Converting LinkedHashMap<dynamic,dynamic> to HashMap<String,List<String>> in dart or flutter

Question

Hi I am new to flutter and dart. I am facing difficulty in typecasting maps. I have saved some data on Firestore in the format of HashMap<String,List<String>>. In my flutter app when I am fetching this data, I am getting it in the format of LinkedHashMap<dynamic,dynamic>. I want to convert this LinkedHashMap into a HashMap<String,List<String>>. Please help.

LinkedHashMap<dynamic,dynamic> mapUserWatchlistCompanies= documentSnapshot.data['Watchlist']; // Fetched data.

HashMap<String,List<String>> hashMapUserWatchlist = ???//How can it be converted? Need help here.

Answer 1

The typecast in dart it's a bit different than you would get in Java or Swift. In dart you have to correctly cast everything when dynamic is involved.

Now in your case you get a LinkedHashMap<dynamic, dynamic>, now in JSON case it's safe to assume that the keys will always be a String value, but the value for a key can be several types (number, string, boolean, map or list).

So if you have a JSON in the following form:

{
  "keyOne": ["One", "Two","whatever"],
  "keyTwp": ["roses", "are", "red"],
  "keyThree":["You", "know", "nothing", "john", "nonw"]
}

Which is a HashMap<String, List<String>>, but dart initially will get a LinkedHashMap<dynamic, dynamic>, you can safely convert it with the following code:

 LinkedHashMap<dynamic, dynamic> theParsedOne = .....
    HashMap<String, List<String>> newMap = HashMap.from(theParsedOne.map((key, value) {
      List<dynamic> values = List.from(value);
      return MapEntry(
          key.toString(),
          values.map((theValue) {
            return theValue.toString();
          }).toList());
    }));