Sorry, my fault. It seems in java you can declare constants in interfaces, but this is frowned upon.

  Interfaces cannot require instance variables to be defined -- only methods.
  (Variables can be defined in interfaces, but they do not behave as might be expected: they are treated as final static.)

(source: https://stackoverflow.com/questions/7311274/attributes-membe...)

(Another useful link explaining that statements resembling instance variables in interfaces are in fact constants: https://coderanch.com/t/178630/certification/Instance-variab...)