Extension Properties

val String.firstChar: Char
    get() = this[0]

fun main() {
    println("Kotlin".firstChar)
}

val List<Int>.average2: Double
    get() = if (this.isEmpty()) 0.0 else this.sum().toDouble() / this.size

fun main() {
    println(listOf(2, 4, 6).average2)
}

var StringBuilder.lastChar: Char
    get() = this[this.length - 1]
    set(value) {
        this.setCharAt(this.length - 1, value)
    }

fun main() {
    val sb = StringBuilder("Kotlin!")
    sb.lastChar = '?'
    println(sb)
}

data class Rectangle(val width: Double, val height: Double)

val Rectangle.area: Double
    get() = width * height

fun main() {
    val rect = Rectangle(4.0, 5.0)
    println("Area: ${rect.area}")
}

1. Trying to give an extension property a backing field (an initial stored value); extension properties cannot have backing fields, only computed getters (and setters).
2. Forgetting an extension var property needs both a custom get() and set() since there is no field to default to.
3. Overusing extension properties for expensive computations, forgetting each access recomputes the value since nothing is cached.

• An extension property is declared as val/var ReceiverType.propertyName: Type get() = ....
• Extension properties cannot have a backing field -- they must be computed through a custom getter (and setter for var).
• Like extension functions, extension properties are resolved statically based on the declared type.
• Extension properties are a clean way to add a derived, read-only fact to an existing type.