Question

I have an Android application with a ViewPager that allows swiping between tabs. When I swipe between tabs I call functions inside the newly active Fragment to update text on that Fragment, and it sometimes throws a NullPointerException but I cannot replicate it on my device. Was wondering if someone could take a look at the code & stack trace below and possibly assist – it would be greatly appreciated. I am not sure if I am missing something or not doing this correctly?

My first thought this morning was that maybe I should be running this on a separate thread as to not disturb the main UI thread or maybe try/catch these functions since they seem to only sometimes crash the app? Thanks in advance.

ViewPagerCode

public void onTabChanged(String tag) {
        //TabInfo newTab = this.mapTabInfo.get(tag);
        int pos = this.mTabHost.getCurrentTab();
        mTabHost.getCurrentTabView().invalidate();
        this.mViewPager.setCurrentItem(pos);



    // calculatePoints on TabChange
    if (pos == 1) {
        TabFragment2 fragTrack = (TabFragment2) fragments.get(1);
        fragTrack.calculatePoints();
        fragTrack.createAndFillDatabase();
    }

    else if (pos == 2) {
        TabFragment3 fragFoodList = (TabFragment3) fragments.get(2);
        fragFoodList.createAndFillDatabase("none");
    }

    this.supportInvalidateOptionsMenu();
}

calculatePoints()

public void calculatePoints() {
        int prefDailyPoints, prefPointsUsed, prefPointsRemain, prefBonusPoints, prefBonusUsed, prefBonusPointsRemain;

        // Get points values and fill in respective fields
        appSettings = PreferenceManager.getDefaultSharedPreferences(currentActivity);
        nonDBData = currentActivity.getSharedPreferences(NONDB_DATA, 0);

        prefDailyPoints = Integer.parseInt(appSettings.getString("pointsAllowed", "0"));
        prefPointsUsed = nonDBData.getInt("pointsUsed", 0);
        //prefPointsRemain = nonDBData.getInt("pointsRemain", prefDailyPoints - prefPointsUsed);
        prefPointsRemain = prefDailyPoints - prefPointsUsed;

        textfieldPointsDaily.setText(Integer.toString(prefDailyPoints));
        textfieldPointsUsed.setText(Integer.toString(prefPointsUsed));
        textfieldPointsRemain.setText(Integer.toString(prefPointsRemain));

        if (prefPointsRemain < 0) {
                textfieldPointsRemain.setTextColor(Color.rgb(255, 0, 0));
                textfieldPointsRemain.setTypeface(null, Typeface.BOLD);
        }
        else {
                textfieldPointsRemain.setTextColor(Color.rgb(0, 0, 0));
                textfieldPointsRemain.setTypeface(null, Typeface.NORMAL);
        }
}

Stack Trace

java.lang.NullPointerException 
at android.preference.PreferenceManager.getDefaultSharedPreferencesName(PreferenceManager.java:371)
at android.preference.PreferenceManager.getDefaultSharedPreferences(PreferenceManager.java:366)
at com.mattdonders.android.wppcalculator.TabFragment2.calculatePoints(TabFragment2.java:667)
at com.mattdonders.android.wppcalculator.TabsViewPagerFragmentActivity.onTabChanged(TabsViewPagerFragmentActivity.java:483)
at android.widget.TabHost.invokeOnTabChangeListener(TabHost.java:446)
at android.widget.TabHost.setCurrentTab(TabHost.java:431) 
at com.mattdonders.android.wppcalculator.TabsViewPagerFragmentActivity.onPageSelected(TabsViewPagerFragmentActivity.java:511)
at android.support.v4.view.ViewPager.scrollToItem(ViewPager.java:539)
at android.support.v4.view.ViewPager.setCurrentItemInternal(ViewPager.java:524)
at android.support.v4.view.ViewPager.onTouchEvent(ViewPager.java:1941)
at android.view.View.dispatchTouchEvent(View.java:7239) 
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2168)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:1903) 
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2174)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:1917) 
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2174)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:1917) 
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2174)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:1917) 
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2174)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:1917) 
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2174)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:1917) 
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2174)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:1917) 
at com.android.internal.policy.impl.PhoneWindow$DecorView.superDispatchTouchEvent(PhoneWindow.java:1953)
at com.android.internal.policy.impl.PhoneWindow.superDispatchTouchEvent(PhoneWindow.java:1405)
at android.app.Activity.dispatchTouchEvent(Activity.java:2410) 
at com.android.internal.policy.impl.PhoneWindow$DecorView.dispatchTouchEvent(PhoneWindow.java:1901)
at android.view.View.dispatchPointerEvent(View.java:7419) 
at android.view.ViewRootImpl.deliverPointerEvent(ViewRootImpl.java:3220)
at android.view.ViewRootImpl.deliverInputEvent(ViewRootImpl.java:3165)
at android.view.ViewRootImpl.doProcessInputEvents(ViewRootImpl.java:4292)
at android.view.ViewRootImpl.enqueueInputEvent(ViewRootImpl.java:4271)
at android.view.ViewRootImpl$WindowInputEventReceiver.onInputEvent(ViewRootImpl.java:4363)
at android.view.InputEventReceiver.dispatchInputEvent(InputEventReceiver.java:179)
at android.os.MessageQueue.nativePollOnce(Native Method) 
at android.os.MessageQueue.next(MessageQueue.java:125) 
at android.os.Looper.loop(Looper.java:124)
at android.app.ActivityThread.main(ActivityThread.java:5039) 
at java.lang.reflect.Method.invokeNative(Native Method) 
at java.lang.reflect.Method.invoke(Method.java:511) 
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560) 
at dalvik.system.NativeStart.main(Native Method)

Answer 1

I changed my code to detect userVisibleHint which was fixed mid-2012 by the Android team. This worked 100% and fixed my issues – thanks everyone!

@Override
public void setUserVisibleHint(boolean isVisibleToUser) {
    super.setUserVisibleHint(isVisibleToUser);

    if (isVisibleToUser == true) { 
        // TO-DO
    }
    else if (isVisibleToUser == false) {
        // TO-DO
    }

}

---

Android Support Library (rev 11) finally fixed the user visible hint
issue, now if you use support library for fragments, then you can
safely use getUserVisibleHint() or override setUserVisibleHint() to
capture the changes as described by gorn’s answer.