Purpose of having abstract child by extending concrete parent

Question

Sometimes, I came across some class design as follow.

abstract class animal {
    public abstract void speak();
}

class dog extends animal {

    @Override
    public void speak() {
        // Do something.
    }

}

abstract class abstract_dog extends dog {
    @Override
    public abstract void speak();
}

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I was wondering, what is the purpose of having an abstract_dog class? Why we "transform" the non-abstract speak method into abstract speak again?

Answer 1

In case you want to create a base class that forces people to override speak, but inherits Dog.