Java 8 Parallel Streams Examples

Few Java 8 examples to execute streams in parallel.

1. BaseStream.parallel()

A simple parallel example to print 1 to 10.

package com.mkyong.java8;
import java.util.stream.IntStream;
public class ParallelExample1 {
    public static void main(String[] args) {
        System.out.println("Normal...");
		IntStream range = IntStream.rangeClosed(1, 10);
        range.forEach(System.out::println);
        System.out.println("Parallel...");
        IntStream range2 = IntStream.rangeClosed(1, 10);
        range2.parallel().forEach(System.out::println);

Output

Normal...
10
Parallel...
10

2. Collection.parallelStream()

Another simple parallel example to print a to z. For collection, we can use parallelStream().

package com.mkyong.java8;
import java.util.ArrayList;
import java.util.List;
public class ParallelExample2 {
    public static void main(String[] args) {
        System.out.println("Normal...");
        List<String> alpha = getData();
        alpha.stream().forEach(System.out::println);
        System.out.println("Parallel...");
        List<String> alpha2 = getData();
        alpha2.parallelStream().forEach(System.out::println);
    private static List<String> getData() {
        List<String> alpha = new ArrayList<>();
        int n = 97;  // 97 = a , 122 = z
        while (n <= 122) {
            char c = (char) n;
            alpha.add(String.valueOf(c));
            n++;
        return alpha;

Output

Normal...
Parallel...

3. Is Stream running in parallel mode?

3.1 We can test it with isParallel()

package com.mkyong.java8;
import java.util.stream.IntStream;
public class ParallelExample3a {
    public static void main(String[] args) {
        System.out.println("Normal...");
        IntStream range = IntStream.rangeClosed(1, 10);
        System.out.println(range.isParallel());         // false
        range.forEach(System.out::println);
        System.out.println("Parallel...");
        IntStream range2 = IntStream.rangeClosed(1, 10);
        IntStream range2Parallel = range2.parallel();
        System.out.println(range2Parallel.isParallel()); // true
        range2Parallel.forEach(System.out::println);

3.2 Or print the current thread name like this:

package com.mkyong.java8;
import java.util.stream.IntStream;
public class ParallelExample3b {
    public static void main(String[] args) {
        System.out.println("Normal...");
        IntStream range = IntStream.rangeClosed(1, 10);
        range.forEach(x -> {
            System.out.println("Thread : " + Thread.currentThread().getName() + ", value: " + x);
        });
        System.out.println("Parallel...");
        IntStream range2 = IntStream.rangeClosed(1, 10);
        range2.parallel().forEach(x -> {
            System.out.println("Thread : " + Thread.currentThread().getName() + ", value: " + x);
        });

Output

Normal...
Thread : main, value: 1
Thread : main, value: 2
Thread : main, value: 3
Thread : main, value: 4
Thread : main, value: 5
Thread : main, value: 6
Thread : main, value: 7
Thread : main, value: 8
Thread : main, value: 9
Thread : main, value: 10
Parallel...
Thread : main, value: 7
Thread : main, value: 6
Thread : ForkJoinPool.commonPool-worker-5, value: 3
Thread : ForkJoinPool.commonPool-worker-7, value: 8
Thread : ForkJoinPool.commonPool-worker-5, value: 5
Thread : ForkJoinPool.commonPool-worker-5, value: 4
Thread : ForkJoinPool.commonPool-worker-3, value: 9
Thread : ForkJoinPool.commonPool-worker-5, value: 1
Thread : ForkJoinPool.commonPool-worker-7, value: 2
Thread : ForkJoinPool.commonPool-worker-9, value: 10

P.S By default, parallel streams use `ForkJoinPool`

4. Calculation

4.1 Java 8 streams to print all prime numbers up to 1 million:

package com.mkyong.java8;
import java.util.ArrayList;
import java.util.List;
import java.util.stream.IntStream;
import java.util.stream.Stream;
public class ParallelExample4 {
    public static void main(String[] args) {
        long count = Stream.iterate(0, n -> n + 1)
                .limit(1_000_000)
                //.parallel()   with this 23s, without this 1m 10s
                .filter(ParallelExample4::isPrime)
                .peek(x -> System.out.format("%s\t", x))
                .count();
        System.out.println("\nTotal: " + count);
    public static boolean isPrime(int number) {
        if (number <= 1) return false;
        return !IntStream.rangeClosed(2, number / 2).anyMatch(i -> number % i == 0);

Result:

• For normal streams, it takes 1 minute 10 seconds.
• For parallel streams, it takes 23 seconds.

P.S Tested with i7-7700, 16G RAM, WIndows 10

4.2 Yet another parallel stream example to find out the average age of a list of employees.

	List<Employee> employees = obj.generateEmployee(10000);
	double age = employees
			.parallelStream()
			.mapToInt(Employee::getAge)
			.average()
			.getAsDouble();
    System.out.println("Average age: " + age);	

5. Case Study

5.1 Parallel streams to increase the performance of a time-consuming save file tasks.

This Java code will generate 10,000 random employees and save into 10,000 files, each employee save into a file.

• For normal stream, it takes 27-29 seconds.
• For parallel stream, it takes 7-8 seconds.

P.S Tested with i7-7700, 16G RAM, WIndows 10

package com.mkyong.java8;
import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectOutputStream;
import java.math.BigDecimal;
import java.math.RoundingMode;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.util.List;
import java.util.Random;
import java.util.stream.Collectors;
import java.util.stream.Stream;
public class ParallelExample5 {
    private static final String DIR = System.getProperty("user.dir") + "/test/";
    public static void main(String[] args) throws IOException {
        Files.createDirectories(Paths.get(DIR));
        ParallelExample5 obj = new ParallelExample5();
        List<Employee> employees = obj.generateEmployee(10000);
		// normal, sequential
        //employees.stream().forEach(ParallelExample5::save); 		// 27s-29s
		// parallel
        employees.parallelStream().forEach(ParallelExample5::save); // 7s-8s
    private static void save(Employee input) {
        try (FileOutputStream fos = new FileOutputStream(new File(DIR + input.getName() + ".txt"));
             ObjectOutputStream obs = new ObjectOutputStream(fos)) {
            obs.writeObject(input);
        } catch (IOException e) {
            e.printStackTrace();
    private List<Employee> generateEmployee(int num) {
        return Stream.iterate(0, n -> n + 1)
                .limit(num)
                .map(x -> {
                    return new Employee(
                            generateRandomName(4),
                            generateRandomAge(15, 100),
                            generateRandomSalary(900.00, 200_000.00)
                    );
                })
                .collect(Collectors.toList());
    private String generateRandomName(int length) {
        return new Random()
                .ints(5, 97, 122) // 97 = a , 122 = z
                .mapToObj(x -> String.valueOf((char) x))
                .collect(Collectors.joining());
    private int generateRandomAge(int min, int max) {
        return new Random()
                .ints(1, min, max)
                .findFirst()
                .getAsInt();
    private BigDecimal generateRandomSalary(double min, double max) {
        return new BigDecimal(new Random()
                .doubles(1, min, max)
                .findFirst()
                .getAsDouble()).setScale(2, RoundingMode.HALF_UP);

package com.mkyong.java8;
import java.io.Serializable;
import java.math.BigDecimal;
public class Employee implements Serializable {
    private static final long serialVersionUID = 1L;
    private String name;
    private int age;
    private BigDecimal salary;
    //getters, setters n etc...