Builder in Java

Builder is a creational design pattern, which allows constructing complex objects step by step.

Unlike other creational patterns, Builder doesn’t require products to have a common interface. That makes it possible to produce different products using the same construction process.

Learn more about Builder

Complexity:

Popularity:

Usage examples: The Builder pattern is a well-known pattern in Java world. It’s especially useful when you need to create an object with lots of possible configuration options.

Builder is widely used in Java core libraries:

java.lang.StringBuilder#append() (unsynchronized)
java.lang.StringBuffer#append() (synchronized)
java.nio.ByteBuffer#put() (also in CharBuffer, ShortBuffer, IntBuffer, LongBuffer, FloatBuffer and DoubleBuffer)
javax.swing.GroupLayout.Group#addComponent()
All implementations java.lang.Appendable

Identification: The Builder pattern can be recognized in a class, which has a single creation method and several methods to configure the resulting object. Builder methods often support chaining (for example, someBuilder.setValueA(1).setValueB(2).create()).

Step-by-step car production

In this example, the Builder pattern allows step by step construction of different car models.

The example also shows how Builder produces products of different kinds (car manual) using the same building steps.

The Director controls the order of the construction. It knows which building steps to call to produce this or that car model. It works with builders only via their common interface. This allows passing different types of builders to the director.

The end result is retrieved from the builder object because the director can’t know the type of resulting product. Only the Builder object knows what does it build exactly.

builders

builders/Builder.java: Common builder interface

package refactoring_guru.builder.example.builders;

import refactoring_guru.builder.example.cars.CarType;
import refactoring_guru.builder.example.components.Engine;
import refactoring_guru.builder.example.components.GPSNavigator;
import refactoring_guru.builder.example.components.Transmission;
import refactoring_guru.builder.example.components.TripComputer;

/**
 * Builder interface defines all possible ways to configure a product.
 */
public interface Builder {
    void setCarType(CarType type);
    void setSeats(int seats);
    void setEngine(Engine engine);
    void setTransmission(Transmission transmission);
    void setTripComputer(TripComputer tripComputer);
    void setGPSNavigator(GPSNavigator gpsNavigator);
}

builders/CarBuilder.java: Builder of car

package refactoring_guru.builder.example.builders;

import refactoring_guru.builder.example.cars.Car;
import refactoring_guru.builder.example.cars.CarType;
import refactoring_guru.builder.example.components.Engine;
import refactoring_guru.builder.example.components.GPSNavigator;
import refactoring_guru.builder.example.components.Transmission;
import refactoring_guru.builder.example.components.TripComputer;

/**
 * Concrete builders implement steps defined in the common interface.
 */
public class CarBuilder implements Builder {
    private CarType type;
    private int seats;
    private Engine engine;
    private Transmission transmission;
    private TripComputer tripComputer;
    private GPSNavigator gpsNavigator;

    public void setCarType(CarType type) {
        this.type = type;
    }

    @Override
    public void setSeats(int seats) {
        this.seats = seats;
    }

    @Override
    public void setEngine(Engine engine) {
        this.engine = engine;
    }

    @Override
    public void setTransmission(Transmission transmission) {
        this.transmission = transmission;
    }

    @Override
    public void setTripComputer(TripComputer tripComputer) {
        this.tripComputer = tripComputer;
    }

    @Override
    public void setGPSNavigator(GPSNavigator gpsNavigator) {
        this.gpsNavigator = gpsNavigator;
    }

    public Car getResult() {
        return new Car(type, seats, engine, transmission, tripComputer, gpsNavigator);
    }
}

builders/CarManualBuilder.java: Builder of a car manual

package refactoring_guru.builder.example.builders;

import refactoring_guru.builder.example.cars.Manual;
import refactoring_guru.builder.example.cars.CarType;
import refactoring_guru.builder.example.components.Engine;
import refactoring_guru.builder.example.components.GPSNavigator;
import refactoring_guru.builder.example.components.Transmission;
import refactoring_guru.builder.example.components.TripComputer;

/**
 * Unlike other creational patterns, Builder can construct unrelated products,
 * which don't have the common interface.
 *
 * In this case we build a user manual for a car, using the same steps as we
 * built a car. This allows to produce manuals for specific car models,
 * configured with different features.
 */
public class CarManualBuilder implements Builder{
    private CarType type;
    private int seats;
    private Engine engine;
    private Transmission transmission;
    private TripComputer tripComputer;
    private GPSNavigator gpsNavigator;

    @Override
    public void setCarType(CarType type) {
        this.type = type;
    }

