Почему у меня есть нулевой указатель SQLite при получении уведомления?

Question

Я создаю приложение, в котором я использую уведомления Firebase. При получении уведомления я хочу сохранить это уведомление в локальном хранилище (SQLite). Но когда я хочу читать из этой базы данных, он отправляет мне нулевой указатель. Где моя ошибка?

Вот мой DataBaseHelper класс:

public class DataBaseHelper extends SQLiteOpenHelper{ 
private static final int DATABASE_VERSION = 1; 
private static final String DATABASE_NAME = "bazaFire.db"; 
private static final String TABLE_NAME = "Messages"; 
private static final String COLUMN_NAME="message_body"; 
private SQLiteDatabase myDB; 
public DataBaseHelper(Context context) { 
    super(context, DATABASE_NAME, null, DATABASE_VERSION); 
} 

@Override 
public void onCreate(SQLiteDatabase db) { 

    db.execSQL("CREATE TABLE " + TABLE_NAME 
      + "(id INTEGER PRIMARY KEY AUTOINCREMENT, message_body TEXT)"); 

} 

@Override 
public void onUpgrade(SQLiteDatabase db, int i, int i1) { 
    db.execSQL("DROP TABLE IF EXISTS " + TABLE_NAME); 
    onCreate(db); 

} 
public void insertMsg(String msg) { 

    SQLiteDatabase db = getWritableDatabase(); 
    Log.d("getWritable", "Okay"); 

    ContentValues cv = new ContentValues(); 

    cv.put(COLUMN_NAME, msg); 
    Log.d("Stavljena", msg); 

    db.insert(TABLE_NAME, null, cv); 

    db.close(); 

} 
public Cursor query(String table, String[] columns, String selectionArgs, String groupBy, String having, String orderBy){ 
    return myDB.query("Messages", null, null, null, null, null, null); 
} 

Вот myFirebaseMessagingService:

public class myFirebaseMessagingService extends FirebaseMessagingService { 

private static final String TAG="MyFirebaseMsgService"; 
@Override 
public void onMessageReceived(RemoteMessage remoteMessage) { 
    super.onMessageReceived(remoteMessage); 
    Log.d("onMessageReceived", "Pozvana funkcija onMessageReceived"); 
    Log.d(TAG, "From " + remoteMessage.getFrom()); 
    Log.d(TAG, "Body " + remoteMessage.getNotification().getBody()); 
    Log.d(TAG, "Location " + remoteMessage.getNotification().getClickAction()); 
    Log.d(TAG, "Value " + remoteMessage.getData().get("click_action")); 
    sendNotification(remoteMessage); 
    Log.d("Function called", "sendNotification"); 
    String message=remoteMessage.getNotification().getBody(); 
    Log.d("baza", message); 
    DataBaseHelper db=new DataBaseHelper(this); 
    db.insertMsg(message); 


} 

private void sendNotification(RemoteMessage remoteMessage) { 
    Intent intent = new Intent(myFirebaseMessagingService.this, MainActivity.class); 
    intent.putExtra("click_action", "goToFragment1"); 
    intent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP); 
    PendingIntent pendingIntent = PendingIntent.getActivity(this, 0, intent, PendingIntent.FLAG_ONE_SHOT); 
    Uri defaultSoundUri = RingtoneManager.getDefaultUri(RingtoneManager.TYPE_NOTIFICATION); 
    NotificationCompat.Builder notification=new NotificationCompat.Builder(this) 
      .setSmallIcon(logo) 
      .setContentText(remoteMessage.getNotification().getBody()) 
      .setContentTitle("Title") 
      .setAutoCancel(true) 
      .setSound(defaultSoundUri) 
      .setContentIntent(pendingIntent); 
    NotificationManager notificationManager = (NotificationManager)getSystemService(Context.NOTIFICATION_SERVICE); 
    notificationManager.notify(0, notification.build()); 
} 

А вот мой MainActivity:

public class MainActivity extends AppCompatActivity { 
Button dugme, dugme2, dugme3; 
Cursor c; 
DataBaseHelper db; 

@Override 
protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    Log.d("onCreate", "ONCREATE"); 
    final Intent intent=getIntent(); 
    setContentView(R.layout.activity_main); 
    String msg = getIntent().getStringExtra("click_action"); 
    FragmentManager fragmentManager = getSupportFragmentManager(); 
    FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction(); 


    if (msg != null) { 
     if (msg.equals("goToFragment1")) { 
      Fragment1 fragment1 = new Fragment1(); 
      fragmentTransaction.replace(R.id.myFragment, fragment1); 
      Log.d("FragmentTransaction", "Fragment je promenjen u onCreate!"); 
      fragmentTransaction.commit(); 
      Log.d("Create", "Kraj onCreatea"); 
     } 
    } 

    dugme = (Button) findViewById(R.id.dugme1); 
    dugme2 = (Button) findViewById(R.id.subscribe); 
    dugme.setOnClickListener(new View.OnClickListener() { 
     @Override 
     public void onClick(View view) { 
      Fragment fragment = null; 
      if (view == dugme) { 
       fragment = new Fragment1(); 
      } 
      FragmentTransaction transaction = getSupportFragmentManager().beginTransaction(); 
      transaction.replace(R.id.myFragment, fragment); 
      transaction.addToBackStack(null); 
      transaction.setTransition(FragmentTransaction.TRANSIT_NONE); 
      transaction.commit(); 
     } 
    }); 


    dugme2.setOnClickListener(new View.OnClickListener() { 
     @Override 
     public void onClick(View view) { 
      FirebaseMessaging.getInstance().subscribeToTopic("android"); 
      Log.d("Log", "Uspesno ste se pretplatili"); 
     } 
    }); 
    try{ 
    c=db.query("Messages", null, null, null, null, null); 
    if(c.moveToFirst()){ 

     do{ 
      Toast.makeText(MainActivity.this, "id: " + c.getString(0) + "IME: " + c.getString(1), Toast.LENGTH_LONG).show(); 
     } 
     while(c.moveToNext());{ 

     } 


    }}catch (Exception e){ 
     throw new Error("Greska!"); 
    } 

И это мой LogCat:

06-15 14:36:26.354 29325-29325/com.example.dev3.notifikacijefragmenti E/AndroidRuntime: FATAL EXCEPTION: main 
                        java.lang.Error: Greska! 
                         at com.example.dev3.notifikacijefragmenti.MainActivity.onCreate(MainActivity.java:82) 
                         at android.app.Activity.performCreate(Activity.java) 
                         at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java) 
                         at android.app.ActivityThread.performLaunchActivity(ActivityThread.java) 
                         at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java) 
                         at android.app.ActivityThread.access$700(ActivityThread.java) 
                         at android.app.ActivityThread$H.handleMessage(ActivityThread.java) 
                         at android.os.Handler.dispatchMessage(Handler.java) 
                         at android.os.Looper.loop(Looper.java) 
                         at android.app.ActivityThread.main(ActivityThread.java) 
                         at java.lang.reflect.Method.invokeNative(Native Method) 
                         at java.lang.reflect.Method.invoke(Method.java:525) 
                         at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java) 
                         at com.android.internal.os.ZygoteInit.main(ZygoteInit.java) 
                         at dalvik.system.NativeStart.main(Native Method) 

Может ли кто-нибудь мне помочь?

Answer 1

Проблема в том, что вы никогда не создавали экземпляр своего класса DataBaseHelper в MainActivity.

Когда вы пытаетесь вызвать:

c=db.query("Messages", null, null, null, null, null); 

db еще null.

Добавьте следующую строку в onCreate():

db = new DataBaseHelper();