Use Of polymorphism?

Question

Employee Class

public class Employee {
protected String name;
protected String jobsheetnumber;

public Employee(String n,String j){
this.name = n;
this.jobsheetnumber = j;
}
public Employee(String name)
{
this.name = name;
}

public String getName() {
    return name;
}

public String getJobsheetnumber() {
    return jobsheetnumber;
}

public void setName(String name) {
    this.name = name;
}

public void setJobsheetnumber(String jobsheetnumber) {
    this.jobsheetnumber = jobsheetnumber;
}



}

Mechanic Class

public class Mechanic extends Employee{


public Mechanic(String name,String jobsheetnumber){
super(name,jobsheetnumber);
}
}

Supervisor Class

public class Supervisor extends Employee{


public Supervisor(String name){
super(name);
}
}

Company Class [snippet]

public class Company {
private String companyname;
private String companyaddress;
private String postalcode;
private String city;
private String country;
private String telephonenumber;
private String faxnumber;
private String province;
private Employee supervisor;
private Employee mechanic;




public Company(String companyname,String companyaddress,String postalcode,String city,String country,String telephonenumber,String faxnumber,String province,String supervisorname,String jobsheetnumber,String mechanicname)
{
 this.companyname = companyname;
 this.companyaddress=companyaddress;
 this.postalcode = postalcode;
 this.city=city;
 this.country=country;
 this.telephonenumber=telephonenumber;
this.faxnumber=faxnumber;
this.province=province;
supervisor = new Supervisor(supervisorname);
mechanic = new Mechanic(mechanicname,jobsheetnumber);




}

Employee Class is the superclass of both Mechanic and Supervisor class .. right now i am using the attributes of Employee i.e name and jobsheetnumber in the subclasses Mechanic and Supervisor Class

the code works fine .. but what if i want to add extended functionality in Mechanic and Supervisor ? then i cannot access those variables because the reference is to the Employee type object.

is this the correct use Of Polymorphism ? that we have to use super() as the constructor each time we create a reference of Supervisor/Mechanic Object ? can we not use extended functionality inside Supervisor and Mechanic Class ?

Answer 1

If you want to call a superclass non-zero-arg constructor then yes, you have to call it explicitly. If the superclass constructor has no arguments then the call will be inserted for you.

The point of polymorphism is so that objects can take care of themselves without having to have the rest of the program micro-manage them, so the outside program refers to them by a reference with the type of an interface or superclass without having to know the exact concrete type. For instance all employees might have a work method, where that work takes a different form for a supervisor than it does for a mechanic; the work method would be overridden by the specific subclasses, and might call the specific methods on the subclasses. So the company can iterate through all the employees and call work on each of them, while work is defined differently for different subclasses of employee.

(In practice using subclasses to describe roles is too inflexible to work, since an employee could have multiple roles, or those roles can change over time. It's usually better to use composition, here assigning Role objects to an Employee.)

Answer 2

A better use of polymorphism would be the same interface (methods) for different implementations. So you can decide which implementation will be used in runtime.

To explain my point i will give a example using your classes.

public class Employee{
    public void work(int hours){ doNothing();}      
}
public class Supervisor extends Employee{
    private Object pen; 
    private Object note;
    @Override
    public void work(int hours){
        observations = superviseWorkers();
        note.write(observations, pen);

    } 

}
public class Mechanic extends Employee{
    private Tool tool;
    private TaskBoard taskBoard;
    @Override
    public void work(int hours){ 
        task = taskBoard.getPendent()
        if(task.canSolveWithTool(tool))
        {
            solveTask(task, tool)
        }
    } 
}

Using example:

employees = new List<Employee>();
employees.add(new Supervisor("foo"));
employees.add(new Mechanic("bar"));

foreach(employee in employees){
   //you don't need to know which kind of employee you are treating here because you are only calling a behavior that all employees have.
   employee.work(8);
}

If in many places in your code you are trying to figure out which object you are dealing with probably you are doing it wrong.

I used your classes in my examples to facilitate your understanding but as Nathan Hughes suggested in this case would be better to use composition instead of inheritance.

Answer 3

I will handle above scenario in two ways.

Solution 1: ( Interface as roles)

1. You can have "state" in Employee object and you can implement role as interface.
2. Employee will have all common attributes & methods. You can override base class method like doWork() in respective Employee implementations.

3. You can add specific behaviour of Mechanic, Supvervisor with use of interfaces.

   public interface ISupervise{
        public void doSupervise();
   }
   
   public class Supervisor extends Employee implements ISupervise{
       public void doSupervise(){
   
       }
   } 
   
   public interface IMechanic{
       public void doMechanicWork();
   }
   
   public class Mechanic extends Employee implements IMechanic{
       public void doMechanicWork(){
   
       }
   }
   

Solution 2: (Decorate role)

Implement Decorator pattern for Employee to play multiple roles. Mechanic and Supervisor will decorate Employee behaviour. Refer to this example for better understanding of Decorator pattern.

Sample code can be found @

When to Use the Decorator Pattern?