The number one mistake you can make is creating a globally accessible class called with a generic name, like Constants. This simply gets littered with garbage and you lose all ability to figure out what portion of your system uses these constants.
Instead, constants should go into the class which "owns" them. Do you have a constant called TIMEOUT? It should probably go into your Communications() or Connection() class. MAX_BAD_LOGINS_PER_HOUR? Goes into User(). And so on and so forth.
The other possible use is Java .properties files when "constants" can be defined at run-time, but not easily user changeable. You can package these up in your .jars and reference them with the Class resourceLoader.
