Builder模式

Builder模式其中的一个作用是在创建类的构造器的时候，把可选的和必须的参数分离。下面是一个例子：

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;
    
    public static class Builder {
        //对象的必选参数
        private final int servingSize;
        private final int servings;
        //对象的可选参数的缺省值初始化
        private int calories = 0;
        private int fat = 0;
        private int carbohydrate = 0;
        private int sodium = 0;
        //只用少数的必选参数作为构造器的函数参数
        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();
        
        NutritionFacts cc = new NutritionFacts.Builder(30, 34).build();
    }

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}

可以通过Builer方式，只写一个构造方法。