多线程操作同一个数组，计算数组元素之和

利用CountDownLatch传入具体的参数。这个参数可以认为是线程的次数（计数器）。每次执行完计数器都会减1.直到为0，主线程开始执行。

public class FiveThreadCount {
	private int[] arr = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26,
			27, 28 };
	private int total = 0;
	public static void main(String[] args) throws InterruptedException{
		FiveThreadCount threadCount=new FiveThreadCount();
		threadCount.test();	
	}
	public void test() throws InterruptedException {
		int length = arr.length;
		CountDownLatch latch = new CountDownLatch(length % 5 == 0 ? 5 : 6);
		System.out.println("length:" + length + ":" + "latch:" + latch.getCount());
		for (int j = 0; j < length; j += length / 5) {
			MyThread m = null;
			if ((j + (length / 5)) <= length) {
				m = new MyThread(arr, j, j + length / 5, latch);
			} else {
				m = new MyThread(arr, j, length, latch);
			}
			new Thread(m).start();
            latch.await();
            System.out.println(total);
		}
	}
	public class MyThread implements Runnable {
		int[] arr;
		int startIndex;
		int endIndex;
		CountDownLatch latch;

		public MyThread(int[] arr, int startIndex, int endIndex, CountDownLatch latch) {
			this.arr = arr;
			this.startIndex = startIndex;
			this.endIndex = endIndex;
			this.latch = latch;
		}

		@Override
		public void run() {
			int sum = 0;
			for (int i = startIndex; i < endIndex; i++) {
				sum += arr[i];
			}
			synchronized (MyThread.class) {
				total += sum;
			}
			System.out.println(Thread.currentThread().getName());
			latch.countDown();
		}
	}

}