You have several options when defining methods on classes. First, let's look at basic examples of each.

class Microwave {
  // basic method  
  void startCooking (){}
  // private method
  // this cannot be called from outside the class itself
  // it is not exposed to _instances_ of the class
  void _startTimer() {}
  // a static method isn't tied to an instance
  // rather, it's logically tied to a class. 
  // static methods are compile-time constant, 
  // so you don't have access to 'this'
  static bool compareWatts(Microwave a, Microwave b) {
    if (a.power > b.power) return -1;
    if (b.power > a.power) return 1;
    // else they're the same
    return 0;
  }
}

class Microwave {
  final int power;
  Microwave(this.power);
  void startCooking() {
    _startTimer();
    print('starting cooking!')
  }
  void _startTimer() {
    print('starting timer!');
  }
  static int compareWatts(Microwave a, Microwave b) {
    if (a.power > b.power) return -1;
    if (b.power > a.power) return 1;
    // else they're the same
    return 0;
  }
}
void main() {
  var averageMicrowave = Microwave(1100);
  var miniMicrowave = Microwave(800);
  var superMicrowave = Microwave(1500);
  // calling a basic instance method
  averageMicrowave.startCooking();
  // calling a static method
  // which you must call on the class, rather than the instance
  print(Microwave.compareWatts(averageMicrowave, superMicrowave));
}