java在RecyclerView布局管理器上获取空指针异常

Question

嗨，伙计们，我有以下错误，我似乎无法解决

我有

java.lang.NullPointerException: Attempt to invoke virtual method 'void 安卓.support.v7.widget.RecyclerView.setLayoutManager(安卓.support.v7.widget.RecyclerView$LayoutManager)' on a null object reference

我的代码与常规示例略有不同，因为我是在片段中而不是在活动中执行此操作的

这是我的密码
我有一个完整片段的xml：

<FrameLayout xmlns:安卓="http://schemas.安卓.com/apk/res/安卓"
    xmlns:tools="http://schemas.安卓.com/tools"
    安卓:layout_width="match_parent"
    安卓:layout_height="match_parent"
    安卓:paddingBottom="@dimen/activity_vertical_margin"
    安卓:paddingLeft="@dimen/activity_horizontal_margin"
    安卓:paddingRight="@dimen/activity_horizontal_margin"
    安卓:paddingTop="@dimen/activity_vertical_margin"
    tools:context="mtr.MainActivityFragmentTwo">

        <TextView
        安卓:layout_width="wrap_content"
        安卓:layout_height="wrap_content"
        安卓:text="@string/grades"
        安卓:textStyle="bold"
        安卓:textSize="40dp"
        安卓:gravity="center_horizontal"
        安卓:layout_gravity="center"
        />

</FrameLayout>

然后我有一个content_片段，它保存着RecyclerView

<?xml version="1.0" encoding="utf-8"?>
<FrameLayout xmlns:安卓="http://schemas.安卓.com/apk/res/安卓"
    xmlns:app="http://schemas.安卓.com/apk/res-auto"
    安卓:layout_width="match_parent"
    安卓:layout_height="match_parent"
    app:layout_behavior="@string/appbar_scrolling_view_behavior">

        <安卓.support.v7.widget.RecyclerView
            安卓:id="@+id/realm_recycler_view"
            安卓:layout_width="match_parent"
            安卓:layout_height="match_parent" />
</FrameLayout>

然后，片段中的代码如下所示

public class MainActivityFragmentTwo extends Fragment {

    private Realm realm;
    private RecyclerView recyclerView;

    public MainActivityFragmentTwo() {

    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        return inflater.inflate(R.layout.fragment_main_activity_fragment_two, container, false);
    }

    @Override
    public void onCreate(Bundle savedInstance) {
        super.onCreate(savedInstance);
        realm.init(getActivity());
        realm = Realm.getDefaultInstance();
        recyclerView = (RecyclerView) getActivity().findViewById(R.id.realm_recycler_view);
        setupRecyclerView();

    }

    public void setupRecyclerView() {
            Log.d("Found grades", "not showing grades tho");
            recyclerView.setLayoutManager(new LinearLayoutManager(this.getContext()));
            recyclerView.setAdapter(new gradeRealmAdapter(this, realm.where(Grade.class).findAllAsync()));
            recyclerView.setHasFixedSize(true);
    }
}

我创建的适配器是

public class gradeRealmAdapter extends RealmRecyclerViewAdapter<Grade, gradeRealmAdapter.GradeViewHolder> {

    private final MainActivityFragmentTwo activity;

    public gradeRealmAdapter (MainActivityFragmentTwo activity, RealmResults<Grade> realmResults) {
        super(activity.getActivity(), realmResults, true);
        this.activity = activity;
    }

    @Override
    public GradeViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
        return new GradeViewHolder(inflater.inflate(R.layout.gradeitem, parent, false));
    }

    @Override
    public void onBindViewHolder(GradeViewHolder holder, int position) {
        Grade grade = getData().get(position);
        holder.title.setText(grade.toString());
    }

    class GradeViewHolder extends RecyclerView.ViewHolder implements View.OnLongClickListener {

        public TextView title;
        public Grade grade;

        public GradeViewHolder(View view) {
            super(view);
            title = (TextView) view.findViewById(R.id.grade_text_view);
            view.setOnLongClickListener(this);
        }

        public boolean onLongClick(View v) {
            activity.deleteItem(grade);
            return true;
        }

    }

}

错误来自下一行的Fragment类

recyclerView.setLayoutManager(new LinearLayoutManager(this.getContext()));

我似乎无法解决这个问题
有人能看出我做错了什么吗

Answer 1

I have an content_fragment which holds the recyclerview

然后，您需要findViewById在布局上，而不是活动的

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    View rootView =  inflater.inflate(R.layout.fragment_main_activity_fragment_two, container, false);

    // Don't use getActivity().findViewById
    recyclerView = (RecyclerView) rootView.findViewById(R.id.realm_recycler_view);

    setupRecyclerView();
    return rootView;
}

然后，您还可以向后进行realm初始化

realm.init(getActivity()); // This throws an exception as well
realm = Realm.getDefaultInstance(); // It is initialized afterwards

因此，在onAttach中也要解决这个问题，在那里你保证会有一些Context

@Override
public void onAttach(Context context) {
    super.onAttach(context);
    realm = Realm.getDefaultInstance(); 
    realm.init(context); 
}

Answer 2

在onViewCreated()上放大回收器视图

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                     Bundle savedInstanceState) {
    View rootView =inflater.inflate(R.layout.fragment_main_activity_fragment_two,container, false);
    recyclerView = (RecyclerView) rootView.findViewById(R.id.realm_recycler_view);

setupRecyclerView();
return rootView;
}

不在onCreate

Answer 3

让我们试着改变

recyclerView.setLayoutManager(new LinearLayoutManager(this.getContext()));

与

recyclerView.setLayoutManager(new LinearLayoutManager(getActivity(), LinearLayoutManager.VERTICAL, false));

并对onCreateView进行一些更新：

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                 Bundle savedInstanceState) {
    View rootView =inflater.inflate(R.layout.fragment_main_activity_fragment_two,container, false);
    recyclerView = (RecyclerView) rootView.findViewById(R.id.realm_recycler_view);
    setupRecyclerView();
    return rootView;
}