Right, creating separate projects and adding the common one in the classpath sounds good (and is probably what Cas meant too, but with a twice of experience, which is very nice
So, to put it in a simple example that I can comprehend with my limited OO knowledge
Say that I have a project "Common" with a method called "build" and now I want game A (in project "A") to have a couple of extra checks (not only can you not build where there is already a block, but you can only build on your own land). I would then extend the Block class in project Common and overwrite the method "build" and add another check (if (getOwner(p)==you) before calling the build method in the superclass which does the original tests. I'd then overwrite all of the block classes (each block type is one class) and make them extend the new version instead of the copied version, and overwrite the method that add blocks, so it adds the new block types instead of the original ones... starting to sound messy
Does that sound about right? It sounds like a lot of maintenance, but I think that it will make it flexible enough and make fixing bugs in the common project automatically fix them in both versions.