遇到多个构造器参数时要考虑用构建器

     考虑用一个类表示包装食品外面显示的营养成分标签，这些标签中有几个域是必需的：每份的含量，每罐的含量以及每份的卡路里，还有一些可选域：钠，歆，铁，总脂肪含量等等！下面以四个可选域举例。

    一般大家习惯采用重叠构造器模式：

public class NutritionFacts1 {

    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 NutritionFacts1(int servingSize,int servings){
        this(servingSize,servings,0);
    }
    
    public NutritionFacts1(int servingSize,int servings,int calories){
        this(servingSize,servings,calories,0);
    }
    
    public NutritionFacts1(int servingSize,int servings,int calories,int fat){
        this(servingSize,servings,calories,fat,0);
    }
    
    public NutritionFacts1(int servingSize,int servings,int calories,int fat,int sodium){
        this(servingSize,servings,calories,fat,sodium,0);
    }
    
    public NutritionFacts1(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 class BuilderTest {

    public static void main(String[] args) {

        NutritionFacts1 nf1 = new NutritionFacts1(240,8,100,0,35,27);
        System.out.println(nf1.toString());
    
    }

}

这仅仅是6个参数，一旦参数增加，问题就来了！客户端代码会很难写，并且比较难以阅读！

     第二种方法是JavaBeans模式：

public class NutritionFacts2 {

    private  int servingSize = -1;
    private  int servings = -1;
    private  int calories = 0;
    private  int fat = 0;
    private  int sodium = 0;
    private  int carbohydrate = 0;
    
    public NutritionFacts2(){}
    
    public void setServingSize(int servingSize){
        this.servingSize = servingSize;
    }
    public void setServings(int servings){
        this.servings = servings;
    }
    public void setCalories(int calories){
        this.calories = calories;
    }
    public void setFat(int fat){
        this.fat = fat;
    }
    public void setSodium(int sodium){
        this.sodium = sodium;
    }
    public void setCarbohydrate(int carbohydrate){
        this.carbohydrate = carbohydrate;
    }
    
}

public class BuilderTest {

    public static void main(String[] args) {

        NutritionFacts2 nf2 = new NutritionFacts2();
        nf2.setServingSize(10);
        nf2.setServings(20);
        nf2.setCalories(1);
        nf2.setCarbohydrate(5);
        nf2.setFat(0);
        nf2.setSodium(10);
    
    }

}

 

JavaBeans模式的缺点：

                            1，构造的过程分到了几个调用中，在构造JavaBeans的时候可能会不一致。

                            2，类无法仅仅通过检验构造器参数的有效性来保证一致性！

                            3，对象的不一致会导致失败，JavaBeans模式阻止了把类做为不可变的可能，需要程序员做额外努力来保证它线程安全。

但是大部分的时候JavaBean是在线程中new出来的，只会供单线程使用，很少会出现多个线程共享JavaBean的情况！

      第三种方法Builder模式，既保证了像重叠构造器模式那样的安全，也能保证像JavaBeans模式那么好的可读性！

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 {
        private int servingSize;
        private 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 BuilderTest {

    public static void main(String[] args) {

        NutritionFacts cocaCola = new NutritionFacts.Builder(240, 8).carbohydrate(4).sodium(5).carbohydrate(6).build();
    
    }

}