The Power of Builder Design Pattern: How to Create Complex Objects with Ease

The Builder Design Pattern is one of the most widely used patterns in software engineering. It is a creational pattern that allows you to separate the construction of complex objects from their representation.

The main idea behind the Builder Design Pattern is to create a separate class (the Builder) that is responsible for creating complex objects. The client code then interacts with the Builder to build the desired object. The Builder uses a step-by-step approach to build the object, and each step can be customized to fit the specific needs of the client.

Note: I'll be discussing the examples in Python and C++

I posted one post before on the same topic. Here, I tried to explain it differently. Please, let me know which tone suits you best. 😄

Problem with traditional approach

Let us see what kind of problem it can solve.

The Builder Pattern solves a very specific problem: Telescoping Constructors. To understand it, let us suppose we have the following constructor definitions for class Vehicle

public Car(int id, String name)
{
    this(id, name, 0, 0);
}

public Car(int id, String name, int number_of_tyres)
{
    this(id, name, number_of_tyres, 0);
}

This might not look like an issue at the earlier stages but if you have eight optional parameters and you want to represent every useful combination, you'll need 256 constructors. This would be very cumbersome and would result in a lot of boilerplate code.

Let's look at an example to see how the Builder Design Pattern works in practice.

Builder Design Pattern Participants

First, let's look at various participants of builder design pattern:

• Product : First, we create the product's blueprint or interface, which will define the steps required to build the product. This interface can be implemented by different classes to produce different products.

• Concrete Builder : Next, we create a concrete builder class that implements the product's interface and provides a method for each step required to build the product.

The concrete builder class also has a method to return the final product.

• Director : Finally, we create a director class that will use the concrete builder to build the product. The director class is responsible for invoking the concrete builder's methods in the correct order to produce the final product.

Builder design pattern Diagrams

Let's see through diagrams how builder design pattern looks or works.
Imagine we're building the same car as above. The car has many attributes, such as the engine type, the number of doors, and the color.

Sequence diagram

Below are the steps required to build a car,

Remember, we can use different concrete builders to create different types of cars with different attributes.

Below is the source code for above sequnce diagram in PlantUML:

@startuml
title Car Building Sequence Diagram

actor Client

Client -> CarDirector: create director
Client -> AudiCarBuilder: create audi builder
CarDirector -> CarDirector: set builder to audi builder
Client -> CarDirector: build car
CarDirector -> AudiCarBuilder: add engine
AudiCarBuilder -> Car: set engine
CarDirector -> AudiCarBuilder: add doors
AudiCarBuilder -> Car: set doors
CarDirector -> AudiCarBuilder: paint car
AudiCarBuilder -> Car: paint
CarDirector -> AudiCarBuilder: get car
AudiCarBuilder -> Car: return car
CarDirector -> Client: return car
@enduml

Class Diagrams

To give you a snapshot, this is how our class skeletons would look like:

In plantUML, it looks as:

@startuml

class Car {
    - engine_type: str
    - num_doors: int
    - color: str
    + set_engine(engine_type: str): void
    + set_doors(num_doors: int): void
    + paint(color: str): void    
}

class AudiCarBuilder {
    - car: Car
    + AudiCarBuilder()
    + add_engine(engine_type: str): void
    + add_doors(num_doors: int): void
    + paint(color: str): void
    + get_car(): Car
}

class CarDirector {
  - builder: ICarBuilder
  + CarDirector(audiCarBuilder: ICarBuilder)
  + build_car(): Car
}

AudiCarBuilder -> Car: 1
CarDirector -> AudiCarBuilder

@enduml

Builder design pattern implementation

Step 1 Declare interface of our final product

First, let's define the product, which is Car.

class Car:
    def __init__(self):
        self.engine_type = None
        self.num_doors = None
        self.color = None

        def add_engine(self, engine_type): 
            self.car.engine_type = engine_type 

        def add_doors(self, num_doors): 
            self.car.num_doors = num_doors 

        def paint(self, color): 
            self.car.color = color    

Our Final product Car has engine type, the number of doors, and the color.

Step 2. Define the Builder class to implement each build step

Next step is to define the concrete builder class that implements the method for each step required to build the above product.

Remember, we also need to define a method to return the final product.

class AudiCarBuilder:
    def __init__(self):
        self.car = Car()

    def add_engine(self, engine_type):
        self.car.set_engine(engine_type)

    def add_doors(self, num_doors):
        self.car.set_doors(num_doors)

    def paint(self, color):
        self.car.paint( = )color)

    def get_car(self):
        return self.car

Apart from defining various methods to build our final product, we also define the method to return it through function get_car().

Step 3.

Lastly, create a director class to build the final product.

Remember to invoke each construction step/method in the correct order to produce the final product. In our example is to first install the engine before attaching doors or starting painting. :happy:

class CarDirector:
    def __init__(self, builder):
        self.builder = builder

    def build_car(self):
        self.builder.add_engine("V8")
        self.builder.add_doors(4)
        self.builder.paint("Red")
        return self.builder.get_car()

Our Builder implementation is now complete. 🙂

Let's use it in our code base. It's too easy now to use.

builder = AudiCarBuilder()
director = CarDirector(builder)
car = director.build_car()

print(f"Engine type: {car.engine_type}")
print(f"Number of doors: {car.num_doors}")
print(f"Color: {car.color}")

How builder pattern helped us

By using the Builder pattern, we can create different concrete builders to build different types of cars with different attributes.

We can also modify the steps required to build a car without modifying the Car class itself, making our code more flexible and easier to maintain.

Complete implementation in C++

Now, I feel it must be easy for you to understand how builder pattern can be written in C++ also. Still, for your reference I'm giving the source code here. 🙂

#include <iostream>
#include <string>

class Car {
private:
    std::string engine_type;
    int num_doors;
    std::string color;
public:
    void set_engine(std::string engine_type) {
        this->engine_type = engine_type;
    }
    void set_doors(int num_doors) {
        this->num_doors = num_doors;
    }
    void paint(std::string color) {
        this->color = color;
    }
    void display() {
        std::cout << "Car with " << engine_type << " engine, " << num_doors << " doors, and " << color << " color." << std::endl;
    }
};

class ICarBuilder {
public:
    virtual void add_engine(std::string engine_type) = 0;
    virtual void add_doors(int num_doors) = 0;
    virtual void paint(std::string color) = 0;
    virtual Car get_car() = 0;
};

class AudiCarBuilder : public ICarBuilder {
private:
    Car car;
public:
    void add_engine(std::string engine_type) {
        car.set_engine(engine_type);
    }
    void add_doors(int num_doors) {
        car.set_doors(num_doors);
    }
    void paint(std::string color) {
        car.paint(color);
    }
    Car get_car() {
        return car;
    }
};

class CarDirector {
private:
    ICarBuilder* builder;
public:
    CarDirector(ICarBuilder* builder) {
        this->builder = builder;
    }
    void build_car() {
        builder->add_engine("V6");
        builder->add_doors(4);
        builder->paint("Red");
    }
};

int main() {
    AudiCarBuilder audiBuilder;
    CarDirector director(&audiBuilder);
    director.build_car();
    Car audiCar = audiBuilder.get_car();
    std::cout << "Audi car: ";
    audiCar.display();
    return 0;
}

💡 Remember, same way you can also add another car builder, say FordCarBuilder

Conclusion

In conclusion, the Builder pattern is a powerful design pattern that can help us create complex objects with many attributes in a more organized and maintainable way. By separating the construction of an object from its representation, we can create different types of objects with different attributes using the same building process.

Photo by Kelly Sikkema on Unsplash