In general, classes that use the Fork/Join Framework follow the following simple algorithm:

// pseudocode Result solve(Problem problem) { if (problem.size < SEQUENTIAL_THRESHOLD) return solveSequentially(problem); else { Result left, right; INVOKE-IN-PARALLEL { left = solve(extractLeftHalf(problem)); right = solve(extractRightHalf(problem)); } return combine(left, right); } }In order to demonstrate this, I have created an example to find the maximum number from a large array using fork/join:

import java.util.Random; import java.util.concurrent.ForkJoinPool; import java.util.concurrent.RecursiveTask; public class MaximumFinder extends RecursiveTask<Integer> { private static final int SEQUENTIAL_THRESHOLD = 5; private final int[] data; private final int start; private final int end; public MaximumFinder(int[] data, int start, int end) { this.data = data; this.start = start; this.end = end; } public MaximumFinder(int[] data) { this(data, 0, data.length); } @Override protected Integer compute() { final int length = end - start; if (length < SEQUENTIAL_THRESHOLD) { return computeDirectly(); } final int split = length / 2; final MaximumFinder left = new MaximumFinder(data, start, start + split); left.fork(); final MaximumFinder right = new MaximumFinder(data, start + split, end); return Math.max(right.compute(), left.join()); } private Integer computeDirectly() { System.out.println(Thread.currentThread() + " computing: " + start + " to " + end); int max = Integer.MIN_VALUE; for (int i = start; i < end; i++) { if (data[i] > max) { max = data[i]; } } return max; } public static void main(String[] args) { // create a random data set final int[] data = new int[1000]; final Random random = new Random(); for (int i = 0; i < data.length; i++) { data[i] = random.nextInt(100); } // submit the task to the pool final ForkJoinPool pool = new ForkJoinPool(4); final MaximumFinder finder = new MaximumFinder(data); System.out.println(pool.invoke(finder)); } }The

`MaximumFinder`

class is a `RecursiveTask`

which is responsible for finding the maximum number from an array. If the size of the array is less than a threshold (5) then find the maximum directly, by iterating over the array. Otherwise, split the array into two halves, recurse on each half and wait for them to complete (`join`

). Once we have the result of each half, we can find the maximum of the two and return it.
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