all android classes should be extended from Activity?

in android, should every class extends Activity? I have a class for reading file. and it does not extends from Activty class. Bu开发者_如何学JAVAt it does not work. So I want to learn all classes should be extended from Activity?

Only classes which are Activity-based UIs should extend from Activity. If your class for reading does not also have a UI to manage, as a stand-alone component, it isn't an Activity.