android.support.v4.app.FragmentManagerImpl catch NullPointerException?

问题:

I need catch NullPointerException by FragmentManagerImpl.
But try ... catch does not work.
How to catch explicit NullPointerException on getItem()?

.......         try {               pagerAdapter = new TFragmentPagerAdapter(getFragmentManager());         } catch (Exception e) {             Log.e(LOG_TAG, e.toString());         }  ..................   private class TFragmentPagerAdapter extends FragmentPagerAdapter {          public TFragmentPagerAdapter(FragmentManager fm) {             super(fm);         }          @Override         public Fragment getItem(int position) {              Fragment fragment = null;             switch (training) {                 case VAR: //                    fragment = getTFragment(position);                     fragment = null;                     break;                 default:                     break;             }              if (fragment == null) {                 throw new RuntimeException();             }              return fragment;         } } 

Stack trace I'm seeing

java.lang.NullPointerException: Attempt to write to field 'android.support.v4.app.FragmentManagerImpl android.support.v4.app.Fragment.mFragmentManager' on a null object reference at android.support.v4.app.BackStackRecord.doAddOp(BackStackRecord.java:417) at android.support.v4.app.BackStackRecord.add(BackStackRecord.java:412) at android.support.v4.app.FragmentPagerAdapter.instantiateItem(FragmentPagerAdapter.java:99) at android.support.v4.view.ViewPager.addNewItem(ViewPager.java:869) at android.support.v4.view.ViewPager.populate(ViewPager.java:1019) at android.support.v4.view.ViewPager.populate(ViewPager.java:951) at android.support.v4.view.ViewPager.onMeasure(ViewPager.java:1473) at android.view.View.measure(View.java:17547) at android.widget.RelativeLayout.measureChildHorizontal(RelativeLayout.java:727) at android.widget.RelativeLayout.onMeasure(RelativeLayout.java:463) at android.view.View.measure(View.java:17547) at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5535) at android.widget.FrameLayout.onMeasure(FrameLayout.java:436) at android.view.View.measure(View.java:17547) at android.support.v4.widget.DrawerLayout.onMeasure(DrawerLayout.java:868) at android.view.View.measure(View.java:17547) at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5535) at android.widget.FrameLayout.onMeasure(FrameLayout.java:436) at android.support.v7.internal.widget.ContentFrameLayout.onMeasure(ContentFrameLayout.java:124) at android.view.View.measure(View.java:17547) at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5535) at android.support.v7.internal.widget.ActionBarOverlayLayout.onMeasure(ActionBarOverlayLayout.java:444) at android.view.View.measure(View.java:17547) at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5535) at android.widget.FrameLayout.onMeasure(FrameLayout.java:436) at android.view.View.measure(View.java:17547) at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5535) at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1436) at android.widget.LinearLayout.measureVertical(LinearLayout.java:722) at android.widget.LinearLayout.onMeasure(LinearLayout.java:613) at android.view.View.measure(View.java:17547) at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5535) at android.widget.FrameLayout.onMeasure(FrameLayout.java:436) at com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2615) at android.view.View.measure(View.java:17547) at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:2015) at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1173) at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1379) at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1061) at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:5885) at android.view.Choreographer$CallbackRecord.run(Choreographer.java:767) at android.view.Choreographer.doCallbacks(Choreographer.java:580) at android.view.Choreographer.doFrame(Choreographer.java:550) at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:753) at android.os.Handler.handleCallback(Handler.java:739) at android.os.Handler.dispatchMessage(Handler.java:95) at android.os.Looper.loop(Looper.java:135) at android.app.ActivityThread.main(ActivityThread.java:5257) at java.lang.reflect.Method.invoke(Native Method) at java.lang.reflect.Method.invoke(Method.java:372) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)

回答1:

The code you posted basically works. Your problem lies elsewhere. Edit your post and show us the code around ActivityThread.java:5257.

For your reference this code shows what you were trying to do in public Fragment getItem(int position)... throw the exception you want and that it can be caught.

package org.jf;  public class SomeClass {      public static void main(String[] args) {         new SomeClass();     }      public SomeClass() {         try {              AClass x = new AClass();             x.getItem(1);         } catch (Exception e) {              System.out.println("Caught");         }     }      class AClass {         public Object getItem(int position) {             int training = 1;             Object fragment = null;             switch (training) {                 case 1: //                    fragment = getTFragment(position);                     fragment = null;                     break;                 default:                     break;             }              if (fragment == null) {                 throw new RuntimeException();             }              return fragment;         }     } } 

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