java - How to Execute AsyncTask in Android - erro in class Zygotelnit -

i have class extends asynctask receive context, string user , string password,

i try execute asynctask, not working.

when execute asynctask class, occurs problem in class zygotelnit

public void run() {         try {             mmethod.invoke(null, new object[] { margs });         } catch (illegalaccessexception ex) {             throw new runtimeexception(ex);         } catch (invocationtargetexception ex) {             throwable cause = ex.getcause();             if (cause instanceof runtimeexception) {                 throw (runtimeexception) cause;             } else if (cause instanceof error) {                 throw (error) cause;             }             throw new runtimeexception(ex);         }     } 

follow below class execute asynctask

mbuttonenter.setonclicklistener(new view.onclicklistener() {         @override         public void onclick(view v) {             username = medittextuser.gettext().tostring();             password = medittextpassword.gettext().tostring();              new loginservice(loginactivity.this, username, password).execute(); 

follow below class asyntask

public class loginservice extends asynctask<string, string, string> {  private string username; private string password; private context context; progressdialog progressdialog = new progressdialog(context);  public loginservice(context context, string username, string password){      this.context = context;     this.username = username;     this.password = password; }  @override protected string doinbackground(string... params) {      string result = "";      try {         url url = new url("http://192.168.0.11:8080/appoint-api/api/usuario/dologin?user=" + this.username + "&senha=" + this.password);         httpurlconnection httpurlconnection = (httpurlconnection) url.openconnection();         httpurlconnection.setrequestmethod("post");          bufferedreader bufferedreader = new bufferedreader(new inputstreamreader(httpurlconnection.getinputstream()));         string inputline;         stringbuilder response = new stringbuilder();          while ((inputline = bufferedreader.readline()) != null) {             response.append(inputline);         }          result = response.tostring();         bufferedreader.close();     } catch (exception e) {         log.d("inputstream", e.getmessage());     }      return result;   } } 

follow error in log cat

08-16 22:23:29.781  18099-18099/br.com.appoint.android d/androidruntime﹕    shutting down vm 08-16 22:23:29.781  18099-18099/br.com.appoint.android w/dalvikvm﹕    threadid=1: thread exiting uncaught exception (group=0x4108b960) 08-16 22:23:29.875  18099-18099/br.com.appoint.android e/androidruntime﹕ fatal exception: main java.lang.nullpointerexception         @ android.app.alertdialog.resolvedialogtheme(alertdialog.java:143)         @ android.app.alertdialog.<init>(alertdialog.java:98)         @ android.app.progressdialog.<init>(progressdialog.java:77)         @ br.com.appoint.android.service.loginservice.<init>(loginservice.java:25)         @ br.com.appoint.android.activity.loginactivity$1.onclick(loginactivity.java:63)         @ android.view.view.performclick(view.java:4452)         @ android.widget.button.performclick(button.java:148)         @ android.view.view$performclick.run(view.java:18428)         @ android.os.handler.handlecallback(handler.java:725)         @ android.os.handler.dispatchmessage(handler.java:92)         @ android.os.looper.loop(looper.java:176)         @ android.app.activitythread.main(activitythread.java:5365)         @ java.lang.reflect.method.invokenative(native method)         @ java.lang.reflect.method.invoke(method.java:511)         @ com.android.internal.os.zygoteinit$methodandargscaller.run(zygoteinit.java:1102)         @ com.android.internal.os.zygoteinit.main(zygoteinit.java:869)         @ dalvik.system.nativestart.main(native method) 

new progressdialog(context); executed before public loginservice(context context, string username, string password), means when progressdialog initialized, context still null, cause npe.

remove progressdialog progressdialog = new progressdialog(context); or initialze progressdialog in public loginservice(context context, string username, string password)