(SOLVED) [1.12.2] Problems with custom item and NBT

[MythicalChromosome]

I'm having two issues with my custom item, both of which I suspect are related to NBT data.

 

Firstly, the addInformation method crashes the game on loading. This might have to do with some of the necessary variables not being initialized at that point in loading but I can't find anything that could help with that issue.

 

Second, right-clicking with the item for some reason causes the entire stack to disappear. Just to make sure it wasn't just using up only one of them, I changed the max stack size and it still removed the entire stack.

 

public class ItemStandArrow extends Item{
	
	private static final String name = "standArrow";
	public ItemStandArrow()
    {
		super();
        setUnlocalizedName(name);
        setCreativeTab(CreativeTabs.TOOLS);
        setMaxStackSize(1);
    }
	
	@Override
	public ActionResult<ItemStack> onItemRightClick(World world, EntityPlayer player, EnumHand hand)
	{
	    NBTTagCompound nbt;
	    ItemStack stack = player.getActiveItemStack();
	    NBTTagList nbttaglist;

	    if (stack.hasTagCompound())
	    {
	        nbt = stack.getTagCompound();
	    }
	    else
	    {
	        nbt = new NBTTagCompound();
	    }
	    
	    if (nbt.hasKey("usedOn")){
	    	nbttaglist = (NBTTagList) nbt.getTag("usedOn");
	    	for (int i = 0; i<nbttaglist.tagCount(); i++){
	    		if (nbttaglist.getStringTagAt(i) == player.getUniqueID().toString()){
	    			System.out.println("Failed to use arrow");
	    			return new ActionResult<ItemStack>(EnumActionResult.FAIL, stack);
	    		}
	    	}
	    }
	    StandHandlingMethods.grantRandomStandFromTable(player, null);
	    System.out.println("Successfully used arrow");
	    return new ActionResult<ItemStack>(EnumActionResult.SUCCESS, stack);
	}
	
	public void addEntityToBlacklist(Entity entity, NBTTagCompound nbt, ItemStack stack){
	    NBTTagList nbttaglist;
	    
	    if (nbt.hasKey("usedOn")){
	    	nbttaglist = (NBTTagList) nbt.getTag("usedOn");
	    }
	    else{
	    	nbttaglist = new NBTTagList();
	    }
	    
	    String entityId = entity.getUniqueID().toString();
	    NBTTagCompound compound = new NBTTagCompound();
	    compound.setString("name", entityId);
	    
	    nbttaglist.appendTag(compound);
	    
	    nbt.setTag("usedOn", nbttaglist);
	    
	    stack.setTagCompound(nbt);
	}

	@Override
	@SideOnly(Side.CLIENT)
	public void addInformation(ItemStack stack, @Nullable World worldIn,
			java.util.List<String> tooltip, ITooltipFlag flagIn) {
		NBTTagCompound nbt = stack.getTagCompound();
		NBTTagList nbttaglist;
		EntityPlayer player = Minecraft.getMinecraft().player;
		
		super.addInformation(stack, worldIn, tooltip, flagIn);
		
		if (nbt.hasKey("usedOn")){
	    	nbttaglist = (NBTTagList) nbt.getTag("usedOn");
	    	for (int i = 0; i<nbttaglist.tagCount(); i++){
	    		if (nbttaglist.getStringTagAt(i) == player.getUniqueID().toString()){
	    			tooltip.add("Already used by " + player.getDisplayName());
	    		}
	    	}
	    }
	}
}

 

Edited January 3, 2018 by MythicalChromosome

[Differentiation]

Post your FML Log(s)?

[Differentiation]

  On 12/31/2017 at 3:13 AM, MythicalChromosome said:

NBTTagList nbttaglist;

Expand  

 

  On 12/31/2017 at 3:13 AM, MythicalChromosome said:

if (nbt.hasKey("usedOn")){ nbttaglist = (NBTTagList) nbt.getTag("usedOn"); for (int i = 0; i<nbttaglist.tagCount(); i++){ if (nbttaglist.getStringTagAt(i) == player.getUniqueID().toString()){ tooltip.add("Already used by " + player.getDisplayName()); } } }

Expand  

What if it doesn't have "usedOn"? "nbttaglist" is null. That's a NullPointerException.

[Tmtravlr]

Hey,

 

Looks like you forgot to check if the NBT tag is null in addInformation (or call stack.hasTagCompound()).

