RDD [（K，V）]是rdd中的一种类型：RDD [（K，V）]？

Question

In spark file of /core/src/main/scala/org/apache/spark/SparkContext.scala, the has the following

implicit def rddToPairRDDFunctions[K, V](rdd: RDD[(K, V)])
      (implicit kt: ClassTag[K], vt: ClassTag[V], ord: Ordering[K] = null) = {
    new PairRDDFunctions(rdd)
  }

I think RDD[(K, V)] is a type for paraemter of rdd. However, how could we define a type like RDD[(K, V)]? The following could NOT be compiled

abstract class RDD[(K, V)]

But the following could.

abstract class RDD[K, V]

So what's RDD[(K, V)]

Answer 1

[(K, V)] is a tuple of K and V, combined as a single argument

[K, V] are two distinct type arguments

And RDD accepts only a single type argument

Answer 2

(K, V) means Tuple2[K, V] , so rdd: RDD[(K, V)] means rdd: RDD[Tuple2[K, V]] . Type RDD should have a single type parameter.

class RDD[T]
new RDD[(Int, String)] // new RDD[Tuple2[Int, String]]