validateRecord routine just allows you to raise exceptions/errors or override. Also you can try to change the content of local fields, but it will not allow you to change CORE records. As it is mentioned in previous answer, you need to implement defaultFieldValues method to make changes on record. 2 important point with respect L3 Java Extensibility:

• HOOK API will be attached with just one Class>method combination. I mean, the functionality to implement should be that just is related with the specific method.
• Java Doc is important start point