 

For the right click one, you should use the player.getHeldItem(hand) instead of getActiveItem(), which is for items you're using over a time period, like food and bows. That is probably empty when that right click is called, which is returning an empty item in the result. 

[Differentiation]

    @Override
    @SideOnly(Side.CLIENT)
    public void addInformation(ItemStack stack, World worldIn, List<String> tooltip, ITooltipFlag flagIn) 
    {
        EntityPlayer playerIn = Minecraft.getMinecraft().player;	
        super.addInformation(stack, worldIn, tooltip, flagIn);
		
        if (stack.getTagCompound() != null && nbt.hasKey("usedOn"))
        {
            NBTTagCompound nbt = stack.getTagCompound();

            if (nbt.getTag("usedOn") != null
            {
                NBTTagList list = (NBTTagList) nbt.getTag("usedOn");
        
                for (int i = 0; i < list.tagCount(); ++i)
                {
                    if (list.getStringTagAt(i) == playerIn.getUniqueID().toString())
                    {
                        tooltip.add("Already used by " + playerIn.getDisplayName());
                    }
                }
            }
        }
    }

There, that's better.

You have to add null checks, not everything is present all the time.

Edited December 31, 2017 by Differentiation

[MythicalChromosome]

  On 12/31/2017 at 3:33 AM, Differentiation said:

    @Override
    @SideOnly(Side.CLIENT)
    public void addInformation(ItemStack stack, World worldIn, List<String> tooltip, ITooltipFlag flagIn) 
    {
        EntityPlayer playerIn = Minecraft.getMinecraft().player;	
        super.addInformation(stack, worldIn, tooltip, flagIn);
		
        if (stack.getTagCompound() != null && nbt.hasKey("usedOn"))
        {
            NBTTagCompound nbt = stack.getTagCompound();

            if (nbt.getTag("usedOn") != null
            {
                NBTTagList list = (NBTTagList) nbt.getTag("usedOn");
        
                for (int i = 0; i < list.tagCount(); ++i)
                {
                    if (list.getStringTagAt(i) == playerIn.getUniqueID().toString())
                    {
                        tooltip.add("Already used by " + playerIn.getDisplayName());
                    }
                }
            }
        }
    }

There, that's better.

You have to add null checks, not everything is present all the time.

Expand  

I'm going to assume this is a typo, but you have nbt.hasKey() being called before nbt is initialized.

Edited December 31, 2017 by MythicalChromosome
I just realized nbt could be moved to be initialized earlier and just nullcheck that instead of stack.getTagCompound(), derp

[Differentiation]

  On 12/31/2017 at 4:25 PM, MythicalChromosome said:

I'm going to assume this is a typo, but you have nbt.hasKey() being called before nbt is initialized.

Expand  

    @Override
    @SideOnly(Side.CLIENT)
    public void addInformation(ItemStack stack, World worldIn, List<String> tooltip, ITooltipFlag flag) 
    {
        EntityPlayer playerIn = Minecraft.getMinecraft().player;	
        super.addInformation(stack, worldIn, tooltip, flag);
		
        if (stack.getTagCompound() != null)
        {
            NBTTagCompound nbt = stack.getTagCompound();

            if (nbt.getTag("usedOn") != null && nbt.hasKey("usedOn"))
            {
                NBTTagList list = (NBTTagList) nbt.getTag("usedOn");
        
                for (int i = 0; i < list.tagCount(); ++i)
                {
                    if (list.getStringTagAt(i) == playerIn.getUniqueID().toString())
                    {
                        tooltip.add("Already used by " + playerIn.getDisplayName());
                    }
                }
            }
        }
    }

Sorry, that's better.

Edited December 31, 2017 by Differentiation

[MythicalChromosome]

Argh, NBT with itemstacks is confusing...

 

Both of my original issues were fixed, but now I have a new one showing up. The item reports a successful usage whenever it's right-clicked with, rather than only the first time it's right-clicked as it's supposed to. The method that's supposed to blacklist entities from using it is definitely being called, but it doesn't seem to be doing anything. I imagine this has something to do with problems reading from or writing to NBT, how would I go about fixing this?

