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RecyclerView not updating on Fragment - Adapter shows null on notifyDataSetChanged()

I am trying to implement a sales app with 3 stages, whereby a user first adds the items, views the cart to checkout and prints a receipt. The first stage is done and works well. I am implementing this using one Activity with a tabbed layout and 3 fragments for each of the tabs.
On the activity for viewing the cart, I am using a RecyclerView which loads well when the activity is created, but once I add an item and try to view the list, there is no update. However, when I print the item list, the item has been added.
I have tried to update the RecyclerView using notifyDataSetChanged() but it gives me a null pointer exception. I have and tried several solutions but none has worked. Kindly assist.
Below is my code.
NewSaleActivity.java
public class NewSaleActivity extends AppCompatActivity implements NavigationView.OnNavigationItemSelectedListener {
private ActivityNewSaleBinding binding;
DrawerLayout drawerLayout;
NavigationView navigationView;
TabLayout tabLayout;
Context context;
ViewPager viewPager;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
binding = ActivityNewSaleBinding.inflate(getLayoutInflater());
setContentView(binding.getRoot());
Toolbar toolbar = findViewById(R.id.new_sale_toolbar);
toolbar.setTitle(R.string.new_sale);
setSupportActionBar(toolbar);
drawerLayout = findViewById(R.id.new_sale_drawer);
navigationView = findViewById(R.id.new_sale_navigation);
navigationView.bringToFront();
navigationView.setNavigationItemSelectedListener(this);
navigationView.setCheckedItem(R.id.nav_sale);
ActionBarDrawerToggle toggle = new ActionBarDrawerToggle(this, drawerLayout, toolbar, R.string.navigation_drawer_open, R.string.navigation_drawer_close);
drawerLayout.addDrawerListener(toggle);
toggle.setDrawerIndicatorEnabled(true);
toggle.syncState();
context = getApplicationContext();
tabLayout = findViewById(R.id.tab_layout);
SectionsPagerAdapter sectionsPagerAdapter = new SectionsPagerAdapter(this, getSupportFragmentManager());
viewPager = binding.viewPager;
viewPager.setAdapter(sectionsPagerAdapter);
viewPager.setOffscreenPageLimit(1);
TabLayout tabs = binding.tabLayout;
tabs.setupWithViewPager(viewPager);
viewPager.addOnPageChangeListener(new TabLayout.TabLayoutOnPageChangeListener(tabLayout));
tabLayout.addOnTabSelectedListener(new TabLayout.OnTabSelectedListener() {
#Override
public void onTabSelected(TabLayout.Tab tab) {
//When cart tab is selected
if(tab.getPosition() == 1) {
CartFragment cartFragment = new CartFragment();
cartFragment.loadItemsInCart();
}
}
#Override
public void onTabUnselected(TabLayout.Tab tab) {
}
#Override
public void onTabReselected(TabLayout.Tab tab) {
}
});
}
#Override
public boolean onNavigationItemSelected(#NonNull MenuItem item) {
switch (item.getItemId()) {
case R.id.nav_home:
Intent mainIntent = new Intent(NewSaleActivity.this, MainActivity.class);
startActivity(mainIntent);
finish();
break;
case R.id.nav_inventory:
Intent inventoryIntent = new Intent(NewSaleActivity.this, InventoryActivity.class);
startActivity(inventoryIntent);
finish();
break;
case R.id.nav_sale:
break;
case R.id.nav_logout:
UserLogout.userLogout(NewSaleActivity.this);
break;
}
drawerLayout.closeDrawer(GravityCompat.START);
return true;
}
}
CartFragment.java
public class CartFragment extends Fragment {
RecyclerView cartRecyclerView;
TextInputLayout grossTotalInput, discountInput, netTotalInput;
Button checkoutButton;
CartItemAdapter cartItemAdapter;
ArrayList<CartItem> cartItemList;
public CartFragment() {
// Required empty public constructor
