RuntimeException在一種情況下。 NullExeption - getBackStackCount（）

Question

只有當我的應用程序關閉（釋放其他應用程序的資源），當我再次打開它時，我得到了這個exaption。如果我自己關閉了應用程序，所有的工作。

11-10 22:00:10.007 6255-6255/com.john.testOne E/AndroidRuntime: FATAL EXCEPTION: main 
11-10 22:00:10.007 6255-6255/com.john.testOne E/AndroidRuntime: java.lang.RuntimeException: Unable to start activity ComponentInfo{com.john.testOne/com.john.testOne.StartPageActivity}: java.lang.NullPointerException 
11-10 22:00:10.007 6255-6255/com.john.testOne E/AndroidRuntime:  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1651) 
11-10 22:00:10.007 6255-6255/com.john.testOne E/AndroidRuntime:  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1667) 
11-10 22:00:10.007 6255-6255/com.john.testOne E/AndroidRuntime:  at android.app.ActivityThread.access$1500(ActivityThread.java:117) 
11-10 22:00:10.007 6255-6255/com.john.testOne E/AndroidRuntime:  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:935) 
11-10 22:00:10.007 6255-6255/com.john.testOne E/AndroidRuntime:  at android.os.Handler.dispatchMessage(Handler.java:99) 
11-10 22:00:10.007 6255-6255/com.john.testOne E/AndroidRuntime:  at android.os.Looper.loop(Looper.java:130) 
11-10 22:00:10.007 6255-6255/com.john.testOne E/AndroidRuntime:  at android.app.ActivityThread.main(ActivityThread.java:3687) 
11-10 22:00:10.007 6255-6255/com.john.testOne E/AndroidRuntime:  at java.lang.reflect.Method.invokeNative(Native Method) 
11-10 22:00:10.007 6255-6255/com.john.testOne E/AndroidRuntime:  at java.lang.reflect.Method.invoke(Method.java:507) 
11-10 22:00:10.007 6255-6255/com.john.testOne E/AndroidRuntime:  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:867) 
11-10 22:00:10.007 6255-6255/com.john.testOne E/AndroidRuntime:  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:625) 
11-10 22:00:10.007 6255-6255/com.john.testOne E/AndroidRuntime:  at dalvik.system.NativeStart.main(Native Method) 
11-10 22:00:10.007 6255-6255/com.john.testOne E/AndroidRuntime: Caused by: java.lang.NullPointerException 
11-10 22:00:10.007 6255-6255/com.john.testOne E/AndroidRuntime:  at com.john.testOne.StartPageActivity.resetToolbarNavigation(StartPageActivity.java:177) 
11-10 22:00:10.007 6255-6255/com.john.testOne E/AndroidRuntime:  at com.john.testOne.BaseFragment.onAttach(BaseFragment.java:25) 
11-10 22:00:10.007 6255-6255/com.john.testOne E/AndroidRuntime:  at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1030) 
11-10 22:00:10.007 6255-6255/com.john.testOne E/AndroidRuntime:  at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1259) 
11-10 22:00:10.007 6255-6255/com.john.testOne E/AndroidRuntime:  at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1241) 
11-10 22:00:10.007 6255-6255/com.john.testOne E/AndroidRuntime:  at android.support.v4.app.FragmentManagerImpl.dispatchCreate(FragmentManager.java:2048) 
11-10 22:00:10.007 6255-6255/com.john.testOne E/AndroidRuntime:  at android.support.v4.app.FragmentController.dispatchCreate(FragmentController.java:154) 
11-10 22:00:10.007 6255-6255/com.john.testOne E/AndroidRuntime:  at android.support.v4.app.FragmentActivity.onCreate(FragmentActivity.java:289) 
11-10 22:00:10.007 6255-6255/com.john.testOne E/AndroidRuntime:  at android.support.v7.app.AppCompatActivity.onCreate(AppCompatActivity.java:61) 
11-10 22:00:10.007 6255-6255/com.john.testOne E/AndroidRuntime:  at com.john.testOne.StartPageActivity.onCreate(StartPageActivity.java:34) 
11-10 22:00:10.007 6255-6255/com.john.testOne E/AndroidRuntime:  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047) 
11-10 22:00:10.007 6255-6255/com.john.testOne E/AndroidRuntime:  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1615) 
11-10 22:00:10.007 6255-6255/com.john.testOne E/AndroidRuntime:  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1667)  
11-10 22:00:10.007 6255-6255/com.john.testOne E/AndroidRuntime:  at android.app.ActivityThread.access$1500(ActivityThread.java:117)  
11-10 22:00:10.007 6255-6255/com.john.testOne E/AndroidRuntime:  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:935)  
11-10 22:00:10.007 6255-6255/com.john.testOne E/AndroidRuntime:  at android.os.Handler.dispatchMessage(Handler.java:99)  
11-10 22:00:10.007 6255-6255/com.john.testOne E/AndroidRuntime:  at android.os.Looper.loop(Looper.java:130)  
11-10 22:00:10.007 6255-6255/com.john.testOne E/AndroidRuntime:  at android.app.ActivityThread.main(ActivityThread.java:3687)  
11-10 22:00:10.007 6255-6255/com.john.testOne E/AndroidRuntime:  at java.lang.reflect.Method.invokeNative(Native Method)  
11-10 22:00:10.007 6255-6255/com.john.testOne E/AndroidRuntime:  at java.lang.reflect.Method.invoke(Method.java:507)  
11-10 22:00:10.007 6255-6255/com.john.testOne E/AndroidRuntime:  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:867)  
11-10 22:00:10.007 6255-6255/com.john.testOne E/AndroidRuntime:  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:625)  
11-10 22:00:10.007 6255-6255/com.john.testOne E/AndroidRuntime:  at dalvik.system.NativeStart.main(Native Method) 

