@EJB(jndi="java:global/moduleName/ConcreteClassName:fullyQualifiedInterfaceName")
ServiceI service;

public abstract class Employee{
    String name;
    SalaryCalcService salaryService;
}

public interface SalaryCalcService {
  public double abstract calculateSalary();
}

public class PerMonthCalc implements SalaryCalcService {
  public double calculateSalary(Employee emp){
     //return something relevant....
  }
}

public class PerHourCalc implements SalaryCalcService {
  public double calculateSalary(Employee emp){
     //return something relevant....
  }
}

SalaryCalcService perMonth = new PerMonthCalc();
SalaryCalcService perHour = new PerHourCalc();

Employee emp1 = new Employee();
emp1.serSalaryService(perMonth);
 //...

for(Employee emp : employees){
   double salary = emp.getSalaryService().calculateSalary();
}