MapsSynchronized sets and mapsContents

Synchronized sets and maps

To get a thread-safe mutable map, you can mix the SynchronizedMap trait 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?"
        }
    }
  }

Mixing in the SynchronizedMap trait.
 

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",
            "Paris" -> "France", "Japan" -> "Tokyo")
res0: scala.collection.mutable.Map[String,String] =
  Map(Paris -> France, 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. !!!explain concurrent map!!!

Next: Concrete Immutable Collection Classes

MapsSynchronized sets and mapsContents