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The Builder Pattern is a software design pattern. The intention is to abstract steps of construction of objects so that different implementations of these steps can construct different representations of objects.

The Builder Pattern separates the construction of a complex object from its representation, so that several different representations can be created depending on the needs of the program.

Often, the Builder Pattern is used to build Products in accordance to the Composite pattern, a structure pattern.

Class Diagram

Builder: Abstract interface for creating objects (product).

Concrete Builder: Provide implementation for Builder. Construct and assemble parts to build the objects.

Director: The Director class is responsible for managing the correct sequence of object creation. It receives a Concrete Builder as a parameter and executes the necessary operations on it.

Product: The Product is the final object that will be created by the Director using Builder.

• Builder focuses on constructing a complex object step by step. Abstract Factory emphasizes a family of product objects (either simple or complex). Builder returns the product as a final step, but as far as the Abstract Factory is concerned, the product gets returned immediately.
• Builder often builds a Composite.
• Often, designs start out using Factory Method (less complicated, more customizable, subclasses proliferate) and evolve toward Abstract Factory, Prototype, or Builder (more flexible, more complex) as the designer discovers where more flexibility is needed.
• Sometimes creational patterns are complementary: Builder can use one of the other patterns to implement which components are built. Abstract Factory, Builder, and Prototype can use Singleton in their implementations.

Consequences of the Builder Pattern

•  A Builder lets you vary the internal representation of the product it builds. It also hides the details of how the product is assembled.
•  Each specific builder is independent of the others and of the rest of the program. This improves modularity and makes the addition of other builders relatively simple.
•  Because each builder constructs the final product step-by-step, depending on the data, you have more control over each final product that a Builder constructs.
• A Builder pattern is somewhat like an Abstract Factory pattern in that both return classes made up of a number of methods and objects. The main difference is that while the Abstract Factory returns a family of related classes, the Builder constructs a complex object step by step depending on the data presented to it.

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