Scala example source code file (MapLike.scala)

This example Scala source code file (MapLike.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, b, b1, boolean, c, iterable, iterator, map, set, this

The MapLike.scala Scala example source code

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

package scala
package collection

import generic._
import mutable.{ Builder, MapBuilder }
import scala.annotation.{migration, bridge}
import parallel.ParMap

/** A template trait for maps, which associate keys with values.
 *
 *  $mapNote
 *  $mapTags
 *  @since 2.8
 *
 *  @define mapNote
 *    '''Implementation note:'''
 *    This trait provides most of the operations of a `Map` independently of its representation.
 *    It is typically inherited by concrete implementations of maps.
 *
 *    To implement a concrete map, you need to provide implementations of the
 *    following methods:
 *    {{{
 *       def get(key: A): Option[B]
 *       def iterator: Iterator[(A, B)]
 *       def + [B1 >: B](kv: (A, B1)): This
 *       def -(key: A): This
 *    }}}
 *    If you wish that methods like `take`, `drop`, `filter` also return the same kind of map
 *    you should also override:
 *    {{{
 *       def empty: This
 *    }}}
 *    It is also good idea to override methods `foreach` and
 *    `size` for efficiency.
 *
 *  @define mapTags
 *  @tparam A    the type of the keys.
 *  @tparam B    the type of associated values.
 *  @tparam This the type of the map itself.
 *
 *  @author  Martin Odersky
 *  @version 2.8
 *
 *  @define coll map
 *  @define Coll Map
 *  @define willNotTerminateInf
 *  @define mayNotTerminateInf
 */
trait MapLike[A, +B, +This <: MapLike[A, B, This] with Map[A, B]]
  extends PartialFunction[A, B]
     with IterableLike[(A, B), This]
     with GenMapLike[A, B, This]
     with Subtractable[A, This]
     with Parallelizable[(A, B), ParMap[A, B]]
{
self =>

  /** The empty map of the same type as this map
   *   @return   an empty map of type `This`.
   */
  def empty: This

  /** A common implementation of `newBuilder` for all maps in terms of `empty`.
   *  Overridden for mutable maps in `mutable.MapLike`.
   */
  override protected[this] def newBuilder: Builder[(A, B), This] = new MapBuilder[A, B, This](empty)

  /** Optionally returns the value associated with a key.
   *
   *  @param  key    the key value
   *  @return an option value containing the value associated with `key` in this map,
   *          or `None` if none exists.
   */
  def get(key: A): Option[B]

  /** Creates a new iterator over all key/value pairs of this map
   *
   *  @return the new iterator
   */
  def iterator: Iterator[(A, B)]

  /** Adds a key/value pair to this map, returning a new map.
   *  @param    kv the key/value pair
   *  @tparam   B1 the type of the value in the key/value pair.
   *  @return   a new map with the new binding added to this map
   *
   *  @usecase  def + (kv: (A, B)): Map[A, B]
   *    @inheritdoc
   */
  def + [B1 >: B] (kv: (A, B1)): Map[A, B1]

  /** Removes a key from this map, returning a new map.
   *  @param    key the key to be removed
   *  @return   a new map without a binding for `key`
   *
   *  @usecase  def - (key: A): Map[A, B]
   *    @inheritdoc
   */
  def - (key: A): This

  /** Tests whether the map is empty.
   *
   *  @return `true` if the map does not contain any key/value binding, `false` otherwise.
   */
  override def isEmpty: Boolean = size == 0

  /**  Returns the value associated with a key, or a default value if the key is not contained in the map.
   *   @param   key      the key.
   *   @param   default  a computation that yields a default value in case no binding for `key` is
   *                     found in the map.
   *   @tparam  B1       the result type of the default computation.
   *   @return  the value associated with `key` if it exists,
   *            otherwise the result of the `default` computation.
   *
   *   @usecase def getOrElse(key: A, default: => B): B
   *     @inheritdoc
   */
  def getOrElse[B1 >: B](key: A, default: => B1): B1 = get(key) match {
    case Some(v) => v
    case None => default
  }

  /** Retrieves the value which is associated with the given key. This
   *  method invokes the `default` method of the map if there is no mapping
   *  from the given key to a value. Unless overridden, the `default` method throws a
   *  `NoSuchElementException`.
   *
   *  @param  key the key
   *  @return     the value associated with the given key, or the result of the
   *              map's `default` method, if none exists.
   */
  def apply(key: A): B = get(key) match {
    case None => default(key)
    case Some(value) => value
  }

