Changes

===RDD Overview===

One of the most important concepts in Spark is a resilient distributed dataset (RDD). RDD is a collection of elements partitioned across the nodes of the cluster that can be operated in parallel. RDDs are created by starting with a file, or an existing Java collection in the driver program, and transforming it. We will introduce some key APIs provided by Spark Core 2.2.1 using Java 8. You can find more information about the RDD here. https://spark.apache.org/docs/2.2.1/rdd-programming-guide.html