Kotlin — Extensions

Extension functions

fun String.isPalindrome(): Boolean {
    return this == this.reversed()
}

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

fun main() {
    println("racecar".isPalindrome()) // true
    println(42.isEven())              // true
}

Extension properties

val String.wordCount: Int
    get() = this.split("\\s+".toRegex()).size

fun main() {
    val text = "Hello World from Kotlin"
    println(text.wordCount) // 4
}

Nullable extensions

fun String?.orDefault(default: String = "N/A"): String {
    return this ?: default
}

fun main() {
    val name: String? = null
    println(name.orDefault())      // N/A
    println("Alice".orDefault())   // Alice
}

Extension in classes

class Calculator {
    fun Int.square() = this * this
    fun Int.cube() = this * this * this
}

fun main() {
    val calc = Calculator()
    with(calc) {
        println(5.square())  // 25
        println(3.cube())    // 27
    }
}

Scope functions

data class Person(var name: String, var age: Int)

fun main() {
    val p = Person("Alice", 30)

    // let
    p.let {
        println("${it.name} is ${it.age}")
    }

    // apply
    p.apply {
        name = "Bob"
        age = 25
    }
    println(p) // Person(name=Bob, age=25)

    // run
    val result = p.run {
        "Name: $name, Age: $age"
    }
    println(result)

    // also
    val nums = mutableListOf(1, 2, 3)
    nums.also { println("List: $it") }.add(4)

    // with
    with(p) {
        println("$name is $age years old")
    }
}

Best practices

• Use extensions for utility functions
• Keep extensions focused and small
• Define extensions where they're used
• Use scope functions for object configuration

Mini Practice

Write Kotlin code that:

1. Creates an extension function for String
2. Uses scope functions (let, apply, run)
3. Creates an extension property
4. Demonstrates with for object configuration

Up Next

In the next lesson, you'll learn about Coroutines — async programming in Kotlin.