Builder pattern

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 (html code, SQL query, X.509 certificate...), 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 (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.

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]

Structure

Builder
Abstract interface for creating objects (product).
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.

//IVSR: Builder Pattern
    public interface IBuilder
    {
        string RunBuilderTask1();
        string RunBuilderTask2();
    }
 
    public class Builder1 : IBuilder
    {
        public string RunBuilderTask1()
        {
            throw new ApplicationException("Task1");
        }
 
        public string RunBuilderTask2()
        {
            throw new ApplicationException("Task2");
        }
    }
 
    public class Builder2 : IBuilder
    {
        public string RunBuilderTask1()
        {
            return "Task3";
        }
 
        public string RunBuilderTask2()
        {
            return "Task4";
        }
    }
 
    public class Director
    {
        public IBuilder CreateBuilder(int type)
        {
            IBuilder builder = null;
            if (type == 1)
                builder = new Builder1();
            else
                builder = new Builder2();
            builder.RunBuilderTask1();
            builder.RunBuilderTask2();
            return builder;
        }
    }

In case of Builder pattern you can see the Director is actually using CreateBuilder to create the instance of the builder. So when the Builder is actually created, we can also invoke a few common task in it.

C++ Example

////// Product declarations and inline impl. (possibly Product.h) //////
class Product{
	public:
		// use this class to construct Product
		class Builder;
 
	private:
		// variables in need of initialization to make valid object
		int i;
		float f;
		char c;
 
		// Only one simple constructor - rest is handled by Builder
		Product( const int i, const float f, const char c ) : i(i), f(f), c(c){}
 
	public:
		// Product specific functionality
		void print();
		void doSomething();
		void doSomethingElse();
};
 
 
class Product::Builder{
	private:
		// variables needed for construction of object of Product class
		int i;
		float f;
		char c;
 
	public:
		// default values for variables
		static const int defaultI = 1;
		static const float defaultF = 3.1415f;
		static const char defaultC = 'a';
 
		// create Builder with default values assigned
		// (in C++11 they can be simply assigned above on declaration instead)
		Builder() : i( defaultI ), f( defaultF ), c( defaultC ){}
 
		// sets custom values for Product creation
		// returns Builder for shorthand inline usage (same way as cout <<)
		Builder& setI( const int i ){ this->i = i; return *this; }
		Builder& setF( const float f ){ this->f = f; return *this; }
		Builder& setC( const char c ){ this->c = c; return *this; }
 
		// prepare specific frequently desired Product
		// returns Builder for shorthand inline usage (same way as cout <<)
		Builder& setProductP(){
			this->i = 42;
			this->f = -1.0f/12.0f;
			this->c = '@';
 
			return *this;
		}
 
		// produce desired Product
		Product build(){
			// here optionaly check variable consistency
			// and also if Product is buildable from given information
 
			return Product( this->i, this->f, this->c );
		}
};

///// Product implementation (possibly Product.cpp) /////
#include <iostream>
 
void Product::print(){
	using namespace std;
 
	cout << "Product internals dump:" << endl;
	cout << "i: " << this->i << endl;
	cout << "f: " << this->f << endl;
	cout << "c: " << this->c << endl;
}
 
void Product::doSomething(){}
void Product::doSomethingElse(){}

//////////////////// Usage of Builder (replaces Director from diagram)
int main(){
	// simple usage
	Product p1 = Product::Builder().setI(2).setF(0.5f).setC('x').build();
	p1.print(); // test p1
 
	// advanced usage
	Product::Builder b;
	b.setProductP();
	Product p2 = b.build(); // get Product P object
	b.setC('!'); // customize Product P
	Product p3 = b.build();
	p2.print(); // test p2
	p3.print(); // test p3
}

See also

References

  1. ↑ Gang Of Four

External links

The Wikibook Computer Science Design Patterns has a page on the topic of: Builder implementations in various languages