Customizing android Fragments in this code

Well, I started studying java for android development this year and now I'm interested in browsing tabs that stay on the toolbar. Well, I was able to understand part of the code, but I could not change the content of the fragments. Could someone give me a light?

typelikethis.

MainActivity.java

packagecom.example.gabriel.testenavegaoportab;importandroid.support.design.widget.TabLayout;importandroid.support.design.widget.Snackbar;importandroid.support.v7.app.AppCompatActivity;importandroid.support.v7.widget.Toolbar;importandroid.support.v4.app.Fragment;importandroid.support.v4.app.FragmentManager;importandroid.support.v4.app.FragmentPagerAdapter;importandroid.support.v4.view.ViewPager;importandroid.os.Bundle;importandroid.view.LayoutInflater;importandroid.view.Menu;importandroid.view.MenuItem;importandroid.view.View;importandroid.view.ViewGroup;importandroid.widget.TextView;publicclassMainActivityextendsAppCompatActivity{privateSectionsPagerAdaptermSectionsPageAdapter;privateViewPagermViewPager;@OverrideprotectedvoidonCreate(BundlesavedInstanceState){super.onCreate(savedInstanceState);setContentView(R.layout.activity_main);Toolbartoolbar=(Toolbar)findViewById(R.id.toolbar);setSupportActionBar(toolbar);mSectionsPageAdapter=newSectionsPagerAdapter(getSupportFragmentManager());mViewPager=(ViewPager)findViewById(R.id.container);mViewPager.setAdapter(mSectionsPageAdapter);TabLayouttabLayout=(TabLayout)findViewById(R.id.tabs);mViewPager.addOnPageChangeListener(newTabLayout.TabLayoutOnPageChangeListener(tabLayout));tabLayout.addOnTabSelectedListener(newTabLayout.ViewPagerOnTabSelectedListener(mViewPager));}@OverridepublicbooleanonCreateOptionsMenu(Menumenu){getMenuInflater().inflate(R.menu.menu_main,menu);returntrue;}@OverridepublicbooleanonOptionsItemSelected(MenuItemitem){intid=item.getItemId();if(id==R.id.action_settings){returntrue;}returnonOptionsItemSelected(item);}publicclassSectionsPagerAdapterextendsFragmentPagerAdapter{publicSectionsPagerAdapter(FragmentManagerfragmentManager){super(fragmentManager);}@OverridepublicFragmentgetItem(intposition){returnPlaceholderFragment.newInstance(position+1);}@OverridepublicintgetCount(){//Mostraumtotalde3tabsreturn3;}}publicstaticclassPlaceholderFragmentextendsFragment{privatestaticfinalStringARG_SECTION_NUMBER="section_number";

    public PlaceholderFragment(){
    }

    public static PlaceholderFragment newInstance(int sectionNumber){
        PlaceholderFragment fragment = new PlaceholderFragment();
        Bundle args= new Bundle();
        args.putInt(ARG_SECTION_NUMBER, sectionNumber);
        fragment.setArguments(args);
        return fragment;
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState){
        View rootView = inflater.inflate(R.layout.fragment_main, container,false);
        TextView textView = (TextView) rootView.findViewById(R.id.section_label);
        textView.setText(getString(R.string.section_format,getArguments().getInt(ARG_SECTION_NUMBER)));
        return rootView;
    }
}
}

activity_main.xml     

<android.support.design.widget.AppBarLayout android:id="@+id/appbar"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:paddingTop="@dimen/appbar_padding_top"
    android:theme="@style/AppTheme.AppBarOverlay">

    <android.support.v7.widget.Toolbar android:id="@+id/toolbar"
        android:layout_width="match_parent"
        android:layout_height="?attr/actionBarSize"
        android:layout_weight="1"
        android:background="?attr/colorPrimary"
        app:layout_scrollFlags="scroll|enterAlways"
        app:popupTheme="@style/AppTheme.PopupOverlay"
        app:title="@string/app_name">

    </android.support.v7.widget.Toolbar>

    <android.support.design.widget.TabLayout
        android:id="@+id/tabs"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_marginEnd="200dp"
        app:tabGravity="center"
        app:tabSelectedTextColor="@color/colorAccent">

        <android.support.design.widget.TabItem
            android:id="@+id/tabItem1"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Tab 1" />

        <android.support.design.widget.TabItem
            android:id="@+id/tabItem2"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Tab 2" />

        <android.support.design.widget.TabItem
            android:id="@+id/tabItem3"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Tab 3" />

    </android.support.design.widget.TabLayout>
    <android.support.v4.view.ViewPager android:id="@+id/container"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_weight="1"
        app:layout_behavior="@string/appbar_scrolling_view_behavior"/>
</android.support.design.widget.AppBarLayout>

fragment_main.xml - Would that be it?     

<TextView android:id="@+id/section_label"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_marginBottom="@dimen/activity_vertical_margin"
    android:layout_marginEnd="@dimen/activity_horizontal_margin"
    android:layout_marginStart="@dimen/activity_horizontal_margin"
    android:layout_marginTop="@dimen/activity_vertical_margin"
    app:layout_constraintLeft_toLeftOf="parent"
    app:layout_constraintTop_toTopOf="@+id/constraintlayout"
    tools:layout_constraintLeft_creator="1"
    tools:layout_constraintTop_creator="1"/>

</android.support.constraint.ConstraintLayout>