How to add elements into ArrayList in HashMap? DART & Flutter

Question

Looking for an example in dart exactly like this question: How to add element into ArrayList in HashMap

  Map<int, List<Message>> user_id_mapped_to_messages_list = new HashMap();

This throws a null insert, what am I missing here?

  user_id_mapped_to_messages_list[user_id].add(message_to_current_user);

Error:

I/flutter (11992): ==============================================
I/flutter (11992): Message is going to the logged in user id of: 11
I/flutter (11992): Message details:
I/flutter (11992): sender: Instance of 'User'
I/flutter (11992): time: 2020-05-11 22:28:26
I/flutter (11992): text: Message 2 from John to Joe
I/flutter (11992): message added to chat group
E/flutter (11992): [ERROR:flutter/lib/ui/ui_dart_state.cc(157)] Unhandled Exception: NoSuchMethodError: The method 'add' was called on null.
E/flutter (11992): Receiver: null
E/flutter (11992): Tried calling: add(Instance of 'Message')
E/flutter (11992): #0      Object.noSuchMethod (dart:core-patch/object_patch.dart:53:5)
E/flutter (11992): #1      _myappnameMessagesRecentChatsState.getConversations.<anonymous closure>.<anonymous closure> 
(package:myappname_app_androidx/widgets/home/messages/messages_v2/recent_chats.dart:158:83)

No items existed... for loop printing the messages in the array list

user_id_mapped_to_messages_list.forEach((k,v) => print('${k}: ${v}'));

can you show the error and the code where yoy are declaring and inserting into the lists — Haidar
– Haidar, Commented May 13, 2020 at 16:47
Added the error, the code where I'm inserting is too much to post... It's basically just: user_id_mapped_to_messages_list[user_id].add(message); — Petro
– Petro, Commented May 13, 2020 at 16:53

Viren V Varasadiya · Accepted Answer · 2020-05-13 17:14:13Z

1

You created user_id_mapped_to_messages_list variable of type Map> but initialised with HashMap because of that you are getting error.

it should be like below.

Map<int, List<Message>> user_id_mapped_to_messages_list = new Map();

Update:

This is happening because when you directly assign or add any value to list when it is not actually initialised, so add following line before a

user_id_mapped_to_messages_list[user_id] = [];

user_id_mapped_to_messages_list[user_id] = [];

    if (user_id_mapped_to_messages_list[user_id] == null) {
      user_id_mapped_to_messages_list[user_id] = [];
      user_id_mapped_to_messages_list[user_id].add(message_to_current_user);
    } else {
      user_id_mapped_to_messages_list[user_id].add(message_to_current_user);
    }

edited May 13, 2020 at 17:14

answered May 13, 2020 at 16:49

Viren V Varasadiya

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