這是我的活動：

public class StartPageActivity extends AppCompatActivity implements BaseFragment.BaseFragmentActions { 

    protected void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     setContentView(R.layout.start_page_activity);; 
     FragmentManager fm = getSupportFragmentManager(); 
     Fragment mFragment = new MainFragment(); 
      fm.beginTransaction().addToBackStack(null) 
        .add(R.id.fragment_container,mFragment) 
        .commit(); 
     initNavigation(); 
    } 
} 

MainFragment延伸BaseFragment ....

和BaseFragment：

public abstract class BaseFragment extends Fragment { 
    private static String TAG = "BaseFragment"; 
    private BaseFragmentActions baseFragmentActions; 
    @Override 
    public void onAttach(Context context) { 
     super.onAttach(context); 
     Log.i(TAG, "onAttach = "); 
     try { 
      baseFragmentActions = (BaseFragmentActions)(StartPageActivity)getActivity(); 
     } catch (ClassCastException e) { 
      throw new ClassCastException(((StartPageActivity)getActivity()).toString() + " must implement interface BaseFragmentActions"); 
     } 
     baseFragmentActions.resetToolbarNavigation(getBackStackCount()!=0); 
    } 

    @Override 
    @CallSuper 
    public void onDetach() { 
     super.onDetach(); 
     baseFragmentActions.resetToolbarNavigation((getBackStackCount() -1)!= 0); 

    } 

    private int getBackStackCount() { 
     int b = getActivity().getSupportFragmentManager().getBackStackEntryCount(); 

     return b; 
    } 

    public interface BaseFragmentActions { 
     public void resetToolbarNavigation(boolean backNavigationEnabled); 
    } 
} 

如果需要

baseFragmentActions.resetToolbarNavigation(getBackStackCount()!=0);  

      ^^ 
      || 
      || 

at com.john.testOne.BaseFragment.onAttach(BaseFragment.java:25) 

我添加resetToolbarNavigation ........

 @Override 
    public void resetToolbarNavigation(boolean backNavigationEnabled) { 
     mActionBar.setDisplayHomeAsUpEnabled(backNavigationEnabled);//------------------177 
//  mActionBar.setHomeAsUpIndicator(R.drawable.ic_add); 
     if (backNavigationEnabled) { 
      mToolbar.setNavigationOnClickListener(new View.OnClickListener() { 
       @Override 
       public void onClick(View view) { 
        // Log.i("resetToolbarNavigation", "setNavigationOnClickListener"); 
        onBackPressed(); 
       } 
      }); 
     } 
     else { 
      initNavigation(); 
      syncState(); 
     } 
    } 

Answer 1

嘗試使用此：

@Override 
public void resetToolbarNavigation(boolean backNavigationEnabled) { 
    setSupportActionBar(mToolbar); 

    getSupportActionBar().setDisplayHomeAsUpEnabled(backNavigationEnabled); 
    if (backNavigationEnabled) { 
     mToolbar.setNavigationOnClickListener(new View.OnClickListener() { 
      @Override 
      public void onClick(View view) { 
       // Log.i("resetToolbarNavigation", "setNavigationOnClickListener"); 
       onBackPressed(); 
      } 
     }); 
    } 
    else { 
     initNavigation(); 
     syncState(); 
    } 
}