This is my ProcessDataFileParallel.Java file. I'm taking numbers from a .txt file and putting it into an array in Java. While it works, I would like to improve the code and possibly change some algorithms to better alternatives.

import java.io.BufferedReader;
import java.io.FileReader;
import java.io.IOException;
import java.util.Scanner;

public class ProcessDataFileParallel {

public static void main(String[] args) throws IOException {
    int[] array = new int[100000];
    int index = 0;
    Scanner inputFile = null;
    Scanner scan = new Scanner(System.in);
    boolean running = false;

    String emptyString;
    FileReader file = new FileReader("dataset529.txt");
    BufferedReader br = new BufferedReader(file);
    emptyString = br.readLine();

    try {
        while ((emptyString = br.readLine()) != null) {
            array[index++] = Integer.parseInt(emptyString);   

    } finally {
        if (br.readLine() == null)  

    inputFile = new Scanner(file);

// Read File from Dataset529.txt
    if(inputFile != null){
        System.out.println("Number of integers in: "  + file);

            while (inputFile.hasNext() && inputFile.hasNextInt()){
                    array[index] = inputFile.nextInt();


        // Print dataset529.txt to Array in Java with For Loop below.
        for (int ai = 0; ai < index; ai++){
            System.out.println("Array Index: " + "(" + ai + ")" + "== >" + " " + array[ai]);

    System.out.println("How many Threads?");  // Scanner -  Ask user how many threads

    int n = scan.nextInt();  // Create variable 'n'  to handle whatever integer the user specifies.  nextInt() is used for the scanner to expect and Int.

    Thread[] myThreads = new Thread[n]; // Variable n placed here to determine how many Threads user wanted
    Worker[] workers = new Worker[n]; // Variable N placed here - the number user specified will determine # of workers.

    int range = array.length/n;  //divides the array into n threads (what scanner specifies)
    for (index = 0; index < n; index ++){
        int start = index*range;
        int end = start + range;
        workers[index] = new Worker(start,end, array);


    for( index = 0; index < n; index++){
        myThreads[index] = new Thread(workers[index]);

    if (running){
         for (Thread t : myThreads) {
             if (t.isAlive()) {
                 running = true;
     } while (running);

     for (Worker worker : workers) {
         System.out.println("Max = " + worker.getMax());


Worker class:

public class Worker implements Runnable {

            private int start;
            private int end;
            private int randomNumbers[];

            int max = Integer.MIN_VALUE;

            public Worker(int start, int end, int[] randomNumbers) {
                this.start = start;
                this.end = end;
                this.randomNumbers = randomNumbers;

            public void run() {
                for (int index = start; index < end; index++) {

                    if (randomNumbers[index] >max)
                        max = randomNumbers[index];

            public int getMax() {
                return max;


Is there any way I can improve/condense my code? Also, is there any way to possibly do alternatives that would be better?

  • \$\begingroup\$ Is this meant to be more like an academic exercise on multi-threading? Also, are you on Java 8? \$\endgroup\$
    – h.j.k.
    Commented Oct 30, 2015 at 3:06

1 Answer 1



If the array length is not a multiplier of the number of threads, you'll miss the remainder! Quick example:

public static void printRange(int length, int n) {
    int range = length / n;
    for (int index = 0; index < n; index ++){
        int start = index * range;
        int end = start + range;
        System.out.printf("[%s, %s)%n", start, end);

// Output for printRange(10, 3):
[0, 3)
[3, 6)
[6, 9)

// Output for printRange(10, 6):
[0, 1)
[1, 2)
[2, 3)
[3, 4)
[4, 5)
[5, 6)

As you can see, you'll miss the 10th element for printRange(10, 3), and up to half the array for printRange(10, 6).

edit: A simplistic approach is to spin a new Thread to handle the remaining elements, but as you can see from the second example, you will end up a relatively high number of five elements for it, whereas the first five threads are only processing (and returning) one element. An alternative approach is to add an extra element per Thread so that you have more threads doing slightly more work, than one single thread doing most of the work.


The preferred way of doing multi-threading since Java 5 is to use an ExecutorService to help you manage the lifecycle of threads. You should read up more on Oracle's tutorial to understand how they are used.

In addition, since you want each thread to compute and return a result for you, you should be looking at the Callable interface. Implementations override the call() method to return an appropriate result.


If you are on Java 7 and above, you should use try-with-resources for efficient handling of the underlying I/O resource:

public static void main(String[] args) {
    try (Scanner scanner = new Scanner(System.in)) {
        // ...

Hard-coding array length

int[] array = new int[100000];

What happens if your file has more than this number of elements? Actually, you also seem to be processing your file twice, once using the BufferedReader approach, and the second using a Scanner. Is this expected?


Getting the max value

Your current implementation forgets to actually compare the max of each partition with each other to arrive at the desired result.

Is this meant to be more like an academic exercise on multi-threading? - myself

The reason why I'm asking is that while it looks like you're looking more at a map-reduce approach to your problem, it may not even be necessary in the first place. Processing 100,000 integers (based on your int[] declaration) or less in a single thread is relatively fast enough on any modern hardware capable of running the JVM. If you are however trying to compare billions of numbers and/or given a (strangely low) memory constraint, then that's when multi-threading will help. However, your question will be quite different at that stage.

