第2条：遇到多个构造器参数时要考虑用构造器

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当我们看帮助文档的时候，有没有想过它是怎么设计构造器的？为什么这么设置？这样设计有什么好处？

第一种情况：重叠构造器(telescoping constructor)

package com.wisdom.effective.java;/** *  * @author Wisdom 第2条：遇到多个构造器参数时要考虑用构造器 */public class TelescopingConstructin {    public static void main(String[] args) {        NutritionFacts nf1 = new NutritionFacts(1, 1);        NutritionFacts nf2 = new NutritionFacts(1, 1, 1);        NutritionFacts nf3 = new NutritionFacts(1, 1, 1, 1);        NutritionFacts nf4 = new NutritionFacts(1, 1, 1, 1, 1);        NutritionFacts nf5 = new NutritionFacts(1, 1, 1, 1, 1, 1);        System.out.println(nf1);        System.out.println(nf2);        System.out.println(nf3);        System.out.println(nf4);        System.out.println(nf5);    }}class NutritionFacts {    private int servingSize; // (ml) required 份量    private int servings; // (ml) required 份    private int calories; // optional 卡路里    private int fat; // (g) optional 脂肪    private int sodium; // (mg) optional 钠    private int carbohydrate; // (g) optional 糖类    public NutritionFacts(int servingSize, int servings) {        this(servingSize, servings, 0);    }    public void setCarbohydrate(int carbohydrate) {        this.carbohydrate = carbohydrate;    }    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 sadium) {        this(servingSize, servings, calories, fat, sadium, 0);    }    public NutritionFacts(int servingSize, int servings, int calories, int fat, int sadium, int carbohydrate) {        this.servingSize = servingSize;        this.servings = servings;        this.calories = calories;        this.fat = fat;        this.sodium = sadium;        this.carbohydrate = carbohydrate;    }}

缺陷：重叠构造器模式可行，但是当有许多参数的时候，客户端会很难编写；如果客户端不小心点到了其中两个参数的顺序，编译器也不会出错，但是程序在运行时会出现错误的行为。

第二种情况：JavaBeans模式

package com.wisdom.effective.java;/** *  * @author Wisdom Effective Java中文版 第2条：遇到多个构造器参数时要考虑用构造器 */public class TelescopingConstructin_2 {    public static void main(String[] args) {        NutritionFacts_2 nf1 = new NutritionFacts_2();        nf1.setServingSize(240);        nf1.setServings(8);        nf1.setCalories(100);        nf1.setSodium(35);        nf1.setCarbohydrate(27);        System.out.println(nf1);    }}class NutritionFacts_2 {    private int servingSize = -1; // (ml) required 份量    private int servings = -1; // (ml) required 份    private int calories = 0; // optional 卡路里    private int fat = 0; // (g) optional 脂肪    private int sodium = 0; // (mg) optional 钠    private int carbohydrate = 0; // (g) optional 糖类    public NutritionFacts_2() {    }    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;    }}

缺陷：1、因为构造过程被分为几个调用中，在构造过程中JavaBeans可能处于不一致的状态。2、JavaBeans模式组织了把类做成不可变的可能，这就需要程序猿付出额外的努力来确保他的线程安全。

第三种情况：Builder模式(即能保证像重叠构造器那样的安全性，也能保证像JavaBeans模式那么好的可读性)，他不直接生成想要的对象，而是让客户端利用所有必要的参数调用类似于setter的方法，来设置每个相关的可选参数。最后，客户端调用无参的build方法来生成不可变的对象。这个builder是它构建的类的静态成员。

package com.wisdom.effective.java;/** *  * @author Wisdom Effective Java中文版 第2条：遇到多个构造器参数时要考虑用构造器 */public class TelescopingConstructin_3 {    public static void main(String[] args) {        NutritionFacts_3 cocaCola = new NutritionFacts_3.Builder(240, 8)                .calories(100).sodium(35).carbohydrate(27).build();        System.out.println(cocaCola);    }}class NutritionFacts_3 {    private int servingSize; // (ml) required 份量    private int servings; // (ml) required 份    private int calories; // optional 卡路里    private int fat; // (g) optional 脂肪    private int sodium; // (mg) optional 钠    private int carbohydrate; // (g) optional 糖类    public static class Builder {        // Required        private int servingSize;        private int servings;        // Optional        private int calories = 0;        private int fat = 0;        private int sodium = 0;        private int carbohydrate = 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 sodium(int val) {            sodium = val;            return this;        }        public Builder carbohydrate(int val) {            carbohydrate = val;            return this;        }        public NutritionFacts_3 build() {            return new NutritionFacts_3(this);        }    }    private NutritionFacts_3(Builder builder) {        servingSize = builder.servingSize;        servings = builder.servings;        calories = builder.calories;        fat = builder.fat;        sodium = builder.sodium;        carbohydrate = builder.carbohydrate;    }}

好处：客户端代码很容易编写，更为重要的是易于阅读。builder模式模拟了具名的可选参数，就像Ada和Python中的一样。

优化版：

package com.wisdom.effective.java;/** *  * @author Wisdom Effective Java中文版 第2条：遇到多个构造器参数时要考虑用构造器 */public class TelescopingConstructin_3 {    public static void main(String[] args) {        NutritionFacts_3 cocaCola = new NutritionFacts_3.Builder(240, 8)                .calories(100).sodium(35).carbohydrate(27).builder();        System.out.println(cocaCola);    }}interface BuilderInter<T>{    public T builder();}class NutritionFacts_3 {    private int servingSize; // (ml) required 份量    private int servings; // (ml) required 份    private int calories; // optional 卡路里    private int fat; // (g) optional 脂肪    private int sodium; // (mg) optional 钠    private int carbohydrate; // (g) optional 糖类    public static class Builder implements BuilderInter<NutritionFacts_3>{        // Required        private int servingSize;        private int servings;        // Optional        private int calories = 0;        private int fat = 0;        private int sodium = 0;        private int carbohydrate = 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 sodium(int val) {            sodium = val;            return this;        }        public Builder carbohydrate(int val) {            carbohydrate = val;            return this;        }//      public NutritionFacts_3 build() {//          return new NutritionFacts_3(this);//      }        public NutritionFacts_3 builder() {            return new NutritionFacts_3(this);        }    }    private NutritionFacts_3(Builder builder) {        servingSize = builder.servingSize;        servings = builder.servings;        calories = builder.calories;        fat = builder.fat;        sodium = builder.sodium;        carbohydrate = builder.carbohydrate;    }}

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