为什么我的片段不能显示来自 recyclerview 的数据？

Question

Before posting here ,i have spent considerable time to figure it out myself,i have removed various errors though. I am developing a little notesapp ,evrything worked fine until i used fragments in my app which contains a recycler view.The problem is that i cannot pass the data retrieved from database to recyclerview adapter .

the error says "Attempt to invoke virtual method 'void com.example.diary.RecyclerAdapter.setdata(java.util.List)' on a null object reference"

i have verified that the data is not empty ,iam confused why do i get a null pointer error.

relevant codes are posted below.

fragment.java

 @Override
public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
    view = inflater.inflate(R.layout.fragment_recents, container, false);
    RecyclerAdapter adapter;
    db = NotesDatabase.createdb(getContext());

    adapter = new RecyclerAdapter(numofitems); 
     retrievetasks();
    layoutManager = new LinearLayoutManager(getActivity());
    binding = DataBindingUtil.setContentView(getActivity(), R.layout.fragment_recents);
    binding.recyclerView.setLayoutManager(layoutManager);
    binding.recyclerView.setAdapter(adapter);
    Log.d("adapeter", "has set");
    return view;
}

public void retrievetasks() {
    try {
        LiveData<List<NotesEntry>> notesEntries = db.Dao().loadalltasks();
        notesEntries.observe(this, new Observer<List<NotesEntry>>() {
            @Override
            public void onChanged(List<NotesEntry> notesEntries) {
                notes = notesEntries;
                Log.d("sizeforall", String.valueOf(notesEntries.size()));
                adapter.setdata(notesEntries); //this is where the error occures }
             });

'adapter.setdata(notesEntries); //this is where the error occures'

adapter.java

 public void setdata(List<NotesEntry> notesEntries) {
    try {
        notesEntryList = notesEntries;
        notifyDataSetChanged();
    } catch (Exception e) {
        e.printStackTrace();
    }
}
public List<NotesEntry> getnotesentries() {
    return notesEntryList;
}
@Override
public void onBindViewHolder(@NonNull RecyclerView.ViewHolder holder, int position) {
    try {
        NotesEntry notesEntries = notesEntryList.get(position);
        id = notesEntries.getId();
        text = notesEntries.getText();
        // b1.setText(String.valueOf(id));
        textview1.setText(text);
    } catch (Exception e) {
        e.printStackTrace();
    }
}
@Override
public int getItemCount() {
    if (!notesEntryList.isEmpty()) {
        Log.d("sizereturned", String.valueOf(notesEntryList.size()));
        return notesEntryList.size();
    } else
        Log.d("sizereturned1", String.valueOf(notesEntryList.size()));
    return 0; }

And if i dont use fragments ,this isuue does not arise.Any help is appreciated.

Answer 1

You need to call retrievetasks() method after initializing the adapter. This is causing the problem

adapter = new RecyclerAdapter(numofitems);
retrievetasks();

Answer 2

The adapter in your retrieveTasks is different than the one you initialized. You are initializing a local instance, which is outside the scope of your retrieveTasks method. Presumably you have a property in the fragment named adapter already, so you can just delete the local instance to initialize the right one:

@Override
public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
    view = inflater.inflate(R.layout.fragment_recents, container, false);
    RecyclerAdapter adapter; // <- DELETE THIS
    db = NotesDatabase.createdb(getContext());
    adapter = new RecyclerAdapter(numofitems);
}

Hope that helps!