 

@Override
	public ActionResult<ItemStack> onItemRightClick(World world, EntityPlayer player, EnumHand hand)
	{
	    NBTTagCompound nbt;
	    ItemStack stack = player.getHeldItem(hand);
	    NBTTagList nbttaglist;

	    if (stack.hasTagCompound())
	    {
	        nbt = stack.getTagCompound();
	    }
	    else
	    {
	        nbt = new NBTTagCompound();
	    }
	    
	    if (nbt.hasKey("usedOn")){
	    	nbttaglist = (NBTTagList) nbt.getTag("usedOn");
	    	for (int i = 0; i<nbttaglist.tagCount(); i++){
	    		if (nbttaglist.getStringTagAt(i) == player.getUniqueID().toString()){
	    			System.out.println("Failed to use arrow");
	    			return new ActionResult<ItemStack>(EnumActionResult.FAIL, stack);
	    		}
	    	}
	    }
	    StandHandlingMethods.grantRandomStandFromTable(player, null); //stub so far
	    addEntityToBlacklist(player, nbt, stack);
	    System.out.println("Successfully used arrow");
	    return new ActionResult<ItemStack>(EnumActionResult.SUCCESS, stack);
	}
	
	public void addEntityToBlacklist(Entity entity, NBTTagCompound nbt, ItemStack stack){
	    NBTTagList nbttaglist;
	    
	    if (nbt.hasKey("usedOn")){
	    	nbttaglist = (NBTTagList) nbt.getTag("usedOn");
	    }
	    else{
	    	nbttaglist = new NBTTagList();
	    }
	    
	    String entityId = entity.getUniqueID().toString();
	    NBTTagCompound compound = new NBTTagCompound();
	    compound.setString("name", entityId);
	    
	    nbttaglist.appendTag(compound);
	    
	    nbt.setTag("usedOn", nbttaglist);
	    
	    System.out.println("Blacklisted " + entityId + " from using this Arrow");
	    
	    stack.setTagCompound(nbt);
	}

	@Override
	@SideOnly(Side.CLIENT)
	public void addInformation(ItemStack stack, @Nullable World worldIn,
			java.util.List<String> tooltip, ITooltipFlag flagIn) {
		
		NBTTagList nbttaglist;
		EntityPlayer player = Minecraft.getMinecraft().player;
		
		
		super.addInformation(stack, worldIn, tooltip, flagIn);
		
		if (stack.getTagCompound() != null){
			NBTTagCompound nbt = stack.getTagCompound();
			if (nbt.getTag("usedOn") != null && nbt.hasKey("usedOn")){
		    	nbttaglist = (NBTTagList) nbt.getTag("usedOn");
		    	for (int i = 0; i<nbttaglist.tagCount(); i++){
		    		if (nbttaglist.getStringTagAt(i) == player.getUniqueID().toString()){
		    			tooltip.add("Already used by " + player.getDisplayName());
		    		}
		    	}
		    }
		}
	}

(Reuploaded code because I changed it somewhat significantly in the interrim)

[Differentiation]

  On 12/31/2017 at 5:16 PM, MythicalChromosome said:

Argh, NBT with itemstacks is confusing...

 

Both of my original issues were fixed, but now I have a new one showing up. The item reports a successful usage whenever it's right-clicked with, rather than only the first time it's right-clicked as it's supposed to. The method that's supposed to blacklist entities from using it is definitely being called, but it doesn't seem to be doing anything. I imagine this has something to do with problems reading from or writing to NBT, how would I go about fixing this?

 

@Override
	public ActionResult<ItemStack> onItemRightClick(World world, EntityPlayer player, EnumHand hand)
	{
	    NBTTagCompound nbt;
	    ItemStack stack = player.getHeldItem(hand);
	    NBTTagList nbttaglist;

	    if (stack.hasTagCompound())
	    {
	        nbt = stack.getTagCompound();
	    }
	    else
	    {
	        nbt = new NBTTagCompound();
	    }
	    
	    if (nbt.hasKey("usedOn")){
	    	nbttaglist = (NBTTagList) nbt.getTag("usedOn");
	    	for (int i = 0; i<nbttaglist.tagCount(); i++){
	    		if (nbttaglist.getStringTagAt(i) == player.getUniqueID().toString()){
	    			System.out.println("Failed to use arrow");
	    			return new ActionResult<ItemStack>(EnumActionResult.FAIL, stack);
	    		}
	    	}
	    }
	    StandHandlingMethods.grantRandomStandFromTable(player, null); //stub so far
	    addEntityToBlacklist(player, nbt, stack);
	    System.out.println("Successfully used arrow");
	    return new ActionResult<ItemStack>(EnumActionResult.SUCCESS, stack);
	}
	
	public void addEntityToBlacklist(Entity entity, NBTTagCompound nbt, ItemStack stack){
	    NBTTagList nbttaglist;
	    
	    if (nbt.hasKey("usedOn")){
	    	nbttaglist = (NBTTagList) nbt.getTag("usedOn");
	    }
	    else{
	    	nbttaglist = new NBTTagList();
	    }
	    
