main.dart

class Calculator {
  double _lastResult = 0;

  double subtract(double a, [double b]) {
    if (b != null) {
      _lastResult = a - b;
    } else {
      _lastResult -= a;
    }
    return _lastResult;
  }
}

void main() {
  var calc = Calculator();

  print(calc.subtract(5, 2));   //  5 - 2 = 3
  print(calc.subtract(1));      //  3 - 1 = 2
  print(calc.subtract(10, 9));  // 10 - 9 = 1
  print(calc.subtract(1));      //  1 - 1 = 0
}