I'm trying to create an Arraylist of players that apply to my plugin (or a HashMap, haven't decided). I was wondering which would take up more space on the ram: an Arraylist of Players or an Arraylist of Player.getEntityId()'s?
That arraylist of players would consist of pointers to the players, not the actual players themselves, so both options take up about the same amount of ram. So, I suggest you use the player object instead of entity ids.