CapabilityTargettable.java

package gigaherz.survivalist.itemcap;

import net.minecraft.entity.Entity;
import net.minecraft.nbt.NBTBase;
import net.minecraft.nbt.NBTTagCompound;
import net.minecraft.util.EnumFacing;
import net.minecraftforge.common.capabilities.Capability;
import net.minecraftforge.common.capabilities.CapabilityInject;
import net.minecraftforge.common.capabilities.CapabilityManager;

import java.util.UUID;
import java.util.concurrent.Callable;

public class CapabilityTargettable
{
    @CapabilityInject(ITargettable.class)
    public static Capability<ITargettable> TARGETTABLE_CAPABILITY = null;

    /**
     * Call this from pre-init if you want to use this capability.
     */
    public static void register()
    {
        CapabilityManager.INSTANCE.register(ITargettable.class, new Storage(), new Callable<ITargettable>()
        {
            @Override
            public ITargettable call() throws Exception
            {
                return new TargettableImpl();
            }
        });
    }

    private static class Storage
            implements Capability.IStorage<ITargettable>
    {
        @Override
        public NBTBase writeNBT(Capability<ITargettable> capability, ITargettable instance, EnumFacing side)
        {
            NBTTagCompound data = new NBTTagCompound();

            data.setString("Target", instance.getTarget().getPersistentID().toString());

            return data;
        }

        @Override
        public void readNBT(Capability<ITargettable> capability, ITargettable instance, EnumFacing side, NBTBase nbt)
        {
            NBTTagCompound data = (NBTTagCompound) nbt;

            UUID target = UUID.fromString(data.getString("Target"));

            // TODO: UUID to Entity -----------------------------------------------------------------------------------------
            Entity targetEntity = null;

            instance.setTarget(targetEntity);
        }
    }
}

ITargettable.java

package gigaherz.survivalist.itemcap;

import net.minecraft.entity.Entity;

public interface ITargettable
{
    Entity getTarget();
    void setTarget(Entity entity);
}

ItemTargettable.java

package gigaherz.survivalist.itemcap;

import net.minecraft.entity.Entity;
import net.minecraft.item.Item;
import net.minecraft.item.ItemStack;
import net.minecraft.nbt.NBTTagCompound;
import net.minecraft.util.EnumFacing;
import net.minecraftforge.common.capabilities.Capability;
import net.minecraftforge.common.capabilities.ICapabilityProvider;
import net.minecraftforge.common.capabilities.ICapabilitySerializable;

public class ItemTargettable extends Item
{
    @Override
    public ICapabilityProvider initCapabilities(ItemStack stack, NBTTagCompound nbt)
    {
        return new ICapabilitySerializable<NBTTagCompound>()
        {
            TargettableImpl instance = new TargettableImpl();

            @Override
            public boolean hasCapability(Capability<?> capability, EnumFacing facing)
            {
                return capability == CapabilityTargettable.TARGETTABLE_CAPABILITY;
            }

            @SuppressWarnings("unchecked")
            @Override
            public <T> T getCapability(Capability<T> capability, EnumFacing facing)
            {
                if (capability == CapabilityTargettable.TARGETTABLE_CAPABILITY)
                    return (T)instance;

                return null;
            }

            @Override
            public NBTTagCompound serializeNBT()
            {
                return (NBTTagCompound)CapabilityTargettable.TARGETTABLE_CAPABILITY.getStorage()
                        .writeNBT(CapabilityTargettable.TARGETTABLE_CAPABILITY, instance, null);
            }

            @Override
            public void deserializeNBT(NBTTagCompound nbt)
            {
                CapabilityTargettable.TARGETTABLE_CAPABILITY.getStorage()
                        .readNBT(CapabilityTargettable.TARGETTABLE_CAPABILITY, instance, null, nbt);
            }
        };
    }

    public void setTarget(ItemStack stack, Entity target)
    {
        if(stack.hasCapability(CapabilityTargettable.TARGETTABLE_CAPABILITY, null))
            stack.getCapability(CapabilityTargettable.TARGETTABLE_CAPABILITY, null).setTarget(target);
    }

    public Entity getTarget(ItemStack stack)
    {
        if(stack.hasCapability(CapabilityTargettable.TARGETTABLE_CAPABILITY, null))
            return stack.getCapability(CapabilityTargettable.TARGETTABLE_CAPABILITY, null).getTarget();
        return null;
    }
}

TargettableImpl.java

package gigaherz.survivalist.itemcap;

import net.minecraft.entity.Entity;

public class TargettableImpl implements ITargettable
{
    private Entity target;

    public Entity getTarget()
    {
        return target;
    }

    public void setTarget(Entity target)
    {
        this.target = target;
    }
}