code: public boolean exists(K k) {
  		Key<K> key = keys.get(k);
  		if(key != null) {
  			return objects.get(key) != null;
  		}
  		return false;
  	}
docs: Returns true if the object with the given key resides in the cache.
  @param k
  @return true if the object with the given key resides in the cache.