Questionis about Item and ItemStack processing

I wonder how does ItemStack's calling function works.

if two ItemStacks were used and called theirs' item's onItemRightClick() function,etc

are they calling the exactly same instance of the item?

if I made an Item like this:

public class WeaponWhatever extends Item {
         
        public Boolean isUsed;

public WeaponWhatever(int par1ID) {
	super(par1ID);
	setItemName("Whateveer it is");
	setTextureFile(ClientProxy.ITEMS_TEXTURE_PATH);
	setIconCoord(0,0);

                isUsed = false;
}


public ItemStack onItemRightClick(ItemStack par1ItemStack, World par2World, EntityPlayer par3EntityPlayer)
    {

              if(!isUsed)System.out.println("Item hasn't been yet used." );
              else System.out.println("Item has been used. ");
              isUsed = true;
              return par1ItemStack;
    }
}

if I put two of the Item in my inventory,and I right click the first one

when I right click the second one,will the result be "Item has been used"?

If so , how can I do to make ItemStacks use seperate same processing func with different local variables?

Thanks a lot