當我回到應用程序時，爲什麼getActivity返回null？

我開發了一個簡單的android應用程序。當我回到應用程序時，爲什麼getActivity返回null？

活動：

public class MainActivity extends AppCompatActivity { 

ArrayList<Fragment> fgs = new ArrayList<>(); 

@Override 
protected void onCreate(Bundle savedInstanceState) { 

    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_main); 

    fgs.add(new MyFragment1()); 
    fgs.add(new MyFragment2()); 

    ViewPager viewPager = (ViewPager) findViewById(R.id.container); 
    viewPager.setAdapter(new FragmentPagerAdapter(getSupportFragmentManager()) { 
     @Override 
     public Fragment getItem(int position) { 
      switch (position) { 
       case 0: 
        return fgs.get(position); 
       case 1: 
        return fgs.get(position); 
       default: 
        return null; 
      } 
     } 

     @Override 
     public int getCount() { 
      return fgs.size(); 
     } 
    }); 
    viewPager.addOnPageChangeListener(new ViewPager.OnPageChangeListener() { 
     @Override 
     public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) { 
     } 
     @Override 
     public void onPageSelected(int position) { 
      switch (position) { 
       case 0: 
        ((MyFragment1) fgs.get(0)).display(); 
        return; 
      } 
     } 
     @Override 
     public void onPageScrollStateChanged(int state) {} 
    }); 
} 

}

片段1

public class MyFragment1 extends Fragment { 

String mString; 

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

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

    mString = "MyFragment1"; 
    ((TextView)v.findViewById(R.id.text)).setText(mString); 
    return v; 
} 

public void display() 
{ 
    Toast.makeText(getActivity(), mString, Toast.LENGTH_SHORT).show(); 

} 
}

Fragment2

public class MyFragment2 extends Fragment { 

String mString; 

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

    View v = inflater.inflate(R.layout.my_fragment, container, false); 
    mString = "MyFragment2"; 

    ((TextView)v.findViewById(R.id.text)).setText(mString); 
    return v; 
} 

}

我測試了一下它的內存是由操作系統發佈的情況下，這個程序behavir（我做了「不要保持活動「檢查。）

在此sitiaion下，當我關閉此應用程序並回到應用程序並滾動到Fragment1時，發生了NullPointerException。
我以爲，當我回到應用程序，一切工作正常。因爲當我回到應用程序，應用程序重新開始。
這是爲什麼發生？我如何解決這個問題？

