Colons
used to be file separators for Mac OS. The new OS X uses the slash as a file separator (like other Unices), but I think it may still allow the colon as a file separator, for Classic and Carbon stuff, I'm not sure.
So, yeah, it's a file separator. What are the legal characters in a filename in
Java? I mean, Unix allows everything except slash. Java must be fairly strict. If they're trying to avoid all platform-specific file separators, that means no /, \, :, and > at least. I'm sure there are probably others, if we dig through enough obscure OSes.