Why to use an empty abstract class instead of an interface?
abstract-class, gwt, interface, java
Solution
I can't really tell for the `Place` (though I have a few ideas, see below), but it was discussed for the `Activity`: https://groups.google.com/d/topic/google-web-toolkit-contributors/V8rhZHiXFRk/discussion
As far as we could go in history, `Place` has always been an abstract class in GWT (and FWIW, the `NOWHERE` place was added long after; note that this commit references a Wave –soon to disappear from the internet– where we can see `interface Place` so it was an interface at some point in time while they designed the API).
Given that the `PlaceHistoryGenerator` (used when you `GWT.create()` a `PlaceHistoryMapper`) looks at the places hierarchy, having an abstract class cuts down a whole lot of edge cases! Imagine your `PlaceHistoryMapper` references a `PlaceTokenizers<Foo>` and `PlaceTokenizer<Bar>` and you have a `class FooBar implements Foo, Bar { }`, which tokenizer should be used? If you don't reference the `FooBar` class explicitly in your `PlaceHistoryMapper`, the generator won't see it (or rather won't look at it), so which kind of code should it generate? And keep in mind that we all want determinism, so the generated code should always be the same. Using a class, the generator can order them by their inheritance tree (from the most specific –most derived– to the least specific), and can safely assume that 2 places whose classes have no particular inheritance relationship are totally distinct, so they can be checked (`instanceof` in the generated code) in any order and still provide a stable result ⇒ determinism.
Disclaimer: I'm the one who reported the ordering issue and then provided the patch, but `Place` was already a class.
Problem
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 ?