}
public static CartFragment newInstance() {
CartFragment fragment = new CartFragment();
Bundle args = new Bundle();
return fragment;
}
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
}
#Override
public void onViewCreated(#NonNull View view, #Nullable Bundle savedInstanceState) {
super.onViewCreated(view, savedInstanceState);
cartRecyclerView = view.findViewById(R.id.cart_recycler_view);
grossTotalInput = view.findViewById(R.id.gross_total_input);
discountInput = view.findViewById(R.id.discount_input);
netTotalInput = view.findViewById(R.id.net_total_input);
checkoutButton = view.findViewById(R.id.checkout_button);
Paper.init(requireContext());
cartItemList = Paper.book().read("cartItems", new ArrayList<CartItem>());
cartRecyclerView.setLayoutManager(new LinearLayoutManager(getContext()));
cartItemAdapter = new CartItemAdapter(cartItemList);
cartRecyclerView.setAdapter(cartItemAdapter);
checkoutButton.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View view) {
checkoutOrder();
}
});
cartItemAdapter.notifyDataSetChanged();
}
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
return inflater.inflate(R.layout.fragment_cart, container, false);
}
public void loadItemsInCart() {
ArrayList<CartItem> cartList = Paper.book().read("cartItems", new ArrayList<CartItem>());
for(CartItem cartItem : cartList) {
Log.d("cartItem", cartItem.getItemName());
}
cartItemAdapter.updateCartList(cartList);
Log.d("loadItemsInCart", "Load Items In Cart is triggered");
}
CartItemAdapter.java
public class CartItemAdapter extends RecyclerView.Adapter<CartItemAdapter.ViewHolder> {
private final ArrayList<CartItem> cartItems;
public CartItemAdapter(ArrayList<CartItem> cartItems) {
this.cartItems = cartItems;
}
public static class ViewHolder extends RecyclerView.ViewHolder {
public TextView itemCode, itemName, quantity, price;
public ViewHolder(View itemView) {
super(itemView);
this.itemCode = itemView.findViewById(R.id.item_code);
this.itemName = itemView.findViewById(R.id.item_name);
this.quantity = itemView.findViewById(R.id.item_quantity);
this.price = itemView.findViewById(R.id.item_price);
}
}
#NonNull
#Override
public ViewHolder onCreateViewHolder(#NonNull ViewGroup parent, int viewType) {
LayoutInflater layoutInflater = LayoutInflater.from(parent.getContext());
View cartListItem = layoutInflater.inflate(R.layout.add_to_cart_item, parent, false);
return new CartItemAdapter.ViewHolder(cartListItem);
}
#Override
public void onBindViewHolder(#NonNull ViewHolder holder, int position) {
holder.itemCode.setText(cartItems.get(position).getItemCode());
holder.itemName.setText(cartItems.get(position).getItemName());
holder.quantity.setText(cartItems.get(position).getQuantity().toString());
holder.price.setText(cartItems.get(position).getTotalAmount().toString());
}
#Override
public int getItemCount() {
return cartItems.size();
}
#SuppressLint("NotifyDataSetChanged")
public void updateCartList(ArrayList<CartItem> cartList) {
cartItems.clear();
this.cartItems.addAll(cartList);
notifyDataSetChanged();
}
}
The error message that I get is as below
java.lang.NullPointerException: Attempt to invoke virtual method 'void app.project.jsml_sales_app.adapters.CartItemAdapter.updateCartList(java.util.ArrayList)' on a null object reference
at app.project.jsml_sales_app.activities.fragments.CartFragment.loadItemsInCart(CartFragment.java:128)
at app.project.jsml_sales_app.activities.NewSaleActivity$1.onTabSelected(NewSaleActivity.java:78)
at com.google.android.material.tabs.TabLayout.dispatchTabSelected(TabLayout.java:1993)
at com.google.android.material.tabs.TabLayout.selectTab(TabLayout.java:1986)

