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第二条 遇到多个构造器参数时，要考虑用构建器

2024-07-02 00:14:59   221人阅读

　　静态工厂和构造器都有个共同的局限性，就是它们都不能够很好的扩展到大量的可选参数。

如：

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 NutritionFacts(int servingSize, int servings){
           this(servingSize, servings, 0);
    }  

    public NutritionFacts(int srvingSize, int servings, int calories){
           this(servingSize, servings, calories, fat, 0);
    }

    public NutritionFacts(int servingSize, int servings, int calories,   int fat ){
           this(servingsSize, servings, calories, fat, 0);
    }
   
    public NutritionFacts(int servingSize, int servings, int calories, int fat, int sodium){
            this(servingsSize, servings, calories, fat, sodium,0);
    }

    public NutritionFacts(int servingSize, int servings, int calories,   int fat, int sodium, int carbohydrate){
            this.servingSize = servingSize;
            this.servings = servings;
            this.calories = caloriers;
            this.fat = fat;
            this.sodium = sodium;
            this.carbohydrate = carbohydrate;
    }

}

我们可以看到利用构造器方式，有多个参数时，很难控制其灵活性。因而我们选择尝试给每个值设置默认值：

public class NutritionFacts{
  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 NutritionFacts(){
     //   setters
     public void setServingSize(int val){
         servingSize = val;
    } 
    
     public void setServings(int val){
          servings = val;
     }
     //....
  }
}

 

  调用初始化

NutritionFacts cocaCola = new NutritionFacts();
cocaCola.setServingSize(240);
cocaCola.setServings(8);
cocaCola.setCalories(100);
cocaCola.setSodium(35);
cocaCola.setCarbohydrate(27);

这是常见的javaBean模式，通过setter方法来构建对象，但是这也阻止了类做成不可变的可能，会有多线程安全问题。

 

那么我们来看看另外一种方式：

public class NutritionFacts{

   private final int servingSize;
   private final int srvings;
   private final int calories;
   private final int fat;
   private final int sodium;
   private final int carbohydrate;

　　// 静态内部类
   public static class Builder{
       
       //　必须元素      
       private final int servingSize;
       private final int servings;
       //　默认元素
       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;
        }

　　　　// 在内部类中返回外部类对象的接口
        public NutritionFacts build(){
            return new NutritionsFacts(this);
        }
   }   

   private NutritionFacts(Builder builder){
        servingSize = builder.servingSize;
        servings = builder.servings;
        calories = builder.calories;
        fat = builder.fat;
        sodium = builder.sodium;
        carbodhydrate = builder.carbodhydrate;
   }
}

 

调用方式：

NutritionFacts fact =  new NutritionFacts.builder(240,8).calories(100).sodium(46).carbohydrate(27).build();

这样通过builder模式，实现了可选参数构建对象实例。


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