Correct use of ForkJoinPool submit and join in Java

Question

I've worked recently on an implementation of a external merge sort algorithm (External Sorting) and my implementation needed to use a multi-threaded approach.

I tried to use ForkJoinPool instead of using the older implementations in Java such as Thread and ExecutorService. The first step of the algorithm requires to read a file, and every x lines collect and send to be sorted and written to file. This action (sort and save) can be done in a separate thread while the main thread reads the next batch. I've written a method to do just that (see below).

My concern is that the actual parallel work is not started when I use ForkJoinPool.commonPool().submit(()->SortAndWriteToFile(lines, fileName)) but instead only when I call task.join() after the loop had finished. That would mean that on a large enough loop I'd be collating the tasks to be run, but not gaining any time running them. When I used invoke instead of submit it seems like I cannot control where the join will be and cannot guarantee all the work was done before moving on.

Is there a more correct way to implement this?

My code is below. The method and two utility methods are listed. I hope this is not too long.

protected int generateSortedFiles (String originalFileName, String destinationFilePrefix) {

    //Number of accumulated sorted blocks of size blockSize
    int blockCount = 0;

    //hold bufferSize number of lines from the file
    List<String> bufferLines = new ArrayList<String>();

    List<ForkJoinTask<?>> taskList = new ArrayList<ForkJoinTask<?>>();

    //Open file to read
    try (Stream<String> fileStream = Files.lines(Paths.get(originalFileName))) {

        //Iterate over BufferSize lines to add them to list.
        Iterator<String> lineItr = fileStream.iterator();

        while(lineItr.hasNext()) {

            //Add bufferSize lines to List
            for (int i=0;i<bufferSize;i++) {
                if (lineItr.hasNext()) {
                    bufferLines.add(lineItr.next());
                }
            }

            //submit the task to sort and write to file in a separate thread
            String fileName= destinationFilePrefix+blockCount+".csv";
            List<String> lines = Collections.unmodifiableList(bufferLines);
            taskList.add(ForkJoinPool.commonPool().submit(
                    ()->SortAndWriteToFile(lines, fileName)));

            blockCount++;
            bufferLines = new ArrayList<String>();
        }
    } catch (IOException e) {
        System.out.println("read from file " +originalFileName + "has failed due to "+e);
    } catch (ArrayIndexOutOfBoundsException e) {
        System.out.println("the index prodived was not available in the file "
                +originalFileName+" and the error is "+e);
    }
    flushParallelTaskList(taskList);

    return blockCount;
}

/**
 * This method takes lines, sorts them and writes them to file
 * @param lines the lines to be sorted
 * @param fileName the filename to write them to
 */
private void SortAndWriteToFile(List<String> lines, String fileName) {
    //Sort lines
    lines = lines.stream()
            .parallel()
            .sorted((e1,e2) -> e1.split(",")[indexOfKey].compareTo(e2.split(",")[indexOfKey]))
            .collect(Collectors.toList());

    //write the sorted block of lines to the destination file.      
    writeBuffer(lines, fileName);

}

/**
 * Wait until all the threads finish, clear the list
 * @param writeList
 */
private void flushParallelTaskList (List<ForkJoinTask<?>> writeList) {
    for (ForkJoinTask<?> task:writeList) {
        task.join();
    }
    writeList.clear();

}