Ekstensi Terbatas dan Ekstensi Companion

private fun String.shout() = uppercase() + "!"
fun main() {
    println("hello".shout()) // HELLO!
}

internal fun List<Int>.sumIfPositive(): Int = filter { it > 0 }.sum()
fun main() {
    println(listOf(-1, 2, 3, -4).sumIfPositive()) // 5
}

class StringHelper(val prefix: String) {
    fun String.withPrefix() = prefix + this
    fun build(): String = "world".withPrefix() // calls extension via instance
}
fun main() {
    println(StringHelper("hello ").build()) // hello world
}

class Repo {
    fun List<Int>.allPositive() = all { it > 0 }
    fun check(xs: List<Int>) = xs.allPositive()
}
fun main() {
    val r = Repo()
    println(r.check(listOf(1, 2, 3))) // true
    // listOf(1).allPositive() // ERROR — no access
}

class Greeter {
    companion object
}
fun Greeter.Companion.morning() = "Good morning!"
fun main() {
    println(Greeter.morning())
}

class User(val name: String) {
    companion object
}
fun User.Companion.guest() = User("Guest")
fun User.Companion.system() = User("System")
fun main() {
    println(User.guest().name)
    println(User.system().name)
}

class Box {
    companion object // required, even if empty
}
fun Box.Companion.create(): Box = Box()
fun main() {
    println(Box.create())
}

// File A:
// fun String.shout() = uppercase()
// File B imports:
// import com.example.shout
// Only File B can call shout()
fun String.shout() = uppercase() + "!"
fun main() { println("hi".shout()) }

class StringTools {
    fun String.greet() = "Hello, $this!"
    fun process(name: String) = with(this) {
        name.greet()
    }
}
fun main() {
    println(StringTools().process("Ada"))
}

class HttpStatus {
    companion object
}
val HttpStatus.Companion.OK_CODE: Int get() = 200
val HttpStatus.Companion.NOT_FOUND: Int get() = 404
fun main() {
    println(HttpStatus.OK_CODE)
    println(HttpStatus.NOT_FOUND)
}

class StringUtils {
    companion object
}
fun StringUtils.Companion.isAlpha(s: String) = s.all { it.isLetter() }
fun main() {
    println(StringUtils.isAlpha("hello"))
    println(StringUtils.isAlpha("hi5"))
}