ViewModel instance - Activity UI - Android Architecture Components

Question

 public class WordViewModel extends AndroidViewModel {

    private WordRepository mRepository;
    private LiveData<List<Word>> mAllWords;

    public WordViewModel(@NonNull Application application) {
        super(application);
        mRepository = new WordRepository(application);
        mAllWords = mRepository.getAllWords();
    }

    LiveData<List<Word>> getAllWords() {
        return mAllWords;
    }

    public void insert(Word word) {
        mRepository.insert(word);
    }
}

mWordViewModel = new ViewModelProvider(this,
                new ViewModelProvider.AndroidViewModelFactory(getApplication()))
                .get(WordViewModel.class);

public class WordRepository {

    private WordDao mWordDao;
    private LiveData<List<Word>> mAllWords;

    public WordRepository(Application application) {

        WordRoomDatabase db = WordRoomDatabase.getDatabase(application);
        mWordDao = db.wordDao();
        mAllWords = mWordDao.getAllWords();
    }

    LiveData<List<Word>> getAllWords() {
        return mAllWords;
    }

    public void insert(Word word) {
        new insertAsyncTask(mWordDao).execute(word);
    }

    private static class insertAsyncTask extends AsyncTask<Word, Void, Void> {

        private WordDao mAsyncTaskDao;

        insertAsyncTask(WordDao dao) {
            mAsyncTaskDao = dao;
        }

        @Override
        protected Void doInBackground(final Word... params) {
            mAsyncTaskDao.insert(params[0]);
            return null;
        }
    }
}

@Database(entities = {Word.class}, version = 1, exportSchema = false)
public abstract class WordRoomDatabase extends RoomDatabase {

    public abstract WordDao wordDao();

    private static WordRoomDatabase INSTANCE;

    public static WordRoomDatabase getDatabase(Context context) {

        if (INSTANCE == null) {
            synchronized (WordRoomDatabase.class) {
                if (INSTANCE == null) {
                    Room.databaseBuilder(context.getApplicationContext(),
                            WordRoomDatabase.class, "word_database")
                            .fallbackToDestructiveMigration()
                            .addCallback(sRoomDatabaseCallback)
                            .build();
                }
            }
        }

        return INSTANCE;
    }

    private static Callback sRoomDatabaseCallback =
            new Callback() {
                @Override
                public void onOpen(@NonNull SupportSQLiteDatabase db) {
                    super.onOpen(db);
                    new PopulateDbAsync(INSTANCE).execute();
                }
            };

    /**
     * Populate the database in the background.
     */
    private static class PopulateDbAsync extends AsyncTask<Void, Void, Void> {

        private final WordDao mDao;
        String[] words = {"dolphin", "crocodile", "cobra"};

        PopulateDbAsync(WordRoomDatabase db) {
            mDao = db.wordDao();
        }

        @Override
        protected Void doInBackground(final Void... params) {
            // Start the app with a clean database every time.
            // Not needed if you only populate the database
            // when it is first created
            mDao.deleteAll();

            for (int i = 0; i <= words.length - 1; i++) {
                Word word = new Word(words[i]);
                mDao.insert(word);
            }
            return null;
        }
    }
}

I am trying to instantiate ViewModel class in my MainActivity's onCreate method but it shows the following error.

Cannot create an instance of class com.example.roomwords_sample.WordViewModel

I also tried using ViewModelProviders.of(this).get(ViewModel.class) but it is still showing the same error. I also looking at other people's answers on similar question but it is not working. I have tried almost everything. Kindly help me out.

Added stack trace

2020-09-20 01:51:37.103 13578-13578/com.example.roomwords_sample E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.roomwords_sample, PID: 13578
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.roomwords_sample/com.example.roomwords_sample.MainActivity}: java.lang.RuntimeException: Cannot create an instance of class com.example.roomwords_sample.WordViewModel
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3449)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3601)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2066)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:223)
        at android.app.ActivityThread.main(ActivityThread.java:7656)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947)
     Caused by: java.lang.RuntimeException: Cannot create an instance of class com.example.roomwords_sample.WordViewModel
        at androidx.lifecycle.ViewModelProvider$AndroidViewModelFactory.create(ViewModelProvider.java:275)
        at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:187)
        at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:150)
        at com.example.roomwords_sample.MainActivity.onCreate(MainActivity.java:32)
        at android.app.Activity.performCreate(Activity.java:8000)
        at android.app.Activity.performCreate(Activity.java:7984)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1309)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3422)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3601) 
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85) 
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135) 
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2066) 
        at android.os.Handler.dispatchMessage(Handler.java:106) 
        at android.os.Looper.loop(Looper.java:223) 
        at android.app.ActivityThread.main(ActivityThread.java:7656) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947) 
     Caused by: java.lang.reflect.InvocationTargetException
        at java.lang.reflect.Constructor.newInstance0(Native Method)
        at java.lang.reflect.Constructor.newInstance(Constructor.java:343)
        at androidx.lifecycle.ViewModelProvider$AndroidViewModelFactory.create(ViewModelProvider.java:267)
        at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:187) 
        at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:150) 
        at com.example.roomwords_sample.MainActivity.onCreate(MainActivity.java:32) 
        at android.app.Activity.performCreate(Activity.java:8000) 
        at android.app.Activity.performCreate(Activity.java:7984) 
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1309) 
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3422) 
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3601) 
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85) 
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135) 
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2066) 
        at android.os.Handler.dispatchMessage(Handler.java:106) 
        at android.os.Looper.loop(Looper.java:223) 
        at android.app.ActivityThread.main(ActivityThread.java:7656) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947) 
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'com.example.roomwords_sample.WordDao com.example.roomwords_sample.WordRoomDatabase.wordDao()' on a null object reference
        at com.example.roomwords_sample.WordRepository.<init>(WordRepository.java:18)
        at com.example.roomwords_sample.WordViewModel.<init>(WordViewModel.java:18)
        at java.lang.reflect.Constructor.newInstance0(Native Method) 
        at java.lang.reflect.Constructor.newInstance(Constructor.java:343) 
        at androidx.lifecycle.ViewModelProvider$AndroidViewModelFactory.create(ViewModelProvider.java:267) 
        at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:187) 
        at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:150) 
        at com.example.roomwords_sample.MainActivity.onCreate(MainActivity.java:32) 
        at android.app.Activity.performCreate(Activity.java:8000) 
        at android.app.Activity.performCreate(Activity.java:7984) 
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1309) 
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3422) 
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3601) 
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85) 
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135) 
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2066) 
        at android.os.Handler.dispatchMessage(Handler.java:106) 
        at android.os.Looper.loop(Looper.java:223) 
        at android.app.ActivityThread.main(ActivityThread.java:7656) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947)

Answer 1

The problem is with your WordRoomDatabase.getDatabase(application) method

When you write:

if (INSTANCE == null) {
    Room.databaseBuilder(context.getApplicationContext(),
        WordRoomDatabase.class, "word_database")
        .fallbackToDestructiveMigration()
        .addCallback(sRoomDatabaseCallback)
        .build();
}

You create the database, but you don't actually assign it to your INSTANCE variable. That means your INSTANCE is always null and WordRoomDatabase.getDatabase(application) never actually returns anything but null.

You can fix your issue and the NullPointerException that comes from it by setting your INSTANCE variable:

if (INSTANCE == null) {
    INSTANCE = Room.databaseBuilder(context.getApplicationContext(),
        WordRoomDatabase.class, "word_database")
        .fallbackToDestructiveMigration()
        .addCallback(sRoomDatabaseCallback)
        .build();
}