  /** Tests whether this map contains a binding for a key.
   *
   *  @param key the key
   *  @return    `true` if there is a binding for `key` in this map, `false` otherwise.
   */
  def contains(key: A): Boolean = get(key).isDefined

  /** Tests whether this map contains a binding for a key. This method,
   *  which implements an abstract method of trait `PartialFunction`,
   *  is equivalent to `contains`.
   *
   *  @param key the key
   *  @return    `true` if there is a binding for `key` in this map, `false` otherwise.
   */
  def isDefinedAt(key: A) = contains(key)

  /** Collects all keys of this map in a set.
   * @return  a set containing all keys of this map.
   */
  def keySet: Set[A] = new DefaultKeySet

  /** The implementation class of the set returned by `keySet`.
   */
  protected class DefaultKeySet extends AbstractSet[A] with Set[A] with Serializable {
    def contains(key : A) = self.contains(key)
    def iterator = keysIterator
    def + (elem: A): Set[A] = (Set[A]() ++ this + elem).asInstanceOf[Set[A]] // !!! concrete overrides abstract problem
    def - (elem: A): Set[A] = (Set[A]() ++ this - elem).asInstanceOf[Set[A]] // !!! concrete overrides abstract problem
    override def size = self.size
    override def foreach[C](f: A => C) = self.keysIterator foreach f
  }

  /** Creates an iterator for all keys.
   *
   *  @return an iterator over all keys.
   */
  def keysIterator: Iterator[A] = new AbstractIterator[A] {
    val iter = self.iterator
    def hasNext = iter.hasNext
    def next() = iter.next()._1
  }

  /** Collects all keys of this map in an iterable collection.
   *
   *  @return the keys of this map as an iterable.
   */
  @migration("`keys` returns `Iterable[A]` rather than `Iterator[A]`.", "2.8.0")
  def keys: Iterable[A] = keySet

  /** Collects all values of this map in an iterable collection.
   *
   *  @return the values of this map as an iterable.
   */
  @migration("`values` returns `Iterable[B]` rather than `Iterator[B]`.", "2.8.0")
  def values: Iterable[B] = new DefaultValuesIterable

  /** The implementation class of the iterable returned by `values`.
   */
  protected class DefaultValuesIterable extends AbstractIterable[B] with Iterable[B] with Serializable {
    def iterator = valuesIterator
    override def size = self.size
    override def foreach[C](f: B => C) = self.valuesIterator foreach f
  }

  /** Creates an iterator for all values in this map.
   *
   *  @return an iterator over all values that are associated with some key in this map.
   */
  def valuesIterator: Iterator[B] = new AbstractIterator[B] {
    val iter = self.iterator
    def hasNext = iter.hasNext
    def next() = iter.next()._2
  }

  /** Defines the default value computation for the map,
   *  returned when a key is not found
   *  The method implemented here throws an exception,
   *  but it might be overridden in subclasses.
   *
   *  @param key the given key value for which a binding is missing.
   *  @throws `NoSuchElementException`
   */
  def default(key: A): B =
    throw new NoSuchElementException("key not found: " + key)

  protected class FilteredKeys(p: A => Boolean) extends AbstractMap[A, B] with DefaultMap[A, B] {
    override def foreach[C](f: ((A, B)) => C): Unit = for (kv <- self) if (p(kv._1)) f(kv)
    def iterator = self.iterator.filter(kv => p(kv._1))
    override def contains(key: A) = self.contains(key) && p(key)
    def get(key: A) = if (!p(key)) None else self.get(key)
  }

  /** Filters this map by retaining only keys satisfying a predicate.
   *  @param  p   the predicate used to test keys
   *  @return an immutable map consisting only of those key value pairs of this map where the key satisfies
   *          the predicate `p`. The resulting map wraps the original map without copying any elements.
   */
  def filterKeys(p: A => Boolean): Map[A, B] = new FilteredKeys(p)

  protected class MappedValues[C](f: B => C) extends AbstractMap[A, C] with DefaultMap[A, C] {
    override def foreach[D](g: ((A, C)) => D): Unit = for ((k, v) <- self) g((k, f(v)))
    def iterator = for ((k, v) <- self.iterator) yield (k, f(v))
    override def size = self.size
    override def contains(key: A) = self.contains(key)
    def get(key: A) = self.get(key).map(f)
  }

  /** Transforms this map by applying a function to every retrieved value.
   *  @param  f   the function used to transform values of this map.
   *  @return a map view which maps every key of this map
   *          to `f(this(key))`. The resulting map wraps the original map without copying any elements.
   */
  def mapValues[C](f: B => C): Map[A, C] = new MappedValues(f)

  // The following 5 operations (updated, two times +, two times ++) should really be
  // generic, returning This[B]. We need better covariance support to express that though.
  // So right now we do the brute force approach of code duplication.

  /** Creates a new map obtained by updating this map with a given key/value pair.
   *  @param    key the key
   *  @param    value the value
   *  @tparam   B1 the type of the added value
   *  @return   A new map with the new key/value mapping added to this map.
   *
   *  @usecase  def updated(key: A, value: B): Map[A, B]
   *    @inheritdoc
   */
  def updated [B1 >: B](key: A, value: B1): Map[A, B1] = this + ((key, value))

  /** Adds key/value pairs to this map, returning a new map.
   *
   *  This method takes two or more key/value pairs. Another overloaded
   *  variant of this method handles the case where a single key/value pair is
   *  added.
   *  @param    kv1 the first key/value pair
   *  @param    kv2 the second key/value pair
   *  @param    kvs the remaining key/value pairs
   *  @tparam   B1  the type of the added values
   *  @return   a new map with the given bindings added to this map
   *
   *  @usecase  def + (kvs: (A, B)*): Map[A, B]
   *    @inheritdoc
   *    @param    kvs the key/value pairs
   */
  def + [B1 >: B] (kv1: (A, B1), kv2: (A, B1), kvs: (A, B1) *): Map[A, B1] =
    this + kv1 + kv2 ++ kvs

  /** Adds all key/value pairs in a traversable collection to this map, returning a new map.
   *
   *  @param    xs  the collection containing the added key/value pairs
   *  @tparam   B1  the type of the added values
   *  @return   a new map with the given bindings added to this map
   *
   *  @usecase  def ++ (xs: Traversable[(A, B)]): Map[A, B]
   *    @inheritdoc
   */
  def ++[B1 >: B](xs: GenTraversableOnce[(A, B1)]): Map[A, B1] =
    ((repr: Map[A, B1]) /: xs.seq) (_ + _)

  /** Returns a new map obtained by removing all key/value pairs for which the predicate
   *  `p` returns `true`.
   *
   *  '''Note:'''    This method works by successively removing elements for which the
   *           predicate is true from this set.
   *           If removal is slow, or you expect that most elements of the set
   *           will be removed, you might consider using `filter`
   *           with a negated predicate instead.
   *  @param p    A predicate over key-value pairs
   *  @return     A new map containing elements not satisfying the predicate.
   */
  override def filterNot(p: ((A, B)) => Boolean): This = {
    var res: This = repr
    for (kv <- this)
      if (p(kv)) res = (res - kv._1).asInstanceOf[This] // !!! concrete overrides abstract problem
    res
  }

  /* Overridden for efficiency. */
  override def toSeq: Seq[(A, B)] = toBuffer[(A, B)]
  override def toBuffer[C >: (A, B)]: mutable.Buffer[C] = {
    val result = new mutable.ArrayBuffer[C](size)
    copyToBuffer(result)
    result
  }

  protected[this] override def parCombiner = ParMap.newCombiner[A, B]

  /** Appends all bindings of this map to a string builder using start, end, and separator strings.
   *  The written text begins with the string `start` and ends with the string
   *  `end`. Inside, the string representations of all bindings of this map
   *  in the form of `key -> value` are separated by the string `sep`.
   *
   *  @param b     the builder to which strings are appended.
   *  @param start the starting string.
   *  @param sep   the separator string.
   *  @param end   the ending string.
   *  @return      the string builder `b` to which elements were appended.
   */
  override def addString(b: StringBuilder, start: String, sep: String, end: String): StringBuilder =
    this.iterator.map { case (k, v) => k+" -> "+v }.addString(b, start, sep, end)

  /** Defines the prefix of this object's `toString` representation.
   *  @return  a string representation which starts the result of `toString` applied to this $coll.
   *           Unless overridden in subclasses, the string prefix of every map is `"Map"`.
   */
  override def stringPrefix: String = "Map"

  override /*PartialFunction*/
  def toString = super[IterableLike].toString

}