SharedPreferences for key-value in Flutter - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ ui_behavior

intermediate

What is the output after saving and reading a value?

Consider this Flutter code snippet that saves a string to SharedPreferences and then reads it back. What will be printed in the console?

Flutter

final prefs = await SharedPreferences.getInstance();
await prefs.setString('username', 'Alice');
final name = prefs.getString('username');
print(name);

A Alice

B Throws an exception

C '' (empty string)

D null

💡 Hint

Remember that setString stores the value and getString retrieves it by the same key.

The code saves the string 'Alice' under the key 'username'. Then it reads it back with getString using the same key, so it prints 'Alice'.

❓ lifecycle

intermediate

When should you initialize SharedPreferences in a Flutter app?

In a Flutter app, when is the best time to call SharedPreferences.getInstance() to ensure data is ready for use?

A In the main() function before runApp()

B After the widget is disposed

C Inside the build() method of a widget

D In the initState() method of a StatefulWidget

💡 Hint

Think about when you want to load data once when the widget is created.

Calling SharedPreferences.getInstance() in initState() ensures the preferences are loaded once when the widget starts, avoiding repeated calls in build().

🔧 Debug

advanced

Why does this code fail to save the value?

Look at this Flutter code snippet. Why does it not save the integer value as expected?

Flutter

void saveScore() async {
  final prefs = await SharedPreferences.getInstance();
  prefs.setInt('score', 100);
}

A The key 'score' is reserved and cannot be used

B setInt() does not exist in SharedPreferences

C SharedPreferences.getInstance() returns a Future, so it must be awaited

D The method saveScore() must be synchronous

💡 Hint

Check how you handle asynchronous calls in Dart.

SharedPreferences.getInstance() returns a Future, so you must await it to get the instance before calling setInt.

🧠 Conceptual

advanced

What happens if you try to read a non-existent key?

If you call prefs.getString('nonexistentKey') on SharedPreferences where the key was never saved, what will be the result?

A It throws a KeyNotFoundException

B It returns null

C It returns an empty string ''

D It returns the string 'null'

💡 Hint

Think about how Dart handles missing keys in maps or similar APIs.

SharedPreferences returns null when a key does not exist, so getString returns null if the key is missing.

❓ navigation

expert

How to update UI after SharedPreferences value changes?

You saved a value in SharedPreferences and want your Flutter widget to update immediately to show the new value. Which approach works best?

A Use setState() after reading the updated value from SharedPreferences

B Restart the app to reload SharedPreferences

C Use a StreamBuilder directly on SharedPreferences

D SharedPreferences automatically notifies widgets to rebuild

💡 Hint

Think about how Flutter widgets update when data changes.

SharedPreferences does not notify widgets automatically. You must read the new value and call setState() to rebuild the widget with updated data.

Practice

1. What is the main purpose of using SharedPreferences in a Flutter app?

easy

A. To save small key-value data that persists after the app closes

B. To create complex database tables

C. To design the app's user interface

D. To handle network requests

Solution

1. Step 1: Understand SharedPreferences role

   SharedPreferences is designed to store simple data like settings or flags in key-value pairs.

2. Step 2: Compare with other options

   Options A, B, and C relate to databases, UI, and networking, which are not the purpose of SharedPreferences.

3. Final Answer:

   To save small key-value data that persists after the app closes -> Option A

4. Quick Check:

   SharedPreferences = persistent small key-value storage [OK]

Hint: Remember: SharedPreferences stores simple data persistently [OK]

Common Mistakes:

• Confusing SharedPreferences with databases
• Thinking it manages UI or network
• Assuming it stores large files

2. Which of the following is the correct way to save a boolean value true with key isLoggedIn using SharedPreferences in Flutter?

easy

A. prefs.putBool('isLoggedIn', true);

B. prefs.saveBool('isLoggedIn', true);

C. prefs.storeBool('isLoggedIn', true);

D. prefs.setBool('isLoggedIn', true);

Solution

1. Step 1: Recall SharedPreferences method names

   The correct method to save a boolean is setBool.

