Retrieving data from firebase

suggest change

I am gonna assume you already know about adding gradle dependencies firebase in android studio. If you don’t just follow the guide from here. Add your app in firebase console, gradle sync android studio after adding dependencies. All dependencies are not needed just firebase database and firebase auth.

Now that we know how data is stored and how to add gradle dependencies let’s see how to use the imported firebase android SDK to retrieve data.

create a firebase database reference

DatabaseReference userDBRef = FirebaseDatabase.getInstance().getReference();
// above statement point to base tree
userDBRef = DatabaseReference.getInstance().getReference().child("user_base")
// points to user_base table JSON (see previous section)

from here you can chain multiple child() method calls to point to the data you are interested in. For example if data is stored as depicted in previous section and you want to point to Bruce Wayne user you can use:

DatabaseReference bruceWayneRef = userDBRef.child("371298");
// 371298 is key of bruce wayne user in JSON structure (previous section)

Or simply pass the whole reference to the JSON object:

DatabaseReference bruceWayneRef = DatabaseReference.getInstance().getReference()
     .child("user_base/371298");
// deeply nested data can also be referenced this way, just put the fully
// qualified path in pattern shown in above code "blah/blah1/blah1-2/blah1-2-3..."

Now that we have the reference of the data we want to fetch, we can use listeners to fetch data in android apps. Unlike the traditional calls where you fire REST API calls using retrofit or volley, here a simple callback listener is required to get the data. Firebase sdk calls the callback methods and you are done.

There are basically two types of listeners you can attach, one is ValueEventListener and the other one is ChildEventListener (described in next section). For any change in data under the node we have references and added listeners to, value event listeners return the entire JSON structure and child event listener returns specific child where the change has happened. Both of these are useful in their own way. To fetch the data from firebase we can add one or more listeners to a firebase database reference (list userDBRef we created earlier).

Here is some sample code (code explanation after code):

userDBRef.addValueEventListener(new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        User bruceWayne = dataSnapshot.child("371298").getValue(User.class);
        // Do something with the retrieved data or Bruce Wayne
    }

    @Override
    public void onCancelled(DatabaseError databaseError) {
        Log.e("UserListActivity", "Error occured");
        // Do something about the error
    });

Did you notice the Class type passed. DataSnapshot can convert JSON data into our defined POJOs, simple pass the right class type.

If your use case does not require the entire data (in our case user_base table) every time some little change occurs or say you want to fetch the data only once, you can use addListenerForSingleValueEvent() method of Database reference. This fires the callback only once.

userDBRef.addListenerForSingleValueEvent(new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        // Do something
    }

    @Override
    public void onCancelled(DatabaseError databaseError) {
        // Do something about the error
    });

Above samples will give you the value of the JSON node. To get the key simply call:

String myKey = dataSnapshot.getKey();

Found a mistake? Have a question or improvement idea? Let me know.

Firebase Realtime Database:

* Firebase Realtime DataBase

* Quick setup

* Firebase Realtime DataBase event handler

* Designing and understanding how to retrieve realtime data from the Firebase Database

