Builder

Origin:

GoF 95

Reason:

Allow a client to construct a complex object, the product, by specifying only its type and content.

Synopsis:

An interface or abstract class, the AbstractBuilder, defines a static method, getInstance that takes the product type and return a Concrete Builder. A Director calls methods of the Concrete Builder to build the product.

Example:

An e-mail gateway transforms messages received in MIME format to specific formats for different e-mail systems. A message is composed of many parts/fields.

Solution:

AbstractBuilder	Interface for building product objects.	has methods "To", "From", etc..
ConcreteBuilder	Implements AbstractBuilder for a specific e-mail system.	say, UnixBuilder, Win95Builder
Director	Calls methods of AbstractBuilder to transform a message.
Client	Calls method getInstance of AbstractBuilder. Passes result to Director to get the product.
Product	Object under construction, subparts are included. ConcreteBuilder sets representation and construction.