[Advanced-java] Another odd question - how to convert String to literal?

[philion@xxxxxxxxxxxxxx (Paul Philion)]

Alvin -

Um... why do you want to?

The easy answer is: Cast it.

A more useful answer, however, it to ask yourself why you need the class
type. If you are going to write literal code against the deserialized
object, then the code has to know what type the object is.

In similar circumstances, I have made the MyClass implement an interface
that all the DB-serialized objects implement. That way, I always cast the
deserialized object into the interface and go from there.

HOWEVER... If you are building something a little more generic, and you
might be storing objects like Customer and Product (that are very different
and don't really share an abstract interface) then you need to build some
sort of flexible Mediator code that examines the object, reads the class
directly form the object (no need for the string class name in the db) and
determines what to do with the object based on the class. Typically, I'll do
a callback-type structure where different classes will register with the
Mediator to be notified when an object of a certain type is deserialized
from the db.

The interaction would go something like this:

- MyClass instance "a" is serialized to the db.
- MyClassHandler registers itself with the Mediator, asking to be notified
when classes of type "MyClass" are deserialized.
- The Mediator stores the reference to the MyClass and to MyClassHandler (in
an internal hashmap, for example).
- MyClass instance "a" is deserialized into Object "x".
- The Mediator gets the class from Object "x" (which is MyClass) and looks
it up.
- The Mediator discovers that MyClassHandler is interested in MyClass
objects.
- The Mediator calls a method on MyClassHandler and passes object "x".
- MyClassHandler knows (by contract, it only registered to be notified of
MyClass instances. An excellent place to but the new 1.4 assertions to work)
that the object "x" is really a MyClass, so it just casts it.
- MyClassHander then does all the work necessary to make stuff happen
correctly with the MyClass instance "a".

I hope this doesn't further confuse stuff for you...

- Paul Philion

----- Original Message -----
From: "Alvin Wang" <xwang@xxxxxxxxxxxxxxxxx>
To: "Advanced-Java" <advanced-java@xxxxxxxxxxxxxxxxxxxxxx>
Sent: Friday, March 29, 2002 11:36 AM
Subject: [Advanced-java] Another odd question - how to convert String to
literal?


> Hi! Thanks for those helping me with my odd question. Maybe they are
naive.
> Here is another one.
>
> I serialized an MyClass object into byte[], and then put the byte[] into
> database, along with the class name "MyClass" as String. Later on, another
> program looks up the databaase, and picks up the byte[] and the class name
> "MyClass". It will deserialize byte[] into Object obj. However, I do not
> know how to convert obj to be a MyClass object. That means I do not know
how
> to convert String "MyClass" to java language class name literal of
MyClass.
> I do not think Reflection will help, because I am not building a new
MyClass
> object.
>
> Can any guru help me out? Thanks!
>
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