flatMapValues
Pass each value in the key-value pair RDD through a flatMap function without changing the keys; this also retains the original RDD's partitioning.
fun <K, V, U> JavaDStream<Tuple2<K, V>>.flatMapValues(flatMapValuesFunc: (V) -> Iterator<U>): JavaDStream<Tuple2<K, U>>
Return a new DStream by applying a flatmap function to the value of each key-value pairs in 'this' DStream without changing the key.