When using InventoryAction.HOTBAR_SWAP in InventoryClickEvent, there is no way to get the other item. I set up a simple test plugin to show the items gettable from the click event (#getCurrentItem and #getCursor), and both are empty when swapping an item from the hotbar to any open inventory slot.

 

@EventHandler
public void onSwap(InventoryClickEvent evt) {
   if (evt.getAction() == InventoryAction.HOTBAR_SWAP)

{      evt.getWhoClicked().sendMessage(evt.getCurrentItem().toString());      evt.getWhoClicked().sendMessage(evt.getCursor().toString());    }

}

 

It would be beneficial to be able to get the item coming into the slot your mouse is over when you swap.