java面试题-多线程数组求和(CyclicBarrier实现）

CyclicBarrier的作用是拦截多个线程，当所有线程到达指定位置时，在开始往下面执行，否则线程等待。
两种构造函数：

public CyclicBarrier(int parties)
public CyclicBarrier(int parties, Runnable barrierAction)

其中parties是线程个数，barrierAction是所有线程到达后执行的任务。
多线程数组求和，首先对数组进行分段，每段一个线程进行求和，当所有线程都完成计算后，在barrierAction中进行整体求和。

public class CyclicBarrierTest implements Runnable {

    CyclicBarrierTest(int count) {
        this.count = count;
        cyclicBarrier = new CyclicBarrier(count, this);
        executorService = Executors.newFixedThreadPool(count);
    }

    private Map<String, Integer> sumMap = new HashMap<>();
    private int count = 4;
    private CyclicBarrier cyclicBarrier;
    private ExecutorService executorService;

    public void sum(int[] arr) {
        int size = arr.length;
        int batch = size / count + 1;
        for (int i = 0; i < count; i++) {
            int end;
            int start;
            start = i * batch;
            end = Math.min(start + batch, size);

            System.out.println(start + " " + end);
            executorService.execute(new Calculator(arr, start, end));
        }
        executorService.shutdown();
    }

    public static void main(String[] args) {

        int[] arr = new int[]{1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11};

        CyclicBarrierTest cyclicBarrierTest = new CyclicBarrierTest(5);
        cyclicBarrierTest.sum(arr);

    }

    class Calculator implements Runnable {

        private int[] arr;
        private int start;
        private int end;

        Calculator(int[] arr, int start, int end) {
            this.arr = arr;
            this.start = start;
            this.end = end;
        }

        @Override
        public void run() {
            int sum = 0;
            for (int i = start; i < end; i++) {
                sum += arr[i];
            }
            sumMap.put(Thread.currentThread().getName(), sum);
            try {
              //  线程等待，当所有线程都到此处时在往下面执行
                cyclicBarrier.await();
            } catch (InterruptedException | BrokenBarrierException e) {
                e.printStackTrace();
            }
        }
    }

    @Override
    public void run() {
        long sum = sumMap.values().stream().reduce(Integer::sum).get();
        System.out.println(sum);
    }
}