The builder pattern helps to separate the construction of a complex object from its representation so that the same construction process can produce different representations(outputs).

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

public Vehicle(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.

Moreover, suppose there is another constructor:

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

Here number_of_headlights has the same type as number_of_tyres mentioned in the earlier constructor. Hence polymorphism concept of having different constructors for different parameters fails here.

Telescoping Constructors Problem

If you use the Builder Pattern instead, you can simply:

Vehicle vehicle = new VehicleBuilder().SetName("Car").SetTyres(4).SetHeadlights(2);

So in short, builder design pattern:

1. allows using the same construction process to create different representations of an object.

2. simplifies the creation of complex objects

Given that the builder pattern sets up a method for constructing new objects, it is understandably categorized as a creational design pattern.

Builder Design Pattern Participants

The Builder Design Pattern has four major participants:

• Director : Director is the one which constructs the complex object by using the builder's interface.

• Builder : Provides an interface to create the components of a complex object/product.

• Concrete Builder : This actor creates all the parts needed of a complex object. It contains a reference to the final product/output. Thus client gets the final product from this actor once object construction is completed.

• Product : This is the final object to be constructed. It provides the interface to build all the parts of the final object and assemble them.

The director performs the decisive process of the builder pattern, the separation of the production of an object/product from the client.

The client :

1. creates a concrete builder class object for the required product.

2. creates a director object.

3. injects the concrete builder class object , created in the first step to the director.

4. asks the director to construct the object.

5. gets the final product from the concrete builder class object.

Builder Design Pattern in Real Life

To explain the builder design pattern, let's take a real-life example of a burger restaurant where a customer orders a meal. The meal can consist of the main item(cheeseburger, chicken burger, etc.), a side item (French fries, etc.), and a drink(coke, coffee).

The order is passed to the front desk executive who acts as a director in this case. He will pass the order to the main chef who in turn asks the respective chefs(concrete builder) to prepare the asked meal. This entire process takes place behind the scenes. The customer does not know what is happening in the kitchen to carry out his order. However, he gets his order served at his table.

Builder Design Pattern UML Diagrams

Let us take a vehicle building example where different types of vehicles are manufactured such as cars, bikes, etc. These are the final products.

IVehicleBuilder is the interface used to create parts of the final product/object. The final assembly process is described in Product.

CarBuilder and BikeBuilder are the concrete implementations of
IVehicleBuilder. The first three methods fit the engine, tires, and headlights to the vehicle. GetVehicle() will return the ultimate product.

Finally, VehiclePlant will be acting as director. It is ultimately responsible for constructing the vehicle, and build the product with IVehicleBuilder interface. It uses the same construct() method to create different types of vehicles.

Class Diagram

The below graphic shows the UML class diagram of a builder pattern consisting of several entities interacting with each other.

Sequence Diagram

Implementation

In C++

IVehicleBuilder class:

class IVehicleBuilder
{
public:
    virtual void FitEngine() = 0;
    virtual void FitTyres() = 0;
    virtual void FitHeadlight() = 0;
    virtual Vehicle GetVehicle() = 0;
};

CarBuilder Class:

class CarBuilder : public IVehicleBuilder
{
private:
    Vehicle vehicle;

public:
    void FitEngine() override
    {
        vehicle.AddPart("Engine Added");
    }

    void FitTyres() override
    {
        vehicle.AddPart("Tyres Inserted");
    }

    void FitHeadlight() override
    {
        vehicle.AddPart("Fitted Headlight");
    }

    Vehicle GetVehicle() override
    {
        return vehicle;
    }
};

Vehicle class:

class Vehicle
{
private:
    vector<string> parts;

public:
    void AddPart(string part)
    {
        parts.emplace_back(part);
    }

    void PrintDetails()
    {
        for (auto &part : parts)
            std::cout << part << "\n";
    }
};

VehiclePlant class:

class VehiclePlant
{
private:
    IVehicleBuilder& vehicleBuilder;

public:
    VehiclePlant(IVehicleBuilder& vBuilder):
    vehicleBuilder(vBuilder)
    {

    }

    void Construct()
    {
        vehicleBuilder.FitEngine();
        vehicleBuilder.FitTyres();
        vehicleBuilder.FitHeadlight();
    }
};

Client program:

int main()
{
    CarBuilder carBuilder;
    VehiclePlant vehiclePlant(carBuilder);

    vehiclePlant.Construct();

    Vehicle vehicle = carBuilder.GetVehicle();

    vehicle.PrintDetails();
}

In Python

from abc import ABC, abstractmethod

class Vehicle:
    def __init__(self):
        self.__parts__ = []

    def AddPart(self, part):
        self.__parts__.append(part)

    def PrintDetails(self):
        for part in self.__parts__:
            print(part)


class IVehicleBuilder(ABC):
    @abstractmethod
    def FitEngine(self):
        pass

