Is there any way to assign NBT data to items/itemstacks.

Any method where you have an ItemStack parameter, you can assign data to it/get data from it using stack.getTagCompound().

That's rather convenient,

 

int length = stack.getTagCompound().getInteger("Length");

 

However, yields a null pointer exception.

 

I assume I would need to set the integer first, but where would I do that?

Uhhh..

 

stack.getTagCompound().setInteger("Length", intValue);

Right, but where would I throw that?

 

Would I need to extend ItemStack and put it in the constructor?

ItemStack can't be extended, it is final. However, getting an integer shouldn't throw NPE as it will always either return a value if it has one or 0 if it does not. Look at the getInteger(String) method in NBTTagCompound and you'll see that it always returns something. The NPE is coming from one of two things (most likely the second) - The stack is null or it's tag compound is null. ItemStack tag compounds are protected against NPE, so first do this:

 

NBTTagCompound nbt = stack.getTagCompound();
	if(nbt == null)
	{
		nbt = new NBTTagCompound();
		stack.setTagCompound(nbt);
	}

 

A specific example of what you're trying to do with the NBT would be helpful, that way I can tell you where to set the int value for retrieval later.

A specific example of what you're trying to do with the NBT would be helpful, that way I can tell you where to set the int value for retrieval later.

 

Not needed, your example works perfectly, thank you.

Good, glad I could help.