What does Underscore “_” before variable name mean

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 variable.

1. Does it also apply to Flutter? Noting that there is no public/protected in Flutter.
2. Will the _ really be private (inaccessible by other classes) or is it just a naming convention?

Variable

 class RandomWordsState extends State<RandomWords> {
  final List<WordPair> _suggestions = <WordPair>[];
  final Set<WordPair> _saved = new Set<WordPair>();
  final TextStyle _biggerFont = const TextStyle(fontSize: 18.0);
  ...
}

3. Does the _ make the Widget private too? In this case, wouldn't the main class be unable to assess the Widget?

Widget

Widget _buildRow(WordPair pair) {
  final bool alreadySaved = _saved.contains(pair);  // Add this line.
  ...
}

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();
  }
}