How to override nullable field with non-nullable one dart

Question

I have a class with a nullable property. I would like to make a superclass that overrides that property with a non nullable one

so

class Example {
String? name;
}

class NamedExample extends Example {
@override
String name;

}

Is there some way to do that? if not how is this goal conventionally accomplished.

I basically want two identical classes except one of them always has a property while it is optional in another.

Answer 1

This is a place for the covariant keyword. Normally it does not make sense to override a parameter's type with its subtype and it is invalid to do so. This keyword tells the analyzer this is intentional. It can be added in either the super or subclass.

Subclass:

class Example {
  String? name;
}

class NamedExample extends Example {
  @override
  covariant String name;
  
  NamedExample(this.name);
}

Superclass:

class Example {
  covariant String? name;
}

class NamedExample extends Example {
  @override
  String name;
  
  NamedExample(this.name);
}