Scala example source code file (List.scala)

This example Scala source code file (List.scala) is included in my "Source Code Warehouse" project. The intent of this project is to help you more easily find Scala source code examples by using tags.

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Scala tags/keywords

a, annotation, any, b, boolean, canbuildfrom, int, list, listbuffer, nil, that

The List.scala Scala example source code

/*                     __                                               *\
**     ________ ___   / /  ___     Scala API                            **
**    / __/ __// _ | / /  / _ |    (c) 2003-2013, LAMP/EPFL             **
**  __\ \/ /__/ __ |/ /__/ __ |    http://scala-lang.org/               **
** /____/\___/_/ |_/____/_/ | |                                         **
**                          |/                                          **
\*                                                                      */

package scala
package collection
package immutable

import generic._
import mutable.{Builder, ListBuffer}
import scala.annotation.tailrec
import java.io._

/** A class for immutable linked lists representing ordered collections
 *  of elements of type.
 *
 *  This class comes with two implementing case classes `scala.Nil`
 *  and `scala.::` that implement the abstract members `isEmpty`,
 *  `head` and `tail`.
 *
 *  This class is optimal for last-in-first-out (LIFO), stack-like access patterns. If you need another access
 *  pattern, for example, random access or FIFO, consider using a collection more suited to this than `List`.
 *
 *  @example {{{
 *  // Make a list via the companion object factory
 *  val days = List("Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday")
 *
 *  // Make a list element-by-element
 *  val when = "AM" :: "PM" :: List()
 *
 *  // Pattern match
 *  days match {
 *    case firstDay :: otherDays =>
 *      println("The first day of the week is: " + firstDay)
 *    case List() =>
 *      println("There don't seem to be any week days.")
 *  }
 *  }}}
 *
 *  ==Performance==
 *  '''Time:''' `List` has `O(1)` prepend and head/tail access. Most other operations are `O(n)` on the number of elements in the list.
 *  This includes the index-based lookup of elements, `length`, `append` and `reverse`.
 *
 *  '''Space:''' `List` implements '''structural sharing''' of the tail list. This means that many operations are either
 *  zero- or constant-memory cost.
 *  {{{
 *  val mainList = List(3, 2, 1)
 *  val with4 =    4 :: mainList  // re-uses mainList, costs one :: instance
 *  val with42 =   42 :: mainList // also re-uses mainList, cost one :: instance
 *  val shorter =  mainList.tail  // costs nothing as it uses the same 2::1::Nil instances as mainList
 *  }}}
 *
 *  @note The functional list is characterized by persistence and structural sharing, thus offering considerable
 *        performance and space consumption benefits in some scenarios if used correctly.
 *        However, note that objects having multiple references into the same functional list (that is,
 *        objects that rely on structural sharing), will be serialized and deserialized with multiple lists, one for
 *        each reference to it. I.e. structural sharing is lost after serialization/deserialization.
 *
 *  @author  Martin Odersky and others
 *  @version 2.8
 *  @since   1.0
 *  @see  [[http://docs.scala-lang.org/overviews/collections/concrete-immutable-collection-classes.html#lists "Scala's Collection Library overview"]]
 *  section on `Lists` for more information.
 *
 *  @define coll list
 *  @define Coll `List`
 *  @define thatinfo the class of the returned collection. In the standard library configuration,
 *    `That` is always `List[B]` because an implicit of type `CanBuildFrom[List, B, That]`
 *    is defined in object `List`.
 *  @define bfinfo an implicit value of class `CanBuildFrom` which determines the
 *    result class `That` from the current representation type `Repr`
 *    and the new element type `B`. This is usually the `canBuildFrom` value
 *    defined in object `List`.
 *  @define orderDependent
 *  @define orderDependentFold
 *  @define mayNotTerminateInf
 *  @define willNotTerminateInf
 */
sealed abstract class List[+A] extends AbstractSeq[A]
                                  with LinearSeq[A]
                                  with Product
                                  with GenericTraversableTemplate[A, List]
                                  with LinearSeqOptimized[A, List[A]]
                                  with Serializable {
  override def companion: GenericCompanion[List] = List

  import scala.collection.{Iterable, Traversable, Seq, IndexedSeq}

  def isEmpty: Boolean
  def head: A
  def tail: List[A]

