第二条 遇到多个构造器参数时要考虑用构造器
这里考虑的是参数多的情况，如果参数个数比较少，那直接采用一般的构造方法就可以了。
书中介绍了写构造方法的时候几种方式：
1. 重叠构造方法模式:
缺点：有许多参数的时候，客户端代码会很难写，而且较难以阅读。
2. javaBeans模式：
缺点：
在构造过程中JavaBean可能处于不一致的状态，类本身无法判断是否有效性。
类做成不可变的可能。
3. builder模式：
优点：
在build方法生成对象的时候，可以做检查，判断是否符合要求
参数灵活
缺点：
创建对象必须先创建构造器，如果对性能要求非常高的应用少用为妙
具体实现代码：
1.重叠构造方法模式：
public class NutritionFacts {
private final int servingSize;
private final int serviings;
private final int calories;
private final int fat;
private int sodium;
private int carbohydrate;
public NutritionFacts(int servingSize, int serviings){
this(servingSize, serviings, 0);
}
public NutritionFacts(int servingSize, int serviings, int calories){
this(servingSize, serviings, calories, 0);
}
public NutritionFacts(int servingSize, int serviings, int calories, int fat){
this(servingSize, serviings, calories, fat,0);
}
public NutritionFacts(int servingSize, int serviings, int calories, int fat, int sodium){
this(servingSize, serviings, calories, fat, sodium,0);
}
public NutritionFacts(int servingSize, int serviings, int calories, int fat, int sodium, int carbohydrate){
this.servingSize = servingSize;
this.serviings = serviings;
this.calories = calories;
this.fat = fat;
this.sodium = sodium;
this.carbohydrate = carbohydrate;
}
}
2. javaBeans模式 代码：
public class NutritionFacts {
private int servingSize;
private int serviings;
private int calories;
private int fat;
private int sodium;
private int carbohydrate;
public NutritionFacts(){}
public void setServingSize(int servingSize) {
this.servingSize = servingSize;
}
public void setServiings(int serviings) {
this.serviings = serviings;
}
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;
}
3. builder模式
public class NutritionFacts {
private final int servingSize;
private final int serviings;
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 serviings;
// 可以为空
private int calories = 0;
private int fat = 0;
private int sodium = 0;
private int carbohydrate = 0;
public Builder(int servingSize, int serviings) {
this.servingSize = servingSize;
this.serviings = serviings;
}
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 build(){
return new NutritionFacts(this);
}
}
public NutritionFacts(Builder builder) {
servingSize = builder.servingSize;
serviings = builder.serviings;
calories = builder.calories;
fat = builder.fat;
sodium = builder.sodium;
carbohydrate = builder.carbohydrate;
}
}
这个调用的时候：
NutritionFacts cocaCola = new NutritionFacts.Builder(11,22).calories(1).fat(2).calories(3).build();