Java ScheduledFuture get List

Question

This code always returns me 10. I think that problem with receiving list of all features. I need to parse every feature and stop execution scheduler when variable limit will equals 5. How can I do this?

static int limit = 0;
static final int testNum = 10;

static ScheduledExecutorService scheduler;
public static void main(String[] args) {
    scheduler = Executors
            .newScheduledThreadPool(5);
    ScheduledFuture<Integer> future = scheduler.schedule(new ScheduledPrinter(), 10, TimeUnit.SECONDS);
    try {
        while (true) {
            System.out.println(future.get());
            if(future.get() != testNum){
                return;
            }
        }
    } catch (InterruptedException e) {
        e.printStackTrace();
    } catch (ExecutionException e) {
        e.printStackTrace();
    }
}
private static class ScheduledPrinter implements Callable<Integer> {
    public Integer call() throws Exception {
        limit++;
        if(limit==5) {
            scheduler.shutdown();
            return limit;
        }
        return testNum;
    }
}

Answer 1

Let's see What's happening here. scheduler.schedule(new ScheduledPrinter(), 10, TimeUnit.SECONDS) runs the ScheduledPrinter.call() only once. Here is the API docs.

What you want is probably a scheduleAtFixedRate. This takes a Runnable instead of a callable, so the code will look something like this:

static volatile int limit = 0; // make it volatile because of *possible* multithreaded access
                               // an AtomicInteger would do too
static final int testNum = 10;

static ScheduledExecutorService scheduler;

public static void main(String[] args) {
    scheduler = Executors
            .newScheduledThreadPool(5);
    // discarding the future. No need to use it here.
    ScheduledFuture<?> future = scheduler.scheduleAtFixedRate(new ScheduledPrinter(), 10L, 10L, TimeUnit.SECONDS);
}

/** Printing and counting happens here **/
private static class ScheduledPrinter implements Runnable {

    @Override
    public void run() {
        limit++;
        if(limit==5) {
            scheduler.shutdown();
            printNum(limit);
        } else {
            printNum(testNum);
        }
    }

    private void printNum(int num) {
        System.out.println(num);
    }
}

Update

OP asked how to return values from Runnable.run() method? Unfortunately, it's impossible. We have to choose between periodical run and a return value because ScheduledExecutorService cannot do both.

It's still possible to get a value out of the Runnable. We must share a reference for this. Here is a rudimentary approach:

    final Queue<Integer> numsPrinted = new ConcurrentLinkedQueue<>(); // a concurrent collection
    ScheduledFuture<?> future = scheduler.scheduleWithFixedDelay( // using scheduleWithFixedDelay because probably this is what you want
            new ScheduledPrinter(numsPrinted), // passing the reference
            10L, 10L, TimeUnit.SECONDS);
    try {
        future.isDone();
        Object obj = future.get(80, TimeUnit.SECONDS); // blocks until 80 secs or until the task is done
        System.out.println(obj);
        System.out.println(Arrays.toString(numsPrinted.toArray()));
    } catch (TimeoutException e) {
        System.out.println(Arrays.toString(numsPrinted.toArray()));
    } catch (InterruptedException | ExecutionException e) {
        e.printStackTrace();
    } 

The ScheduledPrinter now looks like this:

private static class ScheduledPrinter implements Runnable {

    private final Queue<Integer> numsPrinted;

    public ScheduledPrinter(Queue<Integer> numsPrinted) {
        this.numsPrinted = numsPrinted; // storing the reference
    }

    @Override
    public void run() {
        limit++;
        if(limit==5) {
            //scheduler.awaitTermination(timeout, unit)
            scheduler.shutdown();
            storeAndPrintNum(limit);
        } else {
            storeAndPrintNum(testNum);
        }
    }

    private void storeAndPrintNum(int num) {
        numsPrinted.add(num); // using the reference
        System.out.println(num);
    }
}