Elvis operator deep usage in Kotlin - Mini Project: Build & Apply

Elvis Operator Deep Usage

📖 Scenario: You are building a simple contact app that stores user information. Sometimes, some details like phone number or email might be missing. You want to use Kotlin's Elvis operator to provide default values when data is missing.

🎯 Goal: Learn how to use the Elvis operator ?: deeply in Kotlin to handle nullable values and provide fallback defaults in a real-world style contact information scenario.

📋 What You'll Learn

Create a nullable variable for phoneNumber with a null value

Create a nullable variable for email with a null value

Create a variable defaultPhone with the value "000-000-0000"

Create a variable defaultEmail with the value "[e-mail]"

Use the Elvis operator to assign contactPhone to phoneNumber or defaultPhone

Use the Elvis operator to assign contactEmail to email or defaultEmail

Use the Elvis operator deeply to assign finalContact to contactPhone if not null, else contactEmail, else the string "No contact info"

Print the finalContact value

💡 Why This Matters

🌍 Real World

Handling missing or optional user data safely is common in apps like contact managers, messaging, or user profiles.

💼 Career

Understanding Kotlin's Elvis operator helps write concise and safe code that avoids null pointer errors, a key skill for Android developers.

1

Create nullable contact details

Create a nullable variable called phoneNumber and set it to null. Also create a nullable variable called email and set it to null.

Kotlin

# Create nullable variables phoneNumber and email
// Your code here

Hint

Use val phoneNumber: String? = null to create a nullable variable.

2

Create default contact values

Create a variable called defaultPhone with the value "000-000-0000". Create a variable called defaultEmail with the value "[e-mail]".

Kotlin

val phoneNumber: String? = null
val email: String? = null
# Create defaultPhone and defaultEmail variables
// Your code here

Hint

Use val defaultPhone = "000-000-0000" to create a default phone string.

3

Use Elvis operator for contact fallback

Use the Elvis operator ?: to create a variable called contactPhone that is phoneNumber if not null, else defaultPhone. Also create contactEmail that is email if not null, else defaultEmail.

Kotlin

val phoneNumber: String? = null
val email: String? = null
val defaultPhone = "000-000-0000"
val defaultEmail = "[e-mail]"
# Use Elvis operator to assign contactPhone and contactEmail
// Your code here

Hint

Use val contactPhone = phoneNumber ?: defaultPhone to assign with Elvis operator.

4

Use deep Elvis operator and print result

Use the Elvis operator deeply to create a variable called finalContact that is contactPhone if not null, else contactEmail, else the string "No contact info". Then print finalContact.

Kotlin

val phoneNumber: String? = null
val email: String? = null
val defaultPhone = "000-000-0000"
val defaultEmail = "[e-mail]"
val contactPhone = phoneNumber ?: defaultPhone
val contactEmail = email ?: defaultEmail
# Use deep Elvis operator to assign finalContact and print it
// Your code here

Hint

Use val finalContact = contactPhone ?: contactEmail ?: "No contact info" and then println(finalContact).

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