The point of an interface is to specify the public API. An interface has no state. Any variables that you create are really constants (so be careful about making mutable objects in interfaces).
Basically an interface says here are all of the methods that a class that implements it must support.
Because you are just specifying what methods have to be implemented there is no idea of state (no instance variables). If you want to require that every class has a certain variable you need to use an abstract class.
Finally, you should, generally speaking, not use public variables