Java construct a superclass instance with subclass instance

Question

What I was trying to do is like

SuperClass super = convertToSuperClassInstance(SubClass sub);

Clearly sub could directly sign to super, But I need super should be SuperClass(remove useless subclass data), Due to other persist related things, Is there any trick I could do That?

Create new SuperClass and sign all object data to it seems no elegant to me QAQ

I'm afraid you'll have to create your own copy-constructor on the super class. Then you can do SuperClass sup = new SuperClass(sub); — Thilo
– Thilo, Commented Apr 25, 2020 at 8:46
you can cast the object to the superclass, however the overridden methods won't behave like they were of the superclass, more info here: stackoverflow.com/a/21613593/11377112 — Animesh Sahu
– Animesh Sahu, Commented Apr 25, 2020 at 8:49

jurez · Accepted Answer · 2020-04-25 08:46:05Z

4

An instance of subclass will always have all the fields and methods of a superclass. Casting the subclass to superclass (i.e. (SuperClass) subclass) will not remove the data, it will just tell the compiler to apply the rules for SuperClass, which means you will get a compile-time error if you try to access subclass' data. Nevertheless, you can still downcast it back to subclass or use reflection - the data is there. This is by design and you can't get away from it.

If you want to really "remove" the data of a subclass, you need to construct a new instance of a superclass, not subclass. The easiest way would be to add a constructor to superclass that takes a superclass as a parameter and copies all superclass-related data from it.

answered Apr 25, 2020 at 8:46

jurez

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1 Comment

Animesh Sahu Over a year ago

This is neat and the only possible way iirc, but the overridden methods won't behave like they were of the superclass, btw +1, more info here: stackoverflow.com/a/21613593/11377112

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