This week's book giveaway is in the General Computing forum. We're giving away four copies of Arduino in Action and have Martin Evans, Joshua Noble, and Jordan Hochenbaum on-line! See this thread for details.
In my opinion, using an interface just to keep constants is an abomination. Yes, Sun did it in the class libraries, but they also made some other questionable choices, so that shouldn't count for much.
But Java 5 gave us "static import" of a class, which accomplishes much the same, and is IMO a much cleaner way than to implement an interface.