Elvis operator deep usage in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to assign a default value using the Elvis operator.

Kotlin

val name: String? = null
val displayName = name [1] "Unknown"

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

A?.

B==

C!!

D?:

Attempts:

3 left

💡 Hint

Common Mistakes

Using !! instead of ?: causes a crash if the value is null.

Using ?. only safely accesses properties but doesn't provide a default.

The Elvis operator ?: returns the left value if it's not null, otherwise the right value.

2fill in blank

medium

Complete the code to safely get the length of a nullable string or return 0 using the Elvis operator.

Kotlin

val text: String? = null
val length = text?.length [1] 0

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

A?:

B?.

C!!

D==

Attempts:

3 left

💡 Hint

Common Mistakes

Using !! causes an exception if text is null.

Using == is a comparison, not a fallback.

The Elvis operator ?: returns 0 if text?.length is null.

3fill in blank

hard

Fix the error in the code by completing the Elvis operator usage to provide a default value.

Kotlin

fun getUserName(user: User?): String {
    return user?.name [1] "Guest"
}

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

A!!

B?:

C?.

D==

Attempts:

3 left

💡 Hint

Common Mistakes

Using !! causes a crash if user or name is null.

Using ?. does not provide a default value.

Use ?: to return "Guest" if user?.name is null.

4fill in blank

hard

Fill both blanks to use the Elvis operator and safe call to get the length or return -1.

Kotlin

fun getLength(str: String?): Int {
    return str[1]length [2] -1
}

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

A?.

B?:

C!!

D==

Attempts:

3 left

💡 Hint

Common Mistakes

Using !! can cause exceptions if str is null.

Using == is a comparison, not a fallback.

Use ?. to safely access length and ?: to provide -1 if null.

5fill in blank

hard

Fill all three blanks to chain safe calls and Elvis operators to get a nested property or default.

Kotlin

fun getCountry(user: User?): String {
    return user[1]address[2]country [3] "Unknown"
}

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

A?.

B?:

C!!

D==

Attempts:

3 left

💡 Hint

Common Mistakes

Using !! can cause crashes if any property is null.

Using == is a comparison, not a fallback.

Use ?. to safely access nested properties and ?: to provide a default.

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