錯誤日誌：

Exception in MessageQueue callback: handleReceiveCallback java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.res.Resources android.content.Context.getResources()' on a null object reference 
                       at android.widget.Toast.<init>(Toast.java:102) 
                       at android.widget.Toast.makeText(Toast.java:260) 
                       at knkj28.com.experiment.MyFragment1.display(MyFragment1.java:33) 
                       at knkj28.com.experiment.MainActivity$2.onPageSelected(MainActivity.java:52) 
                       at android.support.v4.view.ViewPager.dispatchOnPageSelected(ViewPager.java:1967) 
                       at android.support.v4.view.ViewPager.scrollToItem(ViewPager.java:685) 
                       at android.support.v4.view.ViewPager.setCurrentItemInternal(ViewPager.java:669) 
                       at android.support.v4.view.ViewPager.onTouchEvent(ViewPager.java:2284) 
                       at android.view.View.dispatchTouchEvent(View.java:9991) 
                       at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2663) 
                       at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2344) 
                       at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2669) 
                       at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2358) 
                       at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2669) 
                       at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2358) 
                       at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2669) 
                       at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2358) 
                       at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2669) 
                       at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2358) 
                       at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2669) 
                       at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2358) 
                       at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2669) 
                       at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2358) 
                       at com.android.internal.policy.DecorView.superDispatchTouchEvent(DecorView.java:413) 
                       at com.android.internal.policy.PhoneWindow.superDispatchTouchEvent(PhoneWindow.java:1810) 
                       at android.app.Activity.dispatchTouchEvent(Activity.java:3102) 
                       at android.support.v7.view.WindowCallbackWrapper.dispatchTouchEvent(WindowCallbackWrapper.java:71) 
                       at com.android.internal.policy.DecorView.dispatchTouchEvent(DecorView.java:375) 
                       at android.view.View.dispatchPointerEvent(View.java:10211) 
                       at android.view.ViewRootImpl$ViewPostImeInputStage.processPointerEvent(ViewRootImpl.java:4444) 
                       at android.view.ViewRootImpl$ViewPostImeInputStage.onProcess(ViewRootImpl.java:4312) 
                       at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:3859) 
                       at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:3912) 
                       at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:3878) 
                       at android.view.ViewRootImpl$AsyncInputStage.forward(ViewRootImpl.java:4005) 
                       at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:3886) 
                       at android.view.ViewRootImpl$AsyncInputStage.apply(ViewRootImpl.java:4062) 
                       at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:3859) 
                       at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:3912) 
                       at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:3878) 
                       at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:3886) 
                       at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:3859) 
                       at android.view.ViewRootImpl.deliverInputEvent(ViewRootImpl.java:6220) 
                       at android.view.ViewRootImpl.doProcessInputEvents(ViewRootImpl.java:6194) 
                       at android.view.ViewRootImpl.enqueueInputEvent(ViewRootImpl.java:6155) 
                       at android.view.ViewRootImpl$WindowInputEventReceiver.onInputEvent(ViewRootImpl.java:6323) 
                       at android.view.InputEventReceiver.dispatchInputEvent(InputEventReceiver.java:185) 
                       at android.os.MessageQueue.nativePollOnce(Native Method) 
                       at android.os.MessageQueue.next(MessageQueue.java:331) 
                       at android.os.Looper.loop(Looper.java:219) 
                       at android.app.ActivityThread.main(ActivityThread.java:6217) 
                       at java.lang.reflect.Method.invoke(Native Method) 
                       at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865) 
                      at com.android.internal.os. 
Shutting down VM 


                     --------- beginning of crash 
FATAL EXCEPTION: main 
                     Process: knkj28.com.experiment, PID: 6025 
                     java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.res.Resources android.content.Context.getResources()' on a null object reference 
                      at android.widget.Toast.<init>(Toast.java:102) 
                      at android.widget.Toast.makeText(Toast.java:260) 
                      at knkj28.com.experiment.MyFragment1.display(MyFragment1.java:33) 
                      at knkj28.com.experiment.MainActivity$2.onPageSelected(MainActivity.java:52) 
                      at android.support.v4.view.ViewPager.dispatchOnPageSelected(ViewPager.java:1967) 
                      at android.support.v4.view.ViewPager.scrollToItem(ViewPager.java:685) 
                      at android.support.v4.view.ViewPager.setCurrentItemInternal(ViewPager.java:669) 
                      at android.support.v4.view.ViewPager.onTouchEvent(ViewPager.java:2284) 
                      at android.view.View.dispatchTouchEvent(View.java:9991) 
                      at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2663) 
                      at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2344) 
                      at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2669) 
                      at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2358) 
                      at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2669) 
                      at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2358) 
                      at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2669) 
                      at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2358) 
                      at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2669) 
                      at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2358) 
                      at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2669) 
                      at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2358) 
                      at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2669) 
                      at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2358) 
                      at com.android.internal.policy.DecorView.superDispatchTouchEvent(DecorView.java:413) 
                      at com.android.internal.policy.PhoneWindow.superDispatchTouchEvent(PhoneWindow.java:1810) 
                      at android.app.Activity.dispatchTouchEvent(Activity.java:3102) 
                      at android.support.v7.view.WindowCallbackWrapper.dispatchTouchEvent(WindowCallbackWrapper.java:71) 
                      at com.android.internal.policy.DecorView.dispatchTouchEvent(DecorView.java:375) 
                      at android.view.View.dispatchPointerEvent(View.java:10211) 
                      at android.view.ViewRootImpl$ViewPostImeInputStage.processPointerEvent(ViewRootImpl.java:4444) 
                      at android.view.ViewRootImpl$ViewPostImeInputStage.onProcess(ViewRootImpl.java:4312) 
                      at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:3859) 
                      at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:3912) 
                      at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:3878) 
                      at android.view.ViewRootImpl$AsyncInputStage.forward(ViewRootImpl.java:4005) 
                      at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:3886) 
                      at android.view.ViewRootImpl$AsyncInputStage.apply(ViewRootImpl.java:4062) 
                      at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:3859) 
                      at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:3912) 
                      at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:3878) 
                      at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:3886) 
                      at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:3859) 
                      at android.view.ViewRootImpl.deliverInputEvent(ViewRootImpl.java:6220) 
                      at android.view.ViewRootImpl.doProcessInputEvents(ViewRootImpl.java:6194) 
                      at android.view.ViewRootImpl.enqueueInputEvent(ViewRootImpl.java:6155) 
                      at android.view.ViewRootImpl$WindowInputEventReceiver.onInputEvent(ViewRootImpl.java:6323) 
                      at android.view.InputEventReceiver.dispatchInputEvent(InputEventReceiver.java:185) 
                      at android.os.MessageQueue.nativePollOnce(Native Method) 
                      at android.os.MessageQueue.next(MessageQueue.java:331) 
                      at android.os.Looper.loop(Looper.java:219) 
                      at android.app.ActivityThread.main(ActivityThread.java:6217) 
                      at java.lang.reflect.Method.invoke(Native Method) 
    08-16 13:32:16.701 6025-6025/knkj28.com.experiment E/AndroidRuntime:  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865) 
                      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755)

