Casting subclass to superclass and calling a function that both classes have

问题

public class Test3 {
    public static void main(String[] args) {
        Derived04 b = new Derived04();
        Base04 a = (Base04)b;
        System.out.println(a.i);
        System.out.println(a.f());
    }
}

class Base04 {
    int i=1;
    public int f() {return i;}
}
class Derived04 extends Base04 {
    int i = 2; 
    public int f(){return -i;}
}

Both classes include the f() method so when I cast d onto a which method does it go to when I call a.f()? I thinking since a is declared as Base04 type when you do a.i it gives 1 but then why does it use the method from the subclass?

回答1:

The whole point is about implementing and casting. so when you create a Derived04 then you have implemented a Derived04 class, not its parents Base04 so even if you cast it like that it will run the implemented method not the casting method unless there is a different between f() in both classes. and that's not our case.

回答2:

why does it use the method from the subclass?

You create an object of Derived04 class you just cast it to the type Base04. It is still Derived04 implementation.

来源：https://stackoverflow.com/questions/61421158/casting-subclass-to-superclass-and-calling-a-function-that-both-classes-have