خطا در استفاده از کتابخانه OKhttP

با سلام خدمت همه دوستان . من میخام اطلاعاتی رو به وسیله POST به سرور ارسال کنم و جواب رو بگیرم . از کتابخانه okHttp استفاده کردم که با خطا مواجه شدم . 

سورس برنامه من ..

public void get_obj()  throws IOException {



    String json = bowlingJson("Jesse", "Jake");
String response = post("http://www.roundsapp.com/post", json );


Log.w("NET__", response);



}
public static final MediaType JSON = MediaType.parse("application/json; charset=utf-8");
public OkHttpClient client = new OkHttpClient();

public String bowlingJson(String player1, String player2) {
    return "{'winCondition':'HIGH_SCORE',"
+ "'name':'Bowling',"
+ "'round':4,"
+ "'lastSaved':1367702411696,"
+ "'dateStarted':1367702378785,"
+ "'players':["
+ "{'name':'" + player1 + "','history':[10,8,6,7,8],'color':-13388315,'total':39},"
+ "{'name':'" + player2 + "','history':[6,10,5,10,10],'color':-48060,'total':41}"
+ "]}";
}

public String post (String url, String json )  throws IOException {
    RequestBody body = RequestBody.create(JSON, json);
Request request = new Request.Builder()
            .url(url)
            .post(body)
            .build();
Call call = client.newCall(request);
Response response = null;
    try {
        response = call.execute();
        return response.body().string();
} catch ( IOException e ) {
        e.printStackTrace();
}
    return url;
}

که در تابع زیر فراخونی میکنم

public void get_all_category() throws IOException {
        __net.get_obj();

}

خطاهای دریافتی من

10-06 11:50:30.712    2062-2062/ir.sunit.prestashop3 D/AndroidRuntime﹕ Shutting down VM
10-06 11:50:30.712    2062-2062/ir.sunit.prestashop3 W/dalvikvm﹕ threadid=1: thread exiting with uncaught exception (group=0x418d37d0)
10-06 11:50:30.822    2062-2062/ir.sunit.prestashop3 E/AndroidRuntime﹕ FATAL EXCEPTION: main
    android.os.NetworkOnMainThreadException
            at android.os.StrictMode$AndroidBlockGuardPolicy.onNetwork(StrictMode.java:1117)
            at java.net.InetAddress.lookupHostByName(InetAddress.java:385)
            at java.net.InetAddress.getAllByNameImpl(InetAddress.java:236)
            at java.net.InetAddress.getAllByName(InetAddress.java:214)
            at com.squareup.okhttp.internal.Network$1.resolveInetAddresses(Network.java:29)
            at com.squareup.okhttp.internal.http.RouteSelector.resetNextInetSocketAddress(RouteSelector.java:187)
            at com.squareup.okhttp.internal.http.RouteSelector.nextProxy(RouteSelector.java:156)
            at com.squareup.okhttp.internal.http.RouteSelector.next(RouteSelector.java:98)
            at com.squareup.okhttp.internal.http.HttpEngine.createNextConnection(HttpEngine.java:344)
            at com.squareup.okhttp.internal.http.HttpEngine.connect(HttpEngine.java:327)
            at com.squareup.okhttp.internal.http.HttpEngine.sendRequest(HttpEngine.java:245)
            at com.squareup.okhttp.Call.getResponse(Call.java:267)
            at com.squareup.okhttp.Call$ApplicationInterceptorChain.proceed(Call.java:224)
            at com.squareup.okhttp.Call.getResponseWithInterceptorChain(Call.java:195)
            at com.squareup.okhttp.Call.execute(Call.java:79)
            at ir.sunit.prestashop3.Net.post(Net.java:173)
            at ir.sunit.prestashop3.Net.get_obj(Net.java:141)
            at ir.sunit.prestashop3.SunApp.get_all_category(SunApp.java:132)
            at ir.sunit.prestashop3.FragmentCategoryList.onActivityCreated(FragmentCategoryList.java:37)
            at android.support.v4.app.Fragment.performActivityCreated(Fragment.java:1970)
            at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1051)
            at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1207)
            at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:738)
            at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1572)
            at android.support.v4.app.FragmentManagerImpl.executePendingTransactions(FragmentManager.java:545)
            at android.support.v4.app.FragmentPagerAdapter.finishUpdate(FragmentPagerAdapter.java:141)
            at android.support.v4.view.ViewPager.populate(ViewPager.java:1106)
            at android.support.v4.view.ViewPager.populate(ViewPager.java:952)
            at android.support.v4.view.ViewPager.onMeasure(ViewPager.java:1474)
            at android.view.View.measure(View.java:15247)
            at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:4836)
            at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1390)
            at android.widget.LinearLayout.measureVertical(LinearLayout.java:681)
            at android.widget.LinearLayout.onMeasure(LinearLayout.java:574)
            at android.view.View.measure(View.java:15247)
            at android.support.v4.widget.DrawerLayout.onMeasure(DrawerLayout.java:940)
            at android.view.View.measure(View.java:15247)
            at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:4836)
            at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
            at android.support.v7.internal.widget.ContentFrameLayout.onMeasure(ContentFrameLayout.java:124)
            at android.view.View.measure(View.java:15247)
            at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:4836)
            at android.support.v7.internal.widget.ActionBarOverlayLayout.onMeasure(ActionBarOverlayLayout.java:393)
            at android.view.View.measure(View.java:15247)
            at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:4836)
            at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
            at android.view.View.measure(View.java:15247)
            at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:4836)
            at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1390)
            at android.widget.LinearLayout.measureVertical(LinearLayout.java:681)
            at android.widget.LinearLayout.onMeasure(LinearLayout.java:574)
            at android.view.View.measure(View.java:15247)
            at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:4836)
            at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
            at com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2157)
            at android.view.View.measure(View.ja

به نظرتون باید چکار کنم . البته قبلش اینترنت و کانکشن ها چک میشن . به صورت معمولی هم نتونستم از HttpClient استفاده کنم و اصلا اندروید استادیو نشناختش و البته طبق سرچی که زدم از API -23 به بعد ساپورت میشه و API-8 رو ساپورت نمیکنه ....