In my App, I have two Fragments: FragmentA and FragmentB which are loaded from my MainActivity by the help of an SlidingMenu.
Thanks to Crashlytics I get notified when and where my app crashes. In this case, from time to time the DatabaseHandler in FragmentA is null (for some users), even though it has been initzialized.
This is my Code:
MainActivity
public class MainActivity extends AppCompatActivity implements MyFragment.OnListFragmentInteractionListener, AsyncResponse {
private FragmentA fragmentA = new FragmentA();
private DatabaseHandler databaseHandler = new DatabaseHandler(this);
private NavigationView navigationView;
private DrawerLayout drawer;
private Toolbar toolbar;
// index to identify current nav menu item
private static int navItemIndex = 0;
public static String CURRENT_TAG = MyConstants.TAG_FRAGMENT_A;
// toolbar titles respected to selected nav menu item
private String[] activityTitles;
// flag to load home fragment when user presses back key
private Handler mHandler;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
fragmentA.setDatabaseHandler(this.databaseHandler);
// Init UI
toolbar = (Toolbar) findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
mHandler = new Handler();
drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
navigationView = (NavigationView) findViewById(R.id.nav_view);
fabSendButton = (FloatingActionButton) findViewById(R.id.fab);
// Navigation view header
navHeader = navigationView.getHeaderView(0);
// load toolbar titles from string resources
activityTitles = getResources().getStringArray(R.array.sliding_menu_item_activity_titles);
// initializing navigation menu
setUpNavigationView();
if (savedInstanceState == null) {
navItemIndex = 0;
CURRENT_TAG = MyConstants.TAG_FRAGMENT_A;
loadHomeFragment();
}
}
/***
* Returns respected fragment that user
* selected from navigation menu
*/
private void loadHomeFragment() {
// set toolbar title
setToolbarTitle();
// if user select the current navigation menu again, don't do anything
// just close the navigation drawer
if (getSupportFragmentManager().findFragmentByTag(CURRENT_TAG) != null) {
drawer.closeDrawers();
return;
}
// Sometimes, when fragment has huge data, screen seems hanging
// when switching between navigation menus
// So using runnable, the fragment is loaded with cross fade effect
// This effect can be seen in GMail app
Runnable mPendingRunnable = new Runnable() {
@Override
public void run() {
// update the activity_main_header_with_item content by replacing fragments
Fragment fragment = getFragment();
FragmentTransaction fragmentTransaction = getSupportFragmentManager().beginTransaction();
fragmentTransaction.setCustomAnimations(android.R.anim.fade_in, android.R.anim.fade_out);
fragmentTransaction.replace(R.id.frame, fragment, CURRENT_TAG);
fragmentTransaction.commit();
}
};
// If mPendingRunnable is not null, then add to the message queue
if (mPendingRunnable != null) {
mHandler.post(mPendingRunnable);
}
//Closing drawer on item click
drawer.closeDrawers();
// refresh toolbar menu
invalidateOptionsMenu();
}
private Fragment getFragment() {
switch (navItemIndex) {
case 0:
return this.fragmentA;
case 1:
Fragment B fragmentB = new FragmentB();
fragmentB.setDatabaseHandler(this.databaseHandler);
return fragmentB;
default:
return this.fragmentA;
}
}
private void setToolbarTitle() {
getSupportActionBar().setTitle(activityTitles[navItemIndex]);
}
private void setUpNavigationView() {
//Setting Navigation View Item Selected Listener to handle the item click of the navigation menu
navigationView.setNavigationItemSelectedListener(new NavigationView.OnNavigationItemSelectedListener() {
// This method will trigger on item Click of navigation menu
@Override
public boolean onNavigationItemSelected(MenuItem menuItem) {
//Check to see which item was being clicked and perform appropriate action
switch (menuItem.getItemId()) {
//Replacing the activity_main_header_with_item content with ContentFragment Which is our Inbox View;
case R.id.nav_A:
navItemIndex = 0;
CURRENT_TAG = MyConstants.TAG_FRAGMENT_A;
break;
case R.id.nav_B:
navItemIndex = 1;
CURRENT_TAG = MyConstants.TAG_FRAGMENT_B;
break;
default:
navItemIndex = 0;
}
loadHomeFragment();
return true;
}
});
ActionBarDrawerToggle actionBarDrawerToggle = new ActionBarDrawerToggle(this, drawer, toolbar, R.string.openDrawer, R.string.closeDrawer) {
@Override
public void onDrawerClosed(View drawerView) {
// Code here will be triggered once the drawer closes as we dont want anything to happen so we leave this blank
super.onDrawerClosed(drawerView);
}
@Override
public void onDrawerOpened(View drawerView) {
// Code here will be triggered once the drawer open as we dont want anything to happen so we leave this blank
super.onDrawerOpened(drawerView);
}
};
//Setting the actionbarToggle to drawer layout
drawer.setDrawerListener(actionBarDrawerToggle);
//calling sync state is necessary or else your hamburger icon wont show up
actionBarDrawerToggle.syncState();
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
getMenuInflater().inflate(R.menu.activity_main_header_with_item, menu);
return true;
}
}
FragmentA:
public class FragmentA extends MyFragment implements AsyncResponse {
private View view;
private RecyclerView recyclerView;
public FragmentA() {
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
List<Song> songsList = databaseHandler.getAllSongs();
view = inflater.inflate(R.layout.home_list, container, false);
// Set the adapter
Context context = view.getContext();
recyclerView = (RecyclerView) view.findViewById(R.id.listinclude);
recyclerView.setLayoutManager(new LinearLayoutManager(context));
