Effective Java 第二条：遇到多个构造器参数时要考虑用构建器

<span style="font-family: Arial, Helvetica, sans-serif; background-color: rgb(255, 255, 255);">对于Java处理多参数时，先给大家上两个程序员平时最喜爱用的写法：</span>

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<span style="font-family: Arial, Helvetica, sans-serif; background-color: rgb(255, 255, 255);">第一：重叠构造器</span>

public class NutritionFacts { //重叠构造器    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 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 servings, int servingSize, int caloriesm, int fat) {        this(servings,servingSize,caloriesm,fat,0);    }    public NutritionFacts(int calories, int fat, int servings, int servingSize, int sodium) {        this(calories,fat,servings,servingSize,sodium,0);    }    public NutritionFacts(int calories, int servingSize, int servings, int fat, int sodium, int carbohydrate) {        this.calories = calories;        this.servingSize = servingSize;        this.servings = servings;        this.fat = fat;        this.sodium = sodium;        this.carbohydrate = carbohydrate;    }}

客户端调用：

NutritionFacts facts = new NutritionFacts(12,34,2,5,3,5);

第二：JavaBean模式

public class NutritionFactsJavaBean {  //JavaBean模式    private  int servingSize;    private  int servings;    private  int calories;    private  int fat;    private  int sodium;    private  int carbohydrate;    public NutritionFactsJavaBean() {    }    public void setCalories(int calories) {        this.calories = calories;    }    public void setCarbohydrate(int carbohydrate) {        this.carbohydrate = carbohydrate;    }    public void setFat(int fat) {        this.fat = fat;    }    public void setServings(int servings) {        this.servings = servings;    }    public void setServingSize(int servingSize) {        this.servingSize = servingSize;    }    public void setSodium(int sodium) {        this.sodium = sodium;    }}

客户端调用：

NutritionFactsJavaBean factsJavaBean = new NutritionFactsJavaBean();        factsJavaBean.setCalories(34);        factsJavaBean.setCarbohydrate(33);        factsJavaBean.setFat(333);        factsJavaBean.setServings(345);

以上两种构造器弊端总结：重叠构造器模式可行，但是当有许多参数时，客户端代码会很难编写，难以阅读。JavaBean模式类无法仅仅通过检验构造器参数的有效性保持一致性，并且导致线程不安全。综上，遇到多参数时，统一推荐大家使用构建器，既能有重叠构造器的安全，又能有JavaBean模式的可读性。

遇到多参数，推荐使用构建器：

public class NutritionFactsBuilder { //构建器模式    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{        private final int servingSize;        private final int servings;        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 NutritionFactsBuilder build(){            return new NutritionFactsBuilder(this);        }    }    private NutritionFactsBuilder(Builder builder){        servings = builder.servings;        servingSize = builder.servingSize;        calories = builder.calories;        fat = builder.fat;        sodium = builder.sodium;        carbohydrate = builder.carbohydrate;    }}

客户端调用：

NutritionFactsBuilder builder = new NutritionFactsBuilder.Builder(12,34)                .calories(44)                .carbohydrate(55)                .fat(456)                .sodium(23)                .build();

以上内容整理自《Effective Java》。大家最好运行代码感受一下喔。