Builder Design Pattern





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Environment: Win2K, NT

Creational Pattern Builder

Reference:E. Gamma et al., “Design Patterns: Elements of Reusable
Object-Oriented Software
” ISBN 0-201-63361-2, Addison Wesley, 1995.

The Builder pattern allows a client object to construct a complex object by specifying only its type and content. The client is shielded from the details of the objects construction.

It is a pattern for step-by-step creation of a complex object so that the same construction process can create different representations is the routine in the builder pattern that also makes for finer control over the construction process. All the different builders generally inherit from an abstract builder class that declares the general functions to be used by the director to let the builder create the product in parts.

Builder has a similar motivation to the abstract factory but, whereas in that pattern, the client uses the abstract factory class methods to create its own object, in Builder the client instructs the builder class on how to create the object and then asks it for the result. How the class is put together is up to the Builder class. It’s a subtle difference.

The Builder pattern is applicable when the algorithm for creating a complex object should be independent of the parts that make up the object and how they are assembled and the construction process must allow different representations for the object thats constructed

Example

Below is an example of creating a House, the clients asks the Director (CDirector class) to create a House by calling BuildHouse method which takes a boolean parameter (blnBackyard), the director then creates an Apartment (Concrete Builder) if the blnBackyard is false or a Single Family Home (Concrete Builder) if the blnBackyard is true (both of them implements IHouse interface) and returns IHouse (Abstract Builder) Interface.

The director does the complex building of a House and the client gets back IHouse interface that it codes against without worrying about the creation of House, Rooms, backyard etc.

C#


using System;
using System.Collections;

public interface IHouse
{
bool GetBackyard();
long NoOfRooms();
string Discription();
}

public class CApt:IHouse
{ private bool mblnBackyard;
private Hashtable Rooms;
public CApt()
{ CRoom room;
Rooms = new Hashtable();
room = new CRoom();
room.RoomName = “Master Bedroom”;
Rooms.Add (“room1”,room);

room = new CRoom();
room.RoomName = “Second Bedroom”;
Rooms.Add (“room2”,room);

room = new CRoom();
room.RoomName = “Living Room”;
Rooms.Add (“room3”,room);

mblnBackyard = false;
}

public bool GetBackyard()
{
return mblnBackyard;
}
public long NoOfRooms()
{
return Rooms.Count;
}
public string Discription()
{
IDictionaryEnumerator myEnumerator =
Rooms.GetEnumerator();
string strDiscription;
strDiscription = “This is an Apartment with ” +
Rooms.Count + ” Rooms \n”;
strDiscription = strDiscription +
“This Apartment doesn’t have a backyard \n”;
while (myEnumerator.MoveNext())
{
strDiscription = strDiscription + “\n” +
myEnumerator.Key + “\t” +
((CRoom)myEnumerator.Value).RoomName;
}
return strDiscription;
}
}

public class CSFH:IHouse
{ private bool mblnBackyard;
private Hashtable Rooms;
public CSFH()
{
CRoom room;
Rooms = new Hashtable();

room = new CRoom();
room.RoomName = “Master Bedroom”;
Rooms.Add (“room1”,room);

room = new CRoom();
room.RoomName = “Second Bedroom”;
Rooms.Add (“room2”,room);

room = new CRoom();
room.RoomName = “Third Room”;
Rooms.Add (“room3”,room);

room = new CRoom();
room.RoomName = “Living Room”;
Rooms.Add (“room4”,room);

room = new CRoom();
room.RoomName = “Guest Room”;
Rooms.Add (“room5”,room);

mblnBackyard = true;

}

public bool GetBackyard()
{
return mblnBackyard;
}
public long NoOfRooms()
{
return Rooms.Count;
}
public string Discription()
{
IDictionaryEnumerator myEnumerator =
Rooms.GetEnumerator();
string strDiscription;
strDiscription = “This is an Single Family Home with ”
+ Rooms.Count + ” Rooms \n”;
strDiscription = strDiscription +
“This house has a backyard \n”;
while (myEnumerator.MoveNext())
{
strDiscription = strDiscription +
“\n” + myEnumerator.Key + “\t” +
((CRoom)myEnumerator.Value).RoomName;
}
return strDiscription;
}
}

public interface IRoom
{
string RoomName{get;set;}
}

public class CRoom:IRoom
{
private string mstrRoomName;
public string RoomName
{
get
{
return mstrRoomName;
}
set
{
mstrRoomName = value;
}
}
}

public class CDirector
{
public IHouse BuildHouse(bool blnBackyard)
{
if (blnBackyard)
{
return new CSFH();
}
else
{
return new CApt();
}
}
}

public class Client
{
static void Main(string[] args)
{
CDirector objDirector = new CDirector();
IHouse objHouse;
objHouse = objDirector.BuildHouse(bool.Parse(args[0]));
Console.WriteLine(objHouse.Discription());
}
}

