How to create an Object Model from Another? - flutter

Question

I want to work with vehicles in my App, which can be of many kinds: trucks, vans, etc.
So, I made a model class of the base elements of the vehicle and some processes to work with the model, something like:
import 'dart:convert';
VehicleModel vehicleModelFromJson(String str) => VehicleModel.fromJson(json.decode(str));
String vehicleModelToJson(VehicleModel data) => json.encode(data.toJson());
class VehicleModel {
VehicleModel({
this.model,
this.licenseplate,
});
String? model;
String? licenseplate;
factory VehicleModel.fromJson(Map<String, dynamic> json) => VehicleModel(
model: json["model"],
licenseplate: json["licenseplate"],
);
Map<String, dynamic> toJson() => {
"model": model,
"licenseplate": licenseplate,
};
}
Now I want to make a new type of vehicle based on the old one. Suppose it is a truck, which has the same properties as a vehicle, and also has a load capacity. I read in Exploring Inheritance and Composition in Dart that you simply have to make a class that inherits properties from the other, however I don't know how to do it
Based on this article, I tried to implement something like this:
import 'dart:convert';
import 'package:pathofmyapp/models/vehicle_model.dart';
TruckModel truckModelFromJson(String str) => TruckModel.fromJson(json.decode(str));
String truckModelToJson(TruckModel data) => json.encode(data.toJson());
class TruckModel extends VehicleModel {
TruckModel({
this.model,
this.licenseplate,
this.loadingcapacity,
}) : super (model: model, licenseplate: licenseplate);
String? loadingcapacity;
factory TruckModel.fromJson(Map<String, dynamic> json) => TruckModel(
model: json["model"],
licenseplate: json["licenseplate"],
loadingcapacity: json["loadingcapacity"],
);
Map<String, dynamic> toJson() => {
"model": model,
"licenseplate": licenseplate,
"loadingcapacity": loadingcapacity,
};
}
But an error appears indicating that the model and licenseplate are not a field in the enclosing class. In toJson function appears a suggestion of type Annotate overridden members.
How can I create a Truck Object based on Vehicle Object?

Answer 1

The model and licenseplate arguments have to be forwarded to the super constructor (which is already done in your code), but therefore the this keyword is misplaced in the TruckModel constructor (both fields are defined in the base class VehicleModel).
TruckModel truckModelFromJson(String str) => TruckModel.fromJson(json.decode(str));
String truckModelToJson(TruckModel data) => json.encode(data.toJson());
class TruckModel extends VehicleModel {
// Remove this keyword from the two fields model and licenseplate
TruckModel({
model,
licenseplate,
this.loadingcapacity,
}) : super (model: model, licenseplate: licenseplate);
String? loadingcapacity;
factory TruckModel.fromJson(Map<String, dynamic> json) => TruckModel(
model: json["model"],
licenseplate: json["licenseplate"],
loadingcapacity: json["loadingcapacity"],
);
// Add #override since you are overwriting a method of the base class
#override
Map<String, dynamic> toJson() => {
"model": model,
"licenseplate": licenseplate,
"loadingcapacity": loadingcapacity,
};
}