public class DemoBlockEntity extends BlockEntity implements ImplementedInventory, SidedInventory {
    [...]
    @Override
    public int[] getInvAvailableSlots(Direction var1) {
        // Just return an array of all slots
        int[] result = new int[getItems().size()];
        for (int i = 0; i < result.length; i++) {
            result[i] = i;
        }

        return result;
    }

    @Override
    public boolean canInsert(int slot, ItemStack stack, Direction direction) {
        return direction != Direction.UP;
    }

    @Override
    public boolean canExtract(int slot, ItemStack stack, Direction direction) {
        return true;
    }
}