BeginnerFlutter Fundamentals

What is the difference between StatelessWidget and StatefulWidget?

Short Answer

A StatelessWidget is immutable and rebuilds only when its parent rebuilds; a StatefulWidget holds mutable state via a separate State object and can rebuild itself whenever that state changes.

A StatelessWidget describes part of the UI that depends only on the configuration passed into it and the BuildContext. Once built, it never changes on its own — it's recreated wholesale whenever its parent rebuilds it with new data.

A StatefulWidget is split into two classes: the widget itself (immutable, just configuration) and a State object that Flutter keeps alive across rebuilds. Calling setState() inside the State object marks it dirty and schedules a rebuild of just that subtree, without needing the parent to rebuild.

Code Example

class Counter extends StatefulWidget {
  const Counter({super.key});

  @override
  State<Counter> createState() => _CounterState();
}

class _CounterState extends State<Counter> {
  int _count = 0;

  void _increment() => setState(() => _count++);

  @override
  Widget build(BuildContext context) {
    return ElevatedButton(
      onPressed: _increment,
      child: Text('Count: $_count'),
    );
  }
}

Common Mistakes

×Calling setState() after the widget has been disposed (always guard with `mounted` checks in async callbacks).
×Putting state in a StatelessWidget by mutating a field directly — it won't trigger a rebuild.

Interview Tips

→Explain *why* the split exists: the State object survives widget rebuilds, which is how Flutter preserves things like scroll position and animation controllers.

What is the difference between StatelessWidget and StatefulWidget?

Related Questions

Explain the widget lifecycle in Flutter.

How do you avoid unnecessary widget rebuilds in Flutter?