CSC152 2004F, Class 19: Exceptions

Admin:
* About the need for convo.
  * Thursday's convo meets in Harris
  * Subject: Images of the body, particularly in tech. age
  * Warning: The last time I went to a convo on "images of the body inthe technological age" in Harris, at 12:15, the speake was still going strong
* Advance warning about week 7: Sam absent on Wednesday and Friday.  No class, no assignments.
* Questions on the exam.
* Homework due and assigned

Overview:
* Exceptions, concluded
  * Review
  * Throwing exceptions
  * Creating new exceptions
  * About the homework
* Background
  * Goals of course
  * The project

/Exam Stuff/

* Sam, can you show me a sample Comparator? 
  * See the rebelsky.newvec package, particularly CompareVectorsByRadius
* CompareStudentById and CompareStudentByName should implement Comparator.  They should not be subclasses of Student.
* Evil trick in Java.  Every class implicitly subclasses java.lang.Object (aka Object).  java.lang.Object provides default implementations of all the key methods.
  * toString: Default class@address
  * equals: Default: A lot like eq? in Scheme
  * hashCode: Default: address
  * clone: Default: I don't know
  * others we haven't learned yet
* Must we use exceptions on the exam?
  * No.
* Can we use exceptions on the exam?  If we do, will we earn extra credit?
  * Yes, you may use them.
  * Particularly nice uses may earn you extra credit.
* Detour:
  + Homework grading: 
     "Turned in and works correctly (and well documented)": Check 3
     "Turned in and doesn't work": Minus 2
     "Not turned in": Zero
     "Turned in and suprises me with its beauty": Plus 4
     Mostly checks: B
     Mostly minuses: C
     Mostly zeros: 0
     Mostly plusses: A
  + Exam grading:
     Start with 100
     Each error removes points
     Each "wow!" adds points
     Note: The 80 guaranteed is *after* the ec has been applied
* Documentation on the exam is *optional*

/Exceptions, concluded/
* Review
  * A mechanism for methods to say "I'm sorry, I can't do what you requested"
  * To warn in advance that a method may fail
  	public TYPE NAME(PARAMS)
		throws TYPEOFEXCEPTION,TYPEOFEXCEPTION
  * Lazy solution
  	public TYPE NAME(PARAMS)
		throws EXCEPTION
  * Methods that call exceptional methods have two choices
    * Deal with the problem
    	try {
		EXCEPTIONALMETHOD(PARAMS);
	}
	catch (Exception e) {
		DEALWITHTHEPROBLEM
	}
    * Deal with different kinds of problems differently
    	try {
		EXCEPTIONALMETHOD(PARAMS);
	}
	catch (ExceptionOne e1) {
		DEALWITHONEKINDOFPROBLEM
	}
	catch (ExceptionTwo e2) {
		DEALWITHANOTHERKINDOFPROBLEM
	}
    * Pass the problem on to their caller
    	Declare "throws Exception" in the method header
* Throwing exceptions
	throw new TYPEOFEXCEPTION(PARAMS)

	public int compareTo(Object other)
		throws WrongTypeException,BadDataException
	{
		if (!(other instanceof Vec2D)) {
			throw new WrongTypeException("I can only compare myself to vectors");
		}
		Vec2D ovec = (Vec2D) other;
		if (ovec.radius < 0) {
			throw new BadDataException("Vectors should not have negative radii");
		}
		... // code as before
	} // compareTo(Object)
* Safest strategy: Always use Exception, not specific types of exception
  * Easy
  * Loses expressivity

* Creating new exceptions
public class MyException
	extends Exception
{
	public MyException()
	{
		super();
	}
	public MyException(String note)
	{
		super(note);
	}
} // class MyException

* About the homework
 
/The Project/
* Three aspects to CSC152
  * How to use the equipment: Java [Mostly done]
  * How to design experiments: Programming [Throughout the semester]
  * Basic knowledge of the scientific domain: Algorithms, ADTs, and Data Structures [Starts soon]
* TO learn to program well, you need more than "toy" problems
  * Real errors/difficultlies more likely to occur
  * Communication is central to programming
  * Group work is central to programming
  * Many students learn better by grounding their theory in practice
* Sam's strategy for "real" programming: BIG projects that everyone contributes to
* We have to choose a project
* We have to choose groups
* Tomorrow we will do both