[SOLVED] android.os.NetworkOnMainThreadException for webservice (ksoap)

I am new to android programming and trying to use webservice in this sample program:

I use Android 4.1 and my IDE is Eclipse Juno. I think the programming part is ok, but may be there is a problem about connecting.

package com.example.webserviceexample;

import java.io.IOException;

import org.ksoap2.SoapEnvelope;
import org.ksoap2.SoapFault;
import org.ksoap2.serialization.SoapObject;
import org.ksoap2.serialization.SoapPrimitive;
import org.ksoap2.serialization.SoapSerializationEnvelope;
import org.ksoap2.transport.HttpTransportSE;
import org.xmlpull.v1.XmlPullParserException;

import android.app.Activity;
import android.os.Bundle;
import android.view.Menu;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;

public class MainActivity extends Activity {

    final static String NAMESPACE = "http://tempuri.org/";
    final static String METHOD_NAME = "CelsiusToFahrenheit";
    final static String SOAP_ACTION = "http://tempuri.org/CelsiusToFahrenheit";
    final static String URL = "http://www.w3schools.com/webservices/tempconvert.asmx";

    TextView sonuc;
    EditText deger;
    Button hesapla;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        deger = (EditText) findViewById(R.id.deger);
        sonuc = (TextView) findViewById(R.id.flag);
        hesapla = (Button) findViewById(R.id.hesapla);

        hesapla.setOnClickListener(new OnClickListener() {

            public void onClick(View v) {

                //request info
                SoapObject Request = new SoapObject(NAMESPACE, METHOD_NAME);
                Request.addProperty("Celcius",deger.getText().toString());

                //envelope
                SoapSerializationEnvelope soapEnvelope = new SoapSerializationEnvelope(SoapEnvelope.VER11);
                soapEnvelope.dotNet = true; //.NET = true, php = false

                //putting request to the envelope
                soapEnvelope.setOutputSoapObject(Request);

                //transferring data
                HttpTransportSE aht = new HttpTransportSE(URL); //prepare
                //start
                try {
                        aht.call(SOAP_ACTION, soapEnvelope);
                } 
                catch (IOException e) {
                    e.printStackTrace();
                }
                catch (XmlPullParserException e)
                {
                    e.printStackTrace();
                }

                //waiting and getting response.
                String result;

                try {
                    // we are creating SoapPrimitive Object as waiting for simple variable.
                    result = "Fahrenheit:" + soapEnvelope.getResponse();

                    //writing the result to the textView
                    sonuc.setText(result);
                }
                catch (SoapFault e) {
                    e.printStackTrace();
                }

            }
        });
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        getMenuInflater().inflate(R.menu.activity_main, menu);
        return true;
    }
}

However, I get this error:

08-15 11:45:26.294: E/AndroidRuntime(641): FATAL EXCEPTION: main
08-15 11:45:26.294: E/AndroidRuntime(641): android.os.NetworkOnMainThreadException
08-15 11:45:26.294: E/AndroidRuntime(641):  at android.os.StrictMode$AndroidBlockGuardPolicy.onNetwork(StrictMode.java:1117)
08-15 11:45:26.294: E/AndroidRuntime(641):  at java.net.InetAddress.lookupHostByName(InetAddress.java:385)
08-15 11:45:26.294: E/AndroidRuntime(641):  at java.net.InetAddress.getAllByNameImpl(InetAddress.java:236)
08-15 11:45:26.294: E/AndroidRuntime(641):  at java.net.InetAddress.getAllByName(InetAddress.java:214)
08-15 11:45:26.294: E/AndroidRuntime(641):  at libcore.net.http.HttpConnection.<init>(HttpConnection.java:70)
08-15 11:45:26.294: E/AndroidRuntime(641):  at libcore.net.http.HttpConnection.<init>(HttpConnection.java:50)
08-15 11:45:26.294: E/AndroidRuntime(641):  at libcore.net.http.HttpConnection$Address.connect(HttpConnection.java:341)
08-15 11:45:26.294: E/AndroidRuntime(641):  at libcore.net.http.HttpConnectionPool.get(HttpConnectionPool.java:87)
08-15 11:45:26.294: E/AndroidRuntime(641):  at libcore.net.http.HttpConnection.connect(HttpConnection.java:128)
08-15 11:45:26.294: E/AndroidRuntime(641):  at libcore.net.http.HttpEngine.openSocketConnection(HttpEngine.java:315)
08-15 11:45:26.294: E/AndroidRuntime(641):  at libcore.net.http.HttpEngine.connect(HttpEngine.java:310)
08-15 11:45:26.294: E/AndroidRuntime(641):  at libcore.net.http.HttpEngine.sendSocketRequest(HttpEngine.java:289)
08-15 11:45:26.294: E/AndroidRuntime(641):  at libcore.net.http.HttpEngine.sendRequest(HttpEngine.java:239)
08-15 11:45:26.294: E/AndroidRuntime(641):  at libcore.net.http.HttpURLConnectionImpl.connect(HttpURLConnectionImpl.java:80)
08-15 11:45:26.294: E/AndroidRuntime(641):  at org.ksoap2.transport.ServiceConnectionSE.connect(ServiceConnectionSE.java:76)
08-15 11:45:26.294: E/AndroidRuntime(641):  at org.ksoap2.transport.HttpTransportSE.call(HttpTransportSE.java:146)
08-15 11:45:26.294: E/AndroidRuntime(641):  at org.ksoap2.transport.HttpTransportSE.call(HttpTransportSE.java:95)
08-15 11:45:26.294: E/AndroidRuntime(641):  at com.example.webserviceexample.MainActivity$1.onClick(MainActivity.java:61)
08-15 11:45:26.294: E/AndroidRuntime(641):  at android.view.View.performClick(View.java:4084)
08-15 11:45:26.294: E/AndroidRuntime(641):  at android.view.View$PerformClick.run(View.java:16966)
08-15 11:45:26.294: E/AndroidRuntime(641):  at android.os.Handler.handleCallback(Handler.java:615)
08-15 11:45:26.294: E/AndroidRuntime(641):  at android.os.Handler.dispatchMessage(Handler.java:92)
08-15 11:45:26.294: E/AndroidRuntime(641):  at android.os.Looper.loop(Looper.java:137)
08-15 11:45:26.294: E/AndroidRuntime(641):  at android.app.ActivityThread.main(ActivityThread.java:4745)
08-15 11:45:26.294: E/AndroidRuntime(641):  at java.lang.reflect.Method.invokeNative(Native Method)
08-15 11:45:26.294: E/AndroidRuntime(641):  at java.lang.reflect.Method.invoke(Method.java:511)
08-15 11:45:26.294: E/AndroidRuntime(641):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786)
08-15 11:45:26.294: E/AndroidRuntime(641):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
08-15 11:45:26.294: E/AndroidRuntime(641):  at dalvik.system.NativeStart.main(Native Method)

So what can be the problem?