🔧 Extension Functions

📖 What are Extension Functions?

Extension Functions are a feature that allows you to add new functions to existing classes without modifying them. You can use them just like original methods!

💡 Basic Usage

Your First Extension Function

// Add a new function to String!
fun String.isEmail(): Boolean {
    return this.contains("@") && this.contains(".")
}

fun main() {
    val email = "hong@example.com"

    println(email.isEmail())  // true
    println("invalid".isEmail())  // false
}

Extension Properties

val String.firstChar: Char
    get() = if (this.isNotEmpty()) this[0] else ' '

fun main() {
    println("Hello".firstChar)  // H
    println("Kotlin".firstChar)  // K
}

🎯 Practical Examples

String Utilities

// Phone number format
fun String.toPhoneFormat(): String {
    return if (this.length == 11) {
        "${substring(0, 3)}-${substring(3, 7)}-${substring(7)}"
    } else {
        this
    }
}

// String truncation
fun String.truncate(length: Int): String {
    return if (this.length > length) {
        "${substring(0, length)}..."
    } else {
        this
    }
}

// Extract numbers only
fun String.numbersOnly(): String {
    return this.filter { it.isDigit() }
}

fun main() {
    println("01012345678".toPhoneFormat())  // 010-1234-5678

    val long = "This is a very long text"
    println(long.truncate(10))  // This is a ...

    println("Price: 1,500won".numbersOnly())  // 1500
}

Number Utilities

// Check if even
fun Int.isEven(): Boolean = this % 2 == 0

// Range check
fun Int.isBetween(min: Int, max: Int): Boolean {
    return this in min..max
}

// Currency format
fun Int.toCurrency(): String {
    return "%,d won".format(this)
}

fun main() {
    println(4.isEven())  // true
    println(7.isEven())  // false

    println(50.isBetween(0, 100))  // true

    println(1500000.toCurrency())  // 1,500,000 won
}

Collection Utilities

// Safe second element
fun <T> List<T>.secondOrNull(): T? {
    return if (this.size >= 2) this[1] else null
}

// First index that matches condition
fun <T> List<T>.indexOfFirstOrNull(predicate: (T) -> Boolean): Int? {
    val index = this.indexOfFirst(predicate)
    return if (index >= 0) index else null
}

// Chunk list into groups of n
fun <T> List<T>.chunked(size: Int): List<List<T>> {
    return this.chunked(size)
}

fun main() {
    val numbers = listOf(1, 2, 3, 4, 5)

    println(numbers.secondOrNull())  // 2

    val index = numbers.indexOfFirstOrNull { it > 3 }
    println(index)  // 3 (index)

    println(numbers.chunked(2))  // [[1, 2], [3, 4], [5]]
}

Date/Time (Simple Version)

data class SimpleDate(val year: Int, val month: Int, val day: Int)

fun SimpleDate.format(): String {
    return "%04d-%02d-%02d".format(year, month, day)
}

fun SimpleDate.isWeekend(): Boolean {
    // Simple day of week calculation (Zeller's congruence)
    val y = if (month < 3) year - 1 else year
    val m = if (month < 3) month + 12 else month
    val dayOfWeek = (day + (13 * (m + 1) / 5) + (y % 100) +
                     (y % 100) / 4 + (y / 100) / 4 - 2 * (y / 100)) % 7
    return dayOfWeek == 0 || dayOfWeek == 6  // Sat/Sun
}

fun main() {
    val date = SimpleDate(2024, 12, 25)
    println(date.format())  // 2024-12-25
}

🔍 Nullable Extensions

Null-Safe Extensions

// Null or empty string
fun String?.isNullOrEmpty(): Boolean {
    return this == null || this.isEmpty()
}

// Return default value
fun String?.orDefault(default: String): String {
    return this ?: default
}

fun main() {
    val text: String? = null

    println(text.isNullOrEmpty())  // true
    println(text.orDefault("default value"))  // default value
}

🎨 Advanced Usage

Generic Extensions

// Shuffle collection
fun <T> List<T>.shuffled(): List<T> {
    return this.shuffled()
}

// Conditional transformation
fun <T, R> T.letIf(condition: Boolean, block: (T) -> R): R? {
    return if (condition) block(this) else null
}

fun main() {
    val numbers = listOf(1, 2, 3, 4, 5)
    println(numbers.shuffled())

    val result = "hello".letIf(true) { it.uppercase() }
    println(result)  // HELLO
}

Infix Functions

// Infix notation
infix fun Int.multipliedBy(other: Int): Int {
    return this * other
}

infix fun String.concat(other: String): String {
    return "$this $other"
}

fun main() {
    println(5 multipliedBy 3)  // 15

    val greeting = "Hello" concat "World"
    println(greeting)  // Hello World
}

🛠️ Practical Extension Collection

Validation Extensions

// Email validation
fun String.isValidEmail(): Boolean {
    val pattern = "[a-zA-Z0-9._-]+@[a-z]+\\.+[a-z]+"
    return this.matches(pattern.toRegex())
}

// Password strength
fun String.isStrongPassword(): Boolean {
    return this.length >= 8 &&
           this.any { it.isDigit() } &&
           this.any { it.isUpperCase() } &&
           this.any { it.isLowerCase() }
}

// URL validation
fun String.isValidUrl(): Boolean {
    return this.startsWith("http://") || this.startsWith("https://")
}

fun main() {
    println("hong@example.com".isValidEmail())  // true
    println("Password123".isStrongPassword())   // true
    println("https://kotlin.org".isValidUrl())  // true
}

Conversion Extensions

// CamelCase → snake_case
fun String.toSnakeCase(): String {
    return this.replace(Regex("([a-z])([A-Z])")) {
        "${it.groupValues[1]}_${it.groupValues[2]}"
    }.lowercase()
}

// snake_case → camelCase
fun String.toCamelCase(): String {
    return this.split("_").mapIndexed { index, word ->
        if (index == 0) word else word.capitalize()
    }.joinToString("")
}

fun main() {
    println("userName".toSnakeCase())      // user_name
    println("user_name".toCamelCase())     // userName
}

🤔 Frequently Asked Questions

Q1. Where should I define extension functions?

A: Usually in separate files!

// StringExtensions.kt
fun String.isEmail(): Boolean {
    return this.contains("@")
}

// NumberExtensions.kt
fun Int.isEven(): Boolean {
    return this % 2 == 0
}

// Import when using
import com.example.extensions.*

Q2. What if there's a method with the same name?

A: The existing method takes priority!

class MyClass {
    fun test() {
        println("Original method")
    }
}

// Extension function (won't be called!)
fun MyClass.test() {
    println("Extension function")
}

fun main() {
    MyClass().test()  // Original method
}

Q3. Can I access private members?

A: No, you cannot!

class Person(private val age: Int)

// ❌ Cannot access private
fun Person.getAge(): Int {
    // return this.age  // Error!
    return 0
}

🎬 Conclusion

Make your code more convenient with extension functions!

Key Takeaways:
✅ Add functionality without modifying existing classes
✅ Use fun Type.functionName() format
✅ Can extend nullable types
✅ Cannot access private members
✅ Great for utility functions

Next Step: Learn how to safely handle errors in Exception Handling!