1. 단순 override

void main() {
  TimesTwo tt = TimesTwo(2);
  print(tt.calculate());

  TimesFour tf = TimesFour(2);
  print(tf.calculate());
}

class TimesTwo {
  final int number;

  TimesTwo(
    this.number,
  );

  // method
  int calculate() {
    return number * 2;
  }
}

class TimesFour extends TimesTwo {
  TimesFour(int number) : super(number);

  @override // 
  int calculate() {
    return super.number * 4; // this.number / number 모두 가능
  }
}

2. 부모 클래스의 메서드 활용

class TimesFour extends TimesTwo {
  TimesFour(int number) : super(number);

  @override
  int calculate() {
    return super.calculate() * 2;
  }
}