點選(此處)摺疊或開啟

def main(args: Array[String]) {
//從理論上講，建立一個泛型Array是不可能的，但是可以藉助Manifest實現
//用Manifest[T]的隱式值，儲存執行時T的資訊，在實際執行時，作為一個引數作用在方法執行的上下文中
def arrayMake[T : Manifest](first : T, second : T) = {
val r = new Array[T](2); r(0) = first; r(1) = second; r
}
arrayMake(1,2).foreach(println)
def mkArray[T : ClassTag](elems: T*) = Array[T](elems: _*)
mkArray(42, 13).foreach(println)
mkArray("Japan","Brazil","Germany").foreach(println)
def manif[T](x: List[T])(implicit m: Manifest[T]) = {//原生寫法，隱式引數
if (m <:< manifest[String])
println("List strings")
else
println("Some other type")
}
manif(List("Spark", "Hadoop")) //執行時，會傳入Manifest[String]隱式值
manif(List(1, 2))
manif(List("Scala", 3))
val m = manifest[A[String]]
println(m)
val cm = classManifest[A[String]]
println(cm)
}

由於Manifest在依賴路徑判斷上存在問題，所以，Scala推出來ClassTag、TypeTag用來取代Manifest、classManifest
在Spark中使用最多的就是ClassTag，ClassTag原始碼註釋：
* A `ClassTag[T]` stores the erased class of a given type `T`, accessible via the `runtimeClass`
* field. This is particularly useful for instantiating `Array`s whose element types are unknown
* at compile time.
* ClassTag儲存T被擦出的class資訊，在不知道元素型別而初始化Array時特別有用
*
* `ClassTag`s are a weaker special case of [[scala.reflect.api.TypeTags#TypeTag]]s, in that they
* wrap only the runtime class of a given type, whereas a `TypeTag` contains all static type
* information. That is, `ClassTag`s are constructed from knowing only the top-level class of a
* type, without necessarily knowing all of its argument types. This runtime information is enough
* for runtime `Array` creation.
* ClassTag比TypeTag弱，它只包含執行時class資訊，TypeTag包含所有static型別資訊

ClassTag、Manifest、ClassManifest、TypeTag

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