Maps

A Map is an Iterable consisting of pairs of keys and values (also named mappings or associations). Scala’s Predef object offers an implicit conversion that lets you write key -> value as an alternate syntax for the pair (key, value). For instance Map("x" -> 24, "y" -> 25, "z" -> 26) means exactly the same as Map(("x", 24), ("y", 25), ("z", 26)), but reads better.

The fundamental operations on maps are similar to those on sets. They are summarized in the following table and fall into the following categories:

• Lookup operations apply, get, getOrElse, contains, and isDefinedAt. These turn maps into partial functions from keys to values. The fundamental lookup method for a map is: def get(key): Option[Value]. The operation “m get key” tests whether the map contains an association for the given key. If so, it returns the associated value in a Some. If no key is defined in the map, get returns None. Maps also define an apply method that returns the value associated with a given key directly, without wrapping it in an Option. If the key is not defined in the map, an exception is raised.
• Additions and updates +, ++, updated, which let you add new bindings to a map or change existing bindings.
• Removals -, --, which remove bindings from a map.
• Subcollection producers keys, keySet, keysIterator, values, valuesIterator, which return a map’s keys and values separately in various forms.
• Transformations filterKeys and mapValues, which produce a new map by filtering and transforming bindings of an existing map.

Operations in Class Map

Mutable maps support in addition the operations summarized in the following table.

Operations in Class mutable.Map

The addition and removal operations for maps mirror those for sets. Like sets, mutable maps also support the non-destructive addition operations +, -, and updated, but they are used less frequently because they involve a copying of the mutable map. Instead, a mutable map m is usually updated “in place”, using the two variants m(key) = value or m += (key -> value). There is also the variant m put (key, value), which returns an Option value that contains the value previously associated with key, or None if the key did not exist in the map before.

The getOrElseUpdate is useful for accessing maps that act as caches. Say you have an expensive computation triggered by invoking a function f:

scala> def f(x: String) = {
       println("taking my time."); sleep(100)
       x.reverse }
f: (x: String)String

Assume further that f has no side-effects, so invoking it again with the same argument will always yield the same result. In that case you could save time by storing previously computed bindings of argument and results of f in a map and only computing the result of f if a result of an argument was not found there. One could say the map is a cache for the computations of the function f.

scala> val cache = collection.mutable.Map[String, String]()
cache: scala.collection.mutable.Map[String,String] = Map()

You can now create a more efficient caching version of the f function:

scala> def cachedF(s: String) = cache.getOrElseUpdate(s, f(s))
cachedF: (s: String)String
scala> cachedF("abc")
taking my time.
res3: String = cba
scala> cachedF("abc")
res4: String = cba

Note that the second argument to getOrElseUpdate is “by-name”, so the computation of f("abc") above is only performed if getOrElseUpdate requires the value of its second argument, which is precisely if its first argument is not found in the cache map. You could also have implemented cachedF directly, using just basic map operations, but it would take more code to do so:

def cachedF(arg: String) = cache get arg match {
  case Some(result) => result
  case None =>
    val result = f(x)
    cache(arg) = result
    result
}

Synchronized Sets and Maps

To get a thread-safe mutable map, you can mix the SynchronizedMap trait into whatever particular map implementation you desire. For example, you can mix SynchronizedMap into HashMap, as shown in the code below. This example begins with an import of two traits, Map and SynchronizedMap, and one class, HashMap, from package scala.collection.mutable. The rest of the example is the definition of singleton object MapMaker, which declares one method, makeMap. The makeMap method declares its result type to be a mutable map of string keys to string values.

  import scala.collection.mutable.{Map,
      SynchronizedMap, HashMap}
  object MapMaker {
    def makeMap: Map[String, String] = {
        new HashMap[String, String] with
            SynchronizedMap[String, String] {
          override def default(key: String) =
            "Why do you want to know?"
        }
    }
  }

The first statement inside the body of makeMap constructs a new mutable HashMap that mixes in the SynchronizedMap trait:

new HashMap[String, String] with
  SynchronizedMap[String, String]

Given this code, the Scala compiler will generate a synthetic subclass of HashMap that mixes in SynchronizedMap, and create (and return) an instance of it. This synthetic class will also override a method named default, because of this code:

override def default(key: String) =
  "Why do you want to know?"

If you ask a map to give you the value for a particular key, but it doesn’t have a mapping for that key, you’ll by default get a NoSuchElementException. If you define a new map class and override the default method, however, your new map will return the value returned by default when queried with a non-existent key. Thus, the synthetic HashMap subclass generated by the compiler from the code in the synchronized map code will return the somewhat curt response string, "Why do you want to know?", when queried with a non-existent key.

Because the mutable map returned by the makeMap method mixes in the SynchronizedMap trait, it can be used by multiple threads at once. Each access to the map will be synchronized. Here’s an example of the map being used, by one thread, in the interpreter:

scala> val capital = MapMaker.makeMap  
capital: scala.collection.mutable.Map[String,String] = Map()
scala> capital ++ List("US" -> "Washington",
        "France" -> "Paris", "Japan" -> "Tokyo")
res0: scala.collection.mutable.Map[String,String] =
  Map(France -> Paris, US -> Washington, Japan -> Tokyo)
scala> capital("Japan")
res1: String = Tokyo
scala> capital("New Zealand")
res2: String = Why do you want to know?
scala> capital += ("New Zealand" -> "Wellington")
scala> capital("New Zealand")                    
res3: String = Wellington

You can create synchronized sets similarly to the way you create synchronized maps. For example, you could create a synchronized HashSet by mixing in the SynchronizedSet trait, like this:

import scala.collection.mutable
val synchroSet =
  new mutable.HashSet[Int] with
      mutable.SynchronizedSet[Int]

Finally, if you are thinking of using synchronized collections, you may also wish to consider the concurrent collections of java.util.concurrent instead.