Why to use an empty abstract class instead of an interface?

Question

I use GWT and the Place & Activity mechanism.

It's really sad that Place is a class because my custom place can't extend another class.

When I look at the Place code, I see the following :

public abstract class Place {

  /**
   * The null place.
   */
  public static final Place NOWHERE = new Place() {
  };

}

Seeing that, the Place can be an interface. Is there a good reason that GWT team has chosen to make Place an abstract class instead of an interface ?

And to generalize : is there a good reason to create really empty abstract class vs interface ?

Answer 1

In general I would not define an empty abstract class

However, when I would expect some members in the future, I may decide to use abstract class instead of interface

"class" means: this IS A

"interface" means: this SUPPORTS

For "Place" I could really see both with a little intuitive preference for class

Answer 2

Is there a good reason that GWT team has chosen to make Place an abstract class instead of an interface ?

I can't think of one. But to be realistic, complaining about it on SO is not going to achieve anything.

And to generalize : is there a good reason to create really empty abstract class vs interface ?

Hypothetically, you might do this to ensure that there is a single common base class for future anticipated requirements ... or for some other reason.

But generally speaking it is a bad idea ... IMO.

(Of course, things like this often happen for historical reasons; e.g. the abstract class may have had more members in the past that have since been removed.)