设计思想之构建器

我们都知道，构造器有一个局限性，就是不能很好地扩展到大量的可选参数。如：用一个类来表示包装食品外面显示的营养成分。这些标签中有几个属性是必须的：每份的含量、每罐的含量和每份的卡路里，还有超过20几个属性是可选的。

对于这样的的类，应该用怎样编写构造方法呢?我们一项习惯采用构造方法重载的方式来实现，但这样如果要使创建对象灵活，其构造方法之多可想而知了。那么，让我们来试试使用构建器来实现吧！

public class NutritionFacts{ //营养成分

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 NutritionFacts(Builder builder){

servingSize=builder.servingSize;

servings=builder.servings;

calories=builder.calories;

fat=builder.fat;

sodium=builder.sodium;

carbohydrate=builder.carbohydrate;

}

public static class Builder{

//必要的参数

private final int servingSize;

private final int servings;

//可选参数

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);

}

}

public static void main(String[] args)
{

NutritionFacts cocaCola1=new NutritionFacts.Builder(240,8).build();

NutritionFacts cocaCola2=newNutritionFacts.Builder(240,8).calories(100).build();

NutritionFacts cocaCola3=new NutritionFacts.Builder(240,8).sodium(35).build();

}

}

大家有没有觉得比起构造方法简单、灵活多了呢？如果以后遇到这样的问题，就请大家尝试着用构建器方法来解决吧！

参考：effective java