It could use the same interface as the game, but you'd just start with a blank slate on a very small "mini" world, where you had free reign to place and move any objects you want, and change the floor tiles, etc. etc. Perhaps a cave mode could be included too without too much trouble? I've never really looked at the source code, but it seems like this should be possible.
(Basically, what i'm trying to politely say is that i'd appreciate it if someone did this, because it's been about ten years since i last used java and I don't think i'm up to writing something like this myself
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