How to avoid calling implements at 2 activites?
<p>I have two activities: <code>MainActivity extends ListActivity</code> and <code>Preferences implements BillingController.IConfiguration</code>.</p>

<p>Now I need to call <code>BillingController.someMethod(this)</code> at <code>MainActivity</code>. Such call can be made only if <code>MainActivity implements BillingController.IConfiguration</code>. But this is already implemented at <code>Preferences</code> class. What can I do to avoid implementing that again?</p>
