Objective: Learn interfaces and hashtables.

What are interfaces?
Interfaces are a group of methods with empty bodies. When a class implements an
interface, all methods defined by the interface must be implemented by the class,
this is checked in compile time.


How are interfaces different from abstract classes?
Interface is a contract, so the classes implementing it needs to implement all
methods inside the interface. Interfaces are an empty shell and contains only method
signatures. Consumes less CPU in creating.

Abstract classes can have their own implementation for methods, but if they like
the child to implement its own method they can create an abstract method. Consumes
more CPU.


When to use abstract classes vs concrete classes?
- Abstract classes usually have partial or no implementation. Concrete classes
  always have full implementation of its behavior.
- Concrete classes can be instanciated. Abstract classes cannot.
- Abstract classes may contain abstract methods.


What are hashtables?
Hashtables are key value pairs, where the keys are used to index and find its
associated values. It is dictionary in python. It has O(1) look up time.