I have what is possibly a similar requirement, I would like to create code that allows be to code plugs as in the ProMidi midiIO.plug function.

The midiIO.plug function takes name of the function to be executed as a string.  What I dont seem to be able to find out is how to execute the value of a sting as though it was a function name.

Well, Eclipse comes to the mind: a big application, made of a base module and lot of plugins.
It is based on OSGi for the plugin architecture, but there are alternatives.
Not an easy task...

For something slightly more lightweight, you have to study the system of classloaders in Java.

I think I have asked the question the wrong way.

I want to writea function that allows me to specify that another function is called when a specific event happens.   Much in the way that you can code a function that is run when a mouse if clicked or a key pressed.

I my case I am wanting to have a specific function that is triggered when a specific slider on my midi based Korg Nano Controller is moved and generates a specific midi controller event.

What I would really like to do is hack the source code for ProMidi, but I cannot find it.

What I would really like to do is hack the source code for ProMidi, but I cannot find it.

The sources comes with the download zip, in promidi folder

I was answering the first message, actually.
richard_bd, your request is actually very different from the original one. You want callbacks, which are usually made with wrapping these functions in anonymous (or not) classes.