scala.collection.mutable.HashSet遅い?

Main.scala

import scala.collection.JavaConverters._

object Main extends App {
  def bench(set1: java.util.Set[Int], set2: java.util.Set[Int]) = {
    val start = System.nanoTime
    var sum = 0

    0.until(10).map(_ => {
      val diff = new java.util.HashSet[Int]

      diff.addAll(set1)

      diff.removeAll(set2)

      diff.size
    })

    val end = System.nanoTime

    println("Java Set: %s ms".format((end - start) / 1000.0 / 1000.0))
  }

  def bench(set1: scala.collection.mutable.Set[Int], set2: scala.collection.mutable.Set[Int]) = {
    val start = System.nanoTime
    var sum = 0

    0.until(10).map(_ => {
      val diff = scala.collection.mutable.HashSet.empty[Int]

      diff ++= set1

      // ++= が遅いというのを示すためにここはコメントアウトしておく
      // diff --= set2

      diff.size
    })

    val end = System.nanoTime

    println("Scala Set: %s ms".format((end - start) / 1000.0 / 1000.0))
  }
  

  val values1 = 0.until(100000).filter(_ => math.random < 0.5)
  val values2 = 0.until(100000).filter(_ => math.random < 0.5)

  val javaSet1 = new java.util.HashSet[Int]
  val javaSet2 = new java.util.HashSet[Int]

  javaSet1.addAll(values1.asJava)
  javaSet2.addAll(values2.asJava)

  val scalaSet1 = scala.collection.mutable.HashSet.empty[Int]
  val scalaSet2 = scala.collection.mutable.HashSet.empty[Int]

  scalaSet1 ++= values1
  scalaSet2 ++= values2
  
  for (x <- 0.until(100)) {
    bench(javaSet1, javaSet2)
    bench(scalaSet1, scalaSet2)
    bench(javaSet1, javaSet2)
    bench(scalaSet1, scalaSet2)
  }
}