Com S 362 --- Object-Oriented Analysis and Design

		       EXERCISE: POLYMORPHISM 1
                 (File $Date: 2003/09/29 13:25:52 $)

The purpose of this exercise is for you to learn about the
observer (or publish-subscribe) design pattern. This is a key pattern
used in user interfaces.

As with all exercises, this is to be done individually, not in teams.
And it is due the day this topic is planned to be discussed in class, unless
specified otherwise (see the syllabus at:
http://www.cs.iastate.edu/~cs362/syllabus.shtml). 

As with all exercises, you have two choices for doing the work. You
can either:

  - complete it as specified or
  - write down questions or problems that you had in trying to
    complete it.

If you write down questions or problems you have, these should be
detailed enough so that we can tell that you have read the materials
and thought about them.  (Don't just write: "I couldn't get it to
work; say what you tried and what you didn't understand.)  During the
class where this exercise is discussed, you should try to get help
with these by explaining what you did and what your problems or
confusions are.  Don't be shy; there will be other people with the
same problem, and everyone can learn by discussing these issues.

1. [Observer Pattern]

Read section 23.9 of Craig Larman's book Applying UML and Patterns
(second edition, Prentice Hall PTR, Upper Saddle River, NJ, 2002).

(a) What problem does the observer pattern solve?  Why is that
important for user interfaces?

(b) Suppose we want a GUI for the calculator example we've been
discussing in class to have a "evaluate" button that, when pressed,
evaluates a selected formula (showing the user the result).  Describe
the classes and methods needed to implement that using this pattern.

(c) Can the observer pattern be used for purposes other than GUIs?


WHAT TO HAND IN

You should have at the beginning of class, written answers to the
above questions (or written out questions and problems you encountered
for each part).  Make sure your name is on these. Attach the
printouts, if any, requested above.

ADDITIONAL READINGS

If you have time, read about the Observer pattern in pp. 293-303 of
Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides's book
Design Patterns: Elements of Reusable Object-Oriented Software
(on reserve).