CardView not shown on RecyclerView (In Fragment)

I want to make CardViews visible on a RecyclerView inside a Fragment, but all I get is a infinite loading sign and no cardviews are being showed. I'm very new to Android Programming so please be kind..
MainActivity
public class MainActivity extends AppCompatActivity {
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
FragmentPage fragment = new FragmentPage();
FragmentManager fragmentManager = getSupportFragmentManager();
FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
fragmentTransaction.add(R.id.placeholder, fragment);
fragmentTransaction.commit();
ViewPager viewpager = findViewById(R.id.viewpager);
viewpager.setOffscreenPageLimit(1);
SwipeAdapter adapter = new SwipeAdapter(getSupportFragmentManager());
viewpager.setAdapter(adapter);
viewpager.setPageTransformer(true, new DepthPageTransformer());
viewpager.setCurrentItem(0);
}
}
This is my Adapter class(I followed a Tutorial)
public class ListAdapterIT extends RecyclerView.Adapter {
#NonNull
#Override
public RecyclerView.ViewHolder onCreateViewHolder(#NonNull ViewGroup parent, int viewType) {
return null;
}
#Override
public void onBindViewHolder(#NonNull RecyclerView.ViewHolder holder, int position) {
}
#Override
public int getItemCount() {
return 0;
}
private class ListViewHolder extends RecyclerView.ViewHolder implements View.OnClickListener {
private ImageView mImageView;
public ListViewHolder (View itemView) {
super(itemView);
mImageView = itemView.findViewById(R.id.image2);
itemView.setOnClickListener(this);
}
public void BindViewHolder(int position) {
mImageView.setImageResource(OurData.picturePath[position]);
}
public void onClick(View view) {
}
}
}
This is my Fragment I also think the Problem is in that class
public class FragmentPage extends Fragment {
#Nullable
#Override
public View onCreateView(#NonNull LayoutInflater inflater, #Nullable ViewGroup container, #Nullable
Bundle savedInstanceState) {
View view = inflater.inflate(R.layout.page_fragment, container, false);
ArrayList<Integer> items = new ArrayList<>();
items.add(R.layout.example_item);
RecyclerView recyclerView = (RecyclerView) view.findViewById(R.id.listRecyclerView);
ListAdapterIT listAdapter = new ListAdapterIT() {};
recyclerView.setAdapter(listAdapter);
RecyclerView.LayoutManager layoutManager = new LinearLayoutManager(getActivity());
recyclerView.setLayoutManager(layoutManager);
return view;
}
}

Tab Fragments not showing

I am trying to add tabs in a Fragment (for a bottom navigation bar). However, when I run the app, the tabs don't show up at all. Below is an image of the result:
IMAGE
Home.java -> the fragment that should display 2 tabs which includes BlankFragment.java
CategoryAdapater.java -> sets up the tab names and fragments to display in each tab
BlankFragment.java -> an empty fragment that is shown on each tab
How can I get it to work?
Home.java (the fragment that should show the tabs)
public class Home extends Fragment {
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
// Inflate the layout for this fragment
View rootView = inflater.inflate(R.layout.fragment_home, container, false);
ViewPager viewPager = rootView.findViewById(R.id.viewpager);
CategoryAdapter adapter = new CategoryAdapter(getChildFragmentManager());
viewPager.setAdapter(adapter);
TabLayout tabLayout = rootView.findViewById(R.id.tabs);
tabLayout.setupWithViewPager(viewPager);
return rootView;
}
public Home() {}
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
}
#Override
public void onAttach(Context context) {
super.onAttach(context);
}
#Override
public void onDetach() {
super.onDetach();
}
CategoryAdapater.java
public class CategoryAdapter extends FragmentPagerAdapter {
public CategoryAdapter(FragmentManager fm) {
super(fm);
}
#Override
public Fragment getItem(int position) {
if (position == 0) {
return new BlankFragment();
} else {
return new BlankFragment();
}
}
#Override
public int getCount() {
return 2;
}
#Override
public CharSequence getPageTitle(int position) {
if (position == 0) {
return "Devices";
} else {
return "Numbers";
}
}
}
BlankFragment.java
public class BlankFragment extends Fragment {
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
// Inflate the layout for this fragment
View rootView = inflater.inflate(R.layout.fragment_blank, container, false);
return rootView;
}
public BlankFragment() {}
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
}
#Override
public void onAttach(Context context) {
super.onAttach(context);
}
#Override
public void onDetach() {
super.onDetach();
}
}

adapter addFragment missing
ViewPager viewPager = rootView.findViewById(R.id.viewpager);
CategoryAdapter adapter = new CategoryAdapter(getChildFragmentManager());
BlankFragment frag= new BlankFragment ();
adapter.addFragment(frag);
viewPager.setAdapter(adapter);
TabLayout tabLayout = rootView.findViewById(R.id.tabs);
tabLayout.setupWithViewPager(viewPager);

