Question

I am trying to create a set up where a set of subclasses override a superclass. This superclass contains an abstract method – the return type of which would ideally be that of the object from which this method was called, such that it effectively behaves like this:

public abstract class SuperClass{
  public abstract SuperClass getSelf();
}

public class SubClass extends SuperClass{
  @Override
  public SubClass getSelf(){
    return this;
  }
}

I am unsure if such a thing is possible, as I think return types always have to be the same in order for the override to work – however I have been thinking the answer, should one exist, lies somewhere along this line…

public abstract class SuperClass{
  public abstract <? extends SuperClass> getSelf();
}

public class SubClass extends SuperClass{
  @Override
  public SubClass getSelf(){
    return this;
  }
}

Thanks for any help.

edit: added extends SuperClass to SubClass, duh

Answer 1

This will work:

public abstract class SuperClass{
  public abstract SuperClass getSelf();
}

public class SubClass extends SuperClass{
  @Override
  public SubClass getSelf(){
    return this;
  }
}

Notice I’ve added extends SuperClass to your SubClass definition. The return type of getSelf is referred to as a covariant return type.