flutter-examples/stateful_widget/lib/main.dart

import 'package:flutter/material.dart';

void main() {
  runApp(new MaterialApp(
    home: new MyButton(),
  ));
}

class MyButton extends StatefulWidget {
  @override
  MyButtonState createState() {
    return new MyButtonState();
  }
}

class MyButtonState extends State<MyButton> {
  int counter = 0;
  List<String> strings = ['Flutter', 'is', 'cool', "and","awesome!"];
  String displayedString = "Hello World!";

  void onPressOfButton() {
    setState(() {
      displayedString = strings[counter];
      counter = counter < 4 ? counter + 1 : 0;
    });
  }

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(
        title: new Text("Stateful Widget"),
        backgroundColor: Colors.green,
      ),
      body: new Container(
        child: new Center(
          child: new Column(
            mainAxisAlignment: MainAxisAlignment.center,
            children: <Widget>[
              new Text(displayedString, style: new TextStyle(fontSize: 40.0)),
              new Padding(padding: new EdgeInsets.all(10.0)),
              new RaisedButton(
                child: new Text(
                  "Press me",
                  style: new TextStyle(color: Colors.white),
                ),
                color: Colors.red,
                onPressed: onPressOfButton,
              )
            ],
          ),
        ),
      ),
    );
  }
}