How specialize (override) class fields in Dart?

Issue

I would like to specialize fields (override) when a class is extended or implemented in Dart 2.

See the code:

abstract class Abase {
  String id;
  List<Bbase> bbases;
}

abstract class Bbase {
  String id;
}

class A implements Abase {
  String id;
  String name;

  List<B> bbases; // 'A.bbases=' ('(List<B>) → void') isn't a valid override of 'Abase.bbases=' ('(List<Bbase>) → void').
}

class B implements Bbase {
  String id;
}

class Abase has the generic field List<Bbase> bbase and in the class A I would like to specialize this field with List<B> bbase.

But this is not possible. This error is presented on analyzer or build_runner:

‘A.bbases=’ (‘(List) → void’) isn’t a valid override of
‘Abase.bbases=’ (‘(List) → void’).

1. Is there a reason for does not accept this specialization of fields?
2. Is there another way to specializate fields on extends or
   implements?

Solution

You have to use generics:

abstract class Abase<T extends Bbase> {
  String id;
  List<T> bbases;
}

abstract class Bbase {
  String id;
}

class A implements Abase<B> {
  String id;
  String name;

  List<B> bbases;
}

class B implements Bbase {
  String id;
}

Answered By – Alexandre Ardhuin

Answer Checked By – Terry (FlutterFixes Volunteer)