Transform a callback function to an iterator/list (in Scala or Java). See an example use case: http://scalaenthusiast.wordpress.com/2013/06/12/transform-a-callback-function-to-an-iteratorlist-in-scala/

gistfile1.scala

import java.util.concurrent.ArrayBlockingQueue

trait OptionNextToIterator[T] extends Iterator[T] {
  def getOptionNext: Option[T];

  var answerReady: Boolean = false
  var eof: Boolean = false
  var element: T = _

  def hasNext = {
    prepareNextAnswerIfNecessary
    !eof
  }

  def next = {
    prepareNextAnswerIfNecessary
    if (eof) { throw new NoSuchElementException }
    val retVal = element
    answerReady = false
    retVal
  }

  def prepareNextAnswerIfNecessary {
    if (answerReady) {
      return
    }
    synchronized {
      getOptionNext match {
        case None => eof = true
        case Some(e) => element = e
      }
      answerReady = true
    }
  }
}

trait QueueToIterator[T] extends OptionNextToIterator[T] {
  val queueCapacity: Int
  val queue = new ArrayBlockingQueue[Option[T]](queueCapacity)
  var queueClosed = false

  def queuePut(entry: T) {
    if (queueClosed) { throw new IllegalStateException("The queue has already been closed."); }
    queue.put(Some(entry))
  }

  def queueClose() {
    queueClosed = true
    queue.put(None)
  }

  def getOptionNext = queue.take
}

Transform a callback function to an iterator/list (in Scala)
See an example use case:
http://scalaenthusiast.wordpress.com/2013/06/12/transform-a-callback-function-to-an-iteratorlist-in-scala/

if someone is interested, this can easily be transformed to Java.