Why have you used Comparable? Its documentation says it implies a "natural ordering" which you don't have for employees. You have several possible orderings, eg by name, seniority, salary, ID number, all of which might be valid. Not one "natural" ordering.
So you want to pass a Comparator<Employee> which simply works out the difference between the ID numbers. If they are ordinary positiveints you can simply subtract the IDs to get a result.