@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;
    }
@ 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 ; }
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.