parallele-programmierung/u07-2/README.md

Problem 7.2: Implementation of ExecutorCompletionService

a) Implement the following simplified version of class java.util.concurrent.ExecutorCompletionService:

public class SimpleExecutorCompletionService<ResultType> {
	public SimpleExecutorCompletionService(ExecutorService ex) { … }

	// Submit task to ex for execution:
	public Future<ResultType> submit(Callable<ResultType> task) { … }

	// Fetch result of some finished task if available (else null):
	public Future<ResultType> poll() { … }
}

Use class SimpleExecutorCompletionServiceTester oin the file server to test your class. If your implementation is correct, the numbers printed with "… received result" should appear in mostly¹ ascending order. Method Future.isDone() could be helpful for your implementation.

b) Add the following method to your class:

	// Fetch result of some finished task (blocks until some has finished):
	public Future<ResultType> take() throws InterruptedExecution { … }

Note that you have to change the submit method from (a), too, so that when a task finishes, the threads waiting in take for a task to finish are notified. Use class SimpleExecutorCompletionServiceTester_with_take on the server to test your class.

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1. This order is not really guaranteed: While the "internal" length of each task would indeed produce this order, the total execution time of a task may be longer due to external delays by the thread pool (e.g. how long it takes to start a submitted task) and by other threads (e.g. preemption). ↩︎