flutter How to set the default value for dropdown in dart file?

String dropdownvalue="ford";
DropdownButton(
        value: dropdownvalue,// here we will be giving the default value
        icon: const Icon(Icons.keyboard_arrow_down),
        items: items1.map((String items1) {
          return DropdownMenuItem(
            value: items1,
            child: Text(items1),
          );
        }).toList(),
        onChanged: (String? selectedValue) {
          setState(() {
            dropdownvalue = selectedValue!;
          });
        })

import 'package:flutter/foundation.dart';
import 'package:flutter/material.dart';
void main() {
runApp(MyApp());
}
class MyApp extends StatelessWidget {
@override
Widget build(BuildContext context) {
// TODO: implement build
return MaterialApp(
  title: 'Flutter DropDown Example',
  theme: ThemeData(primarySwatch: Colors.green),
  home: DropDownExample(),
);
}
}
class DropDownExample extends StatefulWidget {
@override
State<DropDownExample> createState() => SampleDropDown();
}
class SampleDropDown extends State<DropDownExample> {
String dropdownvalue = 'Ford';
var items1 = [
'Audi',
'BMW',
'Ford',
'Ferrari',
];
Widget build(BuildContext context) {
return Scaffold(
  appBar: AppBar(
    title: Text("DropDown Default Value Example"),
  ),
  body: Center(
      child: Column(mainAxisAlignment: MainAxisAlignment.center, children: [
    DropdownButton(
        value: dropdownvalue,
        icon: const Icon(Icons.keyboard_arrow_down),
        items: items1.map((String items1) {
          return DropdownMenuItem(
            value: items1,
            child: Text(items1),
          );
        }).toList(),
        onChanged: (String? selectedValue) {
          setState(() {
            dropdownvalue = selectedValue!;
          });
        })
  ])),
);
}
}