Issues retrieving data on Android with Firebase

Question

I'm just learning how to set up Firebase data in Android. I am able to write to the db, but I am unable to read it and display it back on my app. My app is very simple (It just has one main screen for now) and I don't care how the data is displayed on the page as I am just learning - I'm trying to display it in a text view for now.

Here is my Main Activity java:

package com.example.ispotrachel.myapplication;

import android.support.v7.app.ActionBarActivity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.TextView;

import com.firebase.client.DataSnapshot;
import com.firebase.client.Firebase;
import com.firebase.client.FirebaseError;
import com.firebase.client.ValueEventListener;

import java.util.HashMap;
import java.util.Map;


public class MainActivity extends ActionBarActivity {

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    Firebase.setAndroidContext(this);

    Firebase ref = new Firebase("https://blistering-torch-9015.firebaseio.com");

    class User {
        private int birthYear;
        private String fullName;

        public User() {
        }

        public User(String fullName, int birthYear) {
            this.fullName = fullName;
            this.birthYear = birthYear;
        }

        public long getBirthYear() {
            return birthYear;
        }

        public String getFullName() {
            return fullName;
        }
    }

     User user1 = new User("Name 1", 1982);
     User user2 = new User("Name 2", 1972);

     Firebase usersRef = ref.child("users");

     Map<String, User> users = new HashMap<String, User>();
     users.put("user1", user1);
     users.put("user 2", user2);

    usersRef.setValue(users);

    Firebase postRef = ref.child("posts");

    Map<String, String> post1 = new HashMap<String, String>();
    post1.put("author", "user1");
    post1.put("title", "Announcing COBOL, a New Programming Language");
    postRef.push().setValue(post1);

    Map<String, String> post2 = new HashMap<String, String>();
    post2.put("author", "user2");
    post2.put("title", "The Turing Machine");
    postRef.push().setValue(post2);
    ref.addValueEventListener(new ValueEventListener() {


    @Override
    public void onDataChange(DataSnapshot arg0) {
            //System.out.println(arg0.getValue());
            TextView textViewSample = (TextView) findViewById(R.id.sampleTextView);
            textViewSample.setText(arg0.getValue(String.class));
        }

        @Override
        public void onCancelled(FirebaseError firebaseError) {
            System.out.println("the read failed");
        }

    });
}}

Here is my activity_main.xml:

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
 xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent"
 android:layout_height="match_parent"           android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
android:paddingBottom="@dimen/activity_vertical_margin"
tools:context=".MainActivity">


<TextView
    android:id="@+id/sampleTextView"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="@string/hello_world"/>


</LinearLayout>

When I run the app, it crashes. Here is the logcat error message:

12-27 12:15:02.678    1895-1895/com.example.ispotrachel.myapplication I/art﹕ Not late     enabling -Xcheck:jni (already on)
12-27 12:15:02.946    1895-1907/com.example.ispotrachel.myapplication I/art﹕ Background     sticky concurrent mark sweep GC freed 3867(286KB) AllocSpace objects, 0(0B) LOS objects, 11% free, 1009KB/1135KB, paused 2.595ms total 117.910ms
12-27 12:15:03.317    1895-1926/com.example.ispotrachel.myapplication D/OpenGLRenderer﹕ Render dirty regions requested: true
12-27 12:15:03.320    1895-1895/com.example.ispotrachel.myapplication D/﹕ HostConnection::get() New Host Connection established 0xa6667e80, tid 1895
12-27 12:15:03.327    1895-1895/com.example.ispotrachel.myapplication D/Atlas﹕ Validating map...
12-27 12:15:03.391    1895-1926/com.example.ispotrachel.myapplication D/﹕ HostConnection::get() New Host Connection established 0xa66ef3b0, tid 1926
12-27 12:15:03.405    1895-1926/com.example.ispotrachel.myapplication I/OpenGLRenderer﹕ Initialized EGL, version 1.4
12-27 12:15:03.422    1895-1926/com.example.ispotrachel.myapplication D/OpenGLRenderer﹕ Enabling debug mode 0
12-27 12:15:03.439    1895-1926/com.example.ispotrachel.myapplication W/EGL_emulation﹕ eglSurfaceAttrib not implemented
12-27 12:15:03.439    1895-1926/com.example.ispotrachel.myapplication W/OpenGLRenderer﹕ Failed to set EGL_SWAP_BEHAVIOR on surface 0xa66b7880, error=EGL_SUCCESS
12-27 12:15:07.474    1895-1926/com.example.ispotrachel.myapplication W/EGL_emulation﹕ eglSurfaceAttrib not implemented
12-27 12:15:07.474    1895-1926/com.example.ispotrachel.myapplication W/OpenGLRenderer﹕ Failed to set EGL_SWAP_BEHAVIOR on surface 0xa66b7880, error=EGL_SUCCESS
12-27 12:15:13.993    1895-1895/com.example.ispotrachel.myapplication D/AndroidRuntime﹕    Shutting down VM 

