If

傳統If應用如下:

var max: Int
if (a > b) {
    max = a
} else {
    max = b
}

可用單行表達式

val max = if (a > b) a else b

When Expression

Kotlin中採用when關鍵字取代switch
使用方式如下

when (x) {
    1 -> print("x == 1")
    2 -> print("x == 2")
    else -> { // Note the block
        print("x is neither 1 nor 2")
    }
}

如果when多個條件皆處理相同的事物
則可以合併在一起

when (x) {
    0, 1 -> print("x == 0 or x == 1")
    else -> print("otherwise")
}

when的條件除了用常數之外也可用函數表示

when (x) {
  parseInt(s) -> print("s encodes x")
  esle -> print("s does not encode x")
}

也可用in跟in!關鍵字判斷是否在某個range內

when (x) {
    in 1..10 -> print("x is in the range")
    in validNumbers -> print("x is valid")
    !in 10..20 -> print("x is outside the range")
    else -> print("none of the above")
}

用is跟is!檢查參數是否為某種類型(type)

fun hasPrefix(x: Any) = when(x) {
    is String -> x.startsWith("prefix") //x 被自動轉型為string
    else -> false
}

取代if-else判斷式

when {
    x.isOdd() -> print("x is odd")
    x.isEven() -> print("x is even")
    else -> print("x is funny")
}

For loop

For迴圈語法語C#相似

for (item in collection) print(item)

withIndex 可同時獲得key跟value值

for ((index, value) in array.withIndex()) {
    println("the element at $index is $value")
}

While loop

沒甚麼大變化

while (x > 0) {
    x--
}

do {
    val y = retrieveData()
} while (y != null)