public class DemoBlock extends BlockWithEntity {
    [...]
    @Override
    protected ItemActionResult onUseWithItem(ItemStack stack, BlockState state, World world, BlockPos pos, PlayerEntity player, Hand hand, BlockHitResult hit) {
        ...
        if (!player.getStackInHand(hand).isEmpty()) {
            ...
        } else {
            // If the player is not holding anything we'll get give him the items in the block entity one by one

            // Find the first slot that has an item and give it to the player
            if (!blockEntity.getStack(1).isEmpty()) {
                // Give the player the stack in the inventory
                player.getInventory().offerOrDrop(blockEntity.getStack(1));
                // Remove the stack from the inventory
                blockEntity.removeStack(1);
            } else if (!blockEntity.getStack(0).isEmpty()) {
                player.getInventory().offerOrDrop(blockEntity.getStack(0));
                blockEntity.removeStack(0);
            } else {
                return ItemActionResult.PASS_TO_DEFAULT_BLOCK_INTERACTION;
            }
        }
        
        return ItemActionResult.SUCCESS;
    }
}