What question will you ask of this table ...
Who was in class on 11/17?
or
On what days did Bob attend?
I think either way you'll want more than just a Hashmap. In the first example, the date would be the key and you want the map to give you a bunch of people who were there that day. In the second example, the name would be the key and you'd want a bunch of dates that person was present.
Does that make sense so far? While you're looking at the collections tutorial, see if you spot a few choices to hold a "bunch" of names or dates.