Java interfaces - parametric polymorphism

Question

In Java what's the "correct" way to implement an interface where the parameters for a method need parametric polymorphism?

For example, my interface contains:

public int addItem(Object dto);

The interface is implemented by various classes but in each the dto parameter is one of various strongly typed objects such as planeDTO, trainDTO or automobileDTO.

For example, in my planeDAO class:

public int addItem(planeDTO dto) { ... }

Do I simply implement with the dto parameter as Object and then cast to the appropriate type?

jjnguy · Accepted Answer · 2011-01-21 17:24:51Z

4

If the DTOs all inhrerit from a common superclass or implement a common interface you can do:

// DTO is the common superclass/subclass
public interface Addable<E extends DTO> {

    public int addItem(E dto);

}

And your specific implementations can do:

public class PlaneImpl implements Addable<planeDTO> {
    public int addItem(planeDTO dto) { ... }
}

Or, you can simply define your interface to take in the interface/superclass:

// DTO is the common superclass/subclass
public interface Addable {

    public int addItem(DTO dto);

}

Edit:

What you may need to do is the following:

Create interface -

interface AddDto<E> {
    public int addItem(E dto);
}

And implement it in your DAOs.

class planeDAO implements AddDto<planeDTO> {
    public int addItem(planeDTO dto) { ... }
}

edited Jan 21, 2011 at 17:24

answered Jan 21, 2011 at 14:45

jjnguy

139k54 gold badges298 silver badges328 bronze badges

Sign up to request clarification or add additional context in comments.

10 Comments

Bob Over a year ago

There's nothing in common between the DTOs, hence I wanted to use Object in the interface, but I still want type checking in the implementation.

jjnguy Over a year ago

@Bob, if those objects have no common type, then in order to validate type I think you will need a different class for each dto type. It would be very beneficial if you could give the dto's an interface and a few common methods.

Bob Over a year ago

Thanks Justin. I'm not sure what you mean "different class for each dto type" unless you meant different interface for each dto and dao combination? Each DTO holds very different information and I'd like to stick with the DAOs for actual functionality so I don't intend to add any methods to the DTOs. I guess the bottom line is I haven't missed something obvious and what I want isn't possible unless I come at things from a wildly different angle?

jjnguy Over a year ago

@Bob, well in order to use polymorphism like you specify in your question, yes you need to come at this from a very different angle. Polymorphism only works when you have a type hierarchy. You will need something entirely different if you are going to stick with objects that don't follow a specific interface or inherit some common superclass.

Bob Over a year ago

Thanks again, I genuinely do appreciate the feedback. I've played with this quite a bit and even if my DTOs were to inherit from a superclass I still cannot type a parameter as the superclass in the interface and type the same parameter as a child of this superclass in each respective implementation, I thought it would work but it doesn't. And after all, that's what I'm doing anyway except the superclass of my DTOs is Object. I welcome any further thoughts you have.

|

Aaron McIver · Accepted Answer · 2011-01-21 14:45:04Z

0

Why not use an interface which provides the functionality you need and not reference the concrete types?

public int addItem(ITransportationMode mode);

Where planeDTO, trainDTO and automobileDTO all implement ITransportationMode

answered Jan 21, 2011 at 14:45

Aaron McIver

24.8k5 gold badges62 silver badges91 bronze badges

1 Comment

Bob Over a year ago

Thanks Aaron. The DTOs don't have anything in common (I chose them as examples for simplicity only) so ITransportationMode would literally be empty and in the implementation I wouldn't have type checking (assuming I type as ITransportationMode in my implementation too).

Esko Luontola · Accepted Answer · 2011-01-21 14:47:03Z

0

Are you trying to reach for something like double dispatch? What is the behaviour which varies depending on the type of the argument?

answered Jan 21, 2011 at 14:47

Esko Luontola

73.8k17 gold badges119 silver badges129 bronze badges

1 Comment

Bob Over a year ago

I guess it's akin to double dispatch. The DTOs each have their own SQL inserts inside their respective DAO addItem implementation.

Karthik Ramachandran · Accepted Answer · 2011-01-21 14:56:44Z

0

You could also use generic methods:

public interface Addable{
   public <T extends DTO> int addItem(T dto){}
}

You can read more about generic methods at : http://download.oracle.com/javase/tutorial/extra/generics/methods.html

answered Jan 21, 2011 at 14:56

Karthik Ramachandran

12.2k10 gold badges49 silver badges53 bronze badges

Collectives™ on Stack Overflow

Java interfaces - parametric polymorphism

4 Answers 4

10 Comments

1 Comment

1 Comment

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

10 Comments

1 Comment

1 Comment

Comments

Your Answer

Sign up or log in

Post as a guest

Related