Null safety in Flutter - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a nullable String variable.

Flutter

String? name[1]

Drag options to blanks, or click blank then click option'

A;

B:

C!

D=

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting the semicolon at the end.

Using an exclamation mark instead of a semicolon.

In Dart, to declare a nullable variable, you add a question mark after the type. The statement ends with a semicolon.

2fill in blank

medium

Complete the code to safely access the length of a nullable String variable.

Flutter

int? length = name[1].length;

Drag options to blanks, or click blank then click option'

A!

B??

C.

D?

Attempts:

3 left

💡 Hint

Common Mistakes

Using the dot operator directly on a nullable variable causes errors.

Using the exclamation mark forces non-null but can cause runtime errors.

The question mark before the dot safely accesses the property only if the variable is not null.

3fill in blank

hard

Fix the error in the code by forcing non-null access to the variable.

Flutter

int length = name[1].length;

Drag options to blanks, or click blank then click option'

A!

B?

C.

D??

Attempts:

3 left

💡 Hint

Common Mistakes

Using the safe access operator when the variable is expected to be non-null.

Forgetting to handle null values causing runtime errors.

The exclamation mark tells Dart you are sure the variable is not null, so it can access the property directly.

4fill in blank

hard

Fill both blanks to provide a default value if the nullable variable is null.

Flutter

String displayName = name[1] '' [2] 'Guest';

Drag options to blanks, or click blank then click option'

A??

B!

C+

D?

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong operator for null fallback.

Forgetting to concatenate strings properly.

The '??' operator returns the value on its left if not null, otherwise the right. The '+' concatenates strings.

5fill in blank

hard

Fill all three blanks to declare a nullable integer, assign a value, and force non-null access.

Flutter

int? count[1] 5;
int total = count[2] [3] 0;

Drag options to blanks, or click blank then click option'

A=

B!

C??

D;

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up assignment and comparison operators.

Not handling null values properly causing errors.

Use '=' to assign, '!' to assert non-null, and '??' to provide a default if null.

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