HashMap with one or more values in Java

I am new to this forum so if I do not follow some rule, please excuse me. My question is:

How could I create a HashMap that contains one, or several values? I've thought about the ArrayList, but I want each ArrayList to be unique for each key.

All this comes for an exercise of 'Accept the challenge' a platform where you do exercises on programming, I leave the link of the exercise here

Exercise

Thank you very much!