The builder pattern is an object creation software design pattern. Unlike the abstract factory pattern and the factory method pattern whose intention is to enable polymorphism, the intention of the builder pattern is to find a solution to the telescoping constructor anti-pattern that occurs when the increase of object constructor parameter combination leads to an exponential list of constructors. Instead of using numerous constructors, the builder pattern uses another object, a builder, that receives each initialization parameter step by step and then returns the resulting constructed object at once.
The builder pattern has another benefit: It can be used for objects that contain flat data (HTML code, SQL query, X.509 certificate...), that is to say, data that can't be easily edited step by step and hence must be edited at once.
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. Builders are good candidates for a fluent interface.
Maps, Directions, and Place Reviews
Overview
The Builder design pattern is one of the twenty-three well-known GoF design patterns that describe how to solve recurring design problems to design flexible and reusable object-oriented software, that is, objects that are easier to implement, change, test, and reuse.
The Builder design pattern solves problems like:
- 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
Builderobject. - A class delegates object creation to a
Builderobject 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.
See also the UML class and sequence diagram below.
App Building Class Video
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
- Allows you to vary a product's internal representation.
- Encapsulates code for construction and representation.
- Provides control over steps of construction process.
Disadvantages
- Requires creating a separate ConcreteBuilder for each different type of Product.
- Requires the builder classes to be mutable.
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
Pseudocode
We have a Car class. The problem is that a car has many options. The combination of each option would lead to a huge list of constructors for this class. So we will create a builder class, CarBuilder. We will send to the CarBuilder each car option step by step and then construct the final car with the right options:
class Car is Can have GPS, trip computer and various numbers of seats. Can be a city car, a sports car, or a cabriolet. class CarBuilder is method getResult() is output: a Car with the right options Construct and return the car. method setSeats(number) is input: the number of seats the car may have. Tell the builder the number of seats. method setCityCar() is Make the builder remember that the car is a city car. method setCabriolet() is Make the builder remember that the car is a cabriolet. method setSportsCar() is Make the builder remember that the car is a sports car. method setTripComputer() is Make the builder remember that the car has a trip computer. method unsetTripComputer() is Make the builder remember that the car does not have a trip computer. method setGPS() is Make the builder remember that the car has a global positioning system. method unsetGPS() is Make the builder remember that the car does not have a global positioning system. Construct a CarBuilder called carBuilder carBuilder.setSeats(2) carBuilder.setSportsCar() carBuilder.setTripComputer() carBuilder.unsetGPS() car := carBuilder.getResult()
Of course one could dispense with Builder and just do this:
car = new Car(); car.seats = 2; car.type = CarType.SportsCar; car.setTripComputer(); car.unsetGPS(); car.isValid();
So this indicates that the Builder pattern is more than just a means to limit constructor proliferation. It removes what could be a complex building process from being the responsibility of the user of the object that is built. It also allows for inserting new implementations of how an object is built without disturbing the client code.
Examples
C#
The Director assembles a car instance in the example above, delegating the construction to a separate builder object that it has been given to the Director by the Client.
C++
Crystal
F#
In F# we can enforce that a function or method cannot be called with specific data. The example shows a simple way to do it that doesn't fully block a caller from going around it. This way just makes it obvious when someone is doing so in a code base.
Java
Scala
Python
Source of the article : Wikipedia
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