New to Java

Hello again. I've made some progress since my last post ("package ultra basics") but I still have a few questions unresolved.
To reiterate I have code as follows located in the following directory on my Linux system;
/home/adunkin/myJava/world

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

1. To achieve my goal and compile this from anywhere, I have determined that I must use the following command. However why do I have to enter the path twice here? The java command does not require this path repetition.
$ javac -classpath /home/adunkin/myJava/world /home/adunkin/myJava/world/HelloWorld.java

2. However, the following 5 versions also work and this has me baffled. It seems that for the first of these path entries required on the command line, I can enter any fragment of the path to the source code file immediately after the -classpath option. Why do I not have to enter the full path after the -classpath option?

$ javac -classpath /home/adunkin /home/adunkin/myJava/world/HelloWorld.java
$ javac -classpath /myJava /home/adunkin/myJava/world/HelloWorld.java
$ javac -classpath /home /home/adunkin/myJava/world/HelloWorld.java
$ javac -classpath / /home/adunkin/myJava/world/HelloWorld.java
$ javac -classpath /adunkin /home/adunkin/myJava/world/HelloWorld.java

3. However, leave a fragment of the path out before entering the path to the source code file and it does not work. Why not?
$ javac -classpath /home/adunkin/myJava/world/HelloWorld.java
javac: no source files
Usage: javac <options> <source files>

4. This also does not work but I thought it should as it mimics the way the java command is used. Why not?
$ javac -classpath /home/adunkin/myJava/world HelloWorld.java
error: cannot read: HelloWorld.java
1 error

5. The fact that 4. does not work has me confused as the following works;
$ java -classpath /home/adunkin/myJava world.HelloWorld
Hello (cruel) World

It seems that javac commands must be in the form;
$ javac -classpath <any fragment of the path to the source code file> <full path to the source code file>
Whereas java commands take the form;
$ java -classpath <full path to the parent directory of the package containing the class file> packageName.className
Are these two usage statements correct?

Any wise words to help me clear up these problems would be greatly appreciated.
I have studied;

http://java.sun.com/j2se/1.5.0/docs/tooldocs/solaris/javac.html

http://java.sun.com/j2se/1.5.0/docs/tooldocs/solaris/java.html

but could not solve my problems using these documents.