CSC207.01 2014S, Class 09: Classes and Objects, Revisited
=========================================================

_Overview_

* Preliminaries.
    * Admin.
    * Questions.
* Writing fields.
* Writing constructors.
* Writing methods
* Standard methods.
* Lab!

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

### Admin

* Apologies if I'm discombobulated - I had code problems in 151 and 207 
  today.
* You should have received current grades from me.  Let me know if you
  have questions.
* Extra credit: 
    * Convo Feb. 5 at noon in JRC 101: Pussy Riot and Putin.
    * Wednesday Extra, February 5 at 4:30 in 3821, AppDev
    * CS Table Friday: NP Completeness.
    * More?
* New lab partners
* Mentor session tonight at 7:00 in 3821.
* Lots of people absent.  They should be sending me email to explain why.

### Upcoming Work

* Reading: Debugging
* Homework 3, Due Wednesday
* Today's writeup: Exercises 2 and 4
    * Due Wednesday.
    * Subject: CSC 207 Writeup 7: Classes (YOUR NAME(S))

### Questions on the homework

Writing classes
---------------

* A class is a template for making objects
* It includes methods
* It includes fields - the data
* It includes constructors - how to build new objects
* Each of these have a protection level
    * public - anyone
    * protected - class, package, subclass
    * nothing (package) - class and package
    * private - only this class
* In Java, we also have
    * static fields - shared by all objects
    * static methods - can be used without objects
    * constants
    * 
Writing fields
--------------

* A lot like variable declarations

   TYPE NAME;

* Convention: name starts with lowercase letter and uses camelCase
* Can have a protection level. - Usually package or private.
* Becomes usable in *every* object in the class - individual
* Tend to refer to them internally as
    * `this.fieldName` (in the current object)
    * `other.fieldName` (in another object)
* You can drop the `this.` and it is assumed.  But it's ambiguous

    public class Amby
    {
      int x;

      public int multiplyBy(int x)
      {
        return this.x*x;
      } 
    } // class Amby

Writing methods
---------------

* A lot like C

    RETURNTYPE NAME(PARAMS-WITH-TYPES)
    {
      CODE
    }
* Optional protection level
    * Sam's custom: Separate public and non-public methods in your class
* camelCase again
* Can refer to fields

Writing constructors
--------------------

* Look a whole lot like methods
* Name *must* match the name of the class.
* No return type
* No return call

  ClassName(PARAMS)
   {
   } // ClassName(Params)


Standard methods
----------------

* toString()
* equals()
* hashCode() - a number that represents the object
* compareTo()

Lab
---

We'll talk about exceptions on Wednesday.

Why doesn't Sam like

    public Fraction multiply(Fraction multiplicand)
    {
      int productNum = this.num * multiplicand.num;
      int productDenom = this.denom * multiplicand.denom;
      ...
    } // multiply