Ordering of objects by more than one field and get counts

Question

i have an object visit (personid, city, street, place, date)
A person could have visited a same place in the same strret in the same city several times on different dates.
I have a visits 'Set' for a person and I have to get a count of visits he has done to a place..
its basically select count(visits) group by city, street , place.
I know to use interface comparator for a single field.. how to compare multiple fields and get counts??
if you know abt where i can find information please let me know.
Thanks.

Answer 1

For multiple fields, your comparator compares the most significant field first. If they're unequal, it returns +/- accordingly. If they're equal, then it moves on to the next field. And so on, until, finally, if all the relevant fields are equal, then the objects are equal.
Just like what you do when comparing, say, last names. If the first letters are unequal, you're done, else move onto the next letter, and so on..
For the count, you wouldn't use a comparator, as that's for sorting. Exactly how you do it depends on the details of what you're storing and how, which you haven't provided.