ネストされた並列ストリームでは、ScalaのストリームとJavaのストリームで挙動が異なる

NestedParallelStream.scala

import java.util.stream.IntStream

object NestedParallelStream {
  def main(args: Array[String])]: Unit = {
    iterateScalaStream()
    iterateJavaStream()
  }
  
  def iterateScalaStream(): Unit = {
    Stream.range(0, 100)
      .par
      .flatMap(i => {
        Thread.sleep(latency())
        System.out.println("map: " + i)
        Stream.range(0 , 100)
          .par
          .map(j => i * 100 + j)
      })
      .foreach(i => System.out.println(i))
  }
  
  def iterateJavaStream(): Unit = {
    IntStream.range(0, 100)
      .parallel()
      .flatMap(i => {
        Thread.sleep(latency())
        System.out.println("map: " + i)
        IntStream.range(0, 100)
          .parallel()
          .map(j => i * 100 + j)
      }).forEach(i => System.out.println(i))
  }
  
  def latency(): Long = (500 + (Math.random() * 1500)).longValue()
}