The event is not cancelable.

The event does not have a result.

Importare la Classe

Link to importare-la-classe

It might be required for you to import the package if you encounter any issues (like casting an Array), so better be safe than sorry and add the import at the very top of the file.

import crafttweaker.api.event.entity.player.xp.PlayerXpEvent;

Extending PlayerEvent

Link to extending-playerevent

PlayerXpEvent extends PlayerEvent. That means all methods available in PlayerEvent are also available in PlayerXpEvent