I am currently working on a legacy code in Java 1.6 and i found that they have a Constants Interface in the Utils package (Is a quite old Webapps) 
Well, some of the classes import the interface and some just implements it. While i am unable to make some major fixes in the project i would like to use the same solution for every classes that is using this class (almost everything in the projects calls for it).
The content of the class is just a few hundreds of Strings, some static and final and some just public.
Which option is the better approach for this sort of Constants file? extends, implements or just a basic import?