NullPointerException in notifyDataSetChanged()

Question

我试图在过去的19个小时内更新listFragment，但是无法使用notifyDataSetChanged()。我访问了很多关于它的线，但没有使它工作。我有一个父片段，里面有三个ListFragment。ListFragment需要在它们之间传递数据，我可以通过interface实现这一点。信息正在顺利传递。但是ListView没有更新。这是我的尝试代码：

public class SubChaptersListFragment extends SherlockListFragment {

    ArrayAdapter<String> mAdapter = null;
    ArrayList<String> items = new ArrayList<String>();

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        int layout = Build.VERSION.SDK_INT >= Build.VERSION_CODES.HONEYCOMB ? android.R.layout.simple_list_item_activated_1
                : android.R.layout.simple_list_item_1;
        Chapter instance = CompetitiveProgramming.chapterList.get(0);
        for (int i = 0; i < instance.subchapterList.size(); i++) {
            items.add(instance.subchapterList.get(i).subChapterTitle);
        }
        mAdapter = new ArrayAdapter<String>(getSherlockActivity(), layout, items);
        setListAdapter(mAdapter);
    }

    @Override
    public void onStart() {
        super.onStart();
        if (getFragmentManager().findFragmentById(
                R.id.sub_sub_category_fragment) != null) {
            getListView().setChoiceMode(ListView.CHOICE_MODE_SINGLE);
        }
    }

    // this function is invoked from another `fragment` and position is retrieved successfully
    public void updateList(int position) {
        Chapter instance = CompetitiveProgramming.chapterList.get(position);
        items.clear();
        for (int i = 0; i < instance.subchapterList.size(); i++) {
            items.add(instance.subchapterList.get(i).subChapterTitle);
        }
        mAdapter.notifyDataSetChanged(); // nullpointerException in this line
    }

}

编辑日志

08-13 18:33:30.670: E/AndroidRuntime(537): java.lang.NullPointerException
08-13 18:33:30.670: E/AndroidRuntime(537):  at me.kaidul.uhunt.SubChaptersListFragment.updateList(SubChaptersListFragment.java:57)
08-13 18:33:30.670: E/AndroidRuntime(537):  at me.kaidul.uhunt.CompetitiveProgramming.onChapterSelected(CompetitiveProgramming.java:214)
08-13 18:33:30.670: E/AndroidRuntime(537):  at me.kaidul.uhunt.ChaptersListFragment.onListItemClick(ChaptersListFragment.java:60)
08-13 18:33:30.670: E/AndroidRuntime(537):  at android.support.v4.app.ListFragment$2.onItemClick(ListFragment.java:58)
08-13 18:33:30.670: E/AndroidRuntime(537):  at android.widget.AdapterView.performItemClick(AdapterView.java:284)
08-13 18:33:30.670: E/AndroidRuntime(537):  at android.widget.ListView.performItemClick(ListView.java:3382)
08-13 18:33:30.670: E/AndroidRuntime(537):  at android.widget.AbsListView$PerformClick.run(AbsListView.java:1696)
08-13 18:33:30.670: E/AndroidRuntime(537):  at android.os.Handler.handleCallback(Handler.java:587)

我是这样从parentFragment调用片段的：

@Override
public void onChapterSelected(int position) {
    SubChaptersListFragment subChapterFrag = new SubChaptersListFragment();
    if ( subChapterFrag != null) {
        subChapterFrag.updateList(position);
    } else {

    }

}

Answer 1

您的问题比一个简单的空指针要深得多。

问题的症状是，当调用mAdapter时，updateList()为null。更大的问题是，当SubChaptersListFragment调用updateList()时，还没有创建updateList()。

您可能在不考虑生命周期的情况下从一个片段(ChaptersListFragment)直接引用另一个片段(SubChaptersListFragment)。

在进行任何调用之前，您需要确保创建了片段并将其附加到活动中。我建议使用活动的FragmentManager来检查片段的存在，并在必要时创建它。