From this post, I've modified the original armor attribute of my custom item. The biggest change between that post and now is that the attribute multimap is immutable, so it needs to be copied to a mutable map, adjusted, and then copied to an immutable one again. Normally it would be easier to create a new armor material, however the value I need depends on a variable (extraArmor) in the item class. The armor value changes correctly, but only when one instance of the item is present in the inventory. If another one is there, then the armor of the newest instance overwrites the previous item(s). I've tired alternatives such as adding a new armor modifier (Attempt 2) or using an ItemAttributeModifierEvent instead of getAttributeModifiers, but they all face the same problem. I'm not sure why they would change since the extraArmor value isn't static and isn't called or updated statically as far as I'm aware. If anyone can help I'd be grateful.

Attempt 1: Edit original values

Spoiler

	public static<K, V> ListMultimap<K, V> toMutableMultimap(Multimap<K, V> original) {
		ListMultimap<K, V> copy = ArrayListMultimap.create();
		copy.putAll(original);
		return copy;
	}

	public static<K, V> ImmutableMultimap<K, V> toImmutableMultimap(ListMultimap<K, V> original) {
		return new ImmutableMultimap.Builder<K, V>().putAll(original).build();
	}

	@Override
	public Multimap<Attribute, AttributeModifier> getAttributeModifiers(EquipmentSlotType slot, ItemStack stack) {
		Multimap<Attribute, AttributeModifier> multimap = super.getAttributeModifiers(slot, stack);
		ListMultimap<Attribute, AttributeModifier> mutableMultimap = toMutableMultimap(multimap);
		final Collection<AttributeModifier> modifiers = mutableMultimap.get(Attributes.ARMOR);
      
		Item item = stack.getItem();
		if (item instanceof MyItem && slot == this.slot) {
			ArmorItem armorItem = (ArmorItem) item;
			MyItem myItem = (MyItem) item;

			UUID[] uuid_arr = ObfuscationReflectionHelper.getPrivateValue(ArmorItem.class, armorItem, "ARMOR_MODIFIER_UUID_PER_SLOT");
			if (uuid_arr != null) {
				UUID uuid = uuid_arr[slot.getIndex()];
				this.replaceModifier(mutableMultimap, Attributes.ARMOR, uuid, myItem.extraArmor);
			}
		}
		return toImmutableMultimap(mutableMultimap);
	}

	private void replaceModifier(ListMultimap<Attribute, AttributeModifier> multimap, Attribute attribute, UUID id, double newValue) {
		final Collection<AttributeModifier> modifiers = multimap.get(attribute);
		final Optional<AttributeModifier> modifierOptional = 
      modifiers.stream().filter(attributeModifier -> attributeModifier.getId().equals(id)).findFirst();

		if (modifierOptional.isPresent()) {
			final AttributeModifier modifier = modifierOptional.get();
			modifiers.remove(modifier);
			modifiers.add(new AttributeModifier(modifier.getId(), modifier.getName(), modifier.getAmount() + newValue, 	modifier.getOperation()));
		}
	}

 

 

Attempt 2: Add unique modifier

Spoiler

	public static UUID MY_ITEM_ARMOR_UUID = UUID.fromString("b5ba8c15-4e13-437b-9d3a-d80f2849362f");
	public AttributeModifier MY_ITEM_ARMOR_MODIFIER = new AttributeModifier(MY_ITEM_ARMOR_UUID, "My item armor modifier", this.extraArmor, AttributeModifier.Operation.ADDITION);

	@Override
	public Multimap<Attribute, AttributeModifier> getAttributeModifiers(EquipmentSlotType slot, ItemStack stack) {
		Multimap<Attribute, AttributeModifier> multimap = super.getAttributeModifiers(slot, stack);
		ListMultimap<Attribute, AttributeModifier> mutableMultimap = toMutableMultimap(multimap);
		final Collection<AttributeModifier> modifiers = mutableMultimap.get(Attributes.ARMOR);
      
		final Optional<AttributeModifier> modifierOptional = modifiers.stream()
      	.filter(attributeModifier -> attributeModifier.getId().equals(MY_ITEM_ARMOR_UUID)).findFirst();

		if (!modifierOptional.isPresent() && item instanceof MyItem && slot == this.slot) {
			modifiers.add(MY_ITEM_ARMOR_MODIFIER);
		}
		return toImmutableMultimap(mutableMultimap);
	}

 

 

Edited July 5, 20214 yr by urbanxx001

Here's the class for attempt 1. For attempt 2 I've also tried assigning random UUID's and/or random modifier names for each item instance, but they still remain linked.

