将Java样式重构为功能更强大的Scala样式

Question

I want to transform a map of users to their friends to a map of friends to their friends. type Username: String So Map[Username, List[User]] to Map[Username, List[Username]] by making each List[User] into the key.

I have the following code below that is very java like that I want to refactor to make it more idomatic scala code.

val map: Map[String, List[User]] = {
  var m  = collection.mutable.Map[String, collection.mutable.ListBuffer[User]]()
  for(user <- users) {
    for(friend <- user.Friends) {
      if (m.contains(friend)) {
        //m.get(friend.username) += user
      } else {
        m += (friend -> collection.mutable.ListBuffer[User]())
        //m.get(friend.username) += user
      }
    }
  }
  m.map{e =>
    (e._1 -> e._2.toList)}.toMap
}

I started off trying this:

users.map( user =>
  user.friends.flatMap(friend =>

)

As a side note, how do I append to a ListBuffer, for some reason this was not working for me:

m.get(friend.username) += user

And to new-up a listbuffer and append would be done how?

Answer 1

Maybe you would like

    val map: Map[String, List[User]] = {
        val m = new collection.mutable.HashMap[String, collection.mutable.Set[User]]() with scala.collection.mutable.MultiMap[String, User]
        for {
            user <- users
            friend <- user.Friends
        }   m.addBinding(friend.username,user)

        m.mapValues(x ⇒ x.toList).toMap
    }

And second question: method m.get(friend.username) returns Option[ListBuffer[User]]. Option not contains +=.

Update

One line solution

    val map: Map[String, Seq[User]] = {
        users.flatMap(user ⇒ user.Friends.map(_ → user)).groupBy(_._1.username).mapValues(_.map(_._2))
    }