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Singleton pattern:
In software engineering, the singleton design pattern is designed to restrict instantiation of a class to one (or a few) objects. This is useful when exactly one object is needed to coordinate actions across the system. Sometimes it is generalized to systems that operate more efficiently when only one or a few objects exist.
The singleton pattern is implemented by creating a class with a method that creates a new instance of the object if one does not exist. If an instance already exists, it simply returns a reference to that object. To make sure that the object cannot be instantiated any other way, the constructor is made private.
Example implementation
I have created a class called SingleTon. the constructor of this class is made as private to make sure the object cannot be instantiated any other way. In this constructor I created three objects of Job class (which is listed later in this article) add added to a job queue.
The function GetObject returns an instance of SingleTon class. If it exists it returns the existing object reference.
The function GetJOB returns a job object based on the index it receives.
The function Release decrements the count of reference.
Public Class SingleTon
#Region "declaration"
Private Shared instance As SingleTon
Private Shared m_nNofReference As Integer
Private m_ArrJob As ArrayList
#End Region
''' <summary>
'''
''' </summary>
Private Sub New()
'
' TODO: Add constructor logic here
'
m_nNofReference = 0
m_ArrJob = New ArrayList()
'''
'''Add three job types in the queue for
'''the time being
'''
m_ArrJob.Add(New Job(1,"INSERT INTO TABLE"))
m_ArrJob.Add(New Job(2,"DELETE FROM TABLE"))
m_ArrJob.Add(New Job(2,"UPDATE TABLE"))
End Sub
''' <summary>
''' Return the object if it is existing
''' OR create new ONE and return
''' </summary>
Public Shared Function GetObject() As SingleTon
If instance Is Nothing Then
instance = New SingleTon()
End If
m_nNofReference += 1
Return instance
End Function
''' <summary>
'''
''' </summary>
Public Shared Sub Release()
m_nNofReference -= 1
End Sub
''' <summary>
'''
''' </summary>
''' <param name="nJob"></param>
''' <returns></returns>
Public Function GetJOB(ByVal nJob As Integer) As Object
If nJob < m_ArrJob.Count Then
Return m_ArrJob(nJob)
Else
Return Nothing
End If
End Function
End Class
The following is the implementation of class Job.
Public Class Job
#Region "declaration"
Private m_nJobID As Integer
Private m_strJobName As String
#End Region
''' <summary>
'''
''' </summary>
''' <param name="nJID"></param>
''' <param name="strJName"></param>
Public Sub New(ByVal nJID As Integer, ByVal strJName As String)
'
' TODO: Add constructor logic here
'
m_nJobID = nJID
m_strJobName = strJName
End Sub
''' <summary>
'''
''' </summary>
Public ReadOnly Property JOBID() As Integer
Get
Return m_nJobID
End Get
End Property
''' <summary>
'''
''' </summary>
Public ReadOnly Property JOBNAME() As String
Get
Return m_strJobName
End Get
End Property
End Class
The following statement can be used to check functionality of the above code
-
Private objSingleTon As SingleTon = SingleTon.GetObjesct
Private objJob As Job = CType(objSingleTon.GetJOB(0), Job)
-
Dim objSingleTon2 As SingleTon = SingleTon.GetObject()
objJob = CType(objSingleTon.GetJOB(1), Job)
The first line retrieves an object of SingleTon class. In the second line we are requesting for the SingleTon object once again. But at this time it retrieves the existing SingleTon object reference only.
NOTE: THIS ARTICLE IS CONVERTED FROM C# TO VB.NET USING A CONVERSION TOOL. ORIGINAL ARTICLE CAN BE FOUND ON C# CORNER (http://www.c-sharpcorner.com/).
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