app.dart

main() {
  Map json = jsonDecode('{"id": 1, "price": 100, "type": "fruits", "color": "red"}');
  Product product = Product.fromJson(json);

}

class Product {
  int id;
  num price;

  CurrentProduct currentProduct;

  Product.fromJson(Map json)
  {
    this.id = json['id'];
    this.price = json['price'];

    if (json['type'] == 'bottles')
    {
      currentProduct = Bottles(json['type']);
    }
    if (json['type'] == 'fruits')
    {
      currentProduct = Apples(json['red']);
    }

  }

}

abstract class CurrentProduct
{

}

class Bottles implements CurrentProduct {
  String content;
  Bottles(this.content);

}

class Apples implements CurrentProduct {
  String color;
  Apples(this.color);
}