CSC153, Class 40: Polymorphism

Overview:
* Questions from previous class
* Polymorphism
* Interfaces
* Example: Prerequisites
* Abstract Classes

Notes:
* Exam 3 distributed
* Read chapters 1 and 6 of Bailey
* Accepted student visits.
* Cool talk today.
* Sam screws up: Cryptology in the spring.

Why come to Grinnell?
* Grinnell needs more Westies
* Cool small classes with sarcastic professors who
  pick on you regularly.
* Availability of summer research
  Unavailability of graduate courses
* "Relaxed cultural atmosphere"
* Lots of multicultural interaction with the international
  students

Why not come to Grinnell?
* Dates
* Faculty actually expect you to learn
* Students unable to comment thoughtfully on other
  schools (e.g., biases in favor of Grinnell)

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Q. How does higher-orderness relate to polymorphism and
and inheritance?
* The higher-order procedure is effectively polymorphic
  because it can work on many different kinds of data.
* You're changing the basic thing a procedure does.
  You inherit things about the structure
* All three are for reuse

Q: What is multiple inheritance?
A: When you inherit from more than one object.  You gain
the characteristics (e.g., fields, methods) of both structures.

Q. Why have multiple inheritance?
A: For example, you might want to define library book in terms
   of "Book" and "BorrowableThingymajig" (no, that is not
   Katherine's term)

Q: Why not have multiple inheritance?
A: What happens if both superclasses have a method with the
  same name and parameters?  Which do you use? 

Polymorphism, a close friend to inheritance
* Once you know how to do something with a general class of
  object, you know how to do that thing with related objects

Side note: Ocassionally you want to say "These are the capabilities
I expect" without specifying the initial versions of those
capabilities

Solution: Interfaces
Like classes but
* No fields
* No constructors
* No bodies to methods

We've decided what methods are prerequisites should support.
What kinds of prerequisites are there?
* Specific courses (e.g., CSC151 is a prereq for CSC152)
* Choices of classes (e.g., CSC152 or CSC153 before CSC211)
  Generalize: One of two prereqs
* Specific non-Grinnell courses
* Standing (e.g., second year)
* Converse prereqs (not XXX)
* Permission from the professor (left as an exercise for later)
* Combinations of classes (e.g., in order to CS301, you need
  152/153 AND MAT218)
Coreqs are a lot like prereqs, you just look at one more semester

For Monday:
* Think about the rest of PrereqOr