我recyclerview在我的应用程序中做了一个

问题是，java.lang.IllegalStateException: RecyclerView has no LayoutManager即使定义了错误，我也会收到以下错误：

这是ListContentAAR.java文件的代码：

class ListContentAAR {
    String picTag;

    ListContentAAR( String picTag) {
        this.picTag = picTag;
    }

}

这是RVAdapterAAR.java文件的代码：

public class RVAdapterAAR extends RecyclerView.Adapter<RVAdapterAAR.PersonViewHolder> {

    public static class PersonViewHolder extends RecyclerView.ViewHolder {

        CardView cardView;
        TextView pic;

        PersonViewHolder(View itemView) {
            super(itemView);
            cardView = (CardView) itemView.findViewById(R.id.card_accept_request);
            homelessPic = (TextView) itemView.findViewById(R.id.pic_tag);
        }

    }

    List<ListContentAAR> listContentAARs;

    RVAdapterAAR(List<ListContentAAR> listContentAARs) {
        this.listContentAARs = listContentAARs;
    }

    public void onAttachedToRecyclerView(RecyclerView recyclerView) {
        super.onAttachedToRecyclerView(recyclerView);
    }

    public PersonViewHolder onCreateViewHolder(ViewGroup viewGroup, int i) {
        View view = LayoutInflater.from(viewGroup.getContext()).inflate(R.layout.fragment_accept_a_request, viewGroup, false);
        PersonViewHolder personViewHolder = new PersonViewHolder(view);
        return personViewHolder;
    }

    public void onBindViewHolder (PersonViewHolder personViewHolder, int i) {
        personViewHolder.pic.setText(listContentAARs.get(i).homelessPicTag);
    }

    public int getItemCount() {
        return listContentAARs.size();
    }

}

这是AcceptARequest.java文件的代码：

public class AcceptARequest extends Fragment {
    // TODO: Rename parameter arguments, choose names that match
    // the fragment initialization parameters, e.g. ARG_ITEM_NUMBER
    private static final String ARG_PARAM1 = "param1";
    private static final String ARG_PARAM2 = "param2";

    // TODO: Rename and change types of parameters
    private String mParam1;
    private String mParam2;

    private OnFragmentInteractionListener mListener;

    private List<ListContentAAR> listContentAARs;
    private RecyclerView recyclerView;

    public AcceptARequest() {
        // Required empty public constructor
    }

    /**
     * Use this factory method to create a new instance of
     * this fragment using the provided parameters.
     *
     * @param param1 Parameter 1.
     * @param param2 Parameter 2.
     * @return A new instance of fragment AcceptARequest.
     */
    // TODO: Rename and change types and number of parameters
    public static AcceptARequest newInstance(String param1, String param2) {
        AcceptARequest fragment = new AcceptARequest();
        Bundle args = new Bundle();
        args.putString(ARG_PARAM1, param1);
        args.putString(ARG_PARAM2, param2);
        fragment.setArguments(args);
        return fragment;
    }

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        if (getArguments() != null) {
            mParam1 = getArguments().getString(ARG_PARAM1);
            mParam2 = getArguments().getString(ARG_PARAM2);
        }
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Inflate the layout for this fragment

        View rootView = inflater.inflate(R.layout.fragment_accept_a_request, container, false);

        RecyclerView recyclerView = (RecyclerView) rootView.findViewById(R.id.accept_request_list);
        LinearLayoutManager linearLayoutManager = new LinearLayoutManager(getActivity());
        recyclerView.setLayoutManager(linearLayoutManager);
        recyclerView.setHasFixedSize(true);

        initializeData();
        initializeAdapter();

        return rootView;
    }

    private void initializeData(){
        listContentAARs = new ArrayList<>();
        listContentAARs.add(new ListContentAAR("Emma Wilson"));
    }

    private void initializeAdapter(){
        RVAdapterAAR adapter = new RVAdapterAAR(listContentAARs);
        recyclerView.setAdapter(adapter);
    }

    // TODO: Rename method, update argument and hook method into UI event
    public void onButtonPressed(Uri uri) {
        if (mListener != null) {
            mListener.onFragmentInteraction(uri);
        }
    }

    @Override
    public void onDetach() {
        super.onDetach();
        mListener = null;
    }

    /**
     * This interface must be implemented by activities that contain this
     * fragment to allow an interaction in this fragment to be communicated
     * to the activity and potentially other fragments contained in that
     * activity.
     * <p/>
     * See the Android Training lesson <a href=
     * "http://developer.android.com/training/basics/fragments/communicating.html"
     * >Communicating with Other Fragments</a> for more information.
     */
    public interface OnFragmentInteractionListener {
        // TODO: Update argument type and name
        void onFragmentInteraction(Uri uri);
    }
}