    @Override
    public void setSeats(int seats) {
        this.seats = seats;
    }

    @Override
    public void setEngine(Engine engine) {
        this.engine = engine;
    }

    @Override
    public void setTransmission(Transmission transmission) {
        this.transmission = transmission;
    }

    @Override
    public void setTripComputer(TripComputer tripComputer) {
        this.tripComputer = tripComputer;
    }

    @Override
    public void setGPSNavigator(GPSNavigator gpsNavigator) {
        this.gpsNavigator = gpsNavigator;
    }

    public Manual getResult() {
        return new Manual(type, seats, engine, transmission, tripComputer, gpsNavigator);
    }
}

cars

cars/Car.java: Car product

package refactoring_guru.builder.example.cars;

import refactoring_guru.builder.example.components.Engine;
import refactoring_guru.builder.example.components.GPSNavigator;
import refactoring_guru.builder.example.components.Transmission;
import refactoring_guru.builder.example.components.TripComputer;

/**
 * Car is a product class.
 */
public class Car {
    private final CarType carType;
    private final int seats;
    private final Engine engine;
    private final Transmission transmission;
    private final TripComputer tripComputer;
    private final GPSNavigator gpsNavigator;
    private double fuel = 0;

    public Car(CarType carType, int seats, Engine engine, Transmission transmission,
               TripComputer tripComputer, GPSNavigator gpsNavigator) {
        this.carType = carType;
        this.seats = seats;
        this.engine = engine;
        this.transmission = transmission;
        this.tripComputer = tripComputer;
        if (this.tripComputer != null) {
            this.tripComputer.setCar(this);
        }
        this.gpsNavigator = gpsNavigator;
    }

    public CarType getCarType() {
        return carType;
    }

    public double getFuel() {
        return fuel;
    }

    public void setFuel(double fuel) {
        this.fuel = fuel;
    }

    public int getSeats() {
        return seats;
    }

    public Engine getEngine() {
        return engine;
    }

    public Transmission getTransmission() {
        return transmission;
    }

    public TripComputer getTripComputer() {
        return tripComputer;
    }

    public GPSNavigator getGpsNavigator() {
        return gpsNavigator;
    }
}

cars/Manual.java: Manual product

package refactoring_guru.builder.example.cars;

import refactoring_guru.builder.example.components.Engine;
import refactoring_guru.builder.example.components.GPSNavigator;
import refactoring_guru.builder.example.components.Transmission;
import refactoring_guru.builder.example.components.TripComputer;

/**
 * Car manual is another product. Note that it does not have the same ancestor
 * as a Car. They are not related.
 */
public class Manual {
    private final CarType carType;
    private final int seats;
    private final Engine engine;
    private final Transmission transmission;
    private final TripComputer tripComputer;
    private final GPSNavigator gpsNavigator;

    public Manual(CarType carType, int seats, Engine engine, Transmission transmission,
                  TripComputer tripComputer, GPSNavigator gpsNavigator) {
        this.carType = carType;
        this.seats = seats;
        this.engine = engine;
        this.transmission = transmission;
        this.tripComputer = tripComputer;
        this.gpsNavigator = gpsNavigator;
    }

    public String print() {
        String info = "";
        info += "Type of car: " + carType + "\n";
        info += "Count of seats: " + seats + "\n";
        info += "Engine: volume - " + engine.getVolume() + "; mileage - " + engine.getMileage() + "\n";
        info += "Transmission: " + transmission + "\n";
        if (this.tripComputer != null) {
            info += "Trip Computer: Functional" + "\n";
        } else {
            info += "Trip Computer: N/A" + "\n";
        }
        if (this.gpsNavigator != null) {
            info += "GPS Navigator: Functional" + "\n";
        } else {
            info += "GPS Navigator: N/A" + "\n";
        }
        return info;
    }
}

cars/CarType.java

package refactoring_guru.builder.example.cars;

public enum CarType {
    CITY_CAR, SPORTS_CAR, SUV
}

components

components/Engine.java: Product feature 1

package refactoring_guru.builder.example.components;

/**
 * Just another feature of a car.
 */
public class Engine {
    private final double volume;
    private double mileage;
    private boolean started;

    public Engine(double volume, double mileage) {
        this.volume = volume;
        this.mileage = mileage;
    }

    public void on() {
        started = true;
    }

    public void off() {
        started = false;
    }

    public boolean isStarted() {
        return started;
    }

    public void go(double mileage) {
        if (started) {
            this.mileage += mileage;
        } else {
            System.err.println("Cannot go(), you must start engine first!");
        }
    }

    public double getVolume() {
        return volume;
    }

    public double getMileage() {
        return mileage;
    }
}

components/GPSNavigator.java: Product feature 2

package refactoring_guru.builder.example.components;

