学习中的问题

import java.util.*;

public class ObjectTestDrive {
    public static void main(String[] args) {
        ArrayList<Animal> alist = new ArrayList<Animal>();
        Dog d = new Dog();
        alist.add(d);
        Animal a = alist.get(0);
        a.yield();
        System.out.println(a instanceof Dog);
    }
}

class Animal {
    void yield() {
        System.out.println("animal,,,,,,,,,");
    }
}

class Dog extends Animal {
    void yield() {
        System.out.println("wang!wang!");
    }
}

输出结果是：

wang！wang！

true