写Spark代码的时候经常发现rdd没有reduceByKey的方法，这个发生在spark1.2及其以前对版本，因为rdd本身不存在reduceByKey的方法，需要隐式转换成PairRDDFunctions才能访问，因此需要引入Import org.apache.spark.SparkContext._。
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不过到了spark1.3的版本后，隐式转换的放在rdd的object中，这样就会自动被引入，不需要显式引入。
* Defines implicit functions that provide extra functionalities on RDDs of specific types.
* For example, `RDD`.`rddToPairRDDFunctions` converts an RDD into a `PairRDDFunctions` for
* key-value-pair RDDs, and enabling extra functionalities such as `PairRDDFunctions`.`reduceByKey`.
*/
object RDD {
// The following implicit functions were in SparkContext before 1.3 and users had to
// `import SparkContext._` to enable them. Now we move them here to make the compiler find
// them automatically. However, we still keep the old functions in SparkContext for backward
// compatibility and forward to the following functions directly.
implicit def rddToPairRDDFunctions[K, V](rdd: RDD[(K, V)])
(implicit kt: ClassTag[K], vt: ClassTag[V], ord: Ordering[K] = null): PairRDDFunctions[K, V] = {
new PairRDDFunctions(rdd)
}至于什么是隐式转换，简单来讲就是scala偷梁换柱换柱，让隔壁老王来干你干不了的事情了。
