android - AsyncTask behavior confusion -

i have project:

im trying undestand problem , how can solved, point dont know problem. have button , textview. when button clicked procedure called:

android:onclick="pulsaboton" 

and textview show me output.

this main_activity.java

package com.example.pruebasonidos;  import android.app.activity; import android.os.asynctask; import android.os.bundle; import android.view.view; import android.widget.textview;  public class mainactivity extends activity { public string cadena1="", cadena2=""; public textview tv; @override protected void oncreate(bundle savedinstancestate) {      super.oncreate(savedinstancestate);     setcontentview(r.layout.activity_main);     tv = (textview) findviewbyid(r.id.textview01); } public class generamusica extends asynctask<string, integer, integer> {     @override     protected void onpreexecute(){         tv.settext(tv.gettext().tostring()+"pre execute");       }      @override     protected integer doinbackground(string...strings) {          string cadena=strings[0];         tv.settext(tv.gettext().tostring()+cadena);          return null;     }     @override     protected void onpostexecute(integer bytes){         tv.settext(tv.gettext().tostring()+"post execute");      }  } public void pulsaboton(view v) { cadena1="123"; cadena2="111"; tv.settext(""); new generamusica().execute(cadena1); new generamusica().execute(cadena2); } @override public boolean oncreateoptionsmenu(menu menu) {     // inflate menu; adds items action bar if present.     getmenuinflater().inflate(r.menu.main, menu);     return true; }  } 

when pulsaboton clicked, textview1 display this:

preexecutepreexecute123111postexecutepostexecute 

i want output:

preexecute123postexecutepreexecute111postexecute 

what error????

warning: interface modification executed outside of ui thread!

protected integer doinbackground(string...strings) {     ...     tv.settext(tv.gettext().tostring()+cadena);      ... } 

why think asynctask exposes onpreexecute , onpostexecute? couldn't of interface work in doinbackground, before , after async code?

in android, ui needs accessed main thread, foreground thread; doinbackground not run in.

if need post updates ui during execution of asynctask, use publishprogress mechanism.

edit: "properly" accesing interface lot more complex using onpreexecute , onpostexecute. see this blog post. maybe should try loader api, it's less troublesome.