Android 编码规范：（二）遇到多个构造器参数时要考虑用构建器

静态工厂和构造器有个共同的局限性：它们都不能很好地扩展到大量的可选参数。考虑用一个类表示包装食品外面显示的营养成分标签。这些标签中有几个域是必须的：每份的含量、每罐的含量以及每份的卡路里，还有超过20个可选域：总脂肪量、饱和脂肪量、转化脂肪、胆固醇、钠等等。大多数产品在某几个可选域中都会有非零的值。

对于这样的类，应该用哪种构造器或者静态方法来编写呢？

1. 重叠构造器模式

// Telescoping constructor pattern - does not scale well!

public class NutritionFacts {
    private final int servingSize;   // (mL)            required
    private final int servings;      // (per container) required
    private final int calories;      //                 optional
    private final int fat;           // (g)             optional
    private final int sodium;        // (mg)            optional
    private final int carbohydrate;  // (g)             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;
    }

    public static void main(String[] args) {
        NutritionFacts cocaCola =
            new NutritionFacts(240, 8, 100, 0, 35, 27);
    }
}

2. JavaBeans模式

// JavaBeans Pattern - allows inconsistency, mandates mutability public class NutritionFacts { // Parameters initialized to default values (if any) private int servingSize = -1; // Required; no default value 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; } public void setCalories(int val) { calories = val; } public void setFat(int val) { fat = val; } public void setSodium(int val) { sodium = val; } public void setCarbohydrate(int val) { carbohydrate = val; } public static void main(String[] args) { NutritionFacts cocaCola = new NutritionFacts(); cocaCola.setServingSize(240); cocaCola.setServings(8); cocaCola.setCalories(100); cocaCola.setSodium(35); cocaCola.setCarbohydrate(27); } }

3. Builder模式，既能保证安全性，又能保证可读性。

// 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 parameters - initialized to default values
        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;
    }

    public static void main(String[] args) {
        NutritionFacts cocaCola = new NutritionFacts.Builder(240, 8).
            calories(100).sodium(35).carbohydrate(27).build();
    }
}

简而言之，如果类的构造器或者静态工厂中具有多个参数，设计这种类时，Builder模式就是种不错的选择，特别是当大多数参数都是可选的时候。与使用传统的重叠构造器模式相比，使用Builder模式的客户端代码将更易于阅读和编写，构造器也比JavaBeans更加安全。