--------- beginning of crash
12-27 12:15:13.993    1895-1895/com.example.ispotrachel.myapplication E/AndroidRuntime﹕ FATAL EXCEPTION: main
Process: com.example.ispotrachel.myapplication, PID: 1895 com.firebase.client.FirebaseException: Failed to bounce to type 
        at com.firebase.client.DataSnapshot.getValue(DataSnapshot.java:196)
        at com.example.ispotrachel.myapplication.MainActivity$1.onDataChange(MainActivity.java:79) 
        at com.firebase.client.core.ValueEventRegistration$1.run(ValueEventRegistration.java:48)
        at android.os.Handler.handleCallback(Handler.java:739)
        at android.os.Handler.dispatchMessage(Handler.java:95)
        at android.os.Looper.loop(Looper.java:135)
        at android.app.ActivityThread.main(ActivityThread.java:5221)
        at java.lang.reflect.Method.invoke(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:372)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)

 Caused by: com.fasterxml.jackson.databind.JsonMappingException: Can not deserialize instance of java.lang.String out of START_OBJECT token
        at [Source: java.io.StringReader@c8f7297; line: 1, column: 1]
        at com.fasterxml.jackson.databind.DeserializationContext.mappingException(DeserializationContext.java:575)
        at com.fasterxml.jackson.databind.deser.std.StringDeserializer.deserialize(StringDeserializer.java:46)
        at com.fasterxml.jackson.databind.deser.std.StringDeserializer.deserialize(StringDeserializer.java:11)
        at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:2888)
        at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:2034)
        at com.firebase.client.DataSnapshot.getValue(DataSnapshot.java:192)
        at com.example.ispotrachel.myapplication.MainActivity$1.onDataChange(MainActivity.java:79)
        at com.firebase.client.core.ValueEventRegistration$1.run(ValueEventRegistration.java:48)
        at android.os.Handler.dispatchMessage(Handler.java:95)
        at android.os.Looper.loop(Looper.java:135)
        at android.app.ActivityThread.main(ActivityThread.java:5221)
        at java.lang.reflect.Method.invoke(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:372)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)device not found

Answer 1

tl;dr

It looks like you're trying to load a JSON object into a String.

The error message Failed to bounce to type indicates that the Firebase client got back some data, but is unable to convert it into the type you specified.

Details about the problem

You're adding a ValueEventListener to the root of your Firebase here:

ref.addValueEventListener(new ValueEventListener() { ...

In that value event listener you're telling Firebase to load the whole Firebase as a String:

textViewSample.setText(arg0.getValue(String.class));

And this would work fine if you had a very simple Firebase that only contained a single string. However, based on the rest of your code, it looks like your Firebase has a lot more than one String in it :)

How to fix it You have a couple of options:

1. Attach your event listener further down the tree. You could get user's full name as a string:

   ref.child("users/user1/fullName").addValueEventListener(new ValueEventListener() {
   
       @Override
       public void onDataChange(DataSnapshot arg0) {
           TextView textViewSample = (TextView) findViewById(R.id.sampleTextView);
           textViewSample.setText(arg0.getValue(String.class));
       }
       ...
   }
   

2. Or, you can get a whole user, but parse it as a User instead of a String:

   ref.child("users/user1/fullName").addValueEventListener(new ValueEventListener() {
   
       @Override
       public void onDataChange(DataSnapshot arg0) {
           User tempUser = arg0.getValue(User.class);
           TextView textViewSample = (TextView) findViewById(R.id.sampleTextView);
           textViewSample.setText(tempUser.getFullName());
       }
       ...
   }