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Advanced 90 min Java Full project

Chat App in Android Studio with Firebase

Build a real-time chat app with Firebase: users sign up, sign in, see each other and send messages that appear instantly. A big project that ties together authentication, a cloud database and lists.

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What you'll learn

Sign users up and in with Firebase Authentication
Store users and messages in Realtime Database
Show conversations with RecyclerView
Update the chat live as messages arrive

Before you start

Android Studio (any recent version)
Basic Java: variables, methods and if/else
A free Firebase project (console.firebase.google.com)
An emulator or an Android phone

How it fits together

1Sign in or create account
2users/{uid}
3Pick a person
4chats/{chatId}/messages
5Both phones update live

Build it step by step

1. Create a Firebase project

In the Firebase console, add an Android app with your package name (com.example.chatapp to copy the code unchanged) and download google-services.json into the app folder.

2. Turn on sign-in and the database

Authentication → Sign-in method: enable Email/Password.
Realtime Database → Create database. Then open its Rules tab, paste the rules from Required setup, and press Publish. They let only signed-in users see the people list, and only the two people in a chat read it.

3. Add Firebase to the app

In app/build.gradle.kts, add the plugin and dependencies:

kotlin

plugins {
    id("com.google.gms.google-services")
}

dependencies {
    implementation(platform("com.google.firebase:firebase-bom:35.1.0"))
    implementation("com.google.firebase:firebase-auth")
    implementation("com.google.firebase:firebase-database")
}

In the project-level build.gradle.kts, add:

kotlin

plugins {
    id("com.google.gms.google-services") version "4.5.0" apply false
}

Then press Sync Now. Also add <uses-permission android:name="android.permission.INTERNET" /> to the manifest.

4. Plan the data

The database is one big tree:

text

users/
  {uid}: { name, email }
chats/
  {uidA_uidB}/messages/
    {pushId}: { sender, text, time }

A chat's id is the two people's uids in alphabetical order, so both phones open the same chat.

5. Sign in or create an account

LoginActivity uses one screen for both: New here? Create an account shows the name box. createUserWithEmailAndPassword makes the account, then the name and email are saved at users/{uid} so others can find this person. Returning users who are still signed in skip straight to the list.

6. List the people

UsersActivity adds a ValueEventListener to users while the screen is visible. Each time anyone joins, onDataChange gets everyone; leave yourself out and hand the list to UserAdapter.

7. Send a message

In ChatActivity, Send calls messages.push().setValue(...) with the sender, the text and ServerValue.TIMESTAMP. push() creates a key that sorts by time; the server fills in the time, so every phone agrees on the order.

8. Receive messages live

A ChildEventListener calls onChildAdded once for every existing message, then again for each new one. Add it to the adapter and scroll to the bottom. Remove the listener in onDestroy.

9. Draw the bubbles

MessageAdapter has two view types: if you sent the message, use the purple right-hand bubble; otherwise the grey left-hand one. setStackFromEnd(true) starts the list at the newest message.

10. Run it

Run on two emulators (or an emulator and a phone). Create a different account on each, open each other in the list, and chat.

If something goes wrong

"Permission denied": publish the rules from step 2, and make sure you're signed in.
The list is empty: you need a second account; you never see yourself.
Sign-up fails: Email/Password must be enabled, and passwords need at least 6 characters.

What's improved over the video

Sign-in and sign-up share one clean screen, chats use server timestamps, the database rules keep chats private, bubbles follow Material 3, and the message bar stays above the keyboard.

Files affected

Check what to add and what to replace before copying code.

app/build.gradle.kts and build.gradle.ktsAdd these lines
Firebase console → Realtime Database → RulesReplace file
app/src/main/res/layout/activity_chat.xmlAdd these lines
app/src/main/res/layout/activity_login.xmlAdd these lines
app/src/main/res/layout/activity_users.xmlAdd these lines
app/src/main/AndroidManifest.xmlReplace file
app/src/main/res/drawable/bubble_received.xmlAdd these lines
app/src/main/res/drawable/bubble_sent.xmlAdd these lines
app/src/main/java/com/example/chatapp/ChatActivity.javaAdd these lines
app/src/main/res/drawable/circle.xmlAdd these lines
app/src/main/res/values/colors.xmlAdd these lines
app/src/main/res/drawable/ic_back.xmlAdd these lines
app/src/main/res/drawable/ic_logout.xmlAdd these lines
app/src/main/res/drawable/ic_send.xmlAdd these lines
app/src/main/res/drawable/input_box.xmlAdd these lines
app/src/main/res/layout/item_message_received.xmlAdd these lines
app/src/main/res/layout/item_message_sent.xmlAdd these lines
app/src/main/res/layout/item_user.xmlAdd these lines
app/src/main/java/com/example/chatapp/LoginActivity.javaAdd these lines
app/src/main/java/com/example/chatapp/Message.javaAdd these lines
app/src/main/java/com/example/chatapp/MessageAdapter.javaAdd these lines
app/src/main/java/com/example/chatapp/Screens.javaAdd these lines
app/src/main/res/values/strings.xmlReplace file
app/src/main/res/values/themes.xmlReplace file
app/src/main/java/com/example/chatapp/User.javaAdd these lines
app/src/main/java/com/example/chatapp/UserAdapter.javaAdd these lines
app/src/main/java/com/example/chatapp/UsersActivity.javaAdd these lines

Required setup

These are additions to an existing project—not replacement project files.

