is and !is 操作符

is和!is可以用來檢查一個實例是否屬于一種類型

if (obj is String) {
    print(obj.length)
}

if (obj !is String) { // same as !(obj is String)
    print("Not a String")
}
else {
    print(obj.length)
}

Kotlin里經(jīng)過is檢查的變量不用顯示的轉(zhuǎn)型（自動轉(zhuǎn)換）

fun demo(x: Any) {
    if (x is String) {
        print(x.length) // x is automatically cast to String
    }
}

或者

if (x !is String) return
    print(x.length) // x is automatically cast to String

或者

&& 和 || 的右邊

// x is automatically cast to string on the right-hand side of `||`
    if (x !is String || x.length == 0) return

    // x is automatically cast to string on the right-hand side of `&&`
    if (x is String && x.length > 0) {
        print(x.length) // x is automatically cast to String
    }

或者when里

when (x) {
    is Int -> print(x + 1)
    is String -> print(x.length + 1)
    is IntArray -> print(x.sum())
}

如果變量check和usage之間，編譯器無法保證變量不變，則不會做自動轉(zhuǎn)換

• val local variable
• val properties
• var local variable
• var properties

as操作符

通過as轉(zhuǎn)型失敗時會拋出異常，as？轉(zhuǎn)型失敗會返回null

val x: String = y as String

val x: String? = y as? String