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Kotlin Control Flow

if 是表达式, 可以返回值

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

val max = if (a > b) {
    print("Choose a")
    a
} else {
    print("Choose b")
    b
}

when 是表达式, 可以返回值. 顺序匹配直到匹配值, 都不满足就用else分支 下面条件else分支是必须的

• when has a subject of an Boolean, enum, or sealed type, or their nullable counterparts.
• branches of when don’t cover all possible cases for this subject.

when 分支上可以有多个条件, 用,分隔, 条件是任何表达式. 支持in和!in操作, is和!is操作

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")
}

For-in 实现iterator next hasNext三个操作, 就可以使用for-in for-in 支持range表达式 for-in 支持 array, array.indices, array.withIndex

while, do-while

jump expression, 这些表达式可以做为其他表达式的一部分, 返回Nothing Type

• return
• break
• continue

Break and continue labels

loop@ for (i in 1..100) {
    for (j in 1..100) {
        if (...) break@loop
    }
}

Return to labels, 默认return返回外层的函数(lambda, 如果是匿名函数, 那么是返回匿名函数) lambda expression有implicit labels

fun foo() {
    listOf(1, 2, 3, 4, 5).forEach lit@{
        if (it == 3) return@lit // local return to the caller of the lambda - the forEach loop
        if (it == 3) return@forEach // local return to the caller of the lambda - the forEach loop
        print(it)
    }
    print(" done with explicit label")
}

fun foo() {
    listOf(1, 2, 3, 4, 5).forEach(fun(value: Int) {
        if (value == 3) return  // local return to the caller of the anonymous function - the forEach loop
        print(value)
    })
    print(" done with anonymous function")
}

fun foo() {
    run loop@{
        listOf(1, 2, 3, 4, 5).forEach {
            if (it == 3) return@loop // non-local return from the lambda passed to run
            print(it)
        }
    }
    print(" done with nested loop")
}

Exception

Throwable 异常的根类 Kotlin没有检查异常, @Throws可以从别处调用kotlin时来提醒开发者.

throw Exception("Hi There!") // 抛出异常, 是一个表达式, 返回Nothing Type

try-catch 可以是表达式, 可以返回值, finally块不会影响返回值 如果代码正常, 那么返回try块最后的表达式 否则返回catch块最后的表达式

try {
    // some code
} catch (e: SomeException) {
    // handler
} finally {
    // optional finally block
}

Reference

Conditions and loops

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