Why doesn't my jar file know about my classpath

Question

I have created a simple java program to illustrate my point. My java program is called ABC.java and this is what is in it:
import java.lang.String;
public class ABC {
     public static void main(String[] args) {
          String myClassPath;
          myClassPath = System.getProperty("java.class.path");
          System.out.println("Running ABC\n");
          System.out.println(myClassPath);
          System.out.println("\nFinished ABC");
If I run java ABC.class I get my classpath printed out, but if I create a jar file I do not get a classpath printed out.
I am asking this because I have written a java app that uses a class from a third part jar file that is in my class path. If I run the .class version the app works as it should. If I run the jar version of the app I get a NoClassDefFoundError for the class in the third party jar file.
Since the third party jar file may be in a different path on diferent machines I can not hard code the classpath in my manifest file.
If I use the -classpath or -cp switch when I run the jar file my demo app still does not print the classpath as it should.
I hope this is just a bonehead error on my part.
Thanks in advance.

Answer 1

This suggestion would work well if I only needed to support one OS.
The problem is that under Windows the third party JAR would be in a
subdirectory under "C:\Program Files" but under Linux and Solaris the JAR
file might be in a subdirectory under opt. My relative path would be different
for each OS. It would be great if I did not have to have a JAR for each OS.
One of the great things about Java was supposed to be that you could have
one application that ran under several OSes.
I forgot to mention that for me example above I was using JDK 1.5.0_09.