What does Underscore “_” before variable name mean for Flutter

Question

with reference to the Flutter tutorial, I encountered an underscore, _.

I know that in Java, _ is used as a naming convention for a private

Answer 1

From the Dart guide

Unlike Java, Dart doesn’t have the keywords public, protected, and private. If an identifier starts with an underscore (_), it’s private to its library. For details, see Libraries and visibility.

Answer 2

It's not just a naming convention. Underscore fields, classes and methods will only be available in the .dart file where they are defined.

It is common practice to make the State implementation of a widget private, so that it can only be instantiated by the corresponding StatefulWidget:

class MyPage extends StatefulWidget {
  @override
  _MyPageState createState() => _MyPageState();
}

class _MyPageState extends State<MyPage> {
  @override
  Widget build(BuildContext context) {
    return Container();
  }
}