Re: Demoting and Promoting Serialised Objects

"Roedy Green" <look-on@xxxxxxxxxxxxxxxxxxxx> schreef in bericht
news:6kqkc1pjsre6rts11roqhks9m9oq0f6t27@xxxxxxxxxx
>I see a general problem I call demoting and promoting objects.
>
> A Dog gets promoted to a Dalmatian object by gaining extra
> Dalmatian-specific fields.
>
> A Dalmatian gets demoted to Dog object by stripping off its Dalmatian
> specific fields.
>
> The object itself is unchanged. You create a new modified object and
> discard the old one.
>
>
> Why would you do such a thing?
>
> 1. to strip and object down for export, serialisation, or RMI
> transport.

Isn't this where readObject and writeObject would come in handy?

> 2. to strip an object of confidential information before handing [it]
> on.
>
> 3. to avoid having to give confidential code used to process, edit or
> construct your objects when all they will do is view them.
>
> The way you did the equivalent thing in COBOL was a MOVE CORRESPONDING
> that copied field by field. In Java traditionally you do the same
> thing writing code guaranteed to stop working the instant any of the
> class structure changes.
>
> How might you handle this more elegantly?
>
> 1. a class parser then generates code to do the move-corresponding.
>
> 2. Java gain some feature to easily create copy constructor so you
> could say:
>
> something like:
>
> /**
> * cloning constructor
> * Makes a Dog out of any Dog object including a Dalmatian object.
> * The new object will have only the Dog fields and methods, even if
> * cast back to the original type.
> */

You cannot 'cast back to the original type', because it was lost in the
conversion object.

> Dog ( Dog ) { Magic.copy(); }
>
> Where copy is some magic JNI or new Java feature.

Why not reflection?

> The assembler code theoretically could be just a block byte move of
> dog's object size bytes -- extremely fast compared with field by field
> copy.

Field by field copy bytecode could be automagically optimized into just that.


.

Relevant Pages

  • Demoting and Promoting Serialised Objects
    ... I see a general problem I call demoting and promoting objects. ... A Dalmatian gets demoted to Dog object by stripping off its Dalmatian ... Java gain some feature to easily create copy constructor so you ...
    (comp.lang.java.programmer)
  • Re: cast to sub-class or extending instance of super or a.n.other
    ... You need what is called a copy constructor. ... Java has no easy way of composing one. ... working if Dog acquires any more fields. ... A new Dog takes a Dalmatian and strips it down. ...
    (comp.lang.java.programmer)
  • Re: Please help - dog with kidney failure
    ... Our dalmatian hasn't been eating for the last ... > thought she had blood in her urine - we would see blood on the fur. ... > her back for more fluids tomorrow. ... Change dog breeds: quit buying dalamations. ...
    (rec.pets.dogs.health)
  • Re: About the private member
    ... You might use code like your example in a copy constructor ... Dalmatian to Dog. ... Roedy Green Canadian Mind Products ...
    (comp.lang.java.programmer)
  • Re: charming, again
    ... A Dalmatian broke sits.. ... I imagine the owner proofed that dog VERY well, but even then, this HAD to be a worse case scenario, and the sit held. ... Of all the dogs in there, the last one I would have attacked, were I a sentient dog, would have been the Airedale. ... How was the Dalmatian removed? ...
    (rec.pets.dogs.behavior)