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.
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3 Answers
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 
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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.
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oxbow_lakes
As it turns out I can't do this because my "inner" collection is actually an instance of java.util.List, not a Scala Seq. I've asked this question as a follow up: stackoverflow.com/questions/674713/…
From Scala 2.13 use the companion object
import scala.collection.immutable.HashSet val list = List(1,2,3) val set = HashSet.from(list) 