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.
The telescoping constructor anti-pattern 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, that is to say, data that can't be easily
edited.
This type of data cannot be edited step by step and must be edited at once.
The best way to construct such an object is to use a builder class.
Builder often builds a Composite.
Often, designs start out using Factory Method and evolve toward Abstract Factory, Prototype,
or Builder 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.
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.
Structure
Builder Abstract interface for creating objects.
Concrete Builder Provides implementation for Builder.
It is an object able to construct other objects.
Constructs and assembles parts to build the objects.
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()
C# Example This pattern creates object based on the Interface,
but also lets the subclass decide which class to instantiate.
It also has finer control over the construction process.
There is a concept of Director in Builder Pattern implementation.
The director actually creates the object and also runs a few tasks after that.
In case of Builder pattern you can see the Director is actually using CreateBuilder to
create the instance of the builder.
So when the Bulder is actually created, we can also invoke a few common task in it.
See also Currying
References ^ Gang Of Four
External links The JavaWorld article Build user interfaces
without getters and setters shows the complete Java source code for a Builder.
Item 2: Consider a builder by Joshua Bloch