Add Firebase Authentication and Realtime Database

Add these lines

Where: app/build.gradle.kts and build.gradle.kts

Add the plugin and dependencies, put google-services.json in the app folder, then Sync Now.

app/build.gradle.kts and build.gradle.kts

// app/build.gradle.kts
plugins {
    id("com.google.gms.google-services")
}

dependencies {
    implementation(platform("com.google.firebase:firebase-bom:35.1.0"))
    implementation("com.google.firebase:firebase-auth")
    implementation("com.google.firebase:firebase-database")
}

// project-level build.gradle.kts
plugins {
    id("com.google.gms.google-services") version "4.5.0" apply false
}

Database rules

Replace file

Where: Firebase console → Realtime Database → Rules

Paste these rules and press Publish. Only signed-in users can read the people list, and only the two people in a chat can read it.

Firebase console → Realtime Database → Rules

{
  "rules": {
    "users": {
      ".read": "auth != null",
      "$uid": {
        ".write": "auth != null && auth.uid === $uid"
      }
    },
    "chats": {
      "$chatId": {
        ".read": "auth != null && $chatId.contains(auth.uid)",
        ".write": "auth != null && $chatId.contains(auth.uid)"
      }
    }
  }
}

Application code

Select a file to see its code and where it belongs.

This code uses package com.example.chatapp. Change that first line to your own package, the one at the top of your MainActivity.

Add these linesapp/src/main/res/layout/activity_chat.xml

Header, messages, and the type-and-send bar.

activity_chat.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/main"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    tools:context=".ChatActivity">

    <!-- Header: back, avatar and name -->
    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="64dp"
        android:gravity="center_vertical"
        android:orientation="horizontal"
        android:paddingHorizontal="8dp">

        <ImageButton
            android:id="@+id/btnBack"
            android:layout_width="48dp"
            android:layout_height="48dp"
            android:background="?attr/selectableItemBackgroundBorderless"
            android:contentDescription="@string/back"
            android:src="@drawable/ic_back" />

        <TextView
            android:id="@+id/textAvatar"
            style="@style/Avatar"
            android:layout_marginStart="4dp"
            tools:text="A" />

        <TextView
            android:id="@+id/textName"
            android:layout_width="0dp"
            android:layout_height="wrap_content"
            android:layout_marginStart="12dp"
            android:layout_weight="1"
            android:maxLines="1"
            android:textSize="20sp"
            android:textStyle="bold"
            tools:text="Asha" />
    </LinearLayout>

    <androidx.recyclerview.widget.RecyclerView
        android:id="@+id/recyclerMessages"
        android:layout_width="match_parent"
        android:layout_height="0dp"
        android:layout_weight="1"
        android:clipToPadding="false"
        android:paddingVertical="8dp"
        tools:listitem="@layout/item_message_received" />

    <!-- Type and send -->
    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:gravity="bottom"
        android:orientation="horizontal"
        android:padding="12dp">

        <EditText
            android:id="@+id/inputMessage"
            android:layout_width="0dp"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:background="@drawable/input_box"
            android:hint="@string/hint_message"
            android:importantForAutofill="no"
            android:inputType="textMultiLine|textCapSentences"
            android:maxLines="4"
            android:minHeight="52dp"
            android:paddingHorizontal="20dp"
            android:textSize="17sp" />

        <ImageButton
            android:id="@+id/btnSend"
            android:layout_width="52dp"
            android:layout_height="52dp"
            android:layout_marginStart="8dp"
            android:background="@drawable/circle"
            android:contentDescription="@string/send"
            android:src="@drawable/ic_send" />
    </LinearLayout>
</LinearLayout>

Tutorial layout preview

Browser renderer

Approximate illustration of this video’s supplied layout. App logic is not executed.

9:41360 × 640 dp

Back

A

Asha

RecyclerView item 1RecyclerView item 2RecyclerView item 3

Message

Send

100% zoom · 360 × 640 dp layout area

4 preview limitations

Project strings, drawables, dimensions, and theme resources are not loaded; unresolved values are labeled in the preview.
Paste your themes.xml or styles.xml into values.xml to see @style/Avatar.
LinearLayout weights are supported for simple sibling distributions; nested weighted layouts may differ from Android.
RecyclerView items come from your adapter, so the preview shows placeholder rows.

Approximate Android Views rendering, not Android Studio or a device emulator. System bars are simulated; themes need Android Studio for exact results.

Edit a copy in UI Playground

Full source code

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