It‘s an annotation, but the correct name is NonNull:
protected void onSaveInstanceState(@NonNull Bundle outState)
(And also)
import android.support.annotation.NoNNull;
The purpose is to allow the compiler to warn when certain assumptions are being violated (such as a parameter of a method that should always have a value, as in this particular case, although there are others). From the Support Annotations documentation:
The @NonNull annotation can be used to indicate that a given parameter can not be null.
If a local variable is known to be null (for example because some earlier code checked whether it was null), and you pass that as a parameter to a method where that parameter is marked as @NonNull, the IDE will warn you that you have a potential crash.
