NullPointerException getDefaultSharedPreferences/getDefaultSharedPreferencesName

Question

我有一個Android應用程序與ViewPager，允許在標籤之間滑動。當我在標籤頁之間滑動時，我會調用新激活的Fragment中的函數來更新該片段上的文本，並且它有時會拋出NullPointerException，但我無法將其複製到我的設備上。想知道是否有人可以看看下面的代碼&堆棧跟蹤和可能的協助 - 這將不勝感激。我不確定我是否錯過了某些東西，或者沒有正確地做到這一點？

我今天早上第一個想到的是，也許我應該在單獨的線程上運行此操作，以免打擾主UI線程，或者嘗試/捕獲這些函數，因爲它們似乎只是有時會使應用程序崩潰？提前致謝。

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); 
     } 
} 

堆棧跟蹤

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

我改變了我的代碼檢測由Android團隊在2012年中期修復的userVisibleHint。這工作100％，並解決了我的問題 - 謝謝大家！

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

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

} 

---

Android的支持庫（修訂版11）finally fixed the user visible hint issue，現在如果你使用支持庫的片段，那麼你可以 安全地使用getUserVisibleHint（）或覆蓋setUserVisibleHint（）來 捕捉變化由戈恩的回答描述。