Understanding inheritance
When a class inherits from another class, it inherits all the elements that compose the parent class, which is also known as a superclass. The class that inherits the elements is known as a subclass. For example, the Mammal subclass inherits all the properties, instance fields or instance attributes, and class fields or class attributes defined in the Animal superclass.
The Animal abstract class is the baseline for our class hierarchy. We say that it is an abstract class because we shouldn't create instances of the Animal class; instead, we must create instances of the specific subclasses of Animal. However, we must take into account that Swift doesn't allow us to declare a class as an abstract class.
We require each Animal to specify its age, so we will have to specify the age when we create any Animal, that is, any instance of any Animal subclass. The class will define an age property and display a message whenever an animal is created. The class defines three type...
