Safe call operator (?.) in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to safely access the length of the string variable.

Kotlin

val 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 regular dot operator (.) causes a crash if the object is null.

Using the not-null assertion (!!.) is unsafe and can throw exceptions.

The safe call operator ?. allows accessing a property only if the object is not null, preventing null pointer exceptions.

2fill in blank

medium

Complete the code to safely call the function toUpperCase() on a nullable string.

Kotlin

val upper = text[1]toUpperCase()

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

A?:

B.

C!!.

D?.

Attempts:

3 left

💡 Hint

Common Mistakes

Using the regular dot operator (.) causes a null pointer exception if text is null.

Using the not-null assertion (!!.) can crash the program if text is null.

The safe call operator ?. calls the function only if text is not null, otherwise returns null.

3fill in blank

hard

Fix the error in the code by safely accessing the property length of a nullable string.

Kotlin

val len = nullableString[1]length

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

A?.

B!!.

C.

D?:

Attempts:

3 left

💡 Hint

Common Mistakes

Using the regular dot operator (.) causes a null pointer exception.

Using the not-null assertion (!!.) can cause a crash if the value is null.

The safe call operator ?. avoids exceptions by returning null if nullableString is null.

4fill in blank

hard

Fill both blanks to safely call substring on a nullable string and provide a default empty string if null.

Kotlin

val result = nullableStr[1]substring(0, 3) [2] ""

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

A?.

B?:

C!!.

D.

Attempts:

3 left

💡 Hint

Common Mistakes

Using the not-null assertion (!!.) instead of safe call.

Not providing a default value for null cases.

Use ?. to safely call substring only if nullableStr is not null, and ?: to provide a default empty string if it is null.

5fill in blank

hard

Fill all three blanks to safely access the length of a nullable string's trimmed version, or return 0 if null.

Kotlin

val length = nullableStr[1]trim()[2]length [3] 0

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

A?.

B?:

D.

Attempts:

3 left

💡 Hint

Common Mistakes

Using safe call before length unnecessarily.

Not providing a default value for null cases.

First, safely call trim() with ?.. Then access length with a regular dot because trim() returns a non-null string. Finally, use ?: to return 0 if the whole expression is null.

Practice

1. What does the safe call operator ?. do in Kotlin?

easy

A. It forces the program to crash if the object is null.

B. It converts a non-null object to null.

C. It accesses a property or calls a function only if the object is not null.

D. It always returns a default value when the object is null.

Solution

1. Step 1: Understand the purpose of the safe call operator

   The safe call operator ?. is used to safely access properties or call functions on objects that might be null.

2. Step 2: Behavior when object is null

   If the object is null, the operator returns null instead of throwing an error or crashing the program.

3. Final Answer:

   It accesses a property or calls a function only if the object is not null. -> Option C

4. Quick Check:

   Safe call operator = Access if not null [OK]

Hint: Safe call operator skips calls on null objects [OK]

Common Mistakes:

• Thinking it crashes on null
• Confusing with non-null assertion operator (!!)
• Assuming it provides default values

2. Which of the following is the correct syntax to safely call the length property of a nullable String name?

easy

A. val len = name.length

B. val len = name?.length

C. val len = name!!length

D. val len = name?length

Solution

1. Step 1: Identify the safe call syntax

   The safe call operator is ?., used as object?.property or object?.function().

2. Step 2: Check each option

   val len = name.length uses . without safety, which can cause a crash if name is null. val len = name?.length correctly uses ?.. val len = name!!length uses !! incorrectly with property access. val len = name?length uses invalid syntax ?length.

3. Final Answer:

   val len = name?.length -> Option B

4. Quick Check:

   Safe call syntax = ?. [OK]

Hint: Safe call uses ?. between object and property [OK]

Common Mistakes:

• Using . without safe call on nullable
• Confusing !! with ?.
• Writing invalid syntax like ?property

3. What is the output of this Kotlin code?

val text: String? = null
println(text?.length)

medium

A. 0

B. Throws NullPointerException

C. Empty string

D. null

Solution

1. Step 1: Analyze the nullable variable

   The variable text is null, so accessing text?.length uses the safe call operator.

2. Step 2: Evaluate the safe call result

   Since text is null, text?.length returns null instead of crashing or returning 0.

3. Final Answer:

   null -> Option D

4. Quick Check:

   Null safe call on null = null [OK]

Hint: Safe call returns null if object is null [OK]

Common Mistakes:

• Expecting 0 as length of null
• Thinking it throws an exception
• Assuming empty string output

4. Find the error in this Kotlin code snippet:

val user: User? = null
val nameLength = user.length?.toString()

medium

A. The safe call operator should be before length, like user?.length.

B. Safe call operator is used incorrectly on a non-nullable property.

C. The variable user cannot be null.

D. The toString() function cannot be called on an Int.

Solution

1. Step 1: Identify the nullable object and property access

   user is nullable, so accessing length directly without safe call is unsafe.

2. Step 2: Correct safe call placement

   The safe call operator must be used on the nullable object: user?.length. The code user.length?.toString() tries to access length on a nullable object without safety, causing an error.

3. Final Answer:

   The safe call operator should be before length, like user?.length. -> Option A

4. Quick Check:

   Safe call must be on nullable object [OK]

Hint: Put ?. right after nullable variable name [OK]

Common Mistakes:

• Using safe call after property instead of before
• Ignoring nullability of variable
• Misunderstanding function call on nullable

5. Given a nullable list of nullable strings val items: List?, which expression safely gets the length of the first string or returns null if any part is null?

hard

A. items?.get(0)?.length

B. items.get(0).length

C. items!![0]!!.length

D. items?[0].length

Solution

1. Step 1: Understand the nullability

   items can be null, and its elements can also be null. So both the list and the first element might be null.

2. Step 2: Use safe calls to avoid errors

   To safely get the length of the first string, use items?.get(0)?.length. This returns null if items or the first element is null.

3. Final Answer:

   items?.get(0)?.length -> Option A

4. Quick Check:

   Chain safe calls for nested nullables [OK]

Hint: Chain ?. for each nullable level [OK]

Common Mistakes:

• Using !! which throws exception if null
• Missing safe call on list or element
• Using invalid syntax like ?[0]