Builder Pattern - Simple tutorial with an example in Java

Builder design pattern belongs to the creational design pattern group. In Builder Pattern you arrange all the steps used to create an object in one separate class(Builder class).
Assume that you have a class or an interfaces called Vehicle. You can create various types of vehicles which are instances of this class(or interface). For an example an instance can be a car or a motorcycle.
Assume that the below class is your Vehicle class which contains complex methods.(Since I need to keep this tutorial short, I am not using an interface here. But note that the best design is making the vehicle class an interface and implementing the Car and Motorcycle classes using that.)

public class Vehicle {
  String name;
  public void addWheels(int count){}
  public void addController(){}
  public void addFuelTank(){}
  public void setName(String name){this.name = name;}
  public String getName(){return this.name;}

}

Building a car or a motorcycle is a complex and multi stepped process. In such cases you can use a VehicleBuilder to make this process organized. In a VehicleBuilder instance there is an instance of Vehicle class and getVehicle() method returns that after constructed.

This is your VehicleBuilder interface.

public interface VehicleBuilder {
  public void setWheels();
  public void setController();
  public void setFuelTank();
  public void setName();
  public Vehicle getVehicle();
}

You can have multiple implementations of this interface. For an example CarBuilder and MotorcycleBuilder.

public class CarBuilder implements VehicleBuilder {
  Vehicle vehicle = new Vehicle();
  public void setWheels() {
    //Fixing 4 wheels (complex logic)
    vehicle.addWheels(4);
  }
  public void setController() {
    //Fix a steering wheel complex logic
    vehicle.addController();
  }
  public void setFuelTank() {
    //Fixing car fuel tank (complex logic)
    vehicle.addFuelTank();
  }
  public void setName() {
    //seting name (complex logic)
    vehicle.setName("car");
  }
  public Vehicle getVehicle() {
    return vehicle;
  }
}

public class MotorcycleBuilder implements VehicleBuilder {
  Vehicle vehicle = new Vehicle();
  public void setWheels() {
    //Fixing 2 wheels (complex logic)
    vehicle.addWheels(2);
  }
  public void setController() {
    //Fixing a handle (complex logic)
    vehicle.addController();
  }
  public void setFuelTank() {
    //Fixing motorcycle fuel tank (complex logic)
    vehicle.addFuelTank();
  }
  public void setName() {
    //setting name (complex logic)
    vehicle.setName("Motorcycle");
  }
  public Vehicle getVehicle() {
    return vehicle;
  }
}

Now in your class you can use this builders to create cars or motorcycles as below.

public class Test {
  public static void main(String[] args) {
//Building a Car
    VehicleBuilder carBuilder = new CarBuilder();
    carBuilder.setName();
    carBuilder.setWheels();
    carBuilder.setController();
    carBuilder.setFuelTank();
    Vehicle car = carBuilder.getVehicle();
    System.out.println(car.getName());
    

//Building a Motorcycle
    VehicleBuilder motorcycleBuilder = new MotorcycleBuilder();
    motorcycleBuilder.setName();
    motorcycleBuilder.setWheels();
    motorcycleBuilder.setController();
    motorcycleBuilder.setFuelTank();
    Vehicle bike = motorcycleBuilder.getVehicle();
    System.out.println(bike.getName());
  }
}