How to fire an event when a file is not been modified using Java WatchService?

Question

I want to fire an event whenever a file has stopped being modified. I am polling a directory and if the filename matches to "input.csv" then i want to fire an event on that file. Time to check whether the file has been modified is 10 seconds. I am getting a null pointer exception when no event occurs. This is my sample code:

public class MyPoller {
public static void main(String[] args) throws URISyntaxException,
        IOException, InterruptedException {
    Path tmpPath = Paths.get("E:/MyDirectory/DEC");
    WatchService watchService = FileSystems.getDefault().newWatchService();
    // Watching the E:/MyDirectory/DEC directory
    // for MODIFY and DELETE operations
    tmpPath.register(watcput.csvService, StandardWatchEventKinds.ENTRY_MODIFY,
            StandardWatchEventKinds.ENTRY_DELETE);
    for (;;) {
        WatchKey key = watchService.poll(10000,TimeUnit.MILLISECONDS);
         List<WatchEvent<?>> events = key.pollEvents();

        // Poll all the events queued for the key
        for (WatchEvent event : events) {

            switch (event.context().toString()) {
            case "input.csv":
                System.out.println("Modified: " + event.context());
                String filename = "" + event.context();
                System.out.println(" "+filename);

                break;

                default:
                    break;
            }
        }
        boolean valid = key.reset();

        // If the key is invalid, just exit.
        if (!valid) {
            break;
        }
    }

}
}

Answer 1

If no event is generated, then the WatchKey may be null

WatchKey key = watchService.poll(10000,TimeUnit.MILLISECONDS);

Try to do a null check on the key and then proceed to do pollEvents() if it's indeed not null.