android - Dialog Box doesn't changed in Asynctask -

i'm trying update progress bar of asynctask doesn't changed.from doinbackground i'm passing values onprogreessupdate going on there changes doesn't reflect on progreesdailog.

please have , give me reason why happening.

asynctask class

class export extends asynctask<string, integer, string> {          progressdialog dialog;          public export() {              dialog = new progressdialog(inventorycount.this);             dialog.setcancelable(false);          }          protected void onpreexecute() {             super.onpreexecute();             dialog.setmessage("please wait while exporting worksheet..");             dialog.show();             dialog.setprogress(0);             dialog.setmax(100);         } protected string doinbackground(string... results) {             string result = "";             strictmode.threadpolicy policy = new strictmode.threadpolicy.builder()                     .permitall().build();             strictmode.setthreadpolicy(policy);              mis_setting mis_setting = new mis_setting();             dbhelper.getreadabledatabase();             mis_setting = dbhelper.getsetting();             dbhelper.close();             string ipaddress = mis_setting.getipaddress();              string f_url = "http://" + ipaddress                     + "/miswcfservice/service.svc/putmic";              dbhelper.getreadabledatabase();             int count = getcount();             dbhelper.close();             int length = count / 5;              (int = 0; <= length; i++) {                 int progress=(int) ((i / (float) (count / 5)) * 100);                 publishprogress(progress);                 try {                     thread.sleep(500);                 } catch (interruptedexception e) {                     // todo auto-generated catch block                     e.printstacktrace();                 }               }             } protected void onprogressupdate(integer... progress) {             //super.onprogressupdate(progress);             //dialog.setprogress(progress[0]);;             // setprogress(progress[0]);              super.onprogressupdate(progress);                 // update progressbar                 dialog.setprogress(progress[0]);         }          protected void onpostexecute(string result) {              if (result.equals("success")) {                 dialog.dismiss(); //some other stuffs                } } }