Accessing fragment views from parent Activity simple explanation

Question

I have one activity with 4 fragments. Every fragment got its separate class and layout. This is the parent activity class:

    private SectionsPagerAdapter mSectionsPagerAdapter;
private ViewPager mViewPager;


@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    mSectionsPagerAdapter = new SectionsPagerAdapter(getSupportFragmentManager());
    mViewPager = (ViewPager) findViewById(R.id.container);
    mViewPager.setAdapter(mSectionsPagerAdapter);
    TabLayout tabLayout = (TabLayout) findViewById(R.id.tabs);
    tabLayout.setupWithViewPager(mViewPager);
}



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

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    int id = item.getItemId();

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

    return super.onOptionsItemSelected(item);
}



public class SectionsPagerAdapter extends FragmentPagerAdapter {

    public SectionsPagerAdapter(FragmentManager fm) {
        super(fm);
    }

    @Override
    public Fragment getItem(int position) {
            switch (position){
                case 0:
                    return new MainFragment();
                case 1:
                    return new CatalogFragment();
                case 2:
                    return new FolderFragment();
                case 3:
                    return new SettingsFragment();
                default:
                    return null;
            }
    }

    @Override
    public int getCount() {

        return 4;
    }

    @Override
    public CharSequence getPageTitle(int position) {
        switch (position) {
            case 0:
                return "PLAYER";
            case 1:
                return "CATALOG";
            case 2:
                return "FOLDERS";
            case 3:
                return "SETTINGS";
        }
        return null;
    }
}

Fragment classes are only with:

 @Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
   View view = inflater.inflate(R.layout.fragment_main,container,false);
    return view;
}

and a default constructor. In FragmentMain i have 2 TextViews. I want to change them dynamically depending on info within MainActivity. I am using ViewPager. This is my MainActivity XML file:

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent" android:layout_height="match_parent">

<ImageView
    android:layout_width="match_parent"
    android:layout_height="250dp"
    android:id="@+id/imageView"
    android:layout_alignParentTop="true"
    android:layout_alignParentLeft="true"
    android:layout_alignParentStart="false" />

<TextView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:textAppearance="?android:attr/textAppearanceLarge"
    android:text="Large Text"
    android:id="@+id/textView"
    android:layout_below="@+id/imageView"
    android:layout_centerHorizontal="true" />

<TextView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:textAppearance="?android:attr/textAppearanceMedium"
    android:text="Medium Text"
    android:id="@+id/textView2"
    android:layout_below="@+id/textView"
    android:layout_centerHorizontal="true" />

if i am doing this all wrong please correct me! I can findViewById() but i cannot set the text(NullPointer). From the fragment i cannot even findViewById() even thou they are in the MainFragment XML. I've searched a lot and i cannot find easy to understand solution or explanation how this works. Thanks in advance!