这是fragment_accept_a_request.xml文件的代码：

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
                xmlns:tools="http://schemas.android.com/tools"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:paddingLeft="@dimen/activity_horizontal_margin"
                android:paddingRight="@dimen/activity_horizontal_margin"
                android:paddingTop="@dimen/activity_vertical_margin"
                android:paddingBottom="@dimen/activity_vertical_margin"
                xmlns:app="http://schemas.android.com/apk/res-auto"
                tools:context="com.abc.xyz.AcceptARequest">

    <android.support.v7.widget.RecyclerView
        android:id="@+id/accept_request_list"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:background="@android:color/white"
        app:layout_behavior="@string/appbar_scrolling_view_behavior">

        <android.support.v7.widget.CardView
            android:id="@+id/card_accept_request"
            android:layout_width="match_parent"
            android:layout_height="@dimen/card_accept_request"
            app:cardElevation="2dp"
            app:cardUseCompatPadding="true"
            app:contentPadding="10dp">

            <!--<ImageView
                android:id="@+id/pic_accept"
                android:layout_width="match_parent"
                android:layout_height="@dimen/pic_dimen_accept"
                android:layout_gravity="center_horizontal|center_vertical"
                android:layout_alignParentTop="true"/> -->

            <TextView
                android:id="@+id/pic_tag"
                android:layout_width="match_parent"
                android:layout_height="@dimen/pic_dimen_accept"
                android:text="@string/image_view_tag_accept"
                android:gravity="center_horizontal|center_vertical"/>

        </android.support.v7.widget.CardView>

    </android.support.v7.widget.RecyclerView>

</RelativeLayout>

这是MainActivity.java文件的代码：

public class MainActivity extends AppCompatActivity {

    /**
     * The {@link android.support.v4.view.PagerAdapter} that will provide
     * fragments for each of the sections. We use a
     * {@link FragmentPagerAdapter} derivative, which will keep every
     * loaded fragment in memory. If this becomes too memory intensive, it
     * may be best to switch to a
     * {@link android.support.v4.app.FragmentStatePagerAdapter}.
     */
    public SectionsPagerAdapter mSectionsPagerAdapter;

    /**
     * The {@link ViewPager} that will host the section contents.
     */
    private ViewPager mViewPager;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        Typeface typeFace = Typeface.createFromAsset(getAssets(),"fonts/Pacifico.ttf");

        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        TextView mTitle = (TextView) toolbar.findViewById(R.id.toolbar_title);
        mTitle.setTypeface(typeFace);
        toolbar.setTitle(mTitle.getText().toString());
        setSupportActionBar(toolbar);
        getSupportActionBar().setDisplayShowTitleEnabled(false);

        // Create the adapter that will return a fragment for each of the three
        // primary sections of the activity.
        mSectionsPagerAdapter = new SectionsPagerAdapter(this, getSupportFragmentManager());

        // Set up the ViewPager with the sections adapter.
        mViewPager = (ViewPager) findViewById(R.id.container);
        mViewPager.setAdapter(mSectionsPagerAdapter);

        TabLayout tabLayout = (TabLayout) findViewById(R.id.tabs);
        tabLayout.setupWithViewPager(mViewPager);

        /*FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                        .setAction("Action", null).show();
            }
        });*/

    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            Intent settingsIntent = new Intent(MainActivity.this, SettingsActivity.class);
            startActivity(settingsIntent);
        } else if (id == R.id.action_profile) {
            Intent profileIntent = new Intent(MainActivity.this, ProfileActivity.class);
            startActivity(profileIntent);
        } else if (id == R.id.action_send_feedback) {
            Intent sendFeedbackIntent = new Intent(MainActivity.this, SendFeedback.class);
            startActivity(sendFeedbackIntent);
        } else if (id == R.id.action_help) {
            Intent helpIntent = new Intent(MainActivity.this, HelpActivity.class);
            startActivity(helpIntent);
        }

        return super.onOptionsItemSelected(item);
    }

    public class SectionsPagerAdapter extends FragmentPagerAdapter {

        protected Context mContext;

        public SectionsPagerAdapter(Context context, FragmentManager fm) {
            super(fm);
            mContext = context;
        }

        @Override
        public Fragment getItem(int position) {
            // getItem is called to instantiate the fragment for the given page.
            // Return a PlaceholderFragment (defined as a static inner class below).
            // return MainActivity.PlaceholderFragment.newInstance(position + 1);

            switch (position) {
                case 0:
                    return new AcceptARequest();
                case 1:
                    return new PostARequest();
                default:
                    return new AcceptARequest(); // return AcceptARequest() actually
            }
        }

        @Override
        public int getCount() {
            // Show 2 total pages.
            return 2;
        }

        @Override
        public CharSequence getPageTitle(int position) {
            Locale l = Locale.getDefault();
            switch (position) {
                case 0:
                    return "Accept A Request";
                case 1:
                    return "Post A Request";
            }
            return null;
        }
    }

    /**
     * A placeholder fragment containing a simple view.
     */
    public static class PlaceholderFragment extends Fragment {
        /**
         * The fragment argument representing the section number for this
         * fragment.
         */
        private static final String ARG_SECTION_NUMBER = "section_number";

