多个构造器参数时要考虑用构建器

当一个类有大量的可选属性是，要考虑用构建器替代构造器，构造器模式会让类的实例化变得太过复杂。

构建器可以见名之意，并且有很大的灵活性。

构建器构造对象不直接生成想要的对象，而是让客户端利用所有必要的参数调用构造器（或者静态工厂），

得到一个builder对象。然后客户端在builder对象上调用类似于setter的方法，来设置每个相关的可选参数。

最后，客户端调用无参的build方法来生成不可变的对象。这个builder是它构建的类的静态成员类。

示例：

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;

    /**
     * 这里必须用静态方法，否则在没有外部类的情况下无法新建内部类
     */
    public static class Builder {
        //必选属性
        private final int servingSize;
        private final int servings;
        //可选属性
        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 carbohydrate(int val) {
            carbohydrate = val;
            return this;
        }

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

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

        //内部类可以访问外部类的私有成员
        public NutritionFacts build() {
            return new NutritionFacts(this);
        }
    }

    /**
     * 外部类可以访问内部类的私有属性
     */
    private NutritionFacts(Builder builder) {
        servingSize = builder.servingSize;
        servings = builder.servings;
        calories = builder.calories;
        fat = builder.fat;
        sodium = builder.sodium;
        carbohydrate = builder.carbohydrate;
    }

    public String toString() {
        return "[" + servingSize + "," + servings + "," + calories + "," + fat + "," + sodium + "," + carbohydrate + "]";
    }
}

public class Main {
    public static void main(String[] args){
        /**
         * 初始化时，能够见名之意，并且可选属性可以连环赋值，避免了像setter方法一样需要很长的代码。
         */
        NutritionFacts nutritionFacts=new NutritionFacts.Builder(10,20).calories(30).sodium(35).build();
        System.out.println(nutritionFacts.toString());
    }
}

[10,20,30,0,35,0]