how to show dropdownbutton in flutter

dropdownbutton.dart

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatefulWidget {

  @override
  State<StatefulWidget> createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  static List<String> menuItems = <String>[
    'One',
    'Two',
    'Three',
    'Four',
  ];
  final List<DropdownMenuItem<String>> _dropDownMenuItems = menuItems
      .map(
        (String value) => DropdownMenuItem<String>(
              value: value,
              child: Text(value),
            ),
      )
      .toList();
  String _btn1SelectedVal = 'One';

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        body: ListView(
          children: <Widget>[
            ListTile(
              title: Text('DropDownButton with default:'),
              trailing: DropdownButton<String>(
                // Must be one of items.value.
                value: _btn1SelectedVal,
                onChanged: (String newValue) {
                  setState(() {
                    _btn1SelectedVal = newValue;
                  });
                },
                items: this._dropDownMenuItems,
              ),
            ),
          ],
        ),
      ),
    );
  }
}