I'm very confused by Scala's
Scala's mutable and immutable
HashSet implementations are concrete classes which you can instantiate. For example, if you explicitly ask for a new
scala.collection.immutable.HashSet, you will always get a set which is implemented by a hash trie. There are other set implementations, such as
ListSet, which uses a list.
Set is a trait which all the set implementations extend (whereas in Java,
Set is an interface).
Set is also a companion object* with an
apply** method. When you call
Set(...), you're calling this factory method and getting a return value which is some kind of
Set. It might be a
HashSet, but could be some other implementation. According to 2, the default implementation for an immutable set has special representation for empty set and sets size up to 4. Immutable sets size 5 and above and mutable sets all use hashSet.
*In Scala, instead of having static class methods, you can create a singleton
object with the same name as your class or trait. This is called a companion object, and methods you define on it can be called as
ObjectName.method(), similar to how you'd call a static method in Java.
Set(x) is syntactic sugar for