Use execution context to execute concurrent processes using futures. Number of threads is defined in a fixed thread pool. Blocking for a maximum number of minutes, see http://docs.scala-lang.org/style/naming-conventions.html for non-blocking alternative.

ConcurrentExecutionUsingFutures.scala

import java.util.concurrent.Executors
import scala.concurrent.Await
import scala.concurrent.ExecutionContext
import scala.concurrent.Future
import scala.concurrent.duration.DurationInt

/**
 * Use execution context to execute concurrent processes using futures. 
 * Number of threads is defined in a fixed thread pool.
 * Blocking for a maximum number of minutes, see http://docs.scala-lang.org/style/naming-conventions.html 
 * for non-blocking alternative.
 */
object ConcurrentExecutionUsingFutures extends App {
  
  val NumThreads = 4
  val MaxWaitMinutes = 1

  implicit val ec = ExecutionContext.fromExecutorService(Executors.newFixedThreadPool(NumThreads))
  
  val t0 = System.currentTimeMillis()
  
  /**
   * Double letter and wait for 2 seconds
   */
  def getDoubled(letter: String): String = {
    val t1 = System.currentTimeMillis()
    println("letter: " + letter + " (elapsed: "+(t1-t0)+")" + ", thread-name: "+Thread.currentThread().getName())
    Thread.sleep(2000)
    letter+letter
  }
  
  val alphabet = List(
      "a", "b", "c", "d", 
      "e", "f", "g", "h", 
      "i", "j", "k", "l", 
      "m", "n", "o", "p", 
      "q", "r", "s", "t", 
      "u", "v", "w", "x", 
      "y", "z")
      
  val fResult = for(letter <- alphabet) yield Future { getDoubled(letter) } 
  
  // blocking 
  val result: List[String] = Await.result(Future.sequence(fResult.toList), MaxWaitMinutes minutes)
  for(res <- result) {
    println(res)
  }
  
  ec.shutdown()
  
}