public class Home extends Fragment {
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
// Inflate the layout for this fragment
View rootView = inflater.inflate(R.layout.fragment_home, container, false);
ViewPager viewPager = rootView.findViewById(R.id.viewpager);
setupViewPager(viewPager);
CategoryAdapter adapter = new CategoryAdapter(getChildFragmentManager());
viewPager.setAdapter(adapter);
TabLayout tabLayout = rootView.findViewById(R.id.tabs);
tabLayout.setupWithViewPager(viewPager);
return rootView;
}
public Home() {}
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
}
#Override
public void onAttach(Context context) {
super.onAttach(context);
}
private void setupViewPager(ViewPager viewPager) {
ViewPagerAdapter adapter = new
ViewPagerAdapter(getSupportFragmentManager());
adapter.addFragment(new OneFragment(), "Devices");
adapter.addFragment(new TwoFragment(), "Numbers");
viewPager.setAdapter(adapter);
}
#Override
public void onDetach() {
super.onDetach();
}
}
Update Home Class with this code

Reload Fragment, Fragment dont call onCreateView

i have a Fragment(C) with a ViewPager(Adapter) with 2 Fragments(A/B).
When i call the Fragment once time, they load fine.
But when i call the Fragment(C) the secend time, the Fragment A and B dont call the View onCreateView methode and the isn´t loaded.
The Fragment C is called with Fragmetnmanger:
FragmentC fragment = new FragmentC();
getActivity().getSupportFragmentManager().beginTransaction().replace(R.id.adressenmdf_detail_container, fragment).addToBackStack(null).commit();
Fragment C
public BelegeDetails(){
}
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
}
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View rootView = inflater.inflate(R.layout.activity_belege_details, container, false);
//TODO Actionbar noch anpassen
//getSupportActionBar().setDisplayHomeAsUpEnabled(true);
viewPager = (ViewPager) rootView.findViewById(R.id.viewpager);
setupViewPager(viewPager);
tabLayout = (TabLayout) rootView.findViewById(R.id.tabs);
tabLayout.setupWithViewPager(viewPager);
setupTabIcons();
The Setups from Fragment C
private void setupViewPager(ViewPager viewPager) {
ViewPagerAdapter adapter = new ViewPagerAdapter(this.getFragmentManager());
adapter.addFragment(new BelegeDetails1(),"Details" );
adapter.addFragment(new BelegeDetailsPos(), "Positionen");
viewPager.setAdapter(adapter);
}
private void setupTabIcons() {
tabLayout.getTabAt(1).setIcon(R.drawable.ic_belege);
}
class ViewPagerAdapter extends FragmentPagerAdapter {
private final List<Fragment> mFragmentList = new ArrayList<>();
private final List<String> mFragmentTitleList = new ArrayList<>();
public ViewPagerAdapter(FragmentManager manager) {
super(manager);
}
#Override
public Fragment getItem(int position) {
return mFragmentList.get(position);
}
#Override
public int getCount() {
return mFragmentList.size();
}
public void addFragment(Fragment fragment, String title) {
mFragmentList.add(fragment);
mFragmentTitleList.add(title);
}
#Override
public CharSequence getPageTitle(int position) {
return mFragmentTitleList.get(position);
}
}
The Fragment A BelegDetails1 und the Fragment B BelegDetailsPos are normal Fragment Classes like:
public BelegeDetails1() {
// Required empty public constructor
}
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
}
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState.......

try changing this:
ViewPagerAdapter adapter = new ViewPagerAdapter(this.getFragmentManager());
with:
ViewPagerAdapter adapter = new ViewPagerAdapter(getChildFragmentManager());
As you are nesting fragments, you need to use getChildFragmentManager() instead of getFragmentManager()

