Json decode Instance of Item - flutter

Question

I'm trying to save a list of object into a shared preference for easy storage. I'm getting the error [ERROR:flutter/lib/ui/ui_dart_state.cc(198)] Unhandled Exception: FormatException: Unexpected character (at character 1) -> E/flutter ( 7980): Instance of 'History'
Method
Future<void> recordLocation(lat, long) async {
List<History> histories = [];
final prefs = await SharedPreferences.getInstance();
//fetching history
if (prefs.getStringList('history') != null) {
List<String>? prevHistory = prefs.getStringList('history');
histories = prevHistory!
.map<History>((i) => History.fromJson(json.decode(i)))
.toList();
}
//setting history
History current = History(lat.toString(), long.toString());
histories.add(current);
List<String> newHistory = histories.map((i) => i.toString()).toList();
await prefs.setStringList('history', newHistory);
}
History model
class History {
String? latitude;
String? longtitude;
History(this.latitude, this.longtitude);
History.fromJson(Map<String, dynamic> json) {
latitude = json['latitude'];
longtitude = json['longtitude'];
}
Map<String, dynamic> toJson() {
final Map<String, dynamic> data = new Map<String, dynamic>();
data['latitude'] = this.latitude;
data['longtitude'] = this.longtitude;
return data;
}
}

Answer 1

People usually declare the fromJson method as a factory method and data classes as immutable. Try this instead:
class History {
final String latitude;
final String longtitude;
const History({
this.latitude = '',
this.longtitude = '',
});
factory History.fromJson(Map<String, dynamic> json) =>
History(
latitude: json['latitude'] as String? ?? '',
longtitude: json['longtitude'] as String? ?? '',
);
Map<String, dynamic> toJson() => {
'latitude': latitude,
'longtitude': longtitude,
};
}

Answer 2

Edit:
import 'dart:convert';
class History {
String? latitude;
String? longtitude;
History(this.latitude, this.longtitude);
History.fromJson(Map<String, dynamic> json) {
latitude = json['latitude'].toString();
longtitude = json['longtitude'].toString();
}
Map<String, dynamic> toJson() {
final Map<String, dynamic> data = new Map<String, dynamic>();
data['latitude'] = this.latitude;
data['longtitude'] = this.longtitude;
return data;
}
#override
String toString() {
String res = '{"latitude":${latitude ?? ""},"longtitude":${longtitude ?? ""}}';
return res;
}
}
void main(){
List<History> histories = [];
List<String>? prevHistory = [];
for(int i=0;i<10;i++){
History his = History(i.toString(), i.toString());
prevHistory!.add(his.toString());
}
if(prevHistory != null){
histories = prevHistory!.map<History>((i){
return History.fromJson(jsonDecode(i));
}).toList();
}
for(int i=0;i<histories.length;i++){
print(histories[i].toString());
}
}
I have run this code in dartpad and it gives the output as:
{"latitude":0,"longtitude":0}
{"latitude":1,"longtitude":1}
{"latitude":2,"longtitude":2}
{"latitude":3,"longtitude":3}
{"latitude":4,"longtitude":4}
{"latitude":5,"longtitude":5}
{"latitude":6,"longtitude":6}
{"latitude":7,"longtitude":7}
{"latitude":8,"longtitude":8}
{"latitude":9,"longtitude":9}
The problem was with the conversion of string to Map<String, dynamic>
Edit 2:
Here if you do not want to change your history class then just make edit in the toString() method:
String toString() {
String res = '{"latitude":"${latitude ?? ""}","longtitude":"${longtitude ?? ""}"}';
return res;
}