Фрагмент Android не отображается на пейджере просмотра

У меня есть приложение. Я использую макет вкладки «Основное действие» с пейджером просмотра. Я пытался загрузить фрагмент в каждую 3 вкладки на моем пейджере просмотра. Но мой фрагмент не отображается в пейджере просмотра. Я делал это раньше несколько раз, но теперь он не работает !! Не знаю, в чем моя ошибка.

mainactivity.java:

 public class MainActivity extends AppCompatActivity
        implements NavigationView.OnNavigationItemSelectedListener {

    ImageView hamgerMenu;
    DrawerLayout drawer;
    TabLayout tabLayout;
    ViewPager viewPager;
    ViewPagerAdapter adapter;

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


        FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                        .setAction("Action", null).show();
            }
        });

        DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
        NavigationView navigationView = (NavigationView) findViewById(R.id.nav_view);
        navigationView.setNavigationItemSelectedListener(this);

        setupviews();
    }

    private void setupviews() {
        drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
        tabLayout = (TabLayout) findViewById(R.id.tabLayout);
        viewPager = (ViewPager) findViewById(R.id.viewPager);
        adapter = new ViewPagerAdapter(getSupportFragmentManager());
        adapter.addFragments(new FragmentFriends(), "فعالیت ها");
        adapter.addFragments(new FragmentFriends(), "فعالیت ها");
        adapter.addFragments(new FragmentFriends(), "فعالیت ها");
        adapter.addFragments(new FragmentFriends(), "فعالیت ها");
        viewPager.setAdapter(adapter);
        viewPager.setCurrentItem(2);

        tabLayout.setupWithViewPager(viewPager);

        hamgerMenu = (ImageView) findViewById(R.id.img_mainactivity_hambermenu);
        hamgerMenu.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                drawer.openDrawer(Gravity.RIGHT);
            }
        });

    }

    @Override
    public void onBackPressed() {
        if (drawer.isDrawerOpen(Gravity.RIGHT)) {
            drawer.closeDrawer(Gravity.RIGHT);
        } else {
            super.onBackPressed();
        }
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }

    @SuppressWarnings("StatementWithEmptyBody")
    @Override
    public boolean onNavigationItemSelected(MenuItem item) {
        // Handle navigation view item clicks here.
        int id = item.getItemId();

        if (id == R.id.nav_camera) {
            // Handle the camera action
        } else if (id == R.id.nav_gallery) {

        } else if (id == R.id.nav_slideshow) {

        } else if (id == R.id.nav_manage) {

        } else if (id == R.id.nav_share) {

        } else if (id == R.id.nav_send) {

        }

        DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
        drawer.closeDrawer(Gravity.RIGHT);
        return true;
    }
}

viewpagerAdapter:

    public class ViewPagerAdapter extends FragmentPagerAdapter {
    ArrayList<Fragment> fragments;
    ArrayList<String> titles;

    public ViewPagerAdapter(FragmentManager fm) {
        super(fm);
        fragments=new ArrayList<>();
        titles=new ArrayList<>();
    }

    @Override
    public Fragment getItem(int position) {
        return fragments.get(position);
    }

    @Override
    public int getCount() {
        return fragments.size();
    }

    public void addFragments(Fragment fragment,String title){
        fragments.add(fragment);
        titles.add(title);
    }

    @Nullable
    @Override
    public CharSequence getPageTitle(int position) {
        return titles.get(position);
    }
}

фрагмент:

    public class FragmentFriends extends Fragment {
    View view;
    @Nullable
    @Override
    public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
            inflater.inflate(R.layout.friends_fragment,container,false);
            return view;
    }
}

mainactivity.xml:

    <android.support.v4.widget.DrawerLayout xmlns:android = "http://schemas.android.com/apk/res/android"
    xmlns:app = "http://schemas.android.com/apk/res-auto"
    xmlns:tools = "http://schemas.android.com/tools"
    android:id = "@+id/drawer_layout"
    android:layoutDirection = "rtl"
    android:layout_width = "match_parent"
    android:layout_height = "match_parent"
    android:fitsSystemWindows = "true"
    tools:openDrawer = "start">

