GroupByKey with datasets in Spark 2.0 using Java
https://stackoverflow.com/questions/39390912With a DataFrame in Spark 2.0: scala> val data = List ( (1, "a"), (1, "b"), (1, "c"), (2, "a"), (2, "b")).toDF ("c1", "c2") data: org.apache.spark.sql.DataFrame = [c1: …