Why does my ArrayBlockingQueue result in an empty queue after putting a list in it?

Question

This is the first time I have asked a question on StackOverflow. The problem I have is the following:

I have a Producer and Consumer class. In the Producer class I read a file line by line and put these lines of text in a List of Strings. When the list has an x amount of lines. This list gets added to an ArrayBlockingQueue. I have one Producer Thread that is started within the main thread. Besides this I start a couple of Consumer threads. The Consumer thread takes an item from the queue, which should be a list, and walks through this list of lines looking for a particular word. When the word is found it increments a count variable.

What happends is that when the Consumer takes an item from the queue, it says that it is empty. I cannot figure out why because my producer should certainly add it to the queue.

My code looks like this:

Consumer class:

public static class Consumer implements Callable<Integer> {

    int count = 0;

    @Override
    public Integer call() throws Exception {
        List<String> list = new ArrayList<>();
        list = arrayBlockingQueueInput.take();
        do {
            if (!list.isEmpty()){
                for (int i = 0; i < arrayBlockingQueueInput.take().size(); i++) {
                    for (String element : list.get(i).split(" ")) {
                        if (element.equalsIgnoreCase(findWord)) {
                            count++;
                        }
                    }
                }
            } else {
                arrayBlockingQueueInput.put(list);
            }
        } while (list.get(0) != "HALT");
        return count;
    }
}

Producer Class:

public static class Producer implements Runnable {

    @Override
    public void run() {
        try {
            FileReader file = new FileReader("src/testText.txt");
            BufferedReader br = new BufferedReader(file);

            while ((textLine = br.readLine()) != null) {

                if (textLine.isEmpty()) {
                    continue;
                }

                /* Remove punctuation from the text, except of punctuation that is useful for certain words.
                * Examples of these words are don't or re-enter */
                textLine = textLine.replaceAll("[[\\W]&&[^']&&[^-]]", " ");

                /* Replace all double whitespaces with single whitespaces.
                * We will split the text on these whitespaces later */
                textLine = textLine.replaceAll("\\s\\s+", " ");

                textLine = textLine.replaceAll("\\n", "").replaceAll("\\r", "");

                if (results.isEmpty()) {
                    results.add(textLine);
                    continue;
                }
                if (results.size() <= SIZE) {
                    results.add(textLine);
                    if (results.size() == SIZE) {
                        if (arrayBlockingQueueInput.size() == 14){
                            List<String> list = new ArrayList<String>();
                            list.add(HALT);
                            arrayBlockingQueueInput.put(list);
                        } else{
                            arrayBlockingQueueInput.put(results);
                            results.clear();
                        }
                    }
                }
            }
            /* Count the remaining words in the list
             *  (last lines of the file do perhaps not fill up until the given SIZE, therefore need to be counted here)
             *  Fill the list with empty items if the size of the list does not match with the given SIZE */
            while (results.size() != SIZE) {
                results.add("");
            }
            arrayBlockingQueueInput.put(results);
            List<String> list = new ArrayList<String>();
            list.add(HALT);
            arrayBlockingQueueInput.put(list);
            results.clear();
        } catch (InterruptedException e) {
            producerIsRunning = false;
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

Main Class:

public void main() throws IOException, InterruptedException {
    System.out.println("Enter the word you want to find: ");
    Scanner scan = new Scanner(System.in);
    findWord = scan.nextLine();

    System.out.println("Starting...");
    long startTime = System.currentTimeMillis();
    Thread producer = new Thread(new Producer());
    producer.start();
    ExecutorService executorService = Executors.newFixedThreadPool(CORE);

    List<Future<Integer>> futureResults = new ArrayList<Future<Integer>>();

    for (int i = 0; i < CORE; i++) {
        futureResults.add(executorService.submit(new Consumer()));
    }

    executorService.shutdown();

    for (Future<Integer> result : futureResults) {
        try {
            wordsInText += result.get();
        } catch (ExecutionException | InterruptedException e) {
            e.printStackTrace();
        }
    }

    producer.join();

    long stopTime = System.currentTimeMillis();

    System.out.println("The word " + findWord + " appears " + wordsInText + " times in the given text");

    System.out.println("Elapsed time was " + (stopTime - startTime) + " milliseconds.");
}

Can anybody explain why this happends? I would like to add that we try to use a poison pill to notify the consumers that the producer is on HALT.