2. Step 2: Check method correctness

   Options A, B, and D use incorrect method names that do not exist in SharedPreferences API.

3. Final Answer:

   prefs.setBool('isLoggedIn', true); -> Option D

4. Quick Check:

   Save boolean = setBool [OK]

Hint: Use setBool to save booleans in SharedPreferences [OK]

Common Mistakes:

• Using incorrect method names like saveBool or putBool
• Forgetting to await async calls
• Confusing setBool with setString

3. Given the following Flutter code snippet, what will be printed?

final prefs = await SharedPreferences.getInstance();
await prefs.setInt('counter', 5);
int count = prefs.getInt('counter') ?? 0;
print(count);

medium

A. 0

B. null

C. 5

D. Error

Solution

1. Step 1: Analyze saving the value

   The code saves integer 5 with key 'counter' using setInt.

2. Step 2: Analyze reading the value

   It reads the value with getInt. Since 'counter' exists, it returns 5, not the default 0.

3. Final Answer:

   5 -> Option C

4. Quick Check:

   Saved int 5, getInt returns 5 [OK]

Hint: getInt returns saved value or default if missing [OK]

Common Mistakes:

• Assuming getInt returns null instead of default
• Confusing setInt with setString
• Forgetting to await setInt

4. What is wrong with this Flutter code that tries to save a string in SharedPreferences?

final prefs = SharedPreferences.getInstance();
prefs.setString('username', 'Alice');

medium

A. setString method does not exist

B. Missing await before getInstance()

C. Key 'username' cannot be a string

D. Cannot save strings in SharedPreferences

Solution

1. Step 1: Check getInstance usage

   getInstance() returns a Future, so it must be awaited or handled asynchronously.

2. Step 2: Check setString call

   Calling setString on a Future object (not the SharedPreferences instance) causes error.

3. Final Answer:

   Missing await before getInstance() -> Option B

4. Quick Check:

   Always await SharedPreferences.getInstance() [OK]

Hint: Always await getInstance() before using prefs [OK]

Common Mistakes:

• Not awaiting getInstance()
• Calling setString on Future instead of instance
• Confusing async calls with sync

5. You want to save a user's theme preference as a string ('light' or 'dark') and a flag if notifications are enabled (boolean). Which of the following code snippets correctly saves both values using SharedPreferences in Flutter?

hard

A. final prefs = await SharedPreferences.getInstance();
await prefs.setString('theme', 'dark');
await prefs.setBool('notifications', true);

B. final prefs = SharedPreferences.getInstance();
prefs.setString('theme', 'dark');
prefs.setBool('notifications', true);

C. final prefs = await SharedPreferences.getInstance();
prefs.setInt('theme', 'dark');
prefs.setBool('notifications', true);

D. final prefs = await SharedPreferences.getInstance();
await prefs.setInt('theme', 1);
await prefs.setBool('notifications', 'true');

Solution

1. Step 1: Check async usage

   SharedPreferences.getInstance() must be awaited to get the instance before use.

2. Step 2: Check method calls and types

   setString saves the theme string correctly; setBool saves the boolean flag correctly. Both use await for async calls.

3. Step 3: Identify errors in other options

   final prefs = SharedPreferences.getInstance(); prefs.setString('theme', 'dark'); prefs.setBool('notifications', true); misses await on getInstance and set methods; final prefs = await SharedPreferences.getInstance(); prefs.setInt('theme', 'dark'); prefs.setBool('notifications', true); uses setInt('theme', 'dark') causing type error ('dark' not int); final prefs = await SharedPreferences.getInstance(); await prefs.setInt('theme', 1); await prefs.setBool('notifications', 'true'); uses wrong types (setInt for string, boolean as string).

4. Final Answer:

   final prefs = await SharedPreferences.getInstance(); await prefs.setString('theme', 'dark'); await prefs.setBool('notifications', true); -> Option A

5. Quick Check:

   Await getInstance and setX methods with correct types [OK]

Hint: Await all async SharedPreferences calls with correct types [OK]

Common Mistakes:

• Not awaiting async calls
• Using wrong data types for values
• Saving booleans as strings