private IItemHandler createHandler() {
    return new ItemStackHandler(1) {
        @Override
        public boolean isItemValid(int slot, @Nonnull ItemStack stack) {
            return stack.getItem() == ModItems.COPPERCOIN;
        }
    };
}

This, as you can see, would only work with the Copper Coin item. I've tried several ways to make it accept Copper Coin, Silver Coin, and Gold Coin, but they always end up just not accepting literally any item or any item BUT the ones i want. How do i go about doing this?

Edited June 28, 20205 yr by vladilima

return stack.getItem() == ModItems.THING1 || stack.getItem() == ModItems.THING2;

Use equality checks and OR conditionals.

Thanks! I tried this before in a different way, but the way you described it worked!

Use tags.