Getting the max value (part 2)

From Java 8 onwards, there are two approaches of running tasks asynchronously:

  1. ExecutorService to create Futures, and then getting the results from each of them.
  2. Chaining CompletableFutures (Java 8 onwards).

The code below demonstrates both:

public class AsyncMax {

    private static final int MAX = 10;

    public static void main(String[] args) {
        System.out.println("Using ExecutorService:");
        ExecutorService service = Executors.newWorkStealingPool();
        List<Future<Integer>> esResults = IntStream.range(0, MAX).parallel()
                .mapToObj(i -> service.submit(() -> produce(i).get()))
                .map(f -> get(f))
        System.out.println("Using CompletableFuture:");
        CompletableFuture<List<Integer>> cfResults = IntStream.range(0, MAX).parallel()
                .mapToObj(i -> produce(i))
        cfResults.thenApply(results -> results.stream().max(Comparator.naturalOrder()))

For both approaches, IntStream.range(0, MAX).parallel() is used to parallelize the production of MAX integers, via the produce(int) method (shown below). The intermediary esResults and cfResults variables are created solely to highlight where the asynchronous tasks will 'end', to be followed by how the results are collected. One can certainly daisy-chain the method calls completely.

  1. ExecutorService:
    • Start an ExecutorService implementation, the Java 8 newWorkStealingPool() is an example here.
    • submit() a Callable<Integer> that returns an Integer upon completion.
    • Collect the results to a List<Future<Integer>>, i.e. a List of Futures that return Integers.
    • Stream the List by get()-ting (method shown below) each resulting Integer.
    • From the Stream<Integer>, call max(Comparator.naturalOrder()) to get the maximum value, and then print it via ifPresent(Consumer).
    • Last, shutdown() the ExecutorService.
  2. CompletableFuture:
    • Pass each Supplier<Integer> from produce(int) to CompletableFuture.supplyAsync(Supplier).
    • Collect each CompletableFuture<Integer> into a resulting CompletableFuture<List<Integer>> via a custom allOf() Collector (method shown below).
    • From that CompletableFuture, thenApply() it to a Function<List<Integer>, Integer> that derives the maximum value from the List<Integer>.
    • join() to get the result and print it via ifPresent(Consumer).

Supporting methods:

  • This produces the given int after sleep()-ing for some time:

    private static Supplier<Integer> produce(int i) {
        return () -> {
            try {
                Thread.sleep((int) ((1 + Math.random()) * (MAX - i) * 500));
            } catch (InterruptedException e) {
                throw new RuntimeException(e);
            System.out.printf("Returning [%s] from Thread[%s]%n",
                    i, Thread.currentThread().getName());
            return i;
  • This is a Stream-friendly way of get()-ing from a Future, by catching checked Exceptions first:

    private static <T> T get(Future<T> future) {
        try {
            return future.get();
        } catch (InterruptedException | ExecutionException e) {
            throw new RuntimeException(e);
  • This is a Collector implementation for waiting on all CompletableFutures to finish, before returning the List of results (inspired by this article):

    private static <T> Collector<CompletableFuture<T>, ?,
            CompletableFuture<List<T>>> allOf() {
        return Collector.of(ArrayList<CompletableFuture<T>>::new,
                (a, t) -> a.add(t), (a, b) -> { a.addAll(b); return a; },
                a -> CompletableFuture.allOf(
                            a.toArray(new CompletableFuture[a.size()]))
                        .thenApply(v -> a.stream().map(CompletableFuture::join)


Instead of thinking solely on having to synchronize write access on a resulting array, both the ExecutorService or CompletableFuture approaches are more declarative, and IMO the latter more so that the former. This is achieved by letting us express how to generate our asynchronous tasks, followed by what needs to be done after gathering the results. :)

  • \$\begingroup\$ I'm not very good at this stuff, but I'm trying to basically take what I've learned in the past and incorporate Multi-threading. It wasn't my intention to process the file twice. I just wanted to find the max in the file, but split into threads to help speed up the process. Are there any good examples that you know of using executor service using arrays? \$\endgroup\$ Commented Oct 30, 2015 at 4:05
  • \$\begingroup\$ @user3577397 see updated answer. \$\endgroup\$
    – h.j.k.
    Commented Oct 30, 2015 at 6:19
  • 1
    \$\begingroup\$ Yes, it's an academic exercise. I read that this is more practical for dealing with billions of numbers, but it's just meant to teach me the concept. I guess I could change the size of the array to billions, and generate a text file with billions of numbers. Prior to attempting threading, I was able to find the max through 100k numbers with just standard code. Then I attempted to take code I already wrote and add Threading functionality to it. And that's what I came up with above. Thanks for helping me. I'll try to take your tips and improve it \$\endgroup\$ Commented Oct 30, 2015 at 13:59
  • 1
    \$\begingroup\$ @user3577397 see updated answer. In short, don't think about placing the results into a array where you need to worry about synchronizing write access. Use either methods described to better express the steps required. \$\endgroup\$
    – h.j.k.
    Commented Nov 1, 2015 at 13:15
  • 1
    \$\begingroup\$ Thank you, you've been really helpful. I'll try to incorporate these. \$\endgroup\$ Commented Nov 2, 2015 at 17:47

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