	    String entityId = entity.getUniqueID().toString();
	    NBTTagCompound compound = new NBTTagCompound();
	    compound.setString("name", entityId);
	    
	    nbttaglist.appendTag(compound);
	    
	    nbt.setTag("usedOn", nbttaglist);
	    
	    System.out.println("Blacklisted " + entityId + " from using this Arrow");
	    
	    stack.setTagCompound(nbt);
	}

	@Override
	@SideOnly(Side.CLIENT)
	public void addInformation(ItemStack stack, @Nullable World worldIn,
			java.util.List<String> tooltip, ITooltipFlag flagIn) {
		
		NBTTagList nbttaglist;
		EntityPlayer player = Minecraft.getMinecraft().player;
		
		
		super.addInformation(stack, worldIn, tooltip, flagIn);
		
		if (stack.getTagCompound() != null){
			NBTTagCompound nbt = stack.getTagCompound();
			if (nbt.getTag("usedOn") != null && nbt.hasKey("usedOn")){
		    	nbttaglist = (NBTTagList) nbt.getTag("usedOn");
		    	for (int i = 0; i<nbttaglist.tagCount(); i++){
		    		if (nbttaglist.getStringTagAt(i) == player.getUniqueID().toString()){
		    			tooltip.add("Already used by " + player.getDisplayName());
		    		}
		    	}
		    }
		}
	}

(Reuploaded code because I changed it somewhat significantly in the interrim)

Expand  

I tried playing with it myself and it just doesn't seem to work. 
You can do it my way by just adding to player capability. I do it this way:

public ActionResult<ItemStack> onItemRightClick(World worldIn, EntityPlayer playerIn, EnumHand hand)
{
  DinocraftPlayer player = DinocraftPlayer.getEntityPlayer(playerIn);
  
  if (!player.getActions().hasUsedItem1())
  {
    player.setHasUsedItem1(true);
    return new ActionResult<ItemStack>(EnumActionResult.SUCCESS, stack);
  }
  else
  {
    return new ActionResult<ItemStack>(EnumActionResult.FAIL, stack);
  }
}

Now the player can't use this item until they die. And then the process repeats.

 

For your case, it is best to use player capabilities since the item usage data is unique to the player itself and not the item.

Edited December 31, 2017 by Differentiation

[MythicalChromosome]

I thought of using capabilities, but from what I see that makes it so that the item can only be used once in total. The idea I was going for was that the player could find more than one of those items, and could only use each one on themselves once. Your solution isn't quite what I'm going for, but I can't figure out how to make it so that each of the items can only be used once EACH without using NBT... hmm.

[Differentiation]

  On 12/31/2017 at 10:58 PM, MythicalChromosome said:

I thought of using capabilities, but from what I see that makes it so that the item can only be used once in total. The idea I was going for was that the player could find more than one of those items, and could only use each one on themselves once. Your solution isn't quite what I'm going for, but I can't figure out how to make it so that each of the items can only be used once EACH without using NBT... hmm.

Expand  

Wait, wait, wait, wait!! Are you trying to make it so that you can use the item once PER PLAYER?? Or every player can use that item once. So for example, if I use the item and I give it to another player, should they be able to use it?

Edited December 31, 2017 by Differentiation

[MythicalChromosome]

Yeah. I probably should've made that clear earlier, but it should be like this:

1. Each individual item can only be used once per player, i.e. if a player finds another one they can use that item even if they've already used a different one.

2. Each item can be used an infinite number of times, but only once by any individual player, meaning they're reusable by a different person. So yes, if you give it to another player after using it they should also be able to use it.

[Differentiation]

  On 12/31/2017 at 11:46 PM, MythicalChromosome said:

Yeah. I probably should've made that clear earlier, but it should be like this:

1. Each individual item can only be used once per player, i.e. if a player finds another one they can use that item even if they've already used a different one.

2. Each item can be used an infinite number of times, but only once by any individual player, meaning they're reusable by a different person. So yes, if you give it to another player after using it they should also be able to use it.

Expand  

Yeah, I see that's why you used the UUID. I couldn't figure it out myself since I'm not that good at NBTs.

Best of luck figuring out the solution!

