CDK 1.2 code
carbon.setHydrogenCount(4);CDK 1.4 code
carbon.setImplicitHydrogenCount(4);Yeah, that's a simple one. Just to make clear, in both versions the count reflected the number of implicit hydrogens. The getHydrogenCount() suggested, however, to return the number of all hydrogens attached to that atom, that is, the sum of implicit and explicit hydrogens.