不可变对象

不可变对象在多线程的情形下是线程安全的。构建规则可参照如下代码中注释。

//类添加final修饰符，保证类不被继承
public final class ImmutableNutritionFacts {
    //所有成员变量必须私有，并且加上final修饰
    private final int servingSize;
    private final int servings;
    private final int calories;
    private final int fat;
    private final int sodium;
    private final int carbohydrate;
    private final List<String> list;

    //通过构造器初始化所有成员，进行深拷贝(deep copy)
    public ImmutableNutritionFacts(int servingSize, int servings, int calories, int fat,
                                   int sodium, int carbohydrate, List<String> list){
        this.servingSize = servingSize;
        this.servings = servings;
        this.calories = calories;
        this.fat = fat;
        this.sodium = sodium;
        this.carbohydrate = carbohydrate;
        this.list = list.subList(0, list.size()); //deep copy(引用类型)
    }
    //不提供改变成员变量的方法，包括setter
    //在getter方法中，不要直接返回对象本身，而是克隆对象，并返回对象的拷贝
    public int getServingSize() {
        return servingSize;
    }

    public int getServings() {
        return servings;
    }

    public int getCalories() {
        return calories;
    }

    public int getFat() {
        return fat;
    }

    public int getSodium() {
        return sodium;
    }

    public int getCarbohydrate() {
        return carbohydrate;
    }

    public List<String> getList() {
        return list.subList(0, list.size()); //返回对象的拷贝
    }
}