Scala进阶源码实战之六——类型变量

View Bounds 视图界定

package com.dt.scala.type_parameterization//class Pair_NotPerfect[T <: Comparable[T]](val first : T,val second : T){//  def bigger = if(first.compareTo(second) > 0)first else second  //}//  <% 学术名称就是视图界定class Pair_NotPerfect[T <% Comparable[T]](val first : T,val second : T){    def bigger = if(first.compareTo(second) > 0)first else second  }class Pair_Better[T <% Ordered[T]](val first : T,val second : T){    def bigger = if(first > second)first else second  }object View_Bounds {  def main(args: Array[String]) {    val pair = new Pair_NotPerfect("Spark", "Hadoop")    println(pair.bigger)    val pairInt = new Pair_NotPerfect(3, 5) //Int -> RichInt    println(pairInt.bigger)    val pair_Better_String = new Pair_Better("Java", "Scala") //String -> RichString    println(pair_Better_String.bigger)    val pair_Better_Int = new Pair_Better(20, 12)    println(pair_Better_Int.bigger)  }}

Context Bounds

package com.dt.scala.type_parameterizationimport java.util.Comparatorclass Pair_Ordering[T : Ordering] (val first : T, val second : T){  def bigger(implicit ordered: Ordering[T]) = {    if (ordered.compare(first, second) > 0) first else second  }}object Context_Bounds {  def main(args: Array[String]) {    val pair = new Pair_Ordering("Spark", "Hadoop")    println(pair.bigger)    val pairInt = new Pair_Ordering(3, 5)    println(pairInt.bigger)  }}

ClassTag 、Manifest、ClassManifest、TypeTag

package type_paramentsimport scala.reflect.ClassTagclass A[T]object classtag {  println("Welcome to the Scala worksheet")       //> Welcome to the Scala worksheet  def arrayMake[T : Manifest](first : T, second : T) = {      val r = new Array[T](2); r(0) = first; r(1) = second; r                //看下面已经生成了隐式值    }                                             //> arrayMake: [T](first: T, second: T)(implicit evidence$3: Manifest[T])Array[T                                                  //| ]   arrayMake(1,2).foreach(println)                //> 1                                                  //| 2   //实际中用的最多的是ClassTag 可以代替Manifest   //运行时将泛型擦除，通过上下文推断   def mkArray[T : ClassTag](elems: T*) = Array[T](elems: _*)                                                  //> mkArray: [T](elems: T*)(implicit evidence$4: scala.reflect.ClassTag[T])Array                                                  //| [T]   mkArray(42, 13).foreach(println)               //> 42                                                  //| 13   mkArray("Japan","Brazil","Germany").foreach(println)                                                  //> Japan                                                  //| Brazil                                                  //| Germany    def manif[T](x: List[T])(implicit m: Manifest[T]) = {        if (m <:< manifest[String])          println("List strings")        else          println("Some other type")    }                                             //> manif: [T](x: List[T])(implicit m: Manifest[T])Unit    manif(List("Spark", "Hadoop"))            //> List strings    manif(List(1, 2))                         //> Some other type    manif(List("Scala", 3))                   //> Some other type    val m = manifest[A[String]]                   //> m  : Manifest[type_paraments.A[String]] = type_paraments.A[java.lang.String]                                                  //|     println(m)                                //> type_paraments.A[java.lang.String]    val cm = classManifest[A[String]]         //> cm  : ClassManifest[type_paraments.A[String]] = type_paraments.A[java.lang.S                                                  //| tring]    println(cm)                               //> type_paraments.A[java.lang.String]}  

多重界定

class M_A[T]class M_B[T]object Multiple_Bounds {  def main(args: Array[String]) {    implicit val a = new M_A[Int]    implicit val b = new M_B[Int]    def foo[ T : M_A : M_B ](i:T) = println("OK")    foo(2)   }}

类型约束

    def rocky[T](i:T)(implicit ev: T <:< java.io.Serializable) {      print("Life is short,you need spark!") }    rocky("Spark")

Variance
spark 中用的并不多，作为了解