Check out example codes for "flutter stateful widget". It will help you in understanding the concepts better.

Code Example 1

class YellowBird extends StatefulWidget {
  const YellowBird({ Key key }) : super(key: key);

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

class _YellowBirdState extends State<YellowBird> {
  @override
  Widget build(BuildContext context) {
    return Container(color: const Color(0xFFFFE306));
  }
}

Code Example 2

class MyClass extends StatefulWidget {
	@override
    MyClassState createState() => new MyClassState();
}

class MyClassState extends State<MyClass> {
	//Widgets and other code here
}

Learn ReactJs, React Native from akashmittal.com