Spoiler

package com.author.my_mod.group.items;

import com.google.common.collect.*;
import net.minecraft.entity.ai.attributes.*;
import net.minecraft.entity.player.PlayerEntity;
import net.minecraft.inventory.EquipmentSlotType;
import net.minecraft.item.*;
import net.minecraft.util.*;
import net.minecraft.world.World;
import net.minecraftforge.fml.common.ObfuscationReflectionHelper;

import java.util.*;

public class MyItem extends ArmorItem {
	private final EquipmentSlotType slot;
	public int extraArmor = 0;

	public MyItem(IArmorMaterial material, EquipmentSlotType slot, Properties properties) {
		super(material, slot, properties);
		this.slot = slot;
	}

	public static<K, V> ListMultimap<K, V> toMutableMultimap(Multimap<K, V> original) {
		ListMultimap<K, V> copy = ArrayListMultimap.create();
		copy.putAll(original);
		return copy;
	}

	public static<K, V> ImmutableMultimap<K, V> toImmutableMultimap(ListMultimap<K, V> original) {
		return new ImmutableMultimap.Builder<K, V>().putAll(original).build();
	}

	@Override
	public Multimap<Attribute, AttributeModifier> getAttributeModifiers(EquipmentSlotType slot, ItemStack stack) {
		Multimap<Attribute, AttributeModifier> multimap = super.getAttributeModifiers(slot, stack);
		ListMultimap<Attribute, AttributeModifier> mutableMultimap = toMutableMultimap(multimap);

		Item item = stack.getItem();
		if (item instanceof MyItem && slot == this.slot) {
			UUID[] uuid_arr = ObfuscationReflectionHelper.getPrivateValue(ArmorItem.class, (ArmorItem) item, "ARMOR_MODIFIER_UUID_PER_SLOT");
			if (uuid_arr != null) {
				MyItem myItem = (MyItem) item;
				UUID uuid = uuid_arr[slot.getIndex()];
				this.replaceModifier(mutableMultimap, Attributes.ARMOR, uuid, myItem.extraArmor);
			}
		}
		return toImmutableMultimap(mutableMultimap);
	}

	private void replaceModifier(ListMultimap<Attribute, AttributeModifier> multimap, Attribute attribute, UUID id, double newValue) {
		final Collection<AttributeModifier> modifiers = multimap.get(attribute);
		final Optional<AttributeModifier> modifierOptional = modifiers.stream().filter(attributeModifier -> attributeModifier.getId().equals(id)).findFirst();

		if (modifierOptional.isPresent()) {
			final AttributeModifier modifier = modifierOptional.get();
			modifiers.remove(modifier);
			modifiers.add(new AttributeModifier(modifier.getId(), modifier.getName(), modifier.getAmount() + newValue, modifier.getOperation()));
		}
	}

	@Override
	public ActionResult<ItemStack> use(World world, PlayerEntity player, Hand hand) {
		final ItemStack stack = player.getItemInHand(hand);
		if (!world.isClientSide()) {
			this.extraArmor++;
			if (this.extraArmor > 3) {
				this.extraArmor = 0;
			}
		}
		return new ActionResult<>(ActionResultType.CONSUME, stack);
	}

}

 

 

Edited July 5, 20214 yr by urbanxx001

2 minutes ago, urbanxx001 said:

Here's the class

You can't really have an instance field like extraArmor in item classes, since items are singleton and every stack will share the same item. This means that for example if one player uses the item and changes the extraArmor field, it will also change for all other players that also have that item.

1 minute ago, vemerion said:

You can't really have an instance field like extraArmor in item classes, since items are singleton and every stack will share the same item. This means that for example if one player uses the item and changes the extraArmor field, it will also change for all other players that also have that item.

Ah ok. So I should store it in NBT instead?

Just now, urbanxx001 said:

Ah ok. So I should store it in NBT instead?

Yeah, either NBT or a capability should work.