android - substring in AsyncTask cause app to force quit -

i have code:

package com.jamie.translate;  import android.content.context; import android.net.uri; import android.os.asynctask; import android.util.log; import android.widget.edittext; import android.widget.textview;  import org.apache.http.httpresponse; import org.apache.http.client.httpclient; import org.apache.http.client.methods.httpget; import org.apache.http.impl.client.defaulthttpclient;  import java.io.bufferedreader; import java.net.uri;  /**  * created jamie on 25/6/2015.  */ public class translatetexttask extends asynctask<textview, void, string> {     textview t;     string result = "";     string sourcetext = "";     string sourcelanguage = "";     string resultlanguage = "";      public translatetexttask(string _sourcetext, string _sourcelanguage, string _resultlanguage) {         this.sourcetext = _sourcetext;         this.sourcelanguage = _sourcelanguage;         this.resultlanguage = _resultlanguage;     }      protected void d(string tag, string errormessage) {         mainactivity.shownormalmessage(mainactivity.applicationcontext, errormessage);     }      @override     protected string doinbackground(textview... params) {         this.t = params[0];         return gettranslatedtext();     }      final string gettranslatedtext() {         uri.builder builder = new uri.builder();         string langpair;         uri uri = null;         final httpclient httpclient;         final httpget request;         final httpresponse[] response = new httpresponse[1];         final bufferedreader reader = null;         final string jsonresult = "";         string translatedstring;          if(sourcetext.length() > 500) {             mainactivity.showerrormessage(mainactivity.applicationcontext, "source text can max. 500 characters");             return "";         }          d("test", "tag2");try {             string x = ("en - english").substring(0, 2);             string y = resultlanguage.substring(0, 2);              langpair =                     sourcelanguage.substring(0, 2)                             + "|"                             + resultlanguage.substring(0, 2);         } catch (exception e) {             mainactivity.showerrormessage(mainactivity.applicationcontext, "source language or result language invalid.");             return "";         }          d("test", "tag3");try {             uri = new uri(builder.scheme("http")                     .authority("api.mymemory.translated.net")                     .appendpath("get")                     .appendqueryparameter("langpair", langpair)                     .appendqueryparameter("q", sourcetext)                     .build()                     .tostring());         } catch (exception e) {             mainactivity.showerrormessage(mainactivity.applicationcontext, "failed build http request url.");             return "";         }          d("test", "tag4");try {             httpclient = new defaulthttpclient();             request = new httpget();             request.seturi(uri);         } catch (exception e) {             mainactivity.showerrormessage(mainactivity.applicationcontext, "failed build http request.");             return "";         }          d("test", "tag5");try {             // response = httpclient.execute(request);             d("test", "tag6");try {                 response[0] = (httpclient.execute(request));                 return mainactivity.handletranslaterequest(mainactivity.applicationcontext, response[0]);             } catch (exception e) {                 mainactivity.showerrormessage(mainactivity.applicationcontext, e.tostring());                 return "";             }         } catch (exception e) {             mainactivity.showerrormessage(mainactivity.applicationcontext, e.tostring());             // mainactivity.showerrormessage(mainactivity.applicationcontext, "failed complete http request.");             return "";         }     }      protected void onpostexecute(string result) {         t.settext(result);     } } 

and start code by:

new translatetexttask(((edittext) findviewbyid(r.id.source)).gettext(), translatesourcelanguage, translateresultlanguage).execute((edittext) findviewbyid(r.id.result)); 

example values of values of variable in debug process include

((edittext) findviewbyid(r.id.source)).gettext(): "hi" translatesourcelanguage: "en - english" translateresultlanguage: "it - italiano" 

there no internet connection problem. during debug process, set enough breakpoints , found force quit problem happened on line:

string x = ("en - english").substring(0, 2); 

what can solve problem? seems super-simple problem can't stop @ all.

edit

logcat message:

06-25 18:41:04.318    9337-9534/com.jamie.translate e/androidruntime﹕ fatal exception: asynctask #1     process: com.jamie.translate, pid: 9337     java.lang.runtimeexception: error occured while executing doinbackground()             @ android.os.asynctask$3.done(asynctask.java:300)             @ java.util.concurrent.futuretask.finishcompletion(futuretask.java:355)             @ java.util.concurrent.futuretask.setexception(futuretask.java:222)             @ java.util.concurrent.futuretask.run(futuretask.java:242)             @ android.os.asynctask$serialexecutor$1.run(asynctask.java:231)             @ java.util.concurrent.threadpoolexecutor.runworker(threadpoolexecutor.java:1112)             @ java.util.concurrent.threadpoolexecutor$worker.run(threadpoolexecutor.java:587)             @ java.lang.thread.run(thread.java:841)      caused by: java.lang.runtimeexception: can't create handler inside thread has not called looper.prepare()             @ android.os.handler.<init>(handler.java:200)             @ android.os.handler.<init>(handler.java:114)             @ android.widget.toast$tn.<init>(toast.java:376)             @ android.widget.toast.<init>(toast.java:108)             @ android.widget.toast.maketext(toast.java:267)             @ com.jamie.translate.mainactivity.shownormalmessage(mainactivity.java:26)             @ com.jamie.translate.translatetexttask.d(translatetexttask.java:35)             @ com.jamie.translate.translatetexttask.gettranslatedtext(translatetexttask.java:60)             @ com.jamie.translate.translatetexttask.doinbackground(translatetexttask.java:41)             @ com.jamie.translate.translatetexttask.doinbackground(translatetexttask.java:21)             @ android.os.asynctask$2.call(asynctask.java:288)             @ java.util.concurrent.futuretask.run(futuretask.java:237)             at android.os.asynctask$serialexecutor$1.run(asynctask.java:231)             at java.util.concurrent.threadpoolexecutor.runworker(threadpoolexecutor.java:1112)             at java.util.concurrent.threadpoolexecutor$worker.run(threadpoolexecutor.java:587)             at java.lang.thread.run(thread.java:841) 

you must not touch ui (i.e. posting toasts or update textview etc) on doinbackground(). if need update ui need either in onpostexecute() or use onprogressupdate() if fit.