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 |