Java 8 parallel thread takes longer

Question

I am trying to learn about Java 8 parallel thread. I wrote the code below, using Executor first and then using parallel thread. it seems like a parallel thread takes twice (10 seconds) as much time as an Executor (5 seconds) fits. in my opinion, parallel thread should also show similar performance. any idea why parallel thread takes double time? my computer has 8 cores.

/**
 * 
 */
package com.shashank.java8.parallel_stream;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Date;
import java.util.List;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;

/**
 * @author pooja
 *
 */
public class Sample {

    public static int processUrl(String url) {

        try {
            Thread.sleep(5000);
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        System.out.println("Running Thread " + Thread.currentThread());
        return url.length();
    }

    /**
     * @param args
     * @throws Exception
     */
    public static void main(String[] args) throws Exception {
        usingExecutor();
        usingParallelStream();
    }

    public static void usingParallelStream() {

        Date start = new Date();
        // TODO Auto-generated method stub
        int total = buildUrlsList().parallelStream().mapToInt(Sample::processUrl).reduce(0, Integer::sum);
        Date end = new Date();
        System.out.println(total);
        System.out.println((end.getTime() - start.getTime()) / 1000);

    }

    public static void usingExecutor() throws Exception {
        Date start = new Date();
        ExecutorService executorService = Executors.newFixedThreadPool(100);
        List<Future> futures = new ArrayList<>();

        for (String url : buildUrlsList()) {
            futures.add(executorService.submit(() -> processUrl(url)));

        }

        // iterate through the future
        int total = 0;
        for (Future<Integer> future : futures) {
            total += future.get();
        }
        System.out.println(total);
        Date end = new Date();
        System.out.println((end.getTime() - start.getTime()) / 1000);

    }

    public static List<String> buildUrlsList() {
        return Arrays.asList("url1", "url2", "url3", "url4", "url5", "url6", "url7", "url8", "url9");

    }

}

Answer 1

. 8 , parallelStream() 8 . , 5 . (9-) , 5 . . ~ 5 (8 ) + 5 (1 ) = 10 . . :

 public static int processUrl(String url) {

    try {
        Thread.sleep(5000);
    } catch (InterruptedException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
    System.out.println("T[" + Thread.currentThread().getId() + "] finished @[" + System.currentTimeMillis() / 1000 + "]");
    return url.length();
}

, :

T[1] finished @[1494267500]
T[12] finished @[1494267500]
T[17] finished @[1494267500]
T[13] finished @[1494267500]
T[14] finished @[1494267500]
T[16] finished @[1494267500]
T[11] finished @[1494267500]
T[15] finished @[1494267500]
T[12] finished @[1494267505]
36
10

, T [12] 5 "" 8 .

100 . , 9 , 5 , :

T[14] finished @[1494267783]
T[11] finished @[1494267783]
T[19] finished @[1494267783]
T[17] finished @[1494267783]
T[12] finished @[1494267783]
T[16] finished @[1494267783]
T[13] finished @[1494267783]
T[15] finished @[1494267783]
T[18] finished @[1494267783]
36
5

, . ( :-) ).

8 :

ExecutorService executorService = Executors.newFixedThreadPool(8);

, , , . , URL- 8, 9.

, .