    @abstractmethod
    def FitTyres(self):
        pass

    @abstractmethod
    def FitHeadlight(self):
        pass

    @abstractmethod
    def GetVehicle(self):
        pass

class CarBuilder(IVehicleBuilder):
    def __init__(self):
        super().__init__()
        self.__vehicle__ = Vehicle()

    def FitEngine(self):
        self.__vehicle__.AddPart("Engine Added");

    def FitTyres(self):
        self.__vehicle__.AddPart("Tyres Inserted");

    def FitHeadlight(self):
        self.__vehicle__.AddPart("Fitted Headlight");

    def GetVehicle(self):
        return self.__vehicle__;

class VehiclePlant:
    def __init__(self, vBuilder):
        self.__vehicleBuilder__ = vBuilder

    def Construct(self):
        self.__vehicleBuilder__.FitEngine();
        self.__vehicleBuilder__.FitTyres();
        self.__vehicleBuilder__.FitHeadlight();

if __name__ == '__main__':
    carBuilder = CarBuilder()
    vehiclePlant = VehiclePlant (carBuilder);
    vehiclePlant.Construct()
    vehicle = carBuilder.GetVehicle()
    vehicle.PrintDetails()

Note
By default, Python does not provide support for abstract classes. However, Python comes with a module that provides the base for defining Abstract Base classes(ABC). ABC works by decorating methods of the base class as abstract and then registering concrete classes as implementations of the abstract base. A method becomes abstract when decorated with the keyword @abstractmethod.
For Example (Reference taken from geeksforgeeks.com)

from abc import ABC, abstractmethod

class Polygon(ABC):
    @abstractmethod
    def noofsides(self):
        pass


class Triangle(Polygon):
    # overriding abstract method
    def noofsides(self):
        print("I have 3 sides")

# Driver code
R = Triangle()
R.noofsides()

​Variation of the Builder pattern

The builder pattern has a slightly modified version where build methods are chained to each other like below. This method is called the Fluent Builder.

    Vehicle vehicle = carBuilder.FitEngine().FitTyres().FitHeadlight().build();

This is accomplished by returning itself from each of the build methods. The final build() method returns the final object. Moreover, note that you don't need Director class here.

Here, is the alternate implementation:

in C++

IVehicleBuilder Class

class IVehicleBuilder
{
    using self = IVehicleBuilder;
public:
    virtual self& FitEngine() = 0;
    virtual self& FitTyres() = 0;
    virtual self& FitHeadlight() = 0;
    virtual Vehicle GetVehicle() = 0;
};

CarBuilder class

class CarBuilder: public IVehicleBuilder
{
private:
    using self = CarBuilder;
    Vehicle vehicle;

public:
    self& FitEngine() override
    {
        vehicle.AddPart("Engine Added");
        return *this;
    }

    self& FitTyres() override
    {
        vehicle.AddPart("Tyres Inserted");
        return *this;
    }

    self& FitHeadlight() override
    {
        vehicle.AddPart("Fitted Headlight");
        return *this;
    }

    Vehicle GetVehicle() override
    {
        return vehicle;
    }
};

main program

int main()
{
    CarBuilder carBuilder;

    Vehicle vehicle = carBuilder.FitEngine().FitTyres().FitHeadlight().GetVehicle();
    vehicle.PrintDetails();
}

You can also use pointers instead of references. In that case, define Fit...() methods like this:

self* FitEngine() override
{
    vehicle.AddPart("Engine Added");
    return this;
}

Replace your chain calls with the -> operator:

Vehicle vehicle = carBuilder->FitEngine()->FitTyres()->FitHeadlight()->GetVehicle();

You can also replace your GetVehicle() method with function operator to yield the final product:

class CarBuilder: public IVehicleBuilder
{
    ...

    Vehicle operator()()
    {
        return vehicle;
    }
};

int main()
{
    CarBuilder carBuilder;
    Vehicle vehicle = carBuilder.FitEngine().FitTyres().FitHeadlight()();
    vehicle.PrintDetails();
}

in Python

class CarBuilder(IVehicleBuilder):
    def __init__(self):
        super().__init__()
        self.__vehicle__ = Vehicle()

    def FitEngine(self):
        self.__vehicle__.AddPart("Engine Added")
        return self

    def FitTyres(self):
        self.__vehicle__.AddPart("Tyres Inserted");
        return self

    def FitHeadlight(self):
        self.__vehicle__.AddPart("Fitted Headlight");
        return self

    def GetVehicle(self):
        return self.__vehicle__;

if __name__ == '__main__':
    vehicle = CarBuilder().FitEngine().FitTyres().FitHeadlight().GetVehicle()
    vehicle.PrintDetails()

Advantages vs Disadvantages

​Benefits

As the construction process is totally segregated from the final object/product, so you can easily change the steps required to build the product.

As the director isn't aware of the builder process, and the build process doesn't have to change so you can easily insert new concrete builder classes to build a new product.

Disadvantages

There is a close coupling between the product, the concrete builder, and the builder interface. Hence the change in any class requires changes in all the other classes. This makes any changes to the pattern a little difficult.

Also, if we want a mutable object (an object which can be modified after the creational process is over), we should not use this pattern.

Final Words

The builder pattern really is a powerful pattern to solve complex object creation problems.

However, many programmers can be tempted to use a builder design pattern, and overlook simpler and perhaps more elegant solutions. So as a thumb rule, you don't need to use a builder pattern if the object can be constructed easily, and has a limited number of constructor parameters.

A Builder pattern is usually a better candidate if:

• when we want to create a complex object, which is composed of multiple parts and creation steps. Each of these steps might need to follow a specific order.

• We want to decouple the construction process from its representation or output. This will eventually help us to insert more concrete builders whenever needed.