Stateless para Stateful

Apenas converti para StatefulWidget e funcionou, não entendi como posso usar o setState().

class ContactList extends StatefulWidget {


  @override
  _ContactListState createState() => _ContactListState();
}

class _ContactListState extends State<ContactList> {
  @override
  Widget build(BuildContext context) {
    //contacts.add(Contact(0, 'alex', 1000)); teste para aparecer na lista
    return Scaffold(
      appBar: AppBar(
        title: Text('Contacts'),
      ),
      body: FutureBuilder<List<Contact>>(
        // não precisamos de initialData se tratarmos o erro de 'null'!
        initialData: List(),
        future: findAll(),
        // tratando o null
        //Future.delayed(Duration(seconds: 1)).then((value) => findAll()),
        builder: (context, snapshot) {
          switch (snapshot.connectionState) {
            case ConnectionState.none:
              break;
            case ConnectionState.waiting:
              return Center(
                child: Column(
                  mainAxisAlignment: MainAxisAlignment.center,
                  crossAxisAlignment: CrossAxisAlignment.center,
                  children: <Widget>[
                    CircularProgressIndicator(),
                    Text('Loading')
                  ],
                ),
              );
              break;
            case ConnectionState.active:
              break;
            case ConnectionState.done:
              final List<Contact> contacts = snapshot.data;
              return ListView.builder(
                itemBuilder: (context, index) {
                  final Contact contact = contacts[index];
                  return _ContactItem(contact);
                },
                itemCount: contacts.length,
              );
              break;
          }

          return Text('Unknown error');
        },
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: () {
          Navigator.of(context).push(
            MaterialPageRoute(
              builder: (context) => ContactForm(),
            ),
          );
//              .then(
//                (newContact) => debugPrint(newContact.toString()),
//              );
        },
        child: Icon(
          Icons.add,
        ),
      ),
    );
  }
}

class _ContactItem extends StatelessWidget {
  final Contact contact;

  _ContactItem(this.contact);

  @override
  Widget build(BuildContext context) {
    // TODO: implement build
    return Card(
      child: ListTile(
        title: Text(
          contact.name,
          style: TextStyle(
            fontSize: 24.0,
          ),
        ),
        subtitle: Text(
          contact.accountNumber.toString(),
          style: TextStyle(
            fontSize: 16.0,
          ),
        ),
      ),
    );
  }
}