Find the error in this code:

Find the error in this code:

abstract class Animal {
    abstract void sound();
}
class Dog extends Animal {
    void sound() {
        System.out.println("Bark");
    }
}
class Puppy extends Dog {
    void sound() {
        System.out.println("Yip");
    }
}
public class Test {
    public static void main(String[] args) {
        Animal a = new Puppy();
        a.sound();
    }
}