Abstract classes are used to define a class that will be used only to build new classes.
No objects will ever be instantiated from an abstract class. Abstract classes are used to create hie-
rarchies of classes.

Any sub class that extends a super abstract class must implement all methods defined as abstract in the
super class unless the extending class is abstract class.

Abstract classes are great when you know quite a bit about an Object, but not everything.
For instance, say you are writing a program to simulate flying objects and you know that
all of the objects will have width, height, and speed, but you do not know how each one will fly.
The fly properties and behavior will differ for each type of flying object which means
that all of the common properties and behaviors that are already known can be placed in an abstract class.
The specific fly methods will be implemented in each of the specific flying objects.


Description                                             Interface             Abstract Class
===========================================             =========             ==============
Can contain abstract methods                             Yes                   Yes
Can contain non-abstract methods                         No                    Yes
Can contain constructors                                 No                    Yes
Can be instantiated                                      No                    No
Can be extended                                          Yes                   Yes
Can be implemented                                       Yes                   No

Can contain instance variables                           No                    Yes
Can contain final instance variables                     No                    Yes
Can contain final class variables                        Yes                   Yes
Can contain class variables                              No                    Yes
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A class can extend another class.
An abstract class can extend another abstract class.
A class can extend an abstract class.
An abstract class can extend a class.
An interface can extend another interface.

A class can implement an interface.
An abstract class can implement an interface.