Joomla 3 Fatal error: Class cannot extend from interface JController

One of the errors triggered by the usage of extensions created for previous Joomla versions looks like this:

Joomla Fatal error: Class classname cannot extend from interface JController

The reason: It looks like the interface JController has been removed, and now you have to specify which controller you're using.

So, the following code

$controller = JController::getInstance('classname');

will become

$controller = JControllerAdmin::getInstance('classname');

Also, there is one thing you should change:

class ClassnameController extends JController
{
}

will become

class ClassnameController extends JControllerAdmin
{
}