2、遇到多个构造参数的时候需要考虑使用构建器（effective java）

考虑多个构造参数
传统的使用重叠构造函数的写法

采用javabean

采用Builder模式

考虑多个构造参数

一个类的构造参数很多，但是你不是每次构造都用上所有的参数的时候怎么办

传统的使用重叠构造函数的写法

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

//定义了多个构造函数
//顺序都是往上调用的
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);
}
}

这种写法很蠢，参数越多越不好，第二种解决方法：

采用javabean：

//就设置你要的参数
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);
}
}

缺点：

在构造过程中javabean可能处于不一致的状态

javabean模式阻止了把类做成不可变的可能（需要额外的努力来保证线程安全）

采用Builder模式

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;
//一个内部类
//以后维护的时候，添加新参数的时候也很方便
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);
}
}
//构造函数需要传builder
//可以使用单个builder构建多个对象
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) {
//具有安全性和可读性
//build的时候才会验证参数是否正确
NutritionFacts cocaCola = new NutritionFacts.Builder(240, 8)
.calories(100).sodium(35).carbohydrate(27).build();
}
}