Elvis operator deep usage in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Elvis operator with nested nullables

What is the output of this Kotlin code snippet?

Kotlin

val a: String? = null
val b: String? = "Hello"
val c: String? = null

val result = a ?: b ?: c ?: "Default"
println(result)

A null

B Default

C Hello

D Compilation error

💡 Hint

The Elvis operator returns the first non-null value from left to right.

The expression evaluates from left to right: 'a' is null, so it checks 'b' which is "Hello" (non-null), so it returns "Hello".

❓ Predict Output

intermediate

Elvis operator with function calls

What will be printed by this Kotlin code?

Kotlin

fun getName(): String? = null
fun getDefaultName(): String = "Guest"

val name = getName() ?: getDefaultName()
println(name)

A null

B Guest

C Compilation error

D getDefaultName

💡 Hint

Elvis operator calls the right side only if the left side is null.

getName() returns null, so Elvis operator evaluates getDefaultName() which returns "Guest".

❓ Predict Output

advanced

Elvis operator with smart casting and let

What is the output of this Kotlin code?

Kotlin

val input: String? = "Kotlin"

val length = input?.let { it.length } ?: -1
println(length)

A 6

B -1

C null

D Compilation error

💡 Hint

The safe call with let executes only if input is not null.

input is "Kotlin" (not null), so let returns length 6, Elvis operator returns 6.

❓ Predict Output

advanced

Elvis operator with multiple nullable types and side effects

What will be printed by this Kotlin code?

Kotlin

var count = 0

fun getFirst(): String? {
    count++
    return null
}

fun getSecond(): String? {
    count++
    return "Second"
}

val result = getFirst() ?: getSecond() ?: "Default"
println("$result $count")

A Default 1

B Second 1

C Default 2

D Second 2

💡 Hint

Elvis operator evaluates left to right and stops at first non-null.

getFirst() returns null and increments count to 1, getSecond() returns "Second" and increments count to 2, so result is "Second" and count is 2.

❓ Predict Output

expert

Complex Elvis operator with nested expressions and nullables

What is the output of this Kotlin code?

Kotlin

data class User(val name: String?, val nickname: String?)

fun getUser(): User? = User(null, "Nick")

val displayName = getUser()?.name ?: getUser()?.nickname ?: "Unknown"
println(displayName)

A Nick

B Unknown

C Compilation error

D null

💡 Hint

Elvis operator chains check for non-null values from left to right.

getUser() returns User(null, "Nick"). The first call getUser()?.name is null, so Elvis operator checks getUser()?.nickname which is "Nick".

Practice

1.

What does the Elvis operator ?: do in Kotlin?

easy

A. Checks if two values are equal.

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

C. Converts a value to a nullable type.

D. Throws an exception if the left value is null.

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.

3. Final Answer:

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

4. Quick Check:

   Elvis operator returns left if not null else right [OK]

Hint: Elvis operator picks left if not null, else right [OK]

Common Mistakes:

• Thinking it compares two values
• Assuming it throws exceptions
• Confusing it with null checks

2.

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

val result = value1 ??: value2

easy

A. val result = value1 ?: value2

B. val result = value1 ? value2

C. val result = value1 :: value2

D. val result = value1 ?:: value2

Solution

1. Step 1: Identify the correct Elvis operator syntax

   The Elvis operator in Kotlin is written as ?: without spaces.

2. Step 2: Match the syntax with options

   val result = value1 ?: value2 uses ?: correctly, while others use invalid or incorrect symbols.

3. Final Answer:

   val result = value1 ?: value2 -> Option A

4. Quick Check:

   Elvis operator syntax is ?: [OK]

Hint: Elvis operator is always ?: with no spaces [OK]

Common Mistakes:

• Using single question mark ? instead of ?:
• Using double colon :: by mistake
• Adding extra colons or spaces

3.

What is the output of the following Kotlin code?

val a: String? = null
val b: String? = "Hello"
val c: String? = null

val result = a ?: b ?: c ?: "Default"
println(result)

medium

A. Hello

B. null

C. Default

D. Compilation error

Solution

1. Step 1: Evaluate the chained Elvis operators

   Variable a is null, so Elvis operator moves to b. b is "Hello", which is not null.

2. Step 2: Determine the final value

   Since b is not null, result is assigned "Hello" and the rest is ignored.

3. Final Answer:

   Hello -> Option A

4. Quick Check:

   Chained Elvis picks first non-null: Hello [OK]

Hint: Chained Elvis returns first non-null value [OK]

Common Mistakes:

• Assuming it returns null if first is null
• Ignoring chaining and picking last value
• Thinking it causes a compile error

4.

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

val x: String? = null
val y: String = x ?: null

medium

A. Elvis operator cannot be used with nullable types

B. No error, code is correct

C. Variable x should not be nullable

D. Cannot assign null to non-nullable type String

Solution

1. Step 1: Analyze variable types and assignment

   Variable y is declared as non-nullable String, but x ?: null can be null.

2. Step 2: Understand Kotlin's null safety rules

   Assigning null to a non-nullable type causes a compilation error.

3. Final Answer:

   Cannot assign null to non-nullable type String -> Option D

4. Quick Check:

   Non-nullable cannot hold null [OK]

Hint: Non-nullable vars cannot be assigned null [OK]

Common Mistakes:

• Thinking Elvis operator changes nullability
• Assuming nullable can be assigned to non-nullable
• Ignoring Kotlin's strict null safety

5.

Given the following Kotlin code, what will be the output?

fun getName(user: Map): String {
    return user["firstName"] ?: user["nickname"] ?: user["lastName"] ?: "Anonymous"
}

val user1 = mapOf("firstName" to null, "nickname" to null, "lastName" to "Smith")
println(getName(user1))

hard

A. Anonymous

B. Compilation error

C. Smith

D. null

Solution

1. Step 1: Evaluate the map values in order

   firstName and nickname are null, so Elvis operator moves to lastName, which is "Smith".

2. Step 2: Determine the returned value

   Since lastName is not null, the function returns "Smith".

3. Final Answer:

   Smith -> Option C

4. Quick Check:

   Elvis chain returns first non-null: Smith [OK]

Hint: Elvis chains pick first non-null map value [OK]

Common Mistakes:

• Assuming it returns Anonymous despite lastName present
• Confusing null with string "null"
• Thinking map access causes error