It has too much overhead to decide at run-time which superclass needs to be used, whereas interfaces are done at compile time and don't bog down run time. When I said it was possible but the Java developers just decided not to implement it in the programming language, the interviewer seemed a little taken aback and asked if I though the Java developers were too lazy/didn't know how to implement it. This made me second-guess myself because the interviewer gave a semi-shocked reaction. If he doesn't know that they simply didn't want to implement it, he shouldn't be asking the question in an interview. If he is trying to trick candidates, that is unprofessional.