How to make an ArrayList on a PlayerJoinEvent

How would I make a new ArrayList per player when the first join a server (PlayerJoinEvent), so each player has an ArrayList (for friends list plugin)?

I am want an ArrayList, or any other List to be made with the new player's name, and their friends list as the list name

I want this list to be open for command adding so a player can add friends from the server to there friends list. I just don't know how I would make a PlayerJoinEvent create a list based on the player.