Nullable receiver extensions in Kotlin - Mini Project: Build & Apply

Nullable Receiver Extensions

📖 Scenario: Imagine you are building a simple app that processes user comments. Sometimes, the comment text might be missing (null). You want to safely check if a comment is empty or not, even if the comment itself is null.

🎯 Goal: Create a Kotlin extension function with a nullable receiver that checks if a string is null or empty. Then use it to safely check comments.

📋 What You'll Learn

Create a nullable receiver extension function called isNullOrEmptyCustom for String?

The function should return true if the string is null or empty, otherwise false

Create a list of nullable strings called comments with exact values: "Hello", "", null, "Kotlin"

Use a for loop with variables comment to iterate over comments

Inside the loop, print "Empty or null" if comment.isNullOrEmptyCustom() is true, else print the comment text

💡 Why This Matters

🌍 Real World

Nullable receiver extensions help safely add functions to types that can be null, avoiding crashes and making code cleaner.

💼 Career

Understanding nullable receiver extensions is important for Kotlin developers to write safe and readable code, especially when dealing with optional data.

1

Create the list of nullable comments

Create a list called comments with these exact nullable string values: "Hello", "", null, "Kotlin"

Kotlin

# Create the list called comments with the exact values
# Your code here

Hint

Use listOf to create a list with nullable strings.

2

Create the nullable receiver extension function

Create an extension function called isNullOrEmptyCustom with a nullable String? receiver that returns true if the string is null or empty, otherwise false

Kotlin

val comments: List = listOf("Hello", "", null, "Kotlin")

// Create the extension function isNullOrEmptyCustom for String? below
# Your code here

Hint

Use fun String?.isNullOrEmptyCustom() to define the extension function with nullable receiver.

3

Use a for loop to check each comment

Use a for loop with variable comment to iterate over comments. Inside the loop, use comment.isNullOrEmptyCustom() to check if the comment is empty or null.

Kotlin

val comments: List = listOf("Hello", "", null, "Kotlin")

fun String?.isNullOrEmptyCustom(): Boolean {
    return this == null || this.isEmpty()
}

// Add a for loop to iterate over comments and check each comment
# Your code here

Hint

Use for (comment in comments) to loop, then if (comment.isNullOrEmptyCustom()) to check.

4

Print the results

Print the output inside the loop as specified: print "Empty or null" if comment.isNullOrEmptyCustom() is true, otherwise print the comment text.

Kotlin

val comments: List = listOf("Hello", "", null, "Kotlin")

fun String?.isNullOrEmptyCustom(): Boolean {
    return this == null || this.isEmpty()
}

for (comment in comments) {
    if (comment.isNullOrEmptyCustom()) {
        // Print "Empty or null" here
    } else {
        // Print the comment here
    }
}

Hint

Use println to print the correct output inside the loop.

Practice

1. What is the main benefit of using nullable receiver extensions in Kotlin?

easy

A. They prevent any null values from being assigned.

B. They make variables non-nullable automatically.

C. They convert nullable types to non-nullable types permanently.

D. They allow calling functions on nullable types without explicit null checks.

Solution

1. Step 1: Understand nullable receiver extensions

   Nullable receiver extensions let you add functions to types that can be null, so you can call these functions safely without extra null checks.

2. Step 2: Identify the benefit

   This means you can write cleaner code by calling functions directly on nullable objects without risking null pointer exceptions.

3. Final Answer:

   They allow calling functions on nullable types without explicit null checks. -> Option D

4. Quick Check:

   Nullable receiver extensions = call on nullable safely [OK]

Hint: Nullable extensions handle null inside function, no extra checks needed [OK]

Common Mistakes:

• Thinking they convert nullable to non-nullable permanently
• Assuming they prevent null assignments
• Confusing with regular extensions on non-null types

2. Which of the following is the correct syntax to define a nullable receiver extension function isNullOrEmpty for String? in Kotlin?

easy

A. fun String.isNullOrEmpty(): Boolean = this == null || this.isEmpty()

B. fun String.isNullOrEmpty?(): Boolean = this == null || this.isEmpty()

C. fun String?.isNullOrEmpty(): Boolean = this == null || this.isEmpty()

D. fun String?.isNullOrEmpty?(): Boolean = this == null || this.isEmpty()

Solution

1. Step 1: Identify nullable receiver syntax

   To define an extension on a nullable type, use String? as the receiver type with a question mark after the type name.

