public static String fileNameWriterReader = "Task.xml";

{
        taskHashMap.put(task.getId(), task);
           TaskManager.getInstance().taskWrite();

}
    public void taskWrite(){
    // Запись
    write(TaskManager.taskHashMap, fileNameWriterReader);
    }

    public void write(HashMap w, String fileName) {
        try (XMLEncoder encoder = new XMLEncoder(new BufferedOutputStream(
                new FileOutputStream(fileName)))) {
            encoder.writeObject(w);
        } catch(IOException ex) {
            ex.printStackTrace();
        }
    }