How to quit android application programmatically

Question

I Found some codes for quit an Android application programatically. By calling any one of the following code in onDestroy() will it quit application entirely?

Answer 1

I think that application should be kill in some case. For example, there is an app can be used only after login. The login activity has two buttons, 'login' and 'cancel'. When you click 'cancel' button, it definitely means 'Terminate the app'. Nobody wants the app alive in the background. So I agree that some cases need to shut down the app.