Android - WallpaperService why does my Engine have to be a inner class?

Question

Hey,

I'm working on a simple android live wallpaper, I'm following chapter 12 from Hello, Android as my guide.

The bare-bones of a wallpaper service looks like this:

public class MyWallpaper extends WallpaperService {

    private class MyEngine extends Engine {
    //...
    }        

    //...

}

According to the book MyEngine must be an inner class of MyWallpaper. I have no reason to dispute this, but the book offers no explanation as to why this must be so. I prefer not to use inner classes purely for stylistic/aesthetic reasons.

I was wondering if MyEngine actually has to be a private inner class and, if so, why?

Cheers,

Pete

Answer 1

You're supposed to do it this way because class Engine is nested within the abstract class WallpaperService. If you try to make it not nested, your IDE/compiler will tell you something like this:

No enclosing instance of type WallpaperService is accessible to invoke the super constructor. Must define a constructor and explicitly qualify its super constructor invocation with an instance of WallpaperService (e.g. x.super() where x is an instance of WallpaperService).

Which, loosely translated, means "you could do it that way, but it's going to end up uglier than if you just use the nested class."