Question

In order to call some method just once in the app’s lifecycle, not each time
the app is launched, where should I place such method?

In onCreate() or somewhere else?

Answer 1

You can add it to onCreate() and only call the method if it hasn’t been initialized/called previously.

protected void onCreate(Bundle b) {
    if(shouldCall()) { // I know if the method has been called before
        callMethodJustOnce();
    }
}

If you are looking to call this method only once ever, I would take a look at most answers in here recommending using Preferences. But if you are talking about once per time the app is brought to life, this should be achieved in onCreate(), as this should only be called once the app is initialized and started.