scala thread safe HashSet

Question

What are the possible ways to make a HashSet thread safe? Saw some samples as given below.

var test = new mutable.HashSet[Long] with mutable.SynchronizedSet[Long]

SynchronizedSet is deprecated at present. Any suggestions or samples will be very much helpful.

Answer 1

As the API documentation of scala.collection.mutable.SynchronizedSet suggests, you can use java.util.concurrent.ConcurrentHashMap[A, Unit] instead.

If you want it to look like a Set instead of like a Map, then you can use java.util.Collections.newSetFromMap to add a wrapper around the Map to make it look like a Set:

def createSet[T]() = java.util.Collections.newSetFromMap(
  new java.util.concurrent.ConcurrentHashMap[T, java.lang.Boolean])

This will, however, return a Java Set. You can wrap this as a scala.collection.mutable.Set:

def createSet[T]() = {
  import scala.collection.JavaConverters._
  java.util.Collections.newSetFromMap(
    new java.util.concurrent.ConcurrentHashMap[T, java.lang.Boolean]).asScala
}

Now you can create a synchronized set with elements of a specific type, for example Long, like this:

val set = createSet[Long]

Answer 2

Building upon Jespers solution scala thread safe HashSet:

val myConcurrentSet = new java.util.concurrent.ConcurrentHashMap[String, Boolean]().keySet(true)

This way you use only the keys of the Map, thus a Set. Also it's needed to configure the default value for all the values, here as true but could be false as well. Failing to set this default value for key additions, when adding a key the method will raise:

[info]   java.lang.UnsupportedOperationException:
[info]   at java.base/java.util.concurrent.ConcurrentHashMap$KeySetView.add(ConcurrentHashMap.java:4651)

as v will be null in java.base/java.util.concurrent.ConcurrentHashMap.add(K e):

/**...
* Throws:
* ...
* UnsupportedOperationException – if no default mapped value for additions was provided
**/
public boolean add(K e) {
            V v;
            if ((v = value) == null)
                throw new UnsupportedOperationException();
            return map.putVal(e, v, true) == null;
        }