private static void oldJavaWay() {
        Collections.sort(users, new Comparator<User>() {
            public int compare(User u1, User u2) {
                return u1.id.compareTo(u2.id);
            }
        });

        printListOldWay("by ID");

        Collections.sort(users, new Comparator<User>() {
            public int compare(User u1, User u2) {
                return u1.firstName.compareTo(u2.firstName);
            }
        });

        printListOldWay("by FirstName");

        Collections.sort(users, new Comparator<User>() {
            public int compare(User u1, User u2) {
                return u1.lastName.compareTo(u2.lastName);
            }
        });

        printListOldWay("by LastName");

        Collections.sort(users, new Comparator<User>() {
            public int compare(User u1, User u2) {
                return u1.age.compareTo(u2.age);
            }
        });

        printListOldWay("by Age");
    }