为什么我在此代码中得到 null object 参考？

Question

I am working on an app that uses fragments. One fragment will show a list of users, which is working well. Another fragment will show a list of access grades. The access grade list is not working. The code for the access grade list is below:

/**
 * The fragment onCreateView Method
 *
 * @param inflater the layout inflater
 * @param container the container that will hold the fragment
 * @param savedInstanceState the saved instance state
 * @return View object container the fragment view
 */
@Override
public View onCreateView(@NonNull LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
    View view = inflater.inflate(R.layout.fragment_grade_list, container, false);
    assert getArguments() != null;
    String coName = getArguments().getString("CoName");
    String apiUrl = getArguments().getString("ApiUrl");
    update = getArguments().getBoolean("update");
    remove = getArguments().getBoolean("remove");
    new ManagerAdminApi(apiUrl);
    RecyclerView gRecyclerView = view.findViewById(R.id.recyclerViewGradeList);
    LinearLayoutManager gLayoutManager = new LinearLayoutManager(this.getActivity());
    **gRecyclerView.setHasFixedSize(true);**
    gAdapter = new FragmentGradeList.GradeAdapter(gradeList);
    gRecyclerView.setLayoutManager(gLayoutManager);
    gRecyclerView.setAdapter(gAdapter);
    TextView tvTitle = view.findViewById(R.id.tvTitle);
    tvTitle.setText(getString(R.string.grade_list_title, coName));
    readGrades();
    return view;
}

This is virtually the same code I used for the user list, however I am getting this error in the logcat window of Android Studio:

2020-07-15 06:42:08.670 8019-8019/com.phoenixhosman.phoenixdbmanagers E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.phoenixhosman.phoenixdbmanagers, PID: 8019
    java.lang.NullPointerException: Attempt to invoke virtual method 'void androidx.recyclerview.widget.RecyclerView.setHasFixedSize(boolean)' on a null object reference
        at com.phoenixhosman.phoenixdbmanagers.FragmentGradeList.onCreateView(FragmentGradeList.java:68)
        at androidx.fragment.app.Fragment.performCreateView(Fragment.java:2698)
        at androidx.fragment.app.FragmentStateManager.createView(FragmentStateManager.java:310)
        at androidx.fragment.app.FragmentManager.moveToState(FragmentManager.java:1185)
        at androidx.fragment.app.FragmentManager.moveToState(FragmentManager.java:1354)
        at androidx.fragment.app.FragmentManager.moveFragmentToExpectedState(FragmentManager.java:1432)
        at androidx.fragment.app.FragmentManager.moveToState(FragmentManager.java:1495)
        at androidx.fragment.app.BackStackRecord.executeOps(BackStackRecord.java:447)
        at androidx.fragment.app.FragmentManager.executeOps(FragmentManager.java:2167)
        at androidx.fragment.app.FragmentManager.executeOpsTogether(FragmentManager.java:1990)
        at androidx.fragment.app.FragmentManager.removeRedundantOperationsAndExecute(FragmentManager.java:1945)
        at androidx.fragment.app.FragmentManager.execPendingActions(FragmentManager.java:1847)
        at androidx.fragment.app.FragmentManager$4.run(FragmentManager.java:413)
        at android.os.Handler.handleCallback(Handler.java:873)
        at android.os.Handler.dispatchMessage(Handler.java:99)
        at android.os.Looper.loop(Looper.java:193)
        at android.app.ActivityThread.main(ActivityThread.java:6714)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:503)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:911)

It is saying I have a null object reference on the line I have in bold type. I have checked and rechecked my code and I can not find why is it null. My entire fragment code file is here:

/*
    The Phoenix Hospitality Management System
    Database Manager App
    Grade List Fragment Code File
    Copyright (c) 2020 By Troy Marker Enterprises
    All Rights Under Copyright Reserved

    The code in this file was created for use with the Phoenix Hospitality Management System (PHMS).
    Use of this code outside the PHMS is strictly prohibited.
 */
package com.phoenixhosman.phoenixdbmanagers;

import android.content.Context;
import android.graphics.Color;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;
import android.widget.Toast;
import androidx.annotation.NonNull;
import androidx.fragment.app.Fragment;
import androidx.recyclerview.widget.LinearLayoutManager;
import androidx.recyclerview.widget.RecyclerView;
import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;
import java.util.ArrayList;
import java.util.List;
import retrofit2.Call;
import retrofit2.Callback;
import retrofit2.Response;

/**
 * Grade List Fragment code file
 * @author Troy Marker
 * @version 1.0.0
 */
public class FragmentGradeList extends Fragment {
    private RecyclerView.Adapter gAdapter;
    static InterfaceDataPasser dataPasser;
    public static Boolean update;
    public static Boolean remove;
    private final ArrayList<ObjectGrade> gradeList = new ArrayList<>();

    public FragmentGradeList() {
    }

    /**
     * The fragment onCreateView Method
     *
     * @param inflater the layout inflater
     * @param container the container that will hold the fragment
     * @param savedInstanceState the saved instance state
     * @return View object container the fragment view
     */
    @Override
    public View onCreateView(@NonNull LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.fragment_grade_list, container, false);
        assert getArguments() != null;
        String coName = getArguments().getString("CoName");
        String apiUrl = getArguments().getString("ApiUrl");
        update = getArguments().getBoolean("update");
        remove = getArguments().getBoolean("remove");
        new ManagerAdminApi(apiUrl);
        RecyclerView gRecyclerView = view.findViewById(R.id.recyclerViewGradeList);
        LinearLayoutManager gLayoutManager = new LinearLayoutManager(this.getActivity());
        gRecyclerView.setHasFixedSize(true);
        gAdapter = new FragmentGradeList.GradeAdapter(gradeList);
        gRecyclerView.setLayoutManager(gLayoutManager);
        gRecyclerView.setAdapter(gAdapter);
        TextView tvTitle = view.findViewById(R.id.tvTitle);
        tvTitle.setText(getString(R.string.grade_list_title, coName));
        readGrades();
        return view;
    }