來源

2017-08-16 user1692121

你知道如何閱讀代碼，對不對？當您的代碼調用Toast.makeText（）初始化Toast類時，堆棧跟蹤的前三行告訴您Context.getResources（）應用於空引用。看看源代碼：

public Toast(Context context) { 
    mContext = context; 
    mTN = new TN(); 
    mTN.mY = context.getResources().getDimensionPixelSize(
      com.android.internal.R.dimen.toast_y_offset);

所以它意味着你傳遞一個空的上下文給它。在你的身邊，你撥打：

Toast.makeText(getActivity()

這意味着getActivity（）爲空，這是不是一個巨大的驚喜給我給你問的系統執行殺死所有的活動。所以，你應該讓你的代碼準備好讓主機活動爲空。

另外，如果你在閱讀的文檔：

/** 
    * Return the {@link FragmentActivity} this fragment is currently associated with. 
    * May return {@code null} if the fragment is associated with a {@link Context} 
    * instead. 
    */

這意味着你沒有約束力的片段到主機的活動。請務必閱讀如何properly attach the fragment to the activity，也here。

來源

2017-08-16 05:14:12 Alessio

我明白了。但爲什麼我的代碼無法將片段與活動相關聯？它第一次正常工作。即使操作系統強制執行該應用程序，它也應該正常工作，因爲活動和片段都會再次創建，並且生命週期像第一次一樣再次啓動。我誤解了一些問題嗎？ – user1692121

這取決於您如何將片段綁定到活動。看看你的代碼發生了什麼（可能顯示給我們）。另外，調試getActivity（）並查看返回的null。使代碼健壯併爲其做好準備，因爲即使這次你做錯了什麼，一般而言，在片段內部，你不能保證你的宿主活動不會在任何時候被系統所殺死，而且仍然某些片段的代碼（如回調）可能會延遲執行。 – Alessio

public void display() { 
    if (mString.isEmpty()) { 
     mString = getActivity().getClass().getSimpleName(); 
    } 
    Toast.makeText(getActivity(), mString, Toast.LENGTH_SHORT).show(); 
}

來源

2017-08-16 06:03:57

當我回到應用程序時，爲什麼getActivity返回null？

回答

相關問題