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]