Kotlin教程：Elvis运算符(?:)

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。文章源自菜鸟学院-https://www.cainiaoxueyuan.com/ymba/18278.html

在某些情况下，可以声明一个保存空引用的变量。 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，否则使用其他非空值。文章源自菜鸟学院-https://www.cainiaoxueyuan.com/ymba/18278.html

var str: String? = null   
var str2: String? = "May be declare nullable string"

在上面的代码中，str包含一个null值，在访问str的值之前需要执行安全检查，字符串是否包含值。 在传统方法中，使用if...else语句执行此安全检查。文章源自菜鸟学院-https://www.cainiaoxueyuan.com/ymba/18278.html

var len1: Int = if (str != null) str.length else -1  
var len2:  Int = if (str2 != null) str.length else -1

示例代码 -文章源自菜鸟学院-https://www.cainiaoxueyuan.com/ymba/18278.html

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}")
}

执行上面示例代码，得到以下结果 -文章源自菜鸟学院-https://www.cainiaoxueyuan.com/ymba/18278.html

Length of str is -1
Length of str2 is 30

Kotlin提供称为Elvis运算符(?:)的高级运算符，即使条件表达式为空，也返回非空值。 以上if...else运算符可以使用Elvis运算符表示如下：文章源自菜鸟学院-https://www.cainiaoxueyuan.com/ymba/18278.html

var len1:  Int = str?.length ?: -1  
var len2:  Int = str2?.length ?:  -1

Elvis运算符将表达式返回?: -1。 (str ?.length)如果它不为null，否则它将表达式返回?:， 即-1。 仅当左侧返回null时，才会评估Elvis运算符的右侧表达式。文章源自菜鸟学院-https://www.cainiaoxueyuan.com/ymba/18278.html

Kotlin Elvis运算符示例文章源自菜鸟学院-https://www.cainiaoxueyuan.com/ymba/18278.html

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}")
}

执行上面示例代码，得到以下结果 -文章源自菜鸟学院-https://www.cainiaoxueyuan.com/ymba/18278.html

Length of str is -1
Length of str2 is 30

当Kotlin抛出并返回一个表达式时，它们也可以在Elvis运算符的右侧使用。 这可以用于检查函数参数：文章源自菜鸟学院-https://www.cainiaoxueyuan.com/ymba/18278.html

funfunctionName(node: Node): String? {
    val parent = node.getParent() ?: return null
    val name = node.getName() ?: throw IllegalArgumentException("name expected")
    // ...  
}

Kotlin Elvis运算符使用throw和return表达式文章源自菜鸟学院-https://www.cainiaoxueyuan.com/ymba/18278.html

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
}

执行上面示例代码，得到以下结果 -文章源自菜鸟学院-https://www.cainiaoxueyuan.com/ymba/18278.html

str = abc
strLength = 3
strLength2 = 3

check(null,"mango") = null
check("apple","orange") = 
textOne = apple
textTwo = orange

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