Effective Java Item2:Consider a builder when faced with many constructor parameters

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;
    
    public static class Builder{
        //以下两个参数是必须的
        private final int servingSize;
        private final int servings;
        
        private int calories    = 0;
        private int fat         = 0;
        private int sodium      = 0;
        private int carbohydrate= 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 sodium(int val){
            sodium = val;
            return this;
        }
        
        public Builder carbohydrate(int val){
            carbohydrate = 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 class Test {
    public static void main(String[] args) {
        NutritionFacts nf = new NutritionFacts.Builder(200, 80)
                .calories(100).sodium(35).carbohydrate(20).build();
    }
}