Biometric authentication in Android Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ ui_behavior

intermediate

What happens when biometric authentication succeeds?

In an Android app using BiometricPrompt, what is the typical behavior after the user successfully authenticates with fingerprint or face?

A The app proceeds to the secured content or action immediately.

B The app closes automatically without any message.

C The app shows an error message and asks to try again.

D The app restarts the authentication prompt endlessly.

💡 Hint

Think about what the app wants to do after confirming the user is authorized.

When biometric authentication succeeds, the app usually continues to the protected feature or content without interruption.

📝 Syntax

intermediate

Identify the correct way to create a BiometricPrompt.PromptInfo

Which Kotlin code snippet correctly creates a BiometricPrompt.PromptInfo with title "Login" and negative button text "Cancel"?

A

val promptInfo = BiometricPrompt.PromptInfo.Builder()
  .setTitle("Login")
  .setNegativeButtonText("Cancel")
  .build()

B

val promptInfo = BiometricPrompt.PromptInfo()
  .setTitle("Login")
  .setNegativeButtonText("Cancel")
  .build()

C

val promptInfo = BiometricPrompt.PromptInfo.Builder
  .setTitle("Login")
  .setNegativeButtonText("Cancel")
  .build()

D

val promptInfo = BiometricPrompt.PromptInfo.Builder()
  .title("Login")
  .negativeButtonText("Cancel")
  .build()

💡 Hint

Remember to call Builder() as a function and use the correct setter methods.

The Builder class must be instantiated with parentheses, and the setter methods are setTitle and setNegativeButtonText.

❓ lifecycle

advanced

When should you invalidate the BiometricPrompt callback?

In an Android app using BiometricPrompt, when is the best time to invalidate or cancel the biometric authentication callback to avoid memory leaks?

A Right after calling authenticate() method.

B Only when the user cancels the authentication manually.

C In the onDestroy() lifecycle method of the Activity or Fragment.

D Never, the system handles it automatically.

💡 Hint

Think about cleaning up resources when the UI component is no longer visible.

To avoid memory leaks, you should cancel or invalidate biometric callbacks in onDestroy() or equivalent lifecycle methods.

🧠 Conceptual

advanced

Why use BiometricPrompt over FingerprintManager?

Why is it recommended to use BiometricPrompt API instead of the older FingerprintManager API in Android apps?

A BiometricPrompt requires no permissions, while FingerprintManager does.

B BiometricPrompt supports multiple biometric types like face and iris, not just fingerprint.

C FingerprintManager is faster and more secure than BiometricPrompt.

D FingerprintManager works on all Android versions, BiometricPrompt only on the latest.

💡 Hint

Consider the types of biometric authentication supported.

BiometricPrompt is a unified API that supports fingerprint, face, and iris authentication, making it more versatile.

🔧 Debug

expert

What error occurs if you call authenticate() without setting PromptInfo?

Given this Kotlin code snippet: val biometricPrompt = BiometricPrompt(this, executor, callback) biometricPrompt.authenticate() What error will this cause at runtime?

A SecurityException because permissions are missing.

B NullPointerException because callback is null.

C No error, authentication starts with default settings.

D IllegalArgumentException because PromptInfo is missing.

💡 Hint

Check the required parameters for authenticate() method.

The authenticate() method requires a PromptInfo object; calling it without one throws IllegalArgumentException.

Practice

1. What is the main purpose of using BiometricPrompt in an Android app?

easy

A. To access the device camera for scanning QR codes

B. To create a custom login screen with username and password

C. To show the system biometric authentication dialog to the user

D. To manage network connections securely

Solution

1. Step 1: Understand the role of BiometricPrompt

   BiometricPrompt is a system API that shows a standard biometric authentication dialog to the user.

2. Step 2: Identify its main use

   It is used specifically for biometric authentication like fingerprint or face recognition, not for login screens or camera access.

3. Final Answer:

   To show the system biometric authentication dialog to the user -> Option C

4. Quick Check:

   BiometricPrompt = system biometric dialog [OK]

Hint: BiometricPrompt always shows system biometric dialog [OK]

Common Mistakes:

• Confusing biometric prompt with custom login UI
• Thinking it manages network or camera
• Assuming it handles password input

2. Which of the following is the correct way to create a BiometricPrompt instance in Kotlin?

easy

A. val biometricPrompt = BiometricPrompt.create(this, executor, callback)

B. val biometricPrompt = BiometricPrompt(this, executor, callback)

C. val biometricPrompt = BiometricPrompt.newInstance(this, executor, callback)

D. val biometricPrompt = BiometricPrompt.Builder(this).build()

Solution

1. Step 1: Recall BiometricPrompt constructor usage

   In Kotlin, BiometricPrompt is created by calling its constructor with context, executor, and callback.

