Получение NullpointerException при выполнении с singleThreadExecutor и executorServices

Question

Я создал программу, использующую потоки для чтения данных из файла и отображения.Получаю исключение нулевого указателя, первый раз поток 1 печатает первую запись из текстового файла, а вторые данные, которые он удаляет из файла, выбрасывает нулевой указатель

public static void main(String[] args) throws InterruptedException, ExecutionException {
        ExecutorService es = Executors.newSingleThreadExecutor();
        List<String> tasks = getUsersFromFile("new_users.txt");
        DoctorDao dao = new DoctorDao();
        for (String data : tasks) {
            Future<Boolean> result = es.submit(new DoctorTask(data, dao));
            while (result.get()) {
            System.out.println("data stored!!!");
            }
        }
        es.shutdown();
        System.out.println("task done!!!");
    }

    public static List<String> getUsersFromFile(String fileName) {
        List<String> users = new ArrayList<>();
        try (BufferedReader reader = new BufferedReader(new FileReader(new File(fileName)))) {
            String line = null;
            while ((line = reader.readLine()) != null) {
                users.add(line);
            }
        } catch (FileNotFoundException ex) {
            // Logger.getLogger(TestExecutors.class.getName()).log(Level.SEVERE, null, ex);
        } catch (IOException ex) {
            // Logger.getLogger(TestExecutors.class.getName()).log(Level.SEVERE, null, ex);
        }
        return users;

В реализованном классе Callable

    public Boolean call() throws Exception {
        Boolean status = false;
        System.out.println(Thread.currentThread().getName() + " processing record for : " + doctorRecord);
        StringTokenizer tokenizer = new StringTokenizer(this.doctorRecord, ",");
        Doctor doc = null;
        while (tokenizer.hasMoreTokens()) {
            doc = new Doctor();
            doc.setEmailAddress(tokenizer.nextToken());
            doc.setName(tokenizer.nextToken());
            doc.setId(Integer.valueOf(tokenizer.nextToken()));
            status = dao.saveUser(doc);
        }

        return status;
    }

Exception in thread "main" java.util.concurrent.ExecutionException: java.lang.NullPointerException
    at java.util.concurrent.FutureTask.report(FutureTask.java:122)
    at java.util.concurrent.FutureTask.get(FutureTask.java:192)
    at com.executor.demo.readfile.TestDotorExecutor.main(TestDotorExecutor.java:23)
Caused by: java.lang.NullPointerException
    at com.executor.demo.readfile.DoctorDao.saveUser(DoctorDao.java:5)
    at com.executor.demo.readfile.DoctorTask.call(DoctorTask.java:28)
    at com.executor.demo.readfile.DoctorTask.call(DoctorTask.java:1)
    at java.util.concurrent.FutureTask.run(FutureTask.java:266)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
    at java.lang.Thread.run(Thread.java:748)

Answer 1

    I made a mistake in the code i returned a null value from statiDB class, i didnt intialize List Object, so i modified the code as shown below
    **This complete code below was missing in above question**
```
    in dao class
```
    public class DoctorDao {
        public boolean saveUser(Doctor doc) {
            boolean storedStatus = StaticDB.getDoctorList().add(doc);
            return storedStatus;
        }

    }
```
    In DB class
```
    class StaticDB{
    static List<Doctor> docList=null;

         public static List<Doctor> getDoctorList(){
         Doctor doc1=new Doctor(111,"Shalu","ss@gmail.com");
         Doctor doc2=new Doctor(112,"luke","luke@gmail.com");
         docList=new ArrayList<Doctor>(); //This was missing 
         docList.add(doc1);
         docList.add(doc2);
         return docList;