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GregorianCalendar yes, Calendar no.

Does that make sense? There are 3 classes that you should use a factory method for and that is Calendar, DateFormat and NumberFormat. So say the SCJP studying...

Yes you cannot have a final abstract class. There is nothing that says you MUST do what I recommended but its best practice and for your own good really :)

But if you wish you can do just what...

Usually when you create a utility class such as Math you want to mark the constructors as private or protected because it would not make any sense instantiating a class that has only static methods....