what is 'hashKey' in HashOperations of 'Spring Data Redis'

I saw someone use 'redisTemplate.opsForHash' like this:

User user = ...
String key = user.getUserId();
redisTemplate.opsForHash().put(key, key.hashCode(), value);

and I found the API:

http://ift.tt/1BZdH1x

but I don't know what is 'key.hashCode' used for.

And I want to know which is better to store a Object to redis:

1. Object -> byte[]


2. or Object -> Json String -> byte[]


3. 
   

   or use hmset, e.g.

   
   

   
   hmset [userId] name [name] address [address]
   
   

Thank you.