Elvis运算符(?:)用于返回非null值,即使条件表达式为null。它还用于检查值的空安全性。
在某些情况下,无涯教程可以声明一个可以包含空引用的变量。假设一个包含空引用的变量str,在程序中使用str之前,无涯教程将检查它的可空性。如果发现变量str不为null,则其属性将使用,否则将使用其他一些非null值。
链接:https://www.learnfk.comhttps://www.learnfk.com/kotlin/kotlin-elvis-operator.html
来源:LearnFk无涯教程网
var str: String? = null var str2: String? = "May be declare nullable string"
在上面的代码中,字符串str包含一个空值,在访问str的值之前,无涯教程需要执行安全检查,无论字符串是否包含value。在常规方法中,无涯教程使用if ... else语句执行此安全检查。
var len1: Int = if (str != null) str.length else -1 var len2: Int = if (str2 != null) str.length else -1
fun main(args: Array<String>){ var str: String? = null var str2: String? = "May be declare nullable string" var len1: Int = if (str != null) str.length else -1 var len2: Int = if (str2 != null) str2.length else -1 println("Length of str is ${len1}") println("Length of str2 is ${len2}") }
输出:
Length of str is -1 Length of str2 is 30
Kotlin提供了Elvis运算符(?:)的高级运算符,即使条件表达式为null,该运算符也会返回非null值。上面的if...else运算符可以使用Elvis运算符表示如下:
var len1: Int = str?.length ?: -1 var len2: Int = str2?.length ?: -1
fun main(args: Array<String>){ var str: String? = null var str2: String? = "May be declare nullable string" var len1: Int = str ?.length ?: -1 var len2: Int = str2 ?.length ?: -1 println("Length of str is ${len1}") println("Length of str2 is ${len2}") }
输出:
Length of str is -1 Length of str2 is 30
当Kotlin抛出并返回表达式时,它们也可以在Elvis运算符的右侧使用。这可用于检查功能参数:
funfunctionName(node: Node): String? { val parent = node.getParent() ?: return null val name = node.getName() ?: throw IllegalArgumentException("name expected") //... }
Kotlin Elvis 运算符返回表达式
fun main(args: Array<String>){ val fruitName: String = fruits() println(fruitName) } fun fruits(): String{ val str: String? ="abc" val strLength: Int = if(str!= null) str.length else -1 val strLength2: Int = str?.length ?: -1 var string = "str = $str\n"+ "strLength = $strLength\n"+ "strLength2 = $strLength2\n\n" fun check(textOne: String?, textTwo: String?): String?{ val textOne = textOne ?: return null val textTwo = textTwo ?: IllegalArgumentException("text exception") return "\ntextOne = $textOne\n"+ "textTwo = $textTwo\n" } string += "check(null,\"mango\") = ${check(null,"mango")}\n" + "check(\"apple\",\"orange\") = ${check("apple","orange")}\n" return string }
输出:
str = abc strLength = 3 strLength2 = 3 check(null,"mango") = null check("apple","orange") = textOne = apple textTwo = orange
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