尝试从空对象引用上的字段“android.view.View androidx.recyclerview.widget.RecyclerView$ViewHolder.itemView”读取

Question

我正在制作一个应用程序，它从 sql 获取数据并将其放入回收视图中我的数据进入回收视图，它在 logcat 中给了我这个错误。

 java.lang.NullPointerException: Attempt to read from field
       'android.view.View
       androidx.recyclerview.widget.RecyclerView$ViewHolder.itemView' on a
       null object reference
               at androidx.recyclerview.widget.RecyclerView$Adapter.createViewHolder(RecyclerView.java:7079)
               at androidx.recyclerview.widget.RecyclerView$Recycler.tryGetViewHolderForPositionByDeadline(RecyclerView.java:6235)
               at androidx.recyclerview.widget.RecyclerView$Recycler.getViewForPosition(RecyclerView.java:6118)
               at androidx.recyclerview.widget.RecyclerView$Recycler.getViewForPosition(RecyclerView.java:6114)
               at androidx.recyclerview.widget.LinearLayoutManager$LayoutState.next(LinearLayoutManager.java:2303)
               at androidx.recyclerview.widget.LinearLayoutManager.layoutChunk(LinearLayoutManager.java:1627)
               at androidx.recyclerview.widget.LinearLayoutManager.fill(LinearLayoutManager.java:1587)
               at androidx.recyclerview.widget.LinearLayoutManager.onLayoutChildren(LinearLayoutManager.java:665)
               at androidx.recyclerview.widget.RecyclerView.dispatchLayoutStep2(RecyclerView.java:4134)
               at androidx.recyclerview.widget.RecyclerView.dispatchLayout(RecyclerView.java:3851)
               at androidx.recyclerview.widget.RecyclerView.onLayout(RecyclerView.java:4404)
               at android.view.View.layout(View.java:18010)
               at android.view.ViewGroup.layout(ViewGroup.java:5817)
               at android.widget.RelativeLayout.onLayout(RelativeLayout.java:1080)
               at android.view.View.layout(View.java:18010)
               at android.view.ViewGroup.layout(ViewGroup.java:5817)
               at android.widget.FrameLayout.layoutChildren(FrameLayout.java:344)
               at android.widget.FrameLayout.onLayout(FrameLayout.java:281)
               at android.view.View.layout(View.java:18010)
               at android.view.ViewGroup.layout(ViewGroup.java:5817)
               at androidx.appcompat.widget.ActionBarOverlayLayout.onLayout(ActionBarOverlayLayout.java:444)
               at android.view.View.layout(View.java:18010)
               at android.view.ViewGroup.layout(ViewGroup.java:5817)
               at android.widget.FrameLayout.layoutChildren(FrameLayout.java:344)
               at android.widget.FrameLayout.onLayout(FrameLayout.java:281)
               at android.view.View.layout(View.java:18010)
               at android.view.ViewGroup.layout(ViewGroup.java:5817)
               at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1742)
               at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1585)
               at android.widget.LinearLayout.onLayout(LinearLayout.java:1494)

请帮忙，因为它没有让我知道错误来自哪里或类似的东西。

这是我的适配器类：

public class MyAppAdapter extends 

RecyclerView.Adapter<MyAppAdapter.ViewHolder>//has a class viewholder which holds
        {
            private ArrayList<ClassListChat> mOriginalValues; // Original Values
            private ArrayList<ClassListChat> mDisplayedValues;

            public class ViewHolder extends RecyclerView.ViewHolder {

                TextView messageText;
                TextView messageUser;
                TextView messageTime;

                public ViewHolder(@NonNull View itemView) {
                    super(itemView);
                }
            }

            public List <ClassListChat> parkingList;

            public Context context;
            ArrayList<ClassListChat> arraylist;


            private MyAppAdapter(List<ClassListChat> apps, Context context) {
                this.parkingList = apps;
                this.context = context;
                arraylist = new ArrayList<ClassListChat>();
                arraylist.addAll(parkingList);
            }



            @Override
            public MyAppAdapter.ViewHolder onCreateViewHolder(ViewGroup parent,int viewType) {
                View rowView = LayoutInflater.from(parent.getContext()).inflate(R.layout.listcontentstorechat,parent,false);
                ViewHolder viewHolder = new ViewHolder(rowView);
                if (rowView == null) {
                    LayoutInflater inflater = getLayoutInflater();
                    rowView = inflater.inflate(R.layout.listcontentstorechat, parent, false);
                    viewHolder.messageText = (BubbleTextView) rowView.findViewById(R.id.message_text);
                    viewHolder.messageUser = (TextView) rowView.findViewById(R.id.message_user);
                    viewHolder.messageTime = (TextView) rowView.findViewById(R.id.message_time);
                    rowView.setTag(viewHolder);
                } else {
                    viewHolder = (ViewHolder) rowView.getTag();
                }
                // here setting up names and images




                return viewHolder;
            }

            @Override
            public void onBindViewHolder(@NonNull ViewHolder holder, int position) {
                holder.messageText.setText(parkingList.get(position).getMessageText());
                holder.messageUser.setText(parkingList.get(position).getMessageUser());
                holder.messageTime.setText(DateFormat.format("dd-MM-yyyy (HH:mm:ss)", parkingList.get(position).getMessageTime()));
            }

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


I would really appreciate your hard work in helping me!!

编辑：现在它只给了我一个空白的bubbleTextView（我在我的应用程序中实现了），但它没有显示文本、时间、用户也没有任何想法？

这是我的 listcontentstorechat.xml：

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <TextView
        android:layout_alignParentTop="true"
        android:layout_alignParentStart="true"
        android:layout_alignParentLeft="true"
        android:id="@+id/message_user"
        android:textStyle="normal|bold"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" />

    <TextView
        android:layout_alignParentBottom="@+id/message_user"
        android:layout_alignParentEnd="true"
        android:layout_alignParentRight="true"
        android:id="@+id/message_time"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" />

    <com.github.library.bubbleview.BubbleTextView
        android:layout_alignParentStart="true"
        android:layout_alignParentLeft="true"
        android:layout_marginTop="5dp"
        android:layout_below="@id/message_user"
        android:textAppearance="@style/TextAppearance.AppCompat.Body1"
        android:textColor="#FFF"
        android:textSize="18sp"

        android:id="@+id/message_text"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:padding="10dp"
        app:arrowWidth="8dp"
        app:angle="8dp"
        app:arrowHeight="10dp"
        app:arrowPosition="14dp"
        app:arrowLocation="left"
        app:bubbleColor="#333639"
        app:arrowCenter="true"
        />

</RelativeLayout>

这是我的 ClassListChat.java

public class ClassListChat {
    public String messageText;
    public String messageUser;
    public long messageTime;


    public ClassListChat(String messageText, String messageUser) {
        this.messageText = messageText;
        this.messageUser = messageUser;
        messageTime = new Date().getTime();
    }

    public ClassListChat() {
    }

    public String getMessageText() {
        return messageText;
    }

    public void setMessageText(String messageText) {
        this.messageText = messageText;
    }

    public String getMessageUser() {
        return messageUser;
    }

    public void setMessageUser(String messageUser) {
        this.messageUser = messageUser;
    }

    public long getMessageTime() {
        return messageTime;
    }

    public void setMessageTime(long messageTime) {
        this.messageTime = messageTime;
    }

EDIT2：经过一些改进，它现在在 logcat 中返回错误：

 java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference
        at com.example.medicnes.ChatActivity$MyAppAdapter.onBindViewHolder(ChatActivity.java:275)
        at com.example.medicnes.ChatActivity$MyAppAdapter.onBindViewHolder(ChatActivity.java:225)
        at androidx.recyclerview.widget.RecyclerView$Adapter.onBindViewHolder(RecyclerView.java:7065)
        at androidx.recyclerview.widget.RecyclerView$Adapter.bindViewHolder(RecyclerView.java:7107)
        at androidx.recyclerview.widget.RecyclerView$Recycler.tryBindViewHolderByDeadline(RecyclerView.java:6012)
        at androidx.recyclerview.widget.RecyclerView$Recycler.tryGetViewHolderForPositionByDeadline(RecyclerView.java:6279)
        at androidx.recyclerview.widget.RecyclerView$Recycler.getViewForPosition(RecyclerView.java:6118)
        at androidx.recyclerview.widget.RecyclerView$Recycler.getViewForPosition(RecyclerView.java:6114)

Answer 1

onCreateViewHolder() 中的ViewHolder 始终不为空，因为您在此方法中创建的View 不能为空。

onCreateViewHolder() 的构建方式与您用于创建ListView 适配器的getView() 不同

所以，请将您的onCreateViewHolder 更改为

        @Override
        public MyAppAdapter.ViewHolder onCreateViewHolder(ViewGroup parent,int viewType) {
            View rowView = LayoutInflater.from(parent.getContext()).inflate(R.layout.listcontentstorechat,parent,false);
            ViewHolder viewHolder = new ViewHolder(rowView);

            return viewHolder;
        }

编辑： 现在（实际上之前）它不会将文本添加到回收视图中，对此有何帮助？我已经更新了我的问题

你从getItemCount()返回了错误的列表大小，因为你的适配器使用arraylist作为它的数据列表，所以你需要返回它的大小

所以把getItemCount()改成

@Override
 public int getItemCount() {
     if (arraylist != null)
         return arraylist.size();
     else
         return 0;
 }

编辑 2：

 java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference
        at com.example.medicnes.ChatActivity$MyAppAdapter.onBindViewHolder(ChatActivity.java:275)
        at com.example.medicnes.ChatActivity$MyAppAdapter.onBindViewHolder(ChatActivity.java:225)
        at androidx.recyclerview.widget.RecyclerView$Adapter.onBindViewHolder(RecyclerView.java:7065)
        at androidx.recyclerview.widget.RecyclerView$Adapter.bindViewHolder(RecyclerView.java:7107)
        at androidx.recyclerview.widget.RecyclerView$Recycler.tryBindViewHolderByDeadline(RecyclerView.java:6012)
        at androidx.recyclerview.widget.RecyclerView$Recycler.tryGetViewHolderForPositionByDeadline(RecyclerView.java:6279)
        at androidx.recyclerview.widget.RecyclerView$Recycler.getViewForPosition(RecyclerView.java:6118)
        at androidx.recyclerview.widget.RecyclerView$Recycler.getViewForPosition(RecyclerView.java:6114)

您没有使用.findViewById() 在行项目布局中初始化视图，因此将ViewHolder 构造函数更改为

public ViewHolder(@NonNull View itemView) {
    super(itemView);
    messageText = itemView.findViewById(R.id.message_text);
    messageUser = itemView.findViewById(R.id.message_user);
    messageTime = itemView.findViewById(R.id.message_time);
}

Answer 2

您的适配器正在返回空视图。

View rowView = LayoutInflater.from(parent.getContext()).inflate(R.layout.listcontentstorechat,parent,false);

rowView 在这里总是非空的，所以这个条件：

if (rowView == null) {

永远不会被命中，而是代码转到 else 分支

} else {
    viewHolder = (ViewHolder) rowView.getTag();
}

将 viewHolder 替换为 null 并返回。

这部分代码看起来像是试图将旧的 ListView 适配器适配为 RecyclerView 适配器。使用 RecyclerView，您不需要 setTag()/getTag()。

只需创建并初始化您的视图持有者并从方法中返回它。

Answer 3

我遇到了同样的问题，当我尝试在 Fragment 上使用 recyclerView 从 firebase firestore 检索数据并将其放入 recyclerView 时，我无法访问 recyclerView。只需在 findViewById(R.id.recyclerViewid) 前面添加getView()

我之前的代码：

//Declared at the top of the class
RecylcerView recyclerView ;
//In the onViewCreate section
recyclerView = findViewById(R.id.recyclerview)//<--id of the recyclerView

我的代码之后：

//Declared at the top of the class
RecylcerView recyclerView ;
//In the onViewCreate section
recyclerView = (getView).findViewById(R.id.recyclerview)//<--id of the recyclerView