2. Step 2: Check function syntax

   The function name should not have a question mark. So fun String?.isNullOrEmpty() is correct.

3. Final Answer:

   fun String?.isNullOrEmpty(): Boolean = this == null || this.isEmpty() -> Option C

4. Quick Check:

   Nullable receiver = question mark after type [OK]

Hint: Nullable receiver extension uses ? after type, not after function name [OK]

Common Mistakes:

• Putting ? after function name instead of receiver type
• Missing ? on receiver type for nullable
• Using non-nullable receiver for nullable extension

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

fun String?.describe(): String = this ?: "Empty"

fun main() {
  val text1: String? = "Hello"
  val text2: String? = null
  println(text1.describe())
  println(text2.describe())
}

medium

A. Hello\nEmpty

B. Hello\nnull

C. null\nEmpty

D. Empty\nEmpty

Solution

1. Step 1: Understand the extension function behavior

   The function returns the string itself if not null, otherwise returns "Empty".

2. Step 2: Apply to variables

   text1 is "Hello", so describe() returns "Hello". text2 is null, so describe() returns "Empty".

3. Final Answer:

   Hello\nEmpty -> Option A

4. Quick Check:

   Nullable describe returns value or "Empty" [OK]

Hint: ?: operator returns left if not null, else right [OK]

Common Mistakes:

• Assuming null prints as "null" string
• Confusing ?: with !! operator
• Expecting function to throw on null

4. Identify the error in this Kotlin code using a nullable receiver extension:

fun String?.firstChar(): Char = this[0]

fun main() {
  val s: String? = null
  println(s.firstChar())
}

medium

A. Compilation error: Extension function syntax incorrect

B. Compilation error: Cannot call get(0) on nullable receiver without null check

C. No error, prints first character or null

D. Runtime exception: NullPointerException when calling firstChar on null

Solution

1. Step 1: Analyze extension function code

   The function calls this[0] on a nullable receiver String? without a null check or safe call.

2. Step 2: Understand compiler behavior

   Kotlin compiler detects the unsafe access on nullable receiver and produces a compilation error.

3. Final Answer:

   Compilation error: Cannot call get(0) on nullable receiver without null check -> Option B

4. Quick Check:

   Unsafe access on nullable receiver = compile error [OK]

Hint: Nullable receiver extensions must check for null before use [OK]

Common Mistakes:

• Assuming nullable receiver is auto-checked
• Thinking it causes runtime exception instead of compilation error
• Thinking it compiles and prints null instead of error

5. You want to create a nullable receiver extension safeLength() for String? that returns the string length or zero if null. Which implementation is correct and safe?

1) fun String?.safeLength() = this.length ?: 0
2) fun String?.safeLength() = this?.length ?: 0
3) fun String.safeLength() = this.length ?: 0
4) fun String?.safeLength() = if (this == null) null else this.length

hard

A. Option 2

B. Option 1

C. Option 3

D. Option 4

Solution

1. Step 1: Check each option for null safety

   Option 1 calls this.length directly on nullable this, causing error. Option 3 uses non-nullable receiver, so not valid for nullable. Option 4 returns null instead of Int zero.

2. Step 2: Identify correct safe call usage

   Option 2 uses safe call this?.length which returns length or null, then ?: 0 returns zero if null. This is safe and returns Int.

3. Final Answer:

   fun String?.safeLength() = this?.length ?: 0 -> Option A

4. Quick Check:

   Safe call ?. with ?: zero fallback is correct pattern [OK]

Hint: Use safe call ?. and ?: to handle nullable safely [OK]

Common Mistakes:

• Calling length directly on nullable without safe call
• Using non-nullable receiver for nullable extension
• Returning null instead of zero for null input