Comp 212 Lab 04: Visitor Pattern

Introduction

This tutorial consists of exercises on the visitor pattern discussed in class.  It also helps prepare for homework 03.  

I. To Drive or Not To Drive

Consider the following UML model of keys and cars:

You are to do the following:

1. Fill in the concrete methods of all the above concrete classes so that only a specific key will drive a specific car when that car's turnKey() method is called with that key, i.e. call its drive() method. All other keys will call the car's alarm() method.

For instance, suppose we have:

• An object called ford of type Ford, and
• An object called fordKey of type FordKey

then ford.turnKey(fordKey) will cause ford.drive() to be called.

On the other hand, suppose instead we have

• An object called saturnKey of type SaturnKey

then ford.turnKey(saturnKey) will cause ford.alarm() to be called.

2. Fill in the ACar.drive() and ACar.alarm() methods to print out something indicative that the method has been called, e.g. "Car has been driven!" and "Car alarm sounded!".

3. Document your code, of course. It is recommended that you document the methods before you write the method bodies! (Remember Comp210?)

3. Write a test program that clearly tests all possible key and car combinations. Be sure that the test program prints clear messages indicating exactly what is being tested at the moment, i.e. what key-car combination is being used.

II. Student's Social Life

Create the object system that models the following using the Visitor Design pattern:

You're sitting in your room...um...."working"....or maybe not..... Suddenly, the phone rings and some person is on the other end. You need to proffer a response, but the exact response behavior will depend on not only what you've been doing, but who the person on the line is.

Now, there are a couple of scenarios:

1. You've been working diligently on your Comp212 homework and the person calling is:
   1. Your mother. You want say "Oh yes, Mom, I've been working so hard, I haven't got time to even think about going to parties!"
   2. Your Comp212 lab-mate: You want to say "Problem 4's a piece of cake, once you understand double-dispatching!"
   3. Your significant other. You want to say "I'll be done in a minute!"
2. You've been vegging out watching re-runs of the Gong Show and the person calling is:
   1. Your mother. You want say "Of course, I'm working hard! By the way, did I tell you that books cost more than I thought they would?!"
   2. Your Comp212 lab-mate. You want to say "Uh...do you have a clue how to do Problem 1?"
   3. Your significant other. You want to say "I'll be there right now!"

Some things to consider:

• "Saying something" means printing a string to the console.
• There is more than one solution to this problem! The trick is to clearly identify the host vs. the visitor.
• Remember that the abstract host contains a method that will accept (execute) an abstract visitor.
• Use method and class names that correspond to what is happening. Done correctly, your code should be "self documenting", that is, if you read your code, it should sound very similar to what the code is really doing.
• Your code should depend neither on what you were doing nor who is calling!
• In spite of responding without identifying the caller, a bright Rice student like yourself manages to say the appropriate thing anyway.
• No "if"statements are needed. Expect significant grading penalties if you use one!

You are required to do the following:

1. Create a UML diagram that models the above situation.
2. Write all the code, including all the method bodies.
3. Document all your code, of course.
4. Write a test program that clearly demonstrates that your system properly handles all 6 of the above scenarios. Be sure that the test code prints out helpful messages that make it obvious what scenario is being tested at the moment.

III. List Visitor Exercises

First, copy all the java source code for IList hierarchy and its visitor, IListAlgo, from the link http://www.owlnet.rice.edu/~comp212/01-fall/lectures/10/schemeFW/.   This link contains the package schemeFW.  Use the provided java classes as the starting point.

1. Write a visitor called Length to compute the length of IList using direct recursion and without using helpers.  This visitor corresponds to the method length() of scheme.AList .
   
2. Write a visitor called GetMin to compute the minimum of IList, assuming IList contains Integer objects.  GetMin should use a helper visitor called HelpGetMin to compute the minimum.  This corresponds to the getMin() method of scheme.AList as shown in http://www.owlnet.rice.edu/~comp212/01-fall/labs/03/scheme/.
   
3. Write a visitor called Sum to compute the sum of IList, assuming IList contains Integer objects.  Write one version using direct recursion, and one version, called GetSum, using a helper visitor called HelpGetSum.