Week 2 Notes and Questions

 - What is a "class"

   - A way to describe the behaviour and properties of a concept for the
     purposes of programming.  In java, use "methods" or "functions" for the
     behaviour, and "fields", "members" or "attributes" for the properties.
     
   - Example: for a GUI's "window" class, what might some methods be/what
     might some properties be?

   - Classes relate to each other by calling each other's methods.  So it's
     important to decide what methods you need.

   - Usually, the properties of a class are available only to the class
     itself.  Why?  What are the advantages of making the properties of a
     class available to other classes?

   - The idea is that by thinking in terms of "classes" rather than
     "programs", you can design software in a modular reusable way


 - What is a Java "program"

   - java source files are compiled into bytecode by the "javac" command.
     Each class that the compiler finds will be given its own "class" file.

   - there is no "link" stage for Java - all the class files are left
     separate, but they do contain information about what other classes they
     reference. 

   - when the java compiler processes a source file that refers to another
     class, it will look for that class and make sure that (a) it exists, and
     (b) that it is being used properly.  If the class doesn't exist then the
     compiler will try and find the source file for it and compile that too
     (and so on).

   - java compiler is itself a java program.  Class files are the same
     whatever platform.

   - java classes are executed by the JVM.  JVM may be invoked:
     - by "java" command on the command line
     - by browser when running an applet
     - by servlet container (e.g. tomcat)

   - JVM is a "virtual machine".  So it can allow many classes to execute at
     once, independently of each other or communicating.  JVM is platform
     specific but appears exactly the same as far as java classes are
     concerned. 

   - Q: how does JVM know which bit of the class you want to execute?
     A: depends on what context it's being invoked, e.g. "main" or "service" 
     or "appletmain" or whatever.

   - The JVM uses "classpath" to find class files.  When you tell the JVM to
     execute a class, it uses "classpath" to find it, and then recursively
     loads all the other classes which are referenced, again using "classpath"
     to find each of them.  Classpath is like a VMS search list, or a UNIX or
     Windows "path" in that it contains a list of places to look; the main
     difference is that it can include "jar" files as well as directories


Questions for this week (same rules as last time, no asking questions):

- Find out how many classes get loaded when you run "helloworld"

- Produce some html documentation for a program you've written


- write a "hello world" program and another program which says "goodbye
  world".  Now make the "helloworld" program execute the "goodbye world"
  program.  I.e. you'll be able to do:

  % java goodbyeworld
  goodbye world
  % java helloworld
  hello world
  now I will run goodbye world
  goodbye world
  here I am back in hello world
  %

- sign up to the java developer connection, and find out from there three ways
  that you could count the number of words in a string of text.  Write a
  program that uses one of these ways to do this:

  % java countwords "this sentence has five words" "this one doesn't"
  You gave me 2 sentences
  Sentence 1 has 5 words
  Sentence 2 has 3 words