code: @Override
      public Iterable<Long> getUserItemTimestamps(U u, I i) {
          if (userItemTimestamps.containsKey(u) && userItemTimestamps.get(u).containsKey(i)) {
              return userItemTimestamps.get(u).get(i);
          }
          return null;
      }
docs: Method that returns the map with the timestamps between users and items.
  
  @return the map with the timestamps between users and items.