This code created for slcoder's tutorial 'Different between StatelessWidget and StatefulWidget'. you can find the gist for StatelessWidget here: https://gist.github.com/Blasanka/e637bf5a519c02a4e2b84729b27540bf.

statefulwidget_example.dart

// this example is created by slcoder to explain statefulWidget in slcoder's tutorials.

import 'package:flutter/material.dart';

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

int _count = 0;

class MyApp extends StatefulWidget {
  _HomePage createState() => new _HomePage();
}

class _HomePage extends State<MyApp> {
  void _increase() {
    setState(() => _count++);
}
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'Hello World App',
      home: new Scaffold(
        appBar: new AppBar(
          title: new Text('StatelessWidget'),
          backgroundColor: new Color(0xFF8B1122),
        ),
        body: new Center(
          child: new Column(
            mainAxisAlignment: MainAxisAlignment.center,
            children: <Widget>[
              new Text(
                "Up Votes : $_count",
                style: new TextStyle(
                  fontSize: 25.0,
                  color: new Color(0xFF8B1122),
                  fontWeight: FontWeight.w600,
                ),
              ),
              new RaisedButton(
                child: new Text('Vote up', style: new TextStyle(fontSize: 20.0,)),
                color: new Color(0xFF8B1122),
                textColor: Colors.white,
                padding: const EdgeInsets.symmetric(vertical: 10.0, horizontal: 40.0),
                elevation: 10.0,
                splashColor: Colors.white70,
                onPressed: _increase,
              ),
            ],
          ),
        ),
      ),
    );
  }
}