Builder Pattern

June 18, 2024

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/steps.

Golang Implementation

Example: Build houses

The Builder pattern is used when the desired product is complex and requires multiple steps to complete. In this case, several construction methods would be simpler than a single monstrous constructor. The potential problem with the multistage building process is that a partially built and unstable product may be exposed to the client. The Builder pattern keeps the product private until it’s fully built.

In the below code, we see different types of houses (igloo and normalHouse) being constructed by iglooBuilder and normalBuilder. Each house type has the same construction steps. The optional director struct helps to organize the building process.

iBuilder.go

package main

type IBuilder interface {
    setWindowType()
    setDoorType()
    setNumFloor()
    getHouse() House
}

func getBuilder(builderType string) IBuilder {
    if builderType == "normal" {
        return newNormalBuilder()
    }

    if builderType == "igloo" {
        return newIglooBuilder()
    }
    return nil
}

normalBuilder.go

package main

type NormalBuilder struct {
    windowType string
    doorType   string
    floor      int
}

func newNormalBuilder() *NormalBuilder {
    return &NormalBuilder{}
}

func (b *NormalBuilder) setWindowType() {
    b.windowType = "Wooden Window"
}

func (b *NormalBuilder) setDoorType() {
    b.doorType = "Wooden Door"
}

func (b *NormalBuilder) setNumFloor() {
    b.floor = 2
}

func (b *NormalBuilder) getHouse() House {
    return House{
        doorType:   b.doorType,
        windowType: b.windowType,
        floor:      b.floor,
    }
}

iglooBuilder.go

package main

type IglooBuilder struct {
    windowType string
    doorType   string
    floor      int
}

func newIglooBuilder() *IglooBuilder {
    return &IglooBuilder{}
}

func (b *IglooBuilder) setWindowType() {
    b.windowType = "Snow Window"
}

func (b *IglooBuilder) setDoorType() {
    b.doorType = "Snow Door"
}

func (b *IglooBuilder) setNumFloor() {
    b.floor = 1
}

func (b *IglooBuilder) getHouse() House {
    return House{
        doorType:   b.doorType,
        windowType: b.windowType,
        floor:      b.floor,
    }
}

house.go

package main

type House struct {
    windowType string
    doorType   string
    floor      int
}

director.go

package main

type Director struct {
    builder IBuilder
}

func newDirector(b IBuilder) *Director {
    return &Director{
        builder: b,
    }
}

func (d *Director) setBuilder(b IBuilder) {
    d.builder = b
}

func (d *Director) buildHouse() House {
    d.builder.setDoorType()
    d.builder.setWindowType()
    d.builder.setNumFloor()
    return d.builder.getHouse()
}

main.go

package main

import "fmt"

func main() {
	normalBuilder := getBuilder("normal")
	iglooBuilder := getBuilder("igloo")

	director := newDirector(normalBuilder)
	normalHouse := director.buildHouse()

	fmt.Printf("Normal House Door Type: %s\n", normalHouse.doorType)
	fmt.Printf("Normal House Window Type: %s\n", normalHouse.windowType)
	fmt.Printf("Normal House Num Floor: %d\n", normalHouse.floor)

	director.setBuilder(iglooBuilder)
	iglooHouse := director.buildHouse()

	fmt.Printf("\nIgloo House Door Type: %s\n", iglooHouse.doorType)
	fmt.Printf("Igloo House Window Type: %s\n", iglooHouse.windowType)
	fmt.Printf("Igloo House Num Floor: %d\n", iglooHouse.floor)

}

// Normal House Door Type: Wooden Door
// Normal House Window Type: Wooden Window
// Normal House Num Floor: 2

// Igloo House Door Type: Snow Door
// Igloo House Window Type: Snow Window
// Igloo House Num Floor: 1

Rust Implementation

Example: Car & car manual builders

This slightly synthetic example illustrates how you can use the Builder pattern to construct totally different products using the same building process. For example, the trait Builder declares steps for assembling a car. However, depending on the builder implementation, a constructed object can be something different, for example, a car manual. The resulting manual will contain instructions from each building step, making it accurate and up-to-date.

