Add item to the queue in a mutable hashmap?

Question

How to Add items to Queue sitting in a Scala mutable.hashmap?

I tried this:

 val hashMapUserListeners: mutable.HashMap[UUID, mutable.Queue[UUID]]
 hashMapUserListeners.get(uuid) += uuid2

But got this error:

[error]Expression does not convert to assignment because receiver is not assignable.
[error]hashMapUserListeners.get(uuid) += uuid2

Actual Code Snippet:

  def listenUserStatus(actorRef: ActorRef, message: SocketParsedMessage)={
      (message.data \ "userId").validate[UUID] match {
        case s: JsSuccess[UUID] => {
          if(hashMapUserListeners.contains(s.get)){
            if(!hashMapUserListeners.get(s.get).contains(hashMapA2U.get(actorRef))) {
              hashMapUserListeners.get(s.get) += hashMapA2U.get(actorRef)
            }
          } else{
            hashMapUserListeners += (s.get -> new mutable.Queue[UUID]())
          }
        }
        case e: JsError => actorRef ! SocketParsedMessage(
          AllowedSocketMessageTypes.LISTEN_USER_STATUS, Json.obj(
            "success" -> false,
            "message" -> "UserId not provided with request"
          ))

Please refer to documentation: tutorialspoint.com/scala/scala_maps.htm — Pavel
– Pavel, Commented Oct 23, 2017 at 13:03
@Pavel I know how to add items to Hashmap, I am asking about adding more item to quee in Hashmap, without cloning and adding items to it. — Ravinder Payal
– Ravinder Payal, Commented Oct 23, 2017 at 13:06
Its looks like item you are added to the HasMap is immutable, please provide definition for Queue etc — Pavel
– Pavel, Commented Oct 23, 2017 at 13:10
@Pavel I added my actual code snippet, and no queue(mutable.Queue[UUID]) is mutable. — Ravinder Payal
– Ravinder Payal, Commented Oct 23, 2017 at 13:17

Yuval Itzchakov · Accepted Answer · 2017-10-23 13:20:25Z

1

This doesn't work because mutable.HashMap.get returns an Option[Queue[UUID]], not a Queue[UUID]. You'll need to go under the Option using foreach to update the underlying queue:

val maybeUuids: Option[mutable.Queue[UUID]] = hashMapUserListeners.get(uuid)
maybeUuids.foreach(queue => queue += uuid2)

answered Oct 23, 2017 at 13:20

Yuval Itzchakov

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Ravinder Payal · Accepted Answer · 2020-01-04 14:12:46Z

Finally I altered my code to one attached below.

   hashMapUserListeners.get(s.get) match {
        case Some(currentQueue) => {
               if(!currentQueue.contains(s.get)){
                  currentQueue += userId
               }
        }

For those interested in complete code.

 def listenUserStatus(actorRef: ActorRef, message: SocketParsedMessage): Unit = {
    hashMapA2U.get(actorRef) match {
      case Some(user) => {
        (message.data \ "userId").validate[UUID] match {
          case s: JsSuccess[UUID] => {
            hashMapUserListeners.get(s.get) match {
              case Some(x) => {
        case Some(currentQueue) => {
               if(!currentQueue.contains(s.get)){
                  currentQueue += userId
               }
              }
              case None => hashMapUserListeners += (s.get -> new mutable.Queue[UUID]())
            }
          }
          case e: JsError => actorRef ! SocketParsedMessage(
            AllowedSocketMessageTypes.LISTEN_USER_STATUS, Json.obj(
              "success" -> false,
              "message" -> "UserId not provided with request"
            ))
        }
      }
      case None => {
        actorRef ! SocketParsedMessage(
          AllowedSocketMessageTypes.AUTHENTICATE, Json.obj(
            "success" -> false,
            "message" -> "User not authenticated"
          ))
      }
    }
  }

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