  // New methods in List

  /** Adds an element at the beginning of this list.
   *  @param x the element to prepend.
   *  @return  a list which contains `x` as first element and
   *           which continues with this list.
   *
   *  @usecase def ::(x: A): List[A]
   *    @inheritdoc
   *
   *    Example:
   *    {{{1 :: List(2, 3) = List(2, 3).::(1) = List(1, 2, 3)}}}
   */
  def ::[B >: A] (x: B): List[B] =
    new scala.collection.immutable.::(x, this)

  /** Adds the elements of a given list in front of this list.
   *  @param prefix  The list elements to prepend.
   *  @return a list resulting from the concatenation of the given
   *    list `prefix` and this list.
   *
   *  @usecase def :::(prefix: List[A]): List[A]
   *    @inheritdoc
   *
   *    Example:
   *    {{{List(1, 2) ::: List(3, 4) = List(3, 4).:::(List(1, 2)) = List(1, 2, 3, 4)}}}
   */
  def :::[B >: A](prefix: List[B]): List[B] =
    if (isEmpty) prefix
    else if (prefix.isEmpty) this
    else (new ListBuffer[B] ++= prefix).prependToList(this)

  /** Adds the elements of a given list in reverse order in front of this list.
   *  `xs reverse_::: ys` is equivalent to
   *  `xs.reverse ::: ys` but is more efficient.
   *
   *  @param prefix the prefix to reverse and then prepend
   *  @return       the concatenation of the reversed prefix and the current list.
   *
   *  @usecase def reverse_:::(prefix: List[A]): List[A]
   *    @inheritdoc
   */
  def reverse_:::[B >: A](prefix: List[B]): List[B] = {
    var these: List[B] = this
    var pres = prefix
    while (!pres.isEmpty) {
      these = pres.head :: these
      pres = pres.tail
    }
    these
  }

  /** Builds a new list by applying a function to all elements of this list.
   *  Like `xs map f`, but returns `xs` unchanged if function
   *  `f` maps all elements to themselves (as determined by `eq`).
   *
   *  @param f      the function to apply to each element.
   *  @tparam B     the element type of the returned collection.
   *  @return       a list resulting from applying the given function
   *                `f` to each element of this list and collecting the results.
   *
   *  @usecase def mapConserve(f: A => A): List[A]
   *    @inheritdoc
   */
  @inline final def mapConserve[B >: A <: AnyRef](f: A => B): List[B] = {
    // Note to developers: there exists a duplication between this function and `reflect.internal.util.Collections#map2Conserve`.
    // If any successful optimization attempts or other changes are made, please rehash them there too.
    @tailrec
    def loop(mapped: ListBuffer[B], unchanged: List[A], pending: List[A]): List[B] =
      if (pending.isEmpty) {
        if (mapped eq null) unchanged
        else mapped.prependToList(unchanged)
      }
      else {
        val head0 = pending.head
        val head1 = f(head0)

        if (head1 eq head0.asInstanceOf[AnyRef])
          loop(mapped, unchanged, pending.tail)
        else {
          val b = if (mapped eq null) new ListBuffer[B] else mapped
          var xc = unchanged
          while (xc ne pending) {
            b += xc.head
            xc = xc.tail
          }
          b += head1
          val tail0 = pending.tail
          loop(b, tail0, tail0)
        }
      }
    loop(null, this, this)
  }

  // Overridden methods from IterableLike and SeqLike or overloaded variants of such methods

  override def ++[B >: A, That](that: GenTraversableOnce[B])(implicit bf: CanBuildFrom[List[A], B, That]): That = {
    val b = bf(this)
    if (b.isInstanceOf[ListBuffer[_]]) (this ::: that.seq.toList).asInstanceOf[That]
    else super.++(that)
  }

  override def +:[B >: A, That](elem: B)(implicit bf: CanBuildFrom[List[A], B, That]): That = bf match {
    case _: List.GenericCanBuildFrom[_] => (elem :: this).asInstanceOf[That]
    case _ => super.+:(elem)(bf)
  }

  override def toList: List[A] = this

  override def take(n: Int): List[A] = if (isEmpty || n <= 0) Nil else {
    val h = new ::(head, Nil)
    var t = h
    var rest = tail
    var i = 1
    while ({if (rest.isEmpty) return this; i < n}) {
      i += 1
      val nx = new ::(rest.head, Nil)
      t.tl = nx
      t = nx
      rest = rest.tail
    }
    h
  }

  override def drop(n: Int): List[A] = {
    var these = this
    var count = n
    while (!these.isEmpty && count > 0) {
      these = these.tail
      count -= 1
    }
    these
  }

