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.

Practice

1. What is the main purpose of Firebase Authentication in an Android app?

easy

A. To store user data in a database

B. To send push notifications to users

C. To design the app's user interface

D. To securely manage user sign-in and registration

Solution

1. Step 1: Understand Firebase Authentication role

   Firebase Authentication is designed to handle user sign-in and registration securely.

2. Step 2: Differentiate from other Firebase services

   It does not store user data (that's Firestore/Realtime Database), nor handle UI design or notifications.

3. Final Answer:

   To securely manage user sign-in and registration -> Option D

4. Quick Check:

   Firebase Authentication = Secure sign-in [OK]

Hint: Remember Firebase Auth is for user sign-in only [OK]

Common Mistakes:

• Confusing Authentication with database storage
• Thinking it handles UI design
• Assuming it manages notifications

2. Which Kotlin code snippet correctly registers a new user with email and password using Firebase Authentication?

easy

A. firebaseAuth.registerUser(email, password).addOnCompleteListener { ... }

B. firebaseAuth.signInWithEmailAndPassword(email, password).addOnCompleteListener { ... }

C. firebaseAuth.createUserWithEmailAndPassword(email, password).addOnCompleteListener { ... }

D. firebaseAuth.createUser(email, password).addOnCompleteListener { ... }

Solution

1. Step 1: Identify correct Firebase method for registration

   The method to register a new user is createUserWithEmailAndPassword(email, password).

2. Step 2: Check method names in options

   registerUser and createUser do not exist. signInWithEmailAndPassword is for signing in existing users, not registration.

3. Final Answer:

   firebaseAuth.createUserWithEmailAndPassword(email, password).addOnCompleteListener { ... } -> Option C

4. Quick Check:

   Register new user = createUserWithEmailAndPassword [OK]

Hint: Register uses createUserWithEmailAndPassword method [OK]

Common Mistakes:

• Using signInWithEmailAndPassword to register
• Using non-existent methods like createUser or registerUser
• Confusing method names

3. Given this Kotlin code snippet, what will be printed if the user is signed in?

val user = firebaseAuth.currentUser
if (user != null) {
    println("User is signed in")
} else {
    println("No user signed in")
}

medium

A. User is signed in

B. NullPointerException error

C. No user signed in

D. Compilation error

Solution

1. Step 1: Understand currentUser property

   firebaseAuth.currentUser returns the current signed-in user or null if none.

2. Step 2: Analyze the if condition

   If user is not null, it prints "User is signed in"; otherwise, "No user signed in".

3. Final Answer:

   User is signed in -> Option A

4. Quick Check:

   currentUser != null means user signed in [OK]

Hint: currentUser null means no user; else signed in [OK]

Common Mistakes:

• Assuming currentUser throws error if null
• Confusing sign-in check logic
• Expecting compilation errors incorrectly

4. Identify the error in this Kotlin code for signing out a user:

firebaseAuth.signOut
println("User signed out")

medium

A. signOut is not a FirebaseAuth method

B. Missing parentheses after signOut method

C. println should be inside a callback

D. No error, code is correct

Solution

1. Step 1: Check signOut method usage

   signOut is a method and requires parentheses: signOut()

2. Step 2: Verify other options

   signOut exists in FirebaseAuth; println can be outside callback; so only missing parentheses is error.

3. Final Answer:

   Missing parentheses after signOut method -> Option B

4. Quick Check:

   signOut() needs parentheses [OK]

Hint: Methods need () even if no parameters [OK]

Common Mistakes:

• Forgetting parentheses on method calls
• Thinking signOut is a property
• Assuming println must be in callback

5. You want to create a function in Kotlin that registers a user and then immediately signs them in using Firebase Authentication. Which code snippet correctly does this?

hard

A. firebaseAuth.createUserWithEmailAndPassword(email, password).addOnCompleteListener { task -> if (task.isSuccessful) { println("User registered and signed in") } }

B. firebaseAuth.signInWithEmailAndPassword(email, password).addOnCompleteListener { task -> if (task.isSuccessful) { println("User registered and signed in") } }

C. firebaseAuth.createUserWithEmailAndPassword(email, password).addOnCompleteListener { task -> if (task.isSuccessful) { firebaseAuth.signInWithEmailAndPassword(email, password) } }

D. firebaseAuth.createUserWithEmailAndPassword(email, password).addOnCompleteListener { task -> if (task.isSuccessful) { firebaseAuth.signOut() } }

Solution

1. Step 1: Understand createUserWithEmailAndPassword behavior

   This method registers and automatically signs in the user if successful.

2. Step 2: Analyze options for correct flow

   createUserWithEmailAndPassword(...).addOnCompleteListener { task -> if (task.isSuccessful) { println("User registered and signed in") } } registers and confirms success. signInWithEmailAndPassword only signs in existing users. Calling signIn after createUser is redundant. Calling signOut after is incorrect.

3. Final Answer:

   firebaseAuth.createUserWithEmailAndPassword(email, password).addOnCompleteListener { task -> if (task.isSuccessful) { println("User registered and signed in") } } -> Option A

4. Quick Check:

   Registration auto signs in [OK]

Hint: Register auto signs in; no need to call signIn after [OK]

Common Mistakes:

• Calling signIn after createUserWithEmailAndPassword unnecessarily
• Using signIn without registering first
• Signing out immediately after registration