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

[java]
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// 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。

[java]
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// 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。