拡張プロパティと計算拡張

val String.wordCount: Int
    get() = trim().split(Regex("\\s+")).size
println("hello world kotlin".wordCount)  // 3

var MutableList<Int>.first: Int
    get() = this[0]
    set(value) { this[0] = value }
val list = mutableListOf(1, 2, 3)
list.first = 10
println(list)  // [10, 2, 3]

// ERROR: extension property cannot have backing field
// var String.cache: String = ""  // NOT allowed!

// OK: computed only:
val String.isPalindrome: Boolean
    get() = this == this.reversed()

val <T> List<T>.secondOrNull: T?
    get() = if (size >= 2) this[1] else null
val <T> List<T>.isNotNullOrEmpty: Boolean
    get() = isNotEmpty()
println(listOf(1, 2, 3).secondOrNull)  // 2

val Any.isString: Boolean get() = this is String
val String.hasDigits: Boolean get() = any { it.isDigit() }
val Int.isPositive: Boolean get() = this > 0
println(42.isPositive)        // true
println("abc123".hasDigits)   // true

// Property: stateless, simple characteristic
val String.isEmail: Boolean get() = '@' in this
// Function: has parameters
fun String.padCenter(width: Int) = padStart((length + width) / 2)

data class Color(val r: Int, val g: Int, val b: Int) {
    companion object
}
val Color.Companion.RED get() = Color(255, 0, 0)
val Color.Companion.WHITE get() = Color(255, 255, 255)
println(Color.RED)  // Color(r=255, g=0, b=0)

class Formatter {
    val Int.formatted: String
        get() = "%,d".format(this)
    fun format(n: Int) = n.formatted
}
println(Formatter().format(1000000))  // 1,000,000

val Int.dp: Float get() = this * 2.5f
val Float.sp: Float get() = this * 1.0f
// In DSL context:
view.setPadding(16.dp.toInt())

val <T> List<T>.head: T?
    get() = firstOrNull()
val <T> List<T>.tail: List<T>
    get() = drop(1)
println(listOf(1,2,3).head)  // 1
println(listOf(1,2,3).tail)  // [2, 3]