How to stop tab change from running fragment onCreateView in Andriod

I have three tabs using the implementation below and they perform very well. When tab is changed the proper fragment is load and so on. The problem is that, when i get to the last tab and comeback to the first fragment, its like its oncreateview method is always triggered again running the other codes it in causing duplicates. Any help will be greatly appreciated.
//Activity on the tab is based
public class Dashboard extends AppCompatActivity {
private TabLayout tabLayout;
private ViewPager viewPager;
private MyViewPagerAdapter myViewPagerAdapter;
private int[] tabIcon = {R.drawable.ic_home, R.drawable.ic_message, R.drawable.ic_person};
android.support.v7.widget.Toolbar toolbar;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.dashboard);
//Toolbar
toolbar = (android.support.v7.widget.Toolbar) findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
//ViewPager
viewPager = (ViewPager) findViewById(R.id.viewpager);
setupViewPager(viewPager);
//Tablayout
tabLayout = (TabLayout) findViewById(R.id.tabs);
tabLayout.setupWithViewPager(viewPager);
tabLayout.getTabAt(0).setIcon(tabIcon[0]);
tabLayout.getTabAt(1).setIcon(tabIcon[1]);
tabLayout.getTabAt(2).setIcon(tabIcon[2]);
viewPager.addOnPageChangeListener(new TabLayout.TabLayoutOnPageChangeListener(tabLayout));
tabLayout.setOnTabSelectedListener(new TabLayout.OnTabSelectedListener() {
#Override
public void onTabSelected(TabLayout.Tab tab) {
switch(tab.getPosition()) {
case 0:
viewPager.setCurrentItem(0);
toolbar.setTitle("Home");
break;
case 1:
viewPager.setCurrentItem(1);
toolbar.setTitle("Messages");
break;
case 2:
viewPager.setCurrentItem(2);
toolbar.setTitle("Profile");
break;
}
}
#Override
public void onTabUnselected(TabLayout.Tab tab) {
}
#Override
public void onTabReselected(TabLayout.Tab tab) {
}
});
}
private void setupViewPager(ViewPager viewPager){
myViewPagerAdapter = new MyViewPagerAdapter(getSupportFragmentManager());
myViewPagerAdapter.addFragments(new CategoryFragment(), "Categories");
myViewPagerAdapter.addFragments(new MessagesFragment(), "Messages");
myViewPagerAdapter.addFragments(new ProfileFragment(), "Profile");
viewPager.setAdapter(myViewPagerAdapter);
}
//View Pager Adapter
public class MyViewPagerAdapter extends FragmentPagerAdapter {
ArrayList<Fragment> fragments = new ArrayList<>();
ArrayList<String> tabTitles = new ArrayList<>();
public void addFragments(Fragment fragments, String titles){
this.fragments.add(fragments);
this.tabTitles.add(titles);
}
public MyViewPagerAdapter(FragmentManager fm) {
super(fm);
}
#Override
public int getCount() {
return fragments.size();
}
#Override
public Fragment getItem(int position) {
return fragments.get(position);
}
#Override
public CharSequence getPageTitle(int position) {
//return tabTitles.get(position);
return null;
}
}
#Override
public void onBackPressed() {
super.onBackPressed();
}
}
//Main first fragment code
public class CategoryFragment extends Fragment {
private DBHandler dbHandler;
private ListView listView;
private ListAdapter adapter;
ArrayList<Categories> categoriesList = new ArrayList<Categories>();
public CategoryFragment() {
// Required empty public constructor
}
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
// Inflate the layout for this fragment
View view = inflater.inflate(R.layout.fragment_home, container, false);
//Setting up the basic categories
dbHandler = new DBHandler(view.getContext());
//Get Categories from database
final Cursor cursor = dbHandler.getCategories(0);
if (cursor != null) {
if(cursor.moveToFirst()){
do{
Categories categories = new Categories();
categories.set_id(cursor.getInt(0));
categories.set_categoryname(cursor.getString(2));
categories.set_categoriescaption(cursor.getString(3));
categoriesList.add(categories);
}while (cursor.moveToNext());
}
cursor.close();
}
listView = (ListView) view.findViewById(R.id.categories);
adapter = new CategoryAdapter(view.getContext(), R.layout.cursor_row, categoriesList);
listView.setAdapter(adapter);
listView.setOnItemClickListener(
new AdapterView.OnItemClickListener() {
#Override
public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
Integer cid = (int) (long) adapter.getItemId(position);
TextView categoryname = (TextView) view.findViewById(R.id.cursor);
String cname = categoryname.getText().toString();
Intent i = new Intent(view.getContext(), CategoryList.class);
i.putExtra("categoryname", cname);
i.putExtra("categoryid", cid);
startActivity(i);
}
}
);
return view;
}
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
}
}
So when i swipe back here from the last tab. OncreateView runs again. How can i handle that and prevent duplicates. Thank you

By defaults ViewPager retains only 1 page in the view hierarchy in an idle state.So when you swipe to third tab the viepager destroys the first tab in order to retain the second one.
To solve this issue add this line
viewPager.setOffscreenPageLimit(3);
after you set your adapter.
Check here the documentation for more