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

Overview

Preliminaries

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 updated.
- Lessons 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.)

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)

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.

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?

Context:

Design Issues:

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