When a class inherits from another class, it inherits all the elements that compose the parent class, also known as superclass. The class that inherits all the elements of the parent class is known as a subclass. For example, the Mammal subclass inherits all the properties, instance fields or attributes, and class fields or attributes defined in the Animal superclass.
Tip
You don't have to forget what you learned in Chapter 3, Encapsulation of Data, about access modifiers and naming conventions that restrict access to certain members. We must take them into account to determine the inherited members that we will be able to access in subclasses. Some access modifiers and naming conventions applied to members don't allow subclasses to access these members defined in superclasses.
The Animal abstract class is the baseline for our class hierarchy. We require each animal to specify its age; therefore, we will have to specify the age of the animal when we create an instance...