Safe call operator (?.) in Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the safe call operator (?.) do in Kotlin?

It allows you to safely access a property or call a method on an object that might be null, preventing a NullPointerException by returning null instead of crashing.

beginner

How does the safe call operator (?.) behave when the object is null?

When the object is null, the safe call operator returns null immediately without executing the method or property access.

beginner

Example: What is the output of this Kotlin code? val name: String? = null println(name?.length)

The output is 'null' because 'name' is null, so 'name?.length' safely returns null instead of throwing an error.

intermediate

Can the safe call operator (?.) be chained in Kotlin?

Yes, you can chain multiple safe calls like 'obj?.property?.method()' to safely navigate through multiple nullable references.

intermediate

What is the difference between the safe call operator (?.) and the non-null asserted call (!!) in Kotlin?

The safe call operator returns null if the object is null, avoiding exceptions. The non-null asserted call (!!) throws a NullPointerException if the object is null.

What does the safe call operator (?.) return if the object is null?

A null

B Throws NullPointerException

C Empty string

D Zero

The safe call operator returns null when the object is null, preventing exceptions.

Which Kotlin operator throws an exception if the object is null?

A ?.

B ?

C !!

D ?:

The '!!' operator throws a NullPointerException if the object is null.

How would you safely access the length of a nullable String 'text'?

A text.length

B length?.text

C text!!

D text?.length

Using 'text?.length' safely accesses length if 'text' is not null.

Can you chain safe calls like 'obj?.property?.method()' in Kotlin?

A Yes

B No

C Only for properties

D Only for methods

Safe calls can be chained to safely navigate multiple nullable references.

What is the output of this code? val user: User? = null println(user?.name ?: "Unknown")

A null

B Unknown

C Throws exception

D Empty string

The Elvis operator (?:) provides 'Unknown' when 'user?.name' is null.

Explain how the safe call operator (?.) helps prevent null pointer exceptions in Kotlin.

Think about what happens when you try to access something on a null object.

Describe a situation where chaining safe calls (?.) would be useful.

Imagine you have an object inside another object, both could be 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]