Java Stream - 如何验证并行流

import java.util.stream.IntStream;

public class Main {
  public static void main(String[] args) {
    long first = System.currentTimeMillis();
    IntStream.range(0, 4).map(a -> {
      sleep1sec();
      return a;
    }).reduce((a, b) -> a * b).ifPresent(System.out::println);

    long second = System.currentTimeMillis();
    System.out.println("time 1:" + (second - first));

    IntStream.range(0, 4).parallel().map(a -> {
      sleep1sec();
      return a;
    }).reduce((a, b) -> a * b).ifPresent(System.out::println);

    long last = System.currentTimeMillis();
    System.out.println("time 2:" + (last - second));

  }

  private static void sleep1sec() {
    try {
      Thread.sleep(1000);
    } catch (InterruptedException e) {
    }
  }
}