Java Firebase数据库数据类型转换错误

我正在做一个项目，但出现了以下错误：

Fatal Exception: com.google.firebase.database.DatabaseException: Can't convert object of type java.lang.String to type com.example.codechat.InstantMessage
       at com.google.firebase.database.core.utilities.encoding.CustomClassMapper.convertBean(CustomClassMapper.java:436)
       at com.google.firebase.database.core.utilities.encoding.CustomClassMapper.deserializeToClass(CustomClassMapper.java:232)
       at com.google.firebase.database.core.utilities.encoding.CustomClassMapper.convertToCustomClass(CustomClassMapper.java:80)
       at com.google.firebase.database.DataSnapshot.getValue(DataSnapshot.java:203)
       at com.example.codechat.CustomAdapter.getItem(CustomAdapter.java:77)
       at com.example.codechat.CustomAdapter.getView(CustomAdapter.java:96)
       at android.widget.AbsListView.obtainView(AbsListView.java:3073)
       at android.widget.ListView.measureHeightOfChildren(ListView.java:1305)
       at android.widget.ListView.onMeasure(ListView.java:1212)
       at android.view.View.measure(View.java:20236)
       at android.widget.RelativeLayout.measureChildHorizontal(RelativeLayout.java:716)
       at android.widget.RelativeLayout.onMeasure(RelativeLayout.java:462)
       at android.view.View.measure(View.java:20236)
       at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6333)
       at android.widget.FrameLayout.onMeasure(FrameLayout.java:194)
       at androidx.appcompat.widget.ContentFrameLayout.onMeasure(ContentFrameLayout.java:146)
       at android.view.View.measure(View.java:20236)
       at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6333)
       at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1464)
       at android.widget.LinearLayout.measureVertical(LinearLayout.java:747)
       at android.widget.LinearLayout.onMeasure(LinearLayout.java:629)
       at android.view.View.measure(View.java:20236)
       at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6333)
       at android.widget.FrameLayout.onMeasure(FrameLayout.java:194)
       at android.view.View.measure(View.java:20236)
       at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6333)
       at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1464)
       at android.widget.LinearLayout.measureVertical(LinearLayout.java:747)
       at android.widget.LinearLayout.onMeasure(LinearLayout.java:629)
       at android.view.View.measure(View.java:20236)
       at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6333)
       at android.widget.FrameLayout.onMeasure(FrameLayout.java:194)
       at com.android.internal.policy.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:3140)
       at android.view.View.measure(View.java:20236)
       at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:2704)
       at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1656)
       at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1948)
       at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1544)
       at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:7607)
       at android.view.Choreographer$CallbackRecord.run(Choreographer.java:911)
       at android.view.Choreographer.doCallbacks(Choreographer.java:686)
       at android.view.Choreographer.doFrame(Choreographer.java:622)
       at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:897)
       at android.os.Handler.handleCallback(Handler.java:739)
       at android.os.Handler.dispatchMessage(Handler.java:95)
       at android.os.Looper.loop(Looper.java:148)
       at android.app.ActivityThread.main(ActivityThread.java:7402)
       at java.lang.reflect.Method.invoke(Method.java)
       at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1230)
       at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1120)

这是我的自定义适配器：

package com.example.codechat;

import android.app.Activity;
import android.content.Context;
import android.graphics.Color;
import android.view.Gravity;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.BaseAdapter;
import android.widget.LinearLayout;
import android.widget.TextView;

import androidx.annotation.NonNull;
import androidx.annotation.Nullable;

import com.google.firebase.database.ChildEventListener;
import com.google.firebase.database.DataSnapshot;
import com.google.firebase.database.DatabaseError;
import com.google.firebase.database.DatabaseReference;

import java.util.ArrayList;

public class CustomAdapter extends BaseAdapter {
    private Activity myActivity;
    private DatabaseReference myDB;
    private String username;
    private ArrayList<DataSnapshot> dataSnapshots;
DatabaseReference rootRef = FirebaseDatabase.getInstance().getReference();
    DatabaseReference chatsRef = rootRef.child("Chats");
    ValueEventListener valueEventListener = new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            for(DataSnapshot ds : dataSnapshot.getChildren()) {
                dataSnapshots.add(dataSnapshot);
        notifyDataSetChanged();
            }
        }

