MCItemPickupEvent

This class was added by a mod with mod-id crafttweaker. So you need to have this mod installed if you want to use this feature.

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.MCItemPickupEvent;

MCItemPickupEvent extends MCPlayerEvent. That means all methods available in MCPlayerEvent are also available in MCItemPickupEvent

Return Type: IItemStack

MCItemPickupEvent.getStack() as IItemStack
myMCItemPickupEvent.getStack();