  /**
   *  @example {{{
   *  // Given a list
   *  val letters = List('a','b','c','d','e')
   *
   *  // `slice` returns all elements beginning at index `from` and afterwards,
   *  // up until index `until` (excluding index `until`.)
   *  letters.slice(1,3) // Returns List('b','c')
   *  }}}
   */
  override def slice(from: Int, until: Int): List[A] = {
    val lo = scala.math.max(from, 0)
    if (until <= lo || isEmpty) Nil
    else this drop lo take (until - lo)
  }

  override def takeRight(n: Int): List[A] = {
    @tailrec
    def loop(lead: List[A], lag: List[A]): List[A] = lead match {
      case Nil => lag
      case _ :: tail => loop(tail, lag.tail)
    }
    loop(drop(n), this)
  }

  // dropRight is inherited from LinearSeq

  override def splitAt(n: Int): (List[A], List[A]) = {
    val b = new ListBuffer[A]
    var i = 0
    var these = this
    while (!these.isEmpty && i < n) {
      i += 1
      b += these.head
      these = these.tail
    }
    (b.toList, these)
  }
  
  @noinline // TODO - fix optimizer bug that requires noinline (see SI-8334)
  final override def map[B, That](f: A => B)(implicit bf: CanBuildFrom[List[A], B, That]): That = {
    if (bf eq List.ReusableCBF) {
      if (this eq Nil) Nil.asInstanceOf[That] else {
        val h = new ::[B](f(head), Nil)
        var t: ::[B] = h
        var rest = tail
        while (rest ne Nil) {
          val nx = new ::(f(rest.head), Nil)
          t.tl = nx
          t = nx
          rest = rest.tail
        }
        h.asInstanceOf[That]
      }
    }
    else super.map(f)
  }
  
  @noinline // TODO - fix optimizer bug that requires noinline for map; applied here to be safe (see SI-8334)
  final override def collect[B, That](pf: PartialFunction[A, B])(implicit bf: CanBuildFrom[List[A], B, That]): That = {
    if (bf eq List.ReusableCBF) {
      if (this eq Nil) Nil.asInstanceOf[That] else {
        var rest = this
        var h: ::[B] = null
        var x: A = null.asInstanceOf[A]
        // Special case for first element
        do {
          val x: Any = pf.applyOrElse(rest.head, List.partialNotApplied)
          if (x.asInstanceOf[AnyRef] ne List.partialNotApplied) h = new ::(x.asInstanceOf[B], Nil)
          rest = rest.tail
          if (rest eq Nil) return (if (h eq null ) Nil else h).asInstanceOf[That]
        } while (h eq null)
        var t = h
        // Remaining elements
        do {
          val x: Any = pf.applyOrElse(rest.head, List.partialNotApplied)
          if (x.asInstanceOf[AnyRef] ne List.partialNotApplied) {
            val nx = new ::(x.asInstanceOf[B], Nil)
            t.tl = nx
            t = nx
          }
          rest = rest.tail
        } while (rest ne Nil)
        h.asInstanceOf[That]
      }
    }
    else super.collect(pf)
  }
  
  @noinline // TODO - fix optimizer bug that requires noinline for map; applied here to be safe (see SI-8334)
  final override def flatMap[B, That](f: A => GenTraversableOnce[B])(implicit bf: CanBuildFrom[List[A], B, That]): That = {
    if (bf eq List.ReusableCBF) {
      if (this eq Nil) Nil.asInstanceOf[That] else {
        var rest = this
        var found = false
        var h: ::[B] = null
        var t: ::[B] = null
        while (rest ne Nil) {
          f(rest.head).foreach{ b =>
            if (!found) {
              h = new ::(b, Nil)
              t = h
              found = true
            }
            else {
              val nx = new ::(b, Nil)
              t.tl = nx
              t = nx
            }
          }
          rest = rest.tail
        }
        (if (!found) Nil else h).asInstanceOf[That]
      }
    }
    else super.flatMap(f)
  }

  @inline final override def takeWhile(p: A => Boolean): List[A] = {
    val b = new ListBuffer[A]
    var these = this
    while (!these.isEmpty && p(these.head)) {
      b += these.head
      these = these.tail
    }
    b.toList
  }

