Firebase android error “Failed to convert value of type ”

问题

This code is crashing on start;

E/AndroidRuntime: FATAL EXCEPTION: main Process: com.example.manya.eventspage, PID: 2974 com.google.firebase.database.DatabaseException: Failed to convert value of type java.util.HashMap to String at com.google.android.gms.internal.zzamy.zzcg(Unknown Source) at com.google.android.gms.internal.zzamy.zzb(Unknown Source) at com.google.android.gms.internal.zzamy.zza(Unknown Source) 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(Unknown Source) at com.google.android.gms.internal.zzakp.zzcxi(Unknown Source) at com.google.android.gms.internal.zzaks$1.run(Unknown Source) at android.os.Handler.handleCallback(Handler.java:751) at android.os.Handler.dispatchMessage(Handler.java:95) at android.os.Looper.loop(Looper.java:154) at android.app.ActivityThread.main(ActivityThread.java:6077) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755)

the code is

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);
}

The json data is

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

回答1:

Your code fails to take care of the fact that you have a list of events under /events/events. To handle this correctly you have two approaches:

1. Use a ValueEventListener and loop over the child nodes
2. Use a ChildEventListener

The first is closest to your current code and just adds an extra loop:

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
    }
});

The second approach tells Firebase to handle the list in its SDK and surfaces each child:

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);
    }

    ...
});

来源：https://stackoverflow.com/questions/44098562/firebase-android-error-failed-to-convert-value-of-type