..
    DescribeInstancesResult describeInstancesRequest = ec2.describeInstances();
    listEC2Reservations = describeInstancesRequest.getReservations();

    Set<Instance> instances = new HashSet<Instance>();
    for (Reservation reservation : listEC2Reservations) {
        instances.addAll(reservation.getInstances());
    }
    ..

Arrays.sort(instances);

public class MyInstance extends Instance implements Comparable

List<Instance> listInstances = new ArrayList<Instance>(instances);
Collections.sort(listInstances, new Comparator<Instance>() {
   public int compare(Instance i1, Instance i2) {
      return i1.getInstanceId().compareTo(i2.getInstanceId());
   }
});