Extension Functions

fun String.shout(): String = this.toUpperCase() + "!"

fun main() {
    println("hello".shout())
}

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

fun main() {
    println("4 is even: ${4.isEven()}")
    println("7 is even: ${7.isEven()}")
}

fun List<Int>.secondOrNull(): Int? = if (this.size >= 2) this[1] else null

fun main() {
    println(listOf(10, 20, 30).secondOrNull())
    println(listOf(10).secondOrNull())
}

open class Animal
class Dog : Animal()

fun Animal.describe() = "An animal"
fun Dog.describe() = "A dog"

fun main() {
    val animal: Animal = Dog()
    println(animal.describe())
}

1. Assuming an extension function actually modifies the original class; it is resolved statically and does not change the class itself in any way.
2. Expecting extension functions to be polymorphic like member functions; which extension gets called is determined by the declared type at compile time, not the runtime type.
3. Forgetting to import an extension function declared in another package, since it must be imported just like any other top-level function.

• An extension function is declared as fun ReceiverType.functionName(...) { ... }, adding a callable method to an existing type.
• Inside the extension, this refers to the receiver instance the function was called on.
• Extension functions are resolved statically based on the declared type, not dynamically based on the actual runtime type.
• Extension functions do not actually modify the original class; they are just syntactic sugar for a top-level function call.