The Builder design pattern is not the same as the Fluent Interface idiom (that relies on method chaining), although Rust developers sometimes use those terms interchangeably.

Fluent Interface is a way to chain methods for constructing or modifying an object using the following approach:
```
let car = Car::default().places(5).gas(30)
```
It’s pretty elegant way to construct an object. Still, such a code may not be an instance of the Builder pattern.
While the Builder pattern also suggests constructing object step by step, it also lets you build different types of products using the same construction process.

builders/mod.rs

mod car;
mod car_manual;

use crate::components::{CarType, Engine, GpsNavigator, Transmission};

/// Builder defines how to assemble a car.
pub trait Builder {
    type OutputType;
    fn set_car_type(&mut self, car_type: CarType);
    fn set_seats(&mut self, seats: u16);
    fn set_engine(&mut self, engine: Engine);
    fn set_transmission(&mut self, transmission: Transmission);
    fn set_gsp_navigator(&mut self, gps_navigator: GpsNavigator);
    fn build(self) -> Self::OutputType;
}

pub use car::CarBuilder;
pub use car_manual::CarManualBuilder;

builders/car.rs

use crate::{
    cars::Car,
    components::{CarType, Engine, GpsNavigator, Transmission},
};

use super::Builder;

pub const DEFAULT_FUEL: f64 = 5f64;

#[derive(Default)]
pub struct CarBuilder {
    car_type: Option<CarType>,
    engine: Option<Engine>,
    gps_navigator: Option<GpsNavigator>,
    seats: Option<u16>,
    transmission: Option<Transmission>,
}

impl Builder for CarBuilder {
    type OutputType = Car;

    fn set_car_type(&mut self, car_type: CarType) {
        self.car_type = Some(car_type);
    }

    fn set_engine(&mut self, engine: Engine) {
        self.engine = Some(engine);
    }

    fn set_gsp_navigator(&mut self, gps_navigator: GpsNavigator) {
        self.gps_navigator = Some(gps_navigator);
    }

    fn set_seats(&mut self, seats: u16) {
        self.seats = Some(seats);
    }

    fn set_transmission(&mut self, transmission: Transmission) {
        self.transmission = Some(transmission);
    }

    fn build(self) -> Car {
        Car::new(
            self.car_type.expect("Please, set a car type"),
            self.seats.expect("Please, set a number of seats"),
            self.engine.expect("Please, set an engine configuration"),
            self.transmission.expect("Please, set up transmission"),
            self.gps_navigator,
            DEFAULT_FUEL,
        )
    }
}

builders/car_manual.rs

use crate::{
    cars::Manual,
    components::{CarType, Engine, GpsNavigator, Transmission},
};

use super::Builder;

#[derive(Default)]
pub struct CarManualBuilder {
    car_type: Option<CarType>,
    engine: Option<Engine>,
    gps_navigator: Option<GpsNavigator>,
    seats: Option<u16>,
    transmission: Option<Transmission>,
}

/// Builds a car manual instead of an actual car.
impl Builder for CarManualBuilder {
    type OutputType = Manual;

    fn set_car_type(&mut self, car_type: CarType) {
        self.car_type = Some(car_type);
    }

    fn set_engine(&mut self, engine: Engine) {
        self.engine = Some(engine);
    }

    fn set_gsp_navigator(&mut self, gps_navigator: GpsNavigator) {
        self.gps_navigator = Some(gps_navigator);
    }

    fn set_seats(&mut self, seats: u16) {
        self.seats = Some(seats);
    }

    fn set_transmission(&mut self, transmission: Transmission) {
        self.transmission = Some(transmission);
    }

