messed up application menu in LXDE / changing default app

[bennypr0fane]

Hello,
My primary concern here is setting the default application for handling .jar files to OpenJDK, but the problem I'm having with that *may* be related to the fact that my application menu is a mess.
Specifically, I installed LXDE and made it my default desktop. Everything's normal, except in the menu there's an entry called "Other", and it's about 3 screens tall and contains about 130 applications, including ALL the GUI ones that are already in the other sub-menus, plus many command line apps.

This hasn't been a big issue, since I've still been able to find what I'm looking for.
However, when I tried to open a .jar file, it was linked to Eclipse (Java IDE) as the default application. I want to change that to OpenJDK, but it's nowhere to be found in the menus. It's definitely installed though, I checked in Synaptic (and otherwise the .jar file wouldn't be able to run in eclipse either, right?).
1.) Is there a CL tool for assigning default applications to file extensions? What command would I have to use for this specific binding (bcs I'm not very good at making the glyphs in your standard man page into a functioning command, I always get it wrong...)?
2.) What can I do about that mess in the menus? Why are there duplicates for most stuff, but also something is obviously missing?
Thanks, Ben