Using MethodChannel to connect Kotlin or Java with Flutter.

methodchannel.dart

class _MyHomePageState extends State<MyHomePage> {
  static const platform = const MethodChannel('ourproject.sendstring');
  String receivedString = "";

  Future<void> callNativeFunction() async {
    String msg = "Hello from Flutter",data="";
    try {
      final String temp = await platform.invokeMethod('callSendStringFun',{"arg":msg});
      data = temp;
    } on PlatformException catch (e) {
      data = "Failed";
    }
    setState(() {
      receivedString = data;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("CallNative"),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            RaisedButton(
              onPressed: callNativeFunction, //callNativeFunction is called on button pressed
              child: Text("Call Native"),
            ),
            Text('$receivedString'),
          ],
        ),
      ),
    );
  }
}