    fn build(self) -> Manual {
        Manual::new(
            self.car_type.expect("Please, set a car type"),
            self.seats.expect("Please, set a number of seats"),
            self.engine.expect("Please, set an engine configuration"),
            self.transmission.expect("Please, set up transmission"),
            self.gps_navigator,
        )
    }
}

cars/mod.rs

mod car;
mod manual;

pub use car::Car;
pub use manual::Manual;

cars/car.rs

use crate::components::{CarType, Engine, GpsNavigator, Transmission};

pub struct Car {
    car_type: CarType,
    seats: u16,
    engine: Engine,
    transmission: Transmission,
    gps_navigator: Option<GpsNavigator>,
    fuel: f64,
}

impl Car {
    pub fn new(
        car_type: CarType,
        seats: u16,
        engine: Engine,
        transmission: Transmission,
        gps_navigator: Option<GpsNavigator>,
        fuel: f64,
    ) -> Self {
        Self {
            car_type,
            seats,
            engine,
            transmission,
            gps_navigator,
            fuel,
        }
    }

    pub fn car_type(&self) -> CarType {
        self.car_type
    }

    pub fn fuel(&self) -> f64 {
        self.fuel
    }

    pub fn set_fuel(&mut self, fuel: f64) {
        self.fuel = fuel;
    }

    pub fn seats(&self) -> u16 {
        self.seats
    }

    pub fn engine(&self) -> &Engine {
        &self.engine
    }

    pub fn transmission(&self) -> &Transmission {
        &self.transmission
    }

    pub fn gps_navigator(&self) -> &Option<GpsNavigator> {
        &self.gps_navigator
    }
}

cars/manual.rs

use crate::components::{CarType, Engine, GpsNavigator, Transmission};

pub struct Manual {
    car_type: CarType,
    seats: u16,
    engine: Engine,
    transmission: Transmission,
    gps_navigator: Option<GpsNavigator>,
}

impl Manual {
    pub fn new(
        car_type: CarType,
        seats: u16,
        engine: Engine,
        transmission: Transmission,
        gps_navigator: Option<GpsNavigator>,
    ) -> Self {
        Self {
            car_type,
            seats,
            engine,
            transmission,
            gps_navigator,
        }
    }
}

impl std::fmt::Display for Manual {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        writeln!(f, "Type of car: {:?}", self.car_type)?;
        writeln!(f, "Count of seats: {}", self.seats)?;
        writeln!(
            f,
            "Engine: volume - {}; mileage - {}",
            self.engine.volume(),
            self.engine.mileage()
        )?;
        writeln!(f, "Transmission: {:?}", self.transmission)?;
        match self.gps_navigator {
            Some(_) => writeln!(f, "GPS Navigator: Functional")?,
            None => writeln!(f, "GPS Navigator: N/A")?,
        };
        Ok(())
    }
}

components.rs

#[derive(Copy, Clone, Debug)]
pub enum CarType {
    CityCar,
    SportsCar,
    Suv,
}

#[derive(Debug)]
pub enum Transmission {
    SingleSpeed,
    Manual,
    Automatic,
    SemiAutomatic,
}

pub struct Engine {
    volume: f64,
    mileage: f64,
    started: bool,
}

impl Engine {
    pub fn new(volume: f64, mileage: f64) -> Self {
        Self {
            volume,
            mileage,
            started: false,
        }
    }

    pub fn on(&mut self) {
        self.started = true;
    }

    pub fn off(&mut self) {
        self.started = false;
    }

    pub fn started(&self) -> bool {
        self.started
    }

    pub fn volume(&self) -> f64 {
        self.volume
    }

    pub fn mileage(&self) -> f64 {
        self.mileage
    }

    pub fn go(&mut self, mileage: f64) {
        if self.started() {
            self.mileage += mileage;
        } else {
            println!("Cannot go(), you must start engine first!");
        }
    }
}

pub struct GpsNavigator {
    route: String,
}

impl GpsNavigator {
    pub fn new() -> Self {
        Self::from_route(
            "221b, Baker Street, London  to Scotland Yard, 8-10 Broadway, London".into(),
        )
    }

    pub fn from_route(route: String) -> Self {
        Self { route }
    }

    pub fn route(&self) -> &String {
        &self.route
    }
}

director.rs

use crate::{
    builders::Builder,
    components::{CarType, Engine, GpsNavigator, Transmission},
};

