代码最佳实现——02-多个构造器参数时考虑使用构建器-CFANZ编程社区

这一系列的文章，将持续的开始。我是通过阅读《Effective java》，加上网上的博客的学习，进行总结。是这样的《Effective java》这本书中没有代码例子。

多个构造器参数时考虑使用构建器

上面讲到的静态工厂方法有个局限性,就是不能扩展大量的可选参数.当类具有大量可选参数时,我们通常都会采用重叠构造器的方法来解决,即提供多个不同参数的构造器.例如下面的代码:

// Telescoping constructor pattern - does not scale well!
public class NutritionFacts {
    private final int servingSize;   // required
    private final int servings;      //  required
    private final int calories;      // optional
    private final int fat;           // optional
    private final int sodium;        // optional
    private final int carbohydrate;  // optional

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

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

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

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

可以根据需要选择相应的构造器获取实例,但当参数越来越多时,代码就会变得复杂.
还有另外一种办法就是采用JavaBeans模式,提供一个无参的构造器,其他的参数通过setter()方法来设置.

// Telescoping constructor pattern - does not scale well!
public class NutritionFacts {
    private  int servingSize;   // required
    private  int servings;      //  required
    private  int calories;      // optional
    private  int fat;           // optional
    private  int sodium;        // optional
    private  int carbohydrate;  // optional

    public NutritionFacts() {
    }

    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;
    }
}

这样创建实例很容易,代码阅读也清晰.但这种方法可能会造成对象状态的不一致,需要保证线程的安全.

所以这里建议使用第三种方法,就是采用Builder模式,即使用构建器.这种方法不直接生成想要的对象,而是利用必要的参数调用构造器,得到一个builder对象,然后通过builder对象设置可选参数,最后通过build()方法生成对象.

// Builder Pattern
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 {
        // Required parameters
        private final int servingSize;
        private final int servings;
        // Optional
        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 fat(int val) {
            fat = val;
            return this;
        }

        public Builder carbohydrate(int val) {
            carbohydrate = val;
            return this;
        }

        public Builder sodium(int val) {
            sodium = 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;
    }
}

这里bulider的参数设置的方法返回builder本身,方便链式调用,下面是客户端代码

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