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All I desire is to use some concurrent Set (that appears not to exist at all). Java uses java.util.concurrent.ConcurrentHashMap<K, Void> to achieve that behavior. I'd like to do sth similar in Scala so I created instance of Scala HashMap (or Java ConcurrentHashMap) and tried to add some tuples:
val myMap = new HashMap[String, Unit]()
myMap + (("myStringKey", Unit))
This of course crashed the process of compilation as Unit is abstract and final.
How to make this work? Should I use Any/AnyRef instead? I must ensure nobody inserts any value.
Thanks for help
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Instead, Scala has singleton objects. A singleton is a class that can have only one instance, i.e., Object. You create singleton using the keyword object instead of class keyword. Since you can't instantiate a singleton object, you can't pass parameters to the primary constructor.
Unit is a subtype of scala. AnyVal. There is only one value of type Unit , () , and it is not represented by any object in the underlying runtime system. A method with return type Unit is analogous to a Java method which is declared void .
=> is syntactic sugar for creating instances of functions. Recall that every function in scala is an instance of a class. For example, the type Int => String , is equivalent to the type Function1[Int,String] i.e. a function that takes an argument of type Int and returns a String .
If you don't specify any return type of a function, default return type is Unit which is equivalent to void in Java. = : In Scala, a user can create function with or without = (equal) operator. If the user uses it, the function will return the desired value.
You can just use () whose type is Unit:
scala> import scala.collection.mutable.HashMap
import scala.collection.mutable.HashMap
scala> val myMap = new HashMap[String, Unit]()
myMap: scala.collection.mutable.HashMap[String,Unit] = Map()
scala> myMap + ("myStringKey" -> ())
res1: scala.collection.mutable.Map[String,Unit] = Map(myStringKey -> ())
This is a comment taken from Unit.scala:
There is only one value of type
Unit,(), and it is not represented by any object in the underlying runtime system.
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