java在LinearLayoutManager中获取NullPointerException

Question

我得到了这个错误：

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

以下是回收碎片：

public class ThreeFragment extends Fragment{
    private TextView tvEmptyView;
    private RecyclerView mRecyclerView;
    private DataAdapter mAdapter;
    private LinearLayoutManager mLayoutManager;
    private List<Student> studentList;

    protected Handler handler;
    public ThreeFragment() {

    }

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {

        View rootview = inflater.inflate(R.layout.fragment_one, container, false);
        tvEmptyView = (TextView) rootview.findViewById(R.id.empty_view);
        mRecyclerView = (RecyclerView) rootview.findViewById(R.id.my_recycler_view);
        studentList = new ArrayList<Student>();
        handler = new Handler();

        loadData();
        // use this setting to improve performance if you know that changes
        // in content do not change the layout size of the RecyclerView
        mRecyclerView.setHasFixedSize(true);

        mLayoutManager = new LinearLayoutManager(getActivity());
        mLayoutManager.setOrientation(LinearLayoutManager.VERTICAL);
        // use a linear layout manager
        mRecyclerView.setLayoutManager(mLayoutManager);

        // create an Object for Adapter
        mAdapter = new DataAdapter(studentList, mRecyclerView);

        // set the adapter object to the Recyclerview
        mRecyclerView.setAdapter(mAdapter);
        //   mAdapter.notifyDataSetChanged();


        if (studentList.isEmpty()) {
            mRecyclerView.setVisibility(View.GONE);
            tvEmptyView.setVisibility(View.VISIBLE);

        } else {
            mRecyclerView.setVisibility(View.VISIBLE);
            tvEmptyView.setVisibility(View.GONE);
        }

        mAdapter.setOnLoadMoreListener(new OnLoadMoreListener() {
            @Override
            public void onLoadMore() {
                //add null , so the adapter will check view_type and show progress bar at bottom
                studentList.add(null);
                mAdapter.notifyItemInserted(studentList.size() - 1);

                handler.postDelayed(new Runnable() {
                    @Override
                    public void run() {
                        //   remove progress item
                        studentList.remove(studentList.size() - 1);
                        mAdapter.notifyItemRemoved(studentList.size());
                        //add items one by one
                        int start = studentList.size();
                        int end = start + 20;

                        for (int i = start + 1; i <= end; i++) {
                            studentList.add(new Student("Student " + i, "AndroidStudent" + i + "@gmail.com"));
                            mAdapter.notifyItemInserted(studentList.size());
                        }
                        mAdapter.setLoaded();
                        //or you can add all at once but do not forget to call mAdapter.notifyDataSetChanged();
                    }
                }, 2000);

            }
        });
        return rootview;
    }
    // load initial data
    private void loadData() {

        for (int i = 1; i <= 20; i++) {
            studentList.add(new Student("Student " + i, "安卓student" + i + "@gmail.com"));

        }


    }

}

以下是适配器：

public class DataAdapter extends RecyclerView.Adapter {
    private final int VIEW_ITEM = 1;
    private final int VIEW_PROG = 0;

    private List<Student> studentList;

    // The minimum amount of items to have below your current scroll position
    // before loading more.
    private int visibleThreshold = 5;
    private int lastVisibleItem, totalItemCount;
    private boolean loading;
    private OnLoadMoreListener onLoadMoreListener;


    public DataAdapter(List<Student> students, RecyclerView recyclerView) {
        studentList = students;

        if (recyclerView.getLayoutManager() instanceof LinearLayoutManager) {

            final LinearLayoutManager mLayoutManager = (LinearLayoutManager) recyclerView
                    .getLayoutManager();


            recyclerView
                    .addOnScrollListener(new RecyclerView.OnScrollListener() {
                        @Override
                        public void onScrolled(RecyclerView recyclerView,
                                               int dx, int dy) {
                            super.onScrolled(recyclerView, dx, dy);

                            totalItemCount = mLayoutManager.getItemCount();
                            lastVisibleItem = mLayoutManager
                                    .findLastVisibleItemPosition();
                            if (!loading
                                    && totalItemCount <= (lastVisibleItem + visibleThreshold)) {
                                // End has been reached
                                // Do something
                                if (onLoadMoreListener != null) {
                                    onLoadMoreListener.onLoadMore();
                                }
                                loading = true;
                            }
                        }
                    });
        }
    }

    @Override
    public int getItemViewType(int position) {
        return studentList.get(position) != null ? VIEW_ITEM : VIEW_PROG;
    }

    @Override
    public RecyclerView.ViewHolder onCreateViewHolder(ViewGroup parent,
                                                      int viewType) {
        RecyclerView.ViewHolder vh;
        if (viewType == VIEW_ITEM) {
            View v = LayoutInflater.from(parent.getContext()).inflate(
                    R.layout.list_row, parent, false);

            vh = new StudentViewHolder(v);
        } else {
            View v = LayoutInflater.from(parent.getContext()).inflate(
                    R.layout.progress_item, parent, false);

            vh = new ProgressViewHolder(v);
        }
        return vh;
    }

    @Override
    public void onBindViewHolder(RecyclerView.ViewHolder holder, int position) {
        if (holder instanceof StudentViewHolder) {

            Student singleStudent = (Student) studentList.get(position);

            ((StudentViewHolder) holder).tvName.setText(singleStudent.getName());

            ((StudentViewHolder) holder).tvEmailId.setText(singleStudent.getEmailId());

            ((StudentViewHolder) holder).student = singleStudent;

        } else {
            ((ProgressViewHolder) holder).progressBar.setIndeterminate(true);
        }
    }

    public void setLoaded() {
        loading = false;
    }

    @Override
    public int getItemCount() {
        return studentList.size();
    }

    public void setOnLoadMoreListener(OnLoadMoreListener onLoadMoreListener) {
        this.onLoadMoreListener = onLoadMoreListener;
    }


    //
    public static class StudentViewHolder extends RecyclerView.ViewHolder {
        public TextView tvName;

        public TextView tvEmailId;

        public Student student;

        public StudentViewHolder(View v) {
            super(v);
            tvName = (TextView) v.findViewById(R.id.tvName);

            tvEmailId = (TextView) v.findViewById(R.id.tvEmailId);

            v.setOnClickListener(new OnClickListener() {

                @Override
                public void onClick(View v) {
                    Toast.makeText(v.getContext(),
                            "OnClick :" + student.getName() + " \n " + student.getEmailId(),
                            Toast.LENGTH_SHORT).show();

                }
            });
        }
    }

    public static class ProgressViewHolder extends RecyclerView.ViewHolder {
        public ProgressBar progressBar;

        public ProgressViewHolder(View v) {
            super(v);
            progressBar = (ProgressBar) v.findViewById(R.id.progressBar1);
        }
    }
}