how to return Comparator values

Just trying to learn how Comparator works.

Looking at:

import java.util.*;
public class EmpSort {
    static final Comparator<Employee> SENIORITY_ORDER =
                                 new Comparator<Employee>() {
        public int compare(Employee e1, Employee e2) {
            return e2.hireDate().compareTo(e1.hireDate());
        }
    };

    // Employee database
    static final Collection<Employee> employees = ... ;

    public static void main(String[] args) {
        List<Employee>e = new ArrayList<Employee>(employees);
        Collections.sort(e, SENIORITY_ORDER);
        System.out.println(e);
    }
}

http://java.sun.com/docs/books/tutorial/collections/interfaces/order.html

I like the line:

Collections.sort(e, SENIORITY_ORDER); //this will sort the collection
"e" by //SENIORITY_ORDER?

However, how or where is SENIORITY_ORDER ranking defined? I guess
it's in

return e2.hireDate().compareTo(e1.hireDate()); //neg, zero, pos

which returns either a negative, positive or zero. We're only
interested in negative results?

I had a specific question, but sorta answered it while writing it...

anyhow.

-Thufir