Builder

Intent

Separate the construction of a complex object from its representation so that the same construction process can create different representations.

Structure

• Builder - specifies an abstract interface for creating parts of a Product object.
• ConcreteBuilder -
  • 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 - constructs an object using the Builder interface.
• Product -
  • 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.

Notes

The Builder design pattern is a Dynamic Behavior pattern in which the dynamized behavior is object construction. To see this consider the following correspondence between participants.

The Builder participant is often based on a parser. There are two important alternatives for using a parser to create different representations.

• Use the Builder design pattern.
• Construct a parse tree (a Composite) with the parser and use different Visitors to create the different representations.

For large documents the Builder approach uses fewer resources since it does not have to build a large internal structure. The Composite/Visitor approach makes it easier to modify a document.

Examples

The difference between the Builder and Composite/Visitor approaches is best illustrated by considering the two main approaches for dealing with XML documents in the context of creating different representations.

• SAX - different representations can be created by implementing ConcreteBuilders. Each ConcreteBuilder just responds to events signaling the begin and end tags in a document.
• DOM - constructs a Composite; Visitors can create different representations.

See Events vs. Trees for a (somewhat biased) discussion of tradeoffs between the two approaches.