You cannot have a
final method declared in an interface. Fields are always
final but methods are always
abstract (and never
final). You cannot define an interface method that is to be implemented only by classes in the same package.* From section 9.3 of the Java Language Specification:
Every field declaration in the body of an interface is implicitly public, static, and final. It is permitted to redundantly specify any or all of these modifiers for such fields.
and from section 9.4:
Every method declaration in the body of an interface is implicitly public (§6.6).
Every method declaration in the body of an interface is implicitly abstract, so its body is always represented by a semicolon, not a block.
It is permitted, but discouraged as a matter of style, to redundantly specify the public and/or abstract modifier for a method declared in an interface.
* As Paul Bellora points out in a comment, you can make the interface itself package-private (or protected, or even private) if you want to restrict its visibility.