Firebase Authentication in Android Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ ui_behavior

intermediate

What happens after successful Firebase email/password sign-in?

Consider this Kotlin code snippet for Firebase Authentication in an Android app:

auth.signInWithEmailAndPassword(email, password)
  .addOnCompleteListener { task ->
    if (task.isSuccessful) {
      // What happens here?
    } else {
      // Handle failure
    }
  }

What is the expected behavior inside the if (task.isSuccessful) block?

Android Kotlin

auth.signInWithEmailAndPassword(email, password)
  .addOnCompleteListener { task ->
    if (task.isSuccessful) {
      // Navigate to main screen
    } else {
      // Show error message
    }
  }

A The app crashes due to missing exception handling.

B The app logs out the user immediately.

C The app shows a loading spinner indefinitely without navigation.

D The app navigates to the main screen and the user is considered signed in.

💡 Hint

Think about what happens when sign-in succeeds in Firebase Authentication.

When Firebase sign-in succeeds, the user is authenticated and the app typically navigates to the main content screen to allow access.

📝 Syntax

intermediate

Identify the syntax error in Firebase user creation code

Which option contains a syntax error in this Firebase Authentication Kotlin code snippet for creating a user?

auth.createUserWithEmailAndPassword(email, password)
  .addOnCompleteListener(this) { task ->
    if (task.isSuccessful) {
      // User created
    }
  }

Android Kotlin

auth.createUserWithEmailAndPassword(email, password)
  .addOnCompleteListener(this) { task ->
    if (task.isSuccessful) {
      // User created
    }
  }

A auth.createUserWithEmailAndPassword(email, password).addOnCompleteListener(this) { task -> if task.isSuccessful { /* User created */ } }

B auth.createUserWithEmailAndPassword(email, password).addOnCompleteListener(this) { task -> if (task.isSuccessful) { /* User created */ } }

C auth.createUserWithEmailAndPassword(email, password).addOnCompleteListener(this, { task -> if (task.isSuccessful) { /* User created */ } })

D auth.createUserWithEmailAndPassword(email, password).addOnCompleteListener { task -> if (task.isSuccessful) { /* User created */ } }

💡 Hint

Check the syntax of the if statement inside the lambda.

In Kotlin, the if condition must be enclosed in parentheses. Option A misses parentheses around the condition, causing a syntax error.

❓ lifecycle

advanced

What happens if you call FirebaseAuth.getInstance() multiple times?

In an Android app using Firebase Authentication, what is the behavior when you call FirebaseAuth.getInstance() multiple times in different parts of your app?

Android Kotlin

val auth1 = FirebaseAuth.getInstance()
val auth2 = FirebaseAuth.getInstance()

A Each call returns a new independent FirebaseAuth instance.

B The app crashes due to multiple instances.

C Each call returns the same singleton FirebaseAuth instance.

D The second call returns null if called too soon.

💡 Hint

Think about how FirebaseAuth manages its instances internally.

FirebaseAuth uses a singleton pattern, so multiple calls to getInstance() return the same instance managing the user's authentication state.

🔧 Debug

advanced

Why does this Firebase sign-in code never call onComplete?

Given this Kotlin code snippet:

auth.signInWithEmailAndPassword(email, password)
  .addOnCompleteListener { task ->
    if (task.isSuccessful) {
      // Success
    } else {
      // Failure
    }
  }

But the addOnCompleteListener callback never runs. What is the most likely cause?

Android Kotlin

auth.signInWithEmailAndPassword(email, password)
  .addOnCompleteListener { task ->
    if (task.isSuccessful) {
      // Success
    } else {
      // Failure
    }
  }

A The email and password variables are empty strings.

B The app is missing internet permission in AndroidManifest.xml.

C The FirebaseAuth instance was never initialized properly.

D The addOnCompleteListener is called on a background thread.

💡 Hint

Check app permissions related to network access.

Without internet permission, Firebase cannot communicate with its servers, so the sign-in request never completes and the callback is never called.

🧠 Conceptual

expert

What is the effect of calling signOut() on FirebaseAuth instance?

In an Android app using Firebase Authentication, what is the effect of calling FirebaseAuth.getInstance().signOut()?

Android Kotlin

FirebaseAuth.getInstance().signOut()

A It clears the current user session and triggers auth state listeners to update.

B It deletes the user account permanently from Firebase.

C It only clears cached credentials but keeps the user signed in.

D It logs out the user but does not notify any listeners.

💡 Hint

Think about what signOut means for user session and app state.

Calling signOut clears the current user session and triggers any registered auth state listeners to update the UI accordingly.