Scala example source code file (producers.scala)

This example Scala source code file (producers.scala) is included in the DevDaily.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" ^TM.

Java - Scala tags/keywords

actor, int, next, next, none, object, producer, producer, some, stop, tree, tree, undefined, undefined

The Scala producers.scala source code

package examples.actors

import scala.actors.Actor
import scala.actors.Actor._

abstract class Producer[T] {

  /** A signal that the next value should be produced. */
  private val Next = new Object 

  /** A label for an undefined state of the iterators. */
  private val Undefined = new Object

  /** A signal to stop the coordinator. */
  private val Stop = new Object 

  protected def produce(x: T) {
    coordinator ! Some(x)
    receive { case Next => }
  }

  protected def produceValues: Unit

  def iterator = new Iterator[T] {
    private var current: Any = Undefined
    private def lookAhead = {
      if (current == Undefined) current = coordinator !? Next
      current
    }

    def hasNext: Boolean = lookAhead match {
      case Some(x) => true
      case None => { coordinator ! Stop; false }
    }

    def next: T = lookAhead match {
      case Some(x) => current = Undefined; x.asInstanceOf[T]
    }
  }

  private val coordinator: Actor = actor {
    loop {
      react {
        case Next =>
          producer ! Next
          reply {
            receive { case x: Option[_] => x }
          }
        case Stop =>
          exit('stop)
      }
    }
  }

  private val producer: Actor = actor {
    receive { 
      case Next => 
        produceValues
        coordinator ! None 
    }
  }
}

object producers extends Application {

  class Tree(val left: Tree, val elem: Int, val right: Tree)
  def node(left: Tree, elem: Int, right: Tree): Tree = new Tree(left, elem, right)
  def node(elem: Int): Tree = node(null, elem, null)

  def tree = node(node(node(3), 4, node(6)), 8, node(node(9), 10, node(11)))

  class PreOrder(n: Tree) extends Producer[Int] {
    def produceValues = traverse(n) 
    def traverse(n: Tree) {
      if (n != null) {
        produce(n.elem)
        traverse(n.left)
        traverse(n.right)
      }
    }
  }

  class PostOrder(n: Tree) extends Producer[Int] {
    def produceValues = traverse(n) 
    def traverse(n: Tree) {
      if (n != null) {
        traverse(n.left)
        traverse(n.right)
        produce(n.elem)
      }
    }
  }

  class InOrder(n: Tree) extends Producer[Int] {
    def produceValues = traverse(n) 
    def traverse(n: Tree) {
      if (n != null) {
        traverse(n.left)
        produce(n.elem)
        traverse(n.right)
      }
    }
  }

  actor {
    print("PreOrder:")
    for (x <- new PreOrder(tree).iterator) print(" "+x)
    print("\nPostOrder:")
    for (x <- new PostOrder(tree).iterator) print(" "+x)
    print("\nInOrder:")
    for (x <- new InOrder(tree).iterator) print(" "+x)
    print("\n")
  }
}