Well if you don't mix gui stuff with non-gui stuff, then you will have to change only a small part of your application.
Java applications use Swing and Android has its own components that do similar things.
There is not a one to one translation for the components but some are straight forward enough .
In general Swing is more advanced than Android SDK , so if you master it you should be able to convert your app relatively easily.
I find the layouts in android to be somewhat short, and some important components such as JTable are lacking (maybe a tablelayout will do).
I am a _beginner_ in Android myself so here are some difficulties I encountered :
- layouts in general
- tabs (jtabbedpane is easier to handle)
- combo-box (maybe Editbox + Spinner)
- Jpanel (maybe be a FrameLayout will do).