java - ASyncTask with progressdialog and button -

i beginner , had test. did tasks, have problem -

public class httptask extends asynctask<integer, string, string> {####  progressdialog dialog; context context; public httptask(activity activity) {     //init progress dialog     dialog = new progressdialog(context);****     dialog.setprogressstyle(progressdialog.style_horizontal); }  protected void onpreexecute() {     // show progress dialog     dialog.setmessage("loading...");     dialog.setcancelable(false); }  protected string doinbackground(integer... params) {     //freeze system 5 seconds         try {             int seconds = params[0]*5;####             timeunit.seconds.sleep(seconds);         } catch (interruptedexception e) {             e.printstacktrace();         }     return null; }  @override protected void onpostexecute(final string success) {     // if there progress dialog hide     dialog.dismiss(); }  } 

it crashes, when try compile (i showed problems * sign):

08-03 10:43:10.873  29441-29441/? e/androidruntime﹕ fatal exception: main     java.lang.nullpointerexception             @ android.app.alertdialog.resolvedialogtheme(alertdialog.java:142)             @ android.app.alertdialog.<init>(alertdialog.java:98)             @ android.app.progressdialog.<init>(progressdialog.java:77)             @ net.joerichard.androidtest.main.f.httptask.<init>(httptask.java:26)             @ net.joerichard.androidtest.main.f.f_networking_activity$1.onclick(f_networking_activity.java:27)             @ android.view.view.performclick(view.java:4107)             @ android.view.view$performclick.run(view.java:17166)             @ android.os.handler.handlecallback(handler.java:615)             @ android.os.handler.dispatchmessage(handler.java:92)             @ android.os.looper.loop(looper.java:155)             @ android.app.activitythread.main(activitythread.java:5559)             @ java.lang.reflect.method.invokenative(native method)             @ java.lang.reflect.method.invoke(method.java:511)             @ com.android.internal.os.zygoteinit$methodandargscaller.run(zygoteinit.java:1074)             @ com.android.internal.os.zygoteinit.main(zygoteinit.java:841)             @ dalvik.system.nativestart.main(native method) 08-03 10:43:10.913      754-877/? e/embeddedlogger﹕ app crashed! process: net.joerichard.androidtest 08-03 10:43:10.913      754-877/? e/embeddedlogger﹕ app crashed! package: net.joerichard.androidtest v1 (1.0) 08-03 10:43:10.913      754-877/? e/embeddedlogger﹕ application label: androidtest 

this class of main activity.

public class f_networking_activity extends actionbaractivity {  @override protected void oncreate(bundle savedinstancestate) {     super.oncreate(savedinstancestate);     setcontentview(r.layout.activity_f__networking_);      // bdownload: start httptask      button bdownload = (button) findviewbyid(r.id.bdownload);     bdownload.setonclicklistener(new view.onclicklistener() {         @override         public void onclick(view v) {             httptask task = new httptask(f_networking_activity.this);****             task.execute();         }     });  } 

thank answers. have problem (i showed # sign of second problems)

08-03 11:28:18.292      754-877/? e/embeddedlogger﹕ app crashed! process: net.joerichard.androidtest' 08-03 11:28:18.292      754-877/? e/embeddedlogger﹕ app crashed! package: net.joerichard.androidtest v1 (1.0) 08-03 11:28:18.292      754-877/? e/embeddedlogger﹕ application label: androidtest 08-03 11:28:18.292  30544-30726/? e/androidruntime﹕ fatal exception: asynctask #1 java.lang.runtimeexception: error occured while executing doinbackground()         @ android.os.asynctask$3.done(asynctask.java:299)         @ java.util.concurrent.futuretask$sync.innersetexception(futuretask.java:273)         @ java.util.concurrent.futuretask.setexception(futuretask.java:124)         @ java.util.concurrent.futuretask$sync.innerrun(futuretask.java:307)         @ java.util.concurrent.futuretask.run(futuretask.java:137)         @ android.os.asynctask$serialexecutor$1.run(asynctask.java:230)         @ java.util.concurrent.threadpoolexecutor.runworker(threadpoolexecutor.java:1076)         @ java.util.concurrent.threadpoolexecutor$worker.run(threadpoolexecutor.java:569)         @ java.lang.thread.run(thread.java:864)  caused by: java.lang.arrayindexoutofboundsexception: length=0; index=0         @   net.joerichard.androidtest.main.f.httptask.doinbackground(httptask.java:40)         @ net.joerichard.androidtest.main.f.httptask.doinbackground(httptask.java:20)         @     android.os.asynctask$2.call(asynctask.java:287)         @ java.util.concurrent.futuretask$sync.innerrun(futuretask.java:305)         @     java.util.concurrent.futuretask.run(futuretask.java:137)         @     android.os.asynctask$serialexecutor$1.run(asynctask.java:230)         @ java.util.concurrent.threadpoolexecutor.runworker(threadpoolexecutor.java:1076)         @ java.util.concurrent.threadpoolexecutor$worker.run(threadpoolexecutor.java:569)         @ java.lang.thread.run(thread.java:864)  

actually, context null because didn't initialize it.

add 1 line inside httptask:

public httptask(activity activity) {     this.context = activity;     dialog = new progressdialog(context);     dialog.setprogressstyle(progressdialog.style_horizontal); } 

and change context activity this:

activity context; 

now call context anywhere in class.