crear una superclase con dos hijas y que una sea abstracta

class Animal extends Object {
  public String toString() {return "Animal";}
  public void call() {System.out.println(this.toString());}
}

abstract class Mammal extends Animal {
  public String toString() {return "Mammal";}
  public void call() {System.out.println(this.toString());}
}

class Dog extends Mammal {
  public String toString() {return "Dog";}
  public void call() {System.out.println(this.toString());}
}

class Cat extends Mammal {
  public String toString() {return "Cat";}
  public void call() {System.out.println(this.toString());}
}

Animal a = new Animal();
Mammal m = new Mammal();
Dog d = new Dog();
Cat c = new Cat();

a.call();
m.call();
d.call();
c.call();