Chapter 30 Exercise 15, Introduction to Java Programming, Tenth Edition Y. Daniel LiangY.

30.15 (Parallel sum) Implement the following method using Fork/Join to find the
sum of a list.
public static double parallelSum(double[] list)
Write a test program that finds the sum in a list of 9,000,000 double values.

import java.util.concurrent.ForkJoinPool;
import java.util.concurrent.RecursiveTask;

public class Exercise15 {

 public static void main(String[] args) {
  double[] list = new double[9000000];
  for (int i = 0; i < list.length; i++) {
   list[i] = Math.random();
  }
  System.out.print(parallelSum(list));
 }

 public static double parallelSum(double[] list) {
  RecursiveTask<Double> task = new SumTask(list, 0, list.length);
  ForkJoinPool pool = new ForkJoinPool();
  return pool.invoke(task);
 }
 
 private static class SumTask extends RecursiveTask<Double> {
  private static final long serialVersionUID = 1L;
  private final static int THRESHOLD = 1000;
  private double[] list;
  private int low;
  private int high;

  public SumTask(double[] list, int low, int high) {
   this.list = list;
   this.low = low;
   this.high = high;
  }
  
  @Override
  protected Double compute() {
   if (high - low < THRESHOLD) {
    double sum = 0;
    for (int i = low; i < high; i++) {
     sum += list[i];
    }
    return sum;
   } else {
    int mid = (low + high) / 2;
    RecursiveTask<Double> left = new SumTask(list, low, mid);
    RecursiveTask<Double> right = new SumTask(list, mid, high);

    right.fork();
    left.fork();
    return left.join() + right.join();
   }
  }

 }
}