Scala equivalent of new HashSet(Collection)

Question

What is the equivalent Scala constructor (to create an immutable HashSet) to the Java

new HashSet<T>(c)

where c is of type Collection<? extends T>?.

All I can find in the HashSet Object is apply.

Answer 1

The most concise way to do this is probably to use the ++ operator:

import scala.collection.immutable.HashSet
val list = List(1,2,3)
val set = HashSet() ++ list

Answer 2

There are two parts to the answer. The first part is that Scala variable argument methods that take a T* are a sugaring over methods taking Seq[T]. You tell Scala to treat a Seq[T] as a list of arguments instead of a single argument using "seq : _*".

The second part is converting a Collection[T] to a Seq[T]. There's no general built in way to do in Scala's standard libraries just yet, but one very easy (if not necessarily efficient) way to do it is by calling toArray. Here's a complete example.

scala> val lst : java.util.Collection[String] = new java.util.ArrayList
lst: java.util.Collection[String] = []

scala> lst add "hello"
res0: Boolean = true

scala> lst add "world"
res1: Boolean = true

scala> Set(lst.toArray : _*)
res2: scala.collection.immutable.Set[java.lang.Object] = Set(hello, world)

Note the scala.Predef.Set and scala.collection.immutable.HashSet are synonyms.