為什么我在此代碼中得到 null object 參考？

Question

我正在開發一個使用片段的應用程序。 一個片段將顯示一個用戶列表，它運行良好。 另一個片段將顯示訪問等級列表。 訪問等級列表不起作用。 訪問等級列表的代碼如下：

/**
 * 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;
}

這與我用於用戶列表的代碼幾乎相同，但是我在 Android Studio 的 logcat window 中收到此錯誤：

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)

這是說我在粗體字的行上有一個 null object 參考。 我檢查並重新檢查了我的代碼，但找不到為什么它是 null。 我的整個片段代碼文件在這里：

/*
    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;
    }


}

請原諒任何格式錯誤。 感謝您的任何幫助。

這里要求的是我的片段布局文件的代碼：

<?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>

謝謝

Answer 1

Mike M. 這就是問題所在，我有兩種布局，縱向和橫向。 我刪除了景觀，它起作用了。 謝謝你。 我只是好奇。 我在另一個片段中有同樣的東西，有兩種布局，一個很好用。 為什么不插入第二個片段？

我剛剛發現出了什么問題。 顯然我需要回收站視圖的列表項布局。 添加第二個視圖后，它運行良好。