SharedPreferences / DataStore in Android Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ ui_behavior

intermediate

What is the output when reading a missing key from SharedPreferences?

Consider this Kotlin code snippet that reads a value from SharedPreferences:

val prefs = context.getSharedPreferences("settings", Context.MODE_PRIVATE)
val username = prefs.getString("user_name", "Guest")
println(username)

What will be printed if the key "user_name" does not exist in the preferences?

Android Kotlin

val prefs = context.getSharedPreferences("settings", Context.MODE_PRIVATE)
val username = prefs.getString("user_name", "Guest")
println(username)

A "Guest"

B "" (empty string)

C Throws NullPointerException

D null

💡 Hint

Check the default value parameter in getString method.

The getString method returns the default value if the key is missing. Here, "Guest" is the default, so it prints "Guest".

❓ lifecycle

intermediate

When is it safe to read from DataStore in an Android app?

You want to read user preferences stored in DataStore in your Android app.
At which lifecycle event of an Activity is it best to start collecting DataStore data to update the UI?

A Inside onResume()

B Inside onStart()

C Inside onCreate()

D Inside onDestroy()

💡 Hint

Consider when the UI is visible but not yet interactive.

onStart() is called when the Activity becomes visible. Starting to collect DataStore data here ensures UI updates before user interaction.

📝 Syntax

advanced

Which Kotlin code correctly saves a Boolean value using DataStore?

Given a DataStore instance named dataStore, which code snippet correctly saves a Boolean value with key "dark_mode" set to true?

Android Kotlin

val DARK_MODE_KEY = booleanPreferencesKey("dark_mode")

// Choose the correct snippet to save true to this key

A dataStore.updateData { preferences -> preferences[DARK_MODE_KEY] = true }

B dataStore.save { preferences -> preferences[DARK_MODE_KEY] = true }

C dataStore.edit { preferences -> preferences.put(DARK_MODE_KEY, true) }

D dataStore.edit { preferences -> preferences[DARK_MODE_KEY] = true }

💡 Hint

Check the official DataStore API for editing preferences.

The edit function is the correct suspend function to update preferences. updateData and save do not exist, and put is not a method on Preferences.

🔧 Debug

advanced

Why does this SharedPreferences commit not save data?

Look at this Kotlin code:

val prefs = context.getSharedPreferences("prefs", Context.MODE_PRIVATE)
val editor = prefs.edit()
editor.putString("token", "abc123")
// Missing editor.commit() or editor.apply()

What is the reason the data is not saved?

A The editor object is not committed or applied, so changes are not saved.

B The key "token" is invalid and cannot be saved.

C SharedPreferences requires a call to save() instead of commit() or apply().

D Context.MODE_PRIVATE prevents saving data.

💡 Hint

Check what methods finalize changes in SharedPreferences.Editor.

Changes made with putString are only saved after calling commit() or apply() on the editor.

🧠 Conceptual

expert

What is a key advantage of DataStore over SharedPreferences?

Which of the following is a key advantage of using DataStore instead of SharedPreferences in Android development?

A SharedPreferences supports type-safe keys, while DataStore does not.

B DataStore stores data in XML files, making it easier to read manually.

C DataStore supports asynchronous, coroutine-based APIs for safer and more efficient data access.

D DataStore requires no context to access, unlike SharedPreferences.

💡 Hint

Think about modern Kotlin features and thread safety.

DataStore uses Kotlin coroutines and Flow for asynchronous, safe data access, improving performance and avoiding blocking the main thread.