Null safety in Flutter - Practice Problems & Coding Challenges

Challenge - 5 Problems

📝 Syntax

intermediate

Identify the null safety error

Which option will cause a compile-time error due to null safety in Flutter?

Flutter

String name = null;
print(name.length);

A String? name = null; print(name?.length);

B String name = null; print(name.length);

C String name = 'Flutter'; print(name.length);

D String? name = 'Dart'; print(name!.length);

💡 Hint

Check if the variable type allows null values.

Option B declares a non-nullable String but assigns null, which is not allowed in null safety.

❓ ui_behavior

intermediate

Null safety impact on UI rendering

What will be displayed when this Flutter widget runs?

Flutter

String? message;

Widget build(BuildContext context) {
  return Text(message ?? 'No message');
}

A Displays 'No message'

B Displays nothing (empty space)

C Throws a runtime error

D Displays 'null'

💡 Hint

Look at the null-coalescing operator usage.

The '??' operator provides a fallback string when 'message' is null, so 'No message' is shown.

❓ lifecycle

advanced

Null safety and late initialization

What happens if you access a 'late' variable before it is initialized?

Flutter

late String title;

void printTitle() {
  print(title);
}

A Throws LateInitializationError

B Prints null

C Prints empty string

D Prints 'title'

💡 Hint

Consider what 'late' means in Dart null safety.

Accessing a 'late' variable before initialization causes a runtime LateInitializationError.

❓ navigation

advanced

Null safety in route arguments

Given this code, what will happen if 'args' is null?

Flutter

final args = ModalRoute.of(context)?.settings.arguments as String;

print(args.length);

A Prints 0

B Throws a NoSuchMethodError

C Throws a TypeError

D Prints null

💡 Hint

Check the cast and null safety behavior.

Casting null to String with 'as String' causes a TypeError at runtime.

🔧 Debug

expert

Debugging null safety error in widget state

Why does this Flutter stateful widget throw a null safety error?

Flutter

class MyWidget extends StatefulWidget {
  @override
  State<MyWidget> createState() => _MyWidgetState();
}

class _MyWidgetState extends State<MyWidget> {
  late String title;

  @override
  void initState() {
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return Text(title);
  }
}

A Because 'title' should be nullable String

B Because 'initState' is missing super.initState() call

C Because Text widget cannot display variables

D Because 'title' is late but never initialized before use

💡 Hint

Check when 'title' is assigned a value.

The 'late' variable 'title' is used in build without initialization, causing LateInitializationError.

Practice

1. What does the ? symbol mean when used after a type in Flutter null safety?

easy

A. It marks the variable as a constant.

B. It means the variable is always non-null.

C. It means the variable can hold a null value.

D. It makes the variable read-only.

Solution

1. Step 1: Understand the ? usage in Flutter

   The ? after a type means the variable can be null, allowing it to hold either a value or null.

2. Step 2: Compare with non-nullable types

   Without ?, variables must always have a non-null value, so ? explicitly allows null.

3. Final Answer:

   It means the variable can hold a null value. -> Option C

4. Quick Check:

   ? means nullable = A [OK]

Hint: Remember: ? means nullable, can hold null [OK]

Common Mistakes:

• Thinking ? makes variable constant
• Confusing ? with non-nullable
• Assuming ? means read-only

2. Which of the following is the correct way to declare a nullable integer variable in Flutter?

easy

A. int? number;

B. int number?;

C. nullable int number;

D. int number = null;

Solution

1. Step 1: Recall correct nullable syntax

   In Flutter, to declare a nullable type, place ? immediately after the type name, like int?.

2. Step 2: Check each option

   int? number; uses int? number; which is correct. int number?; places ? after variable name, which is invalid. nullable int number; uses a non-existent keyword. int number = null; assigns null to a non-nullable type, causing error.

3. Final Answer:

   int? number; -> Option A

4. Quick Check:

   Nullable type syntax = int? [OK]

Hint: Put ? right after type to make nullable [OK]

Common Mistakes:

• Placing ? after variable name
• Using non-existent keywords like 'nullable'
• Assigning null to non-nullable without ?

3. What will be the output of this Flutter code snippet?

int? a;
int b = a ?? 5;
print(b);

medium

A. null

B. 5

C. 0

D. Compilation error

Solution

1. Step 1: Understand variable initialization

   Variable a is nullable and not initialized, so it is null.

2. Step 2: Evaluate the null-coalescing operator ??

   The expression a ?? 5 means if a is null, use 5 instead.

3. Step 3: Assign and print value

   Since a is null, b gets 5, so print(b) outputs 5.

4. Final Answer:

   5 -> Option B

5. Quick Check:

   ?? gives default if null = 5 [OK]

Hint: Use ?? to provide default when null [OK]

Common Mistakes:

• Assuming uninitialized nullable is zero
• Thinking ?? returns null
• Expecting compilation error for nullable without init

4. Identify the error in this Flutter code snippet:

String? name;
print(name.length);

medium

A. Error: Cannot access property on nullable without check

B. No error, prints null

C. Error: Missing semicolon

D. Error: Variable 'name' not declared

Solution

1. Step 1: Analyze nullable variable usage

   Variable name is nullable and not initialized, so it is null.

2. Step 2: Accessing property on nullable

   Accessing name.length without null check causes error because name might be null.

3. Step 3: Correct usage

   Use null-aware operator like name?.length or check for null before accessing.

4. Final Answer:

   Error: Cannot access property on nullable without check -> Option A

5. Quick Check:

   Access nullable property without check = error [OK]

Hint: Use ?. or check null before property access [OK]

Common Mistakes:

• Assuming nullable property access is safe
• Ignoring null checks
• Confusing syntax errors with runtime errors

5. Given a nullable list of integers List<int>? numbers;, which code safely sums all elements or returns 0 if the list is null?

hard

A. int sum = numbers!.reduce((a, b) => a + b);

B. int sum = numbers.reduce((a, b) => a + b);

C. int sum = numbers == null ? null : numbers.reduce((a, b) => a + b);

D. int sum = numbers?.reduce((a, b) => a + b) ?? 0;

Solution

1. Step 1: Understand nullable list and safe access

   Since numbers can be null, accessing methods directly risks error.

2. Step 2: Analyze each option

   int sum = numbers?.reduce((a, b) => a + b) ?? 0; uses ?. to safely call reduce only if numbers is not null, then ?? 0 to default to 0 if null. int sum = numbers.reduce((a, b) => a + b); calls reduce directly, causing error if null. int sum = numbers!.reduce((a, b) => a + b); forces non-null with !, risky if null. int sum = numbers == null ? null : numbers.reduce((a, b) => a + b); returns null instead of int 0, causing type mismatch.

3. Step 3: Choose safe and correct option

   int sum = numbers?.reduce((a, b) => a + b) ?? 0; safely sums or returns 0 if null.

4. Final Answer:

   int sum = numbers?.reduce((a, b) => a + b) ?? 0; -> Option D

5. Quick Check:

   Safe call + default value = int sum = numbers?.reduce((a, b) => a + b) ?? 0; [OK]

Hint: Use ?. with ?? for safe nullable operations [OK]

Common Mistakes:

• Calling methods on nullable without safe operator
• Using ! without null check
• Returning null instead of default value