unable to retrieve data of a single row using room database

Question

Unable to get particular record from database getting this output.

My output is :

java.lang.RuntimeException: Unable to start activity ComponentInfo{appwork.com.example/appwork.com.example.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.arch.lifecycle.LiveData.observe(android.arch.lifecycle.LifecycleOwner, android.arch.lifecycle.Observer)' on a null object reference

I am using room database, Modelview, Repository and Dao file to get Live data but unable to get particular record from database via following things

    <pre><code>
    public class MainActivity extends AppCompatActivity {

        private NoteViewModel noteViewModel;

        @Override
        protected void onCreate(Bundle savedInstanceState)
        {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_main);

            noteViewModel = 
    ViewModelProviders.of(this).get(NoteViewModel.class);
            noteViewModel.getNote("test7").observe(this, new Observer<Note>()
            {
                   @Override
                   public void onChanged(@Nullable Note note)
                   {
                       Toast.makeText(MainActivity.this,
                            "found title is : " +note.getTitle(),
                            Toast.LENGTH_LONG).show();
                   }
                 });
            }
        }   

    public class NoteViewModel extends AndroidViewModel
    {
        private LiveData<Note> note;

        public NoteViewModel(@NonNull Application application)
        {
            super(application);
    //        note = repository.getNote("");
        }

        public LiveData<Note> getNote(String search)
        {
            repository.getNote(search);
            if(note != null)
            {
                return note;
            }
            return null;
        }
    }


    public class NoteRepository {

        private NoteDao noteDao;
        private LiveData<Note> note;

        public NoteRepository(Application application)
        {
            NoteDatabase noteDatabase = 
    NoteDatabase.getInstance(application);
            noteDao = noteDatabase.noteDao();
        }

        public LiveData<Note> getNote(String search)
        {
            new SearchNoteAsyncTask(noteDao).execute(search);
            if(note != null)
            {
                return note;
            }
            return null;
        }


       public static void asyncFinished(LiveData<Note> results)
       {
            note = results;
       }

        public static class SearchNoteAsyncTask extends AsyncTask<String, Void, LiveData<Note>>
        {
            private NoteDao noteDao;
            private LiveData<Note> note;

            private SearchNoteAsyncTask(NoteDao noteDao)
            {
                this.noteDao = noteDao;
            } 

            public LiveData<Note> doInBackground(String... search)
            {
                return noteDao.getNote(search[0]);
            }

            public void onPostExecute(LiveData<Note> result)
            {
                asyncFinished(result);            
            }
        }
    }

    @Dao
    public interface NoteDao{
    @Query("Select * from note_table where title =:search Order By priority DESC")
        LiveData<Note> getNote(String search);
    }

I am getting response in Repository call but unable to get value from public

static class SearchNoteAsyncTask extends AsyncTask<String, Void, LiveData<Note>>

Any working example will be great!

thanks

Hi Manoj, welcome to SO. This is a NullPointerException, please refer to https://stackoverflow.com/questions/218384/what-is-a-nullpointerexception-and-how-do-i-fix-it — veritas1, Jul 12 '19 at 07:08

Vishal Arora · Answer 1 · 2019-07-12T07:37:27.003

0

Problem is that you are returning a different reference of Livedata to your View in -

public LiveData<Note> getNote(String search){
    repository.getNote(search);
    if(note != null)
    {
        return note; // this is the livedata reference that UI is observing
    }
    return null;
}

And when you get the response from repo you change the reference of livedata -

public static void asyncFinished(LiveData<Note> results) {
    note = results;
}

But your UI is listening to previous LiveData.

Instead of changing the reference, you should change the value in original note livedata.

That would look something like -

public static void asyncFinished(LiveData<Note> results) {
    note.setValue(results.getValue); // not the preferred way though, but it will help you understand why your current implementation doesn't work.
}

And best would be to use Transformation on the original livedata -

public LiveData<Note> getNote(String search){
   return Transformations.map(repository.getNote(search), 
          value -> note.setValue(value));  
}

edited Jul 12 '19 at 07:37

answered Jul 12 '19 at 07:10

Vishal Arora

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public static void asyncFinished(LiveData results) { note.setValue(results.getValue); // not the preferred way though, but it will help you understand why your current implementation doesn't work. } I do not create setValue method in my Note class – Manoj Pawar Jul 12 '19 at 07:39
note is the reference of livedata object - `private LiveData note; ` – Vishal Arora Jul 12 '19 at 07:41
should i make getValue and setValue class in Note class – Manoj Pawar Jul 12 '19 at 08:17
setValue is a method provided by LiveData. – Vishal Arora Jul 12 '19 at 09:51

unable to retrieve data of a single row using room database

1 Answers1