Question

I have the following code, partly in Java, partly in Scala:

Java:

public interface ISubject {
  public void a()
  //...
}

public class CSubject implements ISubject {
  public void a() { /*...*/ }
  public void b() { /*...*/ }
  //...
}

public abstract class AbstractTest {
  ISubject subject;
  //...

  public CSubject generateParticularSubject() {
    return new CSubject();
  }
}

Scala:

object Test extends AbstractTest {

  override val subject: CSubject = generateParticularSubject() /*1*/

  subject.b() /*2*/
}

The issue is: if the line marked ‘1’ is as it is in the code above, the compiler complains that overriding variable subject in class AbstractTest of type ISubject; value subject has incompatible type. If I remove the type annotaion : CSubject in line ‘1’ and the keywords override val, that error disappears but another one appears at line ‘2’: value b is not a member of ISubject.

I understand what these errors say and the way the compiler thinks getting these problems. What I do not understand, is how to get the behavior same as in Java, where implementing an interface member of a class, it is possible to initialize it with any kind of class that implements the interface. What is the way to do this in Scala?

UPDATE: Is it truth that it is impossible to realize such a construct in Scala? The reason why I want to do that is because AbstractTest contains methods that deal with ISubject side of the subject and the child classes who should override subject and define it as a narrower class instance, must implement methods specific to that class. At the same time I want AbstractTest to keep handling all that regards subject and can be handled while it is treated through it’s interface ISubject.

Answer 1

The Java field subject is mutable, therefore it cannot be overridden in Scala using a val. Also, being assignable, it wouldn’t be safe to vary it co-variantly.

You can fix your code by making subject an abstract method. That can be overridden covariantly with a val, as you are doing in your Scala code.

 abstract class AbstractTest {
   public abstract ISubject subject();

   //...

   public CSubject generateParticularSubject() {
     return new CSubject();
   }
 }