Checking for item placed in GUI slot?

[Roboguy99]

Hi there,

I'm trying to make a machine which has 3 slots - 2 inputs and an output. One of the inputs is for anything, and the other is for cobblestone only. What I want is that when you place cobblestone into the bottom slot, it increases a variable and in turn increases a meter on the screen. How can I check for cobblestone in the slot?

 

I have tried

public void updateEntity()
{

	if(this.getStackInSlot(2).equals(Blocks.cobblestone));
	{
		System.out.println("I am awesome!");
	}
}

with no luck. When I attempt to load a world I get a null pointer exception.

 

P.S I know this is a bit off topic, but will help other issues I'm having with the GUI. How can I only let stone be placed in the slot, and how can I only allow items to be taken from the output slot and not placed?

[sequituri]

If there is no stack in slot 2, you're going to get that NPE. Try testing for a 'null' before using .equals()

[coolAlias]

Firstly, you should always check if the stack in the slot is null before doing anything with it, or you will get a null pointer exception.

 

Secondly, an ItemStack will never return true when comparing it to a Block with equals; instead, you need to get the Item and compare that to the ItemBlock:

ItemStack stack = getStackInSlot(2); // or whatever slot

if (stack != null && stack.getItem() == Item.getItemFromBlock(Blocks.cobblestone)) {
// now you know for sure you've got a stack with cobblestone
}

 

Finally, you should look at the Slot class for methods you can use to limit what goes into a slot; you will need to make your own class that extends Slot and add that instead of the generic Slot to your container.