Held: Tuesday, 11 February 2014

Back to Outline 13 - Some Subtleties: References and Automatic Boxing/Unboxing. On to Outline 15 - Inheritance.

Summary

We consider polymorphism, a powerful tool for code reuse (and generalization). In essence, polymorphism says that we can use different kinds of objects for the same purpose, provided they have needed charactistics.

Related Pages

• Reading: Interfaces in Java
• Reading: Polymorphism in Java
• Lab: Polymorphism
• EBoard 14 (Source 14) (HTML 14) (PDF 14)

Overview

• Interfaces.
• Polymorphism.
• An example: Text blocks.

Administrivia

• I will not be holding a review session on Thursday. Sorry.
• Reminder: If you are planning to do work in CS and you haven't started applying for internships and such, you should be doing so soon!
• Reading for Wednesday: Inheritance.
• Today's writeup: Exercise 7 (one of the three options)
  • CSC 207 Writeup 9: Polymorphism
  • Due Friday.
• This lab and the next lab will probably take a class-and-a-half each, particularly as we insert explanations and such.
• I'll reserve a few minutes for questions on the homework.
• I'll reserve a few minutes for questions on the reading.
• I plan to distribute the exam in class tomorrow.
• Extra credit:
  • CS Extra Thursday at 4:30: Stone on Red/Black Trees
  • CS Table Friday at noon: Law, Order, and Computers
  • More?

Interfaces

• Separate the what from the how.
• Only have signatures of methods (plus some other stuff).

Polymorphism

Warning: There is some gap between the ideal of polymorphism and its implementation in many languages, including Java.

• Use related things in the same way.
• Why should you have to write N copies of square? It should work with anything that has a multiply method.
  • And it does work that way in Scheme.
  • But Scheme trusts you. Java is your nanny.
  • (Note: Java does not provide a multipliable interface, so the theory and practice don't match as well as they should.)
• Closely tied to interfaces.
  • If you write a procedure that accepts an interface.
  • Then your procedure will work with anything that implements that interface.
• Not the same as overloading
  • Overloading requires you to write N methods for N types.
  • Polymorphism requires you to write 1 method for N types.
• Example: Text blocks.