I assume I have made a TNT cannon.It fires TNT out.but how can I decrease the TNT amout in inventory?hehe..

Look at the bow's source.

 

Also, I can't help but notice you finish EVERY post with "hehe..". What's with that?

Look at the bow's source.

 

Also, I can't help but notice you finish EVERY post with "hehe..". What's with that?

Its some kind of emotion expression that means quite excited.its my personal-like.ahaha-

Fair enough.

Do something like

if(Minecraft.getMinecraft().thePlayer.inventory.hasItem(ItemStack))

{

Minecraft.getMinecraft.thePlayer.inventory.getItemstack(ItemStack).stackSize--;

}

I'm on mobile so the names aren't 100% accurate

Btw to make a new ItemStack do

ItemStack yourItemStack = new ItemStack(net.minecraft.src.Item.TNT;