Now, we will declare an abstract class named SocialAnimal that implements both the previously defined Sociable interface and the Comparable<Sociable> interface. The latter makes it possible to compare two objects of the Sociable type. In order to implement the Comparable<Sociable> interface, we must implement the compareTo method that receives a Sociable instance and will return the results of comparing their age values. In addition, the class will override the equals method inherited from the java.lang.Object class. We will explain the code for this complex method later, when we review the code for the class. We can read the class declaration as "the SocialAnimal class implements both the Sociable and the Comparable of Sociable interfaces."

The SocialAnimal abstract class has too many lines of code, and therefore, we will use three code snippets instead of just one. The following lines show the first code snippet for the...