Classes and validated state

The idea

A Dart class defines fields and operations for objects. Constructor parameters can initialize fields directly, and named constructors provide meaningful alternative creation paths. A name beginning with an underscore is private to its library, not exclusively to one class. Use a getter to expose a readable value and methods to control changes that must preserve a rule.

Class variables hold references, so assigning an object to another variable creates an alias. Validation should happen before mutation so a failed operation leaves a consistent state. A constructor can reject an invalid starting value with an exception. The same invariant must then remain protected by every later modifying method; constructor checks alone are not sufficient.

class Stock {
  int _count;
  Stock(this._count) {
    if (_count < 0) throw ArgumentError("negative count");
  }
  int get count => _count;
  bool take(int n) {
    if (n < 0 || n > _count) return false;
    _count -= n; return true;
  }
}
void main() {
  final stock = Stock(5);
  print(stock.take(2));
  print(stock.count);
}

true
3

The constructor establishes nonnegative stock. take checks the request before subtracting, so successful calls preserve that rule and rejected calls change nothing.