Thinking in Java之衍生类和基础类的初始化顺序

Posted 2020-06-30 WesTward

《Thinking in Java》书里的例子，我又稍微修改了下代码：

class Real{
    public Real(String index) {
        // TODO Auto-generated constructor stub
        System.out.println("Real()"+index);
    }
}
class Meal{
    Real r= new Real("Meal");
    Meal() {
        // TODO Auto-generated constructor stub
        System.out.println("Meal()");
    }
}
class Bread{
    Bread() {
        // TODO Auto-generated constructor stub
        System.out.println("Bread()");
    }
}
class Cheese{
    Cheese() {
        System.out.println("Cheese()");
    }
}
class Lettuce{
    Lettuce() {
        // TODO Auto-generated constructor stub
        System.err.println("Lettuce()");
    }
}
class Lunch extends Meal{
    Real r= new Real("Lunch");
    Lunch(){
        System.out.println("Lunch()");
    }
}
class PortableLunch extends Lunch{
    Real r= new Real("PortableLunch");
    PortableLunch(){
        System.out.println("PortableLunch()");
    }
}
public class Sandwich extends PortableLunch{
    Bread b= new Bread();
    Cheese c= new Cheese();
    Lettuce l= new Lettuce();
    Sandwich(){
        System.out.println("Sandwich()");
    }
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        new Sandwich();
    }

}

output:

Real()Meal
Meal()
Real()Lunch
Lunch()
Real()PortableLunch
PortableLunch()
Bread()
Cheese()

Lettuce()
Sandwich()

总结：

如果衍生类和基础类都没有static成员，创建衍生类，初始化顺序：一直向上，从根类开始，初始化根类成员，然后根类构造器；然后向下，次根类成员，次根类构造器，以此类推，一直到衍生类本身。