Firebase android 錯誤 "Failed to convert value of type "

Question

此代码在启动时崩溃;

E/Android运行时：致命异常：main 进程： com.example.manya.eventspage， PID： 2974 com.google.firebase.database.DatabaseException：无法将 java.util.HashMap 类型的值转换为字符串 at com.google.android.gms.internal.zzamy.zzcg(未知来源) at com.google.android.gms.internal.zzamy.zzb(未知来源) at com.google.android.gms.internal.zzamy.zza(未知来源) at com.google.firebase.database.DataSnapshot.getValue(Unknown Source) at com.example.manya.eventspage.Events$1.onDataChange(Events.java：46) at com.google.android.gms.internal.zzajp.zza(未知来源) at com.google.android.gms.internal.zzakp.zzcxi(未知来源) at com.google.android.gms.internal.zzaks$1.run(未知来源) at android.os.Handler.handleCallback(Handler.java：751) at android.os.Handler.dispatchMessage(Handler.java：95) at android.os.Looper.loop(Looper.java：154) at android.app.ActivityThread.main(ActivityThread.java：6077) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java：865) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java：755)

代码是

package com.example.manya.eventspage;
import android.app.DatePickerDialog;
import android.app.TimePickerDialog;
import android.content.Context;
import android.content.Intent;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import java.text.*;
import java.text.DateFormat;
import java.util.ArrayList;
import java.util.Calendar;
import java.util.List;
import android.view.Menu;
import android.view.MenuInflater;
import android.view.MenuItem;
import android.view.View;
import android.widget.*;
import com.google.firebase.database.DataSnapshot;
import com.google.firebase.database.DatabaseError;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;
import com.google.firebase.database.ValueEventListener;
public class Events extends AppCompatActivity {
private TextView t;
private static ListView l;
private static ArrayAdapter<String> adapter;
final static ArrayList<String> arrayList = new ArrayList<String>();
FirebaseDatabase database=FirebaseDatabase.getInstance();
protected DatabaseReference myRef;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_events);
t = (TextView) findViewById(R.id.textView3);
l = (ListView) findViewById(R.id.list_view);
myRef=database.getReference().child("events");
myRef.addValueEventListener(new ValueEventListener() {
@Override
public void onDataChange(DataSnapshot dataSnapshot) {
String val=dataSnapshot.child("events").getValue(String.class);
arrayList.add(val);
}
@Override
public void onCancelled(DatabaseError databaseError) {
}
});
// Adapter: You need three parameters 'the context, id of the layout (it will be where the data is shown),
// and the array that contains the data
adapter = new ArrayAdapter<String>(getApplicationContext(), android.R.layout.simple_spinner_item, arrayList);
l.setAdapter(adapter);
}
protected void onResume()
{
super.onResume();
Intent in=getIntent();
if(in.hasExtra(Intent.EXTRA_TEXT))
{

String d=in.getStringExtra(Intent.EXTRA_TEXT);
final DatabaseReference postsRef = database.getReference().child("events");
DatabaseReference newPostRef = postsRef.push();
newPostRef.setValue(new event(d));
arrayList.add(d);
// next thing you have to do is check if your adapter has changed
adapter.notifyDataSetChanged();
}
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
MenuInflater inflater = getMenuInflater();
inflater.inflate(R.menu.menu, menu);
return true;
}
public boolean onOptionsItemSelected(MenuItem item) {
int id = item.getItemId();
if (id == R.id.add_event) {
Context context=Events.this;
Class destclass=addevent.class;
Intent str=new Intent(context,destclass);
startActivity(str);
return true;
}
return super.onOptionsItemSelected(item);
}

json 数据是

{
"events" : {
"events" : {
"-KkfUKAYAriWN7NYqTcx" : {
"event_detail" : "21 05 2017, 20:20TextView"
}
}
}
}

Answer 1

您的代码无法处理您在/events/events下有一个事件列表的事实。要正确处理此问题，您有两种方法：

1. 使用ValueEventListener并遍历子节点
2. 使用ChildEventListener

第一个最接近您当前的代码，只是添加一个额外的循环：

myRef=database.getReference().child("events/events"); // note the change in path
myRef.addValueEventListener(new ValueEventListener() {
@Override
public void onDataChange(DataSnapshot dataSnapshot) {
for (DataSnapshot childSnapshot: dataSnapshot.getChildren()) {
String val=childSnapshot.getValue(String.class);
arrayList.add(val);
}
}
@Override
public void onCancelled(DatabaseError databaseError) {
throw databaseError.toException(); // don't ignore errors
}
});

第二种方法告诉 Firebase 在其 SDK 中处理列表并显示每个子项：

myRef=database.getReference().child("events/events"); // note the change in path
myRef.addChildEventListener(new ChildEventListener() {
@Override
public void onChildAdded(DataSnapshot dataSnapshot, String previousChildKey) {
String val=dataSnapshot.getValue(String.class);
arrayList.add(val);
}
...
});