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I have a fragment with an inner class, something like below:

class MyFragment extends Fragment {
  
  private DataToUiCallbacks uiCallbacks;

  @Override
  public void onActivityCreated(@Nullable Bundle savedInstanceState) {
    super.onActivityCreated(savedInstanceState);
    uiCallbacks.onAttachData(new InnerClass);
  }

  void initialize(DataToUiCallbacks callbacks) {
    uiCallbacks = callbacks;
  }

  private final class InnerClass {
     // no member variables
     void doSomething() {
     .....
     }
  }
}

This fragment is called elsewhere like :

MyFragment myFragment = new MyFragment();
myFragment.initialize();

I get the following NPE :

Caused by: java.lang.NullPointerException
        at com.google.android.com.google.android.myProject.MyFragment$InnerClass.<init>
        at com.google.android.myProject.MyFragment.onActivityCreated
        at android.app.Fragment.performActivityCreated(Fragment.java:2620)
        at android.app.FragmentManagerImpl.moveToState(FragmentManagerImpl.java:1296)
        at android.app.FragmentManagerImpl.moveFragmentToExpectedState(FragmentManagerImpl.java:1549)
        at android.app.FragmentManagerImpl.moveToState(FragmentManagerImpl.java:1611)
        at android.app.FragmentManagerImpl.dispatchMoveToState(FragmentManagerImpl.java:3039)
        at android.app.FragmentManagerImpl.dispatchActivityCreated(FragmentManagerImpl.java:2991)
        at android.app.FragmentController.dispatchActivityCreated(FragmentController.java:178)
        at android.app.Activity.performCreateCommon(Activity.java:6969)
        at android.app.Activity.performCreate(Activity.java:6977)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1213)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2770)

It's definitely happening because of the outer class being null/un-initialized while calling inner class constructor but I am calling it in onActivityCreated(), any idea what's going wrong.

For more context, the compiled code for the Inner class constructor would look like :

The compiled code for the inner constructor init : 
class MyFragment$InnerClass {
  final MyFragment this$0;

  private MyFragment$InnerClass(MyFragment);
    Code:
       0: aload_0
       1: aload_1
       2: putfield      #1        // Field this$0:LMyFragment;
       5: aload_0
       6: invokespecial #2        // Method java/lang/Object."<init>":()V
       9: return
}

I checked the what these mean, looks like for putfield it throws a NPE when objectRef(operand at the top of the stack) is NULL (https://docs.oracle.com/javase/specs/jvms/se7/html/jvms-6.html#jvms-6.5.putfield) so it actually means that while assigning the implicit member variable it threw the exception.

Thanks

Anubha Kushwaha
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  • Does this answer your question? [What is a NullPointerException, and how do I fix it?](https://stackoverflow.com/questions/218384/what-is-a-nullpointerexception-and-how-do-i-fix-it) – Stultuske Sep 25 '20 at 11:16
  • I certainly know what a NPE is, I am more interested in understanding why it's happening – Anubha Kushwaha Sep 25 '20 at 11:17
  • The stack trace mentions `myProject.MyFragment$InnerClass.` . `` means this happens in the constructor. – S.L. Barth Sep 25 '20 at 11:26
  • the code you posted isn't even compilable Java code. let's start with that – Stultuske Sep 25 '20 at 11:26
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    I have not posted the entire code to keep it easy for viewers to understand the code snippet and please be kind to the community :) – Anubha Kushwaha Sep 25 '20 at 11:49
  • @S.L.Barth-ReinstateMonica Right, it's the constructor, it would throw a NPE ideally while assigning the outer class this as implicitly the outerclass this is passed in inner class constructor. – Anubha Kushwaha Sep 25 '20 at 11:51
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    Well done! I don't have time to help further :-( But you've got my vote for showing what you tried. Good luck, hope you'll get an answer! – S.L. Barth Sep 25 '20 at 12:04

0 Answers0