java - ExecutorCompletionService 示例代码是否需要循环？

Question

阅读JDK ExecutorCompletionService文档示例代码

/*
 * Suppose instead that you would like to use the first non-null result
 * of the set of tasks, ignoring any that encounter exceptions,
 * and cancelling all other tasks when the first one is ready:
 *
 * 
 * void solve(Executor e,
 *            Collection<Callable<Result>> solvers)
 *     throws InterruptedException {
 *     CompletionService<Result> ecs
 *         = new ExecutorCompletionService<Result>(e);
 *     int n = solvers.size();
 *     List<Future<Result>> futures
 *         = new ArrayList<Future<Result>>(n);
 *     Result result = null;
 *     try {
 *         for (Callable<Result> s : solvers)
 *             futures.add(ecs.submit(s));
 *         for (int i = 0; i < n; ++i) { //??? what's the purpose for this loop? 
 *             try {
 *                 Result r = ecs.take().get();
 *                 if (r != null) {
 *                     result = r;
 *                     break;
 *                 }
 *             } catch (ExecutionException ignore) {}
 *         }
 *     }
 *     finally {
 *         for (Future<Result> f : futures)
 *             f.cancel(true);
 *     }
 *
 *     if (result != null)
 *         use(result);
 */ 

我觉得这个loop不是必需的，因为take会一直阻塞，直到第一个Task成功，然后直接在那里爆发，我认为爆发时的i永远为零。

这是正确的吗？或者我错过了什么？

 *         for (int i = 0; i < n; ++i) { //??? what's the purpose for this loop? 
 *             try {
 *                 Result r = ecs.take().get();
 *                 if (r != null) {
 *                     result = r;
 *                     break;
 *                 }
 *             } catch (ExecutionException ignore) {}
 *         }

Answer 1

在评论中这样说

* Suppose instead that you would like to use the first non-null result
 * of the set of tasks, ignoring any that encounter exceptions,
 * and cancelling all other tasks when the first one is ready:
 *

如果第一个结果为空(任务已完成但结果为空)，那么它将尝试获取下一个结果

或

如果当前尝试抛出 ExecutionException，则此 for 循环将跳转到下一个元素并尝试获取下一个结果