FreeJ2ME
A J2ME emulator for Windows, Linux, and Raspberry Pi
... is available from the menu (by pressing esc). Preferences are saved and will automatically load.
Games automatically scale with the window size, making it easy to play games designed for smaller screen sizes.
The Java portion can be built using Apache Ant with the build.xml file provided. The Libretro core can be built using the build.sh script. Installation instructions for the Libretro core for RetroPie are included.
Follow the development process here:
https://github.com/hex007/freej2me