从活动中调用片段会引发空指针异常-Android

Question

I am trying to call a List fragment from fragment activity, I m getting null pointer exception.Here is my code,

public class SampleEvents extends FragmentActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_sample_events);
        FragmentManager fm       = getFragmentManager();
        Fragment        fragment = fm.findFragmentById(R.id.fragment_content); 
        if (fragment == null) {
            FragmentTransaction ft = fm.beginTransaction();
            ft.add(R.id.fragment_content, new EventsList());
            ft.commit(); 
        }


    }

Layout of fragment activity

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical" >
  <FrameLayout
        android:id="@+id/fragment_content"
        android:layout_width="fill_parent"
        android:layout_height="fill_parent" />
</LinearLayout>

Layout of the fragment event_list

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="@drawable/background"
    android:orientation="vertical" >
     <TextView
        android:id="@+id/empty_TV"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerHorizontal="true"
        android:layout_centerVertical="true"
        android:textColor="#0c2c0c"
        android:text="No Announcements Found"
        android:textStyle="bold"
        android:textSize="20sp" />
     <ListView
         android:id="@+id/events_list"
         android:padding="10dip"
         android:layout_width="match_parent"
         android:layout_height="wrap_content"
         >
     </ListView>
</RelativeLayout>

Fragment class

public class EventsList extends ListFragment  implements
android.widget.AdapterView.OnItemClickListener{

    View rootView;
    static SharedPreferences sPrefs;
    private NetWorkConnection nc;
    private BaseAlerts alert;
    private ListView events_listview;
    private TextView emptyTV;
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {
        rootView = inflater.inflate(R.layout.events_list, container, false);
        initUI();
        uiListener();
        return rootView;
}

Whats wrong with my code?Can someone help?

The log cat

06-10 19:22:08.248: E/AndroidRuntime(1009): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.ie.minoritybussiness/com.ie.minoritybussiness.dashboard.SampleEvents}: java.lang.RuntimeException: Content has view with id attribute 'android.R.id.list' that is not a ListView class
06-10 19:22:08.248: E/AndroidRuntime(1009):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2059)
06-10 19:22:08.248: E/AndroidRuntime(1009):     at android.app.ActivityThread.access$600(ActivityThread.java:130)
06-10 19:22:08.248: E/AndroidRuntime(1009):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1195)
06-10 19:22:08.248: E/AndroidRuntime(1009):     at android.os.Handler.dispatchMessage(Handler.java:99)
06-10 19:22:08.248: E/AndroidRuntime(1009):     at android.os.Looper.loop(Looper.java:137)
06-10 19:22:08.248: E/AndroidRuntime(1009):     at android.app.ActivityThread.main(ActivityThread.java:4745)
06-10 19:22:08.248: E/AndroidRuntime(1009):     at java.lang.reflect.Method.invokeNative(Native Method)
06-10 19:22:08.248: E/AndroidRuntime(1009):     at java.lang.reflect.Method.invoke(Method.java:511)
06-10 19:22:08.248: E/AndroidRuntime(1009):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786)
06-10 19:22:08.248: E/AndroidRuntime(1009):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
06-10 19:22:08.248: E/AndroidRuntime(1009):     at dalvik.system.NativeStart.main(Native Method)
06-10 19:22:08.248: E/AndroidRuntime(1009): Caused by: java.lang.RuntimeException: Content has view with id attribute 'android.R.id.list' that is not a ListView class
06-10 19:22:08.248: E/AndroidRuntime(1009):     at android.app.ListFragment.ensureList(ListFragment.java:402)
06-10 19:22:08.248: E/AndroidRuntime(1009):     at android.app.ListFragment.onViewCreated(ListFragment.java:203)
06-10 19:22:08.248: E/AndroidRuntime(1009):     at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:843)
06-10 19:22:08.248: E/AndroidRuntime(1009):     at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:1035)

Answer 1

Content has view with id attribute 'android.R.id.list' that is not a ListView class

That looks like your issue. There are 2 reasons I can think of for this to be occuring.

1. You are giving the android id to a non-list view. Check out your XML files for anywhere you are using @android:id/list as an id for a view. Rename it to something else.

2. You are using a ListFragment / ListActivity and are not defining a view with @android:id/list as the id. These classes require that identifier. See this Stack Overflow post for some more details.

Answer 2

Put:

FragmentManager fragmentManager = getSupportFragmentManager();

instead of:

FragmentManager fm = getFragmentManager();

Edit: If you are not using support library than this is probably not the reason for exception. I will give you example how do i do it:

FragmentManager fragmentManager = getSupportFragmentManager();

Fragment fragment = (ListFragment) fragmentManager
            .findFragmentByTag("List");

if (fragment != null) {
    fragmentManager.beginTransaction()
        .add(R.id.fragment, fragment, "List").addToBackStack("List")
        .commit();
}

So, i am not using findFragmentById. I am using findFragmentTag and when i replace (or add) fragment, i put a tag ("List"). ListFragment is custom fragment i created.