I creates a centralized and global repository. Yuk! Constants, as with any other elements, should be declared close to, and at the lowest, scope where they make sense.
I created a Constants class in my gui package and also a Messages class. The first one contains a bit of useful constants, the other one contains hard-coded captions, titles, error messages,... so it will be quiet easy to adapt i18n to the application (if necessary in a future release)