code: @Override
      public void addTimestamp(final U u, final I i, final Long t) {
          Map<I, Set<Long>> userTimestamps = userItemTimestamps.get(u);
          if (userTimestamps == null) {
              userTimestamps = new HashMap<>();
              userItemTimestamps.put(u, userTimestamps);
          }
          Set<Long> timestamps = userTimestamps.get(i);
          if (timestamps == null) {
              timestamps = new HashSet<>();
              userTimestamps.put(i, timestamps);
          }
          timestamps.add(t);
      }
docs: Method that adds a timestamp to the model between a user and an item.
  
  @param u the user.
  @param i the item.
  @param t the timestamp.