  @inline final override def dropWhile(p: A => Boolean): List[A] = {
    @tailrec
    def loop(xs: List[A]): List[A] =
      if (xs.isEmpty || !p(xs.head)) xs
      else loop(xs.tail)

    loop(this)
  }

  @inline final override def span(p: A => Boolean): (List[A], List[A]) = {
    val b = new ListBuffer[A]
    var these = this
    while (!these.isEmpty && p(these.head)) {
      b += these.head
      these = these.tail
    }
    (b.toList, these)
  }

  // Overridden with an implementation identical to the inherited one (at this time)
  // solely so it can be finalized and thus inlinable.
  @inline final override def foreach[U](f: A => U) {
    var these = this
    while (!these.isEmpty) {
      f(these.head)
      these = these.tail
    }
  }

  override def reverse: List[A] = {
    var result: List[A] = Nil
    var these = this
    while (!these.isEmpty) {
      result = these.head :: result
      these = these.tail
    }
    result
  }

  override def foldRight[B](z: B)(op: (A, B) => B): B =
    reverse.foldLeft(z)((right, left) => op(left, right))

  override def stringPrefix = "List"

  override def toStream : Stream[A] =
    if (isEmpty) Stream.Empty
    else new Stream.Cons(head, tail.toStream)

  // Create a proxy for Java serialization that allows us to avoid mutation
  // during de-serialization.  This is the Serialization Proxy Pattern.
  protected final def writeReplace(): AnyRef = new List.SerializationProxy(this)
}

/** The empty list.
 *
 *  @author  Martin Odersky
 *  @version 1.0, 15/07/2003
 *  @since   2.8
 */
@SerialVersionUID(0 - 8256821097970055419L)
case object Nil extends List[Nothing] {
  override def isEmpty = true
  override def head: Nothing =
    throw new NoSuchElementException("head of empty list")
  override def tail: List[Nothing] =
    throw new UnsupportedOperationException("tail of empty list")
  // Removal of equals method here might lead to an infinite recursion similar to IntMap.equals.
  override def equals(that: Any) = that match {
    case that1: scala.collection.GenSeq[_] => that1.isEmpty
    case _ => false
  }
}

/** A non empty list characterized by a head and a tail.
 *  @param hd   the first element of the list
 *  @param tl   the list containing the remaining elements of this list after the first one.
 *  @tparam B   the type of the list elements.
 *  @author  Martin Odersky
 *  @version 1.0, 15/07/2003
 *  @since   2.8
 */
final case class ::[B](override val head: B, private[scala] var tl: List[B]) extends List[B] {
  override def tail : List[B] = tl
  override def isEmpty: Boolean = false
}

/** $factoryInfo
 *  @define coll list
 *  @define Coll `List`
 */
object List extends SeqFactory[List] {
  /** $genericCanBuildFromInfo */
  implicit def canBuildFrom[A]: CanBuildFrom[Coll, A, List[A]] =
    ReusableCBF.asInstanceOf[GenericCanBuildFrom[A]]

  def newBuilder[A]: Builder[A, List[A]] = new ListBuffer[A]

  override def empty[A]: List[A] = Nil

  override def apply[A](xs: A*): List[A] = xs.toList
  
  private[collection] val partialNotApplied = new Function1[Any, Any] { def apply(x: Any): Any = this }

  @SerialVersionUID(1L)
  private class SerializationProxy[A](@transient private var orig: List[A]) extends Serializable {

    private def writeObject(out: ObjectOutputStream) {
      var xs: List[A] = orig
      while (!xs.isEmpty) {
        out.writeObject(xs.head)
        xs = xs.tail
      }
      out.writeObject(ListSerializeEnd)
    }

    // Java serialization calls this before readResolve during de-serialization.
    // Read the whole list and store it in `orig`.
    private def readObject(in: ObjectInputStream) {
      val builder = List.newBuilder[A]
      while (true) in.readObject match {
        case ListSerializeEnd =>
          orig = builder.result()
          return
        case a =>
          builder += a.asInstanceOf[A]
      }
    }

    // Provide the result stored in `orig` for Java serialization
    private def readResolve(): AnyRef = orig
  }
}

/** Only used for list serialization */
@SerialVersionUID(0L - 8476791151975527571L)
private[scala] case object ListSerializeEnd