Visual Basic .NET


Imports System
Imports System.Collections
Imports Microsoft.VisualBasic

Public Interface IHouse
Function GetBackyard() As Boolean
Function NoOfRooms() As Long
Function Discription() As String
End Interface

Public Class CApt
Implements IHouse

Private blnBackYard As Boolean
Private Rooms As Hashtable
Sub New()
Dim room As CRoom
Rooms = New Hashtable()
room = New CRoom()
room.RoomName = “Master Bedroom”
Rooms.Add(“room1”, room)
room = New CRoom()
room.RoomName = “Second Bedroom”
Rooms.Add(“room2”, room)
room = New CRoom()
room.RoomName = “Living Room”
Rooms.Add(“room3”, room)
blnBackYard = False
End Sub

Public Function GetBackyard() As Boolean _
Implements IHouse.GetBackyard
GetBackyard = blnBackYard
End Function

Public Function NoOfRooms() As Long _
Implements IHouse.NoOfRooms
NoOfRooms = Rooms.Count
End Function

Public Function Discription() As String _
Implements IHouse.Discription
Dim myEnumerator As IDictionaryEnumerator = _
Rooms.GetEnumerator
Dim strDiscription As String
strDiscription = “This is an Apartment with ” &_
Rooms.Count & ” Rooms ” & vbLf
strDiscription = strDiscription & _
“This Apartment doesn’t have a backyard” & _
vbLf
With myEnumerator
While .MoveNext
strDiscription = strDiscription & _
vbLf & .Key & vbTab & _
.Value.RoomName
End While
End With
Discription = strDiscription
End Function
End Class

Public Class CSFH
Implements IHouse

Private blnBackYard As Boolean
Private Rooms As Hashtable
Sub New()
Dim room As CRoom
Rooms = New Hashtable()
room = New CRoom()
room.RoomName = “Master Bedroom”
Rooms.Add(“room1”, room)
room = New CRoom()
room.RoomName = “Second Bedroom”
Rooms.Add(“room2”, room)
room = New CRoom()
room.RoomName = “Third Room”
Rooms.Add(“room3”, room)
room = New CRoom()
room.RoomName = “Living Room”
Rooms.Add(“room4”, room)
room = New CRoom()
room.RoomName = “Guest Room”
Rooms.Add(“room5”, room)
blnBackYard = True
End Sub

Public Function GetBackyard() As Boolean _
Implements IHouse.GetBackyard
GetBackyard = blnBackYard
End Function

Public Function NoOfRooms() As Long _
Implements IHouse.NoOfRooms
NoOfRooms = Rooms.Count
End Function

Public Function Discription() As String _
Implements IHouse.Discription
Dim myEnumerator As IDictionaryEnumerator = _
Rooms.GetEnumerator
Dim strDiscription As String
strDiscription = “This is an Single Family Home with ” & _
Rooms.Count & _
” Rooms ” & vbLf
strDiscription = strDiscription & _
“This house has a backyard” & vbLf
With myEnumerator
While .MoveNext
strDiscription = strDiscription & _
vbLf & .Key & vbTab & _
.Value.RoomName
End While
End With
Discription = strDiscription
End Function
End Class

Public Interface IRoom
Property RoomName() As String
End Interface

Public Class CRoom
Implements IRoom
Private mstrRoomName As String
Public Property RoomName() As String _
Implements IRoom.RoomName
Get
RoomName = mstrRoomName
End Get
Set(ByVal value As String)
mstrRoomName = value
End Set
End Property
End Class

Public Class CDirector
Public Function BuildHouse(ByVal blnBackyard _
As Boolean) As IHouse
If blnBackyard Then
BuildHouse = New CSFH()
Else
BuildHouse = New CApt()
End If
End Function
End Class

Public class Client
Public Shared Sub Main(ByVal args() as String)
Dim objDirector As New CDirector()
Dim objHouse as IHouse
objHouse = objDirector.BuildHouse(args(0))
With objHouse
Console.WriteLine(.Discription)
End With
End sub
End class

About the Author

Ashish works as a Sr. Software Developer with Lexign Inc., an end-to-end trusted electronic transaction management software provider (http://www.lexign.com). Ashish has 6 years of experience in designing and developing distributed systems, server technologies and web based software applications using Microsoft technologies, Java, XML and .NET. He holds MCSD (Microsoft Certified Software Developer) and SCJP (Sun Certified Java Programmer) certifications.

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