import java.util.List;
import java.util.concurrent.ForkJoinPool;
import java.util.concurrent.ForkJoinTask;
import java.util.concurrent.RecursiveTask;
 
class Ideone
{
    public static void main(String[] args) {
        // 2 active workers
        ForkJoinPool pool = new ForkJoinPool(2);
 
        // whatever many elements in the list
        List<Integer> original = List.of(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19);
 
        // main task will split up the list into sub threads
        int result = pool.invoke(new ParallelSum(original, 0, original.size()));
        System.out.println("Expected Result: " + original.stream().mapToInt(Integer::intValue).sum());
        System.out.println("Actual Result:   " + result);
    }
 
    static class ParallelSum extends RecursiveTask<Integer> {
        private static final int THRESHOLD = 4;
        private final List<Integer> ints;
        private final int offset, batchSize;
 
        ParallelSum(List<Integer> ints, int offset, int batchSize) {
            this.ints = ints;
            this.offset = offset;
            this.batchSize = batchSize;
        }
 
        @Override
        protected Integer compute() {
            if (batchSize <= THRESHOLD) {
                // process this batch
                int sum = 0;
                for (int i = 0; i < batchSize; i++) {
                    sum += ints.get(offset + i);
                }
                System.out.println(Thread.currentThread().getName() + " processed batch of size: " + batchSize + " from offset: " + offset + " and got sum: " + sum);
                return sum;
            } else {
                // divide and conquer
                int newLength = batchSize / 2;
                var tasks = List.of(
                    new ParallelSum(ints, offset, newLength),
                    // the `bitwise and` helps us to divide odd number of tasks
                    new ParallelSum(ints, offset + newLength, newLength + (batchSize & 1))
                );
                return ForkJoinTask.invokeAll(tasks)
                    .stream()
                    .mapToInt(ForkJoinTask::join)
                    .sum();
            }
        }
    }
}

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