code: public Set<String> keySet() {
  		// prepare container
  		HashSet<String> set = new HashSet<String>();
  		// add container keys
  		set.addAll(container.keySet());
  		// add cache keys
  		set.addAll(cache.keySet());
  		
  		return set;
  	}
docs: returns the keysets of both the container and cache hashmaps