public void save(Campaign campaign) {
   if (campaign.getId() == null) {
     Integer id = NEXT_ID.getAndIncrement();
     campaign.setId(id);
   }
   campaigns.put(campaign.getId(), campaign);
 }
 public Set<Campaign> findByAdvertiserId(int advertiserId) {
   Set<Campaign> results = newHashSet();
   Iterator<Campaign> campaignsIter = campaigns.values().iterator();
   while (campaignsIter.hasNext()) {
     Campaign campaign = campaignsIter.next();
     if (campaign.getAdvertiser().getId().equals(advertiserId)) {
       results.add(campaign);
     }
   }
   return results;
 }