CS 100, Summer 2001
Wednesday, 7/18
Lecture 13
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Announcements: 
+ Assignment 4 due 7/19
+ Prelim 2 on 7/20
+ reading: Skip text file I/O: will do next week;
  instead, bring written question (will collect this time!)
+ Ben and Kiri's office hours moved to Thursday! (from Friday)
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Topics:
+ debugging, part 2
+ exceptions
+ arrays
+ matrices in MATLAB
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+ Assignment 4
  - Planet constructor should NOT check for valid input
    (it's a good thought, but what are you going to do if
     it isn't valid??)  Plus, NoPlanet is *supposed* to be invalid.
  - Planet: keeping track of where it is - I don't actually
    care how you do it (integers for x and y, vs. Position 
    variable, vs... something else?).  This is encapsulation!
    You're designing the class - do whatever you want.  BUT:
    1) do it with good style.  
    2) the INTERFACE shouldn't change (so SpaceTester.java
       should work no matter what you're doing internally)
  - Default constructor: you get to pick the default values 
    for a new Planet.
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+ Debugging, part 2
  - design first.  then code.  then debug.
    - "save now, suffer later"; "suffer now, save later"
    - design algorithms, then implement them
    - design classes, then implement them
  - Debugger on example (ArrayOfTemperatures.java)
    - see debugging guide on Java page off course website
    - debugger program window
    - class browser window
  - breakpoints (set, clear)
  - step over, into, out
  - this is a great tool! It's almost, but not quite, as good
    as having your own personal consultant.  Allows you to
    "peek" into the computer and watch what it's really doing. 
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+ Exceptions
  - way to handle unusual (invalid) cases
  - when something bad happens, the program can "throw" an exception
  - you can also provide code to "catch" the exception
    (handle it in a graceful way)
  EXAMPLE: ExceptionDemo.java
  - syntax:
      try
      {
        // some code
        throw new Exception("Message");
      }
      catch (Exception e)
      {
        // how to handle the exception
      }
  - note catch is NOT a method definition (more like an if-else)
  - analogy from Savitch: line of people tossing rocks with messages
  - flow of control:
    - if no exception thrown, catch block is ignored
    - if an exception is thrown, the rest of the try block is ignored
    - see p. 522 in Savitch

  - sometimes you'll explicitly "throw" the exception; sometimes
    you call methods which might throw exceptions, so you need
    to have a way to catch them.  You'll see this when we do
    File I/O.
  - EXAMPLE: PrimeNumbersSoln.java

  - list of useful exceptions?
  - useful Exception: ArrayOutOfBoundsException
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+ Arrays
  - Review: arrays are objects.  They store multiple items, all of
    the same type.
  - syntax for array declaration
      type[] name = new type[howMany];

  - EXAMPLE: ArrayOfTemperatures.java
  - EXAMPLE: the same thing in MATLAB: aot.m

- initializing arrays: for base types, you can assign values all at once:
    double[] myArray = {1.2, 34.6, 23.6};

is the same as
    double[] myArray = new double[3];
    myArray[0] = 1.2;
    myArray[1] = 34.6;
    myArray[2] = 23.6;

but much shorter.

- indexed array elements as method arguments
  Spaceship2000:    System.out.println(fleet[i]);

- arrays as method arguments
  Spaceship2000:    moveFormation(fleet, 3, 0);

- testing equality
  cannot use == on two arrays (just like with references)
  Instead, must check each element of the two arrays.

- methods that return arrays
  e.g., 
     Spaceship2000[] createFormation(int howMany, int x, int y)
  - what would this method look like?
  - need to create a new array of Spaceship2000's to return
  - then create each one, probably inside a loop
  - then return the new array
 
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PICK UP graded Exercise 7
hand in your Exercise 8!