@Override
 public EmployeeDTO getEmployeeById(long id) {
   Optional<Employee> employee = Optional.ofNullable(employeeRepository.findOne(id));
   if (employee.isPresent()) {
     return mapEmployeeToEmployeeDtoObject(employee.get());
   } else {
     throw new IllegalArgumentException("there is no employee found with id " + id);
   }
 }