Flutter: Type "A" is not a sup type of type "B" even though A extends B-CodePudding

Trying to assign an instance of class NumberTrivia to an instance of class NumberTriviaModel but it is giving me the error: The argument type 'NumberTrivia' can't be assigned to the parameter type 'NumberTriviaModel'.. So I tried to cast it but now I get an uncaught error.

void main() {
  
  NumberTrivia remoteTrivia = NumberTrivia(text : "Text", number : 5);
  cacheNumberTrivia(remoteTrivia as NumberTriviaModel);
}

class NumberTrivia {
  final String text;
  final int number;

  NumberTrivia({
    required this.text,
    required this.number,
  });


}


class NumberTriviaModel extends NumberTrivia {
  NumberTriviaModel({
    required String text,
    required int number,
  }) : super(text: text, number: number);

  factory NumberTriviaModel.fromJson(Map<String, dynamic> json) {
    return NumberTriviaModel(
      text: json['text'],
      number: (json['number'] as num).toInt(),
    );
  }

  Map<String, dynamic> toJson() {
    return {
      'text': text,
      'number': number,
    };
  }
}

void cacheNumberTrivia(NumberTriviaModel triviaToCache) {
    print(triviaToCache.number);
}

but it's giving me the following error

Uncaught Error: TypeError: Instance of 'NumberTrivia': type 'NumberTrivia' is not a subtype of type 'NumberTriviaModel'

CodePudding user response：

Instance of 'NumberTrivia':
  type 'NumberTrivia' is not a subtype of type 'NumberTriviaModel'

This means that NumberTrivia does not extend NumberTriviaModel.

Looking at the code, it is the other way around. NumberTriviaModel extends NumberTrivia, NumberTriviaModel is a sub-type NumberTrivia:

class NumberTriviaModel extends NumberTrivia {
      ! SUBTYPE                 ! TYPE

Your headline states Type "A" is not a sup type of type "B" even though A extends B. But the code does not reflect that.

Instead, the code can be described as Type "A" is not a sup type of type "B" BECAUSE B extends A.

CodePudding user response：

You need to typecasting and so you can try with this

void main() {
  
  NumberTrivia remoteTrivia = NumberTrivia(text : "Text", number : 5);
  cacheNumberTrivia(NumberTriviaModel(text: remoteTrivia.text, number: remoteTrivia.number));
}

N.B: you can able to use all properties of NumberTrivia in NumberTriviaModel but they are not the same object.

CodePudding user response：

The way you are calling the method is wrong. Here A extends B. If you want to access method of A you need to use instance of A. If you have added method in B and tried to access via instance of A, you can use it.

void main() {
  
  NumberTriviaModel remoteTrivia = NumberTriviaModel(text : "Text", number : 5);
  cacheNumberTrivia(remoteTrivia as NumberTriviaModel);
}

class NumberTrivia {
  final String text;
  final int number;

  NumberTrivia({
    required this.text,
    required this.number,
  });


}


class NumberTriviaModel extends NumberTrivia {
  NumberTriviaModel({
    required String text,
    required int number,
  }) : super(text: text, number: number);

  factory NumberTriviaModel.fromJson(Map<String, dynamic> json) {
    return NumberTriviaModel(
      text: json['text'],
      number: (json['number'] as num).toInt(),
    );
  }

  Map<String, dynamic> toJson() {
    return {
      'text': text,
      'number': number,
    };
  }
}

void cacheNumberTrivia(NumberTriviaModel triviaToCache) {
    print(triviaToCache.number);
}