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How to Build an Android Chat Application in Kotlin

· 3 min read
Beniamin Marcu
Full Stack Developer

In this step-by-step tutorial we'll build a real-time Android chat application in Kotlin. This guide reuses the full source code from our premium Android Chat App.

How to Build an Android Chat Application in Kotlin​

android chat application tutorial

To build a fully-fledged Android chat application you should include these core features:

  • Real-time Messaging
  • Photo Messaging
  • Private Messaging
  • Group Chatting
  • Push Notifications
  • Messages Archive
  • Online Status

We leverage Firebase as the backend. More specifically we use:

  • Firebase Auth — user management (Login, Registration, Facebook Login, Google Login, etc.)
  • Firebase Firestore — real-time database for users and messages
  • Firebase Storage — stores photos and videos sent from the app
  • Firebase Cloud Messaging — sends real-time push notifications to message recipients

High-level steps to get started:

  1. Open & Run the Kotlin project in Android Studio
  2. Override google-services.json with your own file (downloaded from Firebase Console). This will wire the chat application to your Firebase project. Enable Firebase Auth providers and Firebase Storage; for Firestore, set read/write rules appropriately during development.
  3. Publish the app to Google Play Store

It looks straightforward — we'll walk through these milestones and also cover enabling push notifications.

Enable Push Notifications Safely​

Firebase Cloud Messaging send requests must run in a trusted environment such as Cloud Functions or your application server. Never embed a service-account key, OAuth token, or legacy server key in the Android app. Anything shipped in the APK can be extracted.

Use this flow instead:

  1. Configure the Android client with your Firebase project's google-services.json.
  2. Retrieve the app instance's FCM registration token and send it to your authenticated backend.
  3. Store tokens with the owning user or installation and update them when Firebase rotates them.
  4. Send notifications from the backend with the Firebase Admin SDK or the FCM HTTP v1 API.
  5. Handle invalid or expired tokens and remove them from the backend.

The backend authenticates with Application Default Credentials, Workload Identity, or another securely stored service-account credential. Follow Firebase's official guides for the trusted server environment and FCM HTTP v1 API.

Test foreground, background, and terminated-app behavior before release. Also verify notification permission handling on supported Android versions.