Java 使用Builder解决构造函数参数过多的问题

Effective Java学习笔记：

  构造函数中，如果需要初始化的参数很多，那么为每种初始化方式都写一个构造函数的话，工作会过于繁重。通过使用Builder模式，可以解决构造函数中参数过多的问题。

定义一个内部类Builder，其成员变量与外部类一致；

为外部类中每一个（可选）变量各定义一个类似setter()的方法；

Builder的构造函数返回外部类的实例。

//Builder Pattern
public class NutritionFacts{
public final int servingSize;
public final int servings;
public final int calories;
public final int fat;
public final int sodium;
public final int carbohydrate;

//静态内部类Builder
public static class Builder{
//必选变量
private final int servingSize;
private final int servings;

//可选变量
private final int calories = 0;
private final int fat = 0;
private final int sodium= 0;
private final int carbohydrate= 0;

//Builder的构造函数
public Builder(int servingSize, int servings){
this
4000
. servingSize = servingSize;
this.servings = servings;
}

//Builder的成员方法返回其自身，所以可以链式调用
//类似于setter()方法
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;
}

//Builder的build方法，返回外部类的实例
public NutritionFacts build(){
return new NutritionFacts(this);
}
}

//外部类的构造函数
private NutritionFacts(Builder build){
servingSize = build.servingSize;
servings = build.servings;
calories = build.calories;
fat = build.fat;
sodium = build.sodium;
carbohydrate = build.carbohydrate;
}
}

示例：

NutritionFacts cocaCola = new NutritionFacts.Builder(240, 8).calories(100).sodium(35).carbohydrate(27).build();