If you looking at a java codebase that already stable and available, take a look at CoffeeMUD. CoffeeMUD is nice if your looking for an 'out-of-the-box ready to go' experience with a web-based game editor.
If you looking for a good project to get you started in using Java, I would recommend the JavaMUD project based through SourceForge.net. JavaMUD project is more inline for those using Java in a more OOB setting, allowing for a load/unload code environment to be utilized (like that of LPMuds).
CoffeeMUD:
JavaMUD:
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