Design Patterns -- Builder

The Builder pattern moves the construction logic for an object outside the class to instantiate. It separates the construction of a complex object from its representation.  This is especially useful when you want to ensure the validity of an object befre instantiatin it and don't want  the associated logic to appear in the target class's constructor. A builder also accommodates step-by-step construction ,when constructing an object with parameters from text or GUI. see pic:

The classes and/or objects participating in this pattern are:

• Builder  (VehicleBuilder)
  • specifies an abstract interface for creating parts of a Product object
• ConcreteBuilder  (MotorCycleBuilder, CarBuilder, ScooterBuilder)
  • constructs and assembles parts of the product by implementing the Builder interface
  • defines and keeps track of the representation it creates
  • provides an interface for retrieving the product
• Director  (Shop)
  • constructs an object using the Builder interface
• Product  (Vehicle)
  • represents the complex object under construction. ConcreteBuilder builds the product's internal representation and defines the process by which it's assembled
  • includes classes that define the constituent parts, including interfaces for assembling the parts into the final result

 To ask other developers to use builder instead of the original constructor, we can make the original constructor protected visibility.

The mail process is like:

Using a Builder also can creates an opportunity to validate the parameters of a object with different reactions to invalid parameters. When invalid parameters are met, the builder can throw an exception.