Thanks for comment. Actually my plan is not to create a good game, but rather to get myself used to jMonkey/Java3D.
That is not a good start... if you don't care about creating a good game, you won't put enough effort into it. Just porting Oolite to a different platform using the same language was a huge effort.
Don't just port Oolite to Java, that would miss a big opportunity to improve upon it. The biggest problem with Oolite is that the original Elite algorithms/ideas are hard-coded in the game engine and the making it pretty well unmoddable except for trival things like new ships/missles/extra planet here or there - ie more of the same.
If you're going to do that much work, redesign it so it is a bit more flexible.
Note this is not a criticism of Oolite - it meets it design goals perfectly. It just means it can't be anything else.