Safe call operator (?.) in Kotlin - Mini Project: Build & Apply

Using the Safe Call Operator (?.) in Kotlin

📖 Scenario: Imagine you have a list of people, and each person may or may not have a phone number. You want to safely print the length of each phone number without causing errors if the phone number is missing.

🎯 Goal: Build a Kotlin program that uses the safe call operator ?. to safely access the length of phone numbers in a list of people.

📋 What You'll Learn

Create a list of people with nullable phone numbers

Create a variable to hold the count of people

Use the safe call operator ?. to get the length of each phone number safely

Print the length of each phone number or null if missing

💡 Why This Matters

🌍 Real World

Handling data where some information might be missing is common in apps, like user profiles with optional phone numbers.

💼 Career

Knowing how to safely access nullable data prevents crashes and bugs in real-world Kotlin applications.

1

Create a list of people with nullable phone numbers

Create a list called people with these exact entries: "Alice" to "12345", "Bob" to null, "Charlie" to "987654321". Use a list of pairs where the second value can be null.

Kotlin

# Your code here

Hint

Use listOf with pairs like "Name" to phoneNumber. Phone number can be null.

2

Create a variable to hold the count of people

Create a variable called count and set it to the size of the people list using people.size.

Kotlin

val people = listOf(
    "Alice" to "12345",
    "Bob" to null,
    "Charlie" to "987654321"
)
# Your code here

Hint

Use val count = people.size to get the number of people.

3

Use the safe call operator to get phone number lengths

Create a list called lengths that contains the length of each phone number using people.map. Use the safe call operator ?. on the phone number to get its length safely.

Kotlin

val people = listOf(
    "Alice" to "12345",
    "Bob" to null,
    "Charlie" to "987654321"
)

val count = people.size
# Your code here

Hint

Use people.map { it.second?.length } to get lengths safely.

4

Print the lengths of phone numbers

Print the lengths list using println(lengths).

Kotlin

val people = listOf(
    "Alice" to "12345",
    "Bob" to null,
    "Charlie" to "987654321"
)

val count = people.size

val lengths = people.map { it.second?.length }
# Your code here

Hint

Use println(lengths) to show the list of lengths.

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]