Should Model Objects Have Interfaces?

Question

I am creating the domain model in my system. When designing my model objects, should I make interfaces for each entity object? People have told me that our web tier should not care about the implementation of an entity and we should be able to swap out implementations, but I'm not sure that would ever happen.

For example, if we have a Teacher class that maintains a List of Students, the getStudents method could either be:

public List<Student> getStudents() {
      return this.students;
}

or this:

public List<Student> getStudents() { 
     return someExternalService.retrieveStudents();
}

I understand this benefit, but what is the general practice?

Answer 1

Unless you have good reason to think you will need to swap-out the model I wouldn't worry about it yet. Based on the YAGNI principle (You ain't gonna need it)

Answer 2

None of the projects I've ever seen or participated in had interfaces for domain entities.

But in one of my projects I had interface NamedEntity:

public interface NamedEntity {
   int getId ();
   String getName ();
}

All domain entities implemented this interface. It gave me a possibility not to create different jsf converters for different entities but create one converter which used this interface instead of concrete domain classes.