Starting Activity from Fragment causes NullPointerException

Question

I'm stuck here with a problem starting an activity from a Button inside a Fragment. Here is my code:

ViewPagerAdapter.java

import java.util.List;  import android.support.v4.app.Fragment; import android.support.v4.app.FragmentManager; import android.support.v4.app.FragmentPagerAdapter;  public class ViewPagerAdapter extends FragmentPagerAdapter {      private List<Fragment> fragments;      public ViewPagerAdapter(FragmentManager fm, List<Fragment> fragments) {         super(fm);         this.fragments = fragments;     }      @Override     public int getCount() {         return this.fragments.size();     }      @Override     public Fragment getItem(int position) {         return this.fragments.get(position);     }  } 

ViewPagerActivity.java

import java.util.List; import java.util.Vector;  import android.os.Bundle; import android.support.v4.app.Fragment; import android.support.v4.app.FragmentActivity; import android.support.v4.view.ViewPager; import android.util.Log; import com.package.here.Class1Overview; import com.package.here.Class1Overview; import com.package.here.Class1Overview;  public class ViewPagerActivity extends FragmentActivity{      @Override     protected void onCreate(Bundle savedInstanceState) {         super.onCreate(savedInstanceState);         super.setContentView(R.layout.viewpager);          ViewPager pager = (ViewPager)super.findViewById(R.id.my_viewpager);         pager.setAdapter(initialisePaging());     }      private ViewPagerAdapter initialisePaging() {          List<Fragment> fragments = new Vector<Fragment>();         fragments.add(Fragment.instantiate(this, Class1Overview.class.getName()));         fragments.add(Fragment.instantiate(this, Class2Overview.class.getName()));         fragments.add(Fragment.instantiate(this, Class3Overview.class.getName()));         return new ViewPagerAdapter(super.getSupportFragmentManager(), fragments);               }  } 

Class1Overview.java (Here is the button)

import android.content.Intent; import android.os.Bundle; import android.support.v4.app.Fragment; import android.view.LayoutInflater; import android.view.View; import android.view.View.OnClickListener; import android.view.ViewGroup; import android.widget.Button; import android.widget.LinearLayout; import com.package.here.R;  public class FuelOverview extends Fragment {     @Override     public View onCreateView(LayoutInflater inflater, ViewGroup container,             Bundle savedInstanceState) {         if (container == null) {             return null;         }         View v = (LinearLayout)inflater.inflate(R.layout.class_one_layout, container, false);         View AddBtn = v.findViewById(R.id.btn_add);         ((Button) AddBtn).setOnClickListener(new OnClickListener() {              @Override             public void onClick(View v) {                 Intent intent = new Intent();                 intent.setClass(getActivity(), ActivityToStart.class);                 startActivity(intent);             }         });          return v;     } } 

And this is my LogCat:

