How to make a Tabs Fragment inside Navigation Drawer Fragment?

Question

I have a navigation drawer with two items(each item is a fragment)

The first item "import" ,should display this Fragment:

Here is the Fragment class:

public class Import extends Fragment {

    @Nullable
    @Override
    public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        return inflater.inflate(R.layout.import_main,container,false);
    }
}

it's layout

 <?xml version="1.0" encoding="utf-8"?>
    <android.support.constraint.ConstraintLayout
        xmlns:android="http://schemas.android.com/apk/res/android" android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:background="@color/colorPrimaryDark">

</android.support.constraint.ConstraintLayout>

The second item "Gallery",should display a TabsLayout that contains 2 fragments (TabLeft and TabRight) :

Here is it's classe:

 @Nullable
    @Override
    public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        return inflater.inflate(R.layout.tabs,container,false);
    }

and the layout:

<?xml version="1.0" encoding="utf-8"?>
<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"
    tools:context=".MainActivity">

    <android.support.design.widget.AppBarLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        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:background="?attr/colorPrimary"
            app:popupTheme="@style/AppTheme.PopupOverlay" />
        <android.support.design.widget.TabLayout
            android:id="@+id/tab_id"
            android:layout_width="match_parent"
            android:layout_height="match_parent">

            <android.support.design.widget.TabItem
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:text="left"/>
            <android.support.design.widget.TabItem
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:text="right"/>
        </android.support.design.widget.TabLayout>

        <android.support.v4.view.ViewPager
            android:layout_width="match_parent"
            android:layout_height="fill_parent"
            android:id="@+id/pager" />

    </android.support.design.widget.AppBarLayout>

    <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>

I have tried to implement that but i get a weird error which is:

java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.abzo.fragmentstut/com.example.abzo.fragmentstut.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.v4.view.ViewPager.setAdapter(android.support.v4.view.PagerAdapter)' on a null object reference

Here is my MainActivity code:

package com.example.abzo.fragmentstut;

import android.os.Bundle;
import android.support.design.widget.FloatingActionButton;
import android.support.design.widget.Snackbar;
import android.support.design.widget.TabLayout;
import android.support.v4.app.FragmentPagerAdapter;
import android.support.v4.view.ViewPager;
import android.view.View;
import android.support.design.widget.NavigationView;
import android.support.v4.view.GravityCompat;
import android.support.v4.widget.DrawerLayout;
import android.support.v7.app.ActionBarDrawerToggle;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.Menu;
import android.view.MenuItem;

public class MainActivity extends AppCompatActivity
        implements NavigationView.OnNavigationItemSelectedListener {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

        ////////
        TabLayout tabLayout = (TabLayout)findViewById(R.id.tab_id);
        ViewPager viewPager = (ViewPager)findViewById(R.id.pager);

        TabPagerAdapter tabsAdapter = new TabPagerAdapter(getSupportFragmentManager());
        viewPager.setAdapter(tabsAdapter);
        //tabLayout.setupWithViewPager(viewPager);

        /////
        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);
        ActionBarDrawerToggle toggle = new ActionBarDrawerToggle(
                this, drawer, toolbar, R.string.navigation_drawer_open, R.string.navigation_drawer_close);
        drawer.addDrawerListener(toggle);
        toggle.syncState();

        NavigationView navigationView = (NavigationView) findViewById(R.id.nav_view);
        navigationView.setNavigationItemSelectedListener(this);
    }

    @Override
    public void onBackPressed() {
        DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
        if (drawer.isDrawerOpen(GravityCompat.START)) {
            drawer.closeDrawer(GravityCompat.START);
        } 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
            getSupportFragmentManager().beginTransaction().replace(R.id.content_main, new Import()).commit();

        } else if (id == R.id.nav_gallery) {
            getSupportFragmentManager().beginTransaction().replace(R.id.content_main, new Tabs()).commit();

        } 

        DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
        drawer.closeDrawer(GravityCompat.START);
        return true;
    }
}

My TabPagerAdapter:

package com.example.abzo.fragmentstut;

import android.support.annotation.Nullable;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentPagerAdapter;

public class TabPagerAdapter extends FragmentPagerAdapter {

    String [] pagetitle = new String []{"one","two"};
    public TabPagerAdapter(FragmentManager fm) {
        super(fm);
    }

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

    @Override
    public Fragment getItem(int position) {

        switch (position)
        {
                case 0:
            TabOne one= new TabOne();
            return one;
                 case 1:
            TabTwo two= new TabTwo();
            return two;
        }
        return null;
    }

    @Override
    public int getCount() {
        return 2;
    }
}

and here is one of the 2 tab classes (fragments): (the 2nd tab class is similar)

  public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        return inflater.inflate(R.layout.tab_left,container,false);
    }

Answer 1

Try this one

 @Override
    public Fragment getItem(int position) {


        switch (position)
        {
            case 0:
                TabOne  one= new TabOne();
                return one;
            deafault:
                 TabTwo two= new TabTwo();
                 return two;
        }
    }

You have to return Fragment here not null. Always try to use default in switch