running mkgmap and mkgmapgui in Mint 19 Mate

[Catdaddy]

Friends,
Off and on for a couple of years I have been seeking an alternative way to update the maps on my Garmin 760DZL LMT. I don't have Windows (but I could) or a fast connection (which I can't get where I live). I found mkgmap and mkgmapgui, and both are in the Software Manager. I can start the mkgmap from the terminal, but it's complexity may be beyond my skill level. I REALLY need to use the mkgmapgui frontend. Unfortunately, I have been unable to get the jar file to open- and I have tried several fixes. The problem APPEARS to me to be in the version of Java installed by default in Mint 19 Mate. The LAST thing I want to do is fool around and crash my PC or render it unusable in some way. Currently installed is openjdk 10.0.2, openjdk runtime environment, and openjdk 64 bit server VM. When I try to open the mkgmapgui jar file with openjdk runtime 11, the terminal flashes up and then disappears. I don't know what else to do or add as info at this time. Does anyone know what I have to do to get this GUI to work? I have gone back to work trucking and my maps are nearly 4 years old. My only other option will be to find someone with Windows and a fast connection willing to do this- and it takes a LOOOONG time using Garmin's software- and not always effective, either. All ideas and help appreciated.