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为什么要在kotlin类构造函数中放置val或var

in-class-initialization kotlin

3

Tord Larsen smaznet · 技术社区 · 7 年前

在下面的第一段代码中学习Kotlin就是 val 关键字在其他代码中没有, 如果和 var

class Person(val firstName: String, val lastName: String) {
}

class Person(firstName: String, lastName: String) {
}

2 回复 | 直到 7 年前

1

20

zapl 7 年前

如果你忽略了 val 或 var https://kotlinlang.org/docs/reference/classes.html

class Person(firstName: String, lastName: String) {
    // directly in val / var declarations
    val firstName = firstName.capitalize()
    val lastName = lastName

    // in init blocks
    val fullName: String
    init {
        fullName = "$firstName $lastName"
    }

    // secondary constructors can only see their own parameters
    // and nothing else can access those
    constructor(fullName: String) : this("", fullName)
}

但它也适用于 delegation using by

interface Named {
    fun getName(): String
}
class Human(private val fname: String, private val lname: String) : Named {
    override fun getName() = "$fname + $lname" // functions need val since
                                               // value is resolved after construction
}
class Person2(firstName: String, lastName: String) : Named by Human(firstName, lastName)

class Person3(human: Human) : Named by human {
    constructor(firstName: String, lastName: String): this(Human(firstName, lastName))
}

或在财产委托中

class Person4(firstName: String, lastName: String) {
    val fullName: String by lazy { "$firstName $lastName" }
}

注意:闭包是在初始化时捕获的,因此当 lazy 最终评估。

2

37

Alex Nolasco 7 年前