    /**
     * Method to read the Grade list from the database
     */
    public void readGrades() {
        gradeList.clear();
        Call<String> call = ManagerAdminApi.getInstance().getApi().grade();
        call.enqueue(new Callback<String>() {
            @Override
            public void onResponse(@NonNull Call<String> call, @NonNull Response<String> response) {
                String body = response.body();
                try {
                    assert body != null;
                    JSONObject obj = new JSONObject(body);
                    if(obj.optString("success").equals("true")) {
                        JSONArray data = obj.getJSONArray("data");
                        for (int i = 0; i < data.length(); i++) {
                            JSONObject datum = data.getJSONObject(i);
                            gradeList.add(new ObjectGrade(
                                    datum.getInt("id"),
                                    datum.getString("grade")
                            ));
                        }
                        gAdapter.notifyDataSetChanged();
                    } else {
                        Toast.makeText(getContext(), obj.optString("message"), Toast.LENGTH_LONG).show();
                    }
                } catch (JSONException e) {
                    e.printStackTrace();
                }
            }
            @Override
            public void onFailure(@NonNull Call<String> call, @NonNull Throwable t) {
            }
        });
    }

    /**
     * recyclerviewUserList display adapter
     */
    public static class GradeAdapter extends RecyclerView.Adapter<FragmentGradeList.GradeAdapter.GradeViewHolder> {
        final private List<ObjectGrade> mGradeList;

        static class GradeViewHolder extends RecyclerView.ViewHolder {
            final TextView tvId;
            final TextView tvGrade;
            GradeViewHolder(View gradeView) {
                super(gradeView);
                tvId = gradeView.findViewById(R.id.tvId);
                tvGrade = gradeView.findViewById(R.id.tvGrade);
            }
        }
        GradeAdapter(ArrayList<ObjectGrade> gradelist) {
            mGradeList = gradelist;
        }
        @NonNull
        @Override
        public FragmentGradeList.GradeAdapter.GradeViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
            View v = LayoutInflater.from(parent.getContext()).inflate(R.layout.fragment_grade_list_item, parent,false);
            return new FragmentGradeList.GradeAdapter.GradeViewHolder(v);
        }
        @Override
        public void onBindViewHolder(@NonNull final FragmentGradeList.GradeAdapter.GradeViewHolder holder, int position) {
            final ObjectGrade currentGrade = mGradeList.get(position);
            holder.tvId.setText(String.valueOf(currentGrade.getId()));
            if (update) {
                holder.tvId.setTextColor(Color.parseColor("#06F522"));
            }
            if (remove) {
                holder.tvId.setTextColor(Color.parseColor("#FF2579"));
            }
            holder.tvGrade.setText(currentGrade.getGrade());
            if (update) {
                holder.tvId.setOnClickListener(v -> dataPasser.onGradeUpdate(currentGrade.getId()));
            }
            if (remove) {
                holder.tvId.setOnClickListener(v -> dataPasser.onGradeRemove(currentGrade.getId()));
            }
        }
        @Override
        public int getItemCount() {
            return mGradeList.size();
        }
    }

    @Override
    public void onAttach(@NonNull Context context) {
        super.onAttach(context);
        dataPasser = (InterfaceDataPasser) context;
    }


}

Please forgive any formatting error. Thank you for any help.

As requested here is the code of my fragment layout file:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="match_parent"
    android:layout_height="match_parent">
    <TextView
        android:id="@+id/tvTitle"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:gravity="center_horizontal"
        android:text="@string/grade_list_title"
        android:textSize="36sp" />
    <LinearLayout
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="center_horizontal"
        android:orientation="horizontal">
        <TextView
            android:layout_width="75dp"
            android:layout_height="wrap_content"
            android:background="@drawable/x_border_3"
            android:gravity="center_horizontal"
            android:text="@string/id"
            android:textAllCaps="true"
            android:textColor="@color/Phoenix_Black" />
        <TextView
            android:layout_width="200dp"
            android:layout_height="wrap_content"
            android:background="@drawable/x_border_3"
            android:gravity="center"
            android:text="@string/grade_1"
            android:textAllCaps="true"
            android:textColor="@color/Phoenix_Black" />
    </LinearLayout>
    <androidx.recyclerview.widget.RecyclerView
        android:id="@+id/recyclerViewGradeList"
        android:layout_width="wrap_content"
        android:layout_height="match_parent"
        android:layout_gravity="center_horizontal"
        android:gravity="center_horizontal"/>
</LinearLayout>

Thanks

Answer 1

Mike M. That was the problem, I had two layouts, portrait and landscape. I removed the landscape and it worked. Thank you. Im just curious. I have the same thing in another fragment, and have two layouts, and that one works fine. Why not ins the second fragment?

I just found out what was wrong. Apparently I needed the list item layout for the recycler view. After I added the second view, it worked fine.