* Denormalization Flat Database Structure

* Understanding firebase JSON database

* Retrieving data from firebase

* Listening for child updates

* Retrieving data with pagination

Table Of Contents

0 Getting Started

1 Layouts

2 Gradle

3 RecyclerView onClickListeners

4 NavigationView

5 Intent

6 JSON handling with org.json

7 Android Studio

8 Resources

9 Data Binding Library

10 Exceptions

11 Getting Calculated View Dimensions

12 AsyncTask

13 SharedPreferences

14 Emulator

15 Material Design

16 Lint Warnings

17 Service

18 Storing Files in Internal and External Storage

19 WebView

20 Project SDK versions

21 RecyclerView

22 Google Maps

23 PorterDuff Mode

24 9-Patch images

25 Android NDK

26 RecyclerView Decorations

27 Camera 2 API

28 ViewPager

29 CardView

30 HttpURLConnection

31 SQLite

32 ADB Android Debug Bridge

33 ButterKnife

34 Supporting different screen resolutions

35 Glide

36 Retrofit2

37 Dialog

38 ACRA

39 GreenDAO

40 Formatting Strings

41 Notifications

42 AlarmManager

43 Fragments

44 Handler

45 Creating Custom Views

46 BroadcastReceiver

47 Activity

48 Snackbar

49 Runtime Permissions in API-23

50 Logging and using Logcat

51 VectorDrawable and AnimatedVectorDrawable

52 Tools Attributes

53 Toast

54 Interfaces

55 Animators

56 Location

57 Theme Style Attribute

58 The Manifest File

59 Parcelable

60 MediaPlayer

61 Multidex and the Dex Method Limit

62 Data Synchronization with Sync Adapter

63 Menu

64 Instant Run in Android Studio

65 Picasso

66 Bluetooth LE API

67 RoboGuice

68 Memory Leaks

69 Universal Image Loader

70 Volley

71 Widgets

72 Date and Time Pickers

73 Integrate Google Sign In

74 In-app billing

75 FloatingActionButton

76 ContentProvider

77 Dagger 2

78 Realm

79 Unit testing in Android with JUnit

80 Android Versions

81 Wi-Fi Connections

82 SensorManager

83 Localization with resources

84 ProgressBar

85 Custom Fonts

86 Vibration

87 Google Awarness APIs

88 Text to Speech (TTS)

89 UI Lifecycle

90 Spinner

91 Data Encryption / Decryption

92 Testing UI with Espresso

93 Writing UI tests

94 GreenRobot EventBus

95 OkHttp

96 Enhancing Performance Using Icon Fonts

97 Handling Depp Links

98 Canvas drawing using SurfaceView

99 Firebase

100 Crash Reporting Tools

101 Check Internet Connectivity

102 Facebook SDK

103 Unzip File

104 Android Places API

105 Creating your own libraries

106 Handling JSON with Gson

107 Device Display Metrics

108 TextView

109 ListView

110 Building Backwards Compatible Apps

111 Loader

112 ProGuard - Obfuscating and Shrinking your code

113 Detect Shake Event

114 Typedef Annotations

115 Capturing Screenshots

116 MVP Architecture

117 Orientation Changes

118 Xposed

119 Security

120 PackageManager

121 ImageView

122 Gesture Detection

123 Doze Mode

124 Android Sound and Media

125 SearchView

126 Camera and Gallery

127 Callback URL

128 Twitter APIs

129 SqlCipher integration

130 Drawables

131 Colors

132 ConstraintLayout

133 RenderScript

134 Fresco

135 Swipe to Refresh

136 AutoCompleteTextView

137 Installing apps with ADB

138 IntentService

139 AdMob

140 Implicit Intents

141 Publish to Play Store

142 Firebase Realtime Database

143 Image Compression

144 Email Validation

145 Keyboard

146 Button

147 TextInputLayout

148 Bottom Sheets

149 CoordinatorLayout and Behaviors

150 EditText

151 Paypal Gateway Integration

152 Firebase App Indexing

153 Firebase Crash Reporting

154 Displaying Google Ads

155 Vk SDK

156 Localized DateTime

157 Count Down Timer

158 Reading Barcode and QR codes

159 Otto Event Bus

160 TransitionDrawable

161 Port Mapping using Cling

162 Creating Overlay always-on-top Windows

163 ExoPlayer

164 Inter-app UI testing with UIAutomator

165 MediaSession

166 Speech to Text Conversion

167 FileProvider

168 Publish .aar file to Apache Archive with Gradle

169 XMPP

170 Authenticator

171 RecyclerView and LayoutManagers

172 AudioManager

173 Job Scheduling

174 Accounts and AccountManager

175 OpenCV

176 Split Screen Multi-Screen Activities

177 Threads

178 MediaStore

179 Time Utils

180 Touch Events

181 Fingerprint API

182 MVVM Architecture

183 BottomNavigationView

184 ORMLite

185 YouTube API

186 TabLayout

187 Retrofit2 with RxJava

188 DayNight Theme AppCompat

189 ShortcutManager

190 LruCache

191 Using Jenkins CI

192 Zip files

193 Vector Drawables

194 Fastlane

195 Define step value for RangeSeekBar

196 Getting started with OpenGL ES 2.0

197 Check Data Connection

198 Java Native Interface (JNI)

199 FileIO

200 Perfomance Optimization

201 Robolectric

202 Moshi

203 Strick Mode Policy

204 Internalization and localization

205 Retrolambda

206 SparseArray

207 Firebase Cloud Messaging

208 Shared Element Transitions

209 Android Things

210 VideoView

211 ViewFlipper

212 Dependency Injection with Dagger 2

213 Formatting phone numbers

214 Storing password securely

215 Android Kernel Optimization

216 Paint

217 AudioTrack

218 ProGuard

219 Create Android Custom ROMs

220 Java on Android

221 Pagination in RecyclerVi

222 Genymotion for Android

223 Handling touch and motion events

224 Creating Splash screen

225 ConstraintSet

226 CleverTap

227 Publish library to Maven repositories

228 ADB shell

229 Ping ICMP

230 AIDL

231 Using Kotlin

232 Autosizing TextView

233 Signing apps for release

234 Context

235 Activity Recognition

236 Secure SharedPreferences

237 Secure SharedPreferences

238 Bitmap Cache

239 Android x86 in VirtualBox

240 JCodec

241 Design Patterns

242 Okio

243 Google SignIn integration

244 TensorFlow

245 Game developmen

246 Notification Channel

247 Bluetooth Low Energy (LE)

248 Leak Canary

249 FuseView

250 SQLite using ContentValues

251 Enhancing Alert Dialogs

252 Hardware Button Events, Intents PTT

253 SpannableString

254 Looper

255 Optimized VideoView

256 Google Drive API

257 Animated AlertDialog Box

258 Annotation Processor

259 SyncAdapter with periodically do sync of data

260 Singleton Class for Toast Message

261 Fastjson

262 Android Architecture Components

263 Jackson

264 Play Store

265 Loading Bitmaps Effectively

266 Getting system font names

267 Smartcard

268 Convert vietnamese string to english

269 Contributors