Edited January 1, 2018 by Differentiation

[MythicalChromosome]

  On 1/1/2018 at 12:02 AM, Differentiation said:

Yeah, I see that's why you used the UUID. I couldn't figure it out myself since I'm not that good at NBTs.

Best of luck figuring out the solution!

Expand  

Heh, well, thanks for your support anyways. If I figure out a solution to what I was looking for I'll post it here.

 

(Btw, happy new year!)

[Differentiation]

  On 1/1/2018 at 12:26 AM, MythicalChromosome said:

Heh, well, thanks for your support anyways. If I figure out a solution to what I was looking for I'll post it here.

 

(Btw, happy new year!)

Expand  

I would like to see the solution if you figure it out. I myself don't have such a good experience with NBT, but I would like to see what you ended up doing.

Happy New Year to you as well. Here in New York, New Year is in 5 hours!  

[MythicalChromosome]

  On 1/1/2018 at 12:34 AM, Differentiation said:

I would like to see the solution if you figure it out. I myself don't have such a good experience with NBT, but I would like to see what you ended up doing.

Happy New Year to you as well. Here in New York, New Year is in 5 hours!  

Expand  

After a bit more debugging, the solution to my problem turned out to be simple (as it usually tends to be...:P)

 

I wasn't checking against the tag correctly. The method I was using was checking against the entire tag as a string, when it should've been checking against the value. Simply changing the method called made it work perfectly as intended:

public class ItemStandArrow extends Item{
	
	private static final String name = "standArrow";
	public ItemStandArrow()
    {
		super();
        setUnlocalizedName(name);
        setCreativeTab(CreativeTabs.TOOLS);
        setMaxStackSize(1);
    }
	
	@Override
	public ActionResult<ItemStack> onItemRightClick(World world, EntityPlayer player, EnumHand hand)
	{
	    NBTTagCompound nbt;
	    ItemStack stack = player.getHeldItem(hand);
	    NBTTagList nbttaglist;

	    if (stack.hasTagCompound())
	    {
	        nbt = stack.getTagCompound();
	    }
	    else
	    {
	        nbt = new NBTTagCompound();
	    }
	    
	    if (nbt.hasKey("usedOn")){
	    	nbttaglist = (NBTTagList) nbt.getTag("usedOn");
	    	for (int i = 0; i<nbttaglist.tagCount(); i++){
	    		System.out.println("Checking index " + i + ": "+ nbttaglist.getStringTagAt(i));
	    		if (nbttaglist.getCompoundTagAt(i).getString("name").equals(player.getUniqueID().toString())){
	    			System.out.println("Failed to use arrow");
	    			return new ActionResult<ItemStack>(EnumActionResult.FAIL, stack);
	    		}
	    	}
	    }
	    StandHandlingMethods.grantRandomStandFromTable(player, null); //stub so far
	    addEntityToBlacklist(player, nbt, stack);
	    System.out.println("Successfully used arrow");
	    return new ActionResult<ItemStack>(EnumActionResult.SUCCESS, stack);
	}
	
	public void addEntityToBlacklist(Entity entity, NBTTagCompound nbt, ItemStack stack){
	    NBTTagList nbttaglist;
	    
	    if (nbt.hasKey("usedOn")){
	    	nbttaglist = (NBTTagList) nbt.getTag("usedOn");
	    }
	    else{
	    	nbttaglist = new NBTTagList();
	    }
	    
	    String entityId = entity.getUniqueID().toString();
	    NBTTagCompound compound = new NBTTagCompound();
	    compound.setString("name", entityId);
	    
	    nbttaglist.appendTag(compound);
	    
	    nbt.setTag("usedOn", nbttaglist);
	    
	    stack.setTagCompound(nbt);
	    
	    System.out.println("Blacklisted " + entityId + " from using this Arrow");
	    
	    
	}

	@Override
	@SideOnly(Side.CLIENT)
	public void addInformation(ItemStack stack, @Nullable World worldIn,
			java.util.List<String> tooltip, ITooltipFlag flagIn) {
		
		NBTTagList nbttaglist;
		EntityPlayer player = Minecraft.getMinecraft().player;
		
		
		super.addInformation(stack, worldIn, tooltip, flagIn);
		
		if (stack.getTagCompound() != null){
			NBTTagCompound nbt = stack.getTagCompound();
			if (nbt.getTag("usedOn") != null && nbt.hasKey("usedOn")){
		    	nbttaglist = (NBTTagList) nbt.getTag("usedOn");
		    	for (int i = 0; i<nbttaglist.tagCount(); i++){
		    		if (nbttaglist.getCompoundTagAt(i).getString("name").equals(player.getUniqueID().toString())){
		    			tooltip.add("Already used by " + player.getName());
		    		}
		    	}
		    }
		}
	}
}

So the problem wasn't in writing to and reading from the ItemStack's NBT data, it was in how I was interpreting the data within the program.

