EmployeeService

public class EmployeeService {

  private final EmployeeDao employeeDao;

  public EmployeeService(final EmployeeDao employeeDao) {
    this.employeeDao = employeeDao;
  }

  public String findManagerName(final Long employeeId) {
    if (null == employeeId) {
      throw new IllegalArgumentException("employeeId must not be null");
    }

    return employeeDao.findManager(employeeId).getName();
  }
}