Article source code: builder.zip
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 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
C# Implementation
using System;
using System.Collections;
public interface IHouse
{
bool GetBackyard();
long NoOfRooms();
string Description();
}
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 Description()
{
IDictionaryEnumerator myEnumerator = Rooms.GetEnumerator();
string strDescription;
strDescription = "This is an Apartment with " + Rooms.Count +
" Rooms \n";
strDescription = strDescription + "This Apartment doesn't have " +
"a backyard \n";
while (myEnumerator.MoveNext())
{
strDescription = strDescription + "\n" + myEnumerator.Key + "\t"
+ ((CRoom)myEnumerator.Value).RoomName;
}
return strDescription;
}
}
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 Description()
{
IDictionaryEnumerator myEnumerator = Rooms.GetEnumerator();
string strDescription;
strDescription = "This is an Single Family Home with " + Rooms.Count
+ " Rooms \n";
strDescription = strDescription + "This house has a backyard \n";
while (myEnumerator.MoveNext())
{
strDescription = strDescription + "\n" + myEnumerator.Key + "\t"
+ ((CRoom)myEnumerator.Value).RoomName;
}
return strDescription;
}
}
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.Description());
}
}
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VB.NET Implementation
Imports System
Imports System.Collections
Imports Microsoft.VisualBasic
Public Interface IHouse
Function GetBackyard() As Boolean
Function NoOfRooms() As Long
Function Description() 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 Description() As String Implements IHouse.Description
Dim myEnumerator As IDictionaryEnumerator = Rooms.GetEnumerator
Dim strDescription As String
strDescription = "This is an Apartment with " & Rooms.Count & _
" Rooms " & vbLf
strDescription = strDescription & "This Apartment doesn't have a " & _
"backyard" & vbLf
With myEnumerator
While .MoveNext
strDescription = strDescription & vbLf & .Key & vbTab & _
.Value.RoomName
End While
End With
Description = strDescription
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 Description() As String Implements IHouse.Description
Dim myEnumerator As IDictionaryEnumerator = Rooms.GetEnumerator
Dim strDescription As String
strDescription = "This is an Single Family Home with " & Rooms.Count _
& " Rooms " & vbLf
strDescription = strDescription & "This house has a backyard" & vbLf
With myEnumerator
While .MoveNext
strDescription = strDescription & vbLf & .Key & vbTab _
& .Value.RoomName
End While
End With
Description = strDescription
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(.Description)
End With
End Sub
End Class
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Reference: E. Gamma et al., "Design Patterns: Elements of Reusable Object-Oriented Software"
ISBN 0-201-63361-2, Addison Wesley, 1995.