Parsing JSON to a Map and Set structure
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 &lt;- tweetJson.get("user")
  idStrObj &lt;- 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 =&gt;
  outstandingUserIds -= id
}