Java Thread How to - Use Fork/Join to Collect results

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Question

We would like to know how to use Fork/Join to Collect results.

Answer

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.ForkJoinPool;
import java.util.concurrent.RecursiveTask;
//from w  w w.j  a  v  a2 s .  c om
public class Main {
  public static void main(String[] args) {
    List<Integer> list = new ArrayList<>();
    long expectedSum = 0;
    for (int i = 0; i < 10000; i++) {
      int random = 1 + (int) (Math.random() * ((100 - 1) + 1));
      list.add(random);
      expectedSum += random;
    }    
    System.out.println("expected sum: " + expectedSum);
    ForkJoinPool forkJoinPool = new ForkJoinPool(Runtime.getRuntime().availableProcessors());
    RecursiveSum recursiveSum = new RecursiveSum(list, 0, list.size());
    long recSum = forkJoinPool.invoke(recursiveSum);
    System.out.println("recursive-sum: " + recSum);
  }
}

class RecursiveSum extends RecursiveTask<Long> {
  static final int THRESHOLD = 1000;
  List<Integer> list;
  int begin;
  int end;

  public RecursiveSum(List<Integer> list, int begin, int end) {
    super();
    this.list = list;
    this.begin = begin;
    this.end = end;
  }
  @Override
  protected Long compute() {
    final int size = end - begin;
    if (size < THRESHOLD) {
      long sum = 0;
      for (int i = begin; i < end; i++)
        sum += list.get(i);
      return sum;
    } else {
      final int middle = begin + ((end - begin) / 2);
      RecursiveSum sum1 = new RecursiveSum(list, begin, middle);
      sum1.fork();
      RecursiveSum sum2 = new RecursiveSum(list, middle, end);
      return sum2.compute() + sum1.join();
    }
  }
}

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