How to work with sqlite in fragments?

Question

Android

Good afternoon, since I have little experience in Android development, I can not figure out the error.
In the main activity (in onCreate), depending on the presence of any data in the database: I get a Cursor and check the number of records in the table, if getCount() == 0, one fragment is loaded (FragmentEmpty), and if getCount()! =0, respectively, another (MainActivityFragment()).
Code from main activity:

@Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        mainActivityFragment = new MainActivityFragment();//at ru.kolpashikov.simplegpstracker.MainActivity.onCreate(MainActivity.java:28)
        fragmentEmpty = new FragmentEmpty();
    }

    @Override
    public void onResume(){
        super.onResume();

        FragmentTransaction fragmentTransaction = getFragmentManager().beginTransaction();

        try{
            try {
                dbHelper = new DBTracksHelper(this);
                dbase = dbHelper.getReadableDatabase(); 
            }catch(IOException e){}

            c = dbase.query(true, Constants.TABLE_NAME, new String[]{"dt"},
                    null, null, null, null, null, null);
            int nCount = c.getCount();
            c.close();

            try {
                dbHelper.close();
                dbase.close();
            }catch(Exception e){}

            if(nCount != 0){
                fragmentTransaction.add(R.id.frameLayout, mainActivityFragment);
            }else {
                fragmentTransaction.add(R.id.frameLayout, fragmentEmpty);
            }
        }catch(SQLiteException e){}

        fragmentTransaction.commit();
    }

MainActivityFragment constructor:

public MainActivityFragment() {
        try{
            dbHelper = new DBTracksHelper(getActivity());
            dbase = dbHelper.getReadableDatabase(); //at ru.kolpashikov.simplegpstracker.MainActivityFragment.<init>(MainActivityFragment.java:37)
        }catch(IOException e){}
        getLoaderManager().initLoader(0, null, this);
    }

And these are the logs:

09-05 11:01:10.653  11765-11765/ru.kolpashikov.simplegpstracker E/AndroidRuntime﹕ FATAL EXCEPTION: main
    java.lang.RuntimeException: Unable to start activity ComponentInfo{ru.kolpashikov.simplegpstracker/ru.kolpashikov.simplegpstracker.MainActivity}: java.lang.NullPointerException
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2062)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2087)
            at android.app.ActivityThread.access$600(ActivityThread.java:133)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1198)
            at android.os.Handler.dispatchMessage(Handler.java:99)
            at android.os.Looper.loop(Looper.java:137)
            at android.app.ActivityThread.main(ActivityThread.java:4793)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:511)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:808)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:575)
            at dalvik.system.NativeStart.main(Native Method)
     Caused by: java.lang.NullPointerException
            at android.database.sqlite.SQLiteOpenHelper.getDatabaseLocked(SQLiteOpenHelper.java:224)
            at android.database.sqlite.SQLiteOpenHelper.getReadableDatabase(SQLiteOpenHelper.java:188)
            at ru.kolpashikov.simplegpstracker.MainActivityFragment.<init>(MainActivityFragment.java:37)
            at ru.kolpashikov.simplegpstracker.MainActivity.onCreate(MainActivity.java:28)
            at android.app.Activity.performCreate(Activity.java:5008)
            at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1079)
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2026)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2087)
            at android.app.ActivityThread.access$600(ActivityThread.java:133)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1198)
            at android.os.Handler.dispatchMessage(Handler.java:99)
            at android.os.Looper.loop(Looper.java:137)
            at android.app.ActivityThread.main(ActivityThread.java:4793)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:511)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:808)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:575)
            at dalvik.system.NativeStart.main(Native Method)

What am I doing wrong?

Answer 1

firstly, the number of records in the table can be obtained using sql, pulling all the data for this is, to put it mildly, inefficient and bad
form, why do you immediately create two fragments in onCreate, I also don’t understand, except perhaps as in a joke ─ one glass of water by the bed if you want drink, the second one is empty if you don’t want to))
you at least display errors if something happened inside, but you don’t know about it and the
first thing that catches your eye in the log is NullPointerException
for good, you need to describe the provider and catch changes in the number of records in the database using CursorLoader , with its help to work in a fragment
Fragment does not need a constructor !
everything you need can be passed to him using the Bundle