Datawatcher in 1.12

Hello, i'm coding a NPC system. To show the NPC's hat, i must use the DataWatcher. So i'm getting the datawatcher of my npc like that :

Code:

npc.getDataWatcher()

The problem is whish isn't exist a method with an id and a byte to use the metadatas for the entities :
http://wiki.vg/Entity_metadata#Player

Have you got an idea ? Thanks for your replies !