Before the advent of smartphones, mobile phones were quite rudimentary and often relying on java applications as a primary programming language. This didn't keep games from being developed for these platforms. Casual simplistic games and rip-offs of retro franchises thrived, but it attracted some genuinely fun games that remained obscure.
The situation is quite different in Japan where mobile hardware was much more developed, only loosely Java-based, and major videogame developers were much more invested in creating unique and high-quality content that's mostly obscure and unpreserved, let alone emulated, today. Those are the very different Galapagos mobile phones (like DoCoMo i-mode, DeNa, RoID...). Some of these games got ported to the inferior Western hardware but these are in the tiny minority.
Earlier black-and-white cell phone games (both in Japan and worldwide) didn't get as much love either when it comes to emulation and preservation of game binaries. But jar files of Java-based non-Japanese cell phones can be still found online with some effort.
Java-based Mobile Phones / J2ME
- MidpX is one of the older emulators. Fixed low resolution (176x220) and compatibility, no handler app support.
- Sj-Boy-JavaEmulator is more compatible than MidpX. Can take snapshots. More resolutions (but still buggy).
- KEmulator has even more features and compatibility (even 3D emulation) than other ones. Has support for custom resolution and full screen (View/Options). You can even set a proxy server for mobile java apps that connect to the internet under options. Requires Java Runtime Environment installed. Is the optimal recommended solution. Last update is 2012, closed-source.
- SDKs for certain Nokia platforms e.g. Series 40 and S60 may still be available, and while the emulators that come with them are made with development in mind, they can also be useful for playing most Java games and Symbian applications.
- PSPKVM is available for cell-phones. Might be the only one that's open-source. Last update is 2009.
- Book mentioning Japanese i-mode emulators that are currently dead, like i-tool.