Elvis operator (?:) for default values in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Kotlin code using the Elvis operator?

Consider the following Kotlin code snippet. What will be printed when it runs?

Kotlin

val name: String? = null
val displayName = name ?: "Guest"
println(displayName)

A Guest

B null

C name

D Compilation error

💡 Hint

The Elvis operator returns the left value if it is not null; otherwise, it returns the right value.

Since 'name' is null, the Elvis operator returns the default value "Guest".

❓ Predict Output

intermediate

What does this Kotlin code print with a non-null value and Elvis operator?

Look at this Kotlin code. What will be the output?

Kotlin

val input: String? = "Hello"
val result = input ?: "Default"
println(result)

A Default

B null

C Compilation error

D Hello

💡 Hint

The Elvis operator returns the left value if it is not null.

Since 'input' is "Hello" (not null), the Elvis operator returns "Hello".

❓ Predict Output

advanced

What is the output of this Kotlin code with nested Elvis operators?

Analyze this Kotlin code snippet. What will be printed?

Kotlin

val a: String? = null
val b: String? = null
val c = a ?: b ?: "Fallback"
println(c)

A a

B null

C Fallback

D Compilation error

💡 Hint

Elvis operators can be chained to provide multiple fallback values.

Both 'a' and 'b' are null, so the chained Elvis operators return "Fallback".

❓ Predict Output

advanced

What error or output does this Kotlin code produce?

Consider this Kotlin code. What will happen when it runs?

Kotlin

val number: Int? = null
val result = number ?: throw IllegalArgumentException("Number required")
println(result)

A Prints null

B Throws IllegalArgumentException with message "Number required"

C Prints 0

D Compilation error

💡 Hint

The Elvis operator can throw an exception if the left side is null.

Since 'number' is null, the Elvis operator throws the specified exception.

🧠 Conceptual

expert

How many items are in the resulting list after applying this Kotlin code with Elvis operator?

Given this Kotlin code, how many elements does the list 'results' contain after execution?

Kotlin

val inputs: List<String?> = listOf("A", null, "B", null, "C")
val results = inputs.map { it ?: "Unknown" }

A 5

B 3

C 2

D 0

💡 Hint

The map function keeps the list size the same, replacing nulls with "Unknown".

The map transforms each element, replacing nulls with "Unknown", but the list size remains 5.

Practice

1. What does the Elvis operator ?: do in Kotlin?

easy

A. Checks if two values are equal

B. Converts a value to a string

C. Throws an exception if the left value is null

D. Returns the left value if it is not null; otherwise returns the right value

Solution

1. Step 1: Understand the Elvis operator purpose

   The Elvis operator ?: is used to provide a default value when the left side is null.

2. Step 2: Analyze the behavior

   If the left value is not null, it returns that value; otherwise, it returns the right value as a fallback.

3. Final Answer:

   Returns the left value if it is not null; otherwise returns the right value -> Option D

4. Quick Check:

   Elvis operator = default value if null [OK]

Hint: Elvis operator gives a fallback value if null [OK]

Common Mistakes:

• Thinking it compares two values
• Assuming it throws an error on null
• Confusing it with string conversion

2. Which of the following is the correct syntax to use the Elvis operator in Kotlin?

easy

A. val result = value ??: defaultValue

B. val result = value ? defaultValue

C. val result = value ?: defaultValue

D. val result = value ?:? defaultValue

Solution

1. Step 1: Recall the correct Elvis operator syntax

   The Elvis operator is written as ?: without any extra characters or spaces.

2. Step 2: Match the syntax with options

   val result = value ?: defaultValue uses ?: correctly between the variable and the default value.

3. Final Answer:

   val result = value ?: defaultValue -> Option C

4. Quick Check:

   Correct Elvis syntax = value ?: defaultValue [OK]

Hint: Elvis operator is ?: exactly, no extra symbols [OK]

Common Mistakes:

• Using single question mark instead of ?:
• Adding extra question marks
• Swapping the order of symbols

3. What will be the output of this Kotlin code?

val name: String? = null
val displayName = name ?: "Guest"
println(displayName)

medium

A. null

B. Guest

C. name

D. Compilation error

Solution

1. Step 1: Check the value of name

   The variable name is explicitly set to null.

2. Step 2: Apply the Elvis operator

   Since name is null, displayName gets the default value "Guest".

3. Final Answer:

   Guest -> Option B

4. Quick Check:

   null ?: "Guest" = "Guest" [OK]

Hint: If left is null, Elvis returns right side [OK]

Common Mistakes:

• Assuming it prints null
• Thinking it prints variable name
• Expecting a compile error

4. Identify the error in this Kotlin code snippet using the Elvis operator:

val input: String? = null
val output = input ? "Default"

medium

A. Incorrect operator used; should be ?: instead of ?

B. Missing right side value after Elvis operator

C. Variable input cannot be nullable

D. No error; code is correct

Solution

1. Step 1: Examine the operator used

   The code uses ? instead of the Elvis operator ?:.

2. Step 2: Understand correct Elvis operator usage

   The Elvis operator must be ?: to provide a default value when the left side is null.

3. Final Answer:

   Incorrect operator used; should be ?: instead of ? -> Option A

4. Quick Check:

   Elvis operator must be ?: not ? [OK]

Hint: Elvis operator always uses ?: not just ? [OK]

Common Mistakes:

• Using single question mark instead of Elvis operator
• Forgetting the default value after ?:
• Assuming nullable variables cause error here

5. Given the following Kotlin code, what will be the value of finalScore?

fun calculateScore(score: Int?): Int {
    return score ?: 0
}

val userScore: Int? = 5
val finalScore = calculateScore(userScore ?: 10)

hard

A. 5

B. 10

C. 0

D. Compilation error

Solution

1. Step 1: Evaluate the expression inside the function call

   The argument is userScore ?: 10. Since userScore is 5 (not null), the expression evaluates to 5.

2. Step 2: Call calculateScore with 5

   The function returns the score if not null, so it returns 5.

3. Final Answer:

   5 -> Option A

4. Quick Check:

   userScore ?: 10 = 5, function returns 5 [OK]

Hint: Elvis operator picks first non-null value [OK]

Common Mistakes:

• Thinking userScore ?: 10 returns 10
• Confusing function return with argument default
• Assuming null causes error here