/**
 * Just another feature of a car.
 */
public class GPSNavigator {
    private String route;

    public GPSNavigator() {
        this.route = "221b, Baker Street, London  to Scotland Yard, 8-10 Broadway, London";
    }

    public GPSNavigator(String manualRoute) {
        this.route = manualRoute;
    }

    public String getRoute() {
        return route;
    }
}

components/Transmission.java: Product feature 3

package refactoring_guru.builder.example.components;

/**
 * Just another feature of a car.
 */
public enum Transmission {
    SINGLE_SPEED, MANUAL, AUTOMATIC, SEMI_AUTOMATIC
}

components/TripComputer.java: Product feature 4

package refactoring_guru.builder.example.components;

import refactoring_guru.builder.example.cars.Car;

/**
 * Just another feature of a car.
 */
public class TripComputer {

    private Car car;

    public void setCar(Car car) {
        this.car = car;
    }

    public void showFuelLevel() {
        System.out.println("Fuel level: " + car.getFuel());
    }

    public void showStatus() {
        if (this.car.getEngine().isStarted()) {
            System.out.println("Car is started");
        } else {
            System.out.println("Car isn't started");
        }
    }
}

director

director/Director.java: Director controls builders

package refactoring_guru.builder.example.director;

import refactoring_guru.builder.example.builders.Builder;
import refactoring_guru.builder.example.cars.CarType;
import refactoring_guru.builder.example.components.Engine;
import refactoring_guru.builder.example.components.GPSNavigator;
import refactoring_guru.builder.example.components.Transmission;
import refactoring_guru.builder.example.components.TripComputer;

/**
 * Director defines the order of building steps. It works with a builder object
 * through common Builder interface. Therefore it may not know what product is
 * being built.
 */
public class Director {

    public void constructSportsCar(Builder builder) {
        builder.setCarType(CarType.SPORTS_CAR);
        builder.setSeats(2);
        builder.setEngine(new Engine(3.0, 0));
        builder.setTransmission(Transmission.SEMI_AUTOMATIC);
        builder.setTripComputer(new TripComputer());
        builder.setGPSNavigator(new GPSNavigator());
    }

    public void constructCityCar(Builder builder) {
        builder.setCarType(CarType.CITY_CAR);
        builder.setSeats(2);
        builder.setEngine(new Engine(1.2, 0));
        builder.setTransmission(Transmission.AUTOMATIC);
        builder.setTripComputer(new TripComputer());
        builder.setGPSNavigator(new GPSNavigator());
    }

    public void constructSUV(Builder builder) {
        builder.setCarType(CarType.SUV);
        builder.setSeats(4);
        builder.setEngine(new Engine(2.5, 0));
        builder.setTransmission(Transmission.MANUAL);
        builder.setGPSNavigator(new GPSNavigator());
    }
}

Demo.java: Client code

package refactoring_guru.builder.example;

import refactoring_guru.builder.example.builders.CarBuilder;
import refactoring_guru.builder.example.builders.CarManualBuilder;
import refactoring_guru.builder.example.cars.Car;
import refactoring_guru.builder.example.cars.Manual;
import refactoring_guru.builder.example.director.Director;

/**
 * Demo class. Everything comes together here.
 */
public class Demo {

    public static void main(String[] args) {
        Director director = new Director();

        // Director gets the concrete builder object from the client
        // (application code). That's because application knows better which
        // builder to use to get a specific product.
        CarBuilder builder = new CarBuilder();
        director.constructSportsCar(builder);

        // The final product is often retrieved from a builder object, since
        // Director is not aware and not dependent on concrete builders and
        // products.
        Car car = builder.getResult();
        System.out.println("Car built:\n" + car.getCarType());


        CarManualBuilder manualBuilder = new CarManualBuilder();

        // Director may know several building recipes.
        director.constructSportsCar(manualBuilder);
        Manual carManual = manualBuilder.getResult();
        System.out.println("\nCar manual built:\n" + carManual.print());
    }

}

OutputDemo.txt: Execution result

Car built:
SPORTS_CAR

Car manual built:
Type of car: SPORTS_CAR
Count of seats: 2
Engine: volume - 3.0; mileage - 0.0
Transmission: SEMI_AUTOMATIC
Trip Computer: Functional
GPS Navigator: Functional