Question

I have a project (in Eclipse) which I’ve turned into an Android Project Library so as to re-use some of the code in another similar project. I think I’ve shot myself in the foot however as I’m getting the error:

Unable to start activity ComponentInfo{com.test.scroller1/com.lib.scrolltest.ScrollTestActivity}: java.lang.ClassCastException: android.app.Application cannot be cast to com.lib.scrolltest.resAppVars

com.lib.scrolltest is my Project Library which instantiates a class extending Application (resAppVars). In the onCreate() method I call:

mRav = (resAppVars) getApplicationContext ();

This way, I can use methods in the mRav object which would otherwise be a lot of duplicated code in other classes (such as passing a query to a generic select statement which returns an ArrayList of results).

What’s the problem here? It seems I’ve hit a limitation in the way I’ve implemented the Application class.

Answer 1

Calling getApplicationContext() returns the Application object for the current application (i.e. the application that owns the activity that onCreate() is running inside of).

Unless you’re doing something strange, you don’t get to pick which Application class is used. There’s even a note in the documentation for Application saying not to do this:

There is normally no need to subclass Application. In most situation,
static singletons can provide the same functionality in a more modular
way. If your singleton needs a global context (for example to register
broadcast receivers), the function to retrieve it can be given a
Context which internally uses Context.getApplicationContext() when
first constructing the singleton.

You should just create a regular shared class inside of your library project. Or if you don’t have a need for the special functionality library projects offer, you can also just use a regular .jar file.

And if you need shared state, just make it a singleton. 😉