Re: A basic question about constants

In article <42e2893e$0$14407$626a14ce@xxxxxxxxxxxx>,
Richard <grin@xxxxxxxx> wrote:

> John B. Matthews a écrit :
> > In article <42e1fbdb$0$29943$636a15ce@xxxxxxxxxxxx>,
> > Richard <grin@xxxxxxxx> wrote:
> >
> >
> >>Hello,
> >>
> >>An extract from the javadoc of javax.sql.rowset.CachedRowSet (an interface):
> >>---
> >>COMMIT_ON_ACCEPT_CHANGES
> >>
> >>static final boolean COMMIT_ON_ACCEPT_CHANGES
> >>
> >>Causes the CachedRowSet object's SyncProvider to commit the changes when
> >>acceptChanges() is called. If set to false, the changes will not be
> >>committed until one of the CachedRowSet interface transaction methods is
> >>called.
> >>
> >>See Also:
> >> commit(), rollback(), Constant Field Values
> >>---
> >>
> >>My basic question is: how is it possible to change the value of a
> >>constant defined in an interface?
> >>
> >>Thanks in advance for your answers.
> >>
> >>Richard
> >
> >
> > AFAIK, you can't: "Every field declaration in the body of an interface
> > is implicitly public, static, and final." [§9.3]

This is true.

> > The value is set by the implementation so you'll know how to handle commits:

My answer here can't be right. The javadoc says "acceptChanges() method
will determine if the COMMIT_ON_ACCEPT_CHANGES is set to true or not,"
implying that it can be changed. But it _can't_ be changed, unless it
implements a different interface.

> But how can the implementation set the value of the constant? It is not
> possible to change the value of a constant and the constant must be
> initialized in the interface.
>
> For example,
>
> public interface I {
> boolean constant = false;
> }
>
> public class AClass implements I {
> static {
> constant = true;
> }
>
> public boolean getConstant() {
> return constant;
> }
> }
>
> does not compile :
> javac AClass.java
> AClass.java:3: cannot assign a value to final variable constant
> constant = true;
>
> There is certainly a thing that I have not understood...

You are correct. I had imagined the implementation somehow "overriding"
the value. That's not Java:-)

> Regards,
>
> Richard
>
> >
> > "Note: The acceptChanges() method will determine if the
> > COMMIT_ON_ACCEPT_CHANGES is set to true or not. If it is set to true,
> > all updates in the synchronization are committed to the data source.
> > Otherwise, the application _must_ explicit[l]y call the commit() or
> > rollback() methods as appropriate." [J2SE 1.5 API]

I wonder now if this is meant to be advice to providers who implement
the CachedRowSet interface. Surely, users don't have to check
COMMIT_ON_ACCEPT_CHANGES to determine if acceptChanges() does the job!
Perhaps COMMIT_ON_ACCEPT_CHANGES being definitively true means that a
conforming implementation does the right thing in acceptChanges(), ie.
call commit if auto_commit is off in the database.

Are you implementing or instantiating?

--
John
jmatthews at wright dot edu
www dot wright dot edu/~john.matthews/
.

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