READING
CHAPTER 13
SECTION 1

- *polymorphism*
  - OO programming feature that allows programmers to send the same message to objects from different classes
  - each class must define it's own implementation
  - the method that is actually called at runtime depends on the runtime class
  - helps write code that is easy to modify and extend
- *polymorphic message*
  - a message whose execution differs depending on the receiving object
- defining subclasses of programmer-defined classes
- all classes automatically inherit from Object if they do not explicitly extend another class
- modeling multiple classes
  - *unrelated classes*
    - classes that are not connected in an inheritance relationship
    - neither one is an ancestor or descendent of the other
    - also do not share a common ancestor (aside from Object)
    - if classes are related, ineffective design model
      - end up duplicating common code
  - use an inheritance hierarchy
- if modeling two related classes, may require three classes when using inheritance
  - once class to model the common features
  - two more classes, one each to model the specific features
- reserved word protected
  - public to descendent classes
  - private to everybody else
- reserved word extends
  - from the idea that a subclass extends, or adds on to, it's superclass
- inheritance class diagrams
  - for protected, use the pound/hash symbol (#) instead of - for private or + for public
  - do not show inherited data members and methods in subclasses
    - instead, use an inheritance arrow from the subclass to the superclass
    - except for 
      - overridden methods (more later), which are included
      - methods that are defined separately in the subclasses

Quick Check:
1. X is the subclass, Y is the superclass
2. The visibility modifier protected allows data members of a superclass to be accessible to the instances of a subclass.