01-05 19:39:00.913: E/AndroidRuntime(10557): Uncaught handler: thread main exiting due to uncaught exception 01-05 19:39:01.034: E/AndroidRuntime(10557): java.lang.RuntimeException: Unable to pause activity {com.package.here/com.package.here.ViewPagerActivity}: java.lang.NullPointerException 01-05 19:39:01.034: E/AndroidRuntime(10557):    at android.app.ActivityThread.performPauseActivity(ActivityThread.java:3162) 01-05 19:39:01.034: E/AndroidRuntime(10557):    at android.app.ActivityThread.performPauseActivity(ActivityThread.java:3119) 01-05 19:39:01.034: E/AndroidRuntime(10557):    at android.app.ActivityThread.handlePauseActivity(ActivityThread.java:3102) 01-05 19:39:01.034: E/AndroidRuntime(10557):    at android.app.ActivityThread.access$2400(ActivityThread.java:119) 01-05 19:39:01.034: E/AndroidRuntime(10557):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1870) 01-05 19:39:01.034: E/AndroidRuntime(10557):    at android.os.Handler.dispatchMessage(Handler.java:99) 01-05 19:39:01.034: E/AndroidRuntime(10557):    at android.os.Looper.loop(Looper.java:123) 01-05 19:39:01.034: E/AndroidRuntime(10557):    at android.app.ActivityThread.main(ActivityThread.java:4363) 01-05 19:39:01.034: E/AndroidRuntime(10557):    at java.lang.reflect.Method.invokeNative(Native Method) 01-05 19:39:01.034: E/AndroidRuntime(10557):    at java.lang.reflect.Method.invoke(Method.java:521) 01-05 19:39:01.034: E/AndroidRuntime(10557):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:860) 01-05 19:39:01.034: E/AndroidRuntime(10557):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:618) 01-05 19:39:01.034: E/AndroidRuntime(10557):    at dalvik.system.NativeStart.main(Native Method) 01-05 19:39:01.034: E/AndroidRuntime(10557): Caused by: java.lang.NullPointerException 01-05 19:39:01.034: E/AndroidRuntime(10557):    at android.support.v4.app.FragmentManagerImpl.saveFragmentBasicState(FragmentManager.java:1576) 01-05 19:39:01.034: E/AndroidRuntime(10557):    at android.support.v4.app.FragmentManagerImpl.saveAllState(FragmentManager.java:1617) 01-05 19:39:01.034: E/AndroidRuntime(10557):    at android.support.v4.app.FragmentActivity.onSaveInstanceState(FragmentActivity.java:481) 01-05 19:39:01.034: E/AndroidRuntime(10557):    at android.app.Activity.performSaveInstanceState(Activity.java:1022) 01-05 19:39:01.034: E/AndroidRuntime(10557):    at android.app.Instrumentation.callActivityOnSaveInstanceState(Instrumentation.java:1180) 01-05 19:39:01.034: E/AndroidRuntime(10557):    at android.app.ActivityThread.performPauseActivity(ActivityThread.java:3144) 01-05 19:39:01.034: E/AndroidRuntime(10557):    ... 12 more 01-05 19:39:01.423: I/dalvikvm(10557): threadid=7: reacting to signal 3 

ViewPager itself is working fine. It seems to be an error pausing the FragmentActivity, but i can't figure it out. The only thing I found was this post on google http://code.google.com/p/android/issues/detail?id=19917

I also got through the google example code to make sure I mainly use the same technique...

Edit: to make sure the ClickListener works, I tried a Toast in the onClick method - works perfectly.

Answer 1

Quick fix is

class MyFragment extends Fragment {      @Override     public void onSaveInstanceState(Bundle outState) {         super.onSaveInstanceState(outState);         setUserVisibleHint(true);     } 

And wait for update in safe mode :)

Answer 2

This is a known problem and I'm shocked to find that no one has filed a bug on the public bug tracker for this.

Your problem stems from this:

if (f.mSavedViewState != null) {     if (result == null) {         result = new Bundle();     }     result.putSparseParcelableArray(             FragmentManagerImpl.VIEW_STATE_TAG, f.mSavedViewState); } if (!f.mUserVisibleHint) {     // Only add this if it's not the default value     result.putBoolean(FragmentManagerImpl.USER_VISIBLE_HINT_TAG, f.mUserVisibleHint); } 

You should notice that if result is null coming into this code and the first if is not triggered you'll get a NullPointerException if we enter the second if.

It should read:

if (f.mSavedViewState != null) {     if (result == null) {         result = new Bundle();     }     result.putSparseParcelableArray(             FragmentManagerImpl.VIEW_STATE_TAG, f.mSavedViewState); } if (!f.mUserVisibleHint) {     if (result == null) {         result = new Bundle();     }     // Only add this if it's not the default value     result.putBoolean(FragmentManagerImpl.USER_VISIBLE_HINT_TAG, f.mUserVisibleHint); } 

I submitted a patch for this a week or two ago: https://android-review.googlesource.com/31261