@Override
public NBTTagCompound writeToNBT(NBTTagCompound compound){
    compound = super.writeToNBT(compound);

    NBTTagCompound items = new NBTTagCompound();
    for(int i = 0; i < inventory.length; i++) {
        ItemStack stack = inventory[i];
        if(stack != null) {
            compound.setTag("" + i, stack.writeToNBT(new NBTTagCompound()));
        }
    }
    compound.setTag("items", items);

    return compound;
}

@Override
public void readFromNBT(NBTTagCompound compound){
    super.readFromNBT(compound);

    NBTTagCompound items = compound.getCompoundTag("items");
    for(int i = 0; i < inventory.length; i++){
        inventory[i] = items.hasKey("" + i) ? ItemStack.loadItemStackFromNBT(items.getCompoundTag("" + i)) : null;
    }
}