kotlin - 检查空变量，然后在其上执行if语句

Question

我正在学习 Kotlin 。我想知道这是解决此问题的最佳方法:
我有一堂简单的课:

class AlternativeCustomerTwo(
    val name: String = "Name Not Provided",
    var age: Int? = null,
    var address: String = "Address not provided"
) 

它具有默认参数，并且可以为空。

我要包括以下字段:

var isApproved: Boolean = false

所以现在我有一个类，看起来像:

class AlternativeCustomerTwo(
    val name: String = "Name Not Provided",
    var age: Int? = null,
    var address: String = "Address not provided"
) {

   var isApproved: Boolean = false}

现在，我想覆盖isApproved的默认 setter ，该 setter 检查年龄是否超过21岁，如果超过21岁，则将其设置为true。像这样:

class AlternativeCustomerTwo(
    val name: String = "Name Not Provided",
    var age: Int? = null,
    var address: String = "Address not provided"
) { 
 var isApproved: Boolean = false
    set(value) {
            if(age >= 21) {
                field = value
            }
    }
}

这里的问题是 var age 。代码无法编译，这是错误:

Error:(19, 20) Kotlin: Operator call corresponds to a dot-qualified call 'age.compareTo(21)' which is not allowed on a nullable receiver 'age'.

经过一番修补后，我实现了所需的功能，如下所示:

class AlternativeCustomerTwo(
    val name: String = "Name Not Provided",
    var age: Int? = null,
    var address: String = "Address not provided"
) {

   var isApproved: Boolean = false
    set(value) {
       age?.let {
           if(it >= 21) {
               field = value
           }
       }
    }
}

如果我这样称呼它:

val customer = AlternativeCustomerTwo(name = "John", age = 120)
customer.isApproved = true

然后打印:true

交替

val customer = AlternativeCustomerTwo(name = "John", age = 12)
    customer.isApproved = true

它打印错误
我的问题是，这是正确的方法，还是我正在做一些可怕的Kotlin？

Answer 1

您正在使用kotlin的两种非常好的语言功能。
首先是let函数。它是一个作用域函数，它接受其调用对象(在您的情况下为 age )并将其提供为lambda的参数(在您的情况下为或)。

第二个是安全 call (？。)功能。仅当调用对象为非null时，安全调用才会继续。

因此，只有在age不为null时，才会调用您的setter。

该技术由Kotlin语言设计师推荐。以下摘录摘自《运行中的 Kotlin 》一书，第6章，第144页

you can use the let function, and call it via a safe call. All the let function does is turn the object on which it’s called into a parameter of the lambda. If you combine it with the safe call syntax, it effectively converts an object of a nullable type on which you call let into a non-null type