Step 1 - Underscore means library-private

class Wallet {
  double _balance = 0;

  void deposit(double amount) {
    if (amount > 0) {
      _balance = _balance + amount;
    }
  }

  double get balance => _balance;
}

void main() {
  Wallet w = Wallet();
  w.deposit(10);
  print(w.balance);
}

• In Dart, privacy is per library (file), not per class.
• Names starting with _ are visible only inside the same .dart file.

Step 2 - Why use private fields

• You control how _balance changes (only through deposit, not random writes).
• You can change internals later without breaking callers if the public API stays stable.

Step 3 - Same file, two classes

class Engine {
  void start() {
    print('vroom');
  }
}

class Car {
  final Engine _engine = Engine();

  void drive() {
    _engine.start();
  }
}

void main() {
  Car c = Car();
  c.drive();
}

• Car uses Engine as an implementation detail; _engine is not part of the public surface of Car from outside this library.

Good habit

• Start public API small: methods and getters callers need, keep fields private when possible.

Practice tasks

• Make a SafeCounter with private _count, public increment(), and a read-only way to see the count.
• Try moving Wallet into a second file and call w._balance from main there; note the analyzer error and why it appears.