Question

If I have a simple marker interface (contains no methods) which contains constants used throughout my application, is there any difference between a class importing the interface and actually implementing the interface?

Interface:

public interface MyConstants {
    String constant1 = "constant1";
    String constant2 = "constant2";
}

Implementing:

public class MyClass implements MyConstants {
    public MyClass(){
        System.out.println(constant1);
    }
}

Importing:

import common.constants.MyConstants
public class MyClass {
    public MyClass(){
        System.out.println(MyConstants.constant1);
    }
}

Answer 1

Implement it would be an anti-pattern:

In the Java programming language, the constant interface pattern
describes the use of an interface solely to define constants, and
having classes implement that interface in order to achieve convenient
syntactic access to those constants. However, since constants are very
often merely an implementation detail, and the interfaces implemented
by a class are part of its exported API, this practice amounts to
putting implementations details into the API, which is considered
inappropriate. [1][2] In general, collecting system constants into
classes independent of behaviour, might create a poor object-oriented
design, because it is often a sign of low cohesion. It is for these
reasons that implementing constants interfaces is considered to be an
anti-pattern.

http://en.wikipedia.org/wiki/Constant_interface

And regarding marker interface you can check it here.