Using a final class for string constants
There won't be any GC overhead since there won't be any instantiated instances to collect.

I think there's a small overhead by accessing a class' static field rather than an in-scope variable ; however, I would consider it largely worth the improved readability/maintainability.

I guess you could also argue you'll use up a little bit more of your permGen / Meta space, but I've never heard it be a consequent problem.