CSC207.01 2014S, Class 05: Classes and Objects
==============================================

_Overview_

* Preliminaries.
    * Admin.
    * A few notes on writeup 3.
    * Questions.
* A brief introduction to objects.
* An exercise: Modeling Ushahidi.

Preliminaries
-------------

### Admin

* Study break at 8 pm tonight in the commons.  Peter is making cookies.
* For second years: Special extra credit for attending the lunch session
  you were invited to.
* I *think* I've responded to all of the writeups I've received as of
  10 pm last night.
* I think I've responded to HW1.
    * [FAQ](../handouts/faq.html) updated.
    * [Lessons](../handouts/lessons.html) added.
* No lab today!  We'll do some small group and large group design.
* Does anyone need a partner for the assignment?
* Extra credit: 
    * CS Extras, Thursday: Ushahidi, Android, and 207 
      by Spencer, Daniel, and Lea.
    * CS Table Friday: The ACM Code of Ethics
    * Convo Feb. 5.  (I'll give my "Why go to convo" lecture closer to the date.)     

### A Few Notes on Writeup 3

* If your code is incorrect, you have an obligation to let me know that
  you know that it is incorrect.
* I'm not sure why we no longer get an error based on the overflow, but
  I'd swear I did last semester.
* A question about changing the preconditions: Are your new preconditions
  biased toward a particular implementation?
* For those of you who said "I'd change the implementation", how would you
* For your unit testing, the following is not a great test statement

    for (int base = -100; base < 100; base++)
      {
        expected = 1;
        for (int power = 0; power < 10; power++)
          {
            assertEquals ("Testing", expected, SampleMethods.expt (base, power));
            expected *= base;
          } // for each power
      } // for each base

    * If the test fails, you'll only see the "Testing" message.  You'd
      probably prefer to see the base and power too.  

            assertEquals ("Testing " + base + "^" + power, 
                          expected, SampleMethods.expt (base, power));
           
    * I'll admit that I often end up doing something like

            int result = SampleMethods.expt (base, power);
            if (result != expected)
              {
                fail ("For " + base + "^" + power + ", expected " +
                      expected + ", got " + result);
              } // if we did not get the expected result.

    * Doesn't 100^10 overflow?  You probably want to have a more sensible
      stopping condition (perhaps involving a while loop).
    * For testing the `double` version, you'll probably need to have a
      "close enough" metric (see documentation).
         assertEquals (message, expected, formula, ACCURACY)
        
### Questions on HW2

_Are you okay with the seemingly inefficient looping (or recursive) solution 
to `isOdd`?_

> Yes.

> But smart programmers who think in C can probably find a more efficient
solution.  

_Can we use a loop for `oddSumTo`?_

> Yes, but you shouldn't need one.

A brief introduction to objects
-------------------------------

* Objects encapsulate data and methods that work with those data.
    * structs
    * plus functions on those structs (methods)
    * plus hiding the fields of the struct
* Kinds of methods
    * Constructors: Build new objects (given some data)
    * Observers: Extract information (e.g., ask for title of library book)
    * Mutators: Change the object
        * Check out a library book
* Like all classification systems, this breaks down rather quickly
    * Complex c = new Complex(1,2);
    * c.multiplyBy(c) - returns a new complex number
        * So is multiplyBy a Constructor?
        * Or is it an observer?
        * Or is it both
* As you design an object
    * What information will the client want to get?
    * What information will we allow the client to change?
    * What information do we need to build a new object?

An exercise: Modeling Ushahidi
------------------------------

Context: 

* Crowdmapping with anonymity and some verification

Design Issues:

* What are the objects you'd want to model this system?
* What methods would you associate with those objects?

Objects

* Map (database?) - Contains the locations of the posts -
* Incident - Information on a single incident
    * Categoriezed, giving you different types of incidents?
* User - With classification (or subclassing) to inform us about capabilities
* Server - Something you submit information to and get information from
* Location - Used for incidents (and maybe for the map)
    * Longitude
    * Latitude
    * Place name (optional)
* Time - Used for incidents

Time

* Constructors
     * Inputs: Year, month, day, hour, minutes, seconds ...
     * Inputs: Timezone, Year, month, day, hour, minutes, seconds ...
     * Inputs: NONE - Give the current time (now)
     * Inputs: Unix time model (milliseconds since "the beginning of time")
     * Inputs: Month and day (rest is implict or unnecessary)
     * Inputs: String description
* Observers
     * getMonth, getDay, getYear, getWeekday, ...
         * With and without timezone
     * getDifferenceBetween (another date)
* Mutators
     * Do we want to edit dates?

Incidents

* Constructors
* Observers
* Mutators