/// Director knows how to build a car.
///
/// However, a builder can build a car manual instead of an actual car,
/// everything depends on the concrete builder.
pub struct Director;

impl Director {
    pub fn construct_sports_car(builder: &mut impl Builder) {
        builder.set_car_type(CarType::SportsCar);
        builder.set_seats(2);
        builder.set_engine(Engine::new(3.0, 0.0));
        builder.set_transmission(Transmission::SemiAutomatic);
        builder.set_gsp_navigator(GpsNavigator::new());
    }

    pub fn construct_city_car(builder: &mut impl Builder) {
        builder.set_car_type(CarType::CityCar);
        builder.set_seats(2);
        builder.set_engine(Engine::new(1.2, 0.0));
        builder.set_transmission(Transmission::Automatic);
        builder.set_gsp_navigator(GpsNavigator::new());
    }

    pub fn construct_suv(builder: &mut impl Builder) {
        builder.set_car_type(CarType::Suv);
        builder.set_seats(4);
        builder.set_engine(Engine::new(2.5, 0.0));
        builder.set_transmission(Transmission::Manual);
        builder.set_gsp_navigator(GpsNavigator::new());
    }
}

main.rs

#![allow(unused)]

mod builders;
mod cars;
mod components;
mod director;

use builders::{Builder, CarBuilder, CarManualBuilder};
use cars::{Car, Manual};
use director::Director;

fn main() {
    let mut car_builder = CarBuilder::default();

    // 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.
    Director::construct_sports_car(&mut car_builder);

    // The final product is often retrieved from a builder object, since
    // Director is not aware and not dependent on concrete builders and
    // products.
    let car: Car = car_builder.build();
    println!("Car built: {:?}\n", car.car_type());

    let mut car_builder_2 = CarBuilder::default();
    Director::construct_suv(&mut car_builder_2);
    let car_2: Car = car_builder_2.build();
    println!("Car built: {:?}\n", car_2.car_type());

    let mut car_builder_3 = CarBuilder::default();
    Director::construct_city_car(&mut car_builder_3);
    let car_3: Car = car_builder_3.build();
    println!("Car built: {:?}\n", car_3.car_type());

    println!("-------------------------------------------------------------------------");

    let mut manual_builder = CarManualBuilder::default();

    // Director may know several building recipes.
    Director::construct_city_car(&mut manual_builder);

    // The final car manual.
    let manual: Manual = manual_builder.build();
    println!("Car manual built:\n{}", manual);

    let mut manual_builder_1 = CarManualBuilder::default();
    Director::construct_suv(&mut manual_builder_1);
    let manual_1: Manual = manual_builder_1.build();
    println!("Car manual built:\n{}", manual_1);

    let mut manual_builder_2 = CarManualBuilder::default();
    Director::construct_sports_car(&mut manual_builder_2);
    let manual_2: Manual = manual_builder_2.build();
    println!("Car manual built:\n{}", manual_2);
}
// Car built: SportsCar
//
// Car built: Suv
//
// Car built: CityCar
//
// -------------------------------------------------------------------------
// Car manual built:
// Type of car: CityCar
// Count of seats: 2
// Engine: volume - 1.2; mileage - 0
// Transmission: Automatic
// GPS Navigator: Functional
//
// Car manual built:
// Type of car: Suv
// Count of seats: 4
// Engine: volume - 2.5; mileage - 0
// Transmission: Manual
// GPS Navigator: Functional
//
// Car manual built:
// Type of car: SportsCar
// Count of seats: 2
// Engine: volume - 3; mileage - 0
// Transmission: SemiAutomatic
// GPS Navigator: Functional