import java.util.ArrayList;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ForkJoinPool;
import java.util.concurrent.ForkJoinTask;
import java.util.concurrent.RecursiveTask;
/**
* ForkJoinPool线程池 分支执行
* RecursiveAction：没有返回值的任务
* RecursiveTask：可以携带返回值的任务
*
* 数列求和 计算1-200000的和
*/
public class CountTask extends RecursiveTask<Long> {
private static final int THRESHOLD = 10000;
private long start;
private long end;
public CountTask(long start, long end) {
this.start = start;
this.end = end;
}
@Override
protected Long compute() {
long sum = 0;
boolean canCompute = (end-start)<THRESHOLD;
if (canCompute){
for (long i = start; i <= end; i++) {
sum += i;
}
}else {
//分100组 每组多少个
long step = (start + end)/100;
ArrayList<CountTask> tasks = new ArrayList<>();
// 第一组的第一个
long pos = start;
for (int i = 0; i < 100; i++) {
//每组的最后一个
long lastOne = pos+step;
if (lastOne>end) lastOne = end;
CountTask task = new CountTask(pos,lastOne);
// 每组的第一个
pos+=step+1;
tasks.add(task);
task.fork();//提交子任务
}
// 子任务结果相加
for (CountTask task : tasks) {
sum+=task.join();
}
}
return sum;
}
public static void main(String[] args){
ForkJoinPool forkJoinPool = new ForkJoinPool();
CountTask task = new CountTask(0,200000L);
ForkJoinTask<Long> result = forkJoinPool.submit(task);
try {
Long res = result.get();//执行get()时，任务没有结束，那么主线程会在get()时等待
System.out.println(res);
} catch (InterruptedException e) {
e.printStackTrace();
} catch (ExecutionException e) {
e.printStackTrace();
}
}
}