无涯教程-Kotlin - when语句 Kotlin，when表达式是返回值的条件表达式时。 Kotlin，when表

Kotlin，when表达式是返回值的条件表达式时。 Kotlin，when表达式替换switch语句。

让无涯教程看一下when表达式的一个简单的例子。

fun main(args: Array<String>){
    var number = 4
    var numberProvided = when(number) {
        1 -> "One"
        2 -> "Two"
        3 -> "Three"
        4 -> "Four"
        5 -> "Five"
        else -> "invalid number"
    }
    println("You provide $numberProvided")
}

输出:

You provide Four

when作为表达式时，不强制使用，它可以像以其他语言一样使用。

例如

fun main(args: Array<String>){
</span><span class="kwd">var</span><span class="pln"> number </span><span class="pun">=</span><span class="pln"> </span><span class="lit">4</span><span class="pln">
</span><span class="kwd">when</span><span class="pun">(</span><span class="pln">number</span><span class="pun">)</span><span class="pln"> </span><span class="pun">{</span><span class="pln">
    </span><span class="lit">1</span><span class="pln"> </span><span class="pun">-&gt;</span><span class="pln"> println</span><span class="pun">(</span><span class="str">"One"</span><span class="pun">)</span><span class="pln">
    </span><span class="lit">2</span><span class="pln"> </span><span class="pun">-&gt;</span><span class="pln"> println</span><span class="pun">(</span><span class="str">"Two"</span><span class="pun">)</span><span class="pln">
    </span><span class="lit">3</span><span class="pln"> </span><span class="pun">-&gt;</span><span class="pln"> println</span><span class="pun">(</span><span class="str">"Three"</span><span class="pun">)</span><span class="pln">
    </span><span class="lit">4</span><span class="pln"> </span><span class="pun">-&gt;</span><span class="pln"> println</span><span class="pun">(</span><span class="str">"Four"</span><span class="pun">)</span><span class="pln">
    </span><span class="lit">5</span><span class="pln"> </span><span class="pun">-&gt;</span><span class="pln"> println</span><span class="pun">(</span><span class="str">"Five"</span><span class="pun">)</span><span class="pln">
    </span><span class="kwd">else</span><span class="pln"> </span><span class="pun">-&gt;</span><span class="pln"> println</span><span class="pun">(</span><span class="str">"invalid number"</span><span class="pun">)</span><span class="pln">
</span><span class="pun">}</span><span class="pln">

}

输出:

Four

无涯教程可以使用条件块中包含的多个语句。

例如

fun main(args: Array<String>){
    var number = 1
    when(number) {
        1 -> {
            println("Monday")
            println("First day of the week")
        }
        7 -> println("Sunday")
        else -> println("Other days")
    }
}

输出:

Monday
First day of the week

无涯教程可以使用用逗号分隔的多个条件分支。当无涯教程需要运行多种选择时需要运行相同的逻辑时使用它。

fun main(args: Array<String>){
    var number = 8
    when(number) {
        3, 4, 5, 6 ->
            println("It is summer season")
        7, 8, 9 ->
            println("It is rainy season")
        10, 11 ->
            println("It is autumn season")
        12, 1, 2 ->
            println("It is winter season")
        else -> println("invalid input")
    }
}

输出:

It is rainy season

当表达式也检查条件时提供的输入范围。使用范围使用..(双点)运算符。在运算符中用于检查值是否属于范围。

例如:

fun main(args: Array<String>){
    var number = 7
    when(number) {
        in 1..5 -> println("Input is provided in the range 1 to 5")
        in 6..10 -> println("Input is provided in the range 6 to 10")
        else -> println("none of the above")
    }
}

输出:

Input is provided in the range 6 to 10

无涯教程-Kotlin - when语句

参考链接