public class DemoBlock extends BlockWithEntity {
    [...]
    @Override
    protected ItemActionResult onUseWithItem(ItemStack stack, BlockState state, World world, BlockPos pos, PlayerEntity player, Hand hand, BlockHitResult hit) {
        if (world.isClient) return ActionResult.SUCCESS;
        
        if (!(world.getBlockEntity(pos) instanceof DemoBlockEntity blockEntity)) {
            return ItemActionResult.PASS_TO_DEFAULT_BLOCK_INTERACTION;
        }

        if (!player.getStackInHand(hand).isEmpty()) {
            // Check what is the first open slot and put an item from the player's hand there
            if (blockEntity.getStack(0).isEmpty()) {
                // Put the stack the player is holding into the inventory
                blockEntity.setStack(0, player.getStackInHand(hand).copy());
                // Remove the stack from the player's hand
                player.getStackInHand(hand).setCount(0);
            } else if (blockEntity.getStack(1).isEmpty()) {
                blockEntity.setStack(1, player.getStackInHand(hand).copy());
                player.getStackInHand(hand).setCount(0);
            } else {
                // If the inventory is full we'll notify the player
                player.sendMessage(Text.literal("The inventory is full! The first slot holds ")
                    .append(blockEntity.getStack(0).getName())
                    .append(" and the second slot holds ")
                    .append(blockEntity.getStack(1).getName()));
            }
        } 
        return ItemActionResult.SUCCESS;
    }
}