Alright, let's dive into Firebase Authentication! This service lets you authenticate users in various ways—email and password, phone numbers, and even third-party providers like Google, Facebook, Twitter, and GitHub. Today, we're focusing on those third-party providers.

First things first, you need to set up a Firebase project and link it to your app.

• Head over to the Firebase console and create a new project. If you already have a Google Cloud Project, you can just select that one.
• Once your project is ready, click on 'Add Firebase to your Android app', fill in the required fields, and register your app.

Next up, let's enable the third-party identity provider you want to use.

• In the Firebase console, go to 'Authentication' and then 'Sign-in method'.
• Pick the provider you want to enable (like Google, Facebook, or Twitter).
• Toggle the 'Enable' switch and fill in the necessary details like 'App ID', 'App secret', and any other required fields.

Each provider will ask for different info, which you can find in their developer console.

Now, let's get to the fun part—coding the sign-in process.

Firebase recommends using the Firebase-UI library for this. It makes the login process super simple, like launching an activity and handling callbacks.

The code will vary for each provider, but here's a quick example for Google Sign-In:

private void signIn() {
    Intent signInIntent = mGoogleSignInClient.getSignInIntent();
    startActivityForResult(signInIntent, RC_SIGN_IN);
}
@Override
public void onActivityResult(int requestCode, int resultCode, Intent data) {
    super.onActivityResult(requestCode, resultCode, data);
    if (requestCode == RC_SIGN_IN) {
        Task<GoogleSignInAccount> task = GoogleSignIn.getSignedInAccountFromIntent(data);
        try {
            GoogleSignInAccount account = task.getResult(ApiException.class);
            firebaseAuthWithGoogle(account);
        } catch (ApiException e) {
            Log.w(TAG, "Google sign in failed", e);
        }
    }
}

In a nutshell, here's what you do:

• Send a sign-in request to the third-party provider.
• Handle the sign-in result.
• Authenticate with Firebase using the provider's credentials.

Once the user is authenticated, you can access their basic profile info and manage their sessions so they stay logged in.

You'll use the FirebaseUser object for this. It has methods to get profile info like display names, profile photos, email, phone number, and provider-specific details.

FirebaseUser user = FirebaseAuth.getInstance().getCurrentUser();
if (user != null) {
    String name = user.getDisplayName();
    String email = user.getEmail();
    Uri photoUrl = user.getPhotoUrl();
}

You can also check if a user is logged in with FirebaseAuth.getInstance().getCurrentUser(). If it returns null, no one is logged in.

Firebase takes care of session management for you, so users stay logged in across app sessions until they explicitly sign out using FirebaseAuth.getInstance().signOut().