(item 2) 생성자에 매개변수가 많다면 빌더를 고려하라

점층적 생성자 패턴

• 필수 인자를 받는 생성자에 선택 인자를 하나씩 추가하면서 여러 생성자 선언
• 필드 수가 많아지면 생성자의 인자가 무슨 필드를 가리키는지 파악이 어려워진다

public class NutritionFacts {
	private final int servingSize; // (mL) required
	private final int servings; // (per container) required
	private final int calories; // optional
	private final int fat; // (g) optional
	private final int sodium; // (mg) optional
	private final int carbohydrate; // (g) optional

	public NutritionFacts(int servingSize, int servings) {
		this(servingSize, servings, 0);
	}

	public NutritionFacts(int servingSize, int servings, int calories) {
		this(servingSize, servings, calories, 0);
	}

	public NutritionFacts(int servingSize, int servings, int calories, int fat) {
		this(servingSize, servings, calories, fat, 0);
	}

	public NutritionFacts(int servingSize, int servings, int calories, int fat,
			int sodium) {
		this(servingSize, servings, calories, fat, sodium, 0);
	}

	public NutritionFacts(int servingSize, int servings, int calories, int fat,
			int sodium, int carbohydrate) {
		this.servingSize = servingSize;
		this.servings = servings;
		this.calories = calories;
		this.fat = fat;
		this.sodium = sodium;
		this.carbohydrate = carbohydrate;
	}

	public static void main(String[] args) {
		NutritionFacts cocaCola = new NutritionFacts(240, 8, 100, 0, 35, 27);
	}
}

자바빈 패턴

• 인자없는 생성자를 호출한 다음 setter를 통해 필드값을 채우는 방법
• 1회의 함수 호출로 객체 생성을 끝낼 수 없다 -> 객체 일관성 깨짐 (thread safe하지 않음)
• setter를 사용하는 코드의 경우 값을 언제든 변경할 수 있어서 유지보수 때 어디에서 호출하는지 일일이 파악해야한다
• freezing?

public class NutritionFacts {
	// Parameters initialized to default values (if any)
	private int servingSize = -1; // Required; no default value
	private int servings = -1; // "     " "      "
	private int calories = 0;
	private int fat = 0;
	private int sodium = 0;
	private int carbohydrate = 0;

	public NutritionFacts() {
	}

	// Setters
	public void setServingSize(int val) {
		servingSize = val;
	}

	public void setServings(int val) {
		servings = val;
	}

	public void setCalories(int val) {
		calories = val;
	}

	public void setFat(int val) {
		fat = val;
	}

	public void setSodium(int val) {
		sodium = val;
	}

	public void setCarbohydrate(int val) {
		carbohydrate = val;
	}

	public static void main(String[] args) {
		NutritionFacts cocaCola = new NutritionFacts();
		cocaCola.setServingSize(240);
		cocaCola.setServings(8);
		cocaCola.setCalories(100);
		cocaCola.setSodium(35);
		cocaCola.setCarbohydrate(27);
	}

builder 패턴

• 자바빈 패턴의 가독성 + 점층적 생성자 패턴의 안정성
• 클래스 내부에 Builder 클래스 선언 / 필수 필드(final)는 Builder 생성자로 / 선택 필드는 Builder 타입 함수로 / 외부 클래스의 객체 생성하는 build() 함수
• return this를 통해 체인 형식으로 사용 가능
• 빌더 패턴은 계층적으로 설계된 클래스와 함께 쓰기에 좋다

public class NutritionFacts {
	private final int servingSize;
	private final int servings;
	private final int calories;
	private final int fat;
	private final int sodium;
	private final int carbohydrate;
    // private final Date expirationDate;

	public static class Builder {
		// Required parameters
		private final int servingSize;
		private final int servings;

		// Optional parameters - initialized to default values
		private int calories = 0;
		private int fat = 0;
		private int carbohydrate = 0;
		private int sodium = 0;

		public Builder(int servingSize, int servings) {
			this.servingSize = servingSize;
			this.servings = servings;
		}

		public Builder calories(int val) {
			calories = val;
			return this;
		}

		public Builder fat(int val) {
			fat = val;
			return this;
		}

		public Builder carbohydrate(int val) {
			carbohydrate = val;
			return this;
		}

		public Builder sodium(int val) {
			sodium = val;
			return this;
		}

		public NutritionFacts build() {
			return new NutritionFacts(this);
		}
	}

	private NutritionFacts(Builder builder) {
		servingSize = builder.servingSize;
		servings = builder.servings;
		calories = builder.calories;
		fat = builder.fat;
		sodium = builder.sodium;
		carbohydrate = builder.carbohydrate;
	}

	public static void main(String[] args) {
		NutritionFacts cocaCola = new NutritionFacts.Builder(240, 8)
				.calories(100).sodium(35).carbohydrate(27).build();
	}

lombok @Builder 사용시

• 클래스 위에 사용할 경우 모든 필드를 매개변수로 받는 생성자가 생긴다
• @AllArgsConstructor(access = AccessLevel.PRIVATE)

계층적 클래스에 적용한 예

• 시뮬레이트한 셀프타입 관용구(simulated self-type)
• 공변 반환 타이핑(covariant return typing)

public abstract class Pizza {
    public enum Topping { HAM, MUSHROOM, ONION, PEPPER, SAUSAGE }
    final Set<Topping> toppings;

    abstract static class Builder<T extends Builder<T>> {
        EnumSet<Topping> toppings = EnumSet.noneOf(Topping.class);
        public T addTopping(Topping topping) {
            toppings.add(Objects.requireNonNull(topping));
            return self();
        }

        abstract Pizza build();

        // Subclasses must override this method to return "this"
        protected abstract T self();
    }
    
    Pizza(Builder<?> builder) {
        toppings = builder.toppings.clone(); // See Item 50
    }
}

public class NyPizza extends Pizza {
    public enum Size { SMALL, MEDIUM, LARGE }
    private final Size size;

    public static class Builder extends Pizza.Builder<Builder> {
        private final Size size;

        public Builder(Size size) {
            this.size = Objects.requireNonNull(size);
        }

        @Override public NyPizza build() {
            return new NyPizza(this);
        }

        @Override protected Builder self() { return this; }
    }

    private NyPizza(Builder builder) {
        super(builder);
        size = builder.size;
    }

    @Override public String toString() {
        return "New York Pizza with " + toppings;
    }
}

public class Calzone extends Pizza {
    private final boolean sauceInside;

    public static class Builder extends Pizza.Builder<Builder> {
        private boolean sauceInside = false; // Default

        public Builder sauceInside() {
            sauceInside = true;
            return this;
        }

        @Override public Calzone build() {
            return new Calzone(this);
        }

        @Override protected Builder self() { return this; }
    }

    private Calzone(Builder builder) {
        super(builder);
        sauceInside = builder.sauceInside;
    }

    @Override public String toString() {
        return String.format("Calzone with %s and sauce on the %s",
                toppings, sauceInside ? "inside" : "outside");
    }
}

public static void main(String[] args) {
        NyPizza pizza = new NyPizza.Builder(SMALL)
                .addTopping(SAUSAGE).addTopping(ONION).build();
        Calzone calzone = new Calzone.Builder()
                .addTopping(HAM).sauceInside().build();
}