To answer the question why we want to do this like this? For school we try to parallelize a certain programming problem. The problem we chose is string matching. We first made a serial solution and a parallel solution. For the next assignment we have to improve our parallel solution and our teacher told us that this is a way to do it.

Thanks in advance!

Nick

Answer 1

You add list to a queue and the clear it:

arrayBlockingQueueInput.put(results);
results.clear();

You need to do something like this to add copy of list to a queue so that clear() will not clean list which is in queue:

arrayBlockingQueueInput.put(new ArrayList<String>(results));
results.clear();

Answer 2

After some help from my teacher he helped us find the problem. There were two mistakes. One was within the producer class. I had code to signal a HALT of the producer within the main while loop. This should not have been done.

Besides this, the .take() I do within the Consumer class before the Do-While should have been done within the do-while loop.

The correct code looks like this:

Consumer class:

public static class Consumer implements Callable<Integer> {

    int count = 0;

    @Override
    public Integer call() throws Exception {
        List<String> list = new ArrayList<>();
        do {
            list = arrayBlockingQueueInput.take();
            if (!list.get(0).equals(HALT)){
                for (int i = 0; i < list.size(); i++) {
                    for (String element : list.get(i).split(" ")) {
                        if (element.equalsIgnoreCase(findWord)) {
                            count++;
                        }
                    }
                }
            } else {
                arrayBlockingQueueInput.put(list);
            }
        } while (!list.get(0).equals(HALT));
        return count;
    }
}

Producer class:

public static class Producer implements Runnable {

    @Override
    public void run() {
        try {
            FileReader file = new FileReader("src/testText.txt");
            BufferedReader br = new BufferedReader(file);

            while ((textLine = br.readLine()) != null) {

                if (textLine.isEmpty()) {
                    continue;
                }

                /* Remove punctuation from the text, except of punctuation that is useful for certain words.
                * Examples of these words are don't or re-enter */
                textLine = textLine.replaceAll("[[\\W]&&[^']&&[^-]]", " ");

                /* Replace all double whitespaces with single whitespaces.
                * We will split the text on these whitespaces later */
                textLine = textLine.replaceAll("\\s\\s+", " ");

                textLine = textLine.replaceAll("\\n", "").replaceAll("\\r", "");

                if (results.isEmpty()) {
                    results.add(textLine);
                    continue;
                }
                if (results.size() <= SIZE) {
                    results.add(textLine);
                    if (results.size() == SIZE) {
                        arrayBlockingQueueInput.put(new ArrayList<String>(results));
                        results.clear();
                    }
                }
            }
            /* Count the remaining words in the list
             *  (last lines of the file do perhaps not fill up until the given SIZE, therefore need to be counted here)
             *  Fill the list with empty items if the size of the list does not match with the given SIZE */
            while (results.size() != SIZE) {
                results.add("");
            }
            arrayBlockingQueueInput.put(new ArrayList<String>(results));
            List<String> list = new ArrayList<String>();
            list.add(HALT);
            arrayBlockingQueueInput.put(list);
            results.clear();
        } catch (InterruptedException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

Main class:

public void main() throws IOException, InterruptedException {
    System.out.println("Enter the word you want to find: ");
    Scanner scan = new Scanner(System.in);
    findWord = scan.nextLine();

    System.out.println("Starting...");
    long startTime = System.currentTimeMillis();
    Thread producer = new Thread(new Producer());
    producer.start();
    ExecutorService executorService = Executors.newFixedThreadPool(CORE);

    List<Future<Integer>> futureResults = new ArrayList<Future<Integer>>();

    for (int i = 0; i < CORE; i++) {
        futureResults.add(executorService.submit(new Consumer()));
    }

    executorService.shutdown();

    for (Future<Integer> result : futureResults) {
        try {
            wordsInText += result.get();
        } catch (ExecutionException | InterruptedException e) {
            e.printStackTrace();
        }
    }

    producer.join();

    long stopTime = System.currentTimeMillis();

    System.out.println("The word " + findWord + " appears " + wordsInText + " times in the given text");

    System.out.println("Elapsed time was " + (stopTime - startTime) + " milliseconds.");
}

Thanks to @Ivan for helping me with the .clear method called on results. Without this the code solution did not work.