Parsing JSON to a Map and Set structure

Question

How do I make these line of codes more scala-ish (shorter?). I still get the Java feeling in it (which I want to stay away from).

import scala.collection.mutable
val outstandingUserIds: mutable.LinkedHashSet[String] = mutable.LinkedHashSet[String]()    

val tweetJson = JacksMapper.readValue[Map[String, AnyRef]](body)     
val userObj = tweetJson.get("user")    
tweetJson.get("user").foreach(userObj => {
  userObj.asInstanceOf[Map[String, AnyRef]].get("id_str").foreach(idStrObj => {
    if (outstandingUserIds.exists(outstandingIdStr => outstandingIdStr.equals(idStrObj))) {
      outstandingUserIds.remove(idStrObj.asInstanceOf[String])
    }
  })
})

Answer 1

If you really want to inflict this JSON library on yourself, your code could be simplified to:

import scala.collection.mutable

val outstandingUserIds = mutable.LinkedHashSet[String]()    

val tweetJson = JacksMapper.readValue[Map[String, AnyRef]](body)     
for {
  userObj <- tweetJson.get("user")
  idStrObj <- userObj.asInstanceOf[Map[String, AnyRef]].get("id_str")
} outstandingUserIds -= idStrObj.asInstanceOf[String]

But I would recommend you use a better JSON library. For example, using the library that comes with the Play! framework:

(body \ "user" \ "id_str").asOpt[String].foreach {id =>
  outstandingUserIds -= id
}

Answer 2

You usually want to avoid mutable collections in Scala, unless you have a good reason.

val outstandingUserIds: Set[String] = ...  // immutable

val userId: Option[String] = tweetJson.get("user")
           .flatMap(userObj => userObj.asInstanceOf[Map[String, AnyRef]].get("id_str"))
           .map(_.asInstanceOf[String])
userId.foreach(id => outstandingUserIds = outstandingUserIds - id)

I wrote the type Option[AnyRef] just for clarity here; you probably would not write it in production code.

You can subtract an element from a Set even if it is not present.

The constant casting is quite annoying. Maybe someone who has experience with such a json library will have better suggestions.