code: @Override
      public Double getUserItemPreference(U u, I i) {
          if (userItemPreferences.containsKey(u) && userItemPreferences.get(u).containsKey(i)) {
              return userItemPreferences.get(u).get(i);
          }
          return Double.NaN;
      }
docs: Method that returns the preference between a user and an item.
  
  @param u the user.
  @param i the item.
  @return the preference between a user and an item or NaN.