import java.util.concurrent.ForkJoinPool; import java.util.concurrent.RecursiveTask; public class ArraySummerForkJoin { // Extend RecursiveTask for tasks that return a result static class SummingRecursiveTask extends RecursiveTask { private static final int THRESHOLD = 10_000; // Threshold for direct computation private final int[] arr; private final int start; private final int end; public SummingRecursiveTask(int[] arr, int start, int end) { this.arr = arr; this.start = start; this.end = end; } @Override protected Long compute() { int length = end - start; if (length <= THRESHOLD) { // Base case: if the chunk is small enough, compute directly long partialSum = 0; for (int i = start; i < end; i++) { partialSum += arr[i]; } return partialSum; } else { // Recursive step: split the task into two sub-tasks int mid = start + length / 2; SummingRecursiveTask leftTask = new SummingRecursiveTask(arr, start, mid); SummingRecursiveTask rightTask = new SummingRecursiveTask(arr, mid, end); // Fork the left task to run asynchronously leftTask.fork(); // Compute the right task (or fork it too) and then join the left long rightResult = rightTask.compute(); // Or rightTask.fork(); then rightTask.join(); long leftResult = leftTask.join(); // Wait for the left task to complete and get its result return leftResult + rightResult; } } } public static long sumForkJoin(int[] arr) { // Create a ForkJoinPool ForkJoinPool forkJoinPool = new ForkJoinPool(); // By default uses availableProcessors() threads SummingRecursiveTask mainTask = new SummingRecursiveTask(arr, 0, arr.length); long totalSum = forkJoinPool.invoke(mainTask); // Invoke the main task and wait for its completion forkJoinPool.shutdown(); // Important: Shut down the pool return totalSum; } public static void main(String[] args) { int arraySize = 1_000_000; int[] largeArray = new int[arraySize]; for (int i = 0; i < arraySize; i++) { largeArray[i] = i % 100; } long startTime = System.nanoTime(); long sum = sumForkJoin(largeArray); long endTime = System.nanoTime(); System.out.println("Multi-threaded (Fork/Join) sum: " + sum); System.out.println("Multi-threaded (Fork/Join) time: " + (endTime - startTime) / 1_000_000.0 + " ms"); } }

Use parallel stream

Using Fork & Join