Builder pattern

The builder pattern is a Software [design pattern|design pattern] that provides a flexible solution to various object creation problems in object-oriented programming. The builder pattern separates the construction of a complex object from its representation. It is one of the 23 classic design patterns described in the book Design Patterns and is sub-categorized as a creational pattern.

Overview

The builder design pattern solves problems like:

How can a class create different representations of a complex object?
How can a class that includes creating a complex object be simplified?

Creating and assembling the parts of a complex object directly within a class is inflexible. It commits the class to creating a particular representation of the complex object and makes it impossible to change the representation later independently from the class.
The builder design pattern describes how to solve such problems:

Encapsulate creating and assembling the parts of a complex object in a separate Builder object.
A class delegates object creation to a Builder object instead of creating the objects directly.

A class can delegate to different Builder objects to create different representations of a complex object.

Definition

The intent of the builder design pattern is to separate the construction of a complex object from its representation. By doing so, the same construction process can create different representations.

Advantages

Advantages of the builder pattern include:

Allows you to vary a product's internal representation.
Encapsulates code for construction and representation.
Provides control over the steps of the construction process.

Disadvantages

Disadvantages of the builder pattern include:

A distinct ConcreteBuilder must be created for each type of product.
Builder classes must be mutable.
May hamper/complicate dependency injection.
In many null-safe languages, the builder pattern defers compile-time errors for unset fields to runtime.

Structure

UML class and sequence diagram

In the above UML class diagram,
the Director class doesn't create and assemble the ProductA1 and ProductB1 objects directly.
Instead, the Director refers to the Builder interface for building the parts of a complex object,
which makes the Director independent of which concrete classes are instantiated.
The Builder1 class implements the Builder interface by creating and assembling the ProductA1 and ProductB1 objects.

The UML sequence diagram shows the run-time interactions:
The Director object calls buildPartA on the Builder1 object, which creates and assembles the ProductA1 object.
Thereafter,
the Director calls buildPartB on Builder1, which creates and assembles the ProductB1 object.

Class diagram

[Image:Builder UML class diagram.svg|500px|center|Builder Structure]
; Builder
; ConcreteBuilder

Examples

A C# example:

namespace Wikipedia.Examples;
///

/// Represents a product created by the builder.
///

public class Bicycle
///

/// The builder abstraction.
///

public interface IBicycleBuilder
///

/// Concrete builder implementation.
///

public class GTBuilder : IBicycleBuilder
///

/// The director.
///

public class MountainBikeBuildDirector
public class Client

The Director assembles a bicycle instance in the example above, delegating the construction to a separate builder object that has been given to the Director by the Client.