OopNote

=Principles=
 * 1) Encapsulation
 * 2) Abstraction
 * 3) Inheritance
 * 4) Polymorphism

=Concept=
 * Public
 * Protected
 * Package
 * Private

Interface
An interface is a group of related methods with empty bodies

Abstract

 * An abstract method is a method that is declared without an implementation.
 * Abstract class
 * is a class that is declared abstract.
 * it may or may not include abstract methods.
 * cannot be instantiated, but they can be subclassed.