2. Step 2: Check each option

   Only val biometricPrompt = BiometricPrompt(this, executor, callback) uses the correct constructor syntax. Options A, C, and D use non-existent methods.

3. Final Answer:

   val biometricPrompt = BiometricPrompt(this, executor, callback) -> Option B

4. Quick Check:

   Constructor call = val biometricPrompt = BiometricPrompt(this, executor, callback) [OK]

Hint: Use constructor directly, no create() or newInstance() methods [OK]

Common Mistakes:

• Using non-existent factory methods
• Trying to use Builder pattern which BiometricPrompt doesn't have
• Confusing with other Android APIs

3. Given this Kotlin code snippet, what will be printed if the user successfully authenticates biometrically?

val callback = object : BiometricPrompt.AuthenticationCallback() {
  override fun onAuthenticationSucceeded(result: BiometricPrompt.AuthenticationResult) {
    println("Success")
  }
  override fun onAuthenticationFailed() {
    println("Failed")
  }
}
// Assume biometricPrompt is initialized and authenticate() is called with this callback

medium

A. Failed

B. Error

C. Nothing

D. Success

Solution

1. Step 1: Understand callback methods

   The onAuthenticationSucceeded method runs when biometric authentication is successful.

2. Step 2: Identify printed output on success

   Inside onAuthenticationSucceeded, it prints "Success". So on success, "Success" is printed.

3. Final Answer:

   Success -> Option D

4. Quick Check:

   Success callback prints "Success" [OK]

Hint: Success callback prints "Success" exactly [OK]

Common Mistakes:

• Confusing onAuthenticationFailed with success
• Assuming error prints on success
• Thinking no output occurs

4. What is wrong with this Kotlin code snippet for biometric authentication?

val biometricPrompt = BiometricPrompt(this, executor, callback)
biometricPrompt.authenticate(promptInfo)

// callback object missing override for onAuthenticationError

medium

A. It will work but won't handle errors properly

B. It will crash because onAuthenticationError is not overridden

C. It will not compile due to missing method

D. It will authenticate without any callback

Solution

1. Step 1: Check callback completeness

   Overriding onAuthenticationError is recommended but not mandatory; missing it won't crash or prevent compilation.

2. Step 2: Understand effect of missing error override

   Without onAuthenticationError, errors won't be handled gracefully, but authentication still runs.

3. Final Answer:

   It will work but won't handle errors properly -> Option A

4. Quick Check:

   Missing error override = no error handling [OK]

Hint: Missing error override causes no error handling, not crash [OK]

Common Mistakes:

• Assuming missing override causes crash
• Thinking code won't compile
• Believing authentication runs without callback

5. You want to add biometric authentication to your app but only if the device supports it and the user has enrolled biometrics. Which Kotlin code snippet correctly checks these conditions before showing the prompt?

val biometricManager = BiometricManager.from(this)
val canAuthenticate = biometricManager.canAuthenticate(BiometricManager.Authenticators.BIOMETRIC_STRONG or BiometricManager.Authenticators.DEVICE_CREDENTIAL)
if (/* condition here */) {
  biometricPrompt.authenticate(promptInfo)
}

hard

A. canAuthenticate == BiometricManager.BIOMETRIC_SUCCESS

B. canAuthenticate != BiometricManager.BIOMETRIC_ERROR_NO_HARDWARE

C. canAuthenticate == BiometricManager.BIOMETRIC_ERROR_NONE_ENROLLED

D. canAuthenticate == BiometricManager.BIOMETRIC_ERROR_HW_UNAVAILABLE

Solution

1. Step 1: Understand canAuthenticate return values

   canAuthenticate returns BIOMETRIC_SUCCESS if biometrics are available and enrolled.

2. Step 2: Choose correct condition to proceed

   Only when canAuthenticate == BIOMETRIC_SUCCESS should the app show the biometric prompt.

3. Final Answer:

   canAuthenticate == BiometricManager.BIOMETRIC_SUCCESS -> Option A

4. Quick Check:

   Check for BIOMETRIC_SUCCESS before authenticating [OK]

Hint: Authenticate only if canAuthenticate equals BIOMETRIC_SUCCESS [OK]

Common Mistakes:

• Checking for error codes instead of success
• Ignoring enrollment status
• Using wrong constants for condition