleryAndName(employee As IEmployee) As String
Dim EmpSalery As Double = CalculateSalery(employee)
Return employee.GetEmployeeNameAndSalery
End Function
Public Function Customize_CalculateSaleryAndName(employee As IEmployee) As String
Dim EmpSalery As Double = CalculateSalery(employee)
Dim EmpName As String = employee.GetEmployeeName
Return "Employee Name is : " & EmpName & " - Employee Salery is : " & EmpSalery & " $"
End Function
End Class
then the result will be like this :
and class diagram will show what exactly happend
Conclusion
Object Oriented Programming will be very interesting topic if you be lazy in extra code writing and also it helps to after-you developers to understand your code and grow the program which you have written but if you are not care to the concept the others developers will not care to you and ruin all your codes which you have created and then start from new project. which using the OOP concept and facilities we can have usable and robust codes .
History
1- ver 2 : Update one has effected , Add Initialize method to avoid creating instance of the base class - 9/6/2013
