public class DemoBlockEntity extends BlockEntity implements ImplementedContainer {
    [...]
    @Override
    public void loadAdditional(ValueInput valueInput) {
        super.loadAdditional(valueInput);
        items.clear();
        ContainerHelper.loadAllItems(valueInput, items);
    }

    @Override
    public void saveAdditional(ValueOutput valueOutput) {
        super.saveAdditional(valueOutput);
        ContainerHelper.saveAllItems(valueOutput, items);
    }
}