FileObserver onCreate Event does not trigger itself in activity

Question

I defined FileObserver class outside the activity

 public class MyFileObserver extends FileObserver {
    public String absolutePath;
    public MyFileObserver(String path) {
        super(path, FileObserver.ALL_EVENTS);
        absolutePath = path;
    }
    @Override
    public void onEvent(int event, String path) {
      if (path == null) {     
   return;
    }

    if ((FileObserver.CREATE & event)!=0) {
        TextView tv = (TextView)findViewById(R.id.text1);

    //Set the text
        tv.setText("CREATED!");
    }

    }
  } 

and created an instance and started watching the file using StartWatching() in main activity

 MyFileObserver fileOb = new MyFileObserver("/mnt/sdcard/");
 fileOb.startWatching();

when i run the application and copy file in the directory being observed OnEvent() does not gets triggered, but when I click the button (used for other purpose defined in main activity) OnEvent triggers and gives the desired result. My question is that why this OnEvent() function does not trigger itself ? Is it because I am calling startWatching() in mainactivity, if this is the case then where should I call this function. Thanks in advance.

here is my onCreate() method .

 @Override
 protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    MyFileObserver fileOb = new MyFileObserver("/mnt/sdcard/");
    fileOb.startWatching();

    FloatingActionButton fab = (FloatingActionButton)                                       findViewById(R.id.fab);
    fab.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                    .setAction("Action", null).show();


        }
    });


}