Functional Programming - Parallelism

Parallelism is a key concept of functional programming where a big task is accomplished by breaking in smaller independent tasks and then these small tasks are completed in a parallel fashion and later combined to give the complete result. With the advent of multi-core processors, this technique helps in faster code execution. Java has Thread based programming support for parallel processing but it is quite tedious to learn and difficult to implement without bugs. Java 8 onwards, stream have parallel method and collections has parallelStream() method to complete tasks in parallel fashion.

Example - Using Serial Stream

FunctionTester.java

package com.tutorialspoint;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

public class FunctionTester {
   public static void main(String[] args) {

      Integer[] intArray = {1, 2, 3, 4, 5, 6, 7, 8 };
      List<Integer> listOfIntegers = new ArrayList<>(Arrays.asList(intArray));

      System.out.println("List using Serial Stream:");
      listOfIntegers
         .stream()
         .forEach(e -> System.out.print(e + " "));
   } 
}

Output

Run the FunctionTester and verify the output.

List using Serial Stream:
1 2 3 4 5 6 7 8  

Example - Using Parallel Stream

FunctionTester.java

package com.tutorialspoint;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

public class FunctionTester {
   public static void main(String[] args) {

      Integer[] intArray = {1, 2, 3, 4, 5, 6, 7, 8 };
      List<Integer> listOfIntegers = new ArrayList<>(Arrays.asList(intArray));

      System.out.println("List using Parallel Stream:");
      listOfIntegers
         .parallelStream()
         .forEach(e -> System.out.print(e + " "));
      System.out.println();

      System.out.println("List using Another Parallel Stream:");
      listOfIntegers
         .stream()
         .parallel()
         .forEach(e -> System.out.print(e + " "));
      System.out.println();

      System.out.println("List using Parallel Stream but Ordered:");
      listOfIntegers
         .parallelStream()
         .forEachOrdered(e -> System.out.print(e + " "));
      System.out.println(); 
   } 
}

Output

Run the FunctionTester and verify the output.

List using Parallel Stream:
6 5 8 7 3 4 2 1 
List using Another Parallel Stream:
6 2 1 7 4 3 8 5 
List using Parallel Stream but Ordered:
1 2 3 4 5 6 7 8