recyclerView.addItemDecoration(new DividerItemDecoration(getActivity(), R.drawable.divider));
setVisibilities(songsList);
this.recyclerViewAdapter = new RecyclerViewAdapter(songsList, mListener, false, true);
recyclerView.setAdapter(this.recyclerViewAdapter);
return view;
}
@Override
public void onActivityCreated(@Nullable Bundle savedInstanceState) {
super.onActivityCreated(savedInstanceState);
}
private void setVisibilities(List<Song> songsList) {
ViewFlipper viewFlipper = (ViewFlipper) view.findViewById(R.id.viewFlipper);
if (songsList.isEmpty() && viewFlipper.getDisplayedChild() == 0) {
viewFlipper.setDisplayedChild(1);
} else if (!songsList.isEmpty() && viewFlipper.getDisplayedChild() == 1) {
viewFlipper.setDisplayedChild(0);
}
}
@Override
public void processFinish(String output) {
// does something
}
}
MyFragment
public class MyFragment extends Fragment {
protected DatabaseHandler databaseHandler;
protected static final String ARG_COLUMN_COUNT = "column-count";
protected int mColumnCount = 1;
protected MyFragment.OnListFragmentInteractionListener mListener;
protected RecyclerViewAdapter recyclerViewAdapter;
public void setDatabaseHandler(DatabaseHandler databaseHandler) {
this.databaseHandler = databaseHandler;
}
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
Timber.i( "onCreate");
if (getArguments() != null) {
mColumnCount = getArguments().getInt(ARG_COLUMN_COUNT);
}
}
@Override
public void onPause() {
Timber.i( "onPause");
super.onPause();
}
@Override
public void onAttach(Context context) {
Timber.i( "onAttach");
super.onAttach(context);
if (context instanceof OnListFragmentInteractionListener) {
mListener = (FragmentA.OnListFragmentInteractionListener) context;
} else {
throw new RuntimeException(context.toString() + " must implement OnListFragmentInteractionListener");
}
}
@Override
public void onDetach() {
Timber.i( "onDetach");
super.onDetach();
mListener = null;
}
public interface OnListFragmentInteractionListener {
// TODO: Update argument type and name
void onListFragmentInteraction(Song item);
}
}
DatabaseHandler:
public class DatabaseHandler extends SQLiteOpenHelper {
// All Static variables
// Database Version
private static final int DATABASE_VERSION = 1;
// Database Name
private static final String DATABASE_NAME = "appName";
// Songs table name
private static final String TABLE_SONGS = "songs";
@Override
public void onCreate(SQLiteDatabase db) {
// create Tables
}
// Upgrading database
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
switch(oldVersion) {
case 1:
//upgrade logic from version 1 to 2
case 2:
//upgrade logic from version 2 to 3
case 3:
//upgrade logic from version 3 to 4
break;
default:
throw new IllegalStateException(
"onUpgrade() with unknown oldVersion " + oldVersion);
}
}
// Getting All Songs
public List<Song> getAllSongs() {
// Select All Query
String selectQuery = "SELECT * FROM " + TABLE_SONGS + " ORDER BY ID DESC";
SQLiteDatabase db = this.getWritableDatabase();
Cursor cursor = db.rawQuery(selectQuery, null);
List<Song> songList = new ArrayList<>();
songList.addAll(getSongsFromCursor(cursor));
cursor.close();
// return title list
return songList;
}
}
Problem:
Fatal Exception: java.lang.RuntimeException
Unable to start activity ComponentInfo{com.myapp.myappname/com.myapp.myappname.ui.activity.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'java.util.List com.myapp.myappname.database.DatabaseHandler.getAllSongs()' on a null object reference
Details:
Fatal Exception: java.lang.RuntimeException: Unable to start activity ComponentInfo{com.myapp.myappname/com.myapp.myappname.ui.activity.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'java.util.List com.myapp.myappname.database.DatabaseHandler.getAllSongs()' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2984)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3045)
at android.app.ActivityThread.-wrap14(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1642)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6776)
at java.lang.reflect.Method.invoke(Method.java)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1496)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1386)
Caused by java.lang.NullPointerException: Attempt to invoke virtual method 'java.util.List com.myapp.myappname.database.DatabaseHandler.getAllSongs()' on a null object reference
at com.myapp.myappname.ui.fragment.FragmentA.onCreateView(FragmentA.java:51)
at android.support.v4.app.Fragment.performCreateView(Fragment.java:2192)
at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1299)
at android.support.v4.app.FragmentManagerImpl.moveFragmentToExpectedState(FragmentManager.java:1528)
at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1595)
at android.support.v4.app.FragmentManagerImpl.dispatchActivityCreated(FragmentManager.java:2900)
at android.support.v4.app.FragmentController.dispatchActivityCreated(FragmentController.java:201)
at android.support.v4.app.FragmentActivity.onStart(FragmentActivity.java:603)
at android.support.v7.app.AppCompatActivity.onStart(AppCompatActivity.java:178)
at com.myapp.myappname.ui.activity.MainActivity.onStart(MainActivity.java:590)
at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1256)
at android.app.Activity.performStart(Activity.java:6972)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2937)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3045)
at android.app.ActivityThread.-wrap14(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1642)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6776)
at java.lang.reflect.Method.invoke(Method.java)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1496)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1386)
