For the exercise I've built the application to detect whether the .csv file has one user record or many.  Depening on the count, the application will fire a Bulk or Single update.

Instructions to run: 

Clone/Download from gitHub project home
Using IntelliJ Java IDE:  File Open  >> Choose pom.xml >> Open As Project

Navigate to /src/mainjava/com/iterable/csvReader

On line 23 and 24, you will see a placeholder to add the path of the 2 csv files- found in src/main/resources..../././
On line 85 and 91, you will see a placeholder to add your API_Key as a query param.

Right click IDE editor and Run project.  

Enjoy!