Effective Java

2019-11-14 21:45:13

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Effective java - 用builder代替构造器

静态工厂和够构造器有一个共同的局限性：遇到大量的参数时无法很好的扩展。

先说说构造器。其实field不多时重叠构造器(telescoping constructor)是个不错的方法，易于编写也易于调用，这种方式在参数数量较少时也很常见。但问题是参数很多(可能越来越多)时，比如(现在已经很难找到对多个参数进行重叠构造的代码了，于是在这里直接引用一下书中的代码)：

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;    }}

我想创建实例的时候，代码会变成这个样子：

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

为了让创建实例的过程变得更加清晰，于是我们有另一中选择——JavaBean模式。这个再熟悉不过了，如下：

public class NutritionFacts {    // Parameters initialized to default values (if any)    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;    }    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;    }}

用起来虽然比只有一行的构造器多几个步骤，但非常清晰：

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);}

相比只通过一个构造器创建实例，JavaBean模式的实例的构造过程被分成了好几个过程。我们完全有可能在属性不完整的情况下使用这个实例。按书中原文就是:

A JavaBean may be in a inconsistent state partway through its construction.

当然，我们也可以做一些操作使其在未完成的情况下无法使用，但我不想因此让一个类变得越来越复杂。另外，我根本无法用JavaBean模式完全排除了将类设计成不可变(Immutable)的可能性。

此时，我们选择使用Builder来解决这一问题。示例如下：

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;    }}

调用起来非常清晰(模拟了具名参数)，而且非常简单：

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

当然，Builder也有缺点。