Mark Minecraft plugin arena as busy?

I'm currently writing a 1vs1 plugin (with quite good success) but there's a serious problem: I currently have 6 arenas. How can I effectively mark an arena as occupied? MFG Johannes

There are different methods. You could save the arenas in ArrayLists, enter them in a config or a database.

Could you tell me how exactly i should go there? The arenas are saved as strings, they are also loaded and the players are successfully thrown into a match. How should I start with the array list exactly? I'm pretty new to code