    <include
        layout = "@layout/app_bar_main"
        android:layout_width = "match_parent"
        android:layout_height = "match_parent" />

    <android.support.design.widget.NavigationView
        android:id = "@+id/nav_view"
        android:layoutDirection = "rtl"
        android:layout_width = "wrap_content"
        android:layout_height = "match_parent"
        android:layout_gravity = "right"
        android:fitsSystemWindows = "true"
        app:headerLayout = "@layout/nav_header_main"
        app:menu = "@menu/activity_main_drawer" />

</android.support.v4.widget.DrawerLayout>

appbarmain.xml:

    <android.support.design.widget.CoordinatorLayout xmlns:android = "http://schemas.android.com/apk/res/android"
    xmlns:app = "http://schemas.android.com/apk/res-auto"
    xmlns:tools = "http://schemas.android.com/tools"
    android:layout_width = "match_parent"
    android:layout_height = "match_parent"
    android:layoutDirection = "rtl"
    tools:context = ".MainActivity">
    <android.support.design.widget.AppBarLayout
        android:layout_width = "match_parent"
        android:layoutDirection = "rtl"
        android:elevation = "0dp"
        android:layout_height = "wrap_content"
        android:theme = "@style/AppTheme.AppBarOverlay">
        <RelativeLayout
            android:id = "@+id/toolbar"
            android:layout_width = "match_parent"
            android:layout_height = "?attr/actionBarSize"
            android:background = "?attr/colorPrimary"
            android:layoutDirection = "rtl">
            <ImageView
                android:id = "@+id/img_mainactivity_hambermenu"
                android:layout_width = "wrap_content"
                android:layout_height = "wrap_content"
                android:layout_alignParentRight = "true"
                android:layout_centerVertical = "true"
                android:layout_marginRight = "8dp"
                android:src = "@drawable/ic_hambermenu" />
            <TextView
                android:layout_width = "wrap_content"
                android:layout_height = "wrap_content"
                android:layout_centerHorizontal = "true"
                android:layout_centerVertical = "true"
                android:text = "@string/app_name"
                android:textColor = "@color/white"
                android:textSize = "18dp" />
        </RelativeLayout>


        <android.support.design.widget.TabLayout
            android:id = "@+id/tabLayout"
            android:background = "@color/colorPrimary"
            app:tabIndicatorColor = "@color/white"
            app:tabSelectedTextColor = "@color/colorAccent"
            app:tabTextColor = "@color/white"
            android:layout_width = "match_parent"
            android:layout_height = "wrap_content"/>
    </android.support.design.widget.AppBarLayout>
    <include layout = "@layout/content_main" />
    <android.support.design.widget.FloatingActionButton
        android:id = "@+id/fab"
        android:layout_width = "wrap_content"
        android:layout_height = "wrap_content"
        android:layout_gravity = "bottom|end"
        android:layout_margin = "@dimen/fab_margin"
        app:srcCompat = "@android:drawable/ic_dialog_email" />
</android.support.design.widget.CoordinatorLayout>

contentmain.xml:

<LinearLayout xmlns:android = "http://schemas.android.com/apk/res/android"
    xmlns:app = "http://schemas.android.com/apk/res-auto"
    xmlns:tools = "http://schemas.android.com/tools"
    android:layout_width = "match_parent"
    android:orientation = "vertical"
    android:layout_height = "match_parent"
    app:layout_behavior = "@string/appbar_scrolling_view_behavior"
    tools:context = ".MainActivity"
    tools:showIn = "@layout/app_bar_main">



    <android.support.v4.view.ViewPager
        android:layoutDirection = "ltr"
        android:id = "@+id/viewPager"
        android:layout_width = "match_parent"
        android:layout_height = "match_parent"/>

</LinearLayout>