Setting things up

You need to set up your environment to use Java properly. Make sure javac and java are in your command path. Use the editor of your choice, but preferably XEmacs!

Create a directory javacourse (or some such) for this course.

Exercise 1 - Part A




/**
 * A class that prints out Hello World.
 *
 * @author (c) everyone who ever wrote a program, beginning of time
 * @version 1.0
 *
 **/

class HelloWorld
  {
  public static void main( String args[] )
    {
    System.out.println( "Hello World!" );
    }
  }

Save this as HelloWorld.java (case is important)

Java code has to finish with the extension .java, and each public class has to be stored in a seperate file of that name.

It is 'compiled' (to the virtual machine code in HelloWorld.class) by the command

javac filename.java

This has been mapped to the key sequence C-c C-v C-c in the Emacs editor (Pulldown menu JDE/Compile).

Run it in a command shell with

java HelloWorld

Alternatively, you can run it in Emacs using the the key sequence C-c C-v C-r (Pulldown menu JDE/RunApp).

Your first Java program!

Points to note:

comments are // for single line comments, or /* .......... */ for anything else
/** ..... */, @author and @version are comments specifically for the self-documentation system called javadoc. Get used to using it, and writing the descriptions of your classes as you go.
everything is part of a class
every program has to have a main, which is declared as public static void
semi-colons terminate line

Moving on...

All the Java classes are well-documented online, in the standard form generated by javadoc. Check it out here - you might like to bookmark this as you'll use it a lot.