Android面试题之Kotlin泛型和reified关键字

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//泛型
class MagicBox<T>(item:T){var available = falseprivate var subject:T = itemfun fetch() : T? {return subject.takeIf { available }}//参数是匿名函数,传入T，返回Rfun <R> fetch(subjectFunction: (T) -> R) : R? {return subjectFunction(subject).takeIf { available }}
}
class Boy(val name:String, val age:Int)
class Man(val name:String, val age:Int)
class Dog(val weight:Int)fun main() {val box1 = MagicBox(Boy("java", 10))val box2 = MagicBox(Dog(10))box1.available = truebox1.fetch()?.run {println("you find $name")}val man = box1.fetch {Man(it.name, it.age+10)}println("${man?.name}, ${man?.age}")
}

泛型in和out关键字

//out
interface Production<out T>{fun product(): T
}//in
interface Consumer<in T>{fun consume(item: T)
}//不变
interface ProductionConsumer<T>{fun product():Tfun consume(item: T)
}open class Food
open class FastFood: Food()
class Burger : FastFood()class FoodStore : Production<Food>{override fun product(): Food {println("Produce food")return Food()}
}
class FastFoodStore : Production<FastFood>{override fun product(): FastFood {println("Produce FastFood")return FastFood()}
}
class BurgerStore : Production<Burger>{override fun product(): Burger {println("Produce Burger")return Burger()}
}class EveryBody: Consumer<Food>{override fun consume(item: Food) {println("consume food")}
}
class ModernPeople: Consumer<FastFood>{override fun consume(item: FastFood) {println("consume FastFood")}
}
class American: Consumer<Burger>{override fun consume(item: Burger) {println("consume FastFood")}
}fun main() {//子类泛型对象可以赋值给父类泛型对象，用outval production1: Production<Food> = FoodStore()val production2: Production<Food> = FastFoodStore()val production3: Production<Food> = BurgerStore()//父类泛型对象可以赋值给子类泛型对象，用inval consumer1: Consumer<Burger> = EveryBody()val consumer2: Consumer<Burger> = ModernPeople()val consumer3: Consumer<Burger> = American()
}

reified关键字

和inline一起使用，reified关键字能帮你检查泛型的参数类型。kotlin不允许对泛型参数T做类型检查，因为泛型参数类型会被类型擦除，也就是说T的类型信息在运行时是不可知的

//泛型 //inline和reified关键字一起使用，以便检查泛型参数类型 
class MagicBox2<T: Human>(){
//随机产生一个对象，如果不是指定类型的对象，则通过backup生成指定类型的对象inline fun <reified T> randomOrBackup(backup:() -> T): T{val items = listOf(Boy2("Jav2k",30),Man2("John3", 45))val random = items.shuffled().first()println(random)return if (random is T) {random} else{backup()}}
}

open class Human(val age: Int) class Boy2(val name:String, age:Int): Human(age) { override fun toString(): String { return "Boy2(name='age')" } 
} 
class Man2(val name:String, age:Int): Human(age) { override fun toString(): String { return "Man2(name='age')" } 
}fun main() { val box1: MagicBox2 = MagicBox2()val subject = box1.randomOrBackup { Boy2("java",23) } println(subject) }

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