A relative path like "../MyJar.jar" has to be relative to something. If you give it to a File object constructor, it will be relative to the current user home directory which you can find in
System Properties. To make it relative to something else, use a File constructor with base path or file and relative path. To make it relative to one of the paths in the Classpath, you can get the classpath, split out all the paths in it, and try each one.
For the classpath, look in System.properties again, or maybe look in a ClassLoader. For the splitting, see String.split().
Show us what you make! If you're still stuck, at least we'll be able to see just where you're stuck.