[B < : A] UpperBound 上界：B类型的上界是A类型，即B类型的父类是A类型
[B>: A] LowerBound 下界：B类型的下界是A类型，即B类型的子类是A类型
[B <% A] ViewBound 表示B类型要转换成A类型，需要一个隐式转换函数
[B : A ] ContextBound 需要一个隐式转换的值
[-A , +B ]
[-A] 逆变，作为参数类型，如果A是T的子类，那么C[T]是C[A]的子类
[B] 协变 作为返回类型，如果B是T的子类，那么C[A]是C[T]的子类
UpperBound
UpperBoundDemo.scala
package day04
/**
* 上界 UpperBound
*/
class UpperBuoudDemo[T <: Comparable[T]] {
def select(first: T,second: T): T = {
if (first.compareTo(second) > 0 ) first else second
}
}
object UpperBuoudDemo {
def main(args: Array[String]): Unit = {
val u = new UpperBuoudDemo[MissRight]
val m1 = new MissRight("biaozi",120)
val m2 = new MissRight("erdiao",465)
val res = u.select(m1,m2)
println(res.name)
}
}
class MissRight(val name: String,val faceValue: Int) extends Comparable[MissRight] {
override def compareTo(o: MissRight): Int = {
this.faceValue - o.faceValue
}
}
ViewBound
ViewBoundDemo.scala
package day04
/**
* [B <% A] ViewBound
*/
class ViewBoundDemo[T <% Ordered[T]] {
def select(first: T,second: T): T = {
if (first > second) first else second
}
}
object ViewBoundDemo {
def main(args: Array[String]): Unit = {
import MyPredef.selectGirl
val viewBoundDemo = new ViewBoundDemo[Girl]
val g1 = new Girl("biaozi",26,120)
val g2 = new Girl("erdiao",24,120)
val res = viewBoundDemo.select(g1,g2)
println(res.name)
}
}
Girl.scala
package day04
class Girl(val name: String, val age: Int, val faceValue: Int) {
}
MyPredef.scala
package day04
object MyPredef {
implicit def fileToRichFile(file: String) = new RichFile(file)
implicit val selectGirl = (g: Girl) => new Ordered[Girl]{
override def compare(that: Girl): Int = {
if (g.faceValue == that.faceValue){
that.age - g.age
}else {
g.faceValue - that.faceValue
}
}
}
}
contextbound
ContextBoundDemo.scala
package day04
/**
* [B : A] ContextBound
*/
class ContextBoundDemo[T: Ordering] {
def select(first: T, second: T): T = {
val ord: Ordering[T] = implicitly[Ordering[T]]
if(ord.gt(first,second)) first else second
}
}
object ContextBoundDemo {
def main(args: Array[String]): Unit = {
import MyPredef.OrderingGirl
val contextBoundDemo = new ContextBoundDemo[Girl]
val g1 = new Girl("shabi",51,21)
val g2 = new Girl("chunhuo",45,21)
val res = contextBoundDemo.select(g1,g2)
println(res.name)
}
}
MyPredef.scala
package day04
object MyPredef {
implicit def fileToRichFile(file: String) = new RichFile(file)
implicit val selectGirl = (g: Girl) => new Ordered[Girl]{
override def compare(that: Girl): Int = {
if (g.faceValue == that.faceValue){
that.age - g.age
}else {
g.faceValue - that.faceValue
}
}
}
implicit object OrderingGirl extends Ordering[Girl]{
override def compare(x: Girl, y: Girl): Int = {
if (x.faceValue == y.faceValue){
y.age - x.age
}else {
x.faceValue - y.faceValue
}
}
}
}
