Encapsulates related code into a self-contained unit, exposing only a public API while hiding internals.

Dart has this built-in — every .dart file is a module, and _ prefix makes members private.

// auth_module.dart

// private — hidden from importers
String _secretKey = 'abc123';

String _hashPassword(String password) => 'hashed_$password';

// public API — visible to importers
class AuthModule {
  static bool login(String user, String password) {
    final hashed = _hashPassword(password);
    print('Logging in $user with key $_secretKey');
    return hashed.isNotEmpty;
  }

  static void logout() => print('Logged out');
}

// main.dart
import 'auth_module.dart';

void main() {
  AuthModule.login('alice', 'pass123'); // works
  AuthModule.logout();                   // works
  // _secretKey;      — compile error, private
  // _hashPassword(); — compile error, private
}