CODEWITHSUNDEEP
classflutterdartextends
Michael
Michael

Reputation: 11

In Dart I'm trying to extend a class while changing the TYPE of one of its properties

Class1 has a property List<ClassA> xyz = [];. ClassA has a number of properties and methods. ClassB extends ClassA adding additional properties and methods. Is there a way for me to create Class2 extending Class1 but change the TYPE of xyz to List<ClassB>? If that is confusing hopefully code below will give an example of what I'm trying to accomplish. Basically the same as overriding a method but overriding a property.

class Game {
  int points;
  String opponent;
  int opponentPoints;
}

class FootballGame extends Game {
  int touchdowns;
  int opponentstouchdowns;
}

class BaseballGame extends Game {
  int homeruns;
  int opponentsHomeruns;
}

class Team {
   String name;
   List<Game> games;

  double winningPercentage() {
    int wins = 0;
    for(var game in games){
      wins += (game.points > game.opponentPoints) ? 1 : 0;        
    }
    return wins / games.length;
  }
}

class FootballTeam extends Team {
  // How do I change the TYPE of the games property to <FootballGame>

}

Upvotes: 1

Views: 1746

Answers (1)

jamesdlin
jamesdlin

Reputation: 90175

You could use the covariant keyword in this case:

class FootballTeam extends Team {
  @override
  covariant List<FootballGame> games;
}

However, be aware that doing so is potentially unsafe; the reason why you need the covariant keyword is to suppress the type error that arises because the override can violate the contract of the base class: the base class advertises that games can be assigned a List<Game>, but such an assignment would be invalid for the derived class. By using the covariant keyword, you disable the type-check and take responsibility for ensuring that you do not violate the contract in practice.

Note that if the games member were final (or were only a getter), then the override (which uses a more specific type) would be safe and wouldn't need to use covariant.

Edit

I had forgotten that I had written this answer when writing a more detailed answer for a similar question.

Upvotes: 7

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