Kotlin 学习手记 -- 泛型、泛型约束、泛型型变，天呐

星投影

星投影在所有逆变点的下限类型是 Nothing, 因此不能用在属性或函数上

说白了只是一个描述符，可以简写泛型参数而已。

fun main() {

val queryMap: QueryMap<*, *> = QueryMap<String, Int>()

queryMap.getKey()

queryMap.getValue()

val f: Function<*, *> = Function<Number, Any>()

//f.invoke()

if (f is Function) {

(f as Function<Number, Any>).invoke(1, Any())

}

maxOf(1, 3)

HashMap<String, List<*>>()

//endregion

val hashMap: HashMap<*, *> = HashMap<String, Int>()

//hashMap.get()

}

class QueryMap<out K : CharSequence, out V : Any> {

fun getKey(): K = TODO()

fun getValue(): V = TODO()

}

fun <T : Comparable<T>> maxOf(a: T, b: T): T {

return if (a > b) a else b

}

class Function<in P1, in P2> {

fun invoke(p1: P1, p2: P2) = Unit

}

泛型擦除（伪泛型）

Java 与 Kotlin 实现机制一样，在运行时擦除真正的类型，C#则会真的生成一个类型去执行。

内联特化：

内联特化在调用的地方会替换到调用处，因此这时类型是确定的了，即已经特化成某个具体类型。通过 fun 前面的关键字 inline 和泛型参数 T 前面的 reified 参数两个来指定泛型参数在调用处实例化。

inline fun <reified T> genericMethod(t: T){

//val t = T()

val ts = Array<T>(3) { TODO() }

val jclass = T::class.java

val list = ArrayList<T>()

if(list is List<*>){

println(list.joinToString())

}

}

class Person(val age: Int, val name: String)

inline fun <reified T> Gson.fromJson(json: String): T = fromJson(json, T::class.java)

fun main() {

val gson = Gson()

val person2: Person = gson.fromJson("""{"age":18,"name":"Bennyhuo"}""")

val person3 = gson.fromJson<Person>("""{"age":18,"name":"Bennyhuo"}""")

}

实例：模仿的 Self Type

ty

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pealias OnConfirm = () -> Unit

typealias OnCancel = () -> Unit

private val EmptyFunction = {}

open class Notification(

val title: String,

val content: String

)

class ConfirmNotification(

title: String,

content: String,

val onConfirm: OnConfirm,

val onCancel: OnCancel

) : Notification(title, content)

interface SelfType<Self> {

val self: Self

get() = this as Self //当前类型强转成 Self 类型

}

//泛型添加约束只能传子类

open class NotificationBuilder<Self: NotificationBuilder<Self>>: SelfType<Self> {

protected var title: String = ""

protected var content: String = ""

fun title(title: String): Self {

this.title = title

return self //返回接口的常量属性即可，运行时就是当前子类实际类型

}

fun content(content: String): Self {

this.content = content

return self

}

open fun build() = Notification(this.title, this.content)

}

class ConfirmNotificationBuilder : NotificationBuilder<ConfirmNotificationBuilder>() {

private var onConfirm: OnConfirm = EmptyFunction

private var onCancel: OnCancel = EmptyFunction

fun onConfirm(onConfirm: OnConfirm): ConfirmNotificationBuilder {

this.onConfirm = onConfirm

return this

}

fun onCancel(onCancel: OnCancel): ConfirmNotificationBuilder {

this.onCancel = onCancel

return this

}

override fun build() = ConfirmNotification(title, content, onConfirm, onCancel)

}

fun main() {

ConfirmNotificationBuilder()

.title("Hello")

.onCancel {

println("onCancel")

}.content("World")

.onConfirm {

println("onConfirmed")

}

.build()

.onConfirm()

}

如果不定义 SelfType 类型，则子类在调用 ConfirmNotificationBuilder().title(“Hello”)之后不能再继续调用子类的 onCancel 方法，因为返回的是父类型，但是实际运行时这个类型是子类型。

实例： 基于泛型实现 Model 实例的注入

import java.util.concurrent.ConcurrentHashMap

import kotlin.reflect.KProperty

abstract class AbsModel {

init {

Models.run { this@AbsModel.register() }

}

}

class DatabaseModel : AbsModel() {

fun query(sql: String): Int = 0

}

class NetworkModel : AbsModel() {

fun get(url: String): String = """{"code": 0}"""

}

class SpModel : AbsModel() {

init {

Models.run { register("SpModel2") }

}

fun hello() = println("HelloWorld")

}

object Models {

private val modelMap = ConcurrentHashMap<String, AbsModel>()

fun AbsModel.register(name: String = this.javaClass.simpleName) {

modelMap[name] = this

}

//String 扩展函数

fun <T: AbsModel> String.get(): T {

return modelMap[this] as T

}

}

fun initModels() {

DatabaseModel()

NetworkModel()

SpModel()