        /**
         * Returns a new instance of this fragment for the given section
         * number.
         */
        public static PlaceholderFragment newInstance(int sectionNumber) {
            PlaceholderFragment fragment = new PlaceholderFragment();
            Bundle args = new Bundle();
            args.putInt(ARG_SECTION_NUMBER, sectionNumber);
            fragment.setArguments(args);
            return fragment;
        }

        public PlaceholderFragment() {
        }

        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container,
                                 Bundle savedInstanceState) {
            View rootView = inflater.inflate(R.layout.fragment_main, container, false);
            TextView textView = (TextView) rootView.findViewById(R.id.section_label);
            textView.setText(getString(R.string.section_format, getArguments().getInt(ARG_SECTION_NUMBER)));
            return rootView;
        }
    }
}

这是stacktrace：

FATAL EXCEPTION: main
                                                                               java.lang.IllegalStateException: RecyclerView has no LayoutManager
                                                                                   at android.support.v7.widget.RecyclerView.generateLayoutParams(RecyclerView.java:3182)
                                                                                   at android.view.LayoutInflater.rInflate(LayoutInflater.java:748)
                                                                                   at android.view.LayoutInflater.rInflate(LayoutInflater.java:749)
                                                                                   at android.view.LayoutInflater.inflate(LayoutInflater.java:489)
                                                                                   at android.view.LayoutInflater.inflate(LayoutInflater.java:396)
                                                                                   at com.humanehelper.humanehelper.AcceptARequest.onCreateView(AcceptARequest.java:75)
                                                                                   at android.support.v4.app.Fragment.performCreateView(Fragment.java:1962)
                                                                                   at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1067)
                                                                                   at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1248)
                                                                                   at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:738)
                                                                                   at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1613)
                                                                                   at android.support.v4.app.FragmentManagerImpl.executePendingTransactions(FragmentManager.java:570)
                                                                                   at android.support.v4.app.FragmentPagerAdapter.finishUpdate(FragmentPagerAdapter.java:141)
                                                                                   at android.support.v4.view.ViewPager.populate(ViewPager.java:1106)
                                                                                   at android.support.v4.view.ViewPager.populate(ViewPager.java:952)
                                                                                   at android.support.v4.view.ViewPager.onMeasure(ViewPager.java:1474)
                                                                                   at android.view.View.measure(View.java:15172)
                                                                                   at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:4814)
                                                                                   at android.support.design.widget.CoordinatorLayout.onMeasureChild(CoordinatorLayout.java:610)
                                                                                   at android.support.design.widget.CoordinatorLayout.onMeasure(CoordinatorLayout.java:677)
                                                                                   at android.view.View.measure(View.java:15172)
                                                                                   at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:4814)
                                                                                   at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
                                                                                   at android.support.v7.widget.ContentFrameLayout.onMeasure(ContentFrameLayout.java:135)
                                                                                   at android.view.View.measure(View.java:15172)
                                                                                   at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:4814)
                                                                                   at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1390)
                                                                                   at android.widget.LinearLayout.measureVertical(LinearLayout.java:681)
                                                                                   at android.widget.LinearLayout.onMeasure(LinearLayout.java:574)
                                                                                   at android.view.View.measure(View.java:15172)
                                                                                   at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:4814)
                                                                                   at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
                                                                                   at android.view.View.measure(View.java:15172)
                                                                                   at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:4814)
                                                                                   at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1390)
                                                                                   at android.widget.LinearLayout.measureVertical(LinearLayout.java:681)
                                                                                   at android.widget.LinearLayout.onMeasure(LinearLayout.java:574)
                                                                                   at android.view.View.measure(View.java:15172)
                                                                                   at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:4814)
                                                                                   at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
                                                                                   at com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2148)
                                                                                   at android.view.View.measure(View.java:15172)
                                                                                   at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:1848)
                                                                                   at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1100)
                                                                                   at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1273)
                                                                                   at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:998)
                                                                                   at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:4212)
                                                                                   at android.view.Choreographer$CallbackRecord.run(Choreographer.java:725)
                                                                                   at android.view.Choreographer.doCallbacks(Choreographer.java:555)
                                                                                   at android.view.Choreographer.doFrame(Choreographer.java:525)
                                                                                   at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:711)
                                                                                   at android.os.Handler.handleCallback(Handler.java:615)
                                                                                   at android.os.Handler.dispatchMessage(Handler.java:92)
                                                                                   at android.os.Looper.loop(Looper.java:137)
                                                                                   at android.app.ActivityThread.main(ActivityThread.java:4745)
                                                                                   at java.lang.reflect.Method.invokeNative(Native Method)
                                                                                   at java.lang.reflect.Method.invoke(Method.java:511)
                                                                                at com

由于我是初学者，所以无法弄清楚为什么会出现以下错误。

请告诉我。

提前致谢。