How to find unique values after groupBy() in spark dataframe ?

[Devi P.V]

Hi all,

I have a dataframe like following,

+---------+----------+
|client_id|Date      |
+-------- +----------+
| a       |2016-11-23|
| b       |2016-11-18|
| a       |2016-11-23|
| a       |2016-11-23|
| a       |2016-11-24|
+---------+----------+

I want to find unique dates of each client_id using spark dataframe.

expected output

a  (2016-11-23, 2016-11-24)
b   2016-11-18

I tried with df.groupBy("client_id").But I don't know how to find distinct
values after groupBy().
How to do this?
Is any other efficient methods are available for doing this ?
I am using scala 2.11.8 & spark 2.0


Thanks