Exchange of items

You are looking for,

if (playerIn.onGround == true)
{
// Do the regeneration thing here
}

To clarify, this only works if you are on the ground. Is that what you want?

Like if you have dirt in the inventory?

Ok, so I tested some methods in MCreator and this is possible without coding. Here is the MCreator method,

Mirror Link

Alternatively, you can add this snippet of code to your item,

    public ActionResult<ItemStack> onItemRightClick(World worldIn, PlayerEntity playerIn, Hand handIn)
    {
        ItemStack itemstack = playerIn.getHeldItem(handIn);
        if (!worldIn.isRemote && playerIn.inventory.hasItemStack(new ItemStack(Blocks.DIRT, 1))) {
            //OPTIONAL BELOW
            playerIn.inventory.clearMatchingItems(p -> new ItemStack(Blocks.DIRT, 1).getItem() == p.getItem(), 1);
            //OPTIONAL^^^ THIS DELETES THE 1 ITEM. YOU CAN CHANGE THE 1 TO WHATEVER AMOUNT YOU DESIRE.
        }
        return ActionResult.resultSuccess(itemstack);
    }

No problem 😁