The builder pattern is a 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.[1]
Overview
The builder design pattern solves problems like:[2]
- How can a class (the same construction process) 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 (without having to change) 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 (the same construction process) 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.[1]
Advantages
Advantages of the builder pattern include:[3]
- 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:[3]
- 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 (creating and assembling) the parts of a complex object,
which makes the Director
independent of which concrete classes are instantiated (which representation is created).
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
- Builder
- Abstract interface for creating objects (product).
- ConcreteBuilder
- Provides implementation for Builder. It is an object able to construct other objects. Constructs and assembles parts to build the objects.
Examples
A C# example:
/// <summary>
/// Represents a product created by the builder.
/// </summary>
public class Bicycle
{
public Bicycle(string make, string model, string colour, int height)
{
Make = make;
Model = model;
Colour = colour;
Height = height;
}
public string Make { get; set; }
public string Model { get; set; }
public int Height { get; set; }
public string Colour { get; set; }
}
/// <summary>
/// The builder abstraction.
/// </summary>
public interface IBicycleBuilder
{
Bicycle GetResult();
string Colour { get; set; }
int Height { get; set; }
}
/// <summary>
/// Concrete builder implementation.
/// </summary>
public class GTBuilder : IBicycleBuilder
{
public Bicycle GetResult()
{
return Height == 29 ? new Bicycle("GT", "Avalanche", Colour, Height) : null;
}
public string Colour { get; set; }
public int Height { get; set; }
}
/// <summary>
/// The director.
/// </summary>
public class MountainBikeBuildDirector
{
private IBicycleBuilder _builder;
public MountainBikeBuildDirector(IBicycleBuilder builder)
{
_builder = builder;
}
public void Construct()
{
_builder.Colour = "Red";
_builder.Height = 29;
}
public Bicycle GetResult()
{
return this._builder.GetResult();
}
}
public class Client
{
public void DoSomethingWithBicycles()
{
var director = new MountainBikeBuildDirector(new GTBuilder());
// Director controls the stepwise creation of product and returns the result.
director.Construct();
Bicycle myMountainBike = director.GetResult();
}
}
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.
See also
Notes
- ^ a b Gamma et al. 1994, p. 97.
- ^ "The Builder design pattern - Problem, Solution, and Applicability". w3sDesign.com. Retrieved 2017-08-13.
- ^ a b "Index of /archive/2010/winter/51023-1/presentations" (PDF). www.classes.cs.uchicago.edu. Retrieved 2016-03-03.
- ^ "The Builder design pattern - Structure and Collaboration". w3sDesign.com. Retrieved 2017-08-12.
References
- Gamma, Erich; Helm, Richard; Johnson, Ralph; Vlissides, John (1994). Design Patterns: Elements of Reusable Object-Oriented Software. Addison-Wesley. ISBN 0-201-63361-2.
External links