11.1 协变和逆变
1)语法
class MyList[+T]{ //协变 } class MyList[-T]{ //逆变 } class MyList[T]{//不变 }
2)说明
协变:Son是Father的子类,则MyList[Son] 也作为MyList[Father]的“子类”。
逆变:Son是Father的子类,则MyList[Son]作为MyList[Father]的“父类”。
不变:Son是Father的子类,则MyList[Father]与MyList[Son]“无父子关系”。
3)实操
package com.yuange.scala.day06 abstract class Animal{ val name: String } abstract class Pet extends Animal{ } class Dog extends Pet{ override val name: String = "狗子" } class Cat extends Pet{ override val name: String = "猫子" } class Lion extends Animal{ override val name: String = "狮子" } class PetContainer[+P](val pet: P){ } object TestGeneric { def testPrint(petContainer: PetContainer[Pet]) = { println(petContainer.pet.name) } def main(args: Array[String]): Unit = { val dog: Dog = new Dog() var cat: Cat = new Cat() var lion: Lion = new Lion() val dogContainer: PetContainer[Dog] = new PetContainer[Dog](dog) val catContainer: PetContainer[Cat] = new PetContainer[Cat](cat) println("dogContainer=" + dogContainer) println("catContainer=" + catContainer) } }
11.2 泛型上下限
1)语法
Class PersonList[T <: Person]{ //泛型上限 } Class PersonList[T >: Person]{ //泛型下限 }
2)泛型的上下限的作用是对传入的泛型进行限定。
3)实操
package com.yuange.scala.day06 abstract class Animal{ val name: String } abstract class Pet extends Animal{ } class Dog extends Pet{ override val name: String = "狗子" } class Cat extends Pet{ override val name: String = "喵咪" } class Lion extends Animal{ override val name: String = "狮子" } class PetContainer[P <: Pet](val pet: P){ } object TestGenericTwo { def testPrint(petContainer: PetContainer[Pet]): Unit ={ println(petContainer.pet.name) } def main(args: Array[String]): Unit = { var dog: Dog = new Dog() var cat: Cat = new Cat() var loin: Lion = new Lion() val dogContainer: PetContainer[Dog] = new PetContainer[Dog](dog) val catContainer: PetContainer[Cat] = new PetContainer[Cat](cat) val loinContainer: PetContainer[Lion] = new PetContainer[Lion](loin) //error /*Error:(39, 24) type arguments [com.yuange.scala.day06.Lion] do not conform to class PetContainer's type parameter bounds [P <: com.yuange.scala.day06.Pet] val loinContainer: PetContainer[Lion] = new PetContainer[Lion](loin) Error:(39, 49) type arguments [com.yuange.scala.day06.Lion] do not conform to class PetContainer's type parameter bounds [P <: com.yuange.scala.day06.Pet] val loinContainer: PetContainer[Lion] = new PetContainer[Lion](loin)*/ } }
11.3 上下文限定
1)语法
def f[A : B](a: A) = println(a) //等同于def f[A](a:A)(implicit arg:B[A])=println(a)
2)上下问限定是将泛型和隐式转换的结合产物,以下两者功能相同,使用上下文限定[A : Ordering]之后,方法内无法使用隐式参数名调用隐式参数,需要通过implicitly[Ordering[A]]获取隐式变量。
3)实操
def f[A:Ordering](a:A,b:A) =implicitly[Ordering[A]].compare(a,b)
def f[A](a: A, b: A)(implicit ord: Ordering[A]) = ord.compare(a, b)
11.4 项目案例
1)需求
(1)获取没有农贸市场的所有省份
(2)获取农产品种类最多的三个省份
(3)获取每个省份中农产品种类最多的三个农贸市场
2)数据准备,找到 product.txt 和 allprovince.txt 文件,下载地址:https://pan.baidu.com/s/1dNIIZn6Tp5_Tee4nRjNF4A 提取码:hs37
3)实操
package com.yuange.scala.day06 import scala.io.Source object TestWordCount { val product: List[String] = Source.fromFile("product.txt","UTF-8").getLines().toList val allprovince: List[String] = Source.fromFile("allprovince.txt","UTF-8").getLines().toList def main(args: Array[String]): Unit = { //1、获取没有农贸市场的所有省份 println("获取没有农贸市场的所有省份:") testOne(allprovince,product) println("-"*100) //2、获取农产品种类最多的三个省份 println("获取农产品种类最多的三个省份:") testTwo(product) println("-"*100) //3、获取每个省份中农产品种类最多的三个农贸市场 println("获取每个省份中农产品种类最多的三个农贸市场:") testThree(product) } //3、获取每个省份中农产品种类最多的三个农贸市场 def testThree(product: List[String]) = { product.filter(_.split("\t").length == 6).map({x => val arr = x.split("\t") (arr(4),arr(3),arr(0)) }).distinct.groupBy{case (province,market,name) => province}.map(x => { x._2.groupBy{case (province,market,name) => market}.map(y => { (x._1,y._1,y._2.size) }).toList.sortBy{case (province,market,num) =>num}.reverse.take(3).foreach(println) }) } //2、获取农产品种类最多的三个省份 def testTwo(product: List[String]) = { product.filter(_.split("\t").length == 6).map( x => { var arr = x.split("\t") (arr(4),arr(0)) }).distinct.groupBy{ case (province,vegetables) => province}.map(x => {(x._1,x._2.size)}).toList.sortBy(_._2).reverse.take(3).foreach(println) } //1、获取没有农贸市场的所有省份 def testOne(allprovince: List[String],product: List[String]) = { // list.filter(x => { // x.split("\t").length == 6(province,market) // }) allprovince.diff(product.filter(_.split("\t").length == 6).map(_.split("\t")(4)).distinct).foreach(println) } }