Extension Functions trong Kotlin

fun String.addExclamation(): String {
    return "$this!"
}
 
fun main() {
    val greeting = "Hello"
    println(greeting.addExclamation())  // Hello!
}

fun Int.isEven() = this % 2 == 0
fun Int.isOdd() = this % 2 != 0
fun Int.squared() = this * this
 
fun main() {
    println(4.isEven())    // true
    println(5.squared())   // 25
}

fun <T> List<T>.secondOrNull(): T? {
    return if (size >= 2) this[1] else null
}
 
fun List<Int>.sumOfSquares(): Int {
    return this.sumOf { it * it }
}
 
fun main() {
    val list = listOf(1, 2, 3)
    println(list.secondOrNull())  // 2
    println(list.sumOfSquares())  // 14
}

val String.firstChar: Char?
    get() = this.firstOrNull()
 
val <T> List<T>.lastIndex: Int
    get() = this.size - 1
 
fun main() {
    println("Hello".firstChar)      // H
    println(listOf(1, 2, 3).lastIndex)  // 2
}

fun String?.orEmpty(): String {
    return this ?: ""
}
 
fun main() {
    val name: String? = null
    println(name.orEmpty())  // "" (empty string)
}

class MyClass {
    companion object
}
 
fun MyClass.Companion.create(): MyClass {
    return MyClass()
}
 
fun main() {
    val instance = MyClass.create()
}

fun <T> T.toSingletonList(): List<T> = listOf(this)
 
fun <T : Comparable<T>> T.coerceIn(min: T, max: T): T {
    return when {
        this < min -> min
        this > max -> max
        else -> this
    }
}
 
fun main() {
    println(5.toSingletonList())  // [5]
    println(10.coerceIn(1, 5))    // 5
}

fun main() {
    val result = StringBuilder().apply {
        append("Hello")
        append(" ")
        append("World")
    }.toString()
    
    println(result)  // Hello World
    
    val length = "Kotlin".let { it.length }
    println(length)  // 6
}

// ✅ Đặt extensions trong file riêng
// StringExtensions.kt
fun String.removeWhitespace() = this.replace(" ", "")
 
// ✅ Dùng cho utility functions
fun LocalDate.isWeekend() = 
    dayOfWeek == DayOfWeek.SATURDAY || dayOfWeek == DayOfWeek.SUNDAY
 
// ❌ Tránh thay đổi behavior cốt lõi
// fun Int.plus(other: Int) = this - other  // Confusing!