 

Hopefully you learned some more about NBT from this, as I did.

[Differentiation]

  On 1/3/2018 at 4:53 PM, MythicalChromosome said:

After a bit more debugging, the solution to my problem turned out to be simple (as it usually tends to be...:P)

 

I wasn't checking against the tag correctly. The method I was using was checking against the entire tag as a string, when it should've been checking against the value. Simply changing the method called made it work perfectly as intended:

public class ItemStandArrow extends Item{
	
	private static final String name = "standArrow";
	public ItemStandArrow()
    {
		super();
        setUnlocalizedName(name);
        setCreativeTab(CreativeTabs.TOOLS);
        setMaxStackSize(1);
    }
	
	@Override
	public ActionResult<ItemStack> onItemRightClick(World world, EntityPlayer player, EnumHand hand)
	{
	    NBTTagCompound nbt;
	    ItemStack stack = player.getHeldItem(hand);
	    NBTTagList nbttaglist;

	    if (stack.hasTagCompound())
	    {
	        nbt = stack.getTagCompound();
	    }
	    else
	    {
	        nbt = new NBTTagCompound();
	    }
	    
	    if (nbt.hasKey("usedOn")){
	    	nbttaglist = (NBTTagList) nbt.getTag("usedOn");
	    	for (int i = 0; i<nbttaglist.tagCount(); i++){
	    		System.out.println("Checking index " + i + ": "+ nbttaglist.getStringTagAt(i));
	    		if (nbttaglist.getCompoundTagAt(i).getString("name").equals(player.getUniqueID().toString())){
	    			System.out.println("Failed to use arrow");
	    			return new ActionResult<ItemStack>(EnumActionResult.FAIL, stack);
	    		}
	    	}
	    }
	    StandHandlingMethods.grantRandomStandFromTable(player, null); //stub so far
	    addEntityToBlacklist(player, nbt, stack);
	    System.out.println("Successfully used arrow");
	    return new ActionResult<ItemStack>(EnumActionResult.SUCCESS, stack);
	}
	
	public void addEntityToBlacklist(Entity entity, NBTTagCompound nbt, ItemStack stack){
	    NBTTagList nbttaglist;
	    
	    if (nbt.hasKey("usedOn")){
	    	nbttaglist = (NBTTagList) nbt.getTag("usedOn");
	    }
	    else{
	    	nbttaglist = new NBTTagList();
	    }
	    
	    String entityId = entity.getUniqueID().toString();
	    NBTTagCompound compound = new NBTTagCompound();
	    compound.setString("name", entityId);
	    
	    nbttaglist.appendTag(compound);
	    
	    nbt.setTag("usedOn", nbttaglist);
	    
	    stack.setTagCompound(nbt);
	    
	    System.out.println("Blacklisted " + entityId + " from using this Arrow");
	    
	    
	}

	@Override
	@SideOnly(Side.CLIENT)
	public void addInformation(ItemStack stack, @Nullable World worldIn,
			java.util.List<String> tooltip, ITooltipFlag flagIn) {
		
		NBTTagList nbttaglist;
		EntityPlayer player = Minecraft.getMinecraft().player;
		
		
		super.addInformation(stack, worldIn, tooltip, flagIn);
		
		if (stack.getTagCompound() != null){
			NBTTagCompound nbt = stack.getTagCompound();
			if (nbt.getTag("usedOn") != null && nbt.hasKey("usedOn")){
		    	nbttaglist = (NBTTagList) nbt.getTag("usedOn");
		    	for (int i = 0; i<nbttaglist.tagCount(); i++){
		    		if (nbttaglist.getCompoundTagAt(i).getString("name").equals(player.getUniqueID().toString())){
		    			tooltip.add("Already used by " + player.getName());
		    		}
		    	}
		    }
		}
	}
}

So the problem wasn't in writing to and reading from the ItemStack's NBT data, it was in how I was interpreting the data within the program.

 

Hopefully you learned some more about NBT from this, as I did.

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A very silly mistake indeed

I'm glad you figured it out.

Happy modding!