READING
CHAPTER 13
SECTION 2
"Using Classes Effectively with Polymorphism"

- advantage of inheritance
  - can put multiple different types in an array 
    - use the common parent class as the array type
- polymorphism
  - allow a single variable to refer to objects from different classes
  - declare variable with a particular type
  - can assign to that variable 
    - any object of that type
    - any object that has that type as an ancestor
  - cannot assign 
    - superclass
    - *sibling classes*
      - classes that share a common ancestor class
- can call a method using a polymorphic variable name
  - at runtime, computer sees which class actually is
  - calls that implementation of the method
- polymorhpic method may call methods implemented in different classes depending on the runtime type of its object
- benefits of polymorphism
  - makes possible smooth and easy extension and modification of a program
    - don't have to change existing code
      - difficult to change existing code, because to ensure it works correctly, must understand completely
- occasionally need to determine the class of a referenced object
  - use instanceof operator
    - left operand is an object
    - right operand is a class name
    - returns a boolean: whether that object is an instance of that class

Quick Check
1.  Cannot do:
    Truck t = new Vehicle();      // because Vehicle is an ancestor of Truck, not a descendent
    Motorcycle m = new Vehicle(); // analogous to above
    Motorcycle m2 = new Truck();  // Motorcycle and Truck are siblings; Truck isn't a descendent of Motorcycle
2.  We can use the instanceof operator to determine whether an object is an instance of a particular class.