Java Coden, Day 2

Object Oriented Programming

sample code

The theory bits:

using object oriented programming typically doesn't gain you any optimizations, but (in theory) it does make it easier to model and build programs that better match the problems we're asked to solve with programming

In java:
CLASS = OBJECT
METHOD = FUNCTION

encapsulation
hiding data and functions
making the pieces of your code into a black box
allows you to go back and re-work that code without breaking things because you've (hopefully) hidden the details of how you actually did the work.
Anything that's public in your classes IS NOT encapsulated, so don't expect to be able to change it willy-nilly after you've started using it. (Or if you do, it will probably be a LOT more work). If you really want to encapsulate something, define it as private.
inheritance
sort of layers of an onion theory. You can tack on additional functionality or data to a class without having to change the original class.
polymorphism
the ability to redefine methods in derived classes to match those new classes
usually a pretty tricky undertaking unless you've thought about making this easy to do when you were building your original classes
a reasonable example is a printArticle method in our class Article. This allows us to print the artcile to System.out (standard out, where-ever that points). If we then created a new class, called BetterArticle, which contained some additional data (like maybe an author listing or such), then we might want to redefine printArticle to also print out the author listing with the article. Over-riding that method to add in the printing of the author listing is using polymorphism.

The practical bits:

When you define a variable (other than a primitive data type), it's a reference.
You can't access non-static methods or data from it until you've created the object.

Creating a referenceCreating an object
String wordlist;String wordlist = new String("one motorcycle harry book");
String wordlist = "one motorcycle harry book"; (implied NEW keyword)

Arrays are objects. Defining int[][] int_matrix = new int[100][100]; doesn't create those integers, it just creates a reference. If you want to use those integers, you have to explicity create them in the array:

for (int x = 0; x < 100; x++) {
for (int y = 0; y < 100; y++) {
int_matrix[x][y] = 0;
}
}

If you don't do this, you'll find yourself looking at a "Null Pointer Exception" - that's when you try to use an object that isn't there. You'll probably see a lot of these, it's a common and easy to make mistake.

Doin it in STYLE

Better yet, read the Ambysoft style guide.

CVS

Concurrent Versioning System: CVS

Network based RCCS system for version control on code. Does require some "set up" though. In our environment, we're using SSH as the transport mechanism. To utilize CVS you need to set a couple of environment variables.

In BASH,KSH,SH:


export CVS_RSH=/usr/local/bin/ssh
export CVSROOT=:ext:heckj@doc.missouri.edu:/usr/local/CVS

In TCSH,CSH:


setenv CVS_RSH /usr/local/bin/ssh
setenv CVSROOT :ext:heckj@doc.missouri.edu:/usr/local/CVS

A CVS Manual is available at http://www.loria.fr/~molli/cvs/doc/cvs_toc.html.

CVS Commands:

cvs --help-commands :
gives you a quick summary listed of cvs commands available to you.
cvs co JavaCourse :
checks out the JavaCourse "tree" from CVS. Builds the tree and it's appropriate filesets into the directory you're currently working in.
cvs status JavaCourse :
gives you the complete status of the CVS tree portion you've specified - in this case "JavaCourse".
cvs update JavaCourse :
updates all the files from the CVS "tree" for the Directory JavaCourse

Looking at the CVS tree from the Web