        @Override
        public void onCancelled(@NonNull DatabaseError databaseError) {
            
        }
    };


       

       

       

    public CustomAdapter(Activity activity, DatabaseReference ref, String name){
        myActivity = activity;
        myDB = ref.child("Chats");
        username = name;
        dataSnapshots = new ArrayList<>();
        myDB.addListenerForSingleValueEvent(valueEventListener);
    }
    static class ViewHolder{
        TextView sender;
        TextView message;
        LinearLayout.LayoutParams layoutParams;
    }
    @Override
    public int getCount() {
        return dataSnapshots.size();
    }

    @Override
    public InstantMessage getItem(int i) {
        DataSnapshot snapshot = dataSnapshots.get(i);
        return snapshot.getValue(InstantMessage.class);
    }

    @Override
    public long getItemId(int i) {
        return 0;
    }

    @Override
    public View getView(int i, View view, ViewGroup viewGroup) {
        if(view == null){
            LayoutInflater layoutInflater = (LayoutInflater) myActivity.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
            view = layoutInflater.inflate(R.layout.chat_list_view, viewGroup, false);
            final ViewHolder viewHolder = new ViewHolder();
            viewHolder.sender = view.findViewById(R.id.text);
            viewHolder.message = view.findViewById(R.id.textView2);
            viewHolder.layoutParams = (LinearLayout.LayoutParams) viewHolder.sender.getLayoutParams();
            view.setTag(viewHolder);
        }
        final InstantMessage message = getItem(i);
        final ViewHolder viewHolder = (ViewHolder) view.getTag();
        boolean isMe = message.getUser().equals(username);//Error
        row_style(isMe, viewHolder);
        String messager = message.getMessage();
        String author = message.getUser();
        viewHolder.sender.setText(author);
        viewHolder.message.setText(messager);
        return view;
    }
    private void row_style(boolean ism, ViewHolder hold){
        if(ism){
            hold.layoutParams.gravity = Gravity.START;
            hold.sender.setTextColor(Color.BLUE);
            hold.message.setBackgroundResource(R.drawable.speech_bubble_orange);
        }else{
            hold.layoutParams.gravity = Gravity.END;
            hold.sender.setTextColor(Color.BLUE);
            hold.message.setBackgroundResource(R.drawable.speech_bubble_green);
        }
    }
    public void confusing(){
        myDB.removeEventListener(valueEventListener);
    }

}

这是我的InstantMessage类：

package com.example.codechat;

public class InstantMessage{
   private String user;
   private String message;
   public InstantMessage(String l, String k){
      user = l;
      message = k;
   }
   public InstantMessage(){

   }
   public String getMessage() {
      return message;
   }

   public String getUser() {
      return user;
   }
}

这是我的数据库：

"Chats" : {
     "-MG3WxkPWaK-SFh7Q5M_" : "ynnh"
}

我找不到与此错误相关的任何内容。 我使用了firebase Crashlytics并获得了堆栈跟踪。 语法与其他语法完全不同。 当我在编辑文本中键入内容时，它崩溃了。 你能帮助我吗？ 编辑： 我有一个新错误：

Fatal Exception: java.lang.NullPointerException: Attempt to invoke virtual method 'boolean java.lang.String.equals(java.lang.Object)' on a null object reference
       at com.example.codechat.CustomAdapter.getView(CustomAdapter.java:88)
       at android.widget.AbsListView.obtainView(AbsListView.java:3073)
       at android.widget.ListView.measureHeightOfChildren(ListView.java:1305)
       at android.widget.ListView.onMeasure(ListView.java:1212)
       at android.view.View.measure(View.java:20236)
       at android.widget.RelativeLayout.measureChildHorizontal(RelativeLayout.java:716)
       at android.widget.RelativeLayout.onMeasure(RelativeLayout.java:462)
       at android.view.View.measure(View.java:20236)
       at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6333)
       at android.widget.FrameLayout.onMeasure(FrameLayout.java:194)
       at androidx.appcompat.widget.ContentFrameLayout.onMeasure(ContentFrameLayout.java:146)
       at android.view.View.measure(View.java:20236)
       at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6333)
       at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1464)
       at android.widget.LinearLayout.measureVertical(LinearLayout.java:747)
       at android.widget.LinearLayout.onMeasure(LinearLayout.java:629)
       at android.view.View.measure(View.java:20236)
       at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6333)
       at android.widget.FrameLayout.onMeasure(FrameLayout.java:194)
       at android.view.View.measure(View.java:20236)
       at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6333)
       at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1464)
       at android.widget.LinearLayout.measureVertical(LinearLayout.java:747)
       at android.widget.LinearLayout.onMeasure(LinearLayout.java:629)
       at android.view.View.measure(View.java:20236)
       at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6333)
       at android.widget.FrameLayout.onMeasure(FrameLayout.java:194)
       at com.android.internal.policy.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:3140)
       at android.view.View.measure(View.java:20236)
       at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:2704)
       at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1656)
       at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1948)
       at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1544)
       at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:7607)
       at android.view.Choreographer$CallbackRecord.run(Choreographer.java:911)
       at android.view.Choreographer.doCallbacks(Choreographer.java:686)
       at android.view.Choreographer.doFrame(Choreographer.java:622)
       at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:897)
       at android.os.Handler.handleCallback(Handler.java:739)
       at android.os.Handler.dispatchMessage(Handler.java:95)
       at android.os.Looper.loop(Looper.java:148)
       at android.app.ActivityThread.main(ActivityThread.java:7402)
       at java.lang.reflect.Method.invoke(Method.java)
       at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1230)
       at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1120)

---

您将收到以下错误：

致命异常：com.google.firebase.database.DatabaseException:无法将java.lang.String类型的对象转换为com.example.codechat.InstantMessage类型

由于以下代码行：

return snapshot.getValue(InstantMessage.class);

这是因为您的

myDB

引用指向实际包含字符串的

Chats

节点，而不是

InstantMessage

对象。要使其正常工作，数据库结构应如下所示：

Firebase-root
  |
  --- Chats
       |
       --- -MG3WxkPWaK-SFh7Q5M_
               |
               --- user: "Alex"
               |
               --- message: "Hi, Jandroid!"

DatabaseReference rootRef = FirebaseDatabase.getInstance().getReference();
DatabaseReference chatsRef = rootRef.child("Chats");
ValueEventListener valueEventListener = new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        for(DataSnapshot ds : dataSnapshot.getChildren()) {
            String user = ds.getKey();
            String message = ds.getValue(String.class);
            Log.d("TAG", user + " / " + message);
        }
    }

    @Override
    public void onCancelled(@NonNull DatabaseError databaseError) {
        Log.d("TAG", databaseError.getMessage()); //Don't ignore potential errors!
    }
};
chatsRef.addListenerForSingleValueEvent(valueEventListener);

因此，正如您所看到的，您不能拥有动态属性，您应该拥有

InstantMessage

类中的确切属性，这样您就可以将子对象映射到

InstantMessage

类的对象中

如果您不想使用POJO类，那么只需从实际结构中获取数据，如下所示：

Firebase-root
  |
  --- Chats
       |
       --- -MG3WxkPWaK-SFh7Q5M_
               |
               --- user: "Alex"
               |
               --- message: "Hi, Jandroid!"

DatabaseReference rootRef = FirebaseDatabase.getInstance().getReference();
DatabaseReference chatsRef = rootRef.child("Chats");
ValueEventListener valueEventListener = new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        for(DataSnapshot ds : dataSnapshot.getChildren()) {
            String user = ds.getKey();
            String message = ds.getValue(String.class);
            Log.d("TAG", user + " / " + message);
        }
    }

    @Override
    public void onCancelled(@NonNull DatabaseError databaseError) {
        Log.d("TAG", databaseError.getMessage()); //Don't ignore potential errors!
    }
};
chatsRef.addListenerForSingleValueEvent(valueEventListener);

日志中的结果将是：

-MG3WxkPWaK-SFh7Q5M_ / ynnh

---

请编辑您的问题，在您的数据库中包含

聊天

中的JSON示例（文本形式，请不要截图）。您可以通过单击溢出菜单中的“导出JSON”链接来实现这一点(⠇) 在您的上。您在

聊天

节点中拥有的应该是

InstantMessage

类型的对象？请用@AlexMamoYes回复，应该是InstantMessage@AlexMamoComments不用于扩展讨论；此对话已被取消。