Elvis operator (?:) for default values in Kotlin

Introduction

The Elvis operator helps you give a default value when something might be missing or null. It keeps your code safe and simple.

When you want to use a value but it might be null, so you need a backup value.

When reading user input that might be empty or missing.

When working with data from the internet or a database that can be null.

When you want to avoid errors caused by null values in your program.

Syntax

Kotlin

val result = value ?: defaultValue

The operator ?: checks if value is null.

If value is not null, it uses value. Otherwise, it uses defaultValue.

Examples

If name is null, displayName becomes "Guest".

Kotlin

val name: String? = null
val displayName = name ?: "Guest"

Since age is 25 (not null), displayAge is 25.

Kotlin

val age: Int? = 25
val displayAge = age ?: 18

If the user enters nothing (null), safeInput will be "No input provided".

Kotlin

val input: String? = readLine()
val safeInput = input ?: "No input provided"

Sample Program

This program shows how the Elvis operator gives default values when variables are null.

Kotlin

fun main() {
    val userName: String? = null
    val displayName = userName ?: "Anonymous"
    println("Hello, $displayName!")

    val userScore: Int? = 42
    val finalScore = userScore ?: 0
    println("Your score is $finalScore.")
}

Important Notes

The Elvis operator is a quick way to avoid writing if-else checks for null.

It only works when the left side can be null.

You can use it with any type, not just strings.

Summary

The Elvis operator ?: provides a default value if something is null.

It helps keep your code clean and safe from null errors.

Use it whenever you want a simple fallback value.

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