Kotlin 中定义类、field关键字，主构造函数和次构造函数详解

class Student {
    //field关键字：Kotlin针对你定义的每个属性，都会产生一个field,一个getter,以及一个setter
    //field用来存储属性数据，不能直接定义field，kotlin会封装field，保护它里面的数据，只暴露给
    //getter和setter方法使用。
    var name: String = "jack"
        get() = field.capitalize()
        set(value) {
            field = value.trim()
        }
    //计算属性
    //计算属性是通过一个覆盖的get或set运算符来定义，这时field就不需要了
    val rolledValue
        get() = (1..9).shuffled().first() //1~9之间的随机数，取第一个
    //防范竞态条件
    var words: String? = "hello"
    fun saySomething() {
        words?.also {
            println("Hello ${it.toUpperCase()}")
        }.also {
            println("Hi $it")
        }
    }
}
fun main() {
    var p = Student()
    println(p.name)
    p.name = " rose "
    println(p.name)
    println(p.rolledValue)
    p.saySomething()
}

Jack
Rose
9
Hello HELLO
Hi hello

/**
 * 主构造函数：我们在Player类的定义头中定义一个主构造函数，使用临时变量为Player的各个属性提供初始值
 * 在Kotlin中，为便于识别临时变量（包括仅引用一次的参数），通常都会以下划线开头的名字命名
 */
class Player(
    _name: String, _age: Int, _isNormal: Boolean
) {
    var name = _name
        get() = field.capitalize()
        set(value) {
            field = value.trim()
        }
    var age = _age
        get() = field.absoluteValue
        set(value) {
            field = value.absoluteValue
        }
    var isNormal = _isNormal
}
fun main() {
    var player = Player("Jack", 30, true)
    player.name = "tool"
    println("${player.name}, ${player.age}, ${player.isNormal}")
}

Tool, 30, true

/**
 * 在主构造函数里定义属性
 * Kotlin允许你不使用临时变量赋值，而是直接用一个同时指定参数和类属性
 * 通常，我们更喜欢用这种方式定义类属性，因为它会减少重复代码
 */
class Player2(
    _name: String, var age: Int, var isNormal: Boolean
) {
    var name = _name
        get() = field.capitalize()
        set(value) {
            field = value.trim()
        }
    //次构造函数
    //我们可以定义多个次构造函数来配置不同的参数组合
    constructor(name: String) : this(name, age = 100, isNormal = false)
    constructor(name: String, age: Int) : this(name, age = 100, isNormal = false) {
        this.name = name.toUpperCase()
    }
}
fun main() {
    val player = Player2("Jack", 30, true)
    player.name = "tool"
    println("${player.name}, ${player.age}, ${player.isNormal}")
    val player2 = Player2("Rose")
    println(player2.name)
    var player21 = Player2("Jacky", 30)
    println("${player21.name},${player21.age}")
}

Tool, 30, true
Rose
JACKY,100

class Player3(
    _name: String, var age: Int = 20, var isNormal: Boolean
) {
    var name = _name
        get() = field.capitalize()
        set(value) {
            field = value.trim()
        }
    //次构造函数
    //我们可以定义多个次构造函数来配置不同的参数组合
    constructor(name: String) : this(name, age = 100, isNormal = false)
    constructor(name: String, age: Int) : this(name, age = 100, isNormal = false) {
        this.name = name.toUpperCase()
    }
}
fun main() {
    var player = Player3(isNormal = false, _name = "Jack")
    println("${player.name}, ${player.age}, ${player.isNormal}")
}

Jack, 20, false