How Extensions Are Resolved

Extensions do not really modify the class. The compiler turns them into normal static functions that take the receiver as the first parameter. Two consequences:

• Which extension is called depends on the declared type of the variable, not the runtime type (no polymorphism).
• If the class already has a member with the same signature, the member always wins.
• Extensions cannot access private members of the receiver.

open class Animal
class Dog : Animal()

fun Animal.sound() = "generic sound"
fun Dog.sound() = "woof"

class Robot { fun beep() = "member beep" }
fun Robot.beep() = "extension beep"   // shadowed: never called

fun main() {
    val a: Animal = Dog()
    println(a.sound())        // declared type is Animal
    println(Dog().sound())
    println(Robot().beep())
}

generic sound
woof
member beep

Top-level extensions are visible in their package; elsewhere import them like any function: import com.shop.utils.toRupees. Keep them in files such as StringExt.kt or ViewExt.kt.