chapter2 创建和销毁对象
何时以及如何创建对象，何时以及如何避免创建对象，如何确保它们能够适时的销毁，以及如何管理对象销毁之前必须进行的各种清理动作。
2.1 用静态工厂方法代替构造器
public static Boolean valueOf(boolean b) {
return b ? Boolean.TRUE : Boolean.FALSE;
}
缺点：仅提供静态工厂方法的主要局限是，没有公共或受保护构造函数的类不能被子类化。
2.2 当构造函数有多个参数时，考虑改用构建器
重载构造器。
JavaBeans模式。
// JavaBeans Pattern - allows inconsistency, mandates mutability
public class NutritionFacts {
// Parameters initialized to default values (if any)
private int servingSize = -1; // Required; no default value
private int servings = -1; // Required; no default value
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; }
}
建造者模式。
// Builder Pattern
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 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 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;
}
}
// 所有参数默认值都在一个位置。构建器的 setter 方法返回构建器本身，这样就可以链式调用，从而得到一个流畅的 API
NutritionFacts cocaCola = new NutritionFacts.Builder(240, 8)
.calories(100).sodium(35).carbohydrate(27).build();
