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.