The builder pattern

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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 object's construction. It is a pattern for step-by-step creation of a complex object so that the same construction process can create different representations. It 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 that's 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.

Diagram

VB.NET implementation

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