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i am developing an application with an actionbarnavigator between two fragments, that means i have two different views for the user, who can swipe between them. the one fragment has a button, which opens another activity to show information from a database in a listview. but when i click on the button, i became a nullpointerexception. i dont know why. i hope that s.o. can help me.

here is the mainactivity

public class MainActivity extends ActionBarActivity implements ActionBar.TabListener {

SectionsPagerAdapter mSectionsPagerAdapter;
Debtor debtor;
Creditor creditor;
public DataBaseHandler myDBHandler;
ViewPager mViewPager;


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

    final ActionBar actionBar = getActionBar();
    actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);

    mSectionsPagerAdapter = new SectionsPagerAdapter(getSupportFragmentManager());
    mViewPager = (ViewPager) findViewById(R.id.pager);
    mViewPager.setAdapter(mSectionsPagerAdapter);

    mViewPager.setOnPageChangeListener(new ViewPager.SimpleOnPageChangeListener() {
                @Override
                public void onPageSelected(int position) {
                    actionBar.setSelectedNavigationItem(position);
                }
            });

    for (int i = 0; i < mSectionsPagerAdapter.getCount(); i++) {
        actionBar.addTab(actionBar.newTab()
                .setText(mSectionsPagerAdapter.getPageTitle(i))
                .setTabListener(this));
    }
}

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

    initFragments();
    openDB();   
}

@Override
protected void onDestroy() {
    super.onDestroy();

    closeDB();
}

private void initFragments() {
    debtor    = (Debtor)mSectionsPagerAdapter.getItem(0);
    creditor  = (Creditor)mSectionsPagerAdapter.getItem(1);
}

private void openDB() {
    myDBHandler = new DataBaseHandler(this);
    myDBHandler.open();
}

private void closeDB() {
    myDBHandler.close();
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    getMenuInflater().inflate(R.menu.main, menu);
    return true;
}


@Override
public void onTabSelected(ActionBar.Tab tab, FragmentTransaction fragmentTransaction) {
    mViewPager.setCurrentItem(tab.getPosition());
}

@Override
public void onTabUnselected(ActionBar.Tab tab, FragmentTransaction fragmentTransaction) {
}

@Override
public void onTabReselected(ActionBar.Tab tab, FragmentTransaction fragmentTransaction) {
}
 }

the fragment class

public class Debtor extends Fragment {

private Button showDebtorsHistory;
MainActivity mainActivity;

public static final String ARG_SECTION_NUMBER = "section_number";

public Debtor(int position) {       
}

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {

    mainActivity = (MainActivity) getActivity();

    View v = inflater.inflate(R.layout.debtor, container, false);

    showDebtorsHistory = (Button)v.findViewById(R.id.button1);

    showDebtorsHistory.setOnClickListener(new OnClickListener() {

        @Override
        public void onClick(View v) {

            Intent i = new Intent(getActivity(), DebtorHistory.class);              
            startActivityForResult(i, 100);
        }
    });

    return v;
}

@Override
public void onAttach(Activity activity) {
    super.onAttach(activity);
    try {
        mainActivity = (MainActivity) activity;
    } 
    catch (ClassCastException e) {
        throw new ClassCastException(activity.toString() +  " Fehler: Keine MainActivity-Instanz!");
    }
}
}

and the activity, which has to be opend, when the button is clicked

public class DebtorHistory extends Activity {

Cursor cursor;
private ListView mainList;
MainActivity mainActivity;

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

    mainList = (ListView)findViewById(R.id.listView_Debtors);
    populateListViewFromDB();

}

@SuppressWarnings("deprecation")
public void populateListViewFromDB() {                                                                          
    cursor = mainActivity.myDBHandler.getAllDifferentNames1();

    String[] colmn = new String[] {"n", "i", "d"};

    int[] tolist = new int[] {R.id.txt_name,                        
                                     R.id.list_userImage,
                                     R.id.datum };

    SimpleCursorAdapter myCursorAdapter 
            = new SimpleCursorAdapter
                        (this, R.layout.history_listrow, cursor, colmn, tolist);

    mainList.setAdapter(myCursorAdapter);
}

}

the logcat message

01-28 15:19:31.967: E/AndroidRuntime(1205): java.lang.RuntimeException: Unable to start activity ComponentInfo{***.debtmanagerpro/***.debtmanagerpro.DebtorHistory}: java.lang.NullPointerException
01-28 15:19:31.967: E/AndroidRuntime(1205):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2073)
01-28 15:19:31.967: E/AndroidRuntime(1205):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2098)
01-28 15:19:31.967: E/AndroidRuntime(1205):     at android.app.ActivityThread.access$600(ActivityThread.java:138)
01-28 15:19:31.967: E/AndroidRuntime(1205):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1204)
01-28 15:19:31.967: E/AndroidRuntime(1205):     at android.os.Handler.dispatchMessage(Handler.java:99)
01-28 15:19:31.967: E/AndroidRuntime(1205):     at android.os.Looper.loop(Looper.java:137)
01-28 15:19:31.967: E/AndroidRuntime(1205):     at android.app.ActivityThread.main(ActivityThread.java:4872)
01-28 15:19:31.967: E/AndroidRuntime(1205):     at java.lang.reflect.Method.invokeNative(Native Method)
01-28 15:19:31.967: E/AndroidRuntime(1205):     at java.lang.reflect.Method.invoke(Method.java:511)
01-28 15:19:31.967: E/AndroidRuntime(1205):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:790)
01-28 15:19:31.967: E/AndroidRuntime(1205):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:557)
01-28 15:19:31.967: E/AndroidRuntime(1205):     at dalvik.system.NativeStart.main(Native Method)
01-28 15:19:31.967: E/AndroidRuntime(1205): Caused by: java.lang.NullPointerException
01-28 15:19:31.967: E/AndroidRuntime(1205):     at ***.debtmanagerpro.DebtorHistory.populateListViewFromDB(DebtorHistory.java:30)
01-28 15:19:31.967: E/AndroidRuntime(1205):     at ***.debtmanagerpro.DebtorHistory.onCreate(DebtorHistory.java:23)
01-28 15:19:31.967: E/AndroidRuntime(1205):     at android.app.Activity.performCreate(Activity.java:5191)
01-28 15:19:31.967: E/AndroidRuntime(1205):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1082)
01-28 15:19:31.967: E/AndroidRuntime(1205):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2037)
01-28 15:19:31.967: E/AndroidRuntime(1205):     ... 11 more

line 23 in DebtorHistory is populateListViewFromDB(); and 30 is cursor = mainActivity.myDBHandler.getAllDifferentNames1();

thanks in advance.

EDIT:

public class DebtorHistory extends Activity {

Cursor cursor;
private ListView mainList;
DataBaseHandler myDBHandler;

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

    mainList = (ListView)findViewById(R.id.listView_Debtors);
    populateListViewFromDB();

}

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

    myDBHandler = new DataBaseHandler(this);
    myDBHandler.open();
}

@SuppressWarnings("deprecation")
public void populateListViewFromDB() {                                                                      //MainList mit Werten aus der db befüllen

    try {
        startManagingCursor(cursor = myDBHandler.getAllDifferentNames1());
    } catch (Exception e) {
        if(cursor == null) {
            Log.e("#################", "is null :(");
        }

    }
    ;

    String[] colm= new String[] {"n", "i", "d"};

    int[] tolist= new int[] {R.id.txt_name, R.id.list_userImage, R.id.datum };

    SimpleCursorAdapter myCursorAdapter 
            = new SimpleCursorAdapter
                        (this, R.layout.history_listrow, cursor, colm, tolist);

    mainList.setAdapter(myCursorAdapter);
}

}
KenMasters
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2 Answers2

1

I think the problem is in the method populateListViewFromDb(). Maybe you're getting a nullpointer exception when trying to get the cursor. Debug that method and see what you get.

rodrigopdl.89
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  • You should have a db helper class. Check out the one in this answer as an example: http://stackoverflow.com/questions/3548533/what-is-a-full-android-database-helper-class-for-an-existing-sqlite-database – rodrigopdl.89 Jan 28 '14 at 15:58
  • You then create an instance of that dbhelper class to use in your activities. – rodrigopdl.89 Jan 28 '14 at 15:59
  • hi rodrigopdl, i have an helper class which works fine. the problem is with the access over the second activity. i changed in my mainactivity and in the second activity the access to my dbhelper class, it opens the db in the onResume method and closes the db in the onPause() method. it opens when the mainactivity is in foreground or the other activity starts. i createt an instance in my activites but the problem still exits. see my edit – KenMasters Jan 28 '14 at 16:29
  • yes, its the same message. i tested the courser in the mainactivity and it works. but unusually it did not work in the second activity. hmmm...!??? – KenMasters Jan 28 '14 at 17:10
  • 1
    Maybe onCreate is being called before onResume? If that is the case, the handler won't be initialized when trying to access the db. – rodrigopdl.89 Jan 28 '14 at 17:25
  • omg, my populateListview method in the oncreate method is executed before the onresume methode, where i initialize my db handler. oh man, that simple error took my time :( thank you rodigropdl. now it works fine ;) – KenMasters Jan 28 '14 at 17:43
0

You cannot simply Access activity from an another activity as you do it.

mainActivity is never initialized in DebtorActivity, so the NullPointerException is reasonable when you call a function of mainActivity in populateListViewFromDb:

cursor = mainActivity.myDBHandler.getAllDifferentNames1();

You will have to find another way to pass the database to the second Activity.

I'd recommend building a helper class for database Access and use that in your Activities.

FD_
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