Davy,
Not every jar file is runnable.
Let's suppose that you create a set of utility classes that you would like to assemble and distribute as a jar file.
It is very probable that none of the classes contained in your jar file has a
main method. Maybe it wouldn't even make any sense to have any
main method in your classes.
Just to begin with, you cannot directly "run" a jar file if there is no
main method to be called in any of its classes. Even if a
main method exists, if you would like to run it this way:
java -jar myjarfile.jar
you would need to include a
Main-Class entry in the jar's manifest file.
In the case of Derby,
you should read its documentation, which explains how you are supposed to use it.