Scala example source code file (MultiMap.scala)

This example Scala source code file (MultiMap.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.

All credit for the original source code belongs to scala-lang.org; I'm just trying to make examples easier to find. (For my Scala work, see my Scala examples and tutorials.)

Scala tags/keywords

a, b, boolean, hashset, map, multimap, none, set, some

The MultiMap.scala Scala example source code

/*                     __                                               *\
**     ________ ___   / /  ___     Scala API                            **
**    / __/ __// _ | / /  / _ |    (c) 2003-2013, LAMP/EPFL             **
**  __\ \/ /__/ __ |/ /__/ __ |                                         **
** /____/\___/_/ |_/____/_/ | |                                         **
**                          |/                                          **
\*                                                                      */



package scala
package collection
package mutable


/** A trait for mutable maps with multiple values assigned to a key.
 *
 *  This class is typically used as a mixin. It turns maps which map `A`
 *  to `Set[B]` objects into multimaps that map `A` to `B` objects.
 *
 *  @example {{{
 *  // first import all necessary types from package `collection.mutable`
 *  import collection.mutable.{ HashMap, MultiMap, Set }
 *
 *  // to create a `MultiMap` the easiest way is to mixin it into a normal
 *  // `Map` instance
 *  val mm = new HashMap[Int, Set[String]] with MultiMap[Int, String]
 *
 *  // to add key-value pairs to a multimap it is important to use
 *  // the method `addBinding` because standard methods like `+` will
 *  // overwrite the complete key-value pair instead of adding the
 *  // value to the existing key
 *  mm.addBinding(1, "a")
 *  mm.addBinding(2, "b")
 *  mm.addBinding(1, "c")
 *
 *  // mm now contains `Map(2 -> Set(b), 1 -> Set(c, a))`
 *
 *  // to check if the multimap contains a value there is method
 *  // `entryExists`, which allows to traverse the including set
 *  mm.entryExists(1, _ == "a") == true
 *  mm.entryExists(1, _ == "b") == false
 *  mm.entryExists(2, _ == "b") == true
 *
 *  // to remove a previous added value there is the method `removeBinding`
 *  mm.removeBinding(1, "a")
 *  mm.entryExists(1, _ == "a") == false
 *  }}}
 *
 *  @define coll multimap
 *  @define Coll `MultiMap`
 *  @author  Matthias Zenger
 *  @author  Martin Odersky
 *  @version 2.8
 *  @since   1
 */
trait MultiMap[A, B] extends Map[A, Set[B]] {
  /** Creates a new set.
   *
   *  Classes that use this trait as a mixin can override this method
   *  to have the desired implementation of sets assigned to new keys.
   *  By default this is `HashSet`.
   *
   *  @return An empty set of values of type `B`.
   */
  protected def makeSet: Set[B] = new HashSet[B]

  /** Assigns the specified `value` to a specified `key`, replacing
   *  the existing value assigned to that `key` if it is equal to
   *  the specified value. Otherwise, simply adds another binding to
   *  the `key`.
   *
   *  @param key    The key to which to bind the new value.
   *  @param value  The value to bind to the key.
   *  @return       A reference to this multimap.
   */
  def addBinding(key: A, value: B): this.type = {
    get(key) match {
      case None =>
        val set = makeSet
        set += value
        this(key) = set
      case Some(set) =>
        set += value
    }
    this
  }

  /** Removes the binding of `value` to `key` if it exists, otherwise this
   *  operation doesn't have any effect.
   *
   *  If this was the last value assigned to the specified key, the
   *  set assigned to that key will be removed as well.
   *
   *  @param key     The key of the binding.
   *  @param value   The value to remove.
   *  @return        A reference to this multimap.
   */
  def removeBinding(key: A, value: B): this.type = {
    get(key) match {
      case None =>
        case Some(set) =>
          set -= value
          if (set.isEmpty) this -= key
    }
    this
  }

  /** Checks if there exists a binding to `key` such that it satisfies the predicate `p`.
   *
   *  @param key   The key for which the predicate is checked.
   *  @param p     The predicate which a value assigned to the key must satisfy.
   *  @return      A boolean if such a binding exists
   */
  def entryExists(key: A, p: B => Boolean